More specifically the SPS bytes in the avcC atom. Note this only fixes the problem for winedmo but not gstreamer. This is because gstreamer 1) does not parse the SPS, 2) will set pixel-aspect-ratio in caps regardless if a pasp atom is present. So there is no way for us to distinguish between 1) pasp being present and having an aspect ratio of 1:1, and 2) pasp not being present but pixel-aspect-ratio having been set to 1:1 by gstreamer as fallback. And because this is the case, I made sure winedmo is tested separately. In winedmo's case, if pasp is not present, we can set the PixelAspectRatio.Denominator in MFVIDEOFORMAT to 0 to distinguish. -- v4: mfsrcsnk: Parse pixel aspect ratio from the avcC atom. mfsrcsnk: Test how pixel aspect ratio metadata in an mp4 file are handled. https://gitlab.winehq.org/wine/wine/-/merge_requests/10321