Shooting for Chroma-Keying


Get great keys, and save time and money in the process!
Updated on February 6, 2011

Basic Rules


* Set and lock your white balance, using a white card, or a preset that matches the type of lighting you're using.
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* Light your green screen evenly, and use green gels or green filtered lights, for a richer green.

* Keep your subject as far from the green screen as possible!

* Backlight your subject to set him/her off from the background, if it won't conflict with your composite.

* Light your subject separately to reduce green spill and give your subject clean edges. If you want a dark subject in your final composite, shoot with good light and darken in post production; after you've applied your keying filters.

* Use a tripod! Unless you really, really know what you're doing, you're not going to be able to composite your shot, if the camera is moving around. Don't even zoom in and out during your shot. This is a rule you can only break if you really know what you're doing, and you've thoroughly tested your plan.

* Focus on the subject! You probably don't want autofocus. Be sure you focus critically. You'll never get a good key around a fuzzy subject; not to mention that your video will look pretty bad.

Lighting the Green Screen


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* Even lighting will allow you to set the luminance chokes to a narrow range, which will make it easier for you to recover areas of your subject that may have picked up too much green (like reflective materials that tend to mirror the green).

* Light shadows on the green screen won't have a dramatic impact on the vector keyer output, but anything that falls close to black (dark shadows) will not contain enough chroma information for keying.

* A green screen that is too bright will be washed out (too white) in the video recording. It's probably better to have the green screen a little on the dark side, but not dark, than to have it washed out with too much white light. That is one reason to use green gels on the lights that are on the green screen.

* Keep your subject lighting separate. Keep your subject as far from the green screen as possible, and adjust the subject lighting so it doesn't wash your green screen out with too much white light.

* Don't let your green lights, if you are using them, cast their light on your subject. If your subject's white shirt is green because they're sitting under a green light, it's likely that shirt will look like the background to the keying software and you can say "bye-bye" to that one. BTW: This is one of those things the luminance chokes in VKey2 can help with. Be sure, if your subject is wearing white, and you think this will be a problem, that the brightness of the white shirt is well above the brightness of the background. This will allow you to isolate the brighter shirt from the key, while still keying the background, as desired.

* When in doubt, light your subject really well and light your green screen at least enough that you get good color saturation across the entire surface. As long as the green is really green; not dark or nearly white, it will probably key just fine. If you get clean edges between your subject and the background, the keyer will try to find those edges and trace them.


Applying the Filters; some quick notes


* Be sure to turn on the Alpha channel in any clip you wish to key with a Quicktime plug-in. Your editor won't know the filter is generating an alpha channel, if you don't tell it.
Note: Using the FxPlug plug-ins, it is not necessary to explicitly turn on the alpha channel, and it is probably better if you do not.

* If you apply your garbage matte before the keying filter, the keying filter will have less work to do, so it will function faster.
Note: When using the FxPlug plug-ins, you might find that you need to apply garbage mattes after the keyer. This is because FxPlug hosts (FCP, Motion, etc.) use premultiplied alpha. You can end up with a thin line along the edge of your garbage matte if you apply it ahead of the vector keyer. The simplest solution is just to move the garbage matte below the vector keyer.

* Apply color correction AFTER your keying filter, unless you need to color correct a bad green for keying, but this is generally a bad idea.

* If you must stack keyers, be sure to turn the green suppression OFF in all but the last (bottom) keying filter. To turn off green suppression, set the green suppression slider to 130 (all the way up).
Note: The FxPlug plug-ins use a different scale for green suppression, and version 2 of VKey2f has color independent spill suppression with several additional controls.

* Set the luminance sliders on VKey2 as tight as you can for the cleanest edges and to preserve your blacks and whites as much as possible. To do this, slide the black cutoff up until you see some of your background keying out, then back off a few clicks to allow for variations. Slide the white cutoff down until you see your background keying out, then back off a few clicks. You may want to turn on "generate matte" while doing this, so it's obvious when you've gone too far.

* If you're doing a test render, to check your settings across your entire clip, turn off the matte blur and, in Final Cut Express/Pro, turn ON motion blur to disable the edge enhancer (slow). You can reverse these settings for the best possible final key.

Sources for Chromakey Stuff


* Colored Gels for your lights: I've purchased gels from various places, but prices vary wildly, and sometimes you must purchase sets of gels that contain lots of gels you may not need and cost $25/set. The best deal I've found (from a vendor I trust) is at Parts-Express. You can try this link, if it still works, it will take you straight to their colored lighting gels.

* Cheap Gaffers Tape: I found some "Duck Tape" brand duct tape in "Island Lime" that is close enough to chroma green to act as gaffers tape on your set. This stuff is made by "Henkel Consumer Adhesives, Inc.", and I found it at the local craft store for $2.99/roll (15 yards).

* Making your background: I tried a few things that worked Ok, but I finally settled on a large piece of muslin cloth that I actually painted with green, exterior latex paint that I purchased from a home improvement store. A gallon of paint goes a long way. I've taken to painting everything green, these days. It's making the lady in my life crazy, but it sure makes it easy to shoot green screen tests when all the walls and appliances in the house are a bright, chroma green!

This just in! One of my customers in Texas found that green spandex makes an excellent background. It's inexpensive, lightweight, and stretches to avoid wrinkles that cause shadows.

* Suspending your background: I've found nothing works like PVC pipe, when you are wanting a giant erector set! I'll try to get some photos, sometime, but I built a 10ft high by 10ft wide (about 3 meters by 3 meters), freestanding, frame with horizontal supports, that I use to setup my green screen where I've got that much clearance. It also comes apart for storage and travel!