aspect ratio ignored in mkv if video decoded by ffdshow

[Isochroma]

If the following conditions are true:

1. media file is a MKV with non 1:1 aspect ratio (set using mkvmerge)
2. player is MPC (any version)
3. video stream in the file is decoded by ffdshow (any version, any supported codec)

then the aspect ratio set in the MKV container will be totally ignored, ie. the video will always display at 1:1.

Now, if ffdshow is not used to decode the video (ie. the xvid codec is used for decoding an xvid video stream), the AR flag is correctly applied on playback. Why?

[fps]

You have to enable the Overlay Mixer in ffdshow (Settings->Output)

[Haali]

That's because ffdshow was changed to ignore dshow AR settings and use the ones present in mpeg bitstream instead.