Really interesting demo, nicely done :) Would be fun if switching the "Target Image" when using the second brush mode in the Growing Demo didn't erase/reset the existing canvas, so we could "stamp" new things on top of other images. Small thing perhaps but I got sad when it disappeared when I wanted to merge a kitten on top of the chameleon but couldn't :(
That's not possible in the current demo but this sounds like an interesting feature to work on and add!
You can, just enable the 'transition' switch.
That seems to be something else? It takes the current image and "transforms" it into the new target.