Effortless Green Screen VFX: Master These 4 Tips for Seamless Compositing
Perfecting Your Green Screen Compositing Process
No symbol or concept is more synonymous with the art of visual effects than the green screen. Since its inception, it’s been a core component of nearly all marriages of reality and vfx artistry. The green screen is a tool just like any other and takes experience and skill to get right.
Frankly, the green screen is taken for granted in today’s world of crunchy key lights and drag and drop effects. Keying is not easy, and a lot has to go right between pre-production and final renders to get a truly masterful composite using green, blue, or any type of chroma key screen.
As many in the industry who’ve worked on smaller independent, commercial, and even student work know: shooting green screens can often go wrong. Though a real compositing wiz can recover a poorly shot screen, you can never fully outrun the age old adage: crap in, crap out. Whether production is strapped on time, budget, or simply experience, implementing any or all of these tips can save a lot of headache in post.
Stand Far Far Away
This is a simple one and the first things most people will tell you about a green screen is it (usually) requires the smallest amount of effort. Positioning your actors and your foreground elements as far as possible from the green screen will easily help to eliminate several problems such as unwanted shadows and spill from your screen.
The limitation here is, of course, the size of your screen. When planning for your shoot consider the goal of 6-10 feet of clearance from your green screen. Given location and resources, this might not always be possible, but it’s the first step to a successful key.
Mind Your Materials
The second basic consideration before you start lighting your screen is to mind your materials. Wardrobe can often be a thorn in your side here but it’s important to think very deliberately about what materials will and won’t play nicely with your screen.
Anything that is reflective, of course, is the worst offender. Try to avoid these situations as best you can with proper planning and consideration. Even bright white clothing made of materials that are going to bounce light more prominently can cause issues.
Oh, and of course, avoid clothing that is the same color of your screen.
Proper Exposure and Lighting
Lighting is the most complicated and difficult part of perfecting your green or blue screen shot. Light, of course, is the ultimate factor when it comes to any work in film or video. It determines everything about how your camera sensor (or film) perceive the world.
In turn, we control the green values which will be delivered to the compositor. Be nice to your post-production team, and give them something good. When going for proper exposure and lighting of your screen, the first understanding is that you don’t want to match the lighting of your background and your foreground. This is a common misconception. Instead, you want to light your screen so that the respective color value is in the upper end of the H&D curve, or 90% range of video.
What this means is that the green values of your screen are exposed at the same level that they would be coming off of a white (red, green, and blue) shirt which was exposed properly. This properly separates the green channel from the foreground colors without clipping your data.
One trick you can use, outlined in the trusty VES Handbook of Visual Effects, is to compare your screen by eye through your camera to a properly exposed 90% reflectance white card. With a filter on your lens to match the screen color, the card will appear as that color. Now, adjust the exposure of your screen until you can no longer distinguish between the white card and the background behind it.
This trick will put your screen perfectly in that sweet spot on the upper end of your exposure.
Even Exposure and Lighting
The biggest complication you’ll run into once you’ve begun to nail down your lighting is the need for even levels across the screen. In an ideal world, your screen is exposed exactly the same from edge to edge and corner to corner.
While reaching perfection may be a hard task, you can focus mostly on the areas which are behind and around your foreground elements and easily come out with a clean and even screen. If you have a light meter, you can simply walk along the screen and check your exposure as you go. You’ll know where you’re too low and where you’re too high.
If you don’t have a light meter, you can try to eyeball it with your camera/monitor if you have the correct channel view settings. Otherwise, you can give the app Green Screener a go. Though it’s not a perfect tool, it will give you some good information while you tweak your screen.
The perfect green screen is rarely possible as budget, time, space, and many other factors aren’t going to be perfectly within your control. Regardless, with a deliberate and careful approach, you can take small steps towards saving time in post and improving the final product as well.
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