JVET-C0043 Arbitrary reference tier for intra directional modes [Y.-J. Chang, P.-H. Lin, C.-L. Lin, J.-S. Tu, C.-C. Lin (ITRI)]

The intra prediction method of the JEM is composed of various kinds of intra prediction modes, i.e., the planar mode, DC mode, and different intra directional modes. Traditionally, only one reference line of samples adjacent to the current prediction unit (PU) is exploited to predict the samples inside the PU. This contribution proposes to increase the number of the exploited reference lines for intra directional modes. Compared to JEM 2.0, the simulation results reportedly show that the proposed method with 2 reference lines can improve up to 0.73% BD-rate for overall test sequence classes and 1.21% BD-rate for the A2 class, featuring 4K sequences with unmanned content. It is further reported that the proposed method with 3 / 4 reference lines can improve up to 1.05% / 1.22% BD-rate for overall classes and 1.73% / 1.98% BD-rate for A2 class.

Reference lines as described above are additional lines/columns from boundary or non-boundary samples of reference blocks. These are used instead of only boundary samples. Computation at the decoder is not increased, but more line buffers are necessary.

Results were only reported for AI. RA simulations were not completed. The method significantly increases encoding time (400% and more for N=4).

There were also results with a “fast” version with 200% for N=4 where the average gain is 0.76%.

Generally, several experts expressed an interest to study this technology, as it gives good gain while not significantly increasing decoder computer decoder.

It was agreed to establish an EE; the EE should also compare against the JEM with a similar complexity of encoder search for conventional intra prediction, where a comparable number of intra prediction modes are tested.