GstGLVideoItem is required to render input video in the overlay's qml.
And currently qmlgloverlay requires to set this GstGLVideoItem to its widget property.
Instead of fetching GstGLVideoItem from the overlay's root object (root-item prop),
and setting it back as a widget (widget prop),
proposing to use found GstGLVideoItem in the current object hierarchy (passed in qml-scene) by default.
Also useful in Python, which solves the issue when casting gpointer <=> QQuickItem* is required.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/919>
Tested on linux with X11/wayland and semi-tested on Windows.
Windows crashes on item destruction however this is better than nothing.
Fix up some win32 build issues on the way with mismatched {} and
G_STMT_{START,END}
Very much in the same spirit as the Gtk GL sink
Two things are provided
1. A QQuickItem subclass that renders out RGBA filled GstGLMemory
buffers that is instantiated from qml.
2. A sink element that will push buffers into (1)
To use
1. Declare the GstGLVideoItem in qml with an appropriate
objectName property set.
2. Get the aforementioned GstGLVideoItem from qml using something like
QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
QObject *rootObject = engine.rootObjects().first();
QQuickItem *videoItem = rootObject->findChild<QQuickItem *> ("videoItem");
3. Set the videoItem on the sink
https://bugzilla.gnome.org/show_bug.cgi?id=752185
