1. Install MediaSDK
2. Make sure corresponding environment variable is set, Windows: INTELMEDIASDKROOT, Linux: MFX_HOME
3. Build with -DWITH_MFX option turned on:

   cmake -DWITH_MFX=ON <path-to-opencv-sources>
   cmake --build .

Media containers are not supported yet, so it is only possible to decode raw video stream stored in a file. It can be extracted from a container manually using the FFmpeg tool (source1, source2) or any other tools:

# H264
ffmpeg -i video.avi -vcodec copy -an -bsf:v h264_mp4toannexb video.264
# H265
ffmpeg -i in.mkv -c:v copy -bsf hevc_mp4toannexb out.h265

Then you can use VideoCapture object to decode the resulting file:

VideoCapture cap(“video.264”, CAP_INTEL_MFX);

Note! The file extension is important, because it will be used to determine the codec. It can be one of .264, .h264, .mp2, .mpeg2, .265 or .hevc.

Use the VideoWriter object:

int fourcc = VideoWriter::fourcc('H', '2', '6', '4');
VideoWriter writer(filename, CAP_INTEL_MFX, fourcc, fps, frameSize, isColor);

Where fourcc can be one of MPG2, H264, X264, AVC , H265 or HEVC.