JVET-R0190 Post-filter hint based on ALF classification [H.-B. Teo, H.-W. Sun, C.-S. Lim (Panasonic)]
This contribution is a further follow-up proposal on JVET-Q0319 and JVET-P0179. In the case when the ALF module is disabled through a bitstream header, a post-filter hint based on ALF SEI is decoded. The proposed SEI message is used to enable the ALF module to filter the reconstructed image for output using the accompanied ALF filter flags and filter coefficients. This contribution provides three examples of possible application cases and their benefit (sharpening, Bokeh effect, and adaptive denoising). Two alternative aspects of ALF SEI signalling were proposed: Aspect 1 uses an APS id to refer to the APS containing the filter coefficients, Aspect 2 signals the filter coefficients in the SEI message itself. Source code for encoder and decoder implementations was reported to be available.
Characteristics of approaches as described by the proponent:
Num | Pre-filter | HEVC post-filter hint | Proposed |
1 | Separate module | Separate module | Built-in to VVC |
2 | Determined by content provider, but typically: Picture level on/off No classification 1 set of filters | Picture level on/off | CTU level on/off |
3 | No classification | 4x4 level luma block classification | |
4 | 1 set of filters for luma 1 set of filters for Cb 1 set of filters for Cr | Up to 25 sets of luma filters 1 set of filters for Cb 1 set of filters for Cr | |
5 | Sharpening original source | At same bit rate, smaller QP used compared to pre-filter | At same bit rate, smaller QP used compared to pre-filter |
ALF ordinarily has CTU-level control (on/off and filter selection). This proposal does not have CTU-level control.
Additional use cases were described. At previous meetings, only the sharpening case had been described.
It was commented that it seems better use the same syntax but not use the APS mechanism to convey the filtering parameters (approach 2). The approach #1 was not in the previous proposals.
It was suggested to consider this for inclusion when a draft of SEI messages is developed.
There was discussion of whether the prior post-filter hint SEI message had been considered of significant interest in applications. It did not seem that this had been very widely used.