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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."
In this exercise we are going to apply the Merge to HDR function, from the Bridge to the sequence of auto-bracketed Raw digital photographs right here. So I am working inside of the Bridge, I've got the Bridge trained on the 17_16bit_HDR Folder. And we've got three DNG image in a rowm Lake Powell_1.dng, Lake Powell_2.dng, Lake Powell_3.dng. Go ahead and click on 1 and Shift+click on 3 to select all three of them. Then I want you to go up to the tools menu and choose Photoshop, and choose this guy right there, Merge to HDR.
Now this is not a fast command. So get ready to watch them paint dry here. You will see a few things happen, first of all of course Windows happily displays this irritating cursor that's telling as they were waiting. It also the cursor you see when the program is about to crash which is a little deceptive. And if you've go to Layers palette up you'll see Photoshop building layers inside the Layers palette, so it's actually adding each one of the images to a layer document, and it's calculating how the images should blend together.
So eventually, it's going to go ahead and run the Auto Align function, and when it does you'll see a progress bar appear on-screen. There it is right now, telling us that it's aligning each one of the selected layers based on its content, that is the Auto Align function, it's very important in our case, because we do have some movement, in the scene. There is a little bit of camera shake, thanks to the fact that I had not tripoded the shot. Now, we are speeding up the process just a little bit so it's going to go slower on your computer. This is actually pretty darn fast machine and it's still not fast enough for us. We want to be able to show this a little bit more quickly to you.
But eventually, you are going to see this dialog box here. In fact this is not really dialog box since it takes up our entire screen, it's more of a window. And one of these it's just a preview of what Photoshop is about to create for you. Nothing too real is going on inside of this dialog box. One real change you can make is to take one of the images out of the loop, one or more of the images out of the loop. So if you have an area of severe banding for example in the sky or you've got some weird color drifting happening in the wall or something along those lines, something that -- some flat surface or gradient surface is going weird on you, then you can try turning off these check boxes to see if that solves the problem. Know however that turning off a check box is a slow process, you turn off the check box and you sit on your hands, and you wait for a moment for Photoshop to go through it again and figure out how the images are going to blend together.
What I am going to do in this image, because we only have three images to work with in the first place, and you can by the way -- you don't have to just merge three images, you can merge like six images, a dozen images, in which case you probably want to be more careful about your circumstances and tripod the shot and so on but you can do that. It just depends on the level of accuracy you want. So more images gives you more accuracy typically, but it also forces you to wait longer as well. Now, we have over here on the right-hand side, we have this White point preview item that shows a Histogram. Well, this is just a preview. Bear that in mind it's just a preview that is allowing you to see where the detail is inside of the image. So if I crank this slider down, I am going to move the white point down to the shadows so that I can see what kind of shadow detail I have available to me down here in the boat region.
And so I can see that it's pretty much all of the detail I would ever want. As I drag down I've got these tiny little shadows left there. So we've got a lot of shadow detail inside this photograph, that's great. And if I drag all the way to the right I can see that I've got an awful lot of highlight details. Well, now it's possible. I am going to go ahead and zoom in here, and I am also going to press and hold the Spacebar so I can get the Hand tool. It's possible we might be losing some detail up here in the farthest corner of the sky, the upper left corner of the sky. As it turns out, we are not. Once we actually put this image together, you'll see that we have tons of sky detail up there. But according to this, I should say fairly misleading preview, it's not always super accurate. According to it we do have a loss of sky detail.
But any way what I suggest you do is just go ahead and move this preview wherever you want, it doesn't matter, is actually what it comes down to. It doesn't matter where you leave this slider. It will inform however what the image looks like when it's first put together inside of Photoshop, but it has no permanence associated with it whatsoever, again it's just a preview function. Just to demonstrate; just how non- representative it is, I am going to move it way over here to the left-hand side and we'll go ahead and zoom back out so that we take in the image, and that of course makes the image look like there is some kind of horrible nuclear winter going on, that's okay, as you'll see.
And then make sure that your Bit Depth is to set to 32 Bit/Channel, that's very important. We don't want to scale down to 16 Bit or 8 Bit, because after all if we did, then we can't let the Luminance levels move through the walls anymore. And that would mean that all of these colors right here that have gone hot on us, that had been clipped to white will stay clipped to white. But if you set your Bit Depth to 32 Bit /Channel you are fine. So go ahead and click OK at this point. And Photoshop will now build the files actually creating a single layer file this time around incidentally. Even though it threw all of those layers on the stack here inside the Layers palette, now it's going to be a single layer file as you are seeing. And it's going to look like, did you see that change? You saw that little change there inside of the Layers palette preview, that thumbnail updated. And that's because it went ahead and changed the preview to the setting that I specified inside the previous window.
All right, so obviously that's wrong, the image looks horrible, that's okay, however. We are going to fix it very easily, because nothing is broken. We are just going to change the preview and begin to work on a file in the next exercise.
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