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Channels are a very important part of Photoshop. It is a way of going in there and being very selective of how something is going to be applied, that how being anything like filters or colorizations--whatever it is you want to do, you apply it in a very specific way. Let's first look at what the channels really are. I have here one of my older images, and I'm going to use this to explain the alpha channel itself. When you select something, say we select something like this, you know that anything you do is going to happen only inside that area.
So if I go in there and start painting, let's just say we're going to go and paint with the paintbrush and make it nice and big so we can really see what we're going to be doing here, and when we start to paint, it only happens inside that area right there, right? Nothing else gets affected. Well, that selection is volatile. When you deselect it, it's gone. But if you save that selection that selection, that selection becomes a channel, an alpha channel to be exact. Now, let's look at that alpha channel. You notice that the area that's selected is white and everything else is black.
This is a way of going in there and selecting. Now an alpha channel is an 8-bit channel so it does have 256 levels in there, the black to white with 254 levels of gray in between. The level of gray will determine the level of the exposure of that image to an effect. Let's just say that we take that same selection right there and we apply a feather to it. We'll go in there and we'll just say give it a feather, a nice big feather of about say 35. That's a big feather. Now it doesn't look like anything happened, but the selection became rounded.
Now again the selection is doing nothing to the image. It is simply segregating part of this image to some kind of an effect. If I saved this selection now, you'll notice it created a second channel, and this one has the graze of that feather. I'm going to deselect and just create a brand-new alpha channel from scratch, which automatically is black because nothing was selected. In this channel, I'm going to throw a gradient from black to white. Now this area here is protecting my artwork.
This area here is exposing it completely. The area in between is exposed based on that level of gray. So when I go back to my artwork and I load that channel, I could load by Command+Clicking here or just coming up here and saying Load Selection and choosing that particular channel, which in this case is number 3. Click OK and you see the marching ants from the 50% gray over to the white. The entire image is in fact selected. I'm going to just hide those marching ants so you can see the effect that it's going to have.
So now I'm going to return my colors to black and white so the canvas is white, and I'm just going to hit the Delete key. Notice that the right side has been deleted 100%, the left side has not, and it's gradually deleted through the grays. Let me undo that. Well, this time we'll go in there and apply some kind of a colorization, like I'll do just that. I'll go into Hue/ Saturation and say Colorize, and we'll colorize it into the blues and increase the Saturation.
You'll notice that the right side is being affected 100%, the left side not at all, and then it's gradually been affected through the area of the grays in the Alpha channel. I'll apply a filter. We'll go in there and just choose a filter like Find Edges. And again you see that the right side has been affected 100%, the left side has been left alone, and it's a gradual filtering through his center where the grays are. In essence the alpha channel is basically a selected mask through which you apply effects to that image.
Now that differs from the layer mask in that it is not exposing the image to be seen; it is exposing the image to be affected, using the same concept of going from white where the exposure is to black where it's protected.
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