draw under darker artwork

[papervampire]

Hi everyone, I feel that there must be a solution to this problem but can't figure it out.

I'm drawing in CS4 with a Wacom tablet. I am using a default brush with flow and opacity jitter set to pressure sensitivity.

I've noticed that if you start a stroke, and without lifting the stylus, double back over your stroke, it will not "add" to the already drawn part, but actually appear to go either "under" it or "over it" depending on how hard you press. i.e., you are not adding to the already drawn lines and making them a certain percentage darker - instead, the already drawn artwork only darkens if you draw darker than it. A lighter line will simply not effect an area that is darker than it.

The problem is that you can only achieve this effect if you draw without releasing the stylus. Is there a way to replicate it between strokes, so that new strokes do not add, but go under or over previous strokes?

I've tried the brush option "behind", but it doesn't do the right thing at all. It seems like I need some kind of reverse multiply!

Any help would be greatly appreciated!

[papervampire]

here's a pictorial example in case I was not clear:

[papervampire]

hi - I've figured out a semi-solution, but I'm still not satisfied.

what I've done is to turn off the flow jitter, and in color dynamics, turn on foreground/background jitter, and set these colours to black and white. I then set the pen tool to darken. however, this isn't quite right; while drawing a continuous line, it's now possible for light to override dark. also, I don't really like the effect of foreground/background jitter as much as flow.

if anyone can think of a better solution I would be so happy. anyone??

ETA actually, this would work perfectly if only, on a continuos stroke, white would not override black. =(