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Now, let's take a look at some workflow examples to see how these Output channels are used for a variety of common situations. Let's start by masking a color grade operation. I'll delete this node. Let's go get Marcie. So we'll punch up our Read node. We'll go to our NUKE WORKSHOP, Lesson 1, and select Marcie, and hook her up to the Viewer. There is a couple of different workflows for masking off an operation like a color grade or blur.
Let's take a look at the inline approach. We'll select Marcie, add a RotoPaint node, select Bezier, draw a little shape. All right! We now have this shape that's in all four channels, RBG and the alpha channel. We'll go back to the RGB and what I am going to do here is I am going to set the Output to alpha channel only. Now we still have a mask in the alpha channel, but as you could see the RGB channels are cleared.
Now, after the RotoPaint node, below it, I can add a Grade node. If I set the Grade node to like a very dark gamma, the entire picture is affected. What I am going to tell the Grade node to do is look in the alpha channel, look here, and use that as a mask for this operation. We do that right here. This pop-up where it says None, we say go get the rgba.alpha channel and use that as a mask. Turn off the Overlay, so you can see it.
And of course we can invert that mask so that it color corrects everything except the face, but I'll turn that back. An alternate workflow is to use the mask input of the Grade node. So I am going to take my RotoPaint node and pop it out over here. Now, the Grade node is complaining because I am telling it to use an alpha channel that no longer exists. So I'll set that back to None. We can now pull the mask input here and hook it up to the RotoPaint node there, and that's another workflow. The key is when to use one or the other is if you want to preserve the fact that there is no alpha channel or you don't want to disturb the alpha channel that's there.
That's when you want to use this Mask Input option. Let's see how we'll do a Pre-multiply operation on a three channel image in order to isolate it for merge. I am going to move my window up here a little bit and make a copy of Marcie. Well, let's clear the Property Bin, add a RotoPaint node, and hook it to our Viewer. So let's draw a little roto shape. One, two, three, four, and close with a Return.
To do a pre-multiply on the three channeled image, the first thing we'll do is set the Roto Shape Output to Alpha, so it only shows up in the alpha channel. There we are. Then we'll set Pre-multiply to RGB. So now the RGB channels have been pre-multiplied. I'll close the RotoPaint node. We now have a four-channeled pre-multiplied image that we can then merge over any background. So I'll just go get a checkerboard here, set that up, drop in a Merge node, hook that up to my checkerboard, and there we go. We're ready for a composite.
I can even add a Transform node to do our resize, rotate, or scale, or any other information of the pre- multiplied foreground that I want. But what if you have a four-channel image? How does that change the set up? Well let's take a look. We'll slide this over. Let's copy this checkerboard. Paste it here. we'll clear the Property Bin, hook that to our Viewer to show you that the checkerboard is in fact a four channel image, all right? So we want to perform a pre- multiply on this four channel image.
Let's see how the workflow is affected. Select the checkerboard, add our RotoPaint, select Bezier, draw a simple shape. All right! We have two different ways we can approach this. We could say that I want the Output to be the alpha channel, but if we look in the alpha channel, don't forget the checkerboard has its own alpha. So that's what we're seeing there. All right! Go back to RGB. I could say pre-multiply the RGB channels, but I've still got the solid alpha channel.
It doesn't match. So what I can do here is I could say Pre-multiply RGBA. So now the alpha channel is also being pre-multiplied by the mask. But there is another workflow. Let me put this back to RGB. So now I am seeing the alpha channel of the original checkerboard. I can now select the Replace option. Well, what that does is it replaces-- remember my output is the alpha channel.
It will replace the alpha channel of the incoming checkerboard with the alpha channel from the RotoPaint node. Now, this is different than doing a pre-multiply on the alpha channel. We'll go back to our RGB channels, and take a look at the next setup. How do we use a RotoPaint node to do a garbage matte on a blue screen? So let's check that out.
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