U.S. patent application number 17/466196 was filed with the patent office on 2021-12-23 for methods for driving electro-optic displays.
The applicant listed for this patent is E INK CORPORATION. Invention is credited to Demetrious Mark HARRINGTON.
Application Number | 20210398476 17/466196 |
Document ID | / |
Family ID | 1000005822907 |
Filed Date | 2021-12-23 |
United States Patent
Application |
20210398476 |
Kind Code |
A1 |
HARRINGTON; Demetrious
Mark |
December 23, 2021 |
METHODS FOR DRIVING ELECTRO-OPTIC DISPLAYS
Abstract
Methods for driving electro-optic displays, especially bistable
displays, include (a) using two-part waveforms, the first part of
which is dependent only upon the initial state of the relevant
pixel; (b) measuring the response of each individual pixel and
storing for each pixel data indicating which of a set of standard
drive schemes are to be used for that pixel; (c) for at least one
transition in a drive scheme, applying multiple different waveforms
to pixels on a random basis; and (d) when updating a limited area
of the display, driving "extra" pixels in an edge elimination
region to avoid edge effects.
Inventors: |
HARRINGTON; Demetrious Mark;
(Cambridge, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
E INK CORPORATION |
Billerica |
MA |
US |
|
|
Family ID: |
1000005822907 |
Appl. No.: |
17/466196 |
Filed: |
September 3, 2021 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16538254 |
Aug 12, 2019 |
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17466196 |
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15207571 |
Jul 12, 2016 |
10380954 |
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16538254 |
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14193081 |
Feb 28, 2014 |
9495918 |
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15207571 |
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61773916 |
Mar 7, 2013 |
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61779413 |
Mar 13, 2013 |
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61771318 |
Mar 1, 2013 |
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61774985 |
Mar 8, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G09G 2320/0257 20130101;
G09G 3/2044 20130101; G09G 2310/0251 20130101; G09G 2340/16
20130101; G02B 26/005 20130101; G02F 1/163 20130101; G09G 2330/021
20130101; G09G 3/2051 20130101; G09G 2320/0295 20130101; G09G 3/38
20130101; G09G 3/2048 20130101; G09G 2320/0693 20130101; G09G
2230/00 20130101; G09G 2300/0426 20130101; G09G 3/344 20130101;
G09G 2310/04 20130101; G09G 3/2007 20130101; G09G 2310/062
20130101; G02F 1/1681 20190101 |
International
Class: |
G09G 3/20 20060101
G09G003/20; G09G 3/34 20060101 G09G003/34; G09G 3/38 20060101
G09G003/38; G02F 1/163 20060101 G02F001/163 |
Claims
1. A method of driving an electro-optic display having a plurality
of pixels using a drive scheme which defines the waveform to be
applied to each pixel for each transition from an initial gray
level to final gray level, and wherein, for at least one transition
in the drive scheme, multiple waveforms are provided in the drive
scheme, and these multiple waveforms are applied to pixels
undergoing the relevant transition on a random basis, so that
different pixels undergoing the same transition experience
different waveforms.
2. A method according to claim 1 further comprising the step of
changing, at intervals, which of the multiple waveform is used for
which said at least one transition at a particular pixel.
3. A method according to claim 1 further comprising tracking the DC
imbalance at each pixel and making the selection from among the
multiple waveforms to reduce the accumulated DC imbalance.
4. A method according to claim 1 wherein multiple waveforms are
provided for each transition in the drive scheme, and any specific
pixel uses the same waveform for each transition.
5. A method according to claim 1 wherein the selection from among
the multiple waveforms is made so that a fixed image is always or
intermittently visible in the background of the display.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation application of U.S.
application Ser. No. 16/538,254, filed on Aug. 12, 2019; which is a
divisional application of U.S. patent application Ser. No.
15/207,571 filed on Jul. 12, 2016, now issued as U.S. Pat. No.
10,380,954; which itself is a divisional application of U.S. patent
application Ser. No. 14/193,081 filed Feb. 28, 2014, now issued as
U.S. Pat. No. 9,495,918. This application claims benefit of (a)
provisional Application Ser. No. 61/771,318, filed Mar. 1, 2013;
(b) provisional Application Ser. No. 61/773,916, filed Mar. 7,
2013; (c) provisional Application Ser. No. 61/774,985, filed Mar.
8, 2013; and (d) provisional Application Ser. No. 61/779,413, filed
Mar. 13, 2013.
[0002] This application is related to U.S. Pat. Nos. 5,930,026;
6,445,489; 6,504,524; 6,512,354; 6,531,997; 6,753,999; 6,825,970;
6,900,851; 6,995,550; 7,012,600; 7,023,420; 7,034,783; 7,116,466;
7,119,772; 7,193,625; 7,202,847; 7,259,744; 7,304,787; 7,312,794;
7,327,511; 7,453,445; 7,492,339; 7,528,822; 7,545,358; 7,583,251;
7,602,374; 7,612,760; 7,679,599; 7,688,297; 7,729,039; 7,733,311;
7,733,335; 7,787,169; 7,952,557; 7,956,841; 7,999,787; 8,077,141;
8,125,501; 8,139,050; 8,174,490; 8,289,250; 8,300,006; and
8,314,784; 9,721,495; 9,726,957; 10,672,350; and U.S. Patent
Applications Publication Nos. 2003/0102858; 2005/0122284;
2005/0179642; 2005/0253777; 2007/0091418; 2007/0103427;
2008/0024429; 2008/0024482; 2008/0136774; 2008/0150888;
2008/0291129; 2009/0174651; 2009/0179923; 2009/0195568;
2009/0322721; 2010/0045592; 2010/0220121; 2010/0220122;
2010/0265561; 2011/0187684; 2011/0193840; 2011/0193841;
2011/0199671; and 2011/0285754.
[0003] The aforementioned patents and applications may hereinafter
for convenience collectively be referred to as the "MEDEOD"
(MEthods for Driving Electro-Optic Displays) applications. The
entire contents of these patents and copending applications, and of
all other U.S. patents and published and copending applications
mentioned below, are herein incorporated by reference.
BACKGROUND OF INVENTION
[0004] The present invention relates to methods for driving
electro-optic displays, especially bistable electro-optic displays,
and to apparatus for use in such methods. Some aspects of the
present invention relate to driving methods which may allow for
reduced "ghosting" and edge effects. Other aspects of the present
invention relate to reduction of noise in the images on
electro-optic displays; such noise may include that referred to as
"grain" or "mottle" and is believed (although the invention is in
no way limited by this belief) to be due to non-uniformities in the
electro-optic material itself. This invention is especially, but
not exclusively, intended for use with particle-based
electrophoretic displays in which one or more types of electrically
charged particles are present in a fluid and are moved through the
fluid under the influence of an electric field to change the
appearance of the display.
[0005] The term "electro-optic", as applied to a material or a
display, is used herein in its conventional meaning in the imaging
art to refer to a material having first and second display states
differing in at least one optical property, the material being
changed from its first to its second display state by application
of an electric field to the material. Although the optical property
is typically color perceptible to the human eye, it may be another
optical property, such as optical transmission, reflectance,
luminescence or, in the case of displays intended for machine
reading, pseudo-color in the sense of a change in reflectance of
electromagnetic wavelengths outside the visible range.
[0006] The term "gray state" is used herein in its conventional
meaning in the imaging art to refer to a state intermediate two
extreme optical states of a pixel, and does not necessarily imply a
black-white transition between these two extreme states. For
example, several of the E Ink patents and published applications
referred to below describe electrophoretic displays in which the
extreme states are white and deep blue, so that an intermediate
"gray state" would actually be pale blue. Indeed, as already
mentioned, the change in optical state may not be a color change at
all. The terms "black" and "white" may be used hereinafter to refer
to the two extreme optical states of a display, and should be
understood as normally including extreme optical states which are
not strictly black and white, for example the aforementioned white
and dark blue states. The term "monochrome" may be used hereinafter
to denote a drive scheme which only drives pixels to their two
extreme optical states with no intervening gray states.
[0007] The terms "bistable" and "bistability" are used herein in
their conventional meaning in the art to refer to displays
comprising display elements having first and second display states
differing in at least one optical property, and such that after any
given element has been driven, by means of an addressing pulse of
finite duration, to assume either its first or second display
state, after the addressing pulse has terminated, that state will
persist for at least several times, for example at least four
times, the minimum duration of the addressing pulse required to
change the state of the display element. It is shown in U.S. Pat.
No. 7,170,670 that some particle-based electrophoretic displays
capable of gray scale are stable not only in their extreme black
and white states but also in their intermediate gray states, and
the same is true of some other types of electro-optic displays.
This type of display is properly called "multi-stable" rather than
bistable, although for convenience the term "bistable" may be used
herein to cover both bistable and multi-stable displays.
[0008] The term "impulse" is used herein in its conventional
meaning of the integral of voltage with respect to time. However,
some bistable electro-optic media act as charge transducers, and
with such media an alternative definition of impulse, namely the
integral of current over time (which is equal to the total charge
applied) may be used. The appropriate definition of impulse should
be used, depending on whether the medium acts as a voltage-time
impulse transducer or a charge impulse transducer.
[0009] Much of the discussion below will focus on methods for
driving one or more pixels of an electro-optic display through a
transition from an initial gray level to a final gray level (which
may or may not be different from the initial gray level). The term
"waveform" will be used to denote the entire voltage against time
curve used to effect the transition from one specific initial gray
level to a specific final gray level. Typically such a waveform
will comprise a plurality of waveform elements; where these
elements are essentially rectangular (i.e., where a given element
comprises application of a constant voltage for a period of time);
the elements may be called "pulses" or "drive pulses". The term
"drive scheme" denotes a set of waveforms sufficient to effect all
possible transitions between gray levels for a specific display. A
display may make use of more than one drive scheme; for example,
the aforementioned U.S. Pat. No. 7,012,600 teaches that a drive
scheme may need to be modified depending upon parameters such as
the temperature of the display or the time for which it has been in
operation during its lifetime, and thus a display may be provided
with a plurality of different drive schemes to be used at differing
temperature etc. A set of drive schemes used in this manner may be
referred to as "a set of related drive schemes." It is also
possible, as described in several of the aforementioned MEDEOD
applications, to use more than one drive scheme simultaneously in
different areas of the same display, and a set of drive schemes
used in this manner may be referred to as "a set of simultaneous
drive schemes."
[0010] Several types of electro-optic displays are known. One type
of electro-optic display is a rotating bichromal member type as
described, for example, in U.S. Pat. Nos. 5,808,783; 5,777,782;
5,760,761; 6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467;
and 6,147,791 (although this type of display is often referred to
as a "rotating bichromal ball" display, the term "rotating
bichromal member" is preferred as more accurate since in some of
the patents mentioned above the rotating members are not
spherical). Such a display uses a large number of small bodies
(typically spherical or cylindrical) which have two or more
sections with differing optical characteristics, and an internal
dipole. These bodies are suspended within liquid-filled vacuoles
within a matrix, the vacuoles being filled with liquid so that the
bodies are free to rotate. The appearance of the display is changed
by applying an electric field thereto, thus rotating the bodies to
various positions and varying which of the sections of the bodies
is seen through a viewing surface. This type of electro-optic
medium is typically bistable.
[0011] Another type of electro-optic display uses an electrochromic
medium, for example an electrochromic medium in the form of a
nanochromic film comprising an electrode formed at least in part
from a semi-conducting metal oxide and a plurality of dye molecules
capable of reversible color change attached to the electrode; see,
for example O'Regan, B., et al., Nature 1991, 353, 737; and Wood,
D., Information Display, 18(3), 24 (March 2002). See also Bach, U.,
et al., Adv. Mater., 2002, 14(11), 845. Nanochromic films of this
type are also described, for example, in U.S. Pat. Nos. 6,301,038;
6,870,657; and 6,950,220. This type of medium is also typically
bistable.
[0012] Another type of electro-optic display is an electro-wetting
display developed by Philips and described in Hayes, R. A., et al.,
"Video-Speed Electronic Paper Based on Electrowetting", Nature,
425, 383-385 (2003). It is shown in U.S. Pat. No. 7,420,549 that
such electro-wetting displays can be made bistable.
[0013] One type of electro-optic display, which has been the
subject of intense research and development for a number of years,
is the particle-based electrophoretic display, in which a plurality
of charged particles move through a fluid under the influence of an
electric field. Electrophoretic displays can have attributes of
good brightness and contrast, wide viewing angles, state
bistability, and low power consumption when compared with liquid
crystal displays. Nevertheless, problems with the long-term image
quality of these displays have prevented their widespread usage.
For example, particles that make up electrophoretic displays tend
to settle, resulting in inadequate service-life for these
displays.
[0014] As noted above, electrophoretic media require the presence
of a fluid. In most prior art electrophoretic media, this fluid is
a liquid, but electrophoretic media can be produced using gaseous
fluids; see, for example, Kitamura, T., et al., "Electrical toner
movement for electronic paper-like display", IDW Japan, 2001, Paper
HCS1-1, and Yamaguchi, Y, et al., "Toner display using insulative
particles charged triboelectrically", IDW Japan, 2001, Paper
AMD4-4). See also U.S. Pat. Nos. 7,321,459 and 7,236,291. Such
gas-based electrophoretic media appear to be susceptible to the
same types of problems due to particle settling as liquid-based
electrophoretic media, when the media are used in an orientation
which permits such settling, for example in a sign where the medium
is disposed in a vertical plane. Indeed, particle settling appears
to be a more serious problem in gas-based electrophoretic media
than in liquid-based ones, since the lower viscosity of gaseous
suspending fluids as compared with liquid ones allows more rapid
settling of the electrophoretic particles.
[0015] Numerous patents and applications assigned to or in the
names of the Massachusetts Institute of Technology (MIT) and E Ink
Corporation describe various technologies used in encapsulated
electrophoretic and other electro-optic media. Such encapsulated
media comprise numerous small capsules, each of which itself
comprises an internal phase containing electrophoretically-mobile
particles in a fluid medium, and a capsule wall surrounding the
internal phase. Typically, the capsules are themselves held within
a polymeric binder to form a coherent layer positioned between two
electrodes. The technologies described in the these patents and
applications include: [0016] (a) Electrophoretic particles, fluids
and fluid additives; see for example U.S. Pat. Nos. 7,002,728; and
7,679,814; [0017] (b) Capsules, binders and encapsulation
processes; see for example U.S. Pat. Nos. 6,922,276; and 7,411,719;
[0018] (c) Films and sub-assemblies containing electro-optic
materials; see for example U.S. Pat. Nos. 6,982,178; and 7,839,564;
[0019] (d) Backplanes, adhesive layers and other auxiliary layers
and methods used in displays; see for example U.S. Pat. Nos.
7,116,318; and 7,535,624; [0020] (e) Color formation and color
adjustment; see for example U.S. Pat. No. 7,075,502; and U.S.
Patent Application Publication No. 2007/0109219; [0021] (f) Methods
for driving displays; see the aforementioned MEDEOD applications;
[0022] (g) Applications of displays; see for example U.S. Pat. Nos.
7,312,784; and 8,009,348; and [0023] (h) Non-electrophoretic
displays, as described in U.S. Pat. Nos. 6,241,921; 6,950,220;
7,420,549 and 8,319,759; and U.S. Patent Application Publication
No. 2012/0293858.
[0024] Many of the aforementioned patents and applications
recognize that the walls surrounding the discrete microcapsules in
an encapsulated electrophoretic medium could be replaced by a
continuous phase, thus producing a so-called polymer-dispersed
electrophoretic display, in which the electrophoretic medium
comprises a plurality of discrete droplets of an electrophoretic
fluid and a continuous phase of a polymeric material, and that the
discrete droplets of electrophoretic fluid within such a
polymer-dispersed electrophoretic display may be regarded as
capsules or microcapsules even though no discrete capsule membrane
is associated with each individual droplet; see for example, the
aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes
of the present application, such polymer-dispersed electrophoretic
media are regarded as sub-species of encapsulated electrophoretic
media.
[0025] A related type of electrophoretic display is a so-called
"microcell electrophoretic display". In a microcell electrophoretic
display, the charged particles and the fluid are not encapsulated
within microcapsules but instead are retained within a plurality of
cavities formed within a carrier medium, typically a polymeric
film. See, for example, U.S. Pat. Nos. 6,672,921 and 6,788,449,
both assigned to Sipix Imaging, Inc.
[0026] Although electrophoretic media are often opaque (since, for
example, in many electrophoretic media, the particles substantially
block transmission of visible light through the display) and
operate in a reflective mode, many electrophoretic displays can be
made to operate in a so-called "shutter mode" in which one display
state is substantially opaque and one is light-transmissive . See,
for example, U.S. Pat. Nos. 5,872,552; 6,130,774; 6,144,361;
6,172,798; 6,271,823; 6,225,971; and 6,184,856. Dielectrophoretic
displays, which are similar to electrophoretic displays but rely
upon variations in electric field strength, can operate in a
similar mode; see U.S. Pat. No. 4,418,346. Other types of
electro-optic displays may also be capable of operating in shutter
mode. Electro-optic media operating in shutter mode may be useful
in multi-layer structures for full color displays; in such
structures, at least one layer adjacent the viewing surface of the
display operates in shutter mode to expose or conceal a second
layer more distant from the viewing surface.
[0027] An encapsulated electrophoretic display typically does not
suffer from the clustering and settling failure mode of traditional
electrophoretic devices and provides further advantages, such as
the ability to print or coat the display on a wide variety of
flexible and rigid substrates. (Use of the word "printing" is
intended to include all forms of printing and coating, including,
but without limitation: pre-metered coatings such as patch die
coating, slot or extrusion coating, slide or cascade coating,
curtain coating; roll coating such as knife over roll coating,
forward and reverse roll coating; gravure coating; dip coating;
spray coating; meniscus coating; spin coating; brush coating; air
knife coating; silk screen printing processes; electrostatic
printing processes; thermal printing processes; ink jet printing
processes; electrophoretic deposition (See U.S. Pat. No.
7,339,715); and other similar techniques.) Thus, the resulting
display can be flexible. Further, because the display medium can be
printed (using a variety of methods), the display itself can be
made inexpensively.
[0028] Other types of electro-optic media may also be used in the
displays of the present invention.
[0029] The bistable or multi-stable behavior of particle-based
electrophoretic displays, and other electro-optic displays
displaying similar behavior (such displays may hereinafter for
convenience be referred to as "impulse driven displays"), is in
marked contrast to that of conventional liquid crystal ("LC")
displays. Twisted nematic liquid crystals are not bi- or
multi-stable but act as voltage transducers, so that applying a
given electric field to a pixel of such a display produces a
specific gray level at the pixel, regardless of the gray level
previously present at the pixel. Furthermore, LC displays are only
driven in one direction (from non-transmissive or "dark" to
transmissive or "light"), the reverse transition from a lighter
state to a darker one being effected by reducing or eliminating the
electric field. Finally, the gray level of a pixel of an LC display
is not sensitive to the polarity of the electric field, only to its
magnitude, and indeed for technical reasons commercial LC displays
usually reverse the polarity of the driving field at frequent
intervals. In contrast, bistable electro-optic displays act, to a
first approximation, as impulse transducers, so that the final
state of a pixel depends not only upon the electric field applied
and the time for which this field is applied, but also upon the
state of the pixel prior to the application of the electric
field.
[0030] Whether or not the electro-optic medium used is bistable, to
obtain a high-resolution display, individual pixels of a display
must be addressable without interference from adjacent pixels. One
way to achieve this objective is to provide an array of non-linear
elements, such as transistors or diodes, with at least one
non-linear element associated with each pixel, to produce an
"active matrix" display. An addressing or pixel electrode, which
addresses one pixel, is connected to an appropriate voltage source
through the associated non-linear element. Typically, when the
non-linear element is a transistor, the pixel electrode is
connected to the drain of the transistor, and this arrangement will
be assumed in the following description, although it is essentially
arbitrary and the pixel electrode could be connected to the source
of the transistor. Conventionally, in high resolution arrays, the
pixels are arranged in a two-dimensional array of rows and columns,
such that any specific pixel is uniquely defined by the
intersection of one specified row and one specified column. The
sources of all the transistors in each column are connected to a
single column electrode, while the gates of all the transistors in
each row are connected to a single row electrode; again the
assignment of sources to rows and gates to columns is conventional
but essentially arbitrary, and could be reversed if desired. The
row electrodes are connected to a row driver, which essentially
ensures that at any given moment only one row is selected, i.e.,
that there is applied to the selected row electrode a voltage such
as to ensure that all the transistors in the selected row are
conductive, while there is applied to all other rows a voltage such
as to ensure that all the transistors in these non-selected rows
remain non-conductive. The column electrodes are connected to
column drivers, which place upon the various column electrodes
voltages selected to drive the pixels in the selected row to their
desired optical states. (The aforementioned voltages are relative
to a common front electrode which is conventionally provided on the
opposed side of the electro-optic medium from the non-linear array
and extends across the whole display.) After a pre-selected
interval known as the "line address time" the selected row is
deselected, the next row is selected, and the voltages on the
column drivers are changed so that the next line of the display is
written. This process is repeated so that the entire display is
written in a row-by-row manner.
[0031] It might at first appear that the ideal method for
addressing such an impulse-driven electro-optic display would be
so-called "general grayscale image flow" in which a controller
arranges each writing of an image so that each pixel transitions
directly from its initial gray level to its final gray level.
However, inevitably there is some error in writing images on an
impulse-driven display. Some such errors encountered in practice
include:
[0032] (a) Prior State Dependence; With at least some electro-optic
media, the impulse required to switch a pixel to a new optical
state depends not only on the current and desired optical state,
but also on the previous optical states of the pixel.
[0033] (b) Dwell Time Dependence; With at least some electro-optic
media, the impulse required to switch a pixel to a new optical
state depends on the time that the pixel has spent in its various
optical states. The precise nature of this dependence is not well
understood, but in general, more impulse is required the longer the
pixel has been in its current optical state.
[0034] (c) Temperature Dependence; The impulse required to switch a
pixel to a new optical state depends heavily on temperature.
[0035] (d) Humidity Dependence; The impulse required to switch a
pixel to a new optical state depends, with at least some types of
electro-optic media, on the ambient humidity.
[0036] (e) Mechanical Uniformity; The impulse required to switch a
pixel to a new optical state may be affected by mechanical
variations in the display, for example variations in the thickness
of an electro-optic medium or an associated lamination adhesive.
Other types of mechanical non-uniformity may arise from inevitable
variations between different manufacturing batches of medium,
manufacturing tolerances and materials variations.
[0037] (f) Voltage Errors; The actual impulse applied to a pixel
will inevitably differ slightly from that theoretically applied
because of unavoidable slight errors in the voltages delivered by
drivers.
[0038] General grayscale image flow suffers from an "accumulation
of errors" phenomenon. For example, imagine that temperature
dependence results in a 0.2 L* (where L* has the usual CIE
definition:
L*=116(R/R.sub.0).sup.1/3-16,
where R is the reflectance and R.sub.0 is a standard reflectance
value) error in the positive direction on each transition. After
fifty transitions, this error will accumulate to 10 L*. Perhaps
more realistically, suppose that the average error on each
transition, expressed in terms of the difference between the
theoretical and the actual reflectance of the display is .+-.0.2
L*. After 100 successive transitions, the pixels will display an
average deviation from their expected state of 2 L*; such
deviations are apparent to the average observer on certain types of
images.
[0039] This accumulation of errors phenomenon applies not only to
errors due to temperature, but also to errors of all the types
listed above. As described in the aforementioned U.S. Pat. No.
7,012,600, compensating for such errors is possible, but only to a
limited degree of precision. For example, temperature errors can be
compensated by using a temperature sensor and a lookup table, but
the temperature sensor has a limited resolution and may read a
temperature slightly different from that of the electro-optic
medium. Similarly, prior state dependence can be compensated by
storing the prior states and using a multi-dimensional transition
matrix, but controller memory limits the number of states that can
be recorded and the size of the transition matrix that can be
stored, placing a limit on the precision of this type of
compensation.
[0040] Thus, general grayscale image flow requires very precise
control of applied impulse to give good results, and empirically it
has been found that, in the present state of the technology of
electro-optic displays, general grayscale image flow is infeasible
in a commercial display.
[0041] It will be appreciated that, whether or not an active matrix
backplane is used, updating of a display requires the preparation
of some form of bitmap to indicate the desired final gray levels of
every pixel in the display. On the other hand, in many cases data
to be shown on the display is stored in a non-bitmap form; for
example, electronic books and similar documents are often stored in
a text-based form, for example as simple ASCII text, as an "epub"
or "mobi" word processing file or as a text mode portable document
file. Other data to be displayed may be stored as a spreadsheet or
presentation file, or as a compressed image or video file.
Conversion of these various stored formats to appropriate bitmaps
for display (usually known as "pre-rendering") makes major demands
upon the data processing capabilities of the display electronics,
especially in the case of portable electronic displays which
typically have substantially lower computing power than a desktop
or laptop personal computer. Pre-rendering of color images is makes
especially large demands upon data processing capabilities.
Furthermore, since users expect E-book readers and similar displays
to respond essentially instantly to user input, in many cases (as
for example when a reader is presented with a menu listing several
possible choices, and which image will appear next depends upon the
selected item on the menu) it is necessary for the display to
pre-render several images in order to avoid delay in presenting the
next image after a choice is made by the user. Such pre-rendering
of multiple images, many of which may never be displayed, occupies
a large amount of memory space and increases the power consumption
of the display, thus reducing the available interval between
battery charges in the case of a portable, battery-powered display.
A first aspect of the present invention relates to reducing or
eliminating the problems.
[0042] A second aspect of the present invention relates to
reduction of noise in images on electro-optic displays. In
practice, it is found that many electro-optic displays suffer from
spatial noise, in the sense that different areas of the display
exhibit different gray levels even though the areas are driven
using the same drive scheme and thus experience the same waveforms.
At least part of such noise appears to be due to non-uniformities
in the electro-optic layer, i.e., grain artifacts. Also often
present are "streak" defects which manifest themselves as elongate
areas having electro-optic responses differing from those of the
surrounding areas. Such grain and streak defects may, if
sufficiently severe, require that part of the electro-optic medium
be discarded, and the defects are thus a significant factor in
yield losses in the production of electro-optic displays. Some
reduction in noise and defects can be achieved by careful choice of
the waveforms and drive schemes to be used in driving the display
but such reduction is limited and grain and streaks are still of
major concern in the manufacture of electro-optic displays.
[0043] A second aspect of the present invention seeks to provide
methods for driving electro-optic displays to allow for more
effective correction of noise than prior art methods.
[0044] A third aspect of the present invention relates to use of
multiple drive schemes. Under some circumstances, it may be
desirable for a single display to make use of multiple drive
schemes. For example, a display capable of more than two gray
levels may make use of a gray scale drive scheme ("GSDS") which can
effect transitions between all possible gray levels, and a
monochrome drive scheme ("MDS") which effects transitions only
between two gray levels, the MDS providing quicker rewriting of the
display that the GSDS. The MDS is used when all the pixels which
are being changed during a rewriting of the display are effecting
transitions only between the two gray levels used by the MDS. For
example, the aforementioned U.S. Pat. No. 7,119,772 describes a
display in the form of an electronic book or similar device capable
of displaying gray scale images and also capable of displaying a
monochrome dialogue box which permits a user to enter text relating
to the displayed images. When the user is entering text, a rapid
MDS is used for quick updating of the dialogue box, thus providing
the user with rapid confirmation of the text being entered. On the
other hand, when the entire gray scale image shown on the display
is being changed, a slower GSDS is used.
[0045] Alternatively, a display may make use of a GSDS
simultaneously with a "direct update" drive scheme ("DUDS"). The
DUDS may have two or more than two gray levels, typically fewer
than the GSDS, but the most important characteristic of a DUDS is
that transitions are handled by a simple unidirectional drive from
the initial gray level to the final gray level, as opposed to the
"indirect" transitions often used in a GSDS, where in at least some
transitions the pixel is driven from an initial gray level to one
extreme optical state, then in the reverse direction to a final
gray level; in some cases, the transition may be effected by
driving from the initial gray level to one extreme optical state,
thence to the opposed extreme optical state, and only then to the
final extreme optical state--see, for example, the drive scheme
illustrated in FIGS. 11A and 11B of the aforementioned U.S. Pat.
No. 7,012,600. Thus, present electrophoretic displays may have an
update time in grayscale mode of about two to three times the
length of a saturation pulse (where "the length of a saturation
pulse" is defined as the time period, at a specific voltage, that
suffices to drive a pixel of a display from one extreme optical
state to the other), or approximately 700-900 milliseconds, whereas
a DUDS has a maximum update time equal to the length of the
saturation pulse, or about 200-300 milliseconds.
[0046] Variation in drive schemes is, however, not confined to
differences in the number of gray levels used. For example, drive
schemes may be divided into global drive schemes, where a drive
voltage is applied to every pixel in the region to which the global
update drive scheme (more accurately referred to as a "global
complete" or "GC" drive scheme) is being applied (which may be the
whole display or some defined portion thereof) and partial update
(or local) drive schemes, where a drive voltage is applied only to
pixels that are undergoing a non-zero transition (i.e., a
transition in which the initial and final gray levels differ from
each other), but no drive voltage is applied during zero
transitions (in which the initial and final gray levels are the
same). An intermediate form a drive scheme (designated a "global
limited" or "GL" drive scheme) is similar to a GC drive scheme
except that no drive voltage is applied to a pixel which is
undergoing a zero, white-to-white transition. In, for example, a
display used as an electronic book reader, displaying black text on
a white background, there are numerous white pixels, especially in
the margins and between lines of text which remain unchanged from
one page of text to the next; hence, not rewriting these white
pixels substantially reduces the apparent "flashiness" of the
display rewriting. However, certain problems remain in this type of
GL drive scheme. Firstly, as discussed in detail in some of the
aforementioned MEDEOD applications, bistable electro-optic media
are typically not completely bistable, and pixels placed in one
extreme optical state gradually drift, over a period of minutes to
hours, towards an intermediate gray level. In particular, pixels
driven white slowly drift towards a light gray color. Hence, if in
a local drive scheme a white pixel is allowed to remain undriven
through a number of page turns, during which other white pixels
(for example, those forming parts of the text characters) are
driven, the freshly updated white pixels will be slightly lighter
than the undriven white pixels, and eventually the difference will
become apparent even to an untrained user.
[0047] Secondly, when an undriven pixel lies adjacent a pixel which
is being updated, a phenomenon known as "blooming" occurs, in which
the driving of the driven pixel causes a change in optical state
over an area slightly larger than that of the driven pixel, because
the electro-optic medium lying between two pixels sees voltages
that are intermediate the voltages applied to the two pixels. In
addition, some electro-optic media, especially electrophoretic
media, react asymmetrically to applied voltages such that if one
pixel is driven with a pulse of one polarity and then with a pulse
of the opposed polarity, while an adjacent pixel is undriven
throughout, a visible edge is left between the two pixels. Such
blooming manifests itself as edge effects along the edges where the
undriven pixels lie adjacent driven pixels. Similar edge effects
occur when using regional updates (where only a particular region
of the display is updated, for example to show an image), except
that with regional updates the edge effects occur at the boundary
of the region being updated. Over time, such edge effects become
visually distracting and must be cleared. Hitherto, such edge
effects (and the effects of color drift in undriven white pixels)
have typically been removed by using a single GC update at
intervals. Unfortunately, use of such an occasional GC update
reintroduces the problem of a "flashy" update, and indeed the
flashiness of the update may be heightened by the fact that the
flashy update only occurs at long intervals.
[0048] A third aspect of the present invention relates to reducing
or eliminating the problems discussed above while still avoiding so
far as possible flashy updates. However, there is an additional
complication in attempting to solve the aforementioned problems,
namely the need for overall DC balance. As discussed in many of the
aforementioned MEDEOD applications, the electro-optic properties
and the working lifetime of displays may be adversely affected if
the drive schemes used are not substantially DC balanced (i.e., if
the algebraic sum of the impulses applied to a pixel during any
series of transitions beginning and ending at the same gray level
is not close to zero). See especially the aforementioned U.S. Pat.
No. 7,453,445, which discusses the problems of DC balancing in
so-called "heterogeneous loops" involving transitions carried out
using more than one drive scheme. A DC balanced drive scheme
ensures that the total net impulse bias at any given time is
bounded (for a finite number of gray states). In a DC balanced
drive scheme, each optical state of the display is assigned an
impulse potential (IP) and the individual transitions between
optical states are defined such that the net impulse of the
transition is equal to the difference in impulse potential between
the initial and final states of the transition. In a DC balanced
drive scheme, any round trip net impulse is required to be
substantially zero.
[0049] Accordingly, the third aspect of the present invention seeks
to provide methods for driving electro-optic displays to reduce the
aforementioned visible edge effects.
SUMMARY OF INVENTION
[0050] According to the first aspect of the present invention, it
has now been found that the aforementioned problems caused by the
need to pre-render multiple images can be reduced if not eliminated
by using waveforms which are in two successive sections, namely a
first section only upon the initial optical state (or possibly the
initial optical state and at least one optical state prior to the
initial optical state) prior to the relevant transition, and a
second section which is dependent upon both the initial optical
state and the final optical state of the relevant transition.
[0051] Accordingly, the first aspect of the present invention
provides a method of driving an electro-optic display comprising a
plurality of pixels. This method comprises storing data
representing an initial state of at least one pixel of the display;
receiving data representing a final state of the at least one
pixel; and applying to the at least one pixel a waveform arranged
to change the optical state of the at least one pixel from the
initial state to the final state. In the method of the present
invention, the or each waveform is in two parts, a first part which
depends upon the initial state of the pixel but not upon the final
state thereof, and a second part which depends upon both the
initial and final states of the pixel.
[0052] This method of the present invention is designed to allow
pre-rendering of the final image after the time at which the final
image is selected, so that only one final image needs to be
pre-rendered, and thus avoiding the problems with prior art drive
schemes which may involve pre-rendering of multiple images, some of
which may never to applied to the display. Using the method of the
present invention, when a host controller determines that it is
necessary to update the display (for example, because a user has
selected an item on a menu), the host controller will send a
message to the display controller to begin an update. The display
controller can then immediately begin the first part of the
waveform to be applied to each pixel, since this first part depends
only upon data already in the possession of the display controller,
primarily the existing state of each pixel, and optionally data
regarding prior states of each pixel, so that this first part of
each waveform can proceed without the display controller needing to
know details of the final image. While the display controller is
executing the first part of each waveform, the host controller
renders the final image, and makes this final image available to
the display controller, so that one the first part of each waveform
is concluded, the second part of each waveform can immediately
commence. In most cases, the period provided by the first part of
each waveform will suffice for the host controller to render the
final image, depending of course upon the type of image to be
produced and the data processing capability of the host controller.
Should additional time be required, a short delay could be provided
before the display controller begins the first part of each
waveform; such a short delay would not substantially affect a
user's perception of the responsiveness of the display.
[0053] The first aspect of the present invention also provides a
display controller adapted to carry out the method of the
invention.
[0054] The second aspect of the present invention provides a method
of driving an electro-optic display having a plurality of pixels.
This method comprises (in a first, testing phase) applying at least
one standard waveform to each pixel, measuring the optical state of
each pixel following application of the standard waveform,
determining, for each pixel, one of a selection of standard drive
schemes to be applied to the pixel, and storing, for each pixel, at
least one selection datum indicating the selected standard drive
scheme. The method further comprises a second (driving) phase,
which comprises:
[0055] storing data representing at least an initial state of each
pixel of the display, and the standard drive schemes;
[0056] receiving input signals representing final gray level of a
plurality of pixels of the display;
[0057] determining from the stored data representing the initial
state, the input signals, the selection data for the relevant
pixels and the standard drive schemes, the impulses necessary to
drive said plurality of pixels to said final gray levels; and
[0058] generating a plurality of output signals representing pixel
voltages to be applied to said plurality of pixels.
[0059] This driving method of the present invention may hereinafter
for convenience be referred to as the "pixel specific driving
method" or "PSD" method of the invention.
[0060] This invention also provides a display controller adapted to
carry out the PSD method of the invention. Such a controller has
provision for storing, for each pixel of the display, at least one
selection datum indicating which of a selection of standard drive
schemes is to be applied to each specific pixel, and for taking
this selection datum into account in determining the waveform to be
applied to each pixel during updating of the display.
[0061] In the PSD method of the present invention, the testing
phase may include driving each pixel of the display to each of the
gray levels which the pixel can display (or at least each of the
gray levels which the drive schemes to be used can display, for
example 16 gray levels). This is conveniently done by driving the
display to show a series of solid images of each gray level while
the display is held in a fixed position. A camera is arranged to
photograph the display, a mapping is effected between pixels of the
camera image and display pixels. The camera image of each of the
solid display images is sampled at the positions corresponding to
the display pixels.
[0062] As already discussed, grain and mottle are artifacts
sometimes manifest on electrophoretic display modules. For certain
displayed material such as a uniform gray tone, the low-spatial
frequency and amplitude of the artifact may be such that it is
visually disturbing. The PSD method of the present invention seeks
to reduce graininess of displayed material by adapting the image
content, or otherwise providing a spatial correction during the
display process, to correct for the local tone offset of the noise,
the correction being produced from a map of the grain offset for
the gray levels at each addressable spatial location (i.e. pixel).
Thus, the PSD method is a form of "active noise cancellation".
[0063] In the simplest forms of this method, the drive schemes
available for updating each pixel are not adjusted, but only the
selection from among available driving signals is altered. For
example, if at a given pixel the grain characteristics make the
pixel too dark, a lighter gray tone level signal could be applied
if the resulting gray tone were predicted to be closer to the
desired. Using the spatial grain offset information, a mapping is
obtained to provide a correction for each gray level input at each
pixel. This mapping can be predetermined and reasonably stored if
the number of gray tone levels is small.
[0064] Nominally, in the PSD method, the drive schemes are chosen
such that the mean lightness of the grain pattern for each gray
level is within some tolerance of a target gray level. In practice,
the actual mean lightness achieved on a given panel can vary from
panel to panel by a significant fraction of the spacing between
gray levels. Empirically it has been found that the light areas
found in the grain pattern are distributed approximately normally
with a standard deviation that is dependent on mean tone (in an
approximately smooth manner) and applied driving scheme. The grain
pattern is spatially correlated at length scales that are important
to the human visual system.
[0065] Given these findings, it will be seen that the simple PSD
scheme for local grain compensation described will lead to a
decrease in the error in placement of the average gray level
achieved from the target gray level. Furthermore, it has been found
that as the grain variance becomes large compared to the spacing
between adjacent gray levels, the resulting variance of the
corrected gray tone level approaches that of a uniform distribution
with a standard deviation of ( 1/12).sup.0.5, or approximately 0.3
levels. See FIGS. 1A and 1B, which show the simulated performance
of the PSD method in baseline grain cancellation for two different
offsets of the nominal gray level placement from the target level.
In both cases, the gray level placement error converges to zero at
large native grain standard deviation, and the resulting standard
deviation converges to that of a uniform distribution on [-0.5,
0.5] levels.
[0066] This decrease in variance is the primary goal of the PSD
method as it reduces the visibility of the grain. However, even at
the resultant reduced levels, the resulting texture pattern can be
visually disturbing because of the spatial correlations in the
grain pattern; when rendering a gray level by choosing the nearest
available gray level at each pixel, large domains of similarly
chosen gray levels can result, and because the human eye is
sensitive to the resulting spatial frequencies, the artifacts
remain readily visible. Accordingly, there is a need for an
improved method for choosing among available gray levels at each
pixel of a display that preserves the mean lightness close to the
target value and reduces the visibility of the corrected grain
pattern, and such an improved method is provided by a second method
of the second aspect of the present invention, in which dithering
or image half-toning techniques are used to place the grain into
higher spatial frequencies so that it is less visible, while
maintaining a tight constraint on the mean value to remain near the
gray level placement target. In most cases this leads to an
increase in total noise variance over the optimal case, but still
provides a decrease in noise visibility.
[0067] The third aspect of the present invention provides a method
of driving an electro-optic display having a plurality of pixels.
This method uses a drive scheme which defines the waveform to be
applied to each pixel for each transition from an initial gray
level to final gray level. For at least one transition in the drive
scheme, multiple waveforms are provided in the drive scheme, and
these multiple waveforms are applied to pixels undergoing the
relevant transition on a random basis, so that different pixels
undergoing the same transition experience different waveforms.
[0068] It will be appreciated that this method of the present
invention requires a display controller which can apply different
waveforms on a pixel-by-pixel basis, as described herein.
[0069] It will be appreciated that the multiple waveforms used for
a particular transition in this method of the present invention
will not necessarily all have the same net impulse, and that in
order to produce random effects which tend to minimize edge effects
and similar visual problem, it is advisable, in the present method,
to change, at intervals, which waveform is used for which
transition at a particular pixel. Hence, unless precautions are
taken, the method could result in the accumulation of DC imbalance.
In many cases, the differences between the net impulse of the
multiple waveforms will be small, and depending upon the type of
electro-optic display used, may be tolerated without significant
damage to the display. Alternatively, the DC imbalance at each
pixel can be tracked (as described in the aforementioned MEDEOD
applications) and the selection of a particular waveform for a
particular transition biased so as to tend to reduce the
accumulated DC imbalance at the particular pixel. In a further
embodiment of the present method, multiple waveforms are provided
for each of the transitions in the drive scheme, and any one pixel
uses the first, second, third etc. waveform for each transition. In
effect, the collection of first, second, third etc. waveform for
each transition form separate drive schemes, with only one of these
drive schemes being applied to a specific pixel at any one time.
(Each of these drive schemes is of the same type or "mode", i.e.,
each of the drive schemes may be a global complete drive scheme or
a global limited drive scheme, but all must be of the same type.
Thus, this embodiment of the present invention differs from the
selective general update drive scheme described in the
aforementioned copending application Ser. No. 13/755,111, in which
a first drive scheme is applied to a non-zero minor proportion of
the pixels of a display during a first update, while a second drive
scheme is applied to the remaining pixels during the first update,
while during a second update following the first update, the first
drive scheme is applied to a different non-zero minor proportion of
the pixels, while the second drive scheme is applied to the
remaining pixels during the second update; in the selective general
update drive scheme the two drive schemes are of different types.)
Each of the separate drive schemes used in this embodiment of the
present invention is desirably itself DC balanced, and desirably
switching of a pixel between two of the separate drive schemes is
effected only when the pixel reaches a particular optical state or
states, such that switching between the drive schemes will not
cause accumulation of DC imbalance. Typically, the particular
optical state or states will include one of the extreme optical
states of the pixel.
[0070] The third aspect of the present invention also provides a
further method of driving an electro-optic display having a
plurality of pixels. This method comprises applying at local drive
scheme to change the optical state of at least one limited area of
the display. The change in the optical state of the at least one
limited area is accompanied by driving pixels in an edge
elimination region at least one pixel wide and substantially
surrounding the at least one limited area. The pixels in the edge
elimination region are first driven from their original gray level
to an intermediate gray level, and then back to their original gray
level (note that since the pixels in the edge elimination region
are outside the at least one limited areas to which the local
update is being applied, the final gray level of the pixels in the
edge elimination region will be the same as their initial gray
level).
[0071] In saying that the edge elimination region "substantially
surrounds" the at least one limited area to which a local drive
scheme is being applied, we mean that the edge elimination region
includes at least pixels which share a common edge with the at
least one limited area. It is not essential, but generally
desirable, that the edge elimination region include pixels which
share only a common corner with the at least one limited area.
[0072] In this "edge elimination" method, the edge elimination
region may be more than one pixel wide, and it is not essential
that all the pixels in the edge elimination region be driven to the
same intermediate gray level; edge elimination may be more
effectively achieved by driving various groups of pixels, and in
particular various groups of pixels at different distances from the
boundary of the at least one limited area, to differing
intermediate gray levels. Furthermore, the method of the present
invention is not limited to driving the pixels in the edge
elimination region to a single intermediate gray level; such edge
elimination pixels may be driven to a series of intermediate gray
levels, and use of multiple intermediate gray levels may be useful
in minimizing edge effects.
[0073] The waveforms applied to the edge elimination region may be
same as those applied by the local drive scheme, or a special "edge
elimination" drive scheme may be used in the edge elimination
region. The exact intermediate gray level used in the edge
elimination region if often not of great concern, since the edge
elimination region will typically be narrow and the visual effect
of changes in the edge elimination region will typically be lost in
the much greater changes occurring in the at least one limited
area; the major consideration is to ensure that no visible edge is
left around the boundary of the at least one limited area following
the transition. In other cases, the details of the waveform used in
the edge elimination region may not be of major importance, and it
may be the transitions effected by the waveform used in the edge
elimination region which is of major importance. It has also been
found that in many cases it is only part of a waveform used for the
local update which is responsible for the formation of the edge
effect and the timing of the waveform used in the edge elimination
region versus the waveform used for the local update may have a
significant effect on the visibility of the edge defects
present.
[0074] The foregoing considerations indicate that in this edge
elimination method, it is generally preferred that the drive scheme
used in the edge elimination region be a special edge elimination
drive scheme, provided that the display controller used can
accommodate the additional drive scheme. Such a special edge
elimination drive scheme can be tuned to give optimum results as
regards edge elimination with specific gray level combinations. As
noted above, edge effects are normally only of concern after the
entire transition has been completed, since a transitory edge
effect during a transition will normally be ignored by a user of
the display. Furthermore, edge effects are often asymmetric, in the
sense that the edge effects are more noticeable when a limited area
of a display transitions to one gray level as opposed to another
gray level. In particular, when a display has a large number of
pixels at the same gray level forming a background of the displayed
image (for example, a large number of white pixels which form the
background of a series of black text, or black lines, on white
background images) and many of these background pixels are updated
only rarely, edge effects are likely to be most severe when a
limited area of the display transitions to the background
color.
[0075] The edge elimination method may be used with both monochrome
and gray scale local drive schemes.
[0076] The present invention also provides a display controller
adapted to carry out the edge elimination method of the invention.
The display controller of the invention is capable of driving an
electro-optic display having a plurality of pixels and comprises
edge detection means for detecting the edges of limited areas of
the display undergoing a transition, and for determining an edge
elimination region at least one pixel wide and substantially
surrounding the at least one limited area, and means for driving
the pixels in the edge elimination region from their original gray
level to an intermediate gray level, and then back to their
original gray level.
[0077] In all the methods of the present invention, the display may
make use of any of the type of electro-optic media discussed above.
Thus, for example, the electro-optic display may comprise a
rotating bichromal member or electrochromic material.
Alternatively, the electro-optic display may comprise an
electrophoretic material comprising a plurality of electrically
charged particles disposed in a fluid and capable of moving through
the fluid under the influence of an electric field. The
electrically charged particles and the fluid may be confined within
a plurality of capsules or microcells. Alternatively, the
electrically charged particles and the fluid may be present as a
plurality of discrete droplets surrounded by a continuous phase
comprising a polymeric material. The fluid may be liquid or
gaseous.
BRIEF DESCRIPTION OF THE DRAWINGS
[0078] As already mentioned, FIGS. 1A and 1B are graphs showing the
simulated performance of the simple PSD method of the present
invention in baseline grain cancellation for two different offsets
of the nominal gray level placement from the target level.
[0079] FIG. 2 is a graph showing the actual reflectance of
individual pixels, and the adjustments made by a PSD method of the
present invention.
[0080] FIG. 3 is a graph showing the improvement in grain achieved
in an electrophoretic display using the simple PSD method of the
present invention as described in Part B below.
[0081] FIGS. 4-6 show a simulation of an application of dithering
and image half-toning techniques to noise reduction in accordance
with the present invention.
[0082] FIGS. 7A and 7B of the accompanying drawings illustrate
schematically the beginning of a transition of a limited area of an
electro-optic display from black to white, and the edge effect
which is produced, using a prior art driving method.
[0083] FIGS. 8A, 8B and 8C are schematic illustrations, similar to
that of FIGS. 7A and 7B, but showing a similar transition using a
driving method according to the third aspect of the present
invention.
[0084] FIG. 8D shows a display driven with no edge elimination
region used.
[0085] FIG. 8E shows the reduced edge effects achieved by a driving
method of the third aspect of present invention.
[0086] FIG. 9 is a block diagram illustrating schematically the
architecture of a prior art display controller for a bistable
electro-optic display.
[0087] FIG. 10 shows the controller memory architecture of the
prior art controller shown in FIG. 9.
[0088] FIG. 11 shows a simple Laplace filter which may be used for
edge detection in a display controller of the present
invention.
[0089] FIGS. 12A and 12B illustrate the use of the Laplace filter
shown in FIG. 11 to detect edge pixels in a display.
[0090] FIG. 13 is schematic diagram, similar to that of FIG. 10, of
the controller memory architecture of a display controller of the
present invention.
[0091] FIG. 14 is a block diagram, similar to that of FIG. 9 but
illustrating schematically the architecture of a display controller
of the present invention using the controller memory architecture
of FIG. 13.
[0092] FIG. 15 shows a lookup table which may be used in the
display controller of FIG. 14.
DETAILED DESCRIPTION
[0093] As already described, the present invention has three main
aspects and provides a variety of methods for improving the images
displayed on electro-optic displays. While the various methods of
the invention will mainly be described separately below, it will be
apparent to those skilled in the technology of electro-optic
displays that in practice a single physical display may make use of
more than one of the methods of the present invention, either
simultaneously or sequentially. For example, a single display might
make use of a two-part waveform according to the first aspect of
the present invention in order to reduce the pre-rendering burden
on the controller, and might also make use of an edge elimination
method according to the third aspect of the present invention in
order to eliminate edge effects in the images displayed.
[0094] Part A: Two-Part Waveforms
[0095] As explained above, the first aspect of the present
invention provides a "two-part waveform" method of driving an
electro-optic display. This method comprises storing data
representing an initial state of at least one pixel of the display;
receiving data representing a final state of the at least one
pixel; and applying to the at least one pixel a waveform arranged
to change the optical state of the at least one pixel from the
initial state to the final state. In the method of the present
invention, the or each waveform is in two parts, a first part which
depends upon the initial state of the pixel but not upon the final
state thereof, and a second part which depends upon both the
initial and final states of the pixel.
[0096] Waveforms suitable for use in this two-part waveform method
can be created in a manner similar to prior art waveforms, as
described in the aforementioned MEDEOD applications, except that
the first part for all waveforms starting from a specific initial
state would have to be the same. For example, in a gray scale drive
scheme (i.e., a drive scheme in which each pixel is capable of
displaying two extreme optical states and at least one gray optical
state intermediate the two extreme optical states), the first part
of each waveform might drive the pixel to a mid-gray level, while
the second part of each waveform would drive it from the mid-gray
level to the final desired gray level. The overall effect would be
that at the end of the first part of the waveform, all pixels would
be at the same mid-gray level so that the initial image would
disappear into a solid mid-gray level image, from which the final
image would emerge. Also, it will be appreciated that the first
part of all waveforms in a drive scheme would have to be the same
length; if necessary, some waveforms could be "padded" with periods
of zero voltage to meet this restriction. Keeping these
restrictions would ensure that each waveform could be broken into
first and second parts for use in the method of the present
invention. If the controller could not play two waveforms back to
back and needed some rest in between, this would also have to be
accounted for by adding that pause into the restrictions. It is
reasonable to assume this is possible since the base waveform
before tuning might allow the first waveform stage to be at least
about 50%, and preferably about 66%, of the total length of the
waveform. Even after tuning many prior art waveforms could be
modified and slightly retuned to line up for the first several
frames of the waveform.
[0097] From the foregoing description, it will be seen that the
two-part waveform aspect of the present invention provides a
driving method and a display controller which allow for rendering
of an image after the waveforms for the transition have already
begun, thus ensuring the host controller need only render one image
at a time and reducing memory and power requirements. The present
method is also useful for coping with sudden changes of mind by the
user of the display. Suppose, for example, that a user reading an
E-book accidentally presses the "Previous Page" button in error,
and immediately corrects his mistake by pressing the "Next Page"
button. In many of the prior art displays described in the
aforementioned MEDEOD applications, a transition cannot be
interrupted once it has begun, since to terminate partially
completed applied waveforms would leave the display in an unknown
state, and might affect the DC balance of the display. Hence, in
this situation the display would have to complete the rewriting of
the display to display the previous page, before proceeding to the
page actually desired. However, when using the method of the
present invention, at the first button press, the host controller
would signal the display controller to begin the first parts of the
relevant waveforms and start to render the (erroneous) next image.
Upon receipt of the second button press, the host controller does
not need to stop the application of the first parts of the relevant
waveforms, since these first parts would be the same for both the
erroneous and correct next images. The host controller could cancel
the rendering of the erroneous next image and begin the rendering
of the correct next image. By the time the first parts of the
waveforms have been applied to the display, or shortly thereafter,
the host controller will have finished rendering the correct next
image, and the display controller can proceed to apply the proper
second parts of the waveforms for the correct next image. The
overall effect is that the correct next image is displayed within
the time frame for a single updating of the display, or a slightly
longer period, rather than taking the full time for two complete
updates of the display, as in the prior art.
[0098] The method of the two-part waveform method requires little
of no modification of the display controller and in practice often
requires only slight modification of waveforms.
[0099] Part B: Pixel Specific Driving Method
[0100] As explained above, the pixel specific driving (PSD) method
of the second aspect of the present invention comprises a first (or
testing phase) which requires applying at least one standard
waveform to each pixel, measuring the optical state of each pixel
following application of the standard waveform, determining, for
each pixel, one of a selection of standard drive schemes to be
applied to the pixel, and storing, for each pixel, at least one
selection datum indicating the selected standard drive scheme. The
phrase "one of a selection of standard drive schemes" should be
interpreted broadly, and is not limited to selecting one of a
limited number of drive schemes all the details of which are
defined in advance. For example, as discussed in the aforementioned
U.S. Pat. No. 7,012,600, the method could make use of one or more
standard drive schemes which contain one or more variable
parameters (for example, the overall length of the waveform or the
lengths of sub-sections thereof), and the parameters to be used for
a specific pixel could be chosen on the basis of data accumulated
in the testing phase. Alternatively, as described with reference to
FIGS. 2 and 3, the selection of the drive scheme may be effected by
using, for a particular transition, a waveform present in a
standard drive scheme but intended, in that drive scheme, for use
in a different transition. The aim of the testing phase is to
accumulate data on the behavior of individual pixels of the display
in order that the most appropriate drive scheme may be applied to
each pixel during the second, driving phase of the method. This
driving phase is carried out in the same way as in the
aforementioned U.S. Pat. No. 7,012,600 and other MEDEOD
applications, except that the drive scheme used is selected
individually for each pixel in order to obtain the gray level
closest to the desired gray level at each pixel despite the
variations in the behavior of individual pixels.
[0101] As already noted, in one form the PSD method, the testing
phase includes driving each pixel of the display to each of the
gray levels which the pixel can display (or at least each of the
gray levels which the drive schemes to be used can display, for
example 16 gray levels). This is conveniently done by driving the
display to show a series of solid images of each gray level while
the display is held in a fixed position. A camera is arranged to
photograph the display, a mapping is effected between pixels of the
camera image and display pixels. The camera image of each of the
solid display images is sampled at the positions corresponding to
the display pixels, achievable at that pixel using the given drive
scheme, and thus represent a pixelwise gray level reproduction
curve. Because of spatial noise, the reflectance actually achieved
using the waveform for the desired gray level at a specific pixel
may not be the optimum one; a waveform associated with a different
final gray level may achieve a reflectance closer to the gray level
desired. FIG. 2 illustrates this approach. In FIG. 2, the lower
curve shows the actual reflectance measured for individual pixels
at gray level 9 on a 16 gray level scale, while the upper curve
shows the adjustments made to the desired gray level to allow for
the errors shown in the lower curve; it will be seen that the
majority of pixels are set to use the waveform for gray level 9 but
certain pixels are set to use the waveforms for gray level 8 or 10.
Applying this concept to all input gray levels at all pixels leads
to a pixelwise lookup table mapping input gray level to an actual
gray level index that gives the best result. Desirably, the desired
reflectance associated with an input grey level is chosen to be the
average of the reflectances of the corresponding solid gray level
image, so that the PSD method of the present invention produces the
same average gray tone as the unmodified drive method. Finally, at
the rendering stage the pixelwise lookup table is used to modify
the voltages applied to the various pixels, substituting the
corresponding entry at each pixel from its individual lookup
table.
[0102] The PSD method has been shown qualitatively to be very
successful, significantly reducing visible grain artifacts in
high-grain panel/waveform systems with no other remediation. In
particular, the method has been shown quantitatively to provide
significant noise reduction in high grain panels; see FIG. 3, which
shows the reduction in grain achieved in an electrophoretic display
using the PSD method. It will be seen that substantial reduction in
grain was achieved in the mid-gray levels (where grain tends to be
most noticeable) although at extreme black and white gray levels
grain slightly increased. The PSD method can also be used to remedy
certain manufacturing defects, such as streaking. By moving partial
control of gray level placement and ghosting from waveform
development to later in the production process, inter-panel
performance variability can be reduced, thus lessening the burden
of tuning waveforms to batches of displays. The PSD method may also
improve production yields by allowing the use of displays which
would otherwise display excessive noise.
[0103] Part C: Use of Dithering and/or Image Half-Toning
Techniques
[0104] As explained above, the second aspect of the present
invention may alternatively apply dithering or image half-toning
techniques to place the grain noise into higher spatial frequencies
so that it is less visible, while maintaining a tight constraint on
the mean gray level value to remain near the gray level placement
target. In most cases this will lead to an increase in total noise
variance, but still provides a decrease in noise visibility.
[0105] Those skilled in dithering and image half-toning techniques
will recognize one unusual feature of the application of such
techniques in the second method of the present invention: unlike
conventional dithering applications, in the second method the gray
levels available are not constant across the entire image but
instead are spatially varying. Conventional dithering algorithms
should be modified to allow for this circumstance by finding an
appropriate generalization. For example, consider dispersed dot
dithering. Normally, an efficient implementation of dispersed dot
dithering is to use a multi-level threshold matrix, but this
technique does not generalize well to spatially varying gray
levels. Instead, one can use dithering in the "screening" sense, by
which a screen function is added to the signal being dithered and
the nearest available level to the resulting image is then sought
at each position; this technique works even if the available levels
are spatially varying. The choice of screen function values,
positions, and amplitude will affect the degree to which the
underlying grain noise is visible as well as the texture and mean
and variance of the resultant image.
[0106] FIGS. 4-6 of the accompanying drawings illustrate a
simulation of such a second method of the present invention. FIG. 4
shows a magnified simulated grain pattern at gray level 8, with a
grain noise standard deviation is 0.5 gray level. FIG. 5 shows the
simulated result of setting each pixel to the nearest available
gray level for the target lightness of gray level 8 according to
the PSD method of the present invention. The nominal (mean) gray
level lightness was 0.25 level too dark, whereas the mapped result
has mean 0.0012 level too dark with a standard deviation of 0.2892
level. However, the rather large domains of similarly chosen gray
tone levels present should be noted. FIG. 6 shows the simulated
result of applying the proposed screening method when choosing the
gray level mapping with a 2.times.2 screen function of:
1/4*[-2 1; 2 -1]
The resulting mean is -0.00012 level with a standard deviation
0.4892. Although the noise variance is larger than in FIG. 5 using
the PSD method alone, the noise is primarily present in the higher
spatial frequencies and is not as noticeable.
[0107] In this method, the screening mask may be applied during
calculation of the grain cancellation gray level mapping, as the
screening mask does not depend on the source image signal. If
sufficient resources are available to compute the dithering
correction on the fly, more advanced methods which use image signal
information can be used, such as a generalization of the error
diffusion algorithm. This would have the benefit of better mean
gray level preservation and more of a blue noise
characteristic.
[0108] From the foregoing description, it will be seen that this
method can further reduce grain and mottle visibility when using an
active matrix noise cancellation approach with fixed driving
signals; tight control over the mean value of the display gray
level can also be obtained.
[0109] Part D: Multiple Waveforms Applied on Random Basis
[0110] As already mentioned, the present invention provides a
"random multiple waveforms" method of driving an electro-optic
display having a plurality of pixels using a drive scheme which
defines the waveform to be applied to each pixel for each
transition from an initial gray level to final gray level. For at
least one transition (and preferably all transitions) in the drive
scheme, multiple waveforms are provided in the drive scheme, and
these multiple waveforms are applied to pixels undergoing the
relevant transition on a random basis, so that different pixels
undergoing the same transition experience different waveforms.
[0111] In the prior art, as discussed in the aforementioned MEDEOD
applications, a drive scheme may have multiple different waveforms
for the same transition depending upon physical parameters such as
temperature, humidity, prior states of the pixel and dwell time
(the time for which the pixel has remained in the same optical
state prior to the transition in question). Multiple different
drive schemes may also be used on separate groups of pixels; see,
for example, the drive scheme described in U.S. Pat. No. 7,012,600,
FIGS. 11A and 11B and the related description, where pixels are
divided into two groups interspersed in a checkerboard or similar
pattern and two different drive schemes are applied to the two
groups. Furthermore, the prior art allows for the use of multiple
simultaneous drive schemes where those schemes are of different
types, for example global complete and local update drive schemes.
However, for any given transition and set of physical parameters,
and a given pixel location, prior art drive schemes have always
used a single waveform. It has been found that such "single
waveform" drive schemes can lead to undesirable ghosting or
variations in gray level across a panel, these undesirable features
are apparently due to temperature variations or unavoidable
variations within the electro-optic layer itself. It has also been
found that repeated updates in a single area where there is a
pattern (such as text) can create an accumulation of ghosting and
edge effects over numerous updates when using a local update drive
scheme. Such ghosting and edge effects tend to be objectionable to
users of a display because over large areas, which are supposed to
be at the same gray level, pixels near to each other are of similar
appearance but have an appearance very different from distant
pixels. The present invention takes advantage of the fact that for
a given transition and type of drive scheme there are, in practice,
typically numerous waveforms with only slight differences in
performance, even though prior art drive schemes choose to use only
one of such numerous waveforms.
[0112] The present invention takes advantages of the existence of
such numerous waveforms (which are degenerate in the sense that
they effect essentially the same transition but are not identical
as regards their voltage against time profiles) by using a
plurality of the degenerate waveforms simultaneously on the same
display, so that the waveform varies pixel by pixel, thus creating
systematic performance variations based on individual pixels (as
opposed to large areas of pixels) making display performance
variation harder to recognize and less objectionable.
[0113] There is no single solution for creating the degenerate
waveforms of similar performance used in the method of the present
invention. The exact update that is performed could be determined
initially but the allocation of particular pixels to particular
waveforms could be reassigned systematically in some rotating
order, or even in a chaotic fashion where there is no obvious
pattern. Whatever system is used, it should ensure that large areas
of the display are not simultaneously updated with the same
waveform for the same features in the image on average.
[0114] The degenerate waveforms that give similar performance can
be created from scratch or may perhaps be more easily created by
modifying a standard waveform in a way that does not much affect
its performance, using standard techniques used to tuning waveforms
for accurate gray level rendition, as described in the
aforementioned MEDEOD applications. Such techniques include
insertion or removal of balanced pulse pairs from a waveform,
insertion or removal of periods of zero voltage within a waveform,
shifting of drive pulses within a waveform, etc.
[0115] Other methods for waveform creation may include making more
temperature brackets than needed and then selecting waveforms from
a range of temperatures. (This has the advantage of reducing
temperature dependence.). One could also create several dwell time
compensated waveforms and select from a range of times, ignoring
the actual dwell times of the individual pixels involved, although
the waveform selection procedure could be biased such that the
probability of a specific waveform being used could be dependent on
its closeness to a particular time or temperature respectively.
[0116] The waveform selection procedure of the present invention
could, in a sense be "inverted" so as to deliberately create a
desired ghost image; for example, the waveform selection procedure
could be chosen such that a company logo was always or
intermittently visible as a "watermark" in the background of a
display.
[0117] Part E: Edge Elimination Driving Methods
[0118] As explained above, the third aspect of the present
invention provides a "edge elimination" method of driving an
electro-optic display having a plurality of pixels. This method
comprises applying at local drive scheme to change the optical
state of at least one limited area of the display. The change in
the optical state of the at least one limited area is accompanied
by driving pixels in an edge elimination region at least one pixel
wide and substantially surrounding the at least one limited area.
The pixels in the edge elimination region are first driven from
their original gray level to an intermediate gray level, and then
back to their original gray level.
[0119] The difference between a prior art local driving method and
the method of the present invention may be appreciated from FIGS.
7A and 7B, and FIGS. 8A-8C of the accompanying drawings. FIGS. 7A
and 7B show a typical prior art local transition occurring in a
monochrome display. A rectangular limited area of the display is
originally black and is surrounded by a white area covering the
rest of the display. A local drive scheme is applied only to the
black pixels within the rectangular limited area to turn the whole
display white; no voltage is applied to any pixel outside the
rectangular limited area. Following the transition, because of the
edge effects discussed above, an outline of the original
rectangular limited area is still visible on the display.
[0120] FIGS. 8A-8C show the same transition as in FIGS. 7A and 7B
but carried out using an edge elimination method of the present
invention. In this method, the transition is a two-step process. In
the first step, an edge elimination region at least one pixel wide,
and desirably wider, extending completely around the rectangular
limited area, is identified and the pixels in this edge elimination
region are driven from white to an intermediate gray level. In the
second step of the process, the pixels in the edge elimination
region are driven back from the intermediate gray level to white,
while the pixels in the rectangular limited area are driven from
black to white (note that FIG. 8B, the upper waveform is that
applied to the black pixels in the rectangular area while the lower
waveform is that applied to the pixels in the edge elimination
region). The outline of the original black rectangular area is not
visible on the display, or at least is much less visible than in
the transition shown in FIGS. 7A and 7B.
[0121] FIGS. 8D and 8E show the edge effect reduction which can be
achieved using the method of the present invention. FIG. 8D shows
that the upper part of the illustrated region was driven using the
prior art method of FIGS. 7A and 7B, with no edge elimination
region, while the lower part of the illustrated region was driven
using the method of FIGS. 8A-8C, with an edge elimination region
driven to an intermediate gray level. FIG. 8E shows the appearance
of the same region as the left side after the whole transition had
been completed, and it will readily be seen that the part of the
illustrated region driven with the prior art method displays an
obvious edge effect, whereas the part of the illustrated region
driven with the method of the present invention has a much less
visible edge effect.
[0122] As previously noted, both the width of the edge elimination
region and the number of gray levels used therein can vary, and a
single pixel within the edge elimination region may undergo more
than one gray level transition during a single transition of the
limited area. The transition of FIGS. 7A and 7B may be represented
symbolically as:
B.fwdarw.W
while that of FIGS. 8A-8E may be represented as (where LG stands
for light gray and DG for dark gray):
B, LG.fwdarw.W
where the edges between B, LG.fwdarw.W and LG, W.fwdarw.W are less
visible than in B, W.fwdarw.W. An alternative to the transition of
FIGS. 8A-8E might involve using two different gray levels within
the edge elimination region and might by symbolically represented
as:
B, LG, DG.fwdarw.W
where the edges between B, LG.fwdarw.W and LG, DG.fwdarw.W are less
visible than in B, W.fwdarw.W.
[0123] Provided the edge elimination region is at least one pixel
wide and substantially surrounding the at least one limited area to
which the local update is being applied, the edge elimination
region may also include pixels within the limited area to which the
local update would normally be applied and adjacent the edges of
this limited area, so that all pixels forming the edge elimination
region are adjacent one another.
[0124] As already mentioned, use of the method of the present
invention may require changes in the display controller used to
drive the display, and the present invention provides a display
controller adapted to carry out the method of the invention. This
display controller is capable of driving an electro-optic display
having a plurality of pixels and comprises edge detection means for
detecting the edges of limited areas of the display undergoing a
transition, and for determining an edge elimination region at least
one pixel wide and substantially surrounding the at least one
limited area, and means for driving the pixels in the edge
elimination region from their original gray level to an
intermediate gray level, and then back to their original gray
level.
[0125] FIG. 9 of the accompanying drawings shows, in schematic
block diagram form, the architecture of a prior art electro-optic
display controller which may be used to carry out the driving
method of FIGS. 7A and 7B. As may be seen from FIG. 9, this
architecture allows the selection of various drive schemes, and use
of their associated lookup tables, in differing areas of the
display. FIG. 10 shows the memory architecture used in the
controller shown in FIG. 9.
[0126] To modify the prior art controller shown in FIGS. 9 and 10
to carry out the method of the present invention, it is first
necessary to enable the controller to detect edges. Methods for
edge detection within digital images are well known to those
skilled in the data processing art and any of the known methods may
be used in the controller of the present invention. For example,
FIG. 11 illustrates a simple Laplace filter for edge detection in a
monochrome image; similar but larger filters are available that can
be used to calculate a gradient over an edge for multi-bit gray
scale images that require edge detection. Running the two
dimensional filter shown in FIG. 11 across the two dimensional data
array of a current image reveals the edges, as schematically
illustrated in FIGS. 12A and 12B, in which FIG. 12A shows the data
array of the image, and FIG. 12B shows the result of applying the
filter shown in FIG. 11.
[0127] The edge map generated in FIGS. 12A and 12B may be stored in
a separate memory region specifically reserved for the edge map, so
that the memory architecture of the controller of the present
invention has the form shown in FIG. 13. Note that it is important
to ensure that no edges occur in a one-pixel wide boundary region
around the periphery of the image, since the filter shown in FIGS.
12A and 12B cannot be properly applied at the boundary pixels of
the display; other filters may require a wider "no edge" region
around the periphery of the image.
[0128] The edge detection process illustrated in FIGS. 12A and 12B
operates on the two dimensional image map data stored in the image
buffer after the display has been updated with this image. The
convert edge step takes the results of the two dimensional filter
pass and either 1) converts all non-zero values to a `1`; 2)
converts only the negative values or positive values to a `1` for a
thinner edge; or 3) converts all non-zero values to a positive
grayscale representation for a gradient. This data is stored in the
edge map portion of the frame buffer and represents the edges
calculated for the image currently displayed.
[0129] When a new image is loaded into the image buffer and the
display controller is commanded to update the display, the
following steps occurs in order for each pixel: [0130] 1) The "next
pixel" value from the update buffer is transferred to the "current
pixel" location; [0131] 2) The "next pixel" location is loaded with
the corresponding pixel data from the image buffer; and [0132] 3)
The "edge pixel" location is loaded with the corresponding pixel
data from the edge map.
[0133] The resultant 3-bit value serves as an index to the proper
drive scheme in the modified controller structure shown in FIG. 14.
This value is also stored in the update buffer region of the frame
buffer for ease of access. The lookup tables shown in FIG. 14 have
the format shown in FIG. 15.
[0134] From the foregoing description, it will be seen that the
present invention provides driving methods and display controllers
capable of substantially reduced edge effects in electrophoretic
and other bistable displays.
[0135] It will be apparent to those skilled in the art that
numerous changes and modifications can be made in the specific
embodiments of the invention described above without departing from
the scope of the invention. Accordingly, the whole of the foregoing
description is to be interpreted in an illustrative and not in a
limitative sense.
* * * * *