Image Processing Reference
In-Depth Information
The computational load is far less than with the DCT.
A smaller computation overhead allows scope for other parts of the encoder to be
implemented with more complex algorithms.
Entropy Coding Improvements
H.264 supports two methods of lossless coding, which are supported at the entropy cod-
ing phase of the coefficients provided by the DCT output. The following two enhanced
operating modes are provided:
Context-adaptive variable-length coding (CAVLC)
Context-adaptive binary-arithmetic coding (CABAC)
The first (CAVLC) is based on a variable-length approach. The conversion tables that
are used to compress the data are computed based on the content of the video. The second
approach (CABAC) works by modifying the coding parameters to improve efficiency.
These would be used in high bit-rate situations such as studio environments.
These new techniques vastly improve the compression available through entropy
Logarithmic Quantization Improvements
Previous quantization tools used a linear quantizer. The H.264 codec implements a loga-
rithmic scale in the quantization parameter settings. Each increment of the quantization
parameter doubles the quantization step size. Figure 13-2 shows the logarithmic
scale, which is oriented in a variety of ways depending on how the mapping must be
carried out.
Logarithmic quantization has been used with audio for some time already because it
plays to the strengths of the human ear and how it responds to sound. The ear is more sen-
sitive at low amplitudes and so the logarithmic squashing of higher amplitudes allows the
samples to be taken more evenly across the range of human sound perception. This tech-
nique is used to make telephone systems work more effectively. The coding schemes
adopted by telecommunications companies are similar but not identical to those used in
the media industry. Useful keywords for further research on the topic are Mu-Law,
U-Law, M-Law, and A-Law.
The subjective effect is that an 8-bit sample sounds as good as a 14-bit sample on a
linear scale. There is therefore some compression already implicitly going on here.
Block-Level Improvements
Many of the improvements to the coding offered by the H.264 codec take place at the mac-
roblock level. Here, we highlight the more important additions.
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