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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."
Now thus far we've spent remarkably little time looking at the Calculations Command just a couple of exercises. That changes starting now. We are going to begin to immerse ourselves in that command. And things we are going to start to get little bit technical. Now, that's not to say this is going to be over your head. Hopefully it won't be. Hopefully you'll be able to keep up with me. But I am going to fire off some technicalities that you right now beginning in this exercise and continuing into the next one, because I want you to have a clear understanding of the two blend modes which are really super useful blend modes that are unique to the Apply Image in Calculations dialog boxes.
And those are the Add and Subtract Modes. And the thing about those modes are, if I were to switch to a layer here inside of this image, you can see that they do not appear anywhere inside the Layers palette. And the reason that they don't appear inside the Layers palette, at least one of the reasons, there are a couple of reasons actually, but one of the reasons that they don't appear inside the Layers palette is because they have additional numerical controls that the Layers palette really can't accommodate. All right, so here is what I want you to do. I want you to go ahead and open this image that's called Add and subtract.psd.
It's found inside of the 14 Calculations folder. And it's going to walk us through how the Add and Subtract blend modes work in just so that you are clear here. These modes appear just inside the Apply Image dialog box if I went over to the Image menu and chose Apply Image. You would see that if were to change the blend mode from Multiply to Add for example or Subtract, in addition to applying this blend mode I would also get two new values, Scale and Offset. And I am going to explain how those values work. So I just want you to see these blend modes do occur inside the Apply Image dialog box. I have said in the past in the previous chapter, I am not all that keen on the Apply Image command, I don't use it all that frequently, because 99% of what it can do you can accomplish more easily using layers, especially adjustment layers.
Anyway, I am going to cancel out of here. But I am a big fan of the Calculations Command. So we are going to be focusing on the Calculations Command here on Add and Subtract inside the Calculations dialog box. And again, just so that we have a frame of reference I am going to go to the Image menu, choose Calculations, and you can see of course the last blend mode that we applied inside of this chapter was Color Dodge. But right there, there is Add, and there is Subtract, and if I choose either of these modes then I get the Offset and Scale values right here. Both modes provide access to these values, none of the other modes do. I am going to cancel out of there.
Now let's focus on the diagram here. I am going to go ahead and hide the Layers palette and I am going to bring up my Layer Comps palette which you can get to of course by choosing the Layer Comps command from the Window menu. And notice right now I am seeing Source #1. Now just again as a refresher, just as a reminder, you may recall inside of the Calculations dialog box, we've got Source #1 at the top here, and Source # 2 at the bottom, and I was telling you, you want to think of it as if Source #2 is sitting on top of Source #1.
So they are actually kind of listed in the wrong order here. Source #2 is on top and Source #1 is on the bottom. So I'll go and cancel out. And by Source #1 and Source #2 I just mean channels inside the image. These could be color bearing channels, they could be Alpha channels. In our case we are going to think in terms of Alpha Channels, and very simple Alpha Channels as well. So this is Source #1, it's something that I am calling the Shadow. It's basically an ellipse that I drew with the Elliptical Marquee tool. I filled it with white against the black background, and then I applied the Gaussian Blur Command with a Radius of 24 pixels.
The next source, the Source #2 item right here if I click in front of Source #2, is the Sun. And the only difference it's still Gaussian Blur with a Radius of 24 pixels. I still drew it with the Ellipse tool. The big difference is that it's a circle instead of an ellipse. And these two guys partially overlap each other. So you can see them back and forth here. Here is Source #1, and here is Source #2. So Source #2 is bigger, it's more circular and it's offset slightly to the right. Here is the way that they overlap. If we advance to the Overlapped Layer Comp you can see the two Alpha Channels as if we are viewing them at the same time in that Quick Mask mode, so that we can see them in color and blue represents the Shadow channels, and Red represents the Sun channel. So wherever we are seeing this purple/violet color here, that's black inside of both channels.
Where we are seeing blue that means that the Shadow channel is black and the Sun channel is white. Where we are seeing Red, that means the Shadow channel is white and the Sun channel is black, and where we are seeing white, both channels are white. Does that make sense? We are seeing the two channels overlap each other. Now, let's focus on how Add works inside of this exercise, and then in the next exercise we'll focus on Subtract. I am going to go ahead and apply the Add Mode. This is what things would look like if I apply the Add blend mode using the Calculations command. And here is that you want to think about it. We are taking Source #2, and we are adding Source #1 to it. So I was telling you Source #1 is sitting on top of Source #2.
so that means we start at the back of the stack here, Source #1, then we add Source #2 to it obviously if we were to flip that equation around it wouldn't matter really where adding is concerned. But once we come to Subtract the order does matter. So best to think of it this way right upfront. And we are really adding Luminance levels, so we are brightening the resulting pixels fairly significantly here. Then we divide by the Scale value and we add the Offset value and we get the result. Now, that's the part that I think sort of twists a lot of people that we are going through Scale and Offset, oh, my goodness! How in the world can we keep track of all these stuff? We'll, all right. Let's say that we've got a pixel that has a Luminance level of 100 inside of the Source #2 channel, which would be the Sun channel. And another pixels of that same location that has a Luminance level of 50 inside of the Shadow channel. Well, then you add them together and you would get a Luminance level of 150 which is slightly brighter than medium gray. Then you would divide that by the Scale value.
The Scale value can be anything between one and two. Obviously, if you divide by one you don't make any change. If you divide by two you are going to take that value down, it can be anything between one and two by the way, it can be decimal values. And the idea is that you can easily add Luminance levels to the point that they get way too bright. Like if you have White plus White, 255 plus 255 that ends up being a Brightness value of more than 500 which is of course off the scale, and use the Scale value to bring it back down to earth essentially. And then you can add the Offset value. The Offset value can be anything between -255 and +255. So you can Subtract or Add using this Offset value. So in our case right here what we are seeing on-screen is Sun plus Shadow, the Sun channel plus the Shadow channel divided by 1.0 which is the Default Scale value plus 0, which is the default Offset value, and you get this effect right here. So bear in mind this was Source #1, this is Source #2, this is what it looks like when you add the two together.
Next, I tried applying that same blend mode, but this time I took the Scale value up to 2.0, and you can see the difference here. This is without the Scale value or the Scale value is set to 1; the default value. And this is a Scale value of 2.0. Things look pretty similar between those two results except that the second result; the scaled result is much softer. So by scaling the effect, by raising the Scale value to it's maximum in this case the 2.O, we are softening the effect, something to bear in mind.
You can also of course offset the effect if you prefer. So I change the Scale value in this case with the Add Offset -128 right here Layer Comp. I change the Scale value back to 1.0, so we are not applying any scale. And I went ahead and changed Offset to -128. So we are subtracting 128, bear in mind that 128 is about a medium gray. So we are subtracting 128 from the picture. And as a result we end up making the highlight effect smaller. So this is what it looked like without any Offset applied. This is what the effect looks like with the Offset applied. But notice that it's also sharp that we have a Sharp effect as a result, because we didn't add any Scale into the picture. Does that make sense? I assure you that if it's like just completely overwhelming you at this point that by time we are done with this chapter you'll have a working knowledge of how to use the Add Mode, because we will be experimenting with it.
But I did want to give a sense of what's going on under the hood. In the next exercise I am going to show you Subtract; I want you to be there for it. I want you to stick with me here if you can. And then following that we will actually put those blend modes into play.
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