A friend posed an interesting problem to me, which I've not been able to solve.

You have two pictures of elements A and B

The first picture has only element A

The second picture Has element B on top of Element A.

We're trying to find a way to extract just element B.  But as the images overlap, this is proving problematic. My first thought was to make a layer mask from A, and apply it inverted to B, but this results in a chunk going missing from B. Obviously, this could be done with lots of manual work selecting the right pixels, but that seems tedious, and something tells me there must be an easier way.

Element a is in exactly the same place in both pictures. So I'm wondering if there's any way to remove common pixels over two layers. Does anyone have any idea?

to clarify, we don't actually have any application for this, but are merely trying to find some technique that can be applied to this. I can create visual aids if it would help explain what I mean.

are you talking about a double film exposure ? or is the area of element b clearly seperated from element a in the 2nd picture ?

here's a visual aid I whipped up. The apple represents element a, the banana is element b.

Using these two images, and preferably without tedious manual work, how can I get a picture of the banana alone?

 try this on for size.

put element "A" on a layer    
put element "A+B" on another

change the blending mode of one of them to "difference"

flatten, invert the colours.  

It sounds like all you wanna do is cut out the banana (element b). This is the same whether or not it's on top of ANYTHING else - you're cutting a foreground object out - what (if anything) is underneath doesn't matter.

This would be using any or all (in combination - particularly if you're dealing with a shallow DOF) of the following - Paths, Lasso Tool, and Masking.

Can you post a pic of the actual image you're working on?
-Lew ;-)

Just re-read your original post again - I think I had it backwards - if you want a finished picture of the APPLE...? (whereas originally I thought you wanted the banana)

Simply put - you're screwed.

The information of the complete object simply isn't there, so you'll have to recreate it one way or another - painting, cloning, etc.

Sorry-
-Lew ;-)

(Again, post the pic you're working on, and we'll see what we can come up with for you as far as any help.)

Quote -  try this on for size.

put element "A" on a layer    
put element "A+B" on another

change the blending mode of one of them to "difference"

flatten, invert the colours.  

Thanks. That's the kind of answer I was looking foi. A bit incomplete, but I worked out the rest from there. Doing this turned the apple a solid color, which made it easy to crete a layer mask showing only the banana

Quote - I
Can you post a pic of the actual image you're working on?

As mentioned in the OP, I don't HAVE a specific picture I'm working on, yet. This is more something that would be good to knoiw for future. Difference blending seems to do the job well.

Okay - tell me specifically if this is right - you want the banana all by itself - minus the apple?

yes

Okay, then it's my first response.

No need to be making A on one layer, B on another, changing blending modes - none of that.

Using Masking, Lasso, Paths - whatever your Tools of choice - just cut A out onto its own layer  - done.