QVideoSink doesn't handle alpha channel

[TheEnigmist]

I've a WebM (VP9) video with alpha channel on it. I can open correctly the video on OBS and it shows as intended, but if I try to use it in QVideoSink using videoFrameChanged to show video frame by frame it shows a black background instead of transparent. I'm using Qt 6.3.0 RC

Here ffmpeg file info :

PixelFormat of each QVideoFrame is Format_BGRA8888 , when converted to QImage becomes Format_ARGB32_Premultiplied , all of them has an alpha channel so why my video show transparency as a black pixel?

EDIT: If I check alpha channel with hasAlpha() on frame.image() is true, QPixmap::fromImage(frame.image) returns false!

connect(videoSink, &QVideoSink::videoFrameChanged, [=](const QVideoFrame& frame) {
		qDebug() << QPixmap::fromImage(frame.toImage()).hasAlpha(); //FALSE
		qDebug() << frame.toImage().hasAlphaChannel(); // TRUE
		videoLabel->setPixmap(QPixmap::fromImage(frame.toImage()));
	});

[SGaist]

Hi,

On which do you see this result ?

[TheEnigmist]

@SGaist Hi,
Actually I'm showing that image on videoLabel wich is a QLabel . I'm using this method since I want to manipulate video with OpenCV or do some image transformation, so my first step is to split video frame by frame

[SGaist]

Why not use OpenCV to read the video stream directly ?

[TheEnigmist]

@SGaist Well because that would be a future work, but maybe it is more simple to start directly with OpenCV than try to do the same first with native Qt code and then add OpenCV to it. I will build OpenCV and try to use it.
How can I set the output of OpenCV frames in my QLabel?

[SGaist]

Convert the cv::Mat to a QImage.

[TheEnigmist]

@SGaist I figured out how to show an image with alpha correctly. But I still cannot show a video/gif with alpha. I'm able to load it without problem in Qt but not with opencv. The only problem with Qt is a video with alpha (webm) that it shown with a background instead of transparency.
While learning how use properly opencv, how can I show videos with alpha channel in Qt?

[SGaist]

From a cursorary look, I think the frame is rendered on a black background in the backend code. You would have to modify this code to achieve what you want.

[TheEnigmist]

@SGaist well I think I cannot edit Qt backend source code...
In OpenCV i've a similar problem with video opened in RGB instead of RGBA

[SGaist]

Sure you can. You can clone the QtMultimedia sources.