Image-based subtitle tracks

[technogeek00]

Use Case Description

From @nicoweilelemental

Image-based subtitles tracks For workflow reasons and charset reasons, some content owners don't include text-based subtitles in the live channels sources that they provide to distributors, but rather image-based subtitles (like DVB-Sub). While it's possible to transform the subtitles as IMSC1 Image Profile as per DASH-IF IOP section 6.4.4, there is no equivalent IMSC1 Image Profile support in the HLS RFC, which means that companies will continue to rely on proprietary forks of the RFC to support these use cases. Could we discuss the integration in the HLS RFC with Apple? Even if it wasn't supported by Apple players, it would be tremendously helpful for interoperability in the rest of the video ecosystem.

Working Notes

• Previously in #2 but separated out to capture submissions independently
• Note something actively included in HLS RFC
• IETF hls-interest list good place for discussion, previous IETF thread that discussed: https://mailarchive.ietf.org/arch/msg/hls-interest/FCfPmXIU6YApjrH7UltpDVSezTA/
• Per @nicoweilelemental, his organization has now created a reference stream for both HLS and DASH

Open Questions

Prioritization Questionaire

Question	Answer
Does the feature relate to an industry streaming use-case?	Yes, Subtitles
- What is the commonality of this case?	Uncommon
- Is this an established or emerging practice?	Declining, movement towards font-based
Does this feature have related mechanisms in both DASH and HLS?	No
- What is the maturity of support in both specifications?	DASH - Mature, HLS - Missing
- What is the maturity of implementation support for both specifications?	DASH - Immature, HLS - N/A
- Are there known interoperability issues between specs?	Yes, HLS explicitly excludes
- Has anyone implemented this interoperably?	Potentially
- Are there features missing in a specification with open proposals for it?	Potentially
Has the industry defined defacto mechanisms not present in both DASH and/or HLS?	Yes
- Why was functionality defined outside of the main specifications?	Usage of image-based IMSC in HLS
- Has the functionality been standardized elsewhere (DASH-IF, CTA, SVA)?	No
- Is the functionality proprietary or openly developed?	Proprietary
- Could the functionality be incorporated into specifications with evangelism?	No, clear stance from Apple

[technogeek00]

Transfer of @nicoweilelemental comment:

Image-based subtitle tracks I agree with the assessment that this is a declining need, mostly related to legacy playback environments. We recently introduced DVB-Sub passthrough for HLS/TS on MediaPackage, but right now we don't see customer demand for forward looking formats like IMSC Image Profile. AS an industry, we should probably not spend more time on it until there is a precise need identified.