Synergies between in-loop and out-of-loop mapping for HDR-PQ content

This paper presents experimentation results related to adaptive video content mapping used as compression tool of HDR-PQ content. The purpose of adaptive video content mapping is to adapt the video signal dynamically depending on its statistical properties in order to better exploit the signal codewords range. Adaptive video content mapping has been investigated during the VVC standard development with two main implementation designs: in-loop mapping, and out-of-loop mapping. In VVC, the in-loop implementation was finally adopted with a tool named “luma mapping with chroma scaling” (LMCS). Thanks to its in-loop implementation, LMCS offers a high flexibility (high temporal adaptivity, content type adaptivity) and implementation benefits (no need to manage several conformance points, one after the decoder, and one after a post-decoder inverse mapping). Nevertheless, it was also observed that for HDR-PQ content, out-of-loop mapping can bring some added coding gains. The paper investigates potential synergies between the in-loop LMCS and out-of-loop mapping. Out-of-loop mapping is implemented as a pre-encoding process, with inverse mapping applied as post-decoding inverse process. An SEI message is used to convey the out-of-loop mapping data aside with the video bitstream. Performance results are reported for HDR-PQ content, with out-of-loop mapping only, and with out-of-loop mapping combined with LMCS.