gstreamer/markdown/design/encoding.md

Encoding and Muxing

Problems this proposal attempts to solve

• Duplication of pipeline code for gstreamer-based applications wishing to encode and or mux streams, leading to subtle differences and inconsistencies across those applications.

• No unified system for describing encoding targets for applications in a user-friendly way.

• No unified system for creating encoding targets for applications, resulting in duplication of code across all applications, differences and inconsistencies that come with that duplication, and applications hardcoding element names and settings resulting in poor portability.

Goals

1. Convenience encoding element

   Create a convenience GstBin for encoding and muxing several streams, hereafter called 'EncodeBin'.

   This element will only contain one single property, which is a profile.

2. Define a encoding profile system

3. Encoding profile helper library

Create a helper library to:

• create EncodeBin instances based on profiles, and

• help applications to create/load/save/browse those profiles.

EncodeBin

Proposed API

EncodeBin is a GstBin subclass.

It implements the GstTagSetter interface, by which it will proxy the calls to the muxer.

Only two introspectable property (i.e. usable without extra API):

• A GstEncodingProfile
• The name of the profile to use

When a profile is selected, encodebin will:

• Add REQUEST sinkpads for all the GstStreamProfile
• Create the muxer and expose the source pad

Whenever a request pad is created, encodebin will:

• Create the chain of elements for that pad
• Ghost the sink pad
• Return that ghost pad

This allows reducing the code to the minimum for applications wishing to encode a source for a given profile:

encbin = gst_element_factory_make ("encodebin, NULL);
g_object_set (encbin, "profile", "N900/H264 HQ", NULL);
gst_element_link (encbin, filesink);

vsrcpad = gst_element_get_src_pad (source, "src1");
vsinkpad = gst_element_get_request\_pad (encbin, "video\_%u");
gst_pad_link (vsrcpad, vsinkpad);

Explanation of the Various stages in EncodeBin

This describes the various stages which can happen in order to end up with a multiplexed stream that can then be stored or streamed.

Incoming streams

The streams fed to EncodeBin can be of various types:

• Video
• Uncompressed (but maybe subsampled)
• Compressed
• Audio
• Uncompressed (audio/x-raw)
• Compressed
• Timed text
• Private streams

Steps involved for raw video encoding

0. Incoming Stream

1. Transform raw video feed (optional)

Here we modify the various fundamental properties of a raw video stream to be compatible with the intersection of: * The encoder GstCaps and * The specified "Stream Restriction" of the profile/target

The fundamental properties that can be modified are: * width/height This is done with a video scaler. The DAR (Display Aspect Ratio) MUST be respected. If needed, black borders can be added to comply with the target DAR. * framerate * format/colorspace/depth All of this is done with a colorspace converter

2. Actual encoding (optional for raw streams)

An encoder (with some optional settings) is used.

3. Muxing

A muxer (with some optional settings) is used.

4. Outgoing encoded and muxed stream

Steps involved for raw audio encoding

This is roughly the same as for raw video, expect for (1)

1. Transform raw audo feed (optional)

We modify the various fundamental properties of a raw audio stream to be compatible with the intersection of: * The encoder GstCaps and * The specified "Stream Restriction" of the profile/target

The fundamental properties that can be modifier are: * Number of channels * Type of raw audio (integer or floating point) * Depth (number of bits required to encode one sample)

Steps involved for encoded audio/video streams

Steps (1) and (2) are replaced by a parser if a parser is available for the given format.

Steps involved for other streams

Other streams will just be forwarded as-is to the muxer, provided the muxer accepts the stream type.

Encoding Profile System

This work is based on:

• The existing GstPreset API documentation system for elements

• The gnome-media GConf audio profile system

• The investigation done into device profiles by Arista and Transmageddon: Research on a Device Profile API, and Research on defining presets usage.

Terminology

• Encoding Target Category A Target Category is a classification of devices/systems/use-cases for encoding.

Such a classification is required in order for: * Applications with a very-specific use-case to limit the number of profiles they can offer the user. A screencasting application has no use with the online services targets for example. * Offering the user some initial classification in the case of a more generic encoding application (like a video editor or a transcoder).

Ex: Consumer devices Online service Intermediate Editing Format Screencast Capture Computer

• Encoding Profile Target A Profile Target describes a specific entity for which we wish to encode. A Profile Target must belong to at least one Target Category. It will define at least one Encoding Profile.

  Examples (with category): Nokia N900 (Consumer device) Sony PlayStation 3 (Consumer device) Youtube (Online service) DNxHD (Intermediate editing format) HuffYUV (Screencast) Theora (Computer)

• Encoding Profile A specific combination of muxer, encoders, presets and limitations.

  Examples: Nokia N900/H264 HQ, Ipod/High Quality, DVD/Pal, Youtube/High Quality HTML5/Low Bandwith, DNxHD

Encoding Profile

An encoding profile requires the following information:

• Name This string is not translatable and must be unique. A recommendation to guarantee uniqueness of the naming could be: /
• Description This is a translatable string describing the profile
• Muxing format This is a string containing the GStreamer media-type of the container format.
• Muxing preset This is an optional string describing the preset(s) to use on the muxer.
• Multipass setting This is a boolean describing whether the profile requires several passes.
• List of Stream Profile

2.3.1 Stream Profiles

A Stream Profile consists of:

• Type The type of stream profile (audio, video, text, private-data)
• Encoding Format This is a string containing the GStreamer media-type of the encoding format to be used. If encoding is not to be applied, the raw audio media type will be used.
• Encoding preset This is an optional string describing the preset(s) to use on the encoder.
• Restriction This is an optional GstCaps containing the restriction of the stream that can be fed to the encoder. This will generally containing restrictions in video width/heigh/framerate or audio depth.
• presence This is an integer specifying how many streams can be used in the containing profile. 0 means that any number of streams can be used.
• pass This is an integer which is only meaningful if the multipass flag has been set in the profile. If it has been set it indicates which pass this Stream Profile corresponds to.

2.4 Example profile

The representation used here is XML only as an example. No decision is made as to which formatting to use for storing targets and profiles.

Nokia N900 Consumer Device Nokia N900/H264 HQ Nokia N900/MP3 Nokia N900/AAC

<gst-encoding-profile>
  <name>Nokia N900/H264 HQ</name>
  <description>
    High Quality H264/AAC for the Nokia N900
  </description>
  <format>video/quicktime,variant=iso</format>
  <streams>
    <stream-profile>
      <type>audio</type>
      <format>audio/mpeg,mpegversion=4</format>
      <preset>Quality High/Main</preset>
      <restriction>audio/x-raw,channels=[1,2]</restriction>
      <presence>1</presence>
    </stream-profile>
    <stream-profile>
      <type>video</type>
      <format>video/x-h264</format>
      <preset>Profile Baseline/Quality High</preset>
      <restriction>
        video/x-raw,width=[16, 800],\
    height=[16, 480],framerate=[1/1, 30000/1001]
      </restriction>
      <presence>1</presence>
    </stream-profile>
  </streams>  
</gst-encoding-profile>

API

A proposed C API is contained in the gstprofile.h file in this directory.

Modifications required in the existing GstPreset system

Temporary preset.

Currently a preset needs to be saved on disk in order to be used.

This makes it impossible to have temporary presets (that exist only during the lifetime of a process), which might be required in the new proposed profile system

Categorisation of presets.

Currently presets are just aliases of a group of property/value without any meanings or explanation as to how they exclude each other.

Take for example the H264 encoder. It can have presets for: * passes (1,2 or 3 passes) * profiles (Baseline, Main, ...) * quality (Low, medium, High)

In order to programmatically know which presets exclude each other, we here propose the categorisation of these presets.

This can be done in one of two ways 1. in the name (by making the name be [:]) This would give for example: "Quality:High", "Profile:Baseline" 2. by adding a new _meta key This would give for example: _meta/category:quality

Aggregation of presets.

There can be more than one choice of presets to be done for an element (quality, profile, pass).

This means that one can not currently describe the full configuration of an element with a single string but with many.

The proposal here is to extend the GstPreset API to be able to set all presets using one string and a well-known separator ('/').

This change only requires changes in the core preset handling code.

This would allow doing the following: gst_preset_load_preset (h264enc, "pass:1/profile:baseline/quality:high");

Points to be determined

This document hasn't determined yet how to solve the following problems:

Storage of profiles

One proposal for storage would be to use a system wide directory (like $prefix/share/gstreamer-0.10/profiles) and store XML files for every individual profiles.

Users could then add their own profiles in ~/.gstreamer-0.10/profiles

This poses some limitations as to what to do if some applications want to have some profiles limited to their own usage.

Helper library for profiles

These helper methods could also be added to existing libraries (like GstPreset, GstPbUtils, ..).

The various API proposed are in the accompanying gstprofile.h file.

Getting user-readable names for formats

This is already provided by GstPbUtils.

Hierarchy of profiles

The goal is for applications to be able to present to the user a list of combo-boxes for choosing their output profile:

 Category 

] # optional, depends on the application [ Profile ]

Convenience methods are offered to easily get lists of categories, devices, and profiles.

Creating Profiles

The goal is for applications to be able to easily create profiles.

The applications needs to be able to have a fast/efficient way to: * select a container format and see all compatible streams he can use with it. * select a codec format and see which container formats he can use with it.

The remaining parts concern the restrictions to encoder input.

Ensuring availability of plugins for Profiles

When an application wishes to use a Profile, it should be able to query whether it has all the needed plugins to use it.

This part will use GstPbUtils to query, and if needed install the missing plugins through the installed distribution plugin installer.

Use-cases researched

This is a list of various use-cases where encoding/muxing is being used.

Transcoding

The goal is to convert with as minimal loss of quality any input file for a target use. A specific variant of this is transmuxing (see below).

Example applications: Arista, Transmageddon

Rendering timelines

The incoming streams are a collection of various segments that need to be rendered. Those segments can vary in nature (i.e. the video width/height can change). This requires the use of identiy with the single-segment property activated to transform the incoming collection of segments to a single continuous segment.

Example applications: PiTiVi, Jokosher

Encoding of live sources

The major risk to take into account is the encoder not encoding the incoming stream fast enough. This is outside of the scope of encodebin, and should be solved by using queues between the sources and encodebin, as well as implementing QoS in encoders and sources (the encoders emitting QoS events, and the upstream elements adapting themselves accordingly).

Example applications: camerabin, cheese

Screencasting applications

This is similar to encoding of live sources. The difference being that due to the nature of the source (size and amount/frequency of updates) one might want to do the encoding in two parts: * The actual live capture is encoded with a 'almost-lossless' codec (such as huffyuv) * Once the capture is done, the file created in the first step is then rendered to the desired target format.

Fixing sources to only emit region-updates and having encoders capable of encoding those streams would fix the need for the first step but is outside of the scope of encodebin.

Example applications: Istanbul, gnome-shell, recordmydesktop

Live transcoding

This is the case of an incoming live stream which will be broadcasted/transmitted live. One issue to take into account is to reduce the encoding latency to a minimum. This should mostly be done by picking low-latency encoders.

Example applications: Rygel, Coherence

Transmuxing

Given a certain file, the aim is to remux the contents WITHOUT decoding into either a different container format or the same container format. Remuxing into the same container format is useful when the file was not created properly (for example, the index is missing). Whenever available, parsers should be applied on the encoded streams to validate and/or fix the streams before muxing them.

Metadata from the original file must be kept in the newly created file.

Example applications: Arista, Transmaggedon

Loss-less cutting

Given a certain file, the aim is to extract a certain part of the file without going through the process of decoding and re-encoding that file. This is similar to the transmuxing use-case.

Example applications: PiTiVi, Transmageddon, Arista, ...

Multi-pass encoding

Some encoders allow doing a multi-pass encoding. The initial pass(es) are only used to collect encoding estimates and are not actually muxed and outputted. The final pass uses previously collected information, and the output is then muxed and outputted.

Archiving and intermediary format

The requirement is to have lossless

CD ripping

Example applications: Sound-juicer

DVD ripping

Example application: Thoggen

Research links

Some of these are still active documents, some other not