195 lines
11 KiB
ReStructuredText
195 lines
11 KiB
ReStructuredText
OBS Studio Backend Design
|
|
=========================
|
|
The OBS Studio backend is powered by the library libobs. Libobs
|
|
provides the main pipeline, the video/audio subsystems, and the general
|
|
framework for all plugins.
|
|
|
|
|
|
Libobs Plugin Objects
|
|
---------------------
|
|
Libobs is designed to be modular, where adding modules will add custom
|
|
functionality. There are four libobs objects that you can make plugins
|
|
for:
|
|
|
|
- :ref:`plugins_sources` -- Sources are used to render video and/or
|
|
audio on stream. Things such as capturing displays/games/audio,
|
|
playing a video, showing an image, or playing audio. Sources can also
|
|
be used to implement audio and video filters.
|
|
|
|
- :ref:`plugins_outputs` -- Outputs allow the ability to output the
|
|
currently rendering audio/video. Streaming and recording are two
|
|
common examples of outputs, but not the only types of outputs.
|
|
Outputs can receive the raw data or receive encoded data.
|
|
|
|
- :ref:`plugins_encoders` -- Encoders are OBS-specific implementations
|
|
of video/audio encoders, which are used with outputs that use
|
|
encoders. x264, NVENC, Quicksync are examples of encoder
|
|
implementations.
|
|
|
|
- :ref:`plugins_services` -- Services are custom implementations of
|
|
streaming services, which are used with outputs that stream. For
|
|
example, you could have a custom implementation for streaming to
|
|
Twitch, and another for YouTube to allow the ability to log in and use
|
|
their APIs to do things such as get the RTMP servers or control the
|
|
channel.
|
|
|
|
*(Author's note: the service API is incomplete as of this writing)*
|
|
|
|
|
|
Libobs Threads
|
|
--------------
|
|
There are three primary threads spawned by libobs on initialization:
|
|
|
|
- The obs_graphics_thread_ function used exclusively for rendering in
|
|
`libobs/obs-video.c`_
|
|
|
|
- The video_thread_ function used exclusively for video encoding/output
|
|
in `libobs/media-io/video-io.c`_
|
|
|
|
- The audio_thread_ function used for all audio
|
|
processing/encoding/output in `libobs/media-io/audio-io.c`_
|
|
|
|
*(Author's note: obs_graphics_thread was originally named
|
|
obs_video_thread; it was renamed as of this writing to prevent confusion
|
|
with video_thread)*
|
|
|
|
|
|
.. _output_channels:
|
|
|
|
Output Channels
|
|
---------------
|
|
Rendering video or audio starts from output channels. You assign a
|
|
source to an output channel via the :c:func:`obs_set_output_source()`
|
|
function. The *channel* parameter can be any number from
|
|
0..(MAX_CHANNELS_-1). You may initially think that this is how you
|
|
display multiple sources at once; however, sources are hierarchical.
|
|
Sources such as scenes or transitions can have multiple sub-sources, and
|
|
those sub-sources in turn can have sub-sources and so on (see
|
|
:ref:`displaying_sources` for more information). Typically, you would
|
|
use scenes to draw multiple sources as a group with specific transforms
|
|
for each source, as a scene is just another type of source. The
|
|
"channel" design allows for highly complex video presentation setups.
|
|
The OBS Studio front-end has yet to even fully utilize this back-end
|
|
design for its rendering, and currently only uses one output channel to
|
|
render one scene at a time. It does however utilize additional channels
|
|
for things such as global audio sources which are set in audio settings.
|
|
|
|
*(Author's note: "Output channels" are not to be confused with output
|
|
objects or audio channels. Output channels are used to set the sources
|
|
you want to output, and output objects are used for actually
|
|
streaming/recording/etc.)*
|
|
|
|
|
|
General Video Pipeline Overview
|
|
-------------------------------
|
|
The video graphics pipeline is run from two threads: a dedicated
|
|
graphics thread that renders preview displays as well as the final mix
|
|
(the obs_graphics_thread_ function in `libobs/obs-video.c`_), and a
|
|
dedicated thread specific to video encoding/output (the video_thread_
|
|
function in `libobs/media-io/video-io.c`_).
|
|
|
|
Sources assigned to output channels will be drawn from channels
|
|
0..(MAX_CHANNELS_-1). They are drawn on to the final texture which will
|
|
be used for output `[1]`_. Once all sources are drawn, the final
|
|
texture is converted to whatever format that libobs is set to (typically
|
|
a YUV format). After being converted to the back-end video format, it's
|
|
then sent along with its timestamp to the current video handler,
|
|
`obs_core_video::video`_.
|
|
|
|
It then puts that raw frame in a queue of MAX_CACHE_SIZE_ in the `video
|
|
output handler`_. A semaphore is posted, then the video-io thread will
|
|
process frames as it's able. If the video frame queue is full, it will
|
|
duplicate the last frame in the queue in an attempt to reduce video
|
|
encoding complexity (and thus CPU usage) `[2]`_. This is why you may
|
|
see frame skipping when the encoder can't keep up. Frames are sent to
|
|
any raw outputs or video encoders that are currently active `[3]`_.
|
|
|
|
If it's sent to a video encoder object (`libobs/obs-encoder.c`_), it
|
|
encodes the frame and sends the encoded packet off to the outputs that
|
|
encoder is connected to (which can be multiple). If the output takes
|
|
both encoded video/audio, it puts the packets in an interleave queue to
|
|
ensure encoded packets are sent in monotonic timestamp order `[4]`_.
|
|
|
|
The encoded packet or raw frame is then sent to the output.
|
|
|
|
|
|
General Audio Pipeline Overview
|
|
-------------------------------
|
|
The audio pipeline is run from a dedicated audio thread in the audio
|
|
handler (the `audio_thread`_ function in `libobs/media-io/audio-io.c`_);
|
|
assuming that AUDIO_OUTPUT_FRAMES_ is set to 1024, the audio thread
|
|
"ticks" (processes audio data) once every 1024 audio samples (around
|
|
every 21 millisecond intervals at 48khz), and calls the audio_callback_
|
|
function in `libobs/obs-audio.c`_ where most of the audio processing is
|
|
accomplished.
|
|
|
|
A source with audio will output its audio via the
|
|
obs_source_output_audio_ function, and that audio data will be appended
|
|
or inserted in to the circular buffer `obs_source::audio_input_buf`_.
|
|
If the sample rate or channel count does not match what the back-end is
|
|
set to, the audio is automatically remixed/resampled via swresample
|
|
`[5]`_. Before insertion, audio data is also run through any audio
|
|
filters attached to the source `[6]`_.
|
|
|
|
Each audio tick, the audio thread takes a reference snapshot of the
|
|
audio source tree (stores references of all sources that output/process
|
|
audio) `[7]`_. On each audio leaf (audio source), it takes the closest
|
|
audio (relative to the current audio thread timestamp) stored in the
|
|
circular buffer `obs_source::audio_input_buf`_, and puts it in
|
|
`obs_source::audio_output_buf`_.
|
|
|
|
Then, the audio samples stored in `obs_source::audio_output_buf`_ of the
|
|
leaves get sent through their parents in the source tree snapshot for
|
|
mixing or processing at each source node in the hierarchy `[8]`_.
|
|
Sources with multiple children such as scenes or transitions will
|
|
mix/process their children's audio themselves via the
|
|
`obs_source_info::audio_render`_ callback. This allows, for example,
|
|
transitions to fade in the audio of one source and fade in the audio of
|
|
a new source when they're transitioning between two sources. The mix or
|
|
processed audio data is then stored in `obs_source::audio_output_buf`_
|
|
of that node similarly, and the process is repeated until the audio
|
|
reaches the root nodes of the tree.
|
|
|
|
Finally, when the audio has reached the base of the snapshot tree, the
|
|
audio of all the sources in each output channel are mixed together for a
|
|
final mix `[9]`_. That final mix is then sent to any raw outputs or
|
|
audio encoders that are currently active `[10]`_.
|
|
|
|
If it's sent to an audio encoder object (`libobs/obs-encoder.c`_), it
|
|
encodes the audio data and sends the encoded packet off to the outputs
|
|
that encoder is connected to (which can be multiple). If the output
|
|
takes both encoded video/audio, it puts the packets in an interleave
|
|
queue to ensure encoded packets are sent in monotonic timestamp order
|
|
`[4]`_.
|
|
|
|
The encoded packet or raw audio data is then sent to the output.
|
|
|
|
.. _obs_graphics_thread: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-video.c#L588-L651
|
|
.. _libobs/obs-audio.c: https://github.com/jp9000/obs-studio/blob/master/libobs/obs-audio.c
|
|
.. _libobs/obs-video.c: https://github.com/jp9000/obs-studio/blob/master/libobs/obs-video.c
|
|
.. _video_thread: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/media-io/video-io.c#L169-L195
|
|
.. _libobs/media-io/video-io.c: https://github.com/jp9000/obs-studio/blob/master/libobs/media-io/video-io.c
|
|
.. _video output handler: https://github.com/jp9000/obs-studio/blob/master/libobs/media-io/video-io.c
|
|
.. _audio_thread: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/media-io/audio-io.c#L241-L282
|
|
.. _libobs/media-io/audio-io.c: https://github.com/jp9000/obs-studio/blob/master/libobs/media-io/audio-io.c
|
|
.. _MAX_CHANNELS: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-defs.h#L20-L21
|
|
.. _[1]: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-video.c#L99-L129
|
|
.. _obs_core_video::video: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-internal.h#L250
|
|
.. _MAX_CACHE_SIZE: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/media-io/video-io.c#L34
|
|
.. _[2]: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/media-io/video-io.c#L431-L434
|
|
.. _[3]: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/media-io/video-io.c#L115-L167
|
|
.. _libobs/obs-encoder.c: https://github.com/jp9000/obs-studio/blob/master/libobs/obs-encoder.c
|
|
.. _[4]: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-output.c#L1382-L1439
|
|
.. _AUDIO_OUTPUT_FRAMES: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/media-io/audio-io.h#L30
|
|
.. _audio_callback: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-audio.c#L367-L485
|
|
.. _obs_source_output_audio: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-source.c#L2578-L2608
|
|
.. _obs_source::audio_input_buf: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-source.c#L1280-L1283
|
|
.. _[5]: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-source.c#L2561-L2563
|
|
.. _[6]: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-source.c#L2591
|
|
.. _[7]: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-audio.c#L393-L415
|
|
.. _obs_source::audio_output_buf: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-internal.h#L580
|
|
.. _[8]: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-audio.c#L417-L423
|
|
.. _obs_source_info::audio_render: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-source.h#L410-L412
|
|
.. _[9]: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/obs-audio.c#L436-L453
|
|
.. _[10]: https://github.com/jp9000/obs-studio/blob/2c58185af3c85f4e594a4c067c9dfe5fa4b5b0a9/libobs/media-io/audio-io.c#L144-L165
|