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Subtract layers' opacity (or something like that)


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dalgard

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Dear Photoshop Gurus,

I don't consider myself a newbie, but compared to a guru I think I must be... :) Here's what I'm trying to achieve:

I want to take a semi-transparent image and split it into two PARTLY OVERLAPPING images. In other words I want part of one image and part of another image to result in the original image when placed on top of one another.

I know which "data" in the overlapping part of the image that should be subtracted since it is repeated in the rest of the image; I am actually working with shadows in web design where this trick would make it much easier using background-repeat.

The problem I guess is that the color is #000000 in the whole of the image - only opacity changes. I've tried working with masks and blend modes, both with all kinds of inverted versions of the part that should be subtracted.

Any ideas? :surprised:

Kris
 

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Oh, another way of explaing this is:

I would like to know a method that will take two identical (!) semi-transparent images consisting only of the color #000000 and subtracting one from the other leaving nothing at all.

I think the solution has to do with alpha layers - or possibly clipping masks; I never really understood their function. In any case, using regular masks could only ever knock out part of the opacity. Maybe if I created the original images using masks in the first place ... but then I guess the question would be more like: which method subtracts two identical greyscale images leaving nothing.

:)
 
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I think I'm getting close.

If I convert the semitransparent layers to grayscale by imposing them on a white background I can use the blend method divide to remove the unwanted part. Only thing is, when trying to go back to transparency from grayscale (using regular masks) it becomes darker - i need to set opacity to 80-82 % to get something similar to the original.

So, can someone tell me how to recreate a semi-transparent image with only black using grayscale masks (applied to an all-black layer)?? :idea:

Actually, I think I will make another thread for this, since this maybe is becoming too complicated...
 
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