JVET-D0113 Variable number of intra modes [V. Seregin, X. Zhao, M. Karczewicz (Qualcomm)]

In this contribution, it is proposed to increase number of intra modes from 67 to 131 for blocks larger than 16x16, which reportedly improves prediction accuracy for large blocks. At the same time, it is proposed to decrease number of intra modes for 4x4 blocks, which reduces the mode signalling for small block sizes. Two thresholds indicating the number of intra modes are signalled in SPS. Simulation results reportedly show 0.17% luma BD rate reduction for all-intra configuration on average.

The first motivation is to increase the number of modes to 131

Then, it is proposed to signal in the SPS how many modes are used for which block sizes. In effect, the encoder complexity stays the same.

Optimum settings of max block sizes are dependent on resolution.

This reportedly reduces bitrate by 0.2%.

The decoder logic becomes more complicated, and parsing becomes dependent on block size. Without that, loss would occur.

Doubling the number of prediction modes and introducing the block size dependency in decoding is undesirable compared to the relative low gain reported.

It was commented that complexity is not only reflected by run time, but also by irregularity that is introduced in decoding.

No action was taken on this.