DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Poynton, Charles, “Wide
Gamut Device-Independent Colour
Image Interchange,” in Proc. International
Broadcasting Convention,
1994 (IEE Conference
Pub. No. 397), 218–222.
Levinthal and Porter introduced
a coding system to accommodate
linear-light (tristimulus) values below
zero and above unity. See Levinthal,
Adam, and Thomas Porter, “Chap:
A SIMD Graphics Processor,” in
Computer Graphics, 18 (3): 77–82
(July 1984, Proc. SIGGRAPH ’84).
DeMarsh, LeRoy E., and Edward J.
Giorgianni, “Color Science for
Imaging Systems,” in Physics Today,
September 1989, 44–52.
Lindbloom, Bruce, “Accurate
Color Reproduction for Computer
Graphics Applications,” in
Computer Graphics, 23 (3): 117–
126 (July 1989).
Hall, Roy, Illumination and Color in
Computer Generated Imagery
(New York: Springer-Verlag,
1989). Sadly, it’s out of print.
Wide-gamut reproduction
For much of the history of color television, cameras
were designed to incorporate assumptions about the
color reproduction capabilities of color CRTs. But nowadays,
video production equipment is being used to
originate images for a much wider range of applications
than just television broadcast. The desire to make
digital cameras suitable for originating images for this
wider range of applications has led to proposals for
video standards that accommodate a wider gamut.
I will introduce the Rec. 1361 transfer function, on
page 265. That transfer function is intended to be the
basis for wide-gamut reproduction in future HDTV
systems. The Rec. 1361 function is intended for use
with RGB tristimulus values having Rec. 709 primaries.
However, the RGB values can occupy a range from
-0.25 to +1.33, well outside the range 0 to 1. The
excursions below zero and above unity allow Rec. 1361
RGB values to represent colors outside the triangle
enclosed by the Rec. 709 primaries. When the
extended R’G’B’ values are matrixed, the resulting
Y’C B C R values lie within the “valid” range: Regions of
Y’C BC R space outside the “legal” RGB cube are
exploited to convey wide-gamut colors. For details, see
C BC R components for Rec. 1361 HDTV, on page 318.
Further reading
For a highly readable short introduction to color image
coding, consult DeMarsh and Giorgianni. For a terse,
complete technical treatment, read Schreiber (cited in
the margin of page 20).
For a discussion of nonlinear RGB in computer graphics,
read Lindbloom’s SIGGRAPH paper.
In a computer graphics system, once light is on its way
to the eye, any tristimulus-based system can accurately
represent color. However, the interaction of light and
objects involves spectra, not tristimulus values. In
computer-generated imagery (CGI), the calculations
actually involve sampled SPDs, even if only three
components are used. Roy Hall discusses these issues.
256 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES