Join Steve Wright for an in-depth discussion in this video A look at different keying workflows, part of VFX Keying: Master Course.
- [Steve Wright] We'll start our grand tour of green screen and blue screen keying by taking a look at the 3 major keying workflows. The mythical one node keyer, the multi-key composite, and what I like to call the uber key method where a master key is assembled from several different keys. This video is a watch only demo to give you an overview of the process. So there are no exercise files. Let's start by taking a look at the mythical one node keyer. The reason I call it mythical is because it almost never works.
We start with our green screen of course and we have our background plate and we plug them into our keyer node and we have a beautiful composite. The advantages are it's very easy, but the disadvantages are it's practically nonexistent. Now here's why this almost never works. Let's come up here to our green screen and I'm going to take my viewer gamma down and my viewer gain up in order to exaggerate the contrast so that you can see how uneven the lighting is here.
Now even though I did a roto to garbage matte the picture and I'm really only keying inside my roto you can see that the green screen here is a lot darker than the green screen down here. So whatever setting works best for the head is not going to work best for the torso. So there is the basic problem trying to do a global setting that works correctly for every part of the picture is usually impossible. So let's put the viewer back. Reset that. We all clear the property bin and rehome the viewer.
The next issue is the color correction. While the ultimatte keyer has a pretty good set of color correction built in none of the other keyers do. So you have to do all the color correction after the comp. So here is the color correction right here I set up after the comp, but here is the issue. This color correction has got to be masked to just the foreground character. Let me open up that grade node, and I did that right here by setting the mask to the alpha channel.
If I don't do that obviously my color correction is going to hit the entire finished composite, and I want to isolate it to my character. So I have to isolate it to the character, and there it is only affecting the character. However, to do that I have to have the alpha channel here. Problem is Mr. key light has to be set up exactly right or you don't get the alpha channel. Let me show you what I mean. I'm going to disable this shuffle node and now if we look at the alpha channel I got a big slab of white.
In order to get the key light to export the alpha channel I have to take the shuffle node here, let me open him up, and turn on the alpha channel and fill it with black. Okay if I fill it with white I don't get my alpha channel out. If I fill it with black the key light key comes out. What's going on is the original background is just a 3 channel image, whereas my shuffle node is producing the 4 channel image, the alpha channel, but it's filled with black.
So Mr. key light wants a 4 channel image with the alpha channel black in order to give you your alpha channel out in order to give you an alpha channel for your green operation. Got all that? Okay. Then we can follow that with an edge blur which uses the same alpha channel. Now let's take a look at the multi-key composite technique. We'll just move over here and we'll take a look right here. I'm going to clear my property bin. So in this workflow we start with the green screen and then we key just the head.
So I could optimize this key for a best key for the head. Then I have another keyer here which is just the torso. These masks isolate the areas in question. So I have 2 different key light nodes, each set correctly for that part of the picture and just to mix it up I added a primatte node here. We can actually see this really did pull a key if I open the viewer gamma a little bit. And I keyed the legs, and again I have a 4 channel key.
So I'll take this key light head composite it to this key light torso right here. Then we can take the primatte legs and composite those in here. Now I have a 4 channel premultiplied image that is ready to composite. I can now do my color correction right here and again because it's a 4 channel premultiplied image I have to enable my unpremultiply option here, and I have to have the alpha channel there to do it, but it's all good, because I do get my alpha channel.
Then we comp that over the background and then do an edge blur. So the advantage with this approach is you do get better keys, but there are some disadvantages. One disadvantage is degrain, grain management. If I try to degrain this green screen in order to get a nice clean matte out of my keyers the problem is I'm going to have to regrain it back here. Now that may not sound like a big problem, but consider this when I degrain this green screen all the grain I've lifted out of it has got the green spill baked into it.
Then I come down here and reapply the grain. I'm reapplying green spilled grain on top of my picture. Not cool. The other problem, and this is arguable more serious is that each one of these nodes is going to apply spill suppression to its part of the picture. The problem is the spill suppression will be slightly different. Even 2 key light nodes will get you different spill suppression, because you've got different adjustments inside the node.
Not to mention a-whole-nother type of keyer. So when we put it all together we're in danger of having different spill suppression treatments from top to bottom of our character. In this video we saw 2 keying workflows and developed an appreciation of their advantages and disadvantages. In the next video we'll see the method preferred by many pros the uber key method.
Note: This course was created by Steve Wright, author of the seminal book, Digital Compositing for Film and Video. We are proud to host this course in our library.
- Creating an uber key
- Keying green-screen vs. blue-screen footage
- Preprocessing footage
- Building a clean plate
- Making luma keys
- Keying on hue and saturation
- Pasting keys together
- Grain management
- Saving time with garbage mattes
- Using spill suppression
- Improving edges
- Color correcting keys
- Sweetening the comp
- Alternative compositing workflows
- Fixing edge problems
- Using KEYLIGHT, Primatte, Ultimatte, and other tools