DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

LUMA
or R’G’B’
Figure 27.4 Coring block
diagram includes complementary
filters that separate
low- and high-frequency
components. The highfrequency
components are
processed by the nonlinear
transfer function in the sketch.
LOWPASS FILTER
NONLINEAR
TRANSFER FUNCTION
HIGHPASS FILTER
input is not a cartoon, you run the risk that coring will
cause it to look like one! In a close-up of a face, skin
texture produces a low-magnitude, high-frequency
component that is not noise. If coring eliminates this
component, the face will take on the texture of plastic.
Coring is liable to introduce spatial artifacts into an
image. Consider an image containing a Persian carpet
that recedes into the distance. The carpet’s pattern will
produce a fairly low spatial frequency in the foreground
(at the bottom of the image); as the pattern recedes
into the background, the spatial frequency of the
pattern becomes higher and its magnitude becomes
lower. If this image is subject to coring, beyond a certain
distance, coring will cause the pattern to vanish.
The viewer will perceive a sudden transition from the
pattern of the carpet to no pattern at all. The viewer
may conclude that beyond a certain distance there is
a different carpet, or no carpet at all.
Chroma transition improvement (CTI)
Color-under VCRs exhibit very poor color difference
bandwidth (evidenced as poor chroma resolution in the
horizontal direction). A localized change in luma may
be faithfully reproduced, but the accompanying change
in color difference components will be spread horizontally.
If you assume that colored areas tend to be
uniformly colored, one way of improving image quality
is to detect localized changes in luma, and use that
information to effect repositioning of color difference
information. Techniques to accomplish this are collectively
known as chroma transition improvement (CTI).
332 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES
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LUMA
or R’G’B’