"Do you mean when you have several other layer and you
create a new layer on top of them and paint
on the new layer, then
the combined transparency (alpha channels) of the layers
under the new layer
is copied to the new layer and the color is used from the
brush?
If you mean this, it would actually be a cool idea. Would be
pretty useful (at least for me :D), but this is not really
a blending mode (i mean, yes it somehow is but not really
:D). I think the brush code would need some modifications
for that. Can you tell me a program that support this (so i
can try it out). I haven't found something like this in Gimp
or Photoshop."
That's the idea, but exactly "the combined transparency (alpha
channels) of the layers under the new layer" is not "copied" to
the new layer, you still paint on the whole page, as with any
other layer blend mode (meaning if you turn it back to Normal
blend mode the "blend effect" disapear…)
I use this painting/layer blend mode on TVPaint (the best
commercial Painting application to me ;) )
(There was an open beta for Linux but it was usable until January
1st, I don't think there is a new one yet. If you can try the win
or mac demo)
Also this blend mode is in Open Source app SYNFIG STUDIO !!! it's
a vector program but the Onto mode is there (with a few original
useful others, like Straight and Straight Onto…)
Look at here:
http://www.synfig.org/wiki/Blend_Method_Parameter