Greenscreen Primer Part 1
With Greenscreen, 80 percent of your post budget is lost on the set...learn how to get it back!
By Alex Lindsay | March 01, 2009
I pulled my first Greenscreen in 1996 working on "Star Wars: Episode 1" pre-viz. The footage was rough (pre-vis handycam footage) and the After Effects Color Difference Key was a complete mystery. Now, the Pixel Corps, we shoot an average of two hours of greenscreen footage a day, mostly 4:4:4 uncompressed. While the tools have progressed significantly, the process itself, and the rules, have changed surprisingly little.
In this article, I'll cover the major issues you need to solve when shooting greenscreen footage. There will be future installments to discuss theory and keying technique. I will argue about 90% of the professionals out there do not use most of the information I will lay out here... and it makes their process much more difficult. I will say, everything we do in the Pixel Corps is designed to lower costs and accelerate delivery speed while constantly improving quality. We're not particular to be particular. We do everything because it shaves time or improves quality.
While some people still use a green cloth and a few shop lights, they are saving money on the front end only to pay for it later, in time and quality, on the back end. This "future" payment can often dwarf the cost of the original shoot. While you can't always get a great screen, most bad screens that we see are shot as talking heads...where there is absolutely no excuse to shoot a rough backing. In reality, you really don't need a super expensive set-up, but a few elements make a difference. The thing to realize is... 80% of the quality of your key happens on-set. If you shoot garbage on set, you will spend countless hours just to survive the composite. If you shoot clean, well lit, plates on set- you will key quickly and easily.
Let's address Greenbacking first (this is the Japanese Term for "Greenscreen" and in some ways, more accurate...as the Japanese often are). I call it "Green" not "Blue" because, well, you should almost never use a bluescreen. Kermit the frog may thought it wasn't easy being green...he obviously wasn't a compositor. Blue is the most disrespected, beaten and shortchanged channel in the digital pipeline. When compression hits...it hits the blue record the hardest. It leaves it blocky, grainy and chattering. You can feel bad for the Blue Record but you shouldn't use it for your keying operations...even out of sympathy. Green, on the other hand, is usually crisp and clean because it actually gets most of the YUV signal in the conversion. Compression will still affect it but not nearly at the level that it affects the Blue.
So, we've decided on green, but what kind of green? You can get Rosco, Composite Components, Wescott, or the green paper at the Pharmacy. At the Pixel Corps, we only use Composite Component's "Digital Green" (and no, we don't get paid to say that). On paper, it shouldn't really work as well as Rosco...but it does by a significant margin. I would not suggest Wescott pop-open screens or anything that feels like corduroy or cloth. Both of these will suffer from reflectance issues and patterns. They are cheaper, and there's a good reason. If you love Roto and lots of extra hours working on the keys...knock yourself out. But I'm lazy and prefer to make the capture process as clean as possible.
Once you've chosen the screen flavor, you need to decide the screen type. The best solution, when you can implement it, is a smooth green painted wall. It's the easiest to light evenly (which is 60% of your challenge). If you can't muster a green wall, Lycra is the next best choice. Again Composite Components Green stretched tight is your best, though not cheapest, bet. You can go out and buy a fancy frame for it but we often use 1" PVC pipe that's 1.5 feet longer and wider than the greenscreen. The nice thing about PVC is that you can get in any city...making it easier to travel. If you are going to go with paint, plan on 4-5 coats. You need it to be really green and really consistent. We also don't "touch-up screens" we repaint the wall every month or two and we really talk care of it. It's expensive, but not as expensive as "fixing it in post."
Our lycra screen hitched to PVC
OK, now we need to light our screen...
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