U.S. patent application number 09/068421 was filed with the patent office on 2002-02-28 for processing image data.
Invention is credited to PETTIGREW, DANIEL.
Application Number | 20020025066 09/068421 |
Document ID | / |
Family ID | 10799868 |
Filed Date | 2002-02-28 |
United States Patent
Application |
20020025066 |
Kind Code |
A1 |
PETTIGREW, DANIEL |
February 28, 2002 |
PROCESSING IMAGE DATA
Abstract
Image data having a plurality of pixel is processed. Each pixel
is represented by three color components (RGB) defining a position
within colorspace. A base color is identified and a distance in
colorspace between an input color and said base color is
calculated. A control value, which may be used as a basis for a
chromasuppress or a chromakey etc, is produced in response to the
calculated distance.
Inventors: |
PETTIGREW, DANIEL; (QUEBEC,
CA) |
Correspondence
Address: |
GATES & COOPER LLP
HOWARD HUGHES CENTER
6701 CENTER DRIVE WEST, SUITE 1050
LOS ANGELES
CA
90045
US
|
Family ID: |
10799868 |
Appl. No.: |
09/068421 |
Filed: |
November 16, 1998 |
PCT Filed: |
September 12, 1997 |
PCT NO: |
PCT/IB97/01104 |
Current U.S.
Class: |
382/162 ;
348/E9.056 |
Current CPC
Class: |
G06T 7/90 20170101; H04N
9/75 20130101; G06T 11/001 20130101 |
Class at
Publication: |
382/162 |
International
Class: |
G06K 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 12, 1996 |
GB |
9619119.2 |
Claims
1. A method of processing image data in which each image has a
plurality of pixels and each pixel is represented by three color
components defining a position within a color-space, comprising
steps of identifying a base color; calculating a distance in
color-space between an input color and said base color; and
producing a control value in relation to said calculated
distance.
2. A method according to claim 1, wherein said color-space
coordinates represent positions on an orthogonal set of axes and
said distance is calculated from the sum of each component
squared.
3. A method according to claim 2, wherein said control value is
calculated from the square root of said sum.
4. A method according to claim 1, wherein color-space coordinates
are transformed onto an alternative set of orthogonal axes.
5. A method according to claim 4, wherein said transformation is
performed with reference to said base color.
6. A method according to claim 1, wherein said base color is
determined from a set of manually selected colors.
7. A method according to claim 6, wherein said base color is
derived by forming a convex hull around said selected colors.
8. A method according to claim 1, wherein said control value is
used to suppress color in areas of color spill.
9. A method according to claim 1, wherein said control value is
used to generate a keying signal.
10. A method according to claim 9, wherein said keying signal
includes a tolerance region and a softness region.
11. Image data processing apparatus including means for defining
image pixels representing color components of a color-space,
comprising means for identifying a base color; calculating means
for calculating distance in color-space between an input color and
said base color; and means for producing a control value in
relation to said calculated distance.
12. Apparatus according to claim 11, including means for
calculating said distance as the sum of each component squared,
wherein said components are defined on an orthogonal set of
axes.
13. Apparatus according to claim 12, wherein said calculating means
is configured to calculate the square root of said sum.
14. Apparatus according to claim 11, including means for
transforming said color-space co-ordinates onto an alternative set
of orthogonal axes.
15. Apparatus according to claim 14, wherein said means for
performing said transformation is configured to perform said
transformation with reference to said base color.
16. Apparatus according to claim 11, including manually operable
means for selecting said base-color.
17. Apparatus according to claim 16, including means for deriving a
base color with reference to a convex hull constructed around
selected colors.
18. Apparatus according to claim 11, including means for
suppressing color signals in areas of color spill under the control
of said control value.
19. Apparatus according to claim 11, including means for keying
video signals in response to said control signal.
20. Apparatus according to claim 19, including means for generating
a keying signal having a tolerance region and a softness region.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method and apparatus for
digitally compositing video image data, wherein first image frames
are derived from a required foreground image recorded against an
unrequired background image such that a compositing or blending
process results in said unrequired background being replaced by a
new background image.
BACKGROUND
[0002] Techniques for modifying image data after the data has been
recorded have been known for some time. Originally, manual
"touching-up" operations were performed directly upon
cinematographic film and later photographic mattes were produced
allowing two or more filmed images to be combined as a composite
image, thereby simulating a visual effect which did not actually
occur in reality.
[0003] Similar techniques have been employed with television and
video signals, originally using analog circuitry arranged to
process analog television signals, either represented as red green
and blue components or as luminance plus chrominance components.
When working with video signals, part of the signal may be removed
or keyed out at particular times defined by a synchronised key
signal or, alternatively, parts of the video signal may be
suppressed to black in response to a suppression signal. These
keying signals and suppression signals traditionally have been
derived from part of the video signal itself, possibly the
luminance signal or possibly the chrominance signals. Thus,
techniques for generating these signals have become known as
luminance keying (luma-keying) and chrominance keying
(chroma-keying) respectively.
[0004] Recently, and particularly in the realms of broadcast
quality post production, video signals have been manipulated as
digital representations where image frames are sampled to produce
an array of picture elements (pixels) with each pixel representing
a color defined by three color components stored as three numerical
values. Thus, traditionally, in video applications, eight bits may
be allocated for red green and blue color components at each pixel
position or, in accordance with alternative processing schemes,
similar allocations of bits may be made for luminance plus color
difference signals.
[0005] Traditionally, scanned cinematographic film has been
processed in an RGB environment with digitized television signals
being processed in a luminance plus color difference signal
environment, usually identified as YUV. General purpose processing
environments have also tended towards a preference for RGB signal
processing. Since cinematographic film is of higher color
definition, typically 12 bits per color component are used, giving
rise to 4096 possible colors per color component. Chroma-keying
techniques are exploited both in film post-production and
television post-production. Image frames for a foreground image may
be derived by recording talent against a background of a particular
color, with a highly saturated blue or a highly saturated green
being particularly preferred. Required portions of the foreground
image should not include colors used in the background image during
the production process. A subsequent post production compositing
process may then be configured to automatically replace the
unrequired background image with a new background image. A key
signal is generated at regions identified as belonging to the
foreground object which is then used to remove the foreground
object from its background. Thus, for example, in action movies
talent may appear to be acting within a highly dangerous
environment where, in reality the action has been recorded in
studio conditions against a green or blue screen background.
Provided that the post production compositing is highly accurate,
it is possible to produce highly realistic illusions which, from a
safety point of view, would not be possible to record directly as a
real production sequence.
[0006] Often, video or film material will have been recorded, for
keying purposes, under less than favourable conditions. Under these
circumstances, distinguishing a first set of colors from a second
set of colors can be particularly difficult. Furthermore, blending
edges are required which represent the interface between the
foreground object and the new background, where a degree of
blending must occur so as to enhance the realism of the effect. If
blending of this type does not occur and hard transitions exist on
pixel boundaries, visible artefacts will be present within the
image and it will be clear to anyone viewing the resulting clip
that the two image parts originated from separate sources.
[0007] A problem with known systems is that it may be difficult to
adjust color volumes so as to ensure that all key colors are within
an internal volume and all non-key colors are outside an external
volume, with the required blending regions being outside the
internal volume, but inside the external volume.
[0008] The term "video" will be used to identify any image signal
consisting of a sequence of image frames arranged to create the
effect of moving action. This includes true video sources, such as
those derived from D1 videotape, in addition to image data derived
from other sources, such as cinematographic film. Thus, as used
herein, high resolution cinematographic film may be digitised to
produce image data which is then be considered herein as "video
data", although not conforming to establish video protocols, when
used in the narrower sense.
SUMMARY OF THE INVENTION
[0009] According to a first aspect of the present invention, there
is provided a method of processing image data in which each image
has a plurality of pixels and each pixel is represented by three
color components defining a position within a color-space,
comprising steps of identifying a base color; calculating a
distance in color-space between an input color and said base color;
and producing a control value in relation to said calculated
distance.
[0010] Preferably, the color-space co-ordinates represent positions
on an orthogonal set of axes and said distance is calculated from
the sum of each component squared. Preferably, said value is
calculated from the square root of said sum. In a preferred
embodiment, color-space co-ordinates are transformed onto an
alternative set of orthogonal axes. Preferably, the transformation
is performed with reference to said base color.
[0011] In a preferred embodiment, the base color is determined from
a set of manually selected colors and said base color may be
derived from said set by forming a convex hull around said selected
colors in color-space.
[0012] In a preferred embodiment, the control value is used to
suppress in areas of color spill. Alternatively, the control value
is used to generate a keying signal and said keying signal may
include a tolerance region and a softness region.
[0013] According to a second aspect of the present invention, there
is provided image data processing apparatus including means for
defining image pixels representing color components of a
color-space, comprising means for identifying a base color;
calculating means for calculating distance in color-space between
an input color and said base color; and means for producing a
control value in relation to said calculated distance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows a compositing station arranged to key
foreground images against new required background images;
[0015] FIG. 2 shows an example of a foreground image to be
composited against a new required background, using the equipment
identified in FIG. 1;
[0016] FIG. 3 illustrates the position of a base color within a
three dimensional color-space said base color being identified on
the unrequired background of the foreground image;
[0017] FIG. 4 illustrates a compositing process in which a
foreground image is composited with a background image, including
processes for key generation and color suppression;
[0018] FIG. 5 details a portion of the image illustrated in FIG.
2;
[0019] FIG. 6A details displayed colors representing individual
pixels scanned from the foreground image, such as the foreground
image shown in FIG. 2;
[0020] FIG. 6B identifies the distance in KAB color-space between a
scanned pixel of the foreground image and the KAB co-ordinate
system origin;
[0021] FIG. 7 details the steps involved in key generation process
405, including a step for identifying the base color of the
unrequired background and a step for applying a condition required
in generation of the matte;
[0022] FIG. 8 details the step identified in FIG. 7 relating to
identification of the unrequired background or base color;
[0023] FIG. 9 details the step identified in FIG. 7 relating to
application of a condition used in generation of the matte;
[0024] FIG. 10 details a relationship between calculated color
distance values and output values stored as matte data, the Figure
showing color distances in a softness zone which provide a grey
level output;
[0025] FIGS. 11A and 11B detail the operation of the color
suppression process shown in FIG. 4;
[0026] FIGS. 12A and 12B illustrate the relocation of co-ordinate
axes in color space as performed in the color suppression process
identified in FIG. 11;
[0027] FIGS. 13A, 13B and 13C illustrate transformation
calculations performed in the color suppression process;
[0028] FIG. 14 details a typical color suppression curve for a
scanned portion of unrequired background on a foreground image;
[0029] FIG. 15 details color-space transformation matrices;
[0030] FIG. 16 illustrates determination of the maximum color
suppression value utilized in the color suppression process shown
in FIG. 4;
[0031] FIG. 17 details procedures for adjustment of hue and
saturation as performed in the color suppression process shown in
FIG. 11;
[0032] FIG. 18 illustrates procedures followed in luminance
restoration as performed in the color suppression process shown in
FIG. 11;
[0033] FIG. 19 shows procedures involved in the adjustment of R, G
and B color components following luminance restoration shown in
FIG. 16.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0034] A compositing station is shown in FIG. 1, in which digital
representations of image frames are processed within an image data
processing system 101. The processing system 101 responds to manual
operation of a stylus 102 against a touch table 103 and supplies
image data to a monitor 104, in the form of control menus and image
clips. An operator may also supply data to the processing system
101 via a manually operable keyboard 105 and images are supplied to
processing device 101 from video player 106.
[0035] An example of an image frame displayed on monitor 104 is
shown in FIG. 2. In this example, the foreground image scene has
been recorded onto cinematographic film in a studio, in which the
required foreground talent has been recorded against a saturated
blue background. The film has been digitized and a compositing
process, effected by system 101, is arranged to remove the
unrequired background and to replace it with a new background
image. The digitized samples consist of an array of picture
elements (pixels) representing red, green and blue color components
which are then displayable to the operator, on monitor 104, as a
video sequence. However, this video sequence is stored on
substantially randomly accessible magnetic disks, to facilitate
non-linear editing and compositing.
[0036] Typically, eight, ten or 12 bits may be allocated to each of
the red, green and blue components which may be visualized as
orthogonal coordinates in a three dimensional color-space. Thus, it
is possible to visualize color as a three dimensional color space
where conventional x, y, z Cartesian spatial co-ordinates are
replaced by Cartesian color co-ordinates of red (R), green (G) and
blue (B).
[0037] Also, like conventional x, y, z-space, color space may be
specified by alternative sets of co-ordinates and relationships
between co-ordinate sets may be mathematically defined by means of
functional transformations, which in turn may be represented by
transformation matrices. Thus, a color space may be qualified by a
particular co-ordinate set such that each coordinate within the
space has a particular value defining a particular color. This same
color may be defined by an alternative co-ordinate set by applying
a matrix transformation upon the numerical values representing the
color. Thus, the co-ordinate set may undergo a transformation in
color space. However, it should be noted that the actual color
itself remains the same and it is merely its representation under
an alternative co-ordinate system which changes.
[0038] A known alternative co-ordinate set in common usage is the
subtractive color set of cyan, yellow, magenta and black (CYMK)
used for printing. Thus, as values within this color space become
larger, pixel values become darker, whereas in RGB space as values
become larger pixel intensities become greater.
[0039] Another known color space is that of luminance plus color
difference. The luminance axis may be considered as the axis of
constant red, green and blue, with color difference values, or hue
and saturation values, being defined at positions on planes
perpendicular to the luminance axis. Thus, color space, defined in
terms of luminance, hue and saturation, represents a cylindrical
color space, where hue is defined as an angle with respect to the
luminance axis. Within the bounds of a system's resolution, an
infinity of color spaces are definable, each having a respective
transformation matrix, such that color defined in a first color
space may be transformed to a second color space, under the
operation of the transformation matrix. However, it should be
stressed that the actual color and its conceptualisation as space
remain constant with its co-ordinate and it is the co-ordinate
definitions which change in accordance with the matrix
transformations.
[0040] In the present example, color components are recorded as
three components representing the primary colors red green and blue
(RGB) with eight bits being assigned to each of these components.
When performing a chroma keying process each RGB color may be
identified as belonging to one of three regions in color space. The
first region represents the full key color of the background which
will often be a relatively small volume of colors having a
relatively high blue component with relatively low red and green
components. Surrounding this volume of colors will be the region of
colors where a transition is to occur. This set will have blue
values that are relatively lower than the hard key set, with red
and green values that are relatively higher. This set of color
values represent pixels where a key will have an intermediate value
(between 1 and 254 in an eight bit system), so as to provide a
degree of blending between the foreground object and the new
background. Thirdly, a region of color space will exist
representing allowed foreground values. In this region, all of the
foreground image will be composited with no blending through to the
background.
[0041] This arrangement of color values within an RGB color space
is illustrated in FIG. 3. A three dimensional color space is
illustrated in which any point within the space is defined by
orthogonal co-ordinates representing red, green and blue
components. Thus, axis 301 represents the intensity of the red
component, axis 302 represents the intensity of the green component
and axis 303 represents the intensity of the blue component. Thus,
every pixel within the image may be mapped onto a region within the
RGB color space. In this color space the value of RGB (0,0,0)
represents black (point 304) and the value of RGB (1,1,1)
represents pure white of maximum intensity (point 305).
[0042] Keying procedures effected by the processor 101 are detailed
in FIG. 4. The compositing process consists of compositing the
foreground image with the background image and therefore it is
necessary to provide a foreground image source 401 in combination
with a background image source 402. These images are processed on a
pixel by pixel basis and on a frame by frame basis. Color keying of
this type is particularly attractive if associated data (the key or
matte) may be established which allows a plurality of frames within
the clip to be keyed in a single pass. Thus, after control
parameters have been established for a particular frame, the same
control parameters may be employed for the other frames within the
clip, thereby allowing the compositing procedure to be effected at
a rate which is much faster than that attainable when generating
keys manually.
[0043] In some circumstances, keying may be effected using a key
generated by external equipment and therefore provision is made for
an external key to be supplied to the process, as illustrated at
403. A key selector 404 is provided which, as shown in its left
orientation, is configured to allow a key signal or matte to be
generated from the foreground image source 401. In its alternative
orientation, when positioned to the right, an external key signal
is supplied to a key (or matte) generation process 405, although
the extent of processing carried out within process 405 will depend
upon the quality of the external key supplied to the system. The
system operates under the assumption that colors may be defined by
three mutually orthogonal axes; effectively defining color as
positions in three dimensional Cartesian RGB color-space.
[0044] Where careful blending of foreground and background is
required the definition of a volume around a particular color of
interest, representing colors where a soft key will be produced,
becomes a highly sensitive matter which, if not accurately defined,
results in the transitional area including regions of the
background which should be cut out and/or not including regions of
the foreground which should be present. Solutions have been
proposed in which this region's color space is defined using highly
sophisticated arrangements of boundary planes, effectively defining
highly complex polyhedral color space. This is an extremely user
intensive and processor intensive operation and would therefore
limit the number of applications where chroma-keying of this type
may be adopted as a realistic solution to a particular compositing
problem.
[0045] By default, this selected color value is supplied to a color
suppression process 406 as shown in FIG. 4, although other colors
may be selected as part of this process. Thus, in order to avoid
the bleeding through of the base color in regions where the key is
soft, the presence of this color in the foreground is suppressed by
process 406.
[0046] The output from color suppression process 406 and the
background image from 402 are supplied to a blending process 407.
The blending process is arranged to select an input from 406, an
input from 402 or a mixture of these two inputs in response to the
key signal generated by the key generation process 405.
[0047] In order to set up a keying procedure, an operator views a
selected frame within a video clip. A portion of the background is
selected, such as portion 201 shown in FIG. 2, thereby identifying
the selected base color to process 405. This background base color
should be totally removed from the composite image and replaced
with a new background, possibly derived from another video clip.
However, the process is complicated by areas of the image where a
soft key is required. These include areas such as the wine bottle
202, glass 203 and regions such as region 204, around the
guitarists hair.
[0048] Region 204 is illustrated in FIG. 5 and shows an 8.times.8
pixel array of key values. The key values are illustrated as
ranging between zero and one, where the base color will produce a
key value of zero, effectively black, with other regions producing
key values greater than zero, up to a maximum of unity. As shown in
FIG. 5, most of the pixels have been set to a value of zero,
representing the presence of the background base color. However,
given the nature of the foreground image, many of the pixels, such
as pixel 501 and pixel 502, have a value which is very close to
zero, thereby resulting in a level of blending being introduced
between the foreground image and the composited background
image.
[0049] Two images are mixed or composited using an associated
key-signal or matte. The key is a monochrome image representing how
a composite image is derived from background and foreground images.
For any given pixel, the key controls which part of the background
and which part of the foreground is to be taken in order to render
the corresponding pixel in the resulting image. When the key for a
given pixel is completely white, only the foreground image is taken
for the resulting pixel and when the key is completely black only
the background is taken for the resulting pixel. However for the
grey pixels, those pixels not being pure white or pure black, the
resulting pixel will be derived from a percentage of the
corresponding foreground pixel and a percentage of the
corresponding background pixel.
[0050] FIG. 6A details displayed colors in RGB color-space
corresponding to individual pixels scanned from a foreground image
comprising a required foreground shot against a blue background.
Red, green and blue color component axes, 601, 602 and 603
respectively are shown with origin 604. In accordance with the
present invention, a central point 605 has been determined as
representing the blue background, following initial selection of an
area of the blue background by manual operations performed by the
operator. Point 605 represents the average color of the blue screen
which is never perfectly blue but a mix of blue, green and red
where blue is the dominant color. Point 605 now forms the origin of
a new orthogonal coordinate system with axes K, A and B
respectively. Pixels derived from the required foreground image are
displaced from the blue screen color, origin 605. Regions are
established around point 605. Region 606 is identified as a
tolerance zone; bounded by a surface 607. Region 608 is identified
as a softness zone, bounded by a further outer surface 609. The
tolerance zone 606 corresponds to pixels that have RGB color
components required to produce a corresponding black matte pixel.
Pixels that lie on the surface of the tolerance zone, 607,
similarly are required to produce a black matte pixel. All pixels
that lie outside the softness zone, that is outside surface 609,
are required to produce white matte pixels. In-between surfaces 607
and 609 scanned pixels of the foreground image represent transition
pixels which are required to have a level of grey so as to blend
foreground and background images in a realistic way. In accordance
with the present invention the grey levels depend on the distance
in color space of the scanned pixel from the color space origin.
For a given scanned image the KAB axes are defined relative to the
new origin 605 and in accordance with the axes of the ellipse, said
ellipse for example being represented by surface 607. Thus the K
axis is situated along the major axis of the ellipse as shown. FIG.
6 simply represents the scanned pixel colors in two dimensions, on
three-dimensional axes for the purpose of illustration. In reality
ellipse 607 and ellipse 609 are three-dimensional ellipsoids.
[0051] Ellipsoids bounded by surfaces 607 and 609, are displayed on
video monitor 104, allowing a video graphics artist (as shown in
FIG. 1) to interactively modify the tolerance and softness
surfaces. Thus the video artist may observe certain groups of
displayed color data lying close to or on surfaces 607 and 609.
[0052] FIG. 6B illustrates color-space distance of a scanned pixel
of the foreground image from the origin of the KAB color-space
co-ordinate system shown in FIG. 6A. The color of the scanned pixel
is represented by the point 610. This point lies at a distance 611
from the origin 605 and has color components, x, y and z
respectively. The distance 611 is given by Pythagoras's theorem in
three-dimensions, equation 612. Thus the distance 611 is given by
the square root of the sum of the squares of the color distance
components. Thus, it is this equation that is used for calculating
distances in color-space relative to a transformed origin.
[0053] In an alternative embodiment it may be preferable to
calculate color space distances in RGB color-space. In this case a
matrix transformation to transform RGB space into KAB color-space
where the origin defines the position of the key color is not
required. If color-space transformation is not incorporated then
calculation of color distances will not be relative to an origin
and the color distances of scanned pixel points will be relative to
the non-zero point representing the color of the unrequired
background. Thus without coordinate transformation the values that
are squared in equation 611 will correspond to differences in each
color component between the point corresponding to the scanned
pixel and the point corresponding to the color of the unrequired
background.
[0054] FIG. 7 illustrates the generation of the key signal by key
generation process 405. At step 701 processor 101 is instructed to
get a foreground clip (clip A) to be processed. At step 702 a frame
of clip A is read and at step 703 a question is asked as to whether
selection parameters are to be set up. If the question asked at
step 703 is answered in the affirmative, a base color is identified
at step 704 and a forward transformation matrix (MF) is calculated.
Following step 704, the tolerance level may be adjusted in response
to instructions provided by the operator, as indicated at step 705.
If the question asked at 703 is answered in the negative.
[0055] Following selection of the pixel, the forward transformation
matrix (MF) is applied to the current pixel being processed at step
707, resulting in co-ordinate axes being transformed from RGB
color-space to KAB color-space; said axes being defined in FIG.
6.
[0056] At step 708 a key generation condition is applied to the
given pixel to create a resulting key pixel which is stored. At
step 709 the pixel in question is displayed in RGB color-space, as
shown in FIG. 6. At step 710 a question is asked as to whether
there is another pixel in the current frame to be processed. If the
question asked at step 710 is answered in the affirmative control
returns to step 706 wherein the next pixel for the frame is
selected and steps 707, 708 and 709 are repeated. If the question
asked at step 710 is answered in the negative a further question is
asked at step 711 as to whether the frame is to be reprocessed.
This question may be answered by the video artist shown in FIG. 1
and if answered in the affirmative control is returned to step 702
wherein the frame is retrieved and reprocessed in accordance with
the steps thereafter. If the question asked at step 711 is answered
in the negative a further question is asked at step 712 as to
whether there is another frame in the clip to be processed. If this
question is answered in the affirmative, control is returned to
step 702 wherein the next frame is read. If the question asked at
712 is answered in the negative, the process for the generation of
the key signal is terminated at step 713.
[0057] Procedure 704 for the identification of a base color
representing the background of the foreground image, is detailed in
FIG. 8. At step 801 the video artist selects background color
points using a color picker or stylus 102 on a touch tablet 103. At
step 802 the selected points, specified as RGB components, are
stored in a buffer. At step 803 processor 101 obtains the next
point stored in the buffer. At step 804 the point selected at step
803 is processed so as to form part of a 3-D convex hull. A convex
hull is the smallest convex surface that contains a given set of
points. A set of points is convex if for any two points in the set,
the points on a straight line segment joining the two points are
also contained within the set.
[0058] Following step 804 a question is asked at step 805 as to
whether there are further selected points to be processed. If this
question is answered in the affirmative, control is returned to
step 802 wherein steps 802 to 805 are repeated. If the question
asked at step 805 is answered in the negative, processor 101 reads
a sampled pixel at step 806. A sampled pixel represents a pixel
color that has been selected by the video artist. The selected
colors are then considered one by one such that only positions in
color-space which actually lie on the surface of a three
dimensional convex hull are retained. Thus, a question is asked at
step 807 as to whether the point lies on the surface of the hull.
If the question is answered in the affirmative, control is passed
to step 808 wherein the point is stored. Following storage of said
point, control is passed to step 809 wherein a further question is
asked as to whether there are any other points to be processed. If
this question is answered in the affirmative control is returned to
step 806 wherein another convex hull point is read and steps 807 to
809 are repeated. If the question asked at step 807 is answered in
the negative control is passed to step 809, the point identified
not being stored in this case. If the question asked at step 809 is
answered in the negative, to the effect that there are no further
points in the generated convex hull to be processed, control is
passed to step 810 where the centre of the convex hull is
calculated. At this point it is also possible to display the shape
of the convex hull. Following step 810 a matrix transformation (MF)
is generated such that the origin of a new co-ordinate system, the
KAB co-ordinate system shown in FIG. 6, is determined as the centre
of the convex hull.
[0059] Step 708 identified in FIG. 7 relating to application of a
matte generation condition with resulting storage of a matte pixel,
is detailed in FIG. 9. At step 901 the current pixel under
consideration, expressed in KAB co-ordinates, is processed so as to
determine the distance of the pixel's color from the convex hull
centre. The distance of the color of this pixel is given by
Pythagoras' theorem as the square root of the sum of the squares of
the color components, wherein the color component distances are the
distances in KAB color space as identified in FIG. 6B. At step 902
a question is asked as to whether the color distance calculated at
step 901 is less than or equal to the tolerance level set by the
video artist at step 705. If this question is answered in the
affirmative control is passed to step 903 wherein the pixel
generated for the key is stored as a black pixel and thus
represents a point on the blue screen background. If the question
asked at step 902 is answered in the negative, control is passed to
step 904 wherein a question is asked as to whether the distance
calculated at step 901 is greater than or equal to unity. This
question corresponds to whether the calculated distance is greater
than or equal to the surface defining the outer surface of the
softness zone, surface 609 shown in FIG. 6A.
[0060] If this question is answered in the affirmative, the pixel
is interpreted as forming part of the required background and
therefore the resulting key pixel is set to white at step 905,
indicating that no blending with the required background is to be
performed for this pixel. If the question asked at step 904 is
answered in the negative, control is directed to step 906 where a
grey level is calculated using a look-up table for the current
pixel. Thus questions resulting in step 906 being implemented are
equivalent to a pixel being identified whose color lies in the
softness region 608 shown in FIG. 6. Such a pixel may lie on an
object boundary and thus will not be appropriate for setting to
either black or white. In this case a level of blending is
required. Following calculation of the grey level at step 906 the
boundary pixel is stored as a grey level matte pixel at step 907.
Pixels identified as background pixels are stored as black matte
pixels at step 903 and pixels identified as required foreground
image pixels are stored as white matte pixels at step 905.
[0061] Step 705 relating to adjustment of a tolerance level is
detailed in FIG. 10. In FIG. 6, the tolerance zone 606 is bounded
by surface 607 and the softness zone 608 is bounded by surface 609.
FIG. 10 is an example of a function defining the grey level of key
pixels resulting from a calculated distance, R. A tolerance level T
is defined by a user and provides a limit in terms of calculated
distance below which an output key pixel is set to black. Thus
calculated distances less than or equal to this limit are output as
black on the key. The value T corresponds to the upper limit of
color distances for which the current pixel being processed is
considered to be part of the unrequired background of the
foreground image. The softness zone is defined as those color
distances greater than T, but less than unity. Pixels being
processed which have associated color distances within these bounds
are processed such that the resulting output key pixel is given a
grey level defined by function 1001. For color distance values
greater than or equal to unity the output key pixel is set to white
and in this case takes the value of the foreground image pixel
color.
[0062] The function shown is by way of example only and other
similar functions may be defined by the video artist. Thus the
video artist may adjust the tolerance level T and also the nature
of the function between the limits T and unity. Thus for example a
non-linear relationship may be provided between input color
distance values and output grey levels for the resulting matte
pixels.
[0063] As an alternative to providing one matrix and one look-up
table for defining tolerance and softness, it is possible to define
two ellipses with two different transformation matrices. In this
configuration, the first matrix may represent tolerance, which is
positioned within a second ellipsis representing softness. In this
way, the two ellipses may have different orientations and different
centres. They are substantially independent; the only limitation
being to the effect that the tolerance ellipsis must be inside the
softness ellipsis.
[0064] When a matte is being generated, the control value will
represent black if the processed pixel lies inside the tolerance
ellipsis. Similarly, the matte value will be set to white if the
pixel under consideration lies outside the softness ellipsis.
In-between, the level of softness will depend upon the distance
between the two ellipses calculated in the direction of a normal
vector relative to the centre of the tolerance ellipsis.
[0065] First image frames are derived from a foreground image
comprising a required foreground image recorded against an
unrequired background, such that a compositing process results in
the unrequired background being substantially replaced by a new
required background image. The procedure comprises the following
basic steps. Firstly, as described a base color of the unrequired
background is identified and defined in three-dimensional
colorspace. Secondly the foreground image data is processed to
determine distance data which represents the distance in
color-space of the foreground image data from the identified color.
Thirdly the foreground image data is processed with reference to
the distance data to produce associated data. The associated data
is generally known as the alpha matte or key signal data. Finally
the image data, comprising both the foreground and background
images, is processed in combination with the associated data to
produce output or composite data.
[0066] Processes for color suppression 406 are detailed in FIGS.
11A and 11B At step 1101 a base color is identified which, by
default, may be the same color specified for the key generation
process 405. Alternatively the base color may be specifically
identified for this process, by the video artist selecting pixels
on the unrequired background using stylus 102 on touch tablet 103.
At step 1102 the video artist is provided with the opportunity to
set color suppression curves for defining the degree of color
suppression to be performed for particular hues. Following this at
step 1103 the foreground image is scanned and at step 1104 the next
pixel in the scanned foreground image is selected. At step 1105 the
luminance of the selected pixel, Lo is calculated in accordance
with the definition of luminance in RGB color-space. Thus as shown
the value of the pixel luminance is given by the R, G and B
components wherein each component is multiplied by a particular
constant. Thus the red component is multiplied by 0.299, the green
component is multiplied by 0.587 and the blue component is
multiplied by 0.114. At step 1106 the hue of the selected pixel is
calculated first to determine the position in terms of color on the
color suppression curves set in step 1102.
[0067] Hue represents the actual color, such as red, green, purple
etc. From a user defined color curve, a color suppression factor is
determined. At step 1107 the co-ordinates of the selected pixel are
transformed from RGB colorspace to an orthogonal KAB color-space.
At step 1108 a value, is calculated for the K color component. This
value represents the value of the K color component for which the
corresponding blue component is equal to the minimum of either the
red component or the green component. This value acts as an upper
limit on the amount of suppression that can be applied to the given
pixel and therefore is alternatively called the clamping level.
[0068] At step 1109 a question is asked as to whether the maximum
allowed suppression is less than the user defined suppression
value, that is whether the suppression factor is greater than the
maximum value for the pixel under consideration. If this question
is answered in the affirmative, a final color suppression value is
calculated. This value is calculated at step 1110 and is equal to
the maximum value plus an additional quantity. The additional
quantity is given by one tenth of the difference between the user
value and the maximum value.
[0069] If the question asked at step 1109 is answered in the
negative, such that the value is not greater than the calculated
maximum value, the final suppression value is set equal to the user
defined value, that is, it is set to the user value as indicated at
step 1111. Following determination of the final suppression value
to be applied to the current pixel under consideration, the K color
component for the pixel is modified in accordance with the final
suppression value calculated. Thus at step 1112 the K color
component value is calculated as the original K color component
value minus the K color component value multiplied by the final
color suppression value. At step 1113 the modified KUV co-ordinates
of the current pixel are transformed back to RGB color-space. Thus
the RGB components resulting will be modified due to procedures
performed at steps 1109 to 1112.
[0070] Following step 1113 the hue and saturation of the final RGB
components are adjusted, at step 1114, in accordance with values
provided by user defined colour curves. At step 1115 the luminance
of the resulting colour of a pixel under consideration is similarly
adjusted in accordance with luminance curves. At step 1116 gain
adjustments are performed on the R, G and B color components, again
using values provided by corresponding curves. At step 1117 a
modified pixel is stored and at step 1118 a question is asked as to
whether there is another pixel to be processed. If this question is
answered in the affirmative, control is returned to step 1104
wherein the next pixel is selected. Alternatively if the question
asked at step 1118 is answered in the negative a further question
is asked at step 1119 as to whether there is another frame in the
clip to be processed. If this question is asked in the affirmative,
control is returned to step 1101. However if the question asked at
step 1119 is answered in the negative, all of the frames in the
clip have been processed and the session relating to color
suppression is terminated at step 1120.
[0071] Color-space considered by the color suppression process 406
at step 1107 in FIG. 11 is illustrated in FIG. 12A. Particular
colors are specified in terms of their red, green and blue
components, which may be represented as x, y and z orthogonal axes
1201, 1202 and 1203. A base color has been selected which,
preferably, should lie at maximum extent upon the blue axis 1203.
However, the selected base color will tend to be slightly off-set
from this preferred position and its actual co-ordinate locations
may be identified as red being equal to 0.1, green being equal to
0.05 and blue being equal to 0.9.
[0072] In the embodiment, colors within the color-space are
redefined with reference to a new orthogonal K, A, B set of axes,
as shown in FIG. 12B. The K axis is rotated such that it now
intersects the identified base color 1204 which is now identified
as a new color-space co-ordinate K. The axes maintain their
orthogonal relationship, resulting in similar transformations being
effected on the A and B axes. Thus, colors within the color-space
may now be defined with reference to the new co-ordinate frame
where each location consists of three components specified as the K
in combination with an A component and a B component.
[0073] The transformation of the new color-space definition is
completed by scaling the axes so that the selected position 1204
occupies a co-ordinate location of 1, 0, 0 in the new co-ordinate
frame. This scaling is effected uniformly throughout the reference
frame, so as to achieve similar scaling with respect to the A and B
axes.
[0074] The transformation required in order to move the red, green
and blue axes onto the KAB axes (which in accordance with
transformation theory may require two rotations and a scaling
operation to be performed) is calculated such that the mathematical
transformation, possibly defined in terms of a transformation
matrix, may be applied to other colors within an input video source
such that these colors are defined in terms of KAB colorspace as an
alternative to being defined in RGB color-space.
[0075] The procedures identified in FIG. 11 from steps 1108 to 1112
may be implemented within the original RGB color-space although it
has been found that more desirable results are obtained, by making
a modification to a new view of color-space, more sympathetic with
the base color. However, it should be appreciated that the
transformation into the new color-space may result in some color
regions being specified using negative numbers.
[0076] Thus, as far as the color suppression process 405 is
concerned, it is necessary to generate a forward matrix (mF) in
order to achieve the transformation of coefficient definitions as
illustrated in FIG. 12(b). Within the color suppression process
406, matrix mF is therefore used to generate color suppression
terms within the KAB color-space. However after suppression in this
color-space has taken place, it is necessary to redefine the image
in conventional RGB color-space so that it may be blended, in
blending process 407, with the background image 402. Consequently,
within the color suppression process 406 it is also necessary to
calculate the reverse or backward matrix (mB) for converting data
defined as KAB into data defined as RGB. Matrices mF and mB are
populated by concatenating the transformation matrices representing
a first rotation, a second rotation and a scaling.
[0077] The particular nature of the rotation will depend upon the
dominant component of the identified base color. If the dominant
component of the selected base color is blue, the rotation of axes
RGB is performed about the green axis, followed by rotation about
the red axis. The angles of rotation about these respective axes
are calculated in accordance with the equations shown in FIG.
13(a). A temporary variable C is calculated, following Pythagoras,
by calculating the square root of the sum of the blue component
squared plus the red component squared. The value of the angle of
rotation about the green axis is then calculated by negating the
arc cosine of the blue component value divided by the temporary
variable C. This is followed by rotation about the red axis, as
illustrated in FIG. 13(a). The positive arc cosine is determined
from the result of dividing temporary variable C by the square root
of C.sup.2 plus the green value squared. Finally, the whole space
is scaled by dividing by the blue component value.
[0078] Similar procedures are effected if the dominant color is
green or red, as defined in FIGS. 13B and 13C respectively.
[0079] The suppression curves set by an operator at step 1102 in
FIG. 11 are illustrated in FIG. 14. A typical window 1401
facilitating a suppression modification by a video operator is
shown.
[0080] Window 1401 comprises three rows, 1402, 1403 and 1404
respectively. Row 1402 displays the colors identified as components
of the unrequired background color. Rows 1402 and 1403 are
subdivided into columns 1405 representing red, 1406 representing
orange, 1407 representing yellow, 1408 representing green, 1409
representing blue, 1410 representing indigo and 1411 representing
violet. Thus, columns 1405 to 1411 represent a continuous color
spectrum. As indicated by curve 1412, representing said identified
colors, the most dominant color in the background has a hue, h,
slightly off-set from the centre of the blue column 1409. Other
hues are incorporated in the scanned background color and therefore
a distribution of colors is observed. Row 1403 displays a curve
generated by the computer to negate the background color
distribution 1412. The color suppression curve 1413 is calculated
as the inverse of curve 1412.
[0081] Curve 1413 may be modified through commands issued by the
video operator via the control buttons present in row 1404. Curve
1413 may therefore be modified in response to actions performed by
the operator to manipulate the position of the screen pointer
(cursor) 1414. Thus cursor 1414 may be moved down or up in response
to the video operator selecting the down-arrow button 1415 or
up-arrow button 1416 respectively. Similarly the operator may move
cursor 1414 in the left or right direction via selection of buttons
1417 and 1418 respectively. Button 1419 is a save button so that
the manipulations performed to curve 1413 by the video operator may
be saved for use in processing a video frame currently being
processed. Buttons 1420 to 1422 are standard window buttons
relating respectively to decreasing window size (1420), increasing
window size (1421) and closing the window (1422). Thus after the
video operator has specified desirable modifications to the color
suppression curve 1413 the window may be closed by the operator
clicking on button 1422 via use of a mouse.
[0082] Color-space transformations effected by color suppression
process 406 are performed using matrices as detailed in FIG. 15.
Thus, step 1107 in FIG. 11 relating to transformation of pixel RGB
co-ordinates to KAB-space is effected by a forward transformation
matrix 1501 which operates on pixel color component data including
control data 1502 to produce new coordinate data 1503. Similarly
the following color suppression steps 1108 to 1112 in FIG. 11 KAB
color-space information 1503 is transformed using a matrix 1504 to
create color suppressed or final RGB data 1505. The backwards
matrix 1504 is effectively the inverse matrix of forward matrix
1501.
[0083] Calculation of clamping levels for maximum suppression
values for a K color component of a given pixel being processed
occurs at step 1108 in FIG. 11. This step is performed in
accordance with the matrix manipulations identified in FIG. 16.
Thus at step 1601, denoting the maximum allowable suppression by
SUPP_MAX, the maximum suppression is defined as the final blue
component wherein said component is equal to the minimum of either
the final red component or the final green component. Thus, for
example, where the minimum of the red and green components is
determined to be red then BF is set equal to RF.
[0084] At step 1602 the final blue component is given by the sum of
the K, A and B color components, each said component multiplied by
the appropriate backward matrix element given by matrix 1505. Thus,
the blue component occurring in the third row of matrix 1505 is
given by the third row of matrix 1503 multiplied by the third row
of matrix 1504. Similarly the red component, RF, is given by the
first row of matrix 1503 multiplied the first row of matrix of 1504
in accordance with the rules of matrix multiplication.
[0085] At step 1603 the corresponding value of the K component for
the case when the final blue component is equal to the final red
component is given by equating the two equations shown in step
1602. At step 1604 the maximum suppression, SUPP_MAX is calculated.
Thus the value KB=R is equal to the K component of the pixel being
processed multiplied by 1 minus the maximum suppression allowable.
Rearranging this equation yields the maximum allowable suppression,
SUPP_MAX as equal to the K component for the case when the blue
component is equal to the red component divided by the K component
for the current pixel, the resulting value being subtracted from
unity. The calculation required to yield the value for SUPP_MAX is
detailed in step 1605. Step 1605 shows the actual calculation
performed, this being the equation shown in step 1604 with the
value 4KB=R substituted from the value determined in step 1603.
[0086] Process 1114 for the adjustment of hue and saturation is
detailed in FIG. 17. At step 1701 pixel components represented in
RGB color-space are transformed to representations in hue,
luminance and saturation (HLS) space. At step 1702 the hue and
saturation levels are adjusted and at step 1703 the pixel is
re-transformed back into RGB color-space.
[0087] Process 1115 for the adjustment of luminance is detailed in
FIG. 18. At step 1801 a new luminance value is calculated from new
components of R, G and B. The new luminance value is derived from
scaling factors for each of the red, green and blue components
which, as shown in FIG. 18, consist of 0.299 for red, 0.587 for
green and 0.114 for blue.
[0088] At step 1802 a difference dL is calculated by subtracting
the new luminance value from the previous luminance value.
Thereafter, at step 1803 pixel luminance values are compensated by
adding appropriately scaled components with respect to the
difference value calculated at step 1802.
[0089] Process 1116 for performing gain adjustments is detailed in
FIG. 19. At step 1901 a red component of the pixel is adjusted in
accordance with manually defined color curves. Similarly, at step
1902 the green component is adjusted and at step 1903 the blue
component is adjusted.
[0090] The present embodiments allow for chroma-key, chroma-matte
and chroma-suppress signals to be generated from sophisticated
manipulations performed within absolute color-space. This is
particularly useful when effecting post-production procedures to
generate alpha signals or control signals from source material that
has been recorded under less than ideal conditions. It can reduce
the number of passes required in order to generate a chroma-key
and, ultimately, may allow keys to be generated from source
material which would not otherwise allow for keying of this
type.
* * * * *