catting movie files together.

Syth

One can cat movie files together and mplayer will play them as if they were actually catted together. However even if all the files are the same size and resolution, there is problems at the joining of 2 parts. The output is something like this:

VDec: vo config request - 352 x 240 (preferred csp: Planar YV12)                                                               
VDec: using Planar YV12 as output csp (no 0)
Movie-Aspect is 1.33:1 - prescaling to correct movie aspect.
VO: [macosx] 352x240 => 352x264 Packed YUY2 
VDec: vo config request - 352 x 240 (preferred csp: Planar YV12)                                                               
VDec: using Planar YV12 as output csp (no 0)
Movie-Aspect is 1.33:1 - prescaling to correct movie aspect.
VO: [macosx] 352x240 => 352x264 Packed YUY2 
V:  20.7 1797/1797  4%  3%  0.0% 0 0

The prescaling to correct movie aspect. parts are where 2 parts meet. If start playing the file at full screen, at the join of 2 parts the window will be resized to normal scale.

I vaguely remember reading that mencoder or transcode could do something to fix this. However I can't remember where. And I don't know enough about video to understand the black magic that is mencoder/transcode. Does anyone know how to get transcode/mencoder to do this or some other tool to fix catted movie files?