[FFmpeg-user] frame number question

[Mark Filipak (ffmpeg)]

For the developers:
At what point are frame numbers assigned?
I would guess: When they leave the decoder. Am I right?

Example:
If '-vf decimate' removes frame[(n+1)%5==2], are frames renumbered at that point or are the frames 
left as frame[0], frame[2], frame[3], frame[4], frame[5], frame[7], frame[8], frame[9], etc.

If you prefer I discover this on my own, that's okay, but I don't know how to put the frame number 
(as text) into the frames so that when I frame-step through the video I can see the frame numbers. 
If you could help with that, I'm pretty sure I can take the rest of the needed steps on my own.

Thanks Kindly,
Mark.