Issue with sub-picture functionality

[vulcano]

I would like to bring to your attention an issue we are facing in our MPEG Immersive video (MIV) experiments related to the VTM project.

Our experiments involve splitting atlases into sub-pictures and encoding them separately with VTM. We then merge them using the VTM sub-picture merging tool. However, we have encountered a problem where there is an offset in rendering from the second frame onwards. After investigating, we have identified that this issue is due to incorrect decoding of the merged bitstream. We suspect that this is a bug in the VTM merge application or in the VTM decoder.

I would like to provide more detailed information on our findings, which can be found in MPEG input contribution m61912 '[MIV] Preliminary experimental result of EE7'. Here is the link to the document: ​https://dms.mpeg.expert/doc_end_user/current_document.php?id=85697&id_meeting=193. A detailed discussion on this issue within MIV Ahg is available here: ​https://mpeg.expert/software/MPEG/MIV/InputDocuments/-/issues/652.

Additionally, we would like to provide the VVC bitstream to recreate this situation and investigate the issue further.

[vulcano]

The original vs. decoded atlas images

[vulcano]

subpic, merged bitstream and encoding cfg file and etc.

[hallapur]

I think the issue is probably with the encoder config paramaters. Here is a quote from software-manual.pdf section 8: "If VTM encoder is used for encoding input bitstreams, it is recommnended that ALF, CCALF, joint chroma coding, LMCS and AMaxBT are disabled. This prevents those tools having parameters with different values in different subpictures which would result in merged bitstream being non-conformant". So those specific tools need to disabled at the encoder to guarantee that merged stream is decodable.

If you want to enable those tools for maximum coding performance, you need to merge original YUV files first (can be done with subpic merge tool) and then use VTM encoder to configure subpictures and encode merged YUV file. There are some subpicture config examples in cfg/partitioning.