Parent Child Compositing

[Kim Mackenzie]

This is the only way coming to mind – basically, use a positive of your Layer C as a mask on one layer, and a negative of it as a mask on the other.

[Filip Vandueren]

Hi Kim,

If both layers were fully opaque to start with, there’s no need to do it for the bottom layer too.
Because at a input value of 50% grey,
both layers would be at 50 % opacity, wich does not add up to 100%

And here’s a faster way;

• Get the three layers situated in your file
• make only layer C visible
• switch to channels and command-click on the RGB-channel -> turns the grayscale equivalent into a selection
• hide layer C, show layer A & B
• Select layer B (you should still sea the ‘marchin ants’ of your complex selection.
• click on the layer-mask icon -> it automatically creates a layer mask that’s equal to the current selection

If that’s wrong, you should invert the mask.

[Kim Mackenzie]

Oh duh 🙂 Yer right, of course.

[Tim Neighbors]

I realize this is a very old post, but thought I’d share… select a layer in photoshop and press control+alt+G (on a PC). This will make it a child of the layer below. Make that child layer a multiply layer and put your mask in it with one of the images the parent (directly below), and the other below that. …just another way to accomplish the same thing.