Most people who dabble in home video editing know about “blue-screening” or “green-screening.” What they may not realize is that it doesn’t take a whole lot of money – or a great deal of know-how – to apply the technique in their own videos. Technically called “chroma keying,” the technique actually allows you to filter out any color of your choosing from one video clip, and superimpose what’s left behind over another, thus giving the illusion that a person or object has a different background than as originally filmed. You’ve seen it on the evening news – the weather person in front of a huge video – which is actually a blank screen. With a little creativity, you can do a whole lot of interesting things with your home video projects by combining chroma keying with other features of your video editing program. We will explore the basics in this post, and follow up with some of the other tricks and techniques in later posts.
First of all, why use a blue or a green screen? Why not red? Or purple? Or a white wall? The answer is that most chroma keying involves “keying” out everything but a human being, and our skin typically contains reds and yellows – but no blue or green. Of course, if you were somehow applying the technique to a blue jay, you would want to use a red or orange screen. But why would you want to do that?
To do this, first you’re going to need a green screen or blue screen. Now, you could spend a hundred bucks buying a fancy screen that folds away neatly, but what I did is to visit a local market where they sell material on huge rolls. If you do that, you’re going to want to find the widest roll possible, and you’ll want smooth material, but not shiny. Look for the minimum amount of “texture”, but still as “flat” – non-glossy – as possible. Get yourself a good 3 or 4 meters of it. And try to avoid wrinkling it on the way home – ideally, roll it up, as the wrinkles will cause problems later.
So assuming you want to place yourself in front of a different background, you want to think about what you’re going to wear. Since you’re going to use a blue or green screen, wear clothing that is as far away from your background color as possible. If you wear a blue shirt and key out blue, your shirt will become transparent like the screen.
Find a flat wall somewhere where you can fully stretch your material on the wall, without any bumps. Outdoors is best, because it simplifies the lighting problem I’ll discuss in a moment. Depending on your wall, use thumbtacks or tape – anything that will hold the material firmly, and allow you to stretch it so it is taut, removing all wrinkles, without damaging the wall or surface. Set up your tripod and make sure you can capture your subject completely, without exposing the wall itself. A bit around the edges is OK, you can fix this later. You want to stand at least a meter away from the wall, without your head poking up beyond the blue screen, for example.
Now let’s talk about lighting. It’s important to get this right. Again, you can go spend hundreds on a fancy lighting kit, but if you’re only going to use this rarely, you can just pick up a few 150-watt and 500-watt halogen work lights for about 10 or 15 bucks apiece. Two of each are enough. They get hot, so you’ll need to figure out a way to mount them at the right height. I bought some inexpensive pieces of 1-inch square wooden poles, made some stands to hold them up, and put nails in at various heights to hang the lights as needed. You are going to use a couple of lights to illuminate yourself for the camera – 150-watt lights are OK for that – but this will throw shadows on the screen. You’ll want to use your bright lights out to the side to illuminate the screen evenly. The idea is to ensure that the blue screen is exactly the same shade of blue all over – not brighter on one side, and no shadows. Also, lighting everything well ensures your editing software can clearly “see” where the blue begins and ends. The following screen shot shows what happens if your blue screen is not illuminated evenly, and your subject is not illuminated enough:
Now check your camera again, and you’re ready to film! Before you tear down your equipment, it’s advisable to test your clip using your software, to ensure you didn’t miss anything. If you have a bit of non-blue wall showing around the edges, you can either increase the size of your clip, or use a cropping tool (depending on your software and your needs) to remove that.
Finally, when you put away your blue or green screen, you don’t want to fold it – or else you’ll have to iron it again before you use it next time. Any wrinkles will have the same effect as a shadow, changing the shade of blue just enough to ruin the chroma key function later. I taped one edge of my screen to a long broomstick and rolled it up. This allows me to unroll it later and hang it, so that the weight of the broomstick helps keep the screen taut.
In a later post, we’ll explore how to combine one or more pieces of chroma-keyed footage with a new background. Good luck!