feat: support l3d-dash (SSR)

[sebastianpiq]

L3D-DASH Support

This pull request introduces the support for Low-Latency & Low-Delay DASH (L3D)

Functionality Overview

The L3D implementation generates individual chunks from full segments, enabling progressive media delivery. The system uses writeSubSegment to handle chunk requests and prepareChunks to divide segments into smaller temporal units. Chunk duration can be configured explicitly via URL parameters.

Main Changes

1. MPD Generation Enhancements (when Low Delay/SSR is enabled)
   Added DASHProfileLinear profile for SSR streaming compliance
   Implemented EssentialProperty with urn:mpeg:dash:ssr:2023 schema reference
   Implemented SupplementalProperty with urn:mpeg:dash:adaptation-set-switching:2016
   Set startWithSAP=1 for proper stream initialization
   Added SegmentSequenceProperties with sapType=1 and cadence=1 to enable playback initialization on any chunk
   Implemented SubNumber on media Segment Template
2. Segment Timeline Improvements
   Introduced calculateK function to compute S@k value for proper segment indexing
   Enhanced sub-segment chunking support for low-latency & low-delay delivery
3. URL Configuration System
   Created SSR UI section in the URL generator for easy configuration
   Added ssrAS and chunkDurSSR URL parameter for enabling Low Delay mode for both video and audio adaptation sets.
   Updated URL generation logic to properly handle low-delay streaming parameters
4. MPD Generation Enhancements (when Low Delay and Low Latency is enabled)
   Implemented EssentialProperty with urn:mpeg:dash:ssr:2023 schema reference
   Implemented SupplementalProperty with urn:mpeg:dash:adaptation-set-switching:2016
   Set startWithSAP=1 for proper stream initialization
   Implemented SubNumber on media Segment Template
   Compute S@k
5. Library and Dependency Updates
   Updated dash-mpd library to v0.13.0 for improved MPD parsing and generation capabilities
   Adapted to new S type struct from the updated dash-mpd library

How to generate a Low Delay URL

1. Select the testpic_2s_low_delay asset
2. Navigate to the SSR section
3. Set the SSR configuration using the following format: adaptationSetId,ssrValue;adaptationSetId,ssrValue;... where ssrValue is the value of ssr EssentialProperty (supports both video and audio adaptation set)
4. Set the chunk duration for the low delay using the following format: adaptationSetId,chunkDuration;adaptationSetId,chunkDuration;... (supports both video and audio adaptation set)

How to generate a L3D (Low Delay & Low Latency) URL

1. Select the testpic_2s_low_delay asset
2. Navigate to the Low latency section
3. Set chunk duration for low-latency
4. Navigate to the SSR section
5. Set the SSR configuration using the following format: adaptationSetId,ssrValue;adaptationSetId,ssrValue;... where ssrValue is the value of ssr EssentialProperty (supports both video and audio adaptation set)
6. Set the chunk duration for low delay using the following format: adaptationSetId,chunkDuration;adaptationSetId,chunkDuration;... (supports both video and audio adaptation set)

Supported Use Cases

• Basic Low Delay to Low Latency Video Content Switching: Start with low delay content and fall back to low-latency content.

Configuration example:
SSR Configuration: 1,0
Switching Logic:
AS1: SSR value="0" → Next reproduction is AS0
AS0: No SSR → Final low-latency regular content

• Progressive Low Delay Video Degradation Chain: Start with the most aggressive low delay setting, progressing through intermediate low-delay configuration, and finally falling back to low-latency content.

Configuration example:
SSR Configuration: 2,1;1,0
chunk duration for low delay: 2,0.2;1,0.5
Switching Logic:
AS2: SSR value="1" → Next reproduction is AS1
AS1: SSR value="0" → Next reproduction is AS0
AS0: No SSR → Final low-latency regular content

• Basic Low Delay to Low Latency Audio Content Switching: Start with low delay video and audio content and fall back to low-latency video and audio content.

Configuration example:
SSR Configuration: `3,2;1,0
Switching Logic:
**AS1 (video) **: SSR value="0" → Next reproduction is AS0 (video)
AS0 (video): No SSR → Final low-latency regular content
**AS3 (audio) **: SSR value="2" → Next reproduction is AS2 (audio)
AS2 (audio): No SSR → Final low-latency regular content

• Switching from low delay to regular renditions (non low latency) should work with the current logic, but haven't been tested since it is out of this PR scope

Wiki Update Suggestion

1. Preparing Content for L3D Assets

Target Wiki Page: Preparing Content for livesim2

Suggested Addition: L3D Content Requirements

Add a new section covering content preparation for L3D streaming:

L3D Content Preparation

For L3D to work, content should be encoded with the following considerations:

Video Encoding Parameters: GOP size must be aligned with the chunk duration in frames (e.g. if your video has a GOP size of 6 frames and a frame rate of 30fps, the GOP duration is 6 / 30 = 0.2 seconds. The chunk duration should be a multiple of this value (e.g., 0.2, 0.4))

FFmpeg sample

ffmpeg
-f lavfi -i smptebars=size=1920x1080:rate=30:duration=8
-vf "drawtext=text='1920x1080@30fps':x=(w-text_w)/2:y=80*(h/1080):fontsize=72*(h/1080):fontcolor=white:box=1:[email protected]:boxborderw=18,
drawtext=timecode='00:00:00:00':rate=30:x=(w-tw)/2:y=280*(h/1080):fontsize=144*(h/1080):fontcolor=white:box=1:[email protected]:boxborderw=30,
drawtext=text='2000 kbit/s - Low Delay':x=(w-text_w)/2:y=480*(h/1080):fontsize=72*(h/1080):fontcolor=white:box=1:[email protected]:boxborderw=18"
-c:v libx264 -g 6 -r 30 -keyint_min 6 -sc_threshold 0 -force_key_frames "expr:gte(t,n_forced*0.2)" -b:v 2M
-y low_delay_1080p.mp4

gpac example

gpac -r
-i sources/low_delay_1080p.mp4:#ASID=1
-i sources/audio.mp4:#ASID=2
-o out/Manifest.mpd:template='r$RepresentationID$_seg$Number$':timescale=1000:segdur=2:dmode=static:profile=live

2. New URL Parameters

Target Wiki Page: URL Parameters

Suggested Addition: Low Delay Parameters

Add documentation for the new Low Delay URL parameter:

Low Delay Parameters

Parameter	Description	Example
ssrAS	Enables Low Delay (SSR) mode on the specified adaptation sets. The following format is required: adaptationSetId,ssrValue;adaptationSetId,ssrValue;... where ssrValue is the next adaptation set id	/livesim2/ssras_{adaptationSetId, ssrValue}/testpic_2s_low_delay/Manifest.mpd

Low Delay Mode Effects: When ssrAS is specified in the URL path, the following modifications are applied to the generated MPD:

• Profile: Set to DASHProfileLinear for L3D compliance
• Essential Property: Includes urn:mpeg:dash:ssr:2023 specification
• SupplementalProperty: Includes urn:mpeg:dash:adaptation-set-switching:2016 specification
• SAP Configuration: AdaptationSet@startWithSAP=1 for proper stream access points
• SegmentSequenceProperties element is included in the low-delay AdaptationSet with the sapType=1 and cadence=1 attributes

Usage Pattern:
/livesim2/[other-params]/ssras_{adaptationSetId, ssrValue}/[asset-name]/Manifest.mpd

Compatibility Notes:

• Low Delay requires Low Delay Chunk Duration URL Parameter
• Low Delay mode works in combination with Low Latency URL parameters
• Low Delay mode works in combination with other URL parameters
• Requires properly encoded content with an appropriate GOP structure

Suggested Addition: Low Delay Chunk Duration Parameters

Add documentation for the new Low Delay Chunk Duration URL parameter:

Low Delay Chunk Duration Parameters

Parameter	Description	Example
chunkDurSSR	Enables sub segment generation on the specified adaptation sets using the specified chunk duration. The following format is required: adaptationSetId,chunkDur;adaptationSetId,chunkDur;... where chunkDur	/livesim2/chunkdurssr_{adaptationSetId, chunkDur}/testpic_2s_low_delay/Manifest.mpd

Low Delay Chunk Duration Mode Effects: When chunkDurSSR is specified in the URL path, the following modifications are applied to the generated MPD:

• Sub-segment Generation: Enhanced timeline generation with chunk duration support

Usage Pattern:
/livesim2/[other-params]/chunkdurssr_{adaptationSetId, chunkDur}/[asset-name]/Manifest.mpd

Compatibility Notes:

• Low Delay Chunk Duration requires Low Delay URL Parameter ssrAS
• Low Delay mode works in combination with other URL parameters
• Requires properly encoded content with an appropriate GOP structure

[tobbee]

Sorry for the long time to respond. Thanks for the PR.
Have you checked this towards any player like dash.js, which is developing this feature as well.

I haven't looked at the code yet, but regarding the documentation, I think it would benefit by a description that L3D is a solution using multiple tools and explain what these tools are. For example, SSR is used in many places, but the SSR abbreviation is not explained.

I plan to look more into this when I'm back from Demuxed.

[tobbee]

There is now a dash.js Release candidate: https://github.com/Dash-Industry-Forum/dash.js/tree/RC_v5.1.0
that includes l3d support. Have you tested against it?

[tobbee]

There was some merge in this PR yesterday, so the git history looks quite strange, see this screenshot from SourceTree

Please fix and force-push a straight line of commits based on "main", and without any loops/merges.

[tobbee]

@cotid-qualabs @sebastianpiq Any update on this?

Would be great if you can update with the playback status with dash.js and other players.

A rebase om the main branch with a linear git commit history for this PR would also be nice before approving and merging this PR.

I think there may be some collisions with PR #272 on Pattern, but I can wait a bit more with that one.