There is cap.hpp which implements a bare minimum IStreamSource over the cv::VideoCapture, and there are more advanced GStreamer-based and oneVPL-based sources are coming:
- Ported GStreamerSource to OpenCV by AsyaPronina · Pull Request #20709 · opencv/opencv · GitHub
- opencv/modules/gapi/src/streaming/onevpl at master · opencv/opencv · GitHub (WIP)
Beware: these implementations may be hard to follow and the “constant size” limitation is still there