TTML/withdrawnProposal028

< Change Proposal Index

IMSC Profile refactoring - WITHDRAWN

• Owner: Nigel Megitt.
• Started: 08/08/14
• Withdrawn: 25/09/14 since there's no longer any need for a 2nd Text profile.

Issues Addressed

• ISSUE-307 Conformance language and processor profile rather than content profile. - OPEN
• ISSUE-315 Maximum number of regions should be a processor constraint - OPEN
• ISSUE-319 HRM should be a processor compliance test - OPEN

Issues impacted

• ISSUE-332 #cellResolution support - PENDING REVIEW

Summary

IMSC 1 currently defines two profiles, a Text and an Image profile, both of which include the requirement to meet the complexity constraints expressed in the HRM. This proposal is to factor out the complexity constraints from the content profiles, while providing a clearly identifiable profile that continues to match what implementors of, say, CFF-TT, have already built. A third profile not constrained in complexity is proposed.

Rationale

The complexity constraints, when applied to a text profile especially, are not appropriate in all use cases. For example, the key constraint in broadcast environments is often delivery bit rate rather than document complexity. The HRM also makes it difficult to add any new features because doing so may require some modification of the HRM, which is a significant extra burden. In particular, features required to allow IMSC to be inclusive of EBU-TT-D, such as #cellResolution (issue-332), ebutts:linePadding and #overflow for example, would be difficult to introduce. Nevertheless, there is a significant community of implementors who wish to continue with the HRM, and their needs should be taken into account.

Proposal

• Factor out the complexity features from the main Text profile, editorially, i.e. into a separate table.
• In place of the current Text profile create a Complexity Constrained Text Profile and a General Text Profile
• The Complexity Constrained Text Profile is as per the current Text Profile, i.e. the content profile features plus the complexity constrained features
• The General Text Profile includes the content profile features as now as well as any features or extensions needed more generally
• In some cases General Text Profile features will be incompatible with Complexity Constrained Text Profile features, i.e. they may be prohibited in one but permitted in the other.
• Optionally the Image Profile could be expressed in terms of content + complexity features too, but since the image profile defines very precisely what should be displayed this is less beneficial.

Edits to be applied

Sections that need to be changed:

• 4.2 Text Profile
  • Change to include the two proposed profiles
• 5.5 Extensions
  • Add extensions for complexity constraints, i.e. HRM and limited maximum number of regions
• 5.7 Hypothetical Render Model
  • Change to say that for documents meeting either of the two complexity constrained profiles requiring the complexity extensions then any sequence of ISDs shall be reproducible using the HRM.
• 6 Text Profile Constraints
  • Rename to Complexity Constrained Text Profile
  • Add new complexity extensions into 6.3 Features
• [new] Add General Text Profile Constraints section
  • with Profile Designator and Features sub-sections
• 7 Image Profile constraints
  • Add new complexity extensions into 7.4 Features
• 8 Hypothetical Render Model
  • Consider adding more granular extension features, e.g. for specific threshold values
• D Extensions
  • List extension features

Impact

This change has no substantive impact on any existing implementations of IMSC, merely changing the name of the text profile. The change permits IMSC to be more inclusive of other profiles and makes the addition of other profile features more straightforward.