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The elusive alpha channel remains one of the most misunderstood yet powerful tools in Photoshop. Alpha channels are collections of luminance data that control the transparency of an image, and they inform just about every aspect of Photoshop. As he builds transitional blended layers, fashions a depth map, makes edge adjustments, and takes on extreme channel mixing, Omni Award-winning expert Deke McClelland teaches Photoshop users that where there's a will, there's a way. Photoshop CS3 Channels and Masks: Advanced Techniques covers mapping texture on an image, turning flesh into stone, using vector masks, working with all different channels, creating a rustic edge effect, and much more. Exercise files accompany the tutorials.
Download Deke's customized keyboard layouts for Channels and Masks from the Exercise Files tab."
If you were with me in the previous exercise, then you will have drawn a boundary around the good area of the final Levels iteration, so that we can combine it with the previous, the first Levels iteration, and we drew that boundary using the Lasso tool and then we feathered it with a radius of 6 pixels. So that's what I am looking at right here. I am working inside incidentally, that same image I opened in the previous exercise, the one that's called Levels iterations.tif. Now, that I have drawn the selection, I am going to go ahead and save it out as an Alpha channel and I am going to do that, first of all, I am going to collapse my Color palette here, so I have a little more room to work. Then I am going to click on this Mask icon down here that allows me to save the selection as a channel.
I am going to go ahead and call this one Iteration Boundary because that's what it is after all. Now, I am going to leave my selection outline intact because I want to show you two different ways that we can work at this point. So let's go ahead and click on 20/0.6/235 in order to make it active, and then I want you to go up to the Image menu and choose the Calculations command. This time around, let's see if we can move this into a location where I can see what I am doing. This time around, I want you to make sure that Source 1 and Source 2 are both set to the same Levels iterations.tif document or whatever the name of your document is. The layer should be background of course because there is only one layer in this image.
Both of the channels are showing up as the active channel right here. I want you to change the second channel to 160/1.3/235 that is the second Levels iteration, and I want you to change the Blend mode to Normal. Now, that may seem like a strange thing to do, what we are seeing is just the Source 1 image. So we are just seeing the 20/0.6/235 channel until we go ahead, and turn on the Mask check-box which allows us to do something different inside of a different portion of the image. Now, that different portion of the image needs to be identified by a channel.
So this Image option right here so once again be set to Levels iterations.tif or whatever the name of your image is. Layer should be background, the Channel however should be set to either selection or iteration boundary. You are going to get exactly the same results either way. So you can just work with the selection outline if you want. I will go ahead and choose selection. Notice what happens in this case, the selection is enclosing the first source channel, and the area outside of the selection is being set to Source 2 here which is exactly the opposite of what we want. So I could reverse the channels; I could set Source 1 to the 160 channel, and Source 2 to the 20 channel, in other words, the first Levels iteration, or I could more simply solve this problem just by turning on the Invert check-box and that gets us the result that we want.
So in other words, the first Levels iteration is outside the selection outline, the second Levels iteration is inside the selection outline, and just to show you here, I could either work from channel B and set to selection or iteration boundary. I will get exactly the same results pixel per pixel which may make you ask well, then why in a world did we save off the selection as iteration boundary as yet another Alpha channel? The reason is, what if we need to come back to it, what if we need to use this selection again in the future, might as well have it handy. But, I want to show you, you can work with the selection outline if you want to. So that's how that Mask check-box works in case you are curious, can be very helpful at times.
Now, go ahead and click OK, and notice that you create yet another Alpha channel, that's good to go, that's got the best of the two iterations. Let's go ahead and call that one Iteration Merge right there. Press the Enter key, and we are ready to modify the channel. That will become the base for our further manipulations here. I am going to press Ctrl+D or Command+D on the Mac to get rid of the Selection Outline because we don't need it anymore, and it's already saved out as this Alpha channel right there. In the next exercise, we will perform some familiar modifications to this Mask using the Dodge and Burn tools.
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