The (somewhat) Weekly Filmschool #2: Chroma Key
Chroma Key (or Green Screen) is one of the most popularly known compositing techniques in our film-literate society. The concept is simple: put a pure-color in places where you want to be transparent later on. It sounds simple in theory, but can be very complex in practice. Fear not though, for it is not out of reach of the average geek! I'll show you how it's done.
Chroma key is really simple and really affordable (which is why it's so overused.) All you need are:
- A vivid green or blue background.
- Multiple lights for lighting the background and your subject. Large fluorescent lights work best for the background. Ideally you don't want to cast any shadows or see any detail in the background.
- A DV camera, or film camera and access to a telecine machine.
- A pretty beefy computer with at least 2GB of RAM, and Adobe AfterEffects.
You can use any color for the chroma key, but you don't want common colors that may make isolating your subject difficult (such as red.) Hence the use of day-glow green and pure blue. You could also use pure black or white ( a "luma" key.) Deciding on what color to use depends on what exactly you're compositing. For instance, on the first Spiderman movie, they shot Spiderman on a green screen because his costume contained a lot of blue. Then they shot the Green Goblin in front of a blue screen because, well, yeah.
You can find fabrics or paint that can make good green screens at a hardware store or any fabric shop for cheap (fuzzy fabrics like velvet work great.) Heck, even construction paper will do the job!
Now you're going to need a program to do the chroma key for you. The sky's the limit in terms of price, especially if you want to do something real-time like what TV meteorologists use. If you have slightly less money than God, I'd suggest Adobe AfterEffects. It's a good, consumer-level program that has other very useful tools. The downside is AfterEffects is one of those programs that requires a rather steep learning curve, because it's not very intuitive to somebody who's never used a post production suite before. But at its core, AfterEffects is simply Photoshop for motion pictures.
The biggest hurdle I think is truly understanding how AE handles your footage. You import video same as any other program, but you do not usually work directly on it. You nest your footage into compositions, which are like canvases. You can have compositions within compositions within compositions to make very complex animations or composited footage.
Ok, now that that's out of the way, let's make some movie magic. I took a picture of a crecent wrench on a green piece of fleece, just for demo purposes.
Open AfterEffects and ignore all the startup prompts. You want a totally blank slate. Import your footage by hitting CRTL+I or CMD+I on the Mac. Or you can left click in the Project window and select Import → Footage.
Hit Ctrl+N or CMD+N to create a new composition. Set it to whatever format you're using. I'm using a still image, but most likely you'll be using NTSC DV (United States) or PAL DV (Everywhere else). You'll also want to set the background color to black under Composition → Composition Settings.
In AfterEffects CS4, you have two tools with which to create a chroma key. You have your basic color key effect , and you also have the Keylight 1.2 plugin. Keylight allows you to do much more than just the basic effect from Adobe, but we'll just stay simple for now.
So double click on Color Key in the Effects & Presets panel on the right. You can go to Keying → Color key, or you can just search for "color key" in the search bar and it'll just pop right up.
In the effects panel, under key Color, use the eyedropper tool to select the color you wish to cut out. At first not much will happen, you have to move the color tolerance up to around 70 to get most of the color out, but not so much that it starts cutting out the subject as well.
Well, that's a fairly decent key. Aside from a hint of green around the edges, it looks pretty much isolated. However, if this was video, it would look like crap. That hint of green, and jaggy edge, would look really cheesy and ruin the effect. No chroma key is ever perfect. 90% of the work is hiding the flaws.
We need to apply another filter. It's the Spill Suppressor effect under the Keying menu. Use the eyedropper tool to carefully pick the color that is spilling onto the subject. Do not pick the background color itself, that will mess up everything. Pick the color that is being cast on the edge.
Now to get rid of the jaggies. Under the Color Key effects controls, there's the Edge Feather slider. Move it until it looks good (but not too fuzzy.)
You might have other stuff in the scene that the keying filters can't cut out. Such as, say, a boom operator or set rigging. The edges of the green screen might not be lit properly. For this, we need a garbage matte (or junk mask.)
It's super easy. In the upper left hand corner is the tool pallet. Select the Pen Tool, and on the subject layer, click to make little points, and make a shape that fits the subject as tightly as possible. This gets tricky with a moving subject, but I won't get into that now.
Finally, import your background put it beneath your subject layer.
Well there you have it. A super fast Chroma Key. It certainly won't get you any jobs, but for the average video jockey, it'll be an indispensable tool for his or her internet videos.
