We are about to switch to a new forum software. Until then we have removed the registration on this forum.
Thanks to GoToLoop's help, the erase effect with a picture overlaying a video has been solved! http://forum.processing.org/two/discussion/4696/erase-effect#Item_7
However, I am currently working on deeper effect with a video overlaying a video, clicking mouse left to erase the upper video to show the below one.
Anymore advices? :)