qmlgloverlay

qmlgloverlay provides a way to render an almost-arbitrary QML scene within GStreamer pipeline using the same OpenGL context that GStreamer uses internally. This avoids attempting to share multiple OpenGL contexts avoiding increased synchronisation points and attempting to share an OpenGL context at runtime which some drivers do not like. The Intel driver on Windows is a notable example of the last point.

qmlgloverlay 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 qmlgloverlay to an already running pipeline
  2. Not having any qmlgloverlay (or qmlglsink) 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 qmlglsink element or a qmlgloverlay element. The recommended usage is to have either element (qmlglsink or qmlgloverlay) 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 a dynamically adding qmlgloverlay (or qmlglsink) to a pipeline case, there are some considerations for ensuring that the window system display and OpenGL contexts are compatible with Qt. When the qmlgloverlay (or qmlglsink) 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 qmlgloverlay and qmlglsink will return the exact same GstGLDisplay object while the pipeline is running regardless of whether any qmlgloverlay or qmlglsink elements are added or removed from the pipeline.

The Qml scene will run at the pace of incoming buffers. One input buffer will cause a render of one output buffer. The timestamps on the input buffers are used to drive the animation time.

Hierarchy

GObject
    ╰──GInitiallyUnowned
        ╰──GstObject
            ╰──GstElement
                ╰──GstBaseTransform
                    ╰──GstGLBaseFilter
                        ╰──GstGLFilter
                            ╰──qmlgloverlay

Factory details

Authors: – Matthew Waters

Classification: – Filter/QML/Overlay

Rank – none

Plugin – qmlgl

Package – GStreamer Good Plug-ins

Pad Templates

sink

video/x-raw(ANY):
         format: { RGBA, BGRA, YV12 }
          width: [ 1, 2147483647 ]
         height: [ 1, 2147483647 ]
      framerate: [ 0/1, 2147483647/1 ]
 texture-target: 2D

Presence – always

Direction – sink

Object type – GstPad

src

video/x-raw(memory:GLMemory):
         format: RGBA
          width: [ 1, 2147483647 ]
         height: [ 1, 2147483647 ]
      framerate: [ 0/1, 2147483647/1 ]
 texture-target: 2D

Presence – always

Direction – src

Object type – GstPad

Signals

qml-scene-destroyed 

qml_scene_destroyed_callback (GstElement * param_0,
                              gpointer udata)
def qml_scene_destroyed_callback (param_0, udata):
    #python callback for the 'qml-scene-destroyed' signal
function qml_scene_destroyed_callback(param_0: GstElement * param_0, udata: gpointer udata): {
    // javascript callback for the 'qml-scene-destroyed' signal
}

Parameters:

param_0 –
No description available
udata –
No description available

Flags: Run Last

qml-scene-initialized 

qml_scene_initialized_callback (GstElement * param_0,
                                gpointer udata)
def qml_scene_initialized_callback (param_0, udata):
    #python callback for the 'qml-scene-initialized' signal
function qml_scene_initialized_callback(param_0: GstElement * param_0, udata: gpointer udata): {
    // javascript callback for the 'qml-scene-initialized' signal
}

Parameters:

param_0 –
No description available
udata –
No description available

Flags: Run Last

Properties

context 

“context” GstGLContext *

Get OpenGL context

Flags : Read

depth-buffer 

“depth-buffer” gboolean

Instructs QML to not use a depth or stencil buffer when rendering.

WARNING: Disabling the depth and stencil buffer may cause QML to incorrectly render the provided scene.

Flags : Read / Write

Default value : true

Since : 1.26

qml-scene 

“qml-scene” gchararray

The contents of the QML scene

Flags : Read / Write

Default value : NULL

qos 

“qos” gboolean

Handle Quality-of-Service events

Flags : Read / Write

Default value : true

root-item 

“root-item” gpointer

The root QQuickItem from the qml-scene used to render

Flags : Read

widget 

“widget” gpointer

The QQuickItem to place the input video in the object hierarchy

Flags : Read / Write

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