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Photoshop Masking and Compositing: Fundamentals is the introductory installment of Deke McClelland's four-part series on making photorealistic compositions in Photoshop. The course shows how to make selections, refine the selections with masks, and then combine them in new ways, using layer effects, blend modes, and other techniques to create a single seamless piece of artwork. Deke introduces the Channels panel and the alpha channel, the key to masking and transparency in Photoshop; reviews the selection tools, including the Color Range tool , Quick Mask mode, and the Refine Edge command; and shows how to blend masked images so they interact naturally.
Here we are looking at the final version of the composition. The remaining steps are to add the synthetic rays of sunlight that are coming out of this upper right cloud. We're also going to add the bird, and that will give me a chance to introduce you to the Calculations command, which allows you to use compositing in order to assemble a base mask. I also want you to open Red kite.psd. Notice we've got some text here that tells you all about the animal, but because we're always working from the composite image when we're creating masks, I need you to turn that text layer off there inside the Layers panel.
Then switch over to the Channels panel, and let's take a look at the channels that we have to work with. If I click in the Red Channel, then we have some dark areas inside the bird, as well as some light regions as well. This bird is another warm-blooded animal, so it's going to resonate most brightly inside the Red channel. Then we have the Green channel, which of course is our detail channel; the one that's closest to the grayscale composite. And finally, we've got the Blue channel, which is ultra bright where the background is concerned. After all, this is a bright blue sky, and the bird is darkest in this channel as well.
So if we were to try to work from one channel, and one channel only, this would be the channel that we would duplicate, and then we'd exaggerate its contrast, and so forth. But notice that we don't really have that much definition around the bird's head. So when you run into situations like this, the best thing to do is composite two channels together in order to create a base mask. You want to blend the channels that have the highest degree of contrast between each other, and in that case, that's going to be this Blue channel, with a very bright sky and the dark bird, and the Red channel, which has the best definition around the head.
Notice we have a very bright head against a somewhat dark sky. So here's how it works. Go ahead and switch to the RGB composite there in the Channels panel, and then I want you to go up to the Image menu, and choose the Calculations command, and that brings up this fairly intricate dialog box. It's a little difficult to understand the first time you see it, but the idea is this: you're taking Source 1, which is a channel, one of the color-bearing channels in our case, and you're mixing it along with Source 2, which is another one of the channels inside the image, and you're doing so subject to a Blend mode, and an Opacity value.
Now, where calculations are concerned, you very rarely change the opacity. You do most of your work by selecting the two channels that you want to mix, and then setting a Blend mode. Now, just about any time that you're masking a warm-blooded creature against a blue background, whether it's a blue screen, or a blue sky, as in our case, you want to set the first channel to red, and you want to set the second channel to blue, and then you want to go ahead and turn on the Invert check box for that Blue channel. And this is a fairly wrote recipe where blue screen images, or blue sky images, is concerned.
Now, where these sorts of mixes are concerned -- and I don't expect you to completely understand the Calculations command in one movie; we're going to be visiting this command over and over again in future courses -- but Source 1 is essentially sitting on top of Source 2. So if you want to get a sense of what the Red Channel looks like by itself, you would set the Blend mode from Multiply, which is its default setting, to Normal, and now you see the Red channel. If you want to see the Blue channel inverted by itself, then you would take the Opacity value down to 0%.
So this gives you a sense of the two channels that we're trying to mix, and I am going to take that Opacity value back up to 100%. Now it's a matter of selecting the best Blend mode. In our case, it's going to be one of the brightening blend modes, that is Lighten, through Lighter Color. And most likely it's either going to be Screen, sometimes Color Dodge is useful, and then finally, Linear Dodge. Let's go ahead and check them out. I will start by selecting the Screen mode, and sure enough, we have a bright bird against a not all that much darker sky.
So we don't have a lot of contrast going on. If we want to make that bird even brighter, then we can up the Blend mode. So we'll start with Color Dodge. That looks pretty darn good, and then next we'll try Linear Dodge (Add). In the case of Linear Dodge, we do indeed white out the bird, so it's completely white at this point, which is great. However, the background is pretty darn bright as well. Now, if you want more control over the brightening process, then you switch your blending mode to Add, and I know I'm running through this pretty quickly, and the reason, once again, is that we are going to be coming back to this dialog box over and over again.
I am going to go ahead and set the mode to Add. We're not going to see any difference whatsoever, and that's because Linear Dodge (Add) is actually the same as the Add mode. That's why Add is in parentheses following Linear Dodge. The difference is that the Add mode provides you with a couple of additional values right here, one of which is the Offset value. What you can do is you can reduce the brightness of your overall composite channel here. Bear in mind that by adding a bunch of luminance levels to each other, your resulting luminance Levels go through the roof.
So these areas that appear white are actually much beyond white, where the math of the Blend mode is concerned. So they're going to remain white even if we start taking this Offset value down. So if I reduce the Offset value to -100, I am subtracting 100 luminance levels from each one of the pixels in the composite view of the image. That darkens the sky, but it also ends up darkening the bird considerably. I found that at -25, we ended up keeping the bird nice and bright, and we darken the background as best we can.
So these are the settings I'm going to recommend. So once again, you want Source 1 set to the channel Red, you want Source 2 set to the channel Blue, with the Invert check box turned on. You want your Blend mode set to Add, and you want to set the Offset value to -25. Now, the result will appear in a new channel. So of course, that's going to be an alpha channel, go ahead and click OK in order to accept that result. Now, let's go ahead and rename this alpha channel. Once again, I want to keep track of how I got to this channel in the first place.
So I am going to say R Minus B. The idea is, a positive R would not be inverted, where a negative B would be inverted. That's just a nomenclature that I use. Then I'll go ahead and enter Add, that's the Blend mode, and -25 for the Offset value, and we've now created our base alpha channel from which we will build the mask for this bird. So again, I realize we ran through that pretty quickly, but I just want you to have a sense for not only how you can use masking to create composites inside of Photoshop, but you can use compositing to create base alpha channels as well.
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