The use of chroma (or color) keying software has gotten much easier in postproduction. The most common techniques involve shooting talent against a blue or green screen and then replacing that colored background with a new one. This process is a staple in filmmaking special effects and has made its way into web video as well.
In order to succeed with chroma key, you want to pay close attention to shooting and lighting.
When shooting chroma key footage, be sure to turn off all the Auto settings on your camera. This means no auto-exposure, auto-white balance or auto-focus. If any of these are left on, the footage you are trying to key will constantly be changing as your subject moves. These constant fluctuations will make it harder for you to get good results. While you’re in changing settings, switch your camera to a progressive frame rate (such as 24p) as you’ll get much cleaner edges on your keyed shots.
You’ll have many choices when selecting a backdrop. While you can simply go and purchase fabric at a fabric store; many will invest in higher-quality backdrops from a video or photography retailer. The most popular backdrops use polyester fabric stretched by a metal frame offers an easy to light surface that avoids wrinkles and shadows. These backdrops can be easily folded and transported. Muslin backdrops are also used, but may require more attention to lighting to avoid wrinkles and bad keys.
Here are a few practical tips for lighting and shooting a chroma key set:
- Even Lighting – It is essential to minimize variation in colors for the backdrop. This means that you must evenly light the background to avoid hot spots. Diffused lighting (such as soft boxes or fluorescent lights) makes this easier.
- Spill is Bad – Be sure your subject doesn’t stand too close to the backdrop, otherwise you’ll get shadows on the backdrop and color spill on the person.
- Keep Your Distance – Try to keep your camera as far away from the screen as possible. It’s better to increase the distance, even if it means some blank edges are showing (you can always crop these out later).
- Avoid fast movement – A fast moving subject creates motion blur. This is typically where keys become bad or obvious.
- Use shallow depth of field – If your camera supports it, lower your aperture. This will help make the background fall out of focus. This is an easy way to hide wrinkles, seams and hot spots.