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463 lines
13 KiB
Text
463 lines
13 KiB
Text
Encoding and Muxing
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-------------------
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Summary
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-------
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A. Problems
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B. Goals
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1. EncodeBin
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2. Encoding Profile System
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3. Helper Library for Profiles
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A. Problems this proposal attempts to solve
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-------------------------------------------
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* Duplication of pipeline code for gstreamer-based applications
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wishing to encode and or mux streams, leading to subtle differences
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and inconsistencies accross those applications.
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* No unified system for describing encoding targets for applications
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in a user-friendly way.
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* No unified system for creating encoding targets for applications,
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resulting in duplication of code accross all applications,
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differences and inconsistencies that come with that duplication,
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and applications hardcoding element names and settings resulting in
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poor portability.
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B. Goals
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--------
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1. Convenience encoding element
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Create a convenience GstBin for encoding and muxing several streams,
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hereafter called 'EncodeBin'.
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This element will only contain one single property, which is a
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profile.
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2. Define a encoding profile system
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2. Encoding profile helper library
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Create a helper library to:
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* create EncodeBin instances based on profiles, and
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* help applications to create/load/save/browse those profiles.
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1. EncodeBin
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------------
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1.1 Proposed API
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----------------
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EncodeBin is a GstBin subclass.
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It implements the GstTagSetter interface, by which it will proxy the
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calls to the muxer.
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Only two introspectable property (i.e. usable without extra API):
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* A GstEncodingProfile*
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* The name of the profile to use
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When a profile is selected, encodebin will:
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* Add REQUEST sinkpads for all the GstStreamProfile
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* Create the muxer and expose the source pad
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Whenever a request pad is created, encodebin will:
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* Create the chain of elements for that pad
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* Ghost the sink pad
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* Return that ghost pad
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This allows reducing the code to the minimum for applications
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wishing to encode a source for a given profile:
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...
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encbin = gst_element_factory_make("encodebin, NULL);
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g_object_set (encbin, "profile", "N900/H264 HQ", NULL);
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gst_element_link (encbin, filesink);
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...
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vsrcpad = gst_element_get_src_pad(source, "src1");
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vsinkpad = gst_element_request_pad_simple (encbin, "video_%d");
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gst_pad_link(vsrcpad, vsinkpad);
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...
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1.2 Explanation of the Various stages in EncodeBin
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--------------------------------------------------
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This describes the various stages which can happen in order to end
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up with a multiplexed stream that can then be stored or streamed.
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1.2.1 Incoming streams
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The streams fed to EncodeBin can be of various types:
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* Video
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* Uncompressed (but maybe subsampled)
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* Compressed
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* Audio
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* Uncompressed (audio/x-raw-{int|float})
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* Compressed
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* Timed text
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* Private streams
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1.2.2 Steps involved for raw video encoding
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(0) Incoming Stream
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(1) Transform raw video feed (optional)
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Here we modify the various fundamental properties of a raw video
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stream to be compatible with the intersection of:
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* The encoder GstCaps and
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* The specified "Stream Restriction" of the profile/target
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The fundamental properties that can be modified are:
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* width/height
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This is done with a video scaler.
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The DAR (Display Aspect Ratio) MUST be respected.
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If needed, black borders can be added to comply with the target DAR.
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* framerate
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* format/colorspace/depth
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All of this is done with a colorspace converter
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(2) Actual encoding (optional for raw streams)
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An encoder (with some optional settings) is used.
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(3) Muxing
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A muxer (with some optional settings) is used.
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(4) Outgoing encoded and muxed stream
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1.2.3 Steps involved for raw audio encoding
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This is roughly the same as for raw video, expect for (1)
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(1) Transform raw audo feed (optional)
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We modify the various fundamental properties of a raw audio stream to
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be compatible with the intersection of:
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* The encoder GstCaps and
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* The specified "Stream Restriction" of the profile/target
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The fundamental properties that can be modifier are:
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* Number of channels
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* Type of raw audio (integer or floating point)
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* Depth (number of bits required to encode one sample)
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1.2.4 Steps involved for encoded audio/video streams
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Steps (1) and (2) are replaced by a parser if a parser is available
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for the given format.
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1.2.5 Steps involved for other streams
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Other streams will just be forwarded as-is to the muxer, provided the
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muxer accepts the stream type.
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2. Encoding Profile System
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--------------------------
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This work is based on:
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* The existing GstPreset system for elements [0]
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* The gnome-media GConf audio profile system [1]
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* The investigation done into device profiles by Arista and
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Transmageddon [2 and 3]
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2.2 Terminology
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---------------
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* Encoding Target Category
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A Target Category is a classification of devices/systems/use-cases
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for encoding.
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Such a classification is required in order for:
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* Applications with a very-specific use-case to limit the number of
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profiles they can offer the user. A screencasting application has
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no use with the online services targets for example.
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* Offering the user some initial classification in the case of a
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more generic encoding application (like a video editor or a
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transcoder).
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Ex:
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Consumer devices
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Online service
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Intermediate Editing Format
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Screencast
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Capture
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Computer
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* Encoding Profile Target
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A Profile Target describes a specific entity for which we wish to
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encode.
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A Profile Target must belong to at least one Target Category.
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It will define at least one Encoding Profile.
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Ex (with category):
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Nokia N900 (Consumer device)
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Sony PlayStation 3 (Consumer device)
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Youtube (Online service)
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DNxHD (Intermediate editing format)
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HuffYUV (Screencast)
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Theora (Computer)
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* Encoding Profile
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A specific combination of muxer, encoders, presets and limitations.
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Ex:
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Nokia N900/H264 HQ
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Ipod/High Quality
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DVD/Pal
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Youtube/High Quality
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HTML5/Low Bandwith
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DNxHD
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2.3 Encoding Profile
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--------------------
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An encoding profile requires the following information:
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* Name
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This string is not translatable and must be unique.
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A recommendation to guarantee uniqueness of the naming could be:
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<target>/<name>
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* Description
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This is a translatable string describing the profile
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* Muxing format
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This is a string containing the GStreamer media-type of the
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container format.
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* Muxing preset
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This is an optional string describing the preset(s) to use on the
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muxer.
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* Multipass setting
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This is a boolean describing whether the profile requires several
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passes.
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* List of Stream Profile
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2.3.1 Stream Profiles
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A Stream Profile consists of:
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* Type
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The type of stream profile (audio, video, text, private-data)
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* Encoding Format
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This is a string containing the GStreamer media-type of the encoding
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format to be used. If encoding is not to be applied, the raw audio
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media type will be used.
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* Encoding preset
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This is an optional string describing the preset(s) to use on the
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encoder.
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* Restriction
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This is an optional GstCaps containing the restriction of the
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stream that can be fed to the encoder.
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This will generally containing restrictions in video
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width/heigh/framerate or audio depth.
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* presence
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This is an integer specifying how many streams can be used in the
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containing profile. 0 means that any number of streams can be
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used.
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* pass
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This is an integer which is only meaningful if the multipass flag
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has been set in the profile. If it has been set it indicates which
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pass this Stream Profile corresponds to.
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2.4 Example profile
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-------------------
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The representation used here is XML only as an example. No decision is
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made as to which formatting to use for storing targets and profiles.
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<gst-encoding-target>
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<name>Nokia N900</name>
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<category>Consumer Device</category>
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<profiles>
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<profile>Nokia N900/H264 HQ</profile>
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<profile>Nokia N900/MP3</profile>
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<profile>Nokia N900/AAC</profile>
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</profiles>
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</gst-encoding-target>
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<gst-encoding-profile>
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<name>Nokia N900/H264 HQ</name>
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<description>
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High Quality H264/AAC for the Nokia N900
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</description>
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<format>video/quicktime,variant=iso</format>
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<streams>
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<stream-profile>
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<type>audio</type>
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<format>audio/mpeg,mpegversion=4</format>
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<preset>Quality High/Main</preset>
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<restriction>audio/x-raw-int,channels=[1,2]</restriction>
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<presence>1</presence>
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</stream-profile>
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<stream-profile>
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<type>video</type>
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<format>video/x-h264</format>
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<preset>Profile Baseline/Quality High</preset>
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<restriction>
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video/x-raw-yuv,width=[16, 800],\
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height=[16, 480],framerate=[1/1, 30000/1001]
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</restriction>
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<presence>1</presence>
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</stream-profile>
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</streams>
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</gst-encoding-profile>
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2.5 API
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-------
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A proposed C API is contained in the gstprofile.h file in this directory.
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2.6 Modifications required in the existing GstPreset system
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-----------------------------------------------------------
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2.6.1. Temporary preset.
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Currently a preset needs to be saved on disk in order to be
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used.
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This makes it impossible to have temporary presets (that exist only
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during the lifetime of a process), which might be required in the
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new proposed profile system
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2.6.2 Categorisation of presets.
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Currently presets are just aliases of a group of property/value
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without any meanings or explanation as to how they exclude each
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other.
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Take for example the H264 encoder. It can have presets for:
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* passes (1,2 or 3 passes)
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* profiles (Baseline, Main, ...)
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* quality (Low, medium, High)
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In order to programmatically know which presets exclude each other,
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we here propose the categorisation of these presets.
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This can be done in one of two ways
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1. in the name (by making the name be [<category>:]<name>)
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This would give for example: "Quality:High", "Profile:Baseline"
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2. by adding a new _meta key
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This would give for example: _meta/category:quality
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2.6.3 Aggregation of presets.
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There can be more than one choice of presets to be done for an
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element (quality, profile, pass).
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This means that one can not currently describe the full
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configuration of an element with a single string but with many.
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The proposal here is to extend the GstPreset API to be able to set
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all presets using one string and a well-known separator ('/').
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This change only requires changes in the core preset handling code.
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This would allow doing the following:
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gst_preset_load_preset (h264enc,
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"pass:1/profile:baseline/quality:high");
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2.7 Points to be determined
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---------------------------
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This document hasn't determined yet how to solve the following
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problems:
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2.7.1 Storage of profiles
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One proposal for storage would be to use a system wide directory
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(like $prefix/share/gstreamer-0.10/profiles) and store XML files for
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every individual profiles.
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Users could then add their own profiles in ~/.gstreamer-0.10/profiles
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This poses some limitations as to what to do if some applications
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want to have some profiles limited to their own usage.
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3. Helper library for profiles
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------------------------------
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These helper methods could also be added to existing libraries (like
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GstPreset, GstPbUtils, ..).
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The various API proposed are in the accompanying gstprofile.h file.
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3.1 Getting user-readable names for formats
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This is already provided by GstPbUtils.
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3.2 Hierarchy of profiles
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The goal is for applications to be able to present to the user a list
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of combo-boxes for choosing their output profile:
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[ Category ] # optional, depends on the application
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[ Device/Site/.. ] # optional, depends on the application
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[ Profile ]
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Convenience methods are offered to easily get lists of categories,
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devices, and profiles.
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3.3 Creating Profiles
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The goal is for applications to be able to easily create profiles.
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The applications needs to be able to have a fast/efficient way to:
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* select a container format and see all compatible streams he can use
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with it.
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* select a codec format and see which container formats he can use
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with it.
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The remaining parts concern the restrictions to encoder
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input.
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3.4 Ensuring availability of plugins for Profiles
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When an application wishes to use a Profile, it should be able to
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query whether it has all the needed plugins to use it.
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This part will use GstPbUtils to query, and if needed install the
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missing plugins through the installed distribution plugin installer.
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* Research links
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Some of these are still active documents, some other not
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[0] GstPreset API documentation
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http://gstreamer.freedesktop.org/data/doc/gstreamer/head/gstreamer/html/GstPreset.html
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[1] gnome-media GConf profiles
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http://www.gnome.org/~bmsmith/gconf-docs/C/gnome-media.html
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[2] Research on a Device Profile API
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http://gstreamer.freedesktop.org/wiki/DeviceProfile
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[3] Research on defining presets usage
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http://gstreamer.freedesktop.org/wiki/PresetDesign
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