Of itu-t sg16 wp3 and iso/iec jtc1/SC29/WG11

JCTVC-O0259 AHG18: On Supporting Higher Bit Depth With Transform Skip Only Changes [K. Misra, S.-H. Kim (Sharp)]

It is asserted that to support higher bit depth the current HEVC range extension draft requires transforms capable of processing a larger data range when compared to HEVC version 1. It is proposed that higher bit depth be supported by changing the transform skip processing path only. It is asserted that the proposed design is desirable since it does not require a change to the core transform processing capabilities. The performance of the proposed design is evaluated using AHG18 anchors and an average luma BD rate degradation was found of approximately 0.6% (12 bit), 1.1% (14 bit), 0.6% (16 bit).

Revision 1 of the document contains experimental results for AHG18 test conditions. It also includes specification text changes reflecting the use of 16-bit clip after the first 1-D inverse transform (which is the same as in HEVC version 1).

As reported, the proponent only tested the low-precision forward transform (not the modification described in O0068). Additional results with the high-precision forward transform can be provided.

The basic proposal is to not extend the inverse transform precision as bit depth increases, but rather to rely on transform skip alone to provide higher precision coding capability. At high bit depth, basically all blocks would be coded with transform skip.

For 16 bit, proposes extending the clipping range by 1 bit, since otherwise the residual would clip valid data.

It was remarked that, if considered, something like this that has an adverse effect on the transform precision should be tested with larger QPs as well and with inter as well as intra coding. It was also commented that having true high dynamic range material is also important. The proponent suggested also testing on even smaller QP values.

It was noted that scaling lists are not useful if the transform is skipped.

There was generally not much interest in this approach due to the adverse effect on the effectiveness of the transform path.