JVET-E0039 A unified condition for affine merge and affine inter mode [J. Lee, S. Kim, J. Lim (LGE)]

In JEM 4.0, enabling conditions for affine merge mode and affine inter mode are different from each other, and the motivation of using two different conditions was suggested to be unjustified. This contribution proposes a unified enabling condition for affine merge mode and affine inter mode. It was reported that the proposed unification has caused negligible BD rate changes and encoder/decoder run-time changes in common test configurations. It was also reported that the current affine control motion vector storing method has a potential mismatch issue between the encoder and decoder when the CU width is equal to 4 (although there is no mismatch in the current JEM 4.0 because affine inter prediction mode is not enabled when the CU width is 4). The contribution proposed to change the current control motion vector storing method to prevent this potential bug.

It was noted that the current implementation of affine MC does not have any problem.

Three unification methods are suggested here, where method 1 would cause a bug (therefore not to be considered), and methods 2 and 3 have very small losses for some classes, and very small gains for other classes; overall there was no impact on average compression performance.

The unification does not seem to provide any relevant simplification. One original motivation of using different criteria came from the observation that affine inter is more frequently used for larger block sizes.

There seemed to be no obvious benefit to making the proposed change, so no action was taken on this.