Help with transparent masking

[Tannin]

Hi all, pretty dumb question here. Sorry about that.

How do I copy empty space in Photoshop? Here is what I mean. I have this so far:

Layer 1: colour map
Layer 2: copy of Layer 1 but in monochrome
Layer 3: copy of Layer 2 with the 1st area of interest erased. (It's a river basin I'm going to be presenting a talk about and showing pictures of.)
Layer 4: copy of Layer 2 with the 2nd area of interest erased. (Another river.)
Layer 5: copy of Layer 2 with the 3rd area of interest erased. (The third river.)

Now I can, for example, turn off all layers except 1 and 4, and "save visible". This provides a monochrome map with the 2nd interesting river valley highlighted in full colour: exactly what I want to illustrate the part of the lecture about that river. Or turn off Layer 4 and replace it with Layer 3 to have the first river valley showing in colour against the greyed-out background, and so on.

Finally - and here is where the problem crops up - I want the grey background but with all three rivers in colour at the same time. This doesn't work! It just looks like Layer 2 and everything is grey. I could copy Layer 2 to make Layer 6 and re-draw (well, re-erase) all of the rivers. That would certainly work, but surely there is a way to get Photoshop to do it for me? What I need is a copy of Layer 2 minus the erased parts of Layers 3, 4, and 5. I imagine that there is a simple way to do this, I just can't think of it.

TIA,

Tony

[ameerat42]

You use layers in Photoshop and you can save them as PSD files, with all layers intact.

You can merge layers that you want to and have others separate.

You should be able to do what you say, even if you then save that as a flattened file (like jpg).

It's an interesting problem which I will try out in a while.
Am.

Oh, and PS: You can Duplicate Layers. Select the layer you want and (here I use the Layers Palette rather than the main menu) duplicate.

[Tannin]

Thanks ameerat. Yes, I'm saving as a PSD file for the master (complete with layers), I'm just making an individual single-layer TIF or JPG file for each of the different views I want for each section of the talk (to mix in with my photographs of the area).

Merging layers, however, is where I strike the problem! I'd better do an example. I'll use an ordinary photograph instead of my map to stay within site copyright rules. Let's imagine I wanted to talk (for some reason) about this scene:

http://tannin.net.au/other/ap/404base.jpg

^ First I show the whole scene, as above.

http://tannin.net.au/other/ap/404mono.jpg

^ Next, I copy the base layer and convert it to monochrome (this is just a working step, I don't use this in my presentation).

http://tannin.net.au/other/ap/404creek.jpg

^ Now I want to talk about the creek, so I do this. (Copy of the mono layer on top of the base layer with the area of interest in the top (mono) layer erased to let the colour show through.

http://tannin.net.au/other/ap/404road.jpg

^ Now I want to talk about the road so I do this, using another copy of the mono layer and again erasing part of it.

http://tannin.net.au/other/ap/404merged.jpg

^ Trouble! Finally, I want to draw the threads of my previous remarks together and talk about both the road and the creek. (In reality, I want to consider all three river systems as a single giant interconnected system. But the road and creek thing will do as an example.) So with the colour layer still underneath I switch on both road layer and creek layer .... but of course, all I see in colour is the part where the erasures overlap. What I need is a way to tell Photoshop to merge the two layers so that it looks like this:

http://tannin.net.au/other/ap/404aim.jpg

^ The desired result. I faked this one just to make an example, but what I need is a way to get Photoshop to do a sort of "reverse merge" of the layers, keeping the content only where both layers show it. I'd be amazed to discover that it isn't possible, I just can't figure out how to do it.

[Steve Axford]

Or PNG files. No layers required and empty is empty.