Open Video file (.gif/.mov/.webm) with alpha channel

I’m trying to open a video file (.gif / .webm) with alpha channel on it, but always it gives me 3 channels and it displays with a solid background in Qt6. If I open a normal image .png with alpha channel it works correctly.
What I’ve done so far is:

VideoCapture capture;, cv::CAP_FFMPEG);
capture.set(cv::CAP_PROP_CONVERT_RGB, false); //to force loading alpha channel
frameRate = capture.get(cv::CAP_PROP_FPS);
capture.get(cv::CAP_PROP_CONVERT_RGB); // gives 0
//in a thread
cv::cvtColor(frame, RGBframe, cv::COLOR_BGRA2RGBA); // to correctly show colors in Qt QImage
img = QImage(, RGBframe.cols, RGBframe.rows, QImage::Format_RGBA8888);

The problem is even I force alpha channel the image frame comes alredy with a black background instead of transparency

OpenCV does not support alpha channel in videos. GIF doesn’t even have one. It just marks some color in its small palette as transparent.