To achieve this pull a hard key of both Dudes (same settings for both) and combine them with an
Outside-node. View the Alpha. Now we have a Mask that shows us only the areas that have been
revealed!



Go on and combine the Dudes using a KeyMix-node and our new Mask. View the result. We have
added new greenscreen material from our Dude at frame 2 to our old Dude at frame 1. So the
changes from frame 1 to frame 2 have taken place only in the blocked areas. Everything else was
left untouched. This is good.

If you think this was tricky, sit down. The real trick comes now.
If you go to frame 3,4,5... and so on in your timeline you will notice that so far our efforts only work
with the current frame and the frame before. All previous results have vanished. Damn!
What we need is a self-updating process. We need to work with the result of all previous steps.
The solution is to create an "output=input" -loop. Most softwares won�t allow such things because
they fear internal feedbacks. Shake users don�t fear anything.