DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 38.13 Because the
quantizer is adjustable, the
quantizer matrix must be
conveyed through a side
channel to the decoder. In
color images, separate quantizer
are used for the luma
and chroma components.
In ISO JPEG, the quantizer matrix is
directly conveyed in the bitstream.
In the DV adaptation of JPEG, several
quantizer matrices are defined
in the standard; the bitstream indicates
which one to use.
The notation mquant is found in the
ITU-T H.261 standard for teleconferencing;
mquant (or MQUANT) is not
found in JPEG or MPEG documents,
but is used informally.
DCT Q VLE
DCT -1
TABLE
Q -1
TABLE
VLE -1
The quantizer matrix could, in principle, be chosen
adaptively to maximize the performance for a particular
image. However, this isn’t practical. JPEG encoders for
still images generally offer a choice of several compression
settings, each associated with a fixed quantizer
that is chosen by the system designer.
Because different quantizer matrices may be associated
with different images, the quantizer matrix must be
conveyed to the decoder, as sketched in Figure 38.13,
either as part of the file, or through a side channel. In
color images, separate quantizers are typically used for
the luma and chroma components. In stillframe applications,
the overhead of this operation is small. In
a realtime system, the overhead of conveying quantizer
matrices with every frame, or even within a frame, is
a burden.
A modified approach to compression ratio control is
adopted in many forms of M-JPEG (and also, as you will
see in the next chapter, in MPEG): Reference luma and
chroma quantizer matrices are established, and all of
their entries are scaled up and down by a single numerical
parameter, the quantizer scale factor (QSF, sometimes
denoted Mquant). The QSF is varied to
accomplish rate control.
As I mentioned, JPEG ordinarily uses luma/chroma
coding, and 4:2:0 chroma subsampling. But the JPEG
standard accommodates R’G’B’ image data without
458 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES