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used for the first and second bin to estimate the probabilities of whether the PU is
partitioned into smaller PUs, and the direction of the PU. Two different contexts
are used for the third bin depending on when the CU size is greater or equal to the
minimum allowed CU size. In the former, the context is based on the probability
of whether asymmetric partitions are used, while in the latter, the context is based
on the probability of whether PART_NxN is used. To reduce the number of regular
coded bins, the fourth bin (for AMP) is bypass coded.
8.4.3
Signaling of Special Coding Modes
HEVC supports two special coding modes, which are invoked on a CU level: the
so-called I_PCM mode and the lossless coding mode. Both modes, albeit similar in
appearance to some degree, serve different purposes and hence, use different syntax
elements for providing different functionalities.
A pcm_flag is sent to indicate whether all samples of the whole CU are coded
with pulse code modulation (PCM), such that prediction, transform, quantization,
and entropy coding as well as their counterparts on the decoder side are simply
bypassed. This I_PCM mode, however, is only allowed for intra-coded CUs with
prediction partitioning mode PART_2Nx2N. 11 The pcm_flag is coded with the
termination mode of the arithmetic coding engine, since in most cases I_PCM mode
is not used, and if it is used, the arithmetic coding engine must be flushed and the
resulting CABAC codeword must be byte aligned before the PCM sample values can
be written directly into the bitstream with fixed length codewords. 12 This procedure
also indicates that the I_PCM mode is in particular useful in cases, where the
statistics of the residual signal would be such that otherwise, an excessive amount of
bits would be generated when applying the regular CABAC residual coding process.
The option of lossless coding , where for coding of the prediction residual both the
transform and quantization (but not the entropy coding) are bypassed, is also enabled
on a CU level and indicated by a regular coded flag called cu_transquant_
bypass_flag . The resulting samples of the losslessly represented residual signal
in the spatial domain are entropy coded by the CABAC residual coding process
(see Sect. 8.6 ), as if they were conventional transform coefficient levels. Note that
in lossless coding mode, both in-loop filters are also bypassed in the reconstruction
process (which is not necessarily the case for I_PCM), such that a mathematically
lossless (local) reconstruction of the input signal is achieved.
11 I_PCM is not allowed for intra-coded 4
4 blocks.
12 Note that the PCM sample bit depth (i.e., wordlength) for luma and chroma samples can be
specified independently in the SPS.
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