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In this exercise I'm going to introduce you to the Alpha Channel. I'm looking at the final version of the toucan from the previous chapter, I've gone ahead and called this file Last- seen toucan.psd and I'm going to switch over to my Channels panel. Notice that I'm seeing all of my channels in grayscale, that's very important, by the way, if you're still seeing errors in color then press Ctrl+K or Command+K on the Mac to bring up the Preferences dialog box. Switch to Interface and turn off the Show Channels in Color check box. Mine is already off so I'll go ahead and click the Cancel button.
Now in addition to the color bearing channels red, green, and blue in this case, the ones that actually contribute to the colors inside the image we have these extra so-called Alpha channels. Now an Alpha Channel can contain a mask which is typically a grayscale representation of a selection outline. It may contain a layer mask which conveys transparency to a layer, or it may contain some other kind of mask. So you might ask why in the world don't we just call these things Masks? Well ultimately the Mask is the thing and the Alpha Channel is the container.
So what I'm going to show you is where the Alpha Channel gets its name, just so you have a sense of what's going on. It's a little bit of math but it's actually easy math, now it's not going to necessarily look like that at first. I'll go ahead and switch over to this image. It's called the origin of alpha.psd, and this is the mathematical expression from which the Alpha Channel gets its name because right there we've got the Greek letter alpha (a). Now this expression comes to us from a guy named Alvy Ray Smith, who actually won an Academy Award for this little bit of math, believe it or not.
And the idea was back in the day, this was around the late 70s. You couldn't build transparency into an image file, so the image and the transparency information were ultimately different files, at least as we think of them today. Now I'm going to switchover to the Layers panel. What this expression allowed us to do was build translucency into an image file, whether that file contains layers, the way they do now, or whether they didn't as back in the old days. In any case let's imagine, I take layer A here inside my Layers panel and I reduce its opacity to 70%.
This expression tells us what a pixel is going to look like at a specific location inside the image. So, for example, this big A represents that pixel on layer A and then the big B represents that same pixel on the layers below layer A, and the expression tells us what that composite pixel is going to look like. So at 70% Opacity, Alpha becomes 0.7 because that's what 70% means, 0.7 times something.
And then to figure out the contribution of the background imagery to that 1 pixel we take 1-0.7 which gives you 0.3 which is the same as 30%. So in other words at any given pixel location we've got 70% of A and 30% of B and that is all that's going on where Alpha is concerned, it's that simple. So the next time you hear me say Alpha Channel which will be very soon, by the way, and very frequent as well, don't be put off by it because the core concept of Alpha is very easy.
What you should be concerned by is that thing that lives inside the Alpha Channel which is the Mask, which is considerably more difficult to create and we'll be learning exactly how these things work beginning in the next exercise.
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