JCTVC-B028 [M. Zhou (TI)] Coding efficiency test on large block size transforms in HEVC
Large block size transforms are a new coding tool proposed to the HEVC for improving coding efficiency. In this contribution, the coding efficiency test results of the large block size transforms are presented. Testing results showed that the large size transforms (32x32 and 64x64) provide an average gain around 1 ~ 2%; the large size transforms are most effective in 1080p (Class B) sequences in which an average gain of 3% (peak gain 6 ~ 9 %) is observed; the additional gain by adding 64x64 transform on the top of 32x32 transform is very limited.
The software that was used was the distributed "stripped version" of the JCTVC-A124 software. For each sequence, 121 frames were coded. It was remarked that coding longer sequences might help.
More detailed summary of the report given: Disabling 64x64 transform leads to a bitrate increase of 0.2% on average, disabling both 64x64 and 32x32 transform leads to an increase of 1.34% on average for hier. B; likewise for IPPP 0.40/1.89; for intra only 0.1-0.2/1.1-1.3. Typically higher gain for class B; Kimono gives largest gain, 9% reduction by using 64x64 maximum compared to 16x16
It was remarked that it may be helpful to measure statistics about how often the transforms are used, broken down according to video resolution classes.
It was also remarked that subjective quality effects should also be studied – in this respect the large block sizes may be more helpful.
Investigation on benefit of large block sizes should be conducted; this should also include a study about the benefit for the various resolutions (i.e. make the usage dependent on the picture size); subjective factors should also be included.
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