Computer graphics processing and selective visual display system – Computer graphics processing – Attributes
Reexamination Certificate
2000-11-29
2004-12-07
Brier, Jeffery (Department: 2672)
Computer graphics processing and selective visual display system
Computer graphics processing
Attributes
C345S603000
Reexamination Certificate
active
06828981
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of display and test equipment for video monitoring, production and the like, and in particular provides a method and apparatus whereby pixel display characteristics are illustrated in a manner that indicates compliance or violation of color gamut limitations applicable to the color space encoding technique being employed.
2. Prior Art
In color video display, a picture field is defined from an array of discrete picture elements. Each individual picture element or pixel comprises three primary-color portions, such as dots or short lines that are closely spaced. A cathode ray tube display, for example, has trios of red, blue and green phosphors. The current amplitude of an electron beam applied to the phosphors causes the red, blue and green dots at each pixel element to emit red, blue and/or green light, respectively. The specific proportion of light emission in the primary colors defines a full spectrum as well as black and white, and the colors of all the pixels in the field make up the picture display.
The signals that drive the light emissions in the primary colors can be encoded in a number of ways. The RGB encoding method provides separate red, blue and green signals, the respective amplitudes of which encode luminance, saturation and hue. Composite encoding uses luminance and color difference signals to encode the same variables of luminance, saturation and hue.
The signal characteristics that encode the different variables are obviously critical to the appearance of the picture. Video production and test equipment typically includes test pattern generators for exercising the video equipment in a predetermined way, and also test equipment such as a display, in which the color characteristics of the signal are illustrated. Such equipment usually includes a vectorscope for displaying information about the color portion of the video signal at any particular time, specifically, hue and saturation. The vector display is a two dimensional polar graph wherein the hue is represented by the angle of the displayed coordinate from the positive X axis, and the saturation is represented by the radius or distance from the origin. The color characteristics of each pixel correspond to a point on the vectorscope display.
Although the vector display is polar wherein angle defines hue and radius defines saturation, the vector display also can be considered simply an XY plot of two color difference components of the video signal. The X and Y ordinates or axes encode I versus Q for NTSC; U versus V for PAL, R-Y versus B-Y for analog component, and so on. The typical test pattern has several color bands wherein all the pixels in the band have given characteristics, and appears as a corresponding number of bright dots at different points in the vector display. A regular video program most often contains many varied combinations of color characteristics that change constantly and appear on the vector display as a changing indistinct shape.
There are three variables that encode the characteristics of a pixel in a display, such as red, blue and green amplitude in RGB, or luminance and two color difference values, for analog components. The three variables are sometimes referred to as a color space and the different encoding techniques are termed different color spaces. The characteristics of each pixel have particular values of three variables. If the variables are plotted as orthogonal axes, those pixel characteristics locate a point in a volume, hence a color space. The volume might be considered a rectilinear volume with mutually perpendicular sides, each of which extends from a minimum value to a maximum value of each of the three variables according to the color space being used. However, the definition of color is made more complex by a number of factors. There are differences in human perception of the respective primary colors whereby different ranges may be appropriate for different values. The three values in color space also are interrelated in some color space definitions, such as color difference definitions which are based on the difference or comparative levels of two colors. As a result of these and other factors, the allowable volume or universe of permissible points in a color space turns out to be an irregular volume within a rectilinear volume circumscribed by maximum and minimum values for each of the three variables in the color space.
To illustrate some of the potential complexity, in component digital video, the color space may be based on a fundamental luminance equation:
Ey
=0.299
Er
+0.587
Eg
+0.114
Eb,
which gives the luminance Ey in terms of the three primaries Er, Eg, and Eb. For the CCIR-601 standard, the two color components which are digitized are:
Cr=KCr
*(
Er−Ey
),
and
Cb=KCb
*(
Eb−Ey
).
They are given by:
Er−Ey
=0.701
R
−0.587
G
−0.114
B
Eb−Ey
=−0.299
R
−0.587
G
+0.886
B
Luminance Y has a permitted range from zero to one. Differences Er−Ey and Eb−Ey have ranges +0.701 to −0.701 and +0.886 to −0.886. They are renormalized by applying coefficients:
KCr
=0.5/0.701=0.713
and
KCb
=0.5/0.886=0.564
which gives re-normalized color differences
ECr
=0.500
R
−0.419
G
−0.081
B
ECb
=−0.169
R
−0.331
G
+0.500
B.
Luminance Y often is quantized or digitized to 220 levels, that is, zero to 220, with “black” being at level 16 (i.e., luminance levels below 16 are blacker-than-black). The decimal value of Y prior to quantization is
Y
=219(
Ey
)+16
where Ey is the 0-1 range continuous version, and Y is either the nearest integer (8-bit version) or the fractional version with two bits of fractional value maintained (10-bit version). Similarly, the color difference components are quantized to 225 levels with an offset or zero level equal to 128 which gives:
Cr
=160(
Er−Ey
)+128
Cb
=126(
Eb−Ey
)+128.
Often, the RGB components have the values 0-255, and the conversions used are:
Y
=0.257
R
+0.504
G
+0.098
B
+16
Cr
=0.439
R
−0.368
G
−0.071
B
+128
Cb
=−0.148
R
−0.291
G
+0.439
B
+128.
In a video device or in a digital processor or other situation, it may be convenient for various reasons to employ a certain one of the encoding techniques for some purposes and a different encoding technique for other purposes. Thus in a color television receiver, for example, video is received and decoded from a composite signal. The receiver processes or converts the color information to separate R, G and B signals to modulate the electron beam current of three separate electron beam guns positioned to excite phosphor dots of the respective color.
A constant problem in video data processing, recordation and replay is encountered because there is a disparity between the allowable ranges of different component and composite color spaces. Combinations of values that are well within the allowed range of a color difference component video system, for example, may produce signal amplitudes that are well outside of the allowable ranges when the signal is transcoded or converted into its equivalent values in composite or RGB color space.
Excursions beyond the permissible bounds of color definition in one or another component or composite color definitions or color spaces are practically impossible to quantify by observing the signals on a waveform monitor. It is possible to envision a waveform monitor that processes video data by transcoding, for example, from color difference video format to RGB format, and then plots the resulting RGB values in a manner that shows when one or more of the transcoded R, G and B values goes out of permissible range. The operator (or other means) then can view or otherwise monitor the excursions directly, and can determine when the R, G and/or B values have gone out of range.
Brier Jeffery
Duane Morris LLP
Good-Johnson Motilewa
Videotek, Inc.
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