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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 I am going to introduce you to luminance blending inside of Photoshop which allows you to either drop out luminance levels inside of the active layer or force the display of luminance levels inside of the layers behind the active layer. And we are going to see how this works using a couple of other images. We are going to take a break from our composition in hand from this almost blue image and we are going to take a look at these two guys right here. Big cloud.jpg and Lightening.jpg. They are both found inside of the 10_Advance_Blend folder and what we want to do is we want to take this lightening as captured by a photographer who goes by the handle Subret at istockphoto.com.
We want to composite it against this image here of the clouds from photographer Don Wilkie. Not only we do want the lightening to look like it's coming from the clouds, but we want to intertwine the lightening inside of this clouds as well. So we want to create a completely all together credible composition here. So how do we go about masking lightening? Well, once upon a time you know before chapter 09 when you mask that fantastic hair image, you might have thought that lightening was a little bit daunting. But now you know it's no big deal right. We can go ahead and switch over to the Channels palette here as I will do and then we could find the high contrast channel which in the case of this image is a Green channel.
We could go ahead and duplicate it by dragging it down to this little page icon at the bottom of the Channels palette. We could rename it mask and then we could bring up the Levels dialog box by pressing Ctrl+L or Command+L on the Mac and then we would say, you know what, we want the lightening to be a lot lighter, but we want the background to be way the heck darker, and there is our mask. It's basically all it takes, right? Then we click OK and we would go ahead and convert that mask into a selection by Ctrl or Command clicking on it, here inside of the Channels palette. Then we will switch back to our RGB image.
Maybe copy the lightening by pressing Ctrl+C or Command+C on the Mac. Then we will switch back to the clouds image and then we paste it by pressing Ctrl+V or Command+V on the Mac like so. And that's actually darn easy to do, but the problem is then we are going to have the blend it, because now we have all this color fringing really kind of shadow fringing going on around the edges or the lightening, and as it turns out this is the harder way to work. Even though wasn't really all that difficult to mask the lightening. Now that we know some of the fundamentals of masking.
It still more work than we all of be doing, because we can take care of this lightening in its entirety without any masking whatsoever using luminance blending. So check it out. Let's go ahead and undo the pasting of that lightening into the clouds image. Let's return to the lightening image, press Ctrl+D or Command+D on the Mac to deselect it. I want you to press Ctrl+A or Command+A on the Mac in order to select all of the image and then press Ctrl+C or Command+C on the Mac in order to copy it. So Ctrl+A, Ctrl+C on the PC.
Command+A, Command+C on the Mac. Let's return to the clouds and do a Ctrl +V or a Command+V on the Mac. Now these images are pasting exactly in the place, because they are of the same size. The lightening in the Big cloud image are of the same width and same height in pixel so they have the same pixel dimension. Now I am going to switch back over to a Layers palette. Let's go ahead and rename this layer lightening. All right, now it's time to start blending these two images together. Step number one is to drop out the dark stuff and keep the light stuff and if you want to keep the light stuff in the image and you want to drop out the dark stuff, the most obvious screen mode has got to be a lighten screen mode right. One of the screen mode inside the lighting family and it's got to be stream. So we will press Shift+Alt+S or Shift+Option+S on the Mac to apply the screen mode and you can see how much work that one mode does by itself. Totally nondestructive, we are not a single pixel inside of either the lightening or the cloud image so we could always change your minds later, if we wanted to.
Next, we need to drop that more of the darks, because we have this overall lightening effect from the lightening layer. We need to drop out more of the darks and we are going to do that using luminance blending. To get to the luminance blending options inside Photoshop, you double-click on the layer. There are actually couple of different ways to do it by the way. You can either double-click on the thumbnail in this case or on the empty area of the layer. I want to double-click on the names, because that allows you to rename the layer right, or you could go down to the fx icons and you could choose blending options. So either of these work. Every single one of these techniques will bring up the Blending Options panel of the gargantuan Layer Style dialog box here.
Down here at the bottom, these two slider bars represent the luminance blending options inside of Photoshop. That's not what they are labeled, but that's what they are. And the This Layer slider bar, I sound ridiculous when I say, the This Layer slider bar, but that's what it is. The This Layer slider bar controls which luminance levels are visible inside of the active layer, the Underlying Layer slider bar which should really be called Underlying Layers, plural, controls which luminance levels are visible inside of the composite view of all of the layers below the active layer.
All right, so we are going to play with the This Layer slider first here. Notice if I drag this white slider triangle right here, I am making light pixels inside of the layer transparent. So I am saying anything that has a luminance level of 123 or lighter, make it transparent, and anything that has a luminance level of 0 for Black up to 123, go ahead and make that visible, go ahead and keep it visible. Well, that's exactly the opposite of what I want to do, right because now I am revealing the inside of the lightening bolts, I am making them transparent, but I am leaving the exterior of the area outside of the lightening visible. Well, I really want to get rid of that stuff. So let's go ahead and make the lightening visible once again by dragging this white slider all the way over to the right and let's make the background invisible by dragging the Black slider over to the right.
So in this case, I am saying that anything that has a brightness value for 129 or darker, make then invisible on the active layer. Anything that has a brightness value of 129 up to 255 which is White, leave that visible subject of course to the Blend mode settings and the Opacity settings as well. Now that ends up producing some very jagged transition. If you zoom in on your layer, you will see that you have all kinds of jaggies at work inside of the lightening bolts.
So we need to soften the transitions by introducing some fuzziness. Remember fuzziness from the Color Range dialog box. Well, we have got fuzziness associated with the luminance blending options as well. But getting to that fuzziness, involves a little bit of trick. If you look very closely at this triangle, this black triangle right here, on you screen. You probably can't make it out on the video. But there is a little cleft in the middle. There is a little line right down the middle of the triangle, and what that's telling you is it's really two triangles in one. You can break these triangles apart and you do that by pressing and holding the Alt key where the Option key on the Mac and dragging the triangle into two pieces like so. And once you drag it apart, you can see that you are introducing fuzziness to your image, to the active layer. So we no longer have those jagged transitions.
I am going to go ahead and move the first half of the triangle here on the left side of the black triangle to 85. you don't have new miracle control incidentally. You can't enter a number. But you can, of course, track the numbers here inside of the dialog box, just keep your eye on them. And I am going to move the right half of the black triangle over to 175. So in this case, when I am saying is anything that has a brightness value of 0 through 85, make that absolutely transparent. Anything that has a brightness value of 175 up to 255, leave that opaque subject to the Blend mode and the Opacity settings and then anything that has a brightness value 85 to 175, ramp it up.
So ramp from Transparency to Opacity, so that we have a nice, soft transition inside of our image. Now I am going to go ahead and click OK in order to accept that modification. Let's zoom out from the image, so we can take in the entire thing, Shift+Tab away the palettes and you can see this is what the lightening looks like now. This was before subject to just the screen mode and nothing else and this is the lightening subject to luminance blending here inside Photoshop.
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