Video filters are split into
QVideoFilterRunnableinstances, similar to
QSGNode. This is necessary to support threaded rendering scenarios. When using the threaded render loop of the Qt Quick scene graph, all rendering happens on a dedicated thread.
QVideoFilterRunnableinstances always live on this thread and all its functions,
run(), the constructor, and the destructor, are guaranteed to be invoked on that thread with the OpenGL context bound.
QAbstractVideoFilterinstances live on the main (GUI) thread, like any other
QQuickIteminstances created from QML.
QVideoFilterRunnableinstances are managed by Qt Multimedia and will be automatically destroyed and recreated when necessary, for example when the scene graph is invalidated or the
QQuickWindowchanges or is closed. Creation happens via the
Indicates that the filter runnable’s associated
QAbstractVideoFilteris the last in the corresponding VideoOutput type’s filters list, meaning that the returned frame is the one that is going to be presented to the scene graph without invoking any further filters.
run(input, surfaceFormat, flags)¶
- Return type
Reimplement this function to perform filtering or computation on the
inputvideo frame. Like the constructor and destructor, this function is always called on the render thread with the OpenGL context bound.
Implementations that do not modify the video frame can simply return
It is safe to access properties of the associated
QAbstractVideoFilterinstance from this function.
surfaceFormatprovides additional information, for example it can be used to determine which way is up in the input image as that is important for filters to operate on multiple platforms with multiple cameras.
flagscontains additional information about the filter’s invocation. For example the
LastInChainflag indicates that the filter is the last in a VideoOutput ‘s associated filter list. This can be very useful in cases where multiple filters are chained together and the work is performed on image data in some custom format (for example a format specific to some computer vision framework). To avoid conversion on every filter in the chain, all intermediate filters can return a
QVideoFramehosting data in the custom format. Only the last, where the flag is set, returns a
QVideoFramein a format compatible with Qt.
QAbstractVideoFiltersubclass to indicate the completion of the operation. For filters that only calculate some results and do not modify the video frame, it is also possible to operate asynchronously. They can queue the necessary operations using the compute API and return from this function without emitting any signals. The signal indicating the completion is then emitted only when the compute API indicates that the operations were done and the results are available. Note that it is strongly recommended to represent the filter’s output data as a separate instance of
QAbstractVideoFilterinstance itself: properties can safely be read in since the gui thread is blocked during that time but writing may become problematic.
Avoid time consuming operations in this function as they block the entire rendering of the application.
The handleType() and pixelFormat() of
inputis completely up to the video decoding backend on the platform in use. On some platforms different forms of input are used depending on the graphics stack. For example, when playing back videos on Windows with the WMF backend,
QVideoFramecontains OpenGL-wrapped Direct3D textures in case of using ANGLE, but regular pixel data when using desktop OpenGL (opengl32.dll). Similarly, the video file format will often decide if the data is RGB or YUV, but this may also depend on the decoder and the configuration in use. The returned video frame does not have to be in the same format as the input, for example a filter with an input of a
QVideoFramebacked by system memory can output a
QVideoFramewith an OpenGL texture handle.
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