I have an XVid file that plays OK in Quicktime but it looks stretched out in MPlayer and VLC. The movie appears to have a custom aspect ratio, which Quicktime manages to ignore.

However, I'd like to be able to remove the custom AR without encoding.

There is a program for Windows called mpeg4modifier, which does this and there are various other tools for changing other formats like mpeg1 etc.

I haven't found any Mac tools that do this though and I wondered if it was possible using ffmpegx. I thought I had it when chose passthrough video and audio and selected decode with quicktime because when opening the partial file, it played correctly in MPlayer. But I quickly realised that it was dumping the video to the uncompressed format.