From the course: VFX Keying: Master Course

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Premultiply and unpremultiply

Premultiply and unpremultiply - Nuke Tutorial

From the course: VFX Keying: Master Course

Premultiply and unpremultiply

- [Narrator] And now for everybody's favorite topic, premultiplied and unpremultiplied images. Here's the rule. Color correction and color space conversion must be done on the unpremultiplied image. In this video, we'll see why this is true and what happens if you get it wrong. So here in my setup I've got this Roto here, which represents a four-channel premultiplied image. Now this is either a CG image or a keyed image that you've already done the premult operation. So the Roto goes to this Grade node, which at the moment has no settings, so no change there, and then we comp over the background and everything looks lovely. However, let's apply a color correction operation. I'm going to take the gamma for the blue channel up to two and ooh, now I've got these ugly blue edges. So let's track this down and see what's going on. Let's go back up to our Roto node and take a look at the Unpremult node. Here's the story. When this Roto was initially created inside the node, it was actually…

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