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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."
All right folks, so far we've seen how 8-bit per channel compares with 16-bit per channel. We've seen some of the benefits that are available to us, when we are working in Photoshop 16-bit per channel mode. Now, let's compare 16-bits per channel to 32-bits per channel. I was telling you at the outside of this whole thing in a live-action piece, how that essentially boils down to several octillion colors inside of an image when you working in the 32-bit per channel mode, which is of course ridiculous. We don't need several octillion colors available to us. What's really great about 32-bit color is we have floating point control, meaning that we can violate our highlights and shadows. It works like this, going back to the analogy with the people in the room playing musical chairs, and of course, the people are the luminance levels.
So we are in this gigantic room, full of chairs in 16-bit per channel mode, and there is more chairs than there are people running around, playing musical chairs. So the people get to always sit down. In 32-bit per channel mode, there is additional rooms full of chairs. It's just a building filled with chairs in different rooms and the people can walk through walls is basically what it comes into; they can move through the walls to get to other chairs if they want to, and of course, just have the octillion chairs out there.
What does that really mean? Well, it means that you can violate white; you can go harder than white, and bring it back later. You can go darker than black and bring it back later. So let me show you what it means. I am working inside of an image called Sam walks cliffs.tif and by the way, I should preface is, this is not a technique based exercise, this is just theory. I am just showing you what's going on so that you understand how the mechanics of 16-bit and 32-bit work. So here I am, looking in an image called Sam walks cliffs.tif, it is a 16-bit per channel image. I did originally open it from a DNG file, brought it in the Camera Raw, processed it, and then, opened it here inside Photoshop in the 16-bit per channel mode. It's available to you inside the 17_16bit_HDR folder.
All right, and this is a picture of my other son Sam. If I show Max, I got to show Sam, it's just the way it is, right? I can't play favorite to, and kind of a cool shot, actually that is shot with a little Leica Camera. I like it, my Leica, and here is what I am going to do. I am going to modify this image, I am going to apply a severe modification and then unapply it, using the Levels command, two applications of the Levels command. All right, so I am going to press Ctrl+L or Command+L on the Mac, and I am going to squeeze the Output Level. So I am going to raise the black output level up to 150, and I am going to reduce the white level to 245. And imagine that, I want this to appear as a background to some text, that I am going to add here inside of Photoshop because I am putting together family album or something along those lines, and this is the opening graphic for the whole thing.
So I am reducing the Output Levels values. I am saying that the darkest color is 150, the lightest color is now 255, so I have compressed the histogram into this space, right there, I'll go ahead and click OK. You can see the result of my modification in the background. Now, let's undo it using the Levels command. Of course, this is not the way you would normally work, but let's say I have applied several operations and then I think, no, I want to recreate the original photograph with all its rich array of luminance levels. So I press Ctrl+L, or Command+L on the Mac. I see that I have got a squeezed histogram here. I'll go ahead and enter those same values that I entered for the black and white point Output Levels into the black and white point Input Levels, so I'll change the first value here to 150, Tab, Tab change the last value to 245, and I'll click OK, and you can see that I have restored the image, there is no banding, there is no posterization, there is no colors pulling apart here.
And in fact, if I press Ctrl+L, or Command+L on the Mac, I see that I have got a nice rich histogram, still there is a few hairs at the top, there is a few spiky-poos at the top, which indicate that we are stressing the image a little bit, but not nearly like we would, if we were working inside the 8-bit per channel mode, but now watch this. If I decided to once again apply those same values, 150 and then, 245, now I am clipping all of these colors, I am saying anything that's black, anything that's 150 or darker make blacks, so that's all going to get clipped, and anything that's 245 or lighter is going to be made white, so that's going to get clipped too, and as a result, Sam looks like he is a world of lava or something.
I don't want him to be in a world of lava. So I am going to press Ctrl+L, or Command+L again, and I think, maybe I could do that whole thing with the Output Levels value, change it to 150, and then, this back to 245, but it doesn't work, because when you violate black and white in 16-bit per channel, it's just as bad violating black and white in 8-bit per channel. So you have clipped colors. We've clipped a lot of shadows, a ton of shadows, and we've clipped a lot a highlights as well, and we've done so permanently unless, of course, we go back, we back step by pressing Ctrl+Alt+Z several times in a row, or Command+Option+Z on the Mac.
All right, so let's go ahead and cancel out of this. I am going to press the F12 key in order to revert this image to its original appearance. So we can see what happens in the 32-bit per channel mode. I am going to go up to the Image menu, and I am going to choose Mode and I am going to choose 32 Bits/Channel, and again, this is not a technique, I am not suggesting you do this with an image, it just something that's possible and it demonstrates how 32-bits per channel works, and what the enormous benefits are. I am going to go ahead and choose the command. Now we have a 32-bit per channel image, and I want to show you something. I am going to get down to this right pointing head in the bottom left corner of the Image window, and I am going to choose Document Sizes, so that we can see this is the size of the image before I upsampled it. So this is the 16-bit per channel image, about 57 MB.
Once I upsampled or upsized it really to 32-bits of data per pixel, per channel, the image now grows to 115 MB in memory, and it's a 10 MP image. By the way, it's not the most enormous image on earth, and yet it is now more than 100 MB of memory, and when we go to save the image, it's going to be ultra mongo large as well, but check this out. It's almost like, it almost defies belief, I'll press Ctrl+L, or Command+L on the Mac, in order to bring up the Levels dialog box, and I'll change the first Input Levels value to 150, and I'll change the second one, the last one that is the white point value to 245.
And so I am clipping, right? I am clipping everything from 150 over, no, those people, and their luminance level of course, but those people just walked through the wall and went to a neighboring room full of chairs. They're still hanging out waiting for us, they just left the building. The building is only this big, right now, but there is some folks that are in neighboring room, so over here, and over here. All right, so I'll show you. I am going to go ahead and click OK. The image looks terrible at this point, right? But watch I'll press Ctrl+L again, Command+L on the Mac, there's little nothing of a histogram left to us, we've got clipped colors over here on the left-hand side, clip shadows, and we've got clipped highlights over in the right, and yet, watch. Watch and wonder, 150 for the first Output Levels value and 245, we called them in, we just knocked on the walls and we said, come back in this room, and they did, and we click OK.
And check it out, everything is back, it's -- I know it gives you goose bumps. All right, I am going to press Ctrl+L or Command+L again to bring up the Level dialog box, and you can see there is our original 32-bit per channel histogram. All right, so it's amazing. So again, that's just the demonstrational thing, just because I can, I am going to fill the screen with the image, tab way the palettes, and show it you. I haven't really done anything to this image. I just like it because Sammy, he was only 4 years old, and he walks up this huge wall like this. I am very proud of him. Of course, he has got his crocks on, that makes him sticky.
But anyway, that's how the 32-bit per channel or HDR mode works. This is the same mode; by the way, it's HDR, High Dynamic Range. We will see some practical uses, or some semi-practical uses I would say, starting in the next exercise.
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