There are four areas to consider when pulling green screen shots, and the better prepared you are to handle these areas the more successful your composite will be. The four areas we will be dealing with are:
Remembering to work smarter, not harder, this article will serve to identify the challenges in keying, both on-set during recording and in post-production. There are numerous sources that go in-depth on the following points that you can find online or at your bookstore (support your local bookstore!), but this article is meant to illustrate the broad stokes for the artist who is already familiar with compositing or would like to refresh themselves on the art.
Green Screen/Blue Screen/Red Screen?
So what color should you shoot against? There are a few good reasons to go with green.
With the vast majority of all digital cameras (even the expensive RED Epics and their ilk), the green channel in RGB composites have the highest luminance value between all three channels. Due to the common Bayer Filter method of CCDs and CMOS sensors at 4:2:2 (see image on the left), there are far more green photosites on the chip (a ratio of 2:1 per other channel); this gives you far more data to work with and the least amount of digital noise. More on the Bayer Filter system later, but it’s also the reason why all the digital cameras that shout about high megapixel values are all basically lying to the consumer and the world (definitely more on the later).
There aren’t a lot of green people walking around, and for that, green is again the preference. It is less likely to conflict with the average person’s skin tones, clothing, or even eye color. The color red is a very dominate shade in making up the hue of various human skin tones – and of course you want to isolate the chromatic range as far as possible when pulling a key so you don’t pull value out of your subject. Of course there are exceptions to this: the subject has green clothing, green make-up, holding a green prop, or you’re including foliage in the shot.
If that wasn’t enough, another reason green screen is the color of choice is that the color green is actually easier to light with tungsten lighting – the color requires a great deal less light to illuminate it to a candle reference point compared to other colors. What does this mean? It costs a lot less in production, time and equipment.
The color blue was traditionally used throughout the decades of optical compositing because of the use of actual filmstock (think: everything Industrial Light & Magic created on the Anderson VistaVision optical printers up until 1993, and much further back as well).
Red screens are used, however they are uncommon. A good candidate for a red screen shoot would be if you are recording inanimate objects (spaceships, miniatures, etc) that contain both green or blue paint or lights, or perhaps other-worldly aliens or monsters using the same colors. Remember, in most software-based keyers, the pulled hue is not limited to shades of green or blue.
It’s important to note that if you plan on using a spare wall for a green screen set, running down to the local hardware store and picking up bright green or blue paint is going to yield you less than desirable results.
Investing in the real, dedicated paint will save you software time in the long run – like Rosco’s Chroma Key or DigiComp paints. These special paints are formulated for proper color density to give you a leg up in light reflectivity, saturation and uniformity. The proper paint isn’t cheap, so if you’re on a budget and use household paint, reinforcing the wall with colored lights of the same color will help (such as Kino Flo bulbs).