JVET-R0146 AHG11: Context coded bin limits for palette coding [J. Gan, C. Rosewarne (Canon)]
To limit the required throughput of the CABAC engine, the number of context coded bins is limited to 1.75 bins per sample. This limit is explicitly calculated and enforced in the residual coding process. In palette coding, the number of context coded bins is not explicitly limited. With the adoption of JVET-Q0504, an “evil” bitstream can be constructed that would reach 1.984 context coded bins/sample. This contribution proposes a number of options for solving this bug. The preferred option explicitly calculates the context coded bins limit for palette mode CUs.
Presented Thu 9 1333 UTC (chaired by JRO)
More investigation is necessary to understand if the worst case is really exceeded, as
- The comparison is only made against the max number of transform coefficients per color component. As palette either codes all color comp jointly, this might only exceed the max number for monochrome case (which only would apply if there was a dedicated monochrome profile)
- In residual coding, additional context coded bins occur for motion, mode etc.
More detailed analysis was presented (see JVET-R0146r2). It is confirmed that the worst case of CC bins in transform coding goes up to 2.125 (with 4x4 blocks, where the bin count for last position affects lowest number of coefficients).
No need for action.