[PD] [Gem] Setting individual pixel alpha values?

[Andrew Johnston]

Hi all,
Please excuse this newbie question, but I'm somewhat stuck.  I'd like to 
be able to set the alpha values of particular pixels in a picture 
somehow.  The idea is to have a picture superimposed on top of a video 
and to be able to change the transparency of individual pixels in the 
still picture to reveal/conceal the video.

I've looked at pix_set, but this requires a full list of all pixels in 
the picture.  What I would like is to be able to specify the coordinates 
of a particular pixel and set its RGBA value.  Seems like the reverse of 
pix_data I think.

Perhaps there is a better method for achieving this effect?  Any tips 
appreciated...

Thanks,
Andrew