From the course: VFX Keying: Master Course

What is spill? - Nuke Tutorial

From the course: VFX Keying: Master Course

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What is spill?

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- [Steve] This chapter is all about the arcane art of spill suppression. The reason spill suppression, or despill, is so troublesome is because it's actually a cheat. We're trying to solve a problem created in three dimensional space by operating on the 2D image. So in this chapter we'll understand where spill comes from, the various tools for removing it, the artifacts that they can introduce, and some advanced spill suppression techniques that will give you much better composites. In this first video, we'll look at what spill suppression is and where it comes from. While spill suppression is a capability that all keyers have, it's a very separate process from pulling the key itself. So here's our setup. We've got a lovely green screen here. We can take a look at the five sources of spill. First of all, of course, are the edges. So let me push in here. And if we look, for example, at this foot, we have pixels that transition from pure foot to pure green screen, and during the transition they're a mix of the green screen and the foot, and that's a source of spill around the edge of everything. Next we have motion blur. Now here we've got a big motion blur leg. The keyer is going to pull a mat all the way out to the outside edge, so all these pixels that are blurred are going to be mixed with the green of the backing color. Another area is any semi transparency like this hair. Let me gamma up the viewer a bit. So anything that's semi transparent, its pixels are going to be a mix of the transparent object's color plus the color of the backing. Then we have bounce light. This is a psych, so we have an enormous amount of bounce light from the floor, so we've got a lot of green on the underside of the characters, but all the walls are throwing bounce light onto our characters, turning them green. And then there's flare. Flare is all the green coming off this screen is entering into the lens and bouncing around and it's kind of laying an overall green fog on top of everything. So all five sources have to be accounted for in doing your spill suppression. So what I've done here is I've taken this key light keyer, and I've pulled a key, but I didn't do any spill suppression just so we can see what happens. If we do our standard premult, you can see the green spill all around the perimeter, mixed into the semi transparent areas, mixed with the motion blur, it's everywhere. You can also see it on the underside of this leg and underneath this elbow, there's your bounce light. If we try to composite it like this, we're going to get a hideous looking composite with green edges all over everything, or blue if it's a blue screen. So we obviously have to do our spill removal. I've got a despill operation right here, so I'm just gonna hook it in. And we can see boom, all taken care of. No green spill at the edges, no green spill in the motion blur, the semi transparent regions, or bounce light, or flare. Now here's an interesting technique. We can take the original green screen and the despilled green screen, and by subtracting one from the other, we can see all the spill. Let me crank up my viewer a bit here. So there's the spill that was removed. Of course, an awful lot of the spill removal was the green backing itself, but the really important stuff is the characters. Back to our spill. So you can see the green fog that's just right on top of everything, plus here's a heavier spill from the bounce light of the floor and underneath the elbow, and of course you can see there's just a lot of spill in those semi transparent and motion blurred objects. I'll rehome the viewer. Now that we've seen the sources of spill, we're ready to take a look at how it's removed.

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