JVET-Y0050 AHG2/AHG9: On the alpha channel information SEI message [Y.-K. Wang (Bytedance)]

In the semantics the alpha channel information SEI message, the alpha_channel_incr_flag is inferred to be equal to 0 when not present. It is asserted that this inference seems unnecessary or even not correct. The flag is not present only when alpha_channel_cancel_flag is equal to 1, in which case no information applies. It is further asserted that, if the absence of the SEI message is also considered, then the value of the flag should depend on the persistency scope of the previous SEI message of this type, and then this inference can be incorrect.

It is therefore proposed to remove the inference rule. Note that this asserted issue applies to both VSEI and HEVC.

Decision: Adopt JVET-Y0050, both for V2 of VSEI and HEVC JVET-Y1005/CDAM.

JVET-Y0050 AHG2/AHG9: On the alpha channel information SEI message [Y.-K. Wang (Bytedance)]

See notes in section 4.2.