Anyone know how to do this?

Seen this masking/transition effect on a few things now…
Very cool, found a vague tutorial on the subject but couldnt get the thing to work

heres what i want to do…
http://www.lomography.com/greenspace/

heres a tutorial…
http://www.actionscript.org/tutorials/advanced/Dynamic_masking_using_the_drawing_API_II/index2.shtml

if you go to the bottom you can see the effect

anone know of a better tutorial, or how to ge this to work…

thanks in advance