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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 the fundamental concepts of a Displacement Map, and then in subsequent exercises we will see some cool uses for them. Now, a Displacement Map is a file whose luminance levels move pixels inside of another image and as a result, you can achieve these custom distortion effects. Now, it is a once a very primitive function and a very powerful function. It's primitive because it was introduced in Photoshop 2.0 and has never been upgraded since. It's one of those orphaned functions that Adobe decided was good enough when they made it and they have never -- nobody has ever had the desire to revisit the feature, even though it needs some enhancements frankly.
It doesn't have a preview for example, as we will see. But it's also very powerful because nothing has taken its place. We have got all kinds of other very powerful distortion function such as, Liquify Filter and golly! You have got Warp Transformations. But neither of them do exactly what Displacement Maps do. All right. So let me show you what's going on here. I have got two images open. One is called Rough trade.psd and it's found inside the 18 Displace maps folder. This is an image that I put together based on a few found images that I rounded up and my own nefarious skills. I did a lot of work on them as well. I made all the colors and stuff. And then we have got this other file that I have open, and this will serve as the Displacement Map.
It's found, you don't have to open this one actually. But it's called B&W.psd and it's found inside the Demap sub- folder that's in the 18 Displace maps folder. And here is what's going on with this file. This is a single channel file. If I bring up the Channels palette, you can see just one channel. Oops! There is the Channels palette. Just one channel, Gray. So it's a grayscale image. All right, I will go ahead and put that away and we have got just three luminance levels going on. We have got medium gray, which is neutral, does nothing and we have got white, which moves pixels up and to the left, and then we have got black, which moves pixels down and to the right.
Now black and white move pixels based on their difference from medium gray. So consider that medium gray has a brightness value of 127, 128, something around there, right. And black has a brightness value; a luminance level of zero and white has a luminance level of 255. And so at most, black will move colors inside of an image, 128 pixels, that is at a 100%. A 100% movement would mean that the pixels inside another image are getting moved 128 pixels down and to the right. And white would move colors a 128 pixels up and to the left, at 100% and we will see what that means in just a moment.
But the reason its 128 is because black is 128 away from medium gray and white is also 128 away from medium gray. So luminance levels, they all move pixels based on their difference from medium gray. Okay, and this will make more sense overtime. You don't want to get to worked up about exactly what the distances are. But you should know what's going on under the hood here. All right. Lets return to the Rough trade image and let's make sure that -- I will go over to the Layers palette here. Let's make sure that the Background layer is active. It is. That's good. And then I am going to go up to the Filter menu. I am going to choose Distort and I am going to choose Displace.
And it brings up this dialog box and you can see what I am talking about. There is no preview. There is no way to see what your effect is going to look like until you go ahead and apply it, which is kind of a pain in the neck. But we have got a Horizontal Scale value and a Vertical Scale value, and these are measured as percentages of that 128 pixel thing I was telling you about. So at 100% for Horizontal Scale, you would move pixels either to the right or to the left as much as 128 pixels. That would be you maximum that things would get moved.
So white would move the colors in one direction and black would move 128 pixels in the other direction. And same with Vertical Scale. It would be down for black and up for white. And you can send these values. It might seem like, wow!, the most you can move anything is 128 pixels. That doesn't seem like it would give you a lot of wiggle room and high resolution images. Well, you can set these values high as 999%, so you can bump it way up there. So you can have thousands of pixels of movement, or more than the thousands of pixels of movement I should say. And then you can also go as low as -999%, if you want to move the black areas up for example, or the white areas down, that kind of thing.
All right. What we are going to do is just leave it set to, leave both values set to their defaults which are 10% a piece. We also have these options here. What do you want to do if the Displacement Map is not as big as the image that you are working on. Now our Displacement Map is exactly the same size as the American Flag image. So it doesn't matter what we set this option to. But if its smaller, do you stretch it to fit. If the Displacement Map is small, do you stretch it to fit or do you tile it, repeat it over and over again? All of our examples, all the examples I am going to show you are going to be Stretch To Fit examples incidentally.
And in undefined areas, the edge pixels basically, should you repeat the edge pixels or should you wrap them around? Should you basically wrap pixels from one side to the other side? We are going to leave these set to their defaults. Stretch To Fit and Repeat Edge Pixels and then click OK. And you will next be invited to choose a Displacement Map because you have to load a Displacement Map. Now here is something to note about Displacement Maps. They have to be saved as .PSD images, so native Photoshop documents, and they have to be flat files. No layers are permitted. They don't have to be grayscale although this guy is a grayscale image. But they have to be flat and they have to be native PSD. They have to be flat because consider Photoshop 2.0 which is where Displacement Maps were introduced and abandoned of course, didn't support layers. Layers didn't come around until Photoshop 3.0 and then it has to be a .PSD document for, I have no idea what reason. I don't know why it can't be a TIFF? But it can't.
So anyway, go ahead and click on B&W. psd in order to select it and then click on the Open button and a moment later, you will see the pixels shift slightly. Now it's not a big shift. Just notice that the pixels, this group, this little block right here, moved slightly up and to the left, and this block right here, moved slightly down and to the right. That's all that happened there. I am going to go ahead and repeat that over and over again a few times. So Ctrl+F, Ctrl+F, Ctrl+F, or just do in Command+F, Command+F, Command+F several times on the Mac, and you can see that the letters keep shifting up and up. Now it's actually the absence of letters. Recall inside of this file here that the letters are set to medium gray. So they are neutral. They are not moving. It's the stuff around and behind the letters that's shifting, in the case of the both the black area and the white area.
And just to give you a sense of what has occurred here, I will go ahead and turn on a couple of other layers. We will turn on the type layer that contains a little bit of white and blackness there and then turn on the strokes layer as well, that stroking the areas that are being affected, and so you can see only that area inside of the rectangle and outside of the letters is being changed and only that area inside this rectangle and outside the letters is being changed as well. So in other words, the colors are shifting exclusively inside that white box and inside that black box. And that's the effect you get. So we are getting these repeated letters over and over again.
Now you might look at this and say, wow! this I believe, this is an old feature, I mean no preview. You can't have layers etcetera, etcetera. But what I am having problems of believing -- this is you talking to me, what I am having problems of believing is that this is a powerful feature. It looks like a totally lame feature. Well, it's not, it's actually a really, really great feature. It's just that this example is a little bit lame. I am keeping it very simple to give you a sense of what's going on there. In the next exercise, we will see just how powerful this feature can be.
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