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 Playback tutorial 9: Digital audio pass-through).

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 Basic tutorial 10: GStreamer tools) 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!

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