U.S. patent application number 10/703748 was filed with the patent office on 2005-05-12 for method for transforming three colors input signals to four or more output signals for a color display.
This patent application is currently assigned to Eastman Kodak Company. Invention is credited to Miller, Michael E., Murdoch, Michael J., Primerano, Bruno.
Application Number | 20050099426 10/703748 |
Document ID | / |
Family ID | 34435580 |
Filed Date | 2005-05-12 |
United States Patent
Application |
20050099426 |
Kind Code |
A1 |
Primerano, Bruno ; et
al. |
May 12, 2005 |
METHOD FOR TRANSFORMING THREE COLORS INPUT SIGNALS TO FOUR OR MORE
OUTPUT SIGNALS FOR A COLOR DISPLAY
Abstract
A method for transforming three color input signals (R, G, B)
corresponding to three gamut defining color primaries to four color
output signals (R', G', B', W) corresponding to the gamut defining
color primaries and one additional color primary W for driving a
display having emitters that emit light corresponding to the to the
four color output signals including calculating a common signal
value S as a function F1 of the three color input signals (R,G,B)
for a current and neighboring pixels; determining a final common
signal value S' based upon the common signals for the current and
neighboring pixels; calculating the three color signals (R',G',B')
by calculating a value of a function F2 of the final common signal
value S' and adding it to each of the three color input signals
(R,G,B); and calculating the output signal W as a function F3 of
the final common signal value S'.
Inventors: |
Primerano, Bruno; (Honeoye
Falls, NY) ; Miller, Michael E.; (Honeoye Falls,
NY) ; Murdoch, Michael J.; (Rochester, NY) |
Correspondence
Address: |
Thomas H. Close
Patent Legal Staff
Eastman Kodak Company
343 State Street
Rochester
NY
14650-2201
US
|
Assignee: |
Eastman Kodak Company
|
Family ID: |
34435580 |
Appl. No.: |
10/703748 |
Filed: |
November 7, 2003 |
Current U.S.
Class: |
345/589 |
Current CPC
Class: |
G09G 3/3208 20130101;
H04N 1/6022 20130101; G09G 3/20 20130101; G09G 2300/0452 20130101;
G09G 5/02 20130101; G09G 2320/02 20130101 |
Class at
Publication: |
345/589 |
International
Class: |
G09G 005/02 |
Claims
What is claimed is:
1. A method for transforming three color input signals (R, G, B)
corresponding to three gamut defining color primaries to four color
output signals (R', G', B', W) corresponding to the gamut defining
color primaries and one additional color primary W for driving a
display having emitters that emit light corresponding to the to the
four color output signals, comprising: a) calculating a common
signal value S as a function F1 of the three color input signals
(R,G,B) for a current and neighboring pixels; b) determining a
final common signal value S' based upon the common signals for the
current and neighboring pixels. c) calculating the three color
signals (R',G',B') by calculating a value of a function F2 of the
final common signal value S' and adding it to each of the three
color input signals (R,G,B); and d) calculating the output signal W
as a function F3 of the final common signal value S'.
2. The method claimed in claim 1, wherein the step of determining
the final common signal S' comprises calculating a weighted average
of the common signals S for the current and neighboring pixels.
3. The method claimed in claim 1, wherein the step of determining
the final common signal S' comprises determining the minimum of the
common signals S for the current pixel and selected neighboring
pixels.
4. The method claimed in claim 1, wherein the step of determining
the final common signal value S' includes: b1) determining a
minimum common signal value for the neighboring pixels; b2)
calculating a weighted average of the minimum common signal value
and the common signal value S for the current pixel; and b3)
determining S' as the minimum of the weighted average and the
common signal S for the current pixel.
5. The method claimed in claim 1, wherein the function F1 is the
minimum of the color input signals (R,G,B).
6. The method claimed in claim 1, wherein the calculated value of
function F2 is negative.
7. The method claimed in claim 1, wherein functions F2 and F3 are
linear functions.
8. The method claimed in claim 7, wherein the values of functions
F2 and F3 have opposite signs.
9. The method claimed in claim 1, wherein the functions F2 and F3
increase in slope with decreasing color saturation represented by
color input signals (R,G,B).
10. The method claimed in claim 1, wherein the functions F2 and F3
increase in slope with increasing luminance represented by color
input signals (R,G,B).
11. The method claimed in claim 1, wherein the functions F2 and F3
are nonlinear, having a smaller slope when the final common signal
S' is high.
12. The method claimed in claim 1, wherein the functions F2 and F3
vary according to the hue represented by the color input signals
(R,G,B).
13. The method claimed in claim 1, wherein the color input signals
(R,G,B) are non-linearly related to intensities of their
corresponding primaries.
14. The method claimed in claim 13, further comprising the steps of
shifting the values of the color input signals by an amount to
better approximate linearity with intensity, and shifting the
values of the three color output signals (R', G', B') by a negative
of the amount that the values of the input color signals were
shifted.
15. The method claimed in claim 1, wherein the display has one or
more further emitters corresponding to additional color primaries,
and further comprising the steps of: e) setting aside one of the
four color output signals (R', G', B', W) and further transforming
the remaining three color output signals to four additional color
output signals (A', B', C', W.sub.2), where A', B,' and C' are the
remaining three further transformed color output signals and
W.sub.2 corresponds to a signal for driving a further additional
color primary, by applying steps a through d, wherein the three of
the four color output signals are treated as the three color input
signals and W.sub.2 is treated as W, and driving the display with
(X, A',B'C',W2), where X is the color output signal that was set
aside in the further transformation; and f) repeating the further
transformation for any number of additional color primaries.
16. The method claimed in claim 15, wherein the choice of the color
output signal that is set aside depends on the relative power
efficiencies of the emitters in the display.
17. The method claimed in claim 1, further comprising; e)
normalizing the color input signals (R,G,B) such that a combination
of equal amounts in each signal produces a color having XYZ
tristimulus values identical to those of the additional color
primary to produce normalized color input signals (Rn,Gn,Bn) and
using the normalized color input signals in steps a) and c) to
calculate the common signal values and the three color signals
(R'G'B'); and f) renormalizing the three color signals (R',G',B')
such that a combination of equal amounts in each signal produces a
color having XYZ tristimulus values identical to those of the
display white point.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to color processing three
color image signals for display on a color OLED display having four
or more color primaries.
BACKGROUND OF THE INVENTION
[0002] Additive color digital image display devices are well known
and are based upon a variety of technologies such as cathode ray
tubes, liquid crystal modulators, and solid-state light emitters
such as Organic Light Emitting Diodes (OLEDs). In a common OLED
color display device a pixel includes red, green, and blue colored
OLEDs. These light emitting color primaries define a color gamut,
and by additively combining the illumination from each of these
three OLEDs, i.e. with the integrative capabilities of the human
visual system, a wide variety of colors can be achieved. OLEDs may
be used to generate color directly using organic materials that are
doped to emit energy in desired portions of the electromagnetic
spectrum, or alternatively, broadband emitting (apparently white)
OLEDs may be attenuated with color filters to achieve red, green
and blue.
[0003] It is possible to employ a white, or nearly white OLED along
with the red, green, and blue OLEDs to improve power efficiency
and/or luminance stability over time. Other possibilities for
improving power efficiency and/or luminance stability over time
include the use of one or more additional non-white OLEDs. However,
images and other data destined for display on a color display
device are typically stored and/or transmitted in three channels,
that is, having three signals corresponding to a standard (e.g.
sRGB) or specific (e.g. measured CRT phosphors) set of primaries.
It is also important to recognize that this data is typically
sampled to assume a particular spatial arrangement of light
emitting elements. In an OLED display device these light emitting
elements are typically arranged side by side on a plane. Therefore
if incoming image data is sampled for display on a three color
display device, the data will also have to be resampled for display
on a display having four OLEDs per pixel rather than the three
OLEDs used in a three channel display device.
[0004] In the field of CMYK printing, conversions known as
undercolor removal or gray component replacement are made from RGB
to CMYK, or more specifically from CMY to CMYK. At their most
basic, these conversions subtract some fraction of the CMY values
and add that amount to the K value. These methods are complicated
by image structure limitations because they typically involve
non-continuous tone systems, but because the white of a subtractive
CMYK image is determined by the substrate on which it is printed,
these methods remain relatively simple with respect to color
processing. Attempting to apply analogous algorithms in continuous
tone additive color systems would cause color errors if the
additional primary is different in color from the display system
white point. Additionally, the colors used in these systems can
typically be overlaid on top of one another therefore there is no
need to spatially resample the data when displaying four
colors.
[0005] In the field of sequential-field color projection systems,
it known to use a white primary in combination with red, green, and
blue primaries. White is projected to augment the brightness
provided by the red, green, and blue primaries, inherently reducing
the color saturation of some, if not all, of the colors being
projected. A method proposed by Morgan et al. in U.S. Pat. No.
6,453,067 issued Sep. 17, 2002, teaches an approach to calculating
the intensity of the white primary dependent on the minimum of the
red, green, and blue intensities, and subsequently calculating
modified red, green, and blue intensities via scaling. The scaling
is ostensibly to try to correct the color errors resulting from the
brightness addition provided by the white, but simple correction by
scaling will never restore, for all colors, all of the color
saturation lost in the addition of white. The lack of a subtraction
step in this method ensures color errors in at least some colors.
Additionally, Morgan's disclosure describes a problem that arises
if the white primary is different in color from the desired white
point of a display device without adequately solving it. The method
simply accepts an average effective white point, which effectively
limits the choice of white primary color to a narrow range around
the white point of the device. Since the red, green, blue, and
white elements are projected to spatially overlap one another,
there is no need to spatially resample the data for display on the
four color device.
[0006] A similar approach is described by Lee et al. in TFT-LCD
with RGBW Color System, SID 03 Digest, pp. 1212-1215, to drive a
color liquid crystal display having red, green, blue, and white
pixels. Lee et al. calculate the white signal as the minimum of the
red, green, and blue signals, then scale the red, green, and blue
signals to correct some, but not all, color errors, with the goal
of luminance enhancement paramount. The method of Lee et al.
suffers from the same color inaccuracy as that of Morgan and no
reference is made to spatially resampling of the incoming three
color data to the array of red, green, blue and white elements.
[0007] In the field of ferroelectric liquid crystal displays,
another method is presented by Tanioka in U.S. Pat. No. 5,929,843,
issued July 27, 1999. Tanioka's method follows an algorithm
analogous to the familiar CMYK approach, assigning the minimum of
the R, G, and B signals to the W signal and subtracting the same
from each of the R, G, and B signals. To avoid spatial artifacts,
the method teaches a variable scale factor applied to the minimum
signal that results in smoother colors at low luminance levels.
Because of its similarity to the CMYK algorithm, it suffers from
the same problem cited above, namely that a white pixel having a
color different from that of the display white point will cause
color errors. Similarly to Morgan et al. (U.S. Pat. No. 6,453,067,
referenced above), the color elements are typically projected to
spatially overlap one another and so there is no need for spatial
resampling of the data.
[0008] While stacked OLED display devices have been discussed in
the prior art, providing full color data at each visible spatial
location, OLED display devices are commonly constructed from
multiple colors of OLEDs that are arranged on a single plane. When
displays provide color light emitting elements that have different
spatial location, it is known to sample the data for the spatial
arrangement. For example, U.S. Pat. No. 5,341,153 issued Aug. 23,
1994 to Benzschawel et al., discusses a method for displaying a
high resolution, color image on a lower resolution liquid crystal
display in which the light emitting elements of different colors
have different spatial locations. Using this method, the spatial
location and the area of the original image that is sampled to
produce a signal for each light emitting element is considered when
sampling the data to a format that provides sub-pixel rendering.
While this patent does mention providing sampling of the data for a
display device having four different color light emitting elements,
it does not provide a method for converting from a traditional
three color image signal to an image signal that is appropriate for
display on a display device having four different color light
emitting elements. Additionally, Benzschawel et al. assumes that
the input data originates from an image file that is higher in
resolution than the display and contains information for all color
light emitting elements at every pixel location.
[0009] The prior art also includes methods for resampling image
data from one intended spatial arrangement of light emitting
elements to a second spatial arrangement of light emitting
elements. U.S. patent application Publication No. 2003/0034992A1,
by Brown Elliott et al., published Feb. 20, 2003, discusses a
method of resampling data that was intended for presentation on a
display device having one spatial arrangement of light emitting
elements having three colors to a display device having a different
spatial arrangement of three color light emitting elements.
Specifically, this patent application discusses resampling three
color data that was intended for presentation on a display device
with a traditional arrangement of light emitting elements to three
color data that is intended for presentation on a display device
with an alternate arrangement of light emitting elements. While it
is possible to resample data from one intended spatial arrangement
to a logical display with spatially overlapping light emitting
elements; performing a conversion from the image three color image
signal to a four color OLED display, and then resampling the data
to the spatial arrangement of the OLED display; is computationally
intensive.
[0010] There is a need, therefore, for an improved method for
transforming three color input signals, bearing images or other
data, to four or more output signals that are not spatially
overlapping.
SUMMARY OF THE INVENTION
[0011] The need is met by providing a method for transforming three
color input signals (R, G, B) corresponding to three gamut defining
color primaries to four color output signals (R', G', B', W)
corresponding to the gamut defining color primaries and one
additional color primary W for driving a display having emitters
that emit light corresponding to the to the four color output
signals that includes calculating a common signal value S as a
function F1 of the three color input signals (R,G,B) for a current
and neighboring pixels; determining a final common signal value S'
based upon the common signals for the current and neighboring
pixels; calculating the three color signals (R',G',B') by
calculating a value of a function F2 of the final common signal
value S' and adding it to each of the three color input signals
(R,G,B); and calculating the output signal W as a function F3 of
the final common signal value S'.
ADVANTAGES
[0012] The present invention has the advantage of providing an
efficient method for transforming a three color input image signal
that may have been sampled for display on a display with three
spatially non-overlapping light emitting elements to a four or more
color image signal in a way that preserves edge information in the
OLED display system.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a prior art CIE 1931 Chromaticity Diagram useful
in describing in-gamut and out-of-gamut colors;
[0014] FIG. 2 is a flow diagram illustrating a portion of the
method of the present invention;
[0015] FIG. 3 is a diagram of a typical display device luminance
intensity response curve;
[0016] FIG. 4 is a drawing of a typical prior art red, green, blue
stripe arrangement of light emitting elements;
[0017] FIG. 5 is a diagram showing intensity when a step edge,
which is centered on the second green light emitting element, is
displayed on the RGB stripe arrangement of light emitting
elements;
[0018] FIG. 6 is a drawing of a typical red, green, blue, white
stripe arrangement of light emitting elements;
[0019] FIG. 7 is a diagram showing intensity when the step edge is
converted to a red, green, blue, white signal using the method
shown in FIG. 2;
[0020] FIG. 8 is a flow diagram illustrating additional steps in
the method of the present invention;
[0021] FIG. 9 is a diagram showing intensity when the step edge is
converted using the additional steps of the present method shown in
FIG. 8;
[0022] FIG. 10 is a flow diagram illustrating alternate additional
steps in the method of the present invention;
[0023] FIG. 11 is a diagram showing intensity when the step edge is
converted using the additional steps of the present method shown in
FIG. 10; and
[0024] FIG. 12 is a flow diagram illustrating typical processing
steps in a system employing a three to four color conversion
process according to the prior art.
DETAILED DESCRIPTION OF THE INVENTION
[0025] The present invention is directed to a method for
transforming three color input signals, bearing images or other
data, to four or more color output signals for display on an
additive display device having four or more color primaries. The
present invention is useful, for example, for converting a 3-color
RGB input color image signal that has been sampled for display on a
display device having three spatially non-overlapping light
emitting elements to a four color signal for driving a four-color
OLED display device having pixels made up of light emitting
elements that each emit light of one of the four colors.
[0026] FIG. 1 shows a 1931 CIE chromaticity diagram displaying
hypothetical representations of the primaries of the four-color
OLED display device. The red primary 2, green primary 4, and blue
primary 6 define a color gamut, bounded by the gamut defining
triangle 8. The additional primary 10 is substantially white,
because it is near the center of the diagram in this example, but
it is not necessarily at the white point of the display. An
alternative additional primary 12 is shown, outside the gamut 8,
the use of which will be described later.
[0027] A given display device has a white point, generally
adjustable by hardware or software via methods known in the art,
but fixed for the purposes of this example. The white point is the
color resulting from the combination of three color primaries, in
this example the red, green, and blue primaries, being driven to
their highest addressable extent. The white point is defined by its
chromaticity coordinates and its luminance, commonly referred to as
xyY values, which may be converted to CIE XYZ tristimulus values by
the following equations: 1 X = x y Y Y = Y Z = ( 1 - x - y ) y
Y
[0028] Noting that all three tristimulus values are scaled by
luminance Y, it is apparent that the XYZ tristimulus values, in the
strictest sense, have units of luminance, such as cd/m.sup.2.
However, white point luminance is often normalized to a
dimensionless quantity with a value of 100, making it effectively
percent luminance. Herein, the term "luminance" will always be used
to refer to percent luminance, and XYZ tristimulus values will be
used in the same sense. Thus, a common display white point of D65
with xy chromaticity values of (0.3127, 0.3290) has XYZ tristimulus
values of (95.0, 100.0, 108.9).
[0029] The display white point and the chromaticity coordinates of
three display primaries, in this example the red, green, and blue
primaries, together specify a phosphor matrix, the calculation of
which is well known in the art. Also well known is that the
colloquial term "phosphor matrix," though historically pertinent to
CRT displays using light-emitting phosphors, may be used more
generally in mathematical descriptions of displays with or without
physical phosphor materials. The phosphor matrix converts
intensities to XYZ tristimulus values, effectively modeling the
additive color system that is the display, and in its inversion,
converts XYZ tristimulus values to intensities.
[0030] The intensity of a primary is herein defined as a value
proportional to the luminance of that primary and scaled such that
the combination of unit intensity of each of the three primaries
produces a color stimulus having XYZ tristimulus values equal to
those of the display white point. This definition also constrains
the scaling of the terms of the phosphor matrix. The OLED display
example, with red, green, and blue primary chromaticity coordinates
of (0.637, 0.3592), (0.2690, 0.6508), and (0.1441, 0.1885),
respectively, with the D65 white point, has a phosphor matrix M3: 2
M3 = [ 56.7 16.0 22.4 32.1 38.7 29.2 0.545 4.76 104 ]
[0031] The phosphor matrix M3 times intensities as a column vector
produces XYZ tristimulus values, as in this equation: 3 M3 .times.
[ I 1 I 2 I 3 ] = [ X Y Z ]
[0032] where I1 is the intensity of the red primary, I2 is the
intensity of the green primary, and I3 is the intensity of the blue
primary.
[0033] It is to be noted that phosphor matrices are typically
linear matrix transformations, but the concept of a phosphor matrix
transform may be generalized to any transform or series of
transforms that leads from intensities to XYZ tristimulus values,
or vice-versa.
[0034] The phosphor matrix may also be generalized to handle more
than three primaries. The current example contains an additional
primary with xy chromaticity coordinates (0.3405, 0.3530)--close to
white, but not at the D65 white point. At a luminance arbitrarily
chosen to be 100, the additional primary has XYZ tristimulus values
of (96.5, 100.0, 86.8). These three values may be appended to
phosphor matrix M3 without modification to create a fourth column,
although for convenience, the XYZ tristimulus values are scaled to
the maximum values possible within the gamut defined by the red,
green, and blue primaries. The phosphor matrix M4 is as follows: 4
M4 = [ 56.7 16.0 22.4 88.1 32.1 38.7 29.2 91.3 0.545 4.76 104 79.3
]
[0035] An equation similar to that presented earlier will allow
conversion of a four-value vector of intensities, corresponding to
the red, green, blue, and additional primaries, to the XYZ
tristimulus values that their combination would have in the display
device: 5 M4 .times. [ I 1 I 2 I 3 I 4 ] = [ X Y Z ]
[0036] In general, the value of a phosphor matrix lies in its
inversion, which allows for the specification of a color in XYZ
tristimulus values and results in the intensities required to
produce that color on the display device. Of course, the color
gamut limits the range of colors whose reproduction is possible,
and out-of-gamut XYZ tristimulus specifications result in
intensities outside the range [0,1]. Known gamut-mapping techniques
may be applied to avoid this situation, but their use is tangential
to the present invention and will not be discussed. The inversion
is simple in the case of 3.times.3 phosphor matrix M3, but in the
case of 3.times.4 phosphor matrix M4 it is not uniquely defined.
The present invention provides a method for assigning intensity
values for all four primary channels without requiring the
inversion of the 3.times.4 phosphor matrix.
[0037] The method of the present invention begins with color
signals for the three gamut-defining primaries, in this example,
intensities of the red, green, and blue primaries. These are
reached either from a XYZ tristimulus value specification by the
above described inversion of phosphor matrix M3 or by known methods
of converting RGB, YCC, or other three-channel color signals,
linearly or nonlinearly encoded, to intensities corresponding to
the gamut-defining primaries and the display white point.
[0038] FIG. 2 shows a flow diagram of the general steps used in the
conversion of a three color image signal to four colors. The three
color input signals (R,G,B) are input 22 to the display. If the
color coordinates of the white material do not match the color
coordinates of the display white point, the optional step of
normalizing 24 the three color input signals (R,G, B) with respect
to the additional primary W. If the optional step of normalizing
was not preformed, the values (R,G,B) would be used in the
following calculations in place of (Rn,Gn,Bn). Following the OLED
example, the red, green, and blue intensities are normalized such
that the combination of unit intensity of each produces a color
stimulus having XYZ tristimulus values equal to those of the
additional primary W. This is accomplished by scaling the red,
green, and blue intensities, shown as a column vector, by the
inverse of the intensities required to reproduce the color of the
additional primary using the gamut-defining primaries: 6 [ 1.010 0
0 0 1.000 0 0 0 1.400 ] .times. [ R G B ] = [ Rn Gn Bn ]
[0039] The normalized signals (Rn, Gn, Bn) 26 are used to calculate
28 a common signal S that is a function F1 (Rn, Gn, Bn). In the
present example, the function F1 is a special minimum function
which chooses the smallest non-negative signal of the three. The
common signal S is used to calculate 30 the value of function
F2(S). In this example, function F2 provides arithmetic
inversion:
F2(S)=-S
[0040] The output of function F2 is added 32 to the normalized
color signals (Rn,Gn,Bn), resulting in normalized output signals
(Rn',Gn',Bn') 34 corresponding to the original primary channels. If
the color coordinates of the white material do not match the color
coordinates of the display white point, the optional step of
normalizing 36 these signals to the display white point by scaling
by the intensities required to reproduce the color of the
additional primary using the gamut-defining primaries, resulting in
the output signals (R',G',B') which correspond to the input color
channels: 7 [ 0.990 0 0 0 1.000 0 0 0 0.715 ] .times. [ Rn ' Gn '
Bn ' ] = [ R ' G ' B ' ]
[0041] The common signal S is used to calculate 40 the value of
function F3(S). In the simple four color OLED example, function F3
is simply the identity function. The output of function F3 is
assigned to the output signal W 42, which is the color signal for
the additional primary W. The four color output signals in this
example are intensities and may be combined into a four-value
vector (R',G',B',W), or in general
(I.sub.1',I.sub.2',I.sub.3',I.sub.4. The 3.times.4 phosphor matrix
M4 times this vector shows the XYZ tristimulus values that will be
produced by the display device: 8 M4 .times. [ I 1 ' I 2 ' I 3 ' I
4 ' ] = [ X Y Z ]
[0042] When, as in this example, function F1 chooses the minimum
non-negative signal, the choice of functions F2 and F3 determine
how accurate the color reproduction will be for in-gamut colors. If
F2 and F3 are both linear functions, F2 having negative slope and
F3 having positive slope, the effect is the subtraction of
intensity from the red, green, and blue primaries and the addition
of intensity to the additional primary. Further, when linear
functions F2 and F3 have slopes equal in magnitude but opposite in
sign, the intensity subtracted from the red, green, and blue
primaries is completely accounted for by the intensity assigned to
the additional primary, preserving accurate color reproduction and
providing luminance identical to the three color system.
[0043] If instead the slope of F3 is greater in magnitude than the
slope of F2, system luminance will be augmented and color accuracy
will degrade, decreasing saturation. If instead the slope of F3 is
lesser in magnitude than the slope of F2, system luminance will be
diminished and color accuracy will degrade, increasing saturation.
If functions F2 and F3 are non-linear functions, color accuracy may
still be preserved, providing F2 is decreasing and F2 and F3 are
symmetric about the independent axis.
[0044] In any of these situations, functions F2 and F3 may be
designed to vary according to the color represented by the color
input signals. For example, they may become steeper as the
luminance increases or the color saturation decreases, or they may
change with respect to the hue of the color input signal (R,G, B).
There are many combinations of functions F2 and F3 that will
provide color accuracy with different levels of utilization of the
additional primary with respect to the gamut-defining primaries.
Additionally, combinations of functions F2 and F3 exist that allow
a trade of color accuracy in favor of luminance. Choice of these
functions in the design or use of a display device will depend on
its intended use and specifications. For example, a portable OLED
display device benefits greatly in terms of power efficiency, and
thus battery life, with maximum utilization of an additional
primary having a higher power efficiency than one or more of the
gamut defining primaries. Use of such a display with a digital
camera or other imaging device demands color accuracy as well, and
the method of the present invention provides both.
[0045] The normalization steps allow for accurate reproduction of
colors within the gamut of the display device regardless of the
color of the additional primary. In the unique case where the color
of the additional primary is exactly the same as the display white
point, these normalization steps reduce to identity functions. In
any other case, the amount of color error introduced by ignoring
the normalization steps depends largely on the difference in color
between the additional primary and the display white point.
[0046] Normalization is especially useful in the transformation of
color signals for display in a display device having an additional
primary outside the gamut defined by the gamut-defining primaries.
Returning to FIG. 1, the additional primary 12 is shown outside the
gamut 8. Because it is out of gamut, reproduction of its color
using the red, green, and blue primaries would require intensities
that exceed the range [0,1]. While physically unrealizable, these
values may be used in calculation. With additional primary
chromaticity coordinates (0.4050, 0.1600), the intensity required
of the green primary is negative, but the same relationship shown
earlier can be used to normalize the intensities: 9 [ 1.000 0 0 0 -
1.411 0 0 0 1.543 ] .times. [ R G B ] = [ Rn Gn Bn ]
[0047] A color outside the gamut of the red, green, and blue
primaries, specifically between the red-blue gamut boundary and the
additional primary, will call for negative intensity for the green
primary and positive intensities for the red and blue primaries.
After this normalization, the red and blue values are negative, and
the green value is positive. The function F1 selects the green as
the minimum non-negative value and the green is replaced in part or
in total by intensity from the additional primary. The negatives
are removed after the additional primary intensity is calculated by
undoing the normalization (i.e. renormalizing): 10 [ 1.000 0 0 0 -
0.709 0 0 0 0.648 ] .times. [ Rn ' Gn ' Bn ' ] = [ R ' G ' B '
]
[0048] The normalization steps preserve color accuracy, clearly
allowing white, near-white, or any other color to be used as an
additional primary in an additive color display. In OLED displays,
the use of a white emitter near but not at the display white point
is very feasible, as is the use of a second blue, a second green, a
second red, or even a gamut-expanding emitter such as yellow or
purple.
[0049] Savings in cost or in processing time may be realized by
using signals that are approximations of intensity in the
calculations. It is well known that image signals are often encoded
non-linearly, either to maximize the use of bit-depth or to account
for the characteristic curve (e.g. gamma) of the display device for
which they are intended. Intensity was previously defined as
normalized to unity at the device white point, but it is clear,
given linear functions in the method, that scaling intensity to
code value 255, peak voltage, peak current, or any other quantity
linearly related to the luminance output of each primary is
possible and will not result in color errors.
[0050] Approximating intensity by using a non-linearly related
quantity, such as gamma-corrected code value, will result in color
errors. However, depending on the deviation from linearity and
which portion of the relationship is used, the errors might be
acceptably small when considering the time or cost savings. For
example, FIG. 3 shows the characteristic curve for an OLED,
illustrating its non-linear intensity response to code value. The
curve has a knee 52 above which it is much more linear in
appearance than below. Using code value to approximate intensity is
probably a bad choice, but subtracting a constant (approximately
175 for the example shown in FIG. 3) to use the knee 52 shown, from
the code value makes a much better approximation. The signals
(R,G,B) provided to the method shown in FIG. 2 are calculated as
follows: 11 [ Rcv Gcv Bcv ] - 175 = [ R G B ]
[0051] The shift is removed after the method shown in FIG. 2 is
completed by using the following step: 12 [ R ' G ' B ' ] + 175 [
Rcv ' Gcv ' Bcv ' ]
[0052] This approximation may save processing time or hardware
cost, because it replaces a lookup operation with simple
addition.
[0053] Utilizing the method shown in FIG. 2 to transform three
color input signals to more than four color output signals requires
successive application of the method shown in FIG. 2. Each
successive application of the method calculates the signal for one
of the additional primaries, and the order of calculation is
determined by the inverse of a priority specified for the primary.
For example, consider an OLED display device having the red, green,
and blue primaries already discussed, having chromaticities (0.637,
0.3592), (0.2690, 0.6508), and (0.1441, 0.1885) respectively, plus
two additional primaries, one slightly yellow having chromaticities
(0.3405, 0.3530) and the other slightly blue having chromaticities
(0.2980, 0.3105). The additional primaries will be referred to as
yellow and light blue, respectively.
[0054] Prioritizing the additional primaries may take into account
luminance stability over time, power efficiency, or other
characteristics of the emitter. In this case, the yellow primary is
more power efficient than the light blue primary, so the order of
calculation proceeds with light blue first, then yellow. Once
intensities for red, green, blue, and light blue have been
calculated, one must be set aside to allow the method to transform
the remaining three signals to four. The choice of the value (X) to
set aside may be arbitrary, but is best chosen to be the signal
which was the source of the minimum calculated by function F1. If
that signal was the green intensity, the method calculates the
yellow intensity based on the red, blue, and light blue
intensities. All five are brought together at the end: red, green,
blue, light blue, and yellow intensities for display. A 3.times.5
phosphor matrix may be created to model their combination in the
display device. This technique may easily be expanded to calculate
signals for any number of additional primaries starting from three
input color signals.
[0055] While this method provides an accurate method of converting
from a three-color input signal to a four or more color signal when
the input data is sampled for a display with spatially overlapping
light emitting elements, the color and luminance distribution along
edges can be disrupted when the input signal has been sampled for
display on a display with non-overlapping light emitting elements.
For example, a three color input signal may be intended to display
an edge on a pixel pattern as shown in FIG. 4. This figure shows a
portion of a display device 54 containing four pixels (56, 58, 60
and 62), each consisting of a repeating pattern of red R, green G
and blue B light emitting elements. These four pixels may be used,
for example, to display a step edge that is centered on the second
green light emitting element 58G and result in a intensity
distribution for each color as shown in FIG. 5
[0056] FIG. 5 shows red 64, green 66, and blue 68 intensities for a
step edge that is centered on the second green light emitting
element 58G in FIG. 4. When a four color conversion algorithm is
applied, the image is displayed on a four element display device,
such as the one shown in FIG. 6. This figure shows a display device
70 with four pixels (72, 74, 76 and 78) each consisting of red R,
green G, blue B, and white W light emitting elements. When an
algorithm, such as the one shown in FIG. 2 is applied to a three
color signal, containing a step edge, such as the one shown in FIG.
5, and displayed on the four color display device 70 shown in FIG.
6, the resulting signal will appear as shown in FIG. 7.
[0057] As shown in FIG. 7, the resulting image of the step edge,
will consist of the green 80 and blue 82 signals in the second
pixel 74 and white 84 signals in the third 76 and fourth 78 pixels.
Notice that the color of the second pixel 74 will be cyan despite
the fact that the step edge is intended to be neutral in color. At
the proper resolution, the resulting image will appear to have a
cyan fringe along the left side of the edge and a red fringe at the
other end of the edge (not shown). A similar phenomenon may result
in a visible fringe or jittering edge if a moving image is being
displayed. To avoid these problems and improve the luminance
distribution along the edge, the method shown in FIG. 2 may be
modified to smooth the transition of the common signal S that was
calculated 28 earlier.
[0058] A flow diagram showing the method of the present invention
is shown in FIG. 8. As shown in this figure, the common signal is
calculated 86 (steps 22 through 26 shown in FIG. 2) for a plurality
of pixels. Neighboring pixels are then selected 88 for inclusion
into a weighted average. Weighting values are then selected 90. A
weighted average of this common signal is then computed 92 for the
current pixel and one or more neighboring pixels. Preferably, this
weighted average would consist of neighboring pixels that include
at least one pixel to the left and one pixel to the right of the
current pixel in the direction that the light emitting elements R,
G, and B in pixels 56, 58, and 60 are displaced from one another.
However the term "neighboring pixels" might also include more than
one pixel to either side of the current pixel and may also include
pixels oriented along other axes, such as orthogonal or diagonal.
For example, employing the common signal S or one pixel to either
side of the current pixel, one might employ the equation:
S'.sub.c=w.sub.1S.sub.(c-1)+w.sub.2S.sub.(c)+w.sub.3S.sub.(c+1)
[0059] to calculate the final common signal S' for the current
pixel c. Where S'.sub.c is the weighted average of the common
signals S from pixels c-1, c, and c+1 and the weights (w.sub.1,
w.sub.2, and w.sub.3) are constants that typically sum to 1 and
might include values such as 0.25, 0.5, and 0.25, respectively.
[0060] While the weights as discussed in this example may be
constants, the weights may also be selected based on the direction
of signal change. For example, the common signal may be compared
between one or more pixels on either side of the current pixel. The
smaller of the two common signals may then be selected and a larger
weighting value applied to the smaller common signal value.
[0061] Optionally, the original common signal S.sub.c for the
current pixel c and the modified common signal S'.sub.c, are then
compared 94 and the minimum of these two values is selected 96 to
be used in place of the common signal S that was calculated 26
earlier. Once the final common signal has been calculated, the
remaining steps (28 through 42) in FIG. 2 are then completed 98
using this common signal. When this algorithm is applied to the
three color input signal in FIG. 5 assuming constant weights of
0.25, 0.5, and 0.25, the signal shown in FIG. 9 results. As shown
in this figure, the resulting signal contains a red signal 100 for
pixel 76, a green signal 102 for pixels 74 and 76, a blue signal
104 for pixels 74 and 76, and white signal 106 for pixels 76 and
78. Because some red energy is present in pixel 76, the cyan fringe
along the leading edge of the step edge is reduced in visibility.
Therefore this method reduces the visibility of fringing.
Additionally, when showing a moving edge, the transition between
light emitting elements will be smoothed, reducing the appearance
of jitter along edges.
[0062] Alternative methods may also be devised. For example, steps
88 through 96 in FIG. 8 may be replaced by the steps shown in FIG.
10. As shown in FIG. 10, the minimum of the common signal is
determined 108 for the relevant neighboring pixels. A weighted
average is calculated 110 using this minimum value and the common
signal for the current pixel. Finally, a minimum of the original
common signal for the current pixel and the value calculated in
step 110 is determined 112. As with the previous method, this
method reduces the visibility of fringing or jittering artifacts
along the edge. FIG. 11 shows the resulting signal when equal
weights are applied in step 110. Once again, this signal contains a
red signal 114 for pixel 76, a green signal 116 for pixels 74 and
76, a blue signal 118 for pixels 74 and 76, and a white signal 120
for pixels 76 and 78. As before, the presence of the red signal 114
in pixel 76 reduces the visibility of cyan fringing along the step
edge.
[0063] A simplification of this method is to simply calculate the
minimum common signal that was calculated 28 earlier across the
neighboring and current pixel and apply this minimum value as the
common signal for the current pixel. This is equivalent to applying
weights in the weighted average step 110 that is 1.0 for the
minimum signal that is determined 108 for the relevant neighboring
pixels.
[0064] While this method has the advantage of reducing color
fringing and other related imaging artifacts, the primary advantage
of this method is not the improvement in quality but the simplified
image processing chain that this algorithm enables. A typical image
processing chain that includes conversion of a three color input
signal to a four color signal is shown in FIG. 12. As shown in this
figure, a high resolution three color signal may be input 130 to
the display system. This signal will ideally represent n pixels of
data where 3n is the number of light emitting elements on the
display device. This signal may then be converted 132 to a four
color signal for each of the 3n signal values, resulting in 4n
values. Finally, the signal may be down sampled 134 from 4n values
to 4/3n values, such that there is one color value for each light
emitting element.
[0065] To reduce the number of processing steps and the processing
power necessary to conduct these steps, steps 132 and 134 would
ideally be reversed. However, when the color conversion process
shown in FIG. 2 is applied, color fringing and other related
artifacts can result. However, when the common signal is either
smoothed or minimized, steps 132 and 134 can be reversed without
the presence of these artifacts. In this case, the color conversion
process 132 must only be conducted for n signal values. Further,
the down sampling step 134 reduces the n signal values to 4/3n
values. As such, a lower powered and lower cost processor may be
applied to complete the necessary processing steps.
[0066] The invention has been described in detail with particular
reference to certain preferred embodiments thereof, but it will be
understood that variations and modifications can be effected within
the spirit and scope of the invention.
Parts List
[0067] 2 red primary chromaticity
[0068] 4 green primary chromaticity
[0069] 6 blue primary chromaticity
[0070] 8 gamut triangle
[0071] 10 additional in-gamut primary chromaticity
[0072] 12 additional out-of-gamut primary chromaticity
[0073] 22 input signals for gamut-defining primaries
[0074] 24 calculate additional primary normalized signals step
[0075] 26 signals normalized to additional primary
[0076] 28 calculate function F1, common signal step
[0077] 30 calculate function F2 of common signal step
[0078] 32 addition step
[0079] 34 output signals normalized to additional primary
[0080] 36 calculate white-point normalized signals step
[0081] 40 calculate function F3 of common signal step
[0082] 42 output signals for additional primary
[0083] 52 knee of curve
[0084] 54 display device
[0085] 56 pixels
[0086] 58 pixels
[0087] 60 pixels
[0088] 62 pixels
[0089] 64 red intensity
[0090] 66 green intensity
[0091] 68 blue intensity
[0092] 70 display device
[0093] 72 pixels
[0094] 74 pixels
[0095] 76 pixels
[0096] 78 pixels
[0097] 80 green signal
[0098] 82 blue signal
[0099] 84 white signal
[0100] 86 calculate common signal step
[0101] 88 select pixels step
[0102] 90 select weighting values step
[0103] 92 compute weighted average step
[0104] 94 compare step
[0105] 96 select value step
[0106] 98 end
[0107] 100 red signal
[0108] 102 green signal
[0109] 104 blue signal
[0110] 106 white signal
[0111] 108 determine common signal step
[0112] 110 calculate weighted average step
[0113] 112 determine minimum step
[0114] 114 red signal
[0115] 116 green signal
[0116] 118 blue signal
[0117] 120 white signal
[0118] 130 input color signal step
[0119] 132 convert step
[0120] 134 down sample step
* * * * *