Media profile: CMAF IMSC1 Image ('im1i')

Provisional media profile

Captions/subtitles using the TTML IMSC1 Image profile defined in CMAF

Name/company of designated proponent

Pierre-Anthony Lemieux (MovieLabs)

Market relevance

1. Device manufacturers which are WAVE members (at least 2): Microsoft, Sony

2. Content providers which are WAVE members (at least 2): Comcast, MovieLabs, Disney

3. Other Wave members (at least total of 6 including the above members): castLabs

Open standards

ISO/IEC 23000-19 Common Media Application Format

Normative references

ISO/IEC 23000-19 Information technology — Coding of audio-visual objects — Part 19: Common media application format (CMAF) for segmented media

W3C Recommendation, TTML Profiles for Internet Media Subtitles and Captions 1.0 (W3C IMSC1), Available at http://www.w3.org/TR/ttml-imsc1

Test and conformance tools

Test Content

The W3C publishes IMSC1 test vectors.

Conformance Tools

dash.js supports IMSC1.

For an Approved media profile:

Does the media profile specification include its ISOBMFF encapsulation?

Yes (see ISO/IEC 23000-19)

Have you tested the media profiles with W3C MSE?

Yes.

Any reported incompatibilities with W3C MSE?

Specification wise, MSE supports this media profile: the MSE spec explicitly allows for the creation of a source buffer providing TextTracks and the media format is packaged in ISO BMFF (as per the CMAF spec) which is included in the MSE Byte Stream registry.

Desktop browsers today don't have support for SourceBuffers containing ISO BMFF packaged IMSC1. Current versions of Chrome, Firefox, Safari and Edge will refuse to add a SourceBuffer for application/mp4;codecs="stpp". Instead a polyfill, as implemented by dash.js, is used.