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docs: design: add adaptive streaming design docs to sitemap

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.. and some small additions to make it clearer what exist
and what's new.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2222>
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Tim-Philipp Müller 2022-04-18 20:04:26 +01:00 committed by GStreamer Marge Bot
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commit d726a839e4
2 changed files with 37 additions and 20 deletions
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54
subprojects/gst-docs/markdown/additional/design/adaptive-demuxer.md
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@ -1,23 +1,31 @@
# Adaptive Demuxers v2
# Adaptive Demuxers for DASH, HLS and Smooth Streaming
The existing adaptive demuxer support in `gst-plugins-bad` has several pitfalls
that prevents improving it easily. The existing design uses a model where an
adaptive streaming element (`dashdemux`, `hlsdemux`) downloads multiplexed
fragments of media, but then relies on other components in the pipeline to
There are two sets of elements implementing client-side adaptive streaming
(HLS, DASH, Microsoft Smooth Streaming) in GStreamer:
- The old legacy elements `dashdemux`, `hlsdemux`, `mssdemux` in the
gst-plugins-bad module.
- New `dashdemux2`, `hlsdemux2`, `mssdemux2` elements in gst-plugins-good
(added in GStreamer 1.22).
The legacy adaptive streaming support in `gst-plugins-bad` had several pitfalls
that prevented improving it easily. The legacy design used a model where an
adaptive streaming element (`dashdemux`, `hlsdemux`) downloaded multiplexed
fragments of media, but then relied on other components in the pipeline to
provide download buffering, demuxing, elementary stream handling and decoding.
The problems with the old design include:
The problems with the old design included:
1. An assumption that fragment streams (to download) are equal to output
(elementary) streams.
* This made it hard to expose `GstStream` and `GstStreamCollection`
describing the available media streams, and by extension it is difficult to
provide efficient stream selection support
describing the available media streams, and by extension made it
difficult to provide efficient stream selection support
2. By performing download buffering outside the adaptive streaming element,
the download scheduling has no visibility into the presentation timeline.
2. By performing download buffering outside the adaptive streaming elements,
the download scheduling had no visibility into the presentation timeline.
* This made it impossible to handle more efficient variant selection and
download strategy
@ -28,22 +36,30 @@ The problems with the old design include:
* Especially with HLS, which does not provide detailed timing information
about the underlying media streams to the same extent that DASH does.
4. Aging design that grew organically since initial adaptive demuxers and miss
better understanding of how they should work in 2020
4. Aging design that grew organically since the initial adaptive demuxer
implementation with a much more limited feature set, and misses a better
understanding of how a feature-rich implementation should work nowadays.
* The code is complicated and interwoven in ways that are hard to follow
* The code was complicated and interwoven in ways that were hard to follow
and reason about.
5. Use of GStreamer pipeline sources for downloading.
* An internal download pipeline that contains a `httpsrc -> queue2 -> src`
chain makes download management, bandwidth estimation and stream parsing
more difficult, and uses a new thread for each download.
* An internal download pipeline that contained a `httpsrc -> queue2 -> src`
chain made download management, bandwidth estimation and stream parsing
more difficult, and used a new thread for each download.
# New design
## High-level overview of the new AdaptiveDemux base class:
The rest of this document describes the new adaptive streaming client
implementation that landed in gst-plugins-good in GStreamer 1.22.
The new elements only work in combination with the "streams-aware"
`playbin3` and `uridecodebin3` elements that support advanced stream
selection functionality, they won't work with the legacy `playbin`
element.
## High-level overview of the new internal AdaptiveDemux2 base class:
* Buffering is handled inside the adaptive streaming element, based on
elementary streams (i.e. de-multiplexed from the downloaded fragments) and

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subprojects/gst-docs/sitemap.txt
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@ -135,7 +135,7 @@
additional/splitup.md
additional/design/index.md
additional/design/MT-refcounting.md
additional/design/TODO.md
additional/design/adaptive-demuxer.md
additional/design/activation.md
additional/design/audiosinks.md
additional/design/buffer.md
@ -204,6 +204,7 @@
additional/design/toc.md
additional/design/tracing.md
additional/design/trickmodes.md
additional/design/TODO.md
additional/rtp.md
additional/videos.md
tools/index.md