qml6glsink

qml6glsink provides a way to render a video stream as a Qml object inside the Qml scene graph. This is achieved by providing the incoming OpenGL textures to Qt as a scene graph object.

qml6glsink will attempt to retrieve the windowing system display connection that Qt is using (#GstGLDisplay). This may be different to any already existing window system display connection already in use in the pipeline for a number of reasons. A couple of examples of this are:

1. Adding qml6glsink to an already running pipeline
2. Not having any qml6glsink element start up before any other OpenGL-based element in the pipeline.

If one of these scenarios occurs, then there will be multiple OpenGL contexts in use in the pipeline. This means that either the pipeline will fail to start up correctly, a downstream element may reject buffers, or a complete GPU->System memory->GPU transfer is performed for every buffer.

The requirement to avoid this is that all elements share the same GstGLDisplay object and as Qt cannot currently share an existing window system display connection, GStreamer must use the window system display connection provided by Qt. This window system display connection can be retrieved by either a qml6glsink element, a qml6gloverlay element, or a qml6glmixer element. The recommended usage is to have either element (qml6glsink, or qml6gloverlay, or qml6glmixer) be the first to propagate the GstGLDisplay for the entire pipeline to use by setting either element to the READY element state before any other OpenGL element in the pipeline.

In the dynamically adding qml6glsink (or qml6gloverlay, or qml6glmixer) to a pipeline case, there are some considerations for ensuring that the window system display and OpenGL contexts are compatible with Qt. When the qml6gloverlay (or qml6glsink, or qml6glmixer) element is added and brought up to READY, it will propagate it's own GstGLDisplay using the GstContext mechanism regardless of any existing GstGLDisplay used by the pipeline previously. In order for the new GstGLDisplay to be used, the application must then set the provided GstGLDisplay containing GstContext on the pipeline. This may effectively cause each OpenGL element to replace the window system display and also the OpenGL context it is using. As such this process may take a significant amount of time and resources as objects are recreated in the new OpenGL context.

All instances of qml6glsink, qml6gloverlay, and qml6glmixer will return the exact same GstGLDisplay object while the pipeline is running regardless of whether any qml6glsink or qml6gloverlay elements are added or removed from the pipeline.