If the source of the video is in digital format, then you can overlay
digital content.  I've done this, but not with a PIC, but a pretty large
FPGA.  Essentially, you need to count lines and pixels and either sum or
modify the pixel data at the location you need it to be at.  Its all in the
digital domain and very easy to control at that level.  Then of course, you
need to send the data out and convert back to analog and in PAL standards.
In the world of visual simulation, thats how its done most the time for
HUD's and target overlays.

If your in the analog domain, then I suppose you could do some sort of video
summer, but it needs to be component video (RGB/H/V) to do it right and make
sure that the start of each field lines up with each other (easier said than
done...believe me. Its hard enough to do it in the digital domain, let alone
the analog domain).

But doing it with a PIC or SX....not sure I'd attempt it.  Sometimes you
can't fit a square peg in a round hole.

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