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Claims  |
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We claim:
1. A method for compressing color video data in a video communication
system having means for producing a color video signal for a plurality of
video picture frames, with each picture frame comprising a plurality of
scan lines composed of a plurality of pixels, each scan line having a
starting pixel and an ending pixel, and each pixel in each said frame
comprising three digital color component signals of first, second and
third digital word sizes, respectively, said method comprising the steps
of:
(a) determining a luminance function for each pixel based upon at least one
of said three digital color component signals;
(b) determining at least one decision parameter for at least a substantial
portion of the pixels in the scan lines of said picture frame based upon
the difference of said luminance function between pixels at least one
predetermined distance from at least one other pixel on each scan line;
(c) determining the rate of change of said at least one decision parameter
for each of said pixels for which said luminance function difference has
been determined;
(d) comparing said rate of change of said at least one decision parameter
with a corresponding adaptive rate of change threshold to determine which
of said pixels represent decision points;
(e) reducing the word size of at least one of said three digital color
components signals to provide one to three reduced digital color component
signals of fourth, fifth, and sixth digital word sizes, respectively, for
each said pixel; and
(f) coding said plurality of pixels in each scan line as a plurality of
combinations of pixel run lengths and said three reduced color component
signals for each said run length, said run lengths being determined
between said starting pixel for each scan line, intermediate points
selected from the group of said decision points and points intermediate
said decision points, and said ending pixel, and each run length being of
a seventh digital word size.
2. The method of claim 1, wherein the step of comparing said rates of
change with an adaptive rate of change threshold to determine said
decision points includes comparing at least one said decision parameters
with an adaptive cumulative difference threshold.
3. The method of claim 1, wherein said first, second, and third digital
word sizes are equal.
4. The method of claim 1, wherein at least two of said first, second, and
third digital word sizes are equal.
5. The method of claim 1, wherein each of said first, second and third
digital word sizes are different.
6. The method of claim 1, wherein said color video signal is RGB, and said
three digital color component signals represent red, green, and blue color
video components.
7. The method of claim 1, wherein said three digital color component
signals represent cyan, magenta, and yellow color video components.
8. The method of claim 1, wherein said three digital color components are
based upon hue, saturation, and intensity of said color video signal.
9. The method of claim 1, wherein the digital word size of each of said
digital color components is six bits.
10. The method of claim 1, wherein each said combination of run length and
reduced color components is coded as at least a 16 bit digital word.
11. The method of claim 1, further including the step of concatenating run
lengths in said plurality of combinations of run lengths and reduced color
components in a scan line which are associated with reduced color
components whose differences are less than a predetermined color
difference threshold.
12. The method of claim 1, wherein said three digital color component
signals represent first and second colors and a parameter based upon
luminance of the color video signal.
13. The method of claim 12, wherein the word size of each of said digital
color components is reduced to 4 bits.
14. The method of claim 13, wherein each said combination of run length and
reduced color components is coded as at least a 16 bit digital word.
15. The method of claim 1, wherein the step of determining said luminance
function comprises summing each of said three digital color component
signals.
16. The method of claim 15, wherein said luminance function has a digital
word size of 8 bits.
17. The method of claim 15 wherein the step of determining said luminance
function includes weighting of the sum of said three digital color
component signals with respect to one or more of said three digital color
component signals.
18. The method of claim 1, wherein a first decision parameter is determined
from the difference in said luminance function between a first pixel and a
second proximate pixel a distance of one pixel away from said first pixel
on a scan line.
19. The method of claim 18, wherein a second decision parameter is
determined from differences between said proximate pixel differences
20. The method of claim 18, wherein a cumulative decision parameter is
determined from summing said proximate pixel differences.
21. The method of claim 1, wherein a first decision parameter is determined
from the difference in said luminance function between a first pixel and a
second proximate pixel a distance of two pixels away from said first pixel
on a scan line.
22. The method of claim 21, wherein a second decision parameter is
determined from differences between said proximate pixel differences.
23. The method of claim 21, wherein a cumulative decision parameter is
determined from summing said proximate pixel differences.
24. A system for compressing color video data for use in a video
communication system having means for producing a color video signal for a
plurality of video picture frames, with each picture frame comprising a
plurality of scan lines composed of a plurality of pixels, each scan line
having a staring pixel and an ending pixel, and each pixel in said frame
comprising three digital color component signals of first, second and
third digital word sizes, respectively, said system comprising:
(a) means for determining a luminance function for each pixel based upon at
least one of said three digital color component signals;
(b) means for determining at least one decision parameter for at least a
substantial portion of the pixels in the scan lines of said picture frame
based upon the difference of said luminance function between pixels at
least one predetermined distance from at least one other pixel on each
scan line;
(c) means for determining the rates of change of said at least one decision
parameter for each of said pixels for which said luminance function
difference has been determined;
(d) means for comparing said rate of change of said at least one decision
parameter with a corresponding adapative rate of change threshold to
determine which of said pixels represent decision points;
(e) means for reducing the word size of at least one of said three digital
color components signals to provide one to three reduced digital color
component signals of fourth, fifth, and sixth digital word sizes,
respectively, for each said pixel;
(f) means for coding said plurality of pixels in each scan line as a
plurality of combinations of pixel run lengths and said three reduced
color component signals for each said run length, said run lengths being
determined between said starting pixel for each scan line, intermediate
points selected from the group of said decision points and points
intermediate said decision points, and said ending pixel, and each run
length being of a seventh digital word size.
25. The system of claim 24, wherein said means for comparing said rates of
change includes means for comparing at least one said decision parameters
with an adaptive cumulative threshold.
26. The system of claim 24, wherein said first, second, and third digital
word sizes are equal.
27. The system of claim 24, wherein at least two of said first, second, and
third digital word sizes are equal.
28. The system of claim 24, wherein each of said first, second and third
digital word sizes are different.
29. The system of claim 24, wherein said color video signal is RGB, and
said three digital color component signals represent red, green, and blue
color video components.
30. The system of claim 24, wherein said three digital color component
signals represent cyan, magenta, and yellow color video components.
31. The system of claim 24, wherein said three digital color component
signals represent first and second colors and a parameter based upon
luminance of the color video signal.
32. The system of claim 24, wherein said three digital color components are
based upon hue, saturation, and intensity of said color video signal.
33. The system of claim 24, wherein said means for coding said plurality of
pixels in each scan line is adapted to code each said combination of run
length and reduced color components as at least a 16 bit digital word.
34. The system of claim 24, further including means for concatenating run
lengths in said plurality of combinations of run lengths and reduced color
components in a scan line which are associated with reduced color
components whose differences are less than a predetermined color
difference threshold.
35. The system of claim 24, wherein the digital word size of each of said
digital color components is six bits.
36. The system of claim 35, wherein said means for reducing the word size
of at least one of said three digital color component signals is adapted
to reduce the word size of each of said digital color components to 4
bits.
37. The system of claim 36, wherein said means for coding said plurality of
pixels in each scan line is adapted to code each said combination of run
length and reduced color components as at least a 16 bit digital word.
38. The system of claim 24, wherein said means for determining said
luminance function comprises means for summing each of said three digital
color component signals
39. The system of claim 38, wherein said luminance function has a digital
word size of 8 bits.
40. The system of claim 38, wherein said means for determining said
luminance function includes means for weighting of the sum of said three
digital color component signals with respect to one or more of said three
digital color component signals.
41. In a camera for use in a video communication system, said camera being
of the type producing a color video signal for a plurality of video
picture frames, with each picture frame comprising a plurality of scan
lines composed of a plurality of pixels, each scan line having a starting
pixel and an ending pixel, and each pixel in said frame comprising three
digital color component signals of first, second and third digital word
sizes, respectively, the improvement in said camera comprising:
(a) means for determining a luminance function for each pixel based upon at
least one of said three digital color component signals;
(b) means for determining at least one decision parameter for at least a
substantial portion of the pixels in the scan lines of said picture frame
based upon the difference of said luminance function between pixels at
least one predetermined distance from at least one other pixel on each
scan line;
(c) means for determining the rate of change of said at least one decision
parameter for each of said pixels for which said luminance function
difference has been determined;
(d) means for comparing said rate of change of said at least one decision
parameters with a corresponding adaptive rate of change threshold to
determine which of said pixels represent decision points;
(e) means for reducing the word size of at least one of said three reduced
digital color components signals to provide one to three reduced digital
color component signals of fourth, fifth, and sixth digital word sizes,
respectively, for each said pixel;
(f) means for coding said plurality of pixels in each scan line as a
plurality of combinations of pixel run lengths and said three reduced
color component signals for each said run length, said run lengths being
determined between said starting pixel for each scan line, intermediate
points selected from the group of said decision points and points
intermediate said decision points, and said ending pixel, and each run
length being of a seventh digital word size.
42. The camera of claim 41, wherein said means for comparing said rates of
change includes means for comparing at least one said decision parameter
with an adaptive cumulative threshold.
43. The camera of claim 41, wherein said first, second, and third digital
word sizes are equal.
44. The camera of claim 41, wherein at least two of said first, second, and
third digital word sizes are equal.
45. The camera of claim 41, wherein each of said first, second, and third
digital word sizes are different.
46. The camera of claim 41, wherein said color video signal is RGB, and
said three digital color component signals represent red, green, and blue
color video components.
47. The camera of claim 41, wherein said three digital color component
signals represent cyan, magenta, and yellow color video components.
48. The camera of claim 41, wherein said three digital color component
signals represent first and second colors and a parameter based upon
luminance of the color video signal.
49. The camera of claim 41, wherein said three digital color component are
based upon hue, saturation, and intensity of said color video signal.
50. The camera of claim 41, wherein said means for coding said plurality of
pixels in each scan line is adapted to code each said combination of run
length and reduced color components as at least a 16 bit digital word.
51. The camera of claim 41, further including means for concatenating run
lengths in said plurality of combinations of run lengths and reduced color
components in a scan line which are associated with reduced color
components whose differences are less than a predetermined color
difference threshold.
52. The camera of claim 41, wherein the digital word size of each of said
digital color components is six bits.
53. The camera of claim 52, wherein said means for reducing the word size
of at least one of said three digital color component signals is adapted
to reduce the word size of each of said digital color components to 4
bits.
54. The camera of claim 53, wherein said means for coding said plurality of
pixels in each scan line is adapted to code each said combination of run
length and reduced color components as at least 16 bit digital word.
55. The camera of claim 41, wherein said means for determining said
luminance function comprises means for summing each of said three digital
color component signals.
56. The camera of claim 51, wherein said luminance function has a digital
word size of 8 bits.
57. The camera of claim 55, wherein said means for determining said
luminance function includes means for weighting of the sum of said three
digital color component signals with respect to one of said three digital
color component signals. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to information signal processing, and in
particular to the field of processing time sequential information signals,
such as video signals, for the purpose of compressing the amount of
information to be transferred from an encoding site to a decoding site. A
particular use of the invention is in the communication of color video
data over telephone lines.
2. Prior Art
Encoding of digital television signals ordinarily requires a transmission
rate of approximately 200 Mbits/s. Recent developments in coding systems
have permitted the transmission rate to be cut to less than 2 Mbits/s.
Coding systems using block oriented analysis of video picture frames and
processing by a conventional hybrid discrete cosine transform (DCT)
coefficient permit transmission at rates of between 64 Kbits/s and 384
Kbits/s. Such a system is described in Gerken and Schiller, "A Low
Bit-Rate Image Sequence Coder Combining A Progressive DPCM On Interleaved
Rasters With A Hybrid DCT Technique", IEEE Journal on Selected Areas in
Communications, Vol. SAC-5, No. 7, August, 1987. Adaptive coding
techniques applied to such DCT processing have allowed video data
transmission at rates as low as one to two bits per pixel, as is described
in Chen and Smith, "Adaptive Coding of Monochrome and Color Images", IEEE
Transactions on Communications, Vol. COM-25, No. 11, Nov. 19, 1977.
However, information transmitted at such low data rates seriously affects
the ability to reconstruct a sufficient number of frames per second so
that a real time picture is acceptable to a viewer. High capacity
telephone lines are available which will carry transmission at a rate of
up to 1.544 Mbits/s, but such lines are extremely expensive at a dedicated
use rate, and are still quite expensive at a scheduled use rate. Lower
capacity telephone lines are available which permit transmission at rates
of up to 56 Kbits/s and 64 Kbits/s. Relatively expensive video digital and
coding devices are commercially available which will transmit a video
signal at 56,000 bits per second, so that it is necessary to utilize a
combination of a device of this nature with the high capacity 1.544
Mbits/s telephone line to allow a framing speed much faster than about one
frame per second. The current transmission rate limit of ordinary
telephone lines approaches 18,000 bits per second, so that transmission of
real time sequencing of video pictures over ordinary telephone lines has
been viewed in the prior art as not being feasible.
Various schemes for reducing the amount of redundancy of information to be
transmitted in a digital video signal have been used. One technique is to
utilize a slow scan camera; and another technique is to transmit every nth
scanning line for each frame. Another technique involves the sending of
only those parts of a picture frame which are deemed to be important or to
have changed in some significant manner, by dividing the picture frame
into a number of segments or blocks which are typically 3.times.3 or
4.times.4 groups of pixels, and analyzing the content of the blocks. These
techniques tend to also reduce the resolution of the video picture.
Another technique in the reduction of transmission time which does not
decrease the resolution of a picture transmitted is run length encoding.
In run length encoding, the scan lines of a picture frame are encoded as a
value of the color content of a series of pixels and the length of the
sequence of pixels having that value or range of values. The values may be
a measure of the amplitude of a video signal, or other properties of such
video signals, such as luminance or chrominance. An example of a system
which utilizes run length coding of amplitude of video signals is U.S.
Pat. No. 3,609,244 (Mounts). In that system, a frame memory also
determines frame to frame differences, so that only those differences from
one frame to the next are to be transmitted. Another example of a method
for transmitting video signals as compressed run length values which also
utilizes statistical coding of frequent values to reduce the number of
bits required to represent data is U.S. Pat. No. 4,420,771 (Pirsch).
Ideally, compression of color video information to allow real time
sequencing of picture frames at a rate of up to 15 frames per second, and
at bit rates as low as 11,500 bits per second would be desirable, to allow
the communication of color video data over ordinary telephone lines. A
video data compression system able to achieve equivalent data transmission
rates as systems using higher quality telephone lines with more efficient
and less costly equipment than is currently available would also be
desirable.
SUMMARY OF THE INVENTION
The present invention provides for a method and system for compressing
color video data in a video communication system, in which a luminance
funcion is utilized to determine differences between the luminance of
pixels in the scan lines of the picture, to determine the rates of change
about certain decision points in each scan line, and in which the digital
word size of the color values is reduced. Thereafter pixels in the scan
lines of the picture are coded as a series of run lengths of the digitally
compressed color values.
Briefly, and in general terms, the method for compressing color video data
according to the present invention is for use in a video communication
system having means for producing a color video signal comprising three
digital color component signals of first, second and third digital word
sizes, and includes determining a luminance function for each pixel based
upon the digital color signals; determining at least one decision
parameter based upon differences in said luminance function between pixels
a given distance from one another; determining the rate of change of the
decision parameter for each of those pixels; comparing the rates of change
of the decision parameter with one or more threshold values to determine
decision points; reducing the word size of digital color signals to
provide reduced digital color signals of fourth, fifth and sixth digital
word sizes; and coding the pixels in scan lines as combinations of run
lengths and the digitally reduced color signals.
The invention also provides generally for a system for compressing color
video data for use in a video communications system having means for
producing a color video signal comprising three digital color signals of
first, second and third digital word sizes, and having means for
determining a luminance function for each pixel based upon the digital
color signals; the data compression system comprising means for
determining at least one decision parameter based upon differences in said
luminance function between pixels a given distance from one another; means
for determining the rate of change of at least one decision parameter for
each of the pixels; means for comparing the rates of change of at least
one decision parameter with one or more threshold values to determine
decision points; means for reducing the word size of the digital color
signals to give reduced digital color signals of fourth, fifth, and sixth
digital word sizes; and means for coding the pixels in scan lines as
combinations of run lengths of the digitally reduced color signals. The
invention also provides for a camera which includes the data compression
system.
In a preferred embodiment, the digital color component signals are RGB, and
the color component word sizes are equal. The digital word size of the
digital color components is preferably initially six bits per each
component color, and the luminance function is determined with an accuracy
based upon the six bit digital color values. Thereafter the word size of
the digital color components is reduced to four bits each, and the run
length and color components are coded together as a bit stream of combined
run length and color information in sixteen bit digital words.
Other aspects and advantages of the invention will become apparent from the
following detailed description and the accompanying drawings illustrating
by way of example the features of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the system and method for compressing
color video data in a video communication system;
FIG. 2 is a luminance plot across one scan line in a video picture;
FIG. 3 shows a run length representation of features in a video scan line;
and
FIG. 4 shows a run length representation of transitions about slope
decision points of a video scan line.
DETAILED DESCRIPTION OF THE INVENTION
As is shown in the drawings for purposes of illustration, the invention is
embodied in a method and system for compressing color video data in a
video communication system having means for producing a color video signal
for a plurality of picture frames, with each picture frame comprising a
plurality of scan lines composed of a plurality of pixels, each scan line
having a starting pixel and an ending pixel, and each pixel in each | | |
