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gstreamer/docs/design/encoding.txt
Edward Hervey d0efe18de5 docs/design: Fixups/additions based on feedback
2009-10-19 18:32:23 +02:00

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Encoding and Muxing
-------------------
Summary
-------
A. Problems
B. Goals
1. EncodeBin
2. Encoding Profile System
3. Helper Library for Profiles
A. 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 accross 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 accross all applications,
differences and inconsistencies that come with that duplication,
and applications hardcoding element names and settings resulting in
poor portability.
B. 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
2. 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.
1. EncodeBin
------------
1.1 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_%d");
gst_pad_link(vsrcpad, vsinkpad);
...
1.2 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.
1.2.1 Incoming streams
The streams fed to EncodeBin can be of various types:
* Video
* Uncompressed (but maybe subsampled)
* Compressed
* Audio
* Uncompressed (audio/x-raw-{int|float})
* Compressed
* Timed text
* Private streams
1.2.2 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
1.2.3 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)
1.2.4 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.
1.2.5 Steps involved for other streams
Other streams will just be forwarded as-is to the muxer, provided the
muxer accepts the stream type.
2. Encoding Profile System
--------------------------
This work is based on:
* The existing GstPreset system for elements [0]
* The gnome-media GConf audio profile system [1]
* The investigation done into device profiles by Arista and
Transmageddon [2 and 3]
2.2 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.
Ex (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.
Ex:
Nokia N900/H264 HQ
Ipod/High Quality
DVD/Pal
Youtube/High Quality
HTML5/Low Bandwith
DNxHD
2.3 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:
<target>/<name>
* 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.
<gst-encoding-target>
<name>Nokia N900</name>
<category>Consumer Device</category>
<profiles>
<profile>Nokia N900/H264 HQ</profile>
<profile>Nokia N900/MP3</profile>
<profile>Nokia N900/AAC</profile>
</profiles>
</gst-encoding-target>
<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-int,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-yuv,width=[16, 800],\
height=[16, 480],framerate=[1/1, 30000/1001]
</restriction>
<presence>1</presence>
</stream-profile>
</streams>
</gst-encoding-profile>
2.5 API
-------
A proposed C API is contained in the gstprofile.h file in this directory.
2.6 Modifications required in the existing GstPreset system
-----------------------------------------------------------
2.6.1. 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
2.6.2 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 [<category>:]<name>)
This would give for example: "Quality:High", "Profile:Baseline"
2. by adding a new _meta key
This would give for example: _meta/category:quality
2.6.3 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");
2.7 Points to be determined
---------------------------
This document hasn't determined yet how to solve the following
problems:
2.7.1 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.
3. 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.
3.1 Getting user-readable names for formats
This is already provided by GstPbUtils.
3.2 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
[ Device/Site/.. ] # optional, depends on the application
[ Profile ]
Convenience methods are offered to easily get lists of categories,
devices, and profiles.
3.3 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.
3.4 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.
* Research links
Some of these are still active documents, some other not
[0] GstPreset API documentation
http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstPreset.html
[1] gnome-media GConf profiles
http://www.gnome.org/~bmsmith/gconf-docs/C/gnome-media.html
[2] Research on a Device Profile API
http://gstreamer.freedesktop.org/wiki/DeviceProfile
[3] Research on defining presets usage
http://gstreamer.freedesktop.org/wiki/PresetDesign
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