DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

MPEG’s VLE tables are standardized;
they do not need to be
transmitted with each sequence
or each picture.
Figure 38.11 Reconstruction
error is shown in this matrix of
differences between original
sample values and reconstructed
sample values. Original sample
values are not perfectly reconstructed,
but discarding information
according to the spatial
frequency response of human
vision ensures that the errors will
not be too perceptible.
differences from that. In essence, the previous DC term
is used as a predictor for the current term. Separate
predictors are maintained for Y’, C B, and C R.
Zero AC coefficients are collapsed, and the string is
represented in {run length, level} pairs, as shown in
Figure 38.10:
{1: -2}, {0: -1}, {0: -1}, {0: -1}, {2: -1}, EOB
Figure 38.10 VLE {run length, level} pairs
A JPEG encoder has one or more VLE tables that map
the set of {run length, level} pairs to variable-length
bitstrings; pairs with high probability are assigned short
bitstrings. JPEG has no standard VLE tables; however,
sample tables given in a nonnormative appendix are
often used. Typically, there are two tables, one for luma
and one for color differences. The tables used for an
image are included at the head of the JPEG bitstream,
and thereby conveyed to the decoder.
JPEG decoding
Decompression is achieved by performing the inverse of
the encoder operations, in reverse order. Figure 38.11
shows the matrix of differences between original
sample values and reconstructed sample values for this
example – the reconstruction error. As is typical of
JPEG, the original sample values are not perfectly
reconstructed. However, discarding information
according to the spatial frequency response of human
vision ensures that the errors introduced during
compression will not be too perceptible.
ε
=
-5 -2 0 1 1 -1 -1 -1
-4 1 1 2 3 0 0 0
-5 -1 3 5 0 -1 0 1
-1 0 1 -2 -1 0 2 4
-4 -3 -3 -1 0 -5 -3 -1
-2 -2 -3 -3 -2 -3 -1 0
2 1 -1 1 0 -4 -2 -1
4 3 0 0 1 -3 -1 0
456 DIGITAL VIDEO AND HDTV ALGORITHMS AND INTERFACES