JVET-AP0216 AHG9: On SEI extensions for legacy devices [R. Skupin, Y. Sanchez, T. M. Borges, C. Hellge, T. Schierl (HHI)]

When encountering unknown payload beyond the expected SEI message payload, legacy decoders shall ignore the unknown payload bits and process the SEI message payload up to the unknown extension bits.

This contribution argues that this behaviour may not always be desirable and proposes to add a mechanism that allows encoders to indicate whether SEI payload extension bits are required to correctly process the SEI message by a legacy device that cannot parse a particular SEI message payload extension.

It is proposed to consider adding the signalling to the draft text for the new version of AVC and VSEI v5 as well as for future versions of HEVC and VVC and future codecs.

The mechanism consists of adding a flag (sei_payload_ext_indispensable_flag) to the sei_payload structure() in the codec specifications and a corresponding flag (sei_ext_indispensable_flag) to future SEI message extensions in VSEI, controlled by presence variables to accommodate legacy devices.

The value of SeiExtIndispensablePresntFlag would be done by the codec specification for each SEI message.

It was noted that the SEI manifest message can indicate essentiality of a full SEI message.

It was suggested that a new SEI message can be defined if an extension is indispensable.

A question was raised about the need for identifying indispensable in particular extensions in multiple extensions.

Further study.