DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

Figure 38.3 JPEG block
diagram shows the encoder
(at the top), which performs
the discrete cosine transform
(DCT), quantization (Q), and
variable-length encoding
(VLE). The decoder (at the
bottom) performs the
inverse of each of these
operations, in reverse order.
Inverse quantization (IQ) has
no relation to IQ color difference
components.
DISCRETE
COSINE
TRANSFORM QUANTIZE
DCT Q VLE
INVERSE
DCT
DCT -1
DCT -1
INVERSE
QUANTIZE
Q -1
Q -1
VARIABLE-
LENGTH
ENCODE
VARIABLE-
LENGTH
DECODE
VLE -1
VLE -1
In principle, the DCT algorithm could be applied to any
block size, from 2×2 up to the size of the whole image,
perhaps 512×512. (DCT is most efficient when applied
to a matrix whose dimensions are powers of two.) The
choice of 8×8 blocks of luma for the application of DCT
in video represents a compromise between a block size
small enough to minimize storage and processing overheads,
but large enough to effectively exploit image
redundancy.
The DCT operation discards picture information to
which vision is insensitive. Surprisingly, though, the
JPEG standard itself makes no reference to perceptual
uniformity. Because JPEG’s goal is to represent visually
important information, it is important that so-called
RGB values presented to the JPEG algorithm are first
subject to a nonlinear transform such as that outlined in
Nonlinear image coding, on page 12, that mimics vision.
JPEG block diagram
The JPEG block diagram in Figure 38.3 shows, at the
top, the three main blocks of a JPEG encoder: the
discrete cosine transform (DCT) computation (sometimes
called forward DCT, FDCT), quantization (Q), and
variable-length encoding (VLE). The decoder (at the
bottom of Figure 38.3) performs the inverse of each of
these operations, in reverse order. The inverse DCT is
sometimes denoted IDCT; inverse quantization is sometimes
denoted IQ.
450 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES