http://library.creativecow.net has a very nice and long article about On Set Keying. That’s basically when they use a green screen in studios and use the computer to put reality and virtual together live on Set. It is something quite important to place objects or actors in the green screen scene for example, also to make sure lighting is well adjusted etc.
Story by Andrew Shanks source
As an on set VFX Supervisor, one of your jobs is to make sure the greenscreens/bluescreens on a shoot get lit evenly and that spill is controlled as much as possible (thankfully most gaffers and DoPs (Director of photography) these days are pretty well versed in what is required which means our job is easier). Traditional tools to get the screen evenly lit and exposed are light meters (one of the most important tools a DP or Gaffer has in their toolbox), as well as using a waveform monitor sometimes on studio shoots (having it just pointed at a green/bluescreen you can see where unevenly lit patches can be). An easier technique DoPs often use is to switch on zebra patterns in the cameras viewfinder then rock the exposure up and down (overexposing the image) until you find a point where you can see hotspots or darker patches that need the lighting tweaked to correct them.
The above is fine for getting the screen evenly lit, but everyone on set usually wants to get a sense, even if its just rough, of what they’re about to shoot in regards to how it will look in the final scene (not just from a logistics point of view but also it can help motivate actors or solidify in their minds exactly what they are meant to be doing (e.g. “ah, so that ball on a stick is a dragon and we’re on a clifftop, riiiiiight”). In the past people have used vision mixers, but these setups are a pain when moving round to various locations (as playback VCR, mixer and power supply have to be lugged round). A popular method in the past 5 years or so has been capturing shots with a digital still camera to produce a test key on the laptop (which is a good way to go as you can load it into a compositing app on your laptop, pull a rough key (I like to keep it almost a one click key, as it then shows up the bad areas we might want to re-light), then display the greyscale matte it produces (a great visual guide to see where problems of spill are on the foreground, or where patches of potentially problematic uneven light might be on the background). The downside with the still image option is just that, it is still, you can’t see things update as you swing the light round (you can keep taking photos and transferring them, but it isn’t a very quick interactive workflow).
In recent years applications for using your laptop as a monitor for your camera (especially for use with HD cameras) have started to come out (e.g. Scopebox (mac) and Adobe On Location (PC)). Most of the early options don’t have keying capability unfortunately.
There is a lot more in this article, I therefore invite you to check it out on it’s rightful owner website here
