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Basic tutorial 16: Platform-specific elements
Goal
Even though GStreamer is a multiplatform framework, not all the elements
are available on all platforms. For example, the video sinks
depend heavily on the underlying windowing system, and a different one
needs to be selected depending on the platform. You normally do not need
to worry about this when using elements like playbin or
autovideosink, but, for those cases when you need to use one of the
sinks that are only available on specific platforms, this tutorial hints
you some of their peculiarities.
Cross Platform
glimagesink
This video sink is based on
OpenGL or OpenGL ES. It supports rescaling
and filtering of the scaled image to alleviate aliasing. It implements
the VideoOverlay interface, so the video window can be re-parented
(embedded inside other windows). This is the video sink recommended on
most platforms. In particular, on Android and iOS, it is the only
available video sink. It can be decomposed into
glupload ! glcolorconvert ! glimagesinkelement to insert further OpenGL
hardware accelerated processing into the pipeline.
Linux
ximagesink
A standard RGB only X-based video sink. It implements the VideoOverlay
interface, so the video window can be re-parented (embedded inside
other windows). It does not support scaling or color formats other
than RGB; it has to be performed by different means (using the
videoscale element, for example).
xvimagesink
An X-based video sink, using the X Video Extension (Xv). It implements the VideoOverlay interface, so the video window can be re-parented (embedded inside other windows). It can perform scaling efficiently, on the GPU. It is only available if the hardware and corresponding drivers support the Xv extension.
alsasink
This audio sink outputs to the sound card via ALSA (Advanced Linux Sound Architecture). This sink is available on almost every Linux platform. It is often seen as a “low level” interface to the sound card, and can be complicated to configure (See the comment on ).
pulsesink
This sink plays audio to a PulseAudio server. It is a higher level abstraction of the sound card than ALSA, and is therefore easier to use and offers more advanced features. It has been known to be unstable on some older Linux distributions, though.
Mac OS X
osxvideosink
This is the video sink available to GStreamer on Mac OS X. It is also
possible to draw using glimagesink using OpenGL.
osxaudiosink
This is the only audio sink available to GStreamer on Mac OS X.
Windows
directdrawsink
This is the oldest of the Windows video sinks, based on Direct Draw. It requires DirectX 7, so it is available on almost every current Windows platform. It supports rescaling and filtering of the scaled image to alleviate aliasing.
dshowvideosink
This video sink is based on Direct Show. It can use different rendering back-ends, like EVR, VMR9 or VMR7, EVR only being available on Windows Vista or more recent. It supports rescaling and filtering of the scaled image to alleviate aliasing. It implements the VideoOverlay interface, so the video window can be re-parented (embedded inside other windows).
d3dvideosink
This video sink is based on Direct3D and it’s the most recent Windows video sink. It supports rescaling and filtering of the scaled image to alleviate aliasing. It implements the VideoOverlay interface, so the video window can be re-parented (embedded inside other windows).
directsoundsink
This is the default audio sink for Windows, based on Direct Sound, which is available in all Windows versions.
dshowdecwrapper
Direct Show is a multimedia
framework similar to GStreamer. They are different enough, though, so
that their pipelines cannot be interconnected. However, through this
element, GStreamer can benefit from the decoding elements present in
Direct Show. dshowdecwrapper wraps multiple Direct Show decoders so
they can be embedded in a GStreamer pipeline. Use the gst-inspect-1.0 tool
(see ) to see the
available decoders.
Android
openslessink
This is the only audio sink available to GStreamer on Android. It is based on OpenSL ES.
openslessrc
This is the only audio source available to GStreamer on Android. It is based on OpenSL ES.
androidmedia
android.media.MediaCodec
is an Android specific API to access the codecs that are available on
the device, including hardware codecs. It is available since API level
16 (JellyBean) and GStreamer can use it via the androidmedia plugin
for audio and video decoding. On Android, attaching the hardware
decoder to the glimagesink element can produce a high performance
zero-copy decodebin pipeline.
ahcsrc
This video source can capture from the cameras on Android devices, it is part of the androidmedia plugin and uses the android.hardware.Camera API.
iOS
osxaudiosink
This is the only audio sink available to GStreamer on iOS.
iosassetsrc
Source element to read iOS assets, this is, documents stored in the
Library (like photos, music and videos). It can be instantiated
automatically by playbin when URIs use the
assets-library:// scheme.
iosavassetsrc
Source element to read and decode iOS audiovisual assets, this is,
documents stored in the Library (like photos, music and videos). It can
be instantiated automatically by playbin when URIs use the
ipod-library:// scheme. Decoding is performed by the system, so
dedicated hardware will be used if available.
Conclusion
This tutorial has shown a few specific details about some GStreamer
elements which are not available on all platforms. You do not have to
worry about them when using multiplatform elements like playbin or
autovideosink, but it is good to know their personal quirks if
instancing them manually.
It has been a pleasure having you here, and see you soon!