| JCTVC-O0132 AHG5/AHG9: On support for alpha channel in HEVC [M. Naccari, M. Mrak (BBC)]
Initially reviewed in BoG.
Further discussed Thu 31st (GJS).
Alpha channel signals are used in professional (studio) video coding applications. An alpha channel complements the information contained in the main bitstream by providing, for each image sample, its degree of transparency. At the 13th meeting in Incheon it was decided that alpha channel will be supported in the Range Extensions of HEVC using auxiliary pictures. Moreover, at the same meeting it was also decided that the depth information used in 3D video coding will be also transmitted using the auxiliary pictures. At the 14th meeting in Vienna, contribution JCTVC-N0063/JCTVC-E0049 proposed a framework to transmit auxiliary data as an additional scalability dimension. This contribution builds on the framework described in JCTVC-N0063/JCTVC-E0049 and proposes high level syntax changes to support functionalities relative to the use of alpha channel in frame composition. The parameters and data suggested for alpha channel manipulation are proposed in the form of an SEI message.
Proposed syntax elements:
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alpha_opaque_value
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alpha_transparent_value
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alpha_threshold_type
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if( alpha_threshold_type == 1 ), alpha_threshold_binary
A contributor asked about the alpha channel parameters in AVC. That includes an indication of pre-multiplication and some other parameters.
The contributor said that this was mostly the same as in AVC except not supporting the pre-multiplication, which would probably be good to indicate, and adding binary clipping.
AVC also has an alpha_incr_flag regarding scaling interpretation of alpha.
The ue(v) coding was questioned and it was suggested that it might be better to indicate the bit depth and use bit-depth-dependent FLC for the parameters.
It was agreed that something like this is desirable to have. Further study was encouraged to decide on details.
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