DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

A sensor element is a photosite.
In a “one-chip” camera, hardware
or firmware performs spatial interpolation
to reconstruct R, G, and
B at each photosite. In a “threechip”
camera, the dichroic filters
are mounted on one or two glass
blocks. In optical engineering, a
glass block is called a prism, but it
is not the prism that separates the
colors, it is the dichroic filters.
Figure 22.3, on page 244
Figure 22.4, on page 245
Figure 22.5, on page 246
Figure 22.6, on page 247
Figure 22.7, on page 248
Figure 22.8, on page 249
drive the display. Consider these display primaries:
monochromatic red at 600 nm, monochromatic green
at 550 nm, and monochromatic blue at 470 nm. The
3×3 matrix of Equation 22.2 can be used to process
XYZ values into components suitable to drive that
display. Such signal processing is not just desirable; it is
a necessity for achieving accurate color reproduction!
Every color video camera or digital still camera needs to
sense the image through three different spectral characteristics.
Digital still cameras and consumer camcorders
typically have a single area array CCD sensor (“one
chip”); each 2×2 tile of the array has sensor elements
covered by three different types of filter. Typically, filters
appearing red, green, and blue are used; the green filter
is duplicated onto two of the photosites in the 2×2 tile.
This approach loses light, and therefore sensitivity.
A studio video camera separates incoming light using
dichroic filters operating as beam splitters; each component
has a dedicated CCD sensor (“3 CCD”). Such an
optical system separates different wavelength bands
without absorbing any light, achieving high sensitivity.
Figure 22.7 shows the set of spectral sensitivity functions
implemented by the beam splitter and filter
(“prism”) assembly of an actual video camera. The functions
are positive everywhere across the spectrum, so
the filters are physically realizable. However, rather poor
color reproduction will result if these signals are used
directly to drive a display having Rec. 709 primaries.
Figure 22.8 shows the same set of camera analysis functions
processed through a 3×3 matrix transform. The
transformed components will reproduce color more
accurately – the more closely these curves resemble the
ideal Rec. 709 CMFs of Figure 22.5, the more accurate
the camera’s color reproduction will be.
In theory, and in practice, using a linear matrix to
process the camera signals can capture and reproduce
all colors correctly. However, capturing all of the colors
is seldom necessary in practice, as I will explain in the
Gamut section below. Also, capturing the entire range
of colors would incur a noise penalty, as I will describe
in Noise due to matrixing, on page 252.
CHAPTER 22 COLOR SCIENCE FOR VIDEO 243