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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 see the effects of editing a continuous tone full color digital photograph in both the 8 bit and 16 bit per channel spaces. Now we are going to see how 16 bit per channel actually provides us with the benefit even if we start things off with an 8-bit per channel JPEG image like this one right here. This image by the way is called Max by water.jpg found inside the 17_16-bit HDR folder. It's a photograph that I shot off my eldest son Max here dipping his toes into Lake Powell and it's actually a beautiful photograph. It's got great colors. We just need to bring those colors to life because right now the luminance is really muted. So we need to increase the contrast and if I were to press Ctrl+L or Command+L on the Mac to bring up the Levels dialog box. You can see that we have a very smooth histogram. The problem is it's all bunched up in the center right there. All the luminance levels are bunched up in this relatively small space.
So we need to bring them out. We need to stretch them apart and we are going to be doing that in this exercise of course. So I am going to go ahead and Cancel out of this dialog box for now and we are going to see the effects of applying our modifications in both 8 bit and 16 bit as I say. So let me Shift+Tab away my palettes and I am going to make a duplicate of this image by going up to the Image menu and choosing the Duplicate command and if you want to see the effects of these modifications you want to do the same thing. So choose the Duplicate command and I am going to go ahead and call this Max 16-bit or something along those lines.
Bear in mind that this is 16 bit so the information per channel per pixel. So on a channel by channel basis. The 8-bit per channel image is really 8+8+8, so it's 24 bits of the information per pixel where as a 16-bit is really 16+16+16 so we have got 48 bits of information per pixel. So I will go ahead and click OK. That's just a function of working inside of a full color RGB file and I am going to go ahead and move Max over a little bit and go to the Window menu and choose Arrange and then I am going to choose this command right there, Tile Vertically.
So we can see the images side by side and I will go ahead and zoom in on Max over here on the right and I need to convert him to 16-bit. So I am going to go up to the Image menu and I am going to choose Mode and I am going to choose 16 Bits/Channel. Now that doesn't spontaneously generate more color information. It just gives you a bigger room to work in. So I frequently make the musical chairs analogy, basically I will go ahead and choose this command here 16 Bits/Channel. Same darn image we have before and the image on left, we have got 256 different chairs and if the color is have to run around the chairs and we take one of the chairs away then two of the colors have to sit on the same chair and one of the colors have to leave the building.
Basically we are harming the image every time we apply a color correction is what it comes down to. If we are working inside of this image we still only have 256 different luminance level per channel but we have got 32000 chairs in the building. So we could sit there all day and take chairs out of the room and you know all the colors get to run around as much as they want. They still get to sit down in independent chairs. So that's all what it does. We just added more chairs to the room essentially inside of this image on right. The number of luminance levels that are occupying the space is still the same. But check it out. Let's go ahead and apply a couple of pretty big corrections here. The first thing I am going to do and I am working on the 8 bit/channel image on left.
I am going to press Ctrl+U or Command+U on the Mac in order to bring up the Hue/Saturation dialog box and I am going to increase the saturation value to 30 okay and you might want to do that as well. Just to bring out some more saturation values here and then click OK and then I am going to go into the Curves dialog box by pressing Ctrl+M or Command+M on the Mac and I am going to go ahead and move this white slider over to an Input level of 170. See that right there.
So 170 is going to 255 and then we will take this black slider and drag it over to the right until the input value becomes 30. So 30 is becoming zero. So we are stretching out the histogram of course. Now that makes Max appear a little too pink in my opinion. So I am going to switch over to the red channel here and I am going to drag the middle. I am going to click in the middle of that line and drag it down slightly in order to remove some of the pink from Max, just a little bit actually. Because we still want a healthy amount of pink in the boy because he is my son. So he is you know fairly pale, except when he gets out he was actually looking fairly sunned in this case but also these rocks tend to be very reddish as well.
So we don't want to subtract too much red and then I am going to go over to the green channel and I am going to click in the center and I am going to drag that up a little bit just to add a little tiny bit of green to the image here and we might want to add a little bit of blue too. You could go ahead and switch over to the blue channel here, then add a little bit of blue if you want to, just to increase the saturation of the blue water here. Then I am going to go back to RGB and really you can apply any modifications you want. These are just ones that I am suggesting. I am going to go ahead and switch back to RGB and I am going to click right about there on the graph and I am going to drag it down in order to darken things up. Fairly precipitously I want to add some heft to this image, make it darker and I think actually this looks pretty good. If you ask me it does. I will go ahead and click OK in order to accept that modification and you can see that we have got this little hint of brown highlight going on from the rocks reflecting in the water. So it's a good looking image actually once we get done modifying it.
But it's a bad looking histogram. Let's press Ctrl+L or Command+L on the Mac to checkout that histogram and you can see that we have got a lot of gaps in this histogram. Meaning that we have a lot of gaps in this histogram meaning that we have a lot of choppy transitions inside of the image. So there is some possible posterization, there might be some color banding as well. Who knows what? But a kind of worst of all in my opinion. If I were to hand this off to a client for example. They would be able to see that I have been in this file and that I applied you know some forms of static color modifications to the image and that I have got a bunch of gaps inside of my histogram. And I am not so sure. I like to share that with clients or anybody for that matter.
I like a nice smooth histogram even when I give an image to you by the way, a little secret. They have got some nice smooth histogram that's because I have monkeyed with the colors carefully. All right, so I am going to go ahead and cancel out of this. Let's go ahead and replay those very same color modifications here inside the 16 bit/channel image. I am going to press Ctrl+Alt+U or Command+Option+U on the Mac in order to replay the Hue/Saturation modification, of saturation plus 30, click OK. Now I am going to press Ctrl+Alt+M or Command+Option+M on the Mac in order to bring up those same Curves modification right here that we have applied before and I will click OK in order to apply the command and you can see it looks the same. Once again this is the same image for all intensive purposes plus we started from the very same file right.
And they look the same on screen because we are viewing the image on essentially 8 bit/channel device. But I am going to press Ctrl+L or Command+L on the Mac. Look at that histogram. It has been modified. It's got some fillers going up there. So it has some gaps on the top end of the histogram but the larger body of the histogram is still in great shape which is basically demonstrating to us that we have smoother transitions. Just by virtue of the fact that we entered 16 bit and applied our color modifications in the 16 bit/channel mode and I will go ahead and cancel out of there.
What I would do at this point. After I have applied my color modifications and I know I am done with those color modifications, I would go up to the Image menu. I would choose Mode and I would choose 8 Bits/Channel in order to convert this image back to an 8 Bits/Channel image because it's silly to leave it in 16 bit; it is not providing us that much utility and that reduces the file size. Notice before I will go ahead and point down to this lower area and 16 bit this image was 46 megs so pretty darn big. After I convert it back to 8 Bits/Channel we can see that it's 23 megabytes essentially and I will press Ctrl+L or Command+L on the Mac. The histogram ends up looking smooth or still.
Now that we have converted back to 8 Bits of data per channel and therefore somebody is going to look at this histogram. That's a the kind of histogram you get with an original unmodified image. No body is really going to know for sure that I have been here and that's what I want. I don't want you to know I have been here. You don't want me to know that you have been there. You know we don't need to know this about each other. All right, anyway we also have smoother color transitions inside this image. So we are going to have less risk of banding and posterization when we go to print. So the moral of the story, even when you are modifying an 8 Bits/Channel image, the JPEG image here, for example, it's a good idea to convert it over to 16 bit color, make your modifications and then convert it back to 8 Bits/Channel and if all you are doing is just modifying the colors in the photograph in order to make them look better. You better off applying those modifications as static modifications from the Image Adjustments menu right here then you are using adjustment layers.
So the technique I just showed you is better than adjustment layers if all you are doing is correcting a standard photograph. All right, in the next exercise, I am going to show you how it gets even better if we open the image into 16 bit in the first place. If we actually start with a 16 bit/channel image.
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