83 How does MPEG achieve compression? Sample:
Description
This article is from the MPEG FAQ, by Frank Gadegast phade@cs.tu-berlin.de with numerous contributions by others.
83 How does MPEG achieve compression? Sample:
The two chrominace pictures (Cb, Cr) possess only half the resolution
in both the horizontal and vertical direction as the luminance picture
(Y). This is the definition of the 4:2:0 chroma format. Most
television displays require that at least the vertical chrominance
resolution matches the luminance (4:2:2 chroma format). Computer
displays may further still demand that the horizontal resolution also
be equivalent (4:4:4 chroma format). There are a variety of filtering
methods for interpolating the chrominance samples to match the sample
density of luminance. However, the official location or center of the
lower resolution chrominance sample should influence the filter design
(relative taps weights), otherwise the chrominance plane can appear to
be shifted by a fractional sample in the wrong direction.
The subsampled MPEG-1 chroma position has a center exactly half way
between the four nearest neighboring luminance samples. To be
consistent with the subsampled chrominance positions of 4:2:2
television signals, MPEG-2 moved the center of the chrominance samples
to be co-located horizontally with the luminance samples.
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