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Further compression can be achieved by exploiting the statistical redundancy in
the resulting quantised DCT coefficients. This can be achieved with run-length
or entropy encoding, such as Huffman encoding. The general idea behind these
encoders is that the most frequently occurring values are assigned shorter
codes, with longer codes representing the less frequent values.
Particularly, for DCT coefficients, it is better to select the input to the
run-length encoding by applying a special zig-zag block scanning pattern, as
shown in Figure 2.4. This scanning ensures that the correlation of
adjacent samples will be maximal, increasing the overall coding efficiency.
Figure 2.4:
The zig-zag scanning pattern for the DCT coefficients.
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Isaac Kokkinidis
1998-08-27