Difference between layers

DD
Posted By
David Dyer-Bennet
Mar 22, 2005
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2173
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1
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I want to create a layer whose content is the difference between two other layers. Pixels not changes should be transparent. There ought to be some way to do this with "apply" or "calculations". In fact it looks like I can take the difference between two *channels* in calculations and get what I want. So with sufficient time and effort I could build up a layer from that. And if that’s the only way I could record it as an action to make it simple. But first I’m wondering if anybody knows an easier way?

(This is to support a particular photo editing crotchet of mine; I like to leave the background layer unmodified, and make all the retouches and curves adjustments and such using layers or adjustment layers. This allows me to *change my mind* later without loss, among other things. But there are situations where using blending modes for retouching makes a huge difference. Currently I rubber-stamp or layer-via-copy the areas I need to do that in onto my retouch layer and then work on them there, but that leaves me with lots of unaltered pixels duplicated on the retouch layer, so it’s inelegant. This difference between layers thing would let me edit a background copy relatively freely, and then extract the differences.)

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Tom Nelson
Mar 23, 2005
Here’s an easy way:
1. Copy the background layer twice
2. On the top copy, make your changes. Change the blend mode to Exclusion. Everything which is the same between the top and middle layers goes pure black.
3. Merge the top two layers. You now have a mostly black layer in Normal mode.
4. With the tolerance of the Magic Wand set to 0, click a black area. All the unchanged parts of the image are selected.
5. Delete the selection. What remains is the content of the top layer (from step 2) which is different than the original image.

Tom Nelson
Tom Nelson Photography

In article , David Dyer-Bennet
wrote:

I want to create a layer whose content is the difference between two other layers. Pixels not changes should be transparent. There ought to be some way to do this with "apply" or "calculations". In fact it looks like I can take the difference between two *channels* in calculations and get what I want. So with sufficient time and effort I could build up a layer from that. And if that’s the only way I could record it as an action to make it simple. But first I’m wondering if anybody knows an easier way?

(This is to support a particular photo editing crotchet of mine; I like to leave the background layer unmodified, and make all the retouches and curves adjustments and such using layers or adjustment layers. This allows me to *change my mind* later without loss, among other things. But there are situations where using blending modes for retouching makes a huge difference. Currently I rubber-stamp or layer-via-copy the areas I need to do that in onto my retouch layer and then work on them there, but that leaves me with lots of unaltered pixels duplicated on the retouch layer, so it’s inelegant. This difference between layers thing would let me edit a background copy relatively freely, and then extract the differences.)

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