Drawing on video with Isadora

Alfred said:

i want to draw LIVE with a wacom pad thin white lines onto the video. the lines should dissapear slowly. any ideas how to do that?

And then Zola said ...

You could do it with the shapes actor going through a luminance keys actor (foreground input) that feeds back to its Background input. You'll have to put another video actor (in bypass mode if you prefer) between the output and the input of the Luminance Key actor for the virtual cable to accept to go there.

So here it is: