[MPlayer-dev-eng] Where is video frame processing in source code?

[Jesse Gordon]

Where in the source code for mplayer do I find a place where I can 
insert my own code to analyze and modify the video frames before they 
are displayed?

Specifically, I wish to add focus detection and indication. I know how 
to do focus detection in C, but I just can't find in the source code 
where (or what variables/buffers) I can analyze and modify each frame 
with my own code.

Thank you very much,

Jesse Gordon