Game Development Reference
In-Depth Information
Fig. 6.3
CBAC encoder block diagram
coding, where the final stage of arithmetic encoding together with a subsequent
context updating takes place. This process is similar as CABAC. However, each of
the three modules is different from that in CABAC which will be discussed in the
following subsections:
The symbol sequence is modeled as a high-orderMarkov process, and compressed
by context-based arithmetic coding. A key issue in context modeling of an input
symbol sequence is how to balance the desire of using a high-order context modeling
technique against the context model cost. As we know from the view of information
theory, the higher the conditional entropy, the more an observer can predict the state
of a variable, knowing the state of the other variables. Therefore, if the context
order is not sufficiently high, it will not be able to capture all the local statistical
behaviors of the source sequence. But on the other hand, if the order of the model
is too high, there will not be enough samples to accurately estimate the context
parameters, causing context dilution problem. To solve this problem, CBAC adopts
a novel context quantization technique that generates only limited context states out
of a very large causal context, as described below.
6.3.2 Symbol Binarization and Binary Arithmetic Coder
For the nonbinary valued symbols, e.g.,
Level
and
Run
, they should perform bina-
rization before sending to arithmetic coder. The typical binarization process is as
follows:
•
The signed integer Level is represented by sign (0/1: +/-) and the unary bits of its
magnitude (
absLe
v
el
).
•
The positive integer
Run
is simply represented by unary bits.
Table
6.1
lists part of the mapping from unsigned numbers to unary binary repre-
sentation. We can see that the binarized representation has a regular code structure,
which is a concatenation of a prefix code and a suffix code. Given a nonnegative
number
N
, the prefix part consists of
n
−
1 zeros and the suffix part is fixed to be a
one.
