Picture on Picture?

Parabola i have noticed in some games (Dave's Quest, Goldrush) that there apear to be picture files with something like a tree that end up on top of another picture in the game. like the cut away stairwell in Dave's Quest.

Could someone please explain this technique or point me in the direction of some info about it? thanx :-\
AGI1122 It is really simple to do: v38 = x; load.pic(v38); overlay.pic(v38); discard.pic(v38); show.pic(); Just change x to the picture you want to overlay over your current pic. All it does is draw the new pic on the old pic.
Parabola thanx for that chris :)
Andrew_Baker In fact, if you're really clever, you can create multiple pic files that when used in different combinations, can create a polynomially greater number of total pictures.


| **** ** ***** *(*|
| ** ** *** |
night pic (upper half)
| -*- |
| /|\ |
day pic (upper half)
mountains (lower half)

This is a small and crappy example, but you could use these three pics to make the mountains be under a day-time or night-time sky. With additional lower-half pics, the same day or night sky would double the possibilities. Just a suggestion.
Kon-Tiki Wow, never knew that was possible. That certainly saves a lot of time