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EXPERTS An editor’s perspective: Chroma-key by Larry Jordan W hen it comes to chroma-key (also called “green-screen key”) the phrase “Don’t worry, we’ll fix it in post.” is a recipe for disaster, because if you don’t shot your key right during production, it will be painful, time- consuming and expensive to fix it later during editing. I know, because I’m an editor. Bad chroma-keys are a train wreck and it isn’t even the editor’s fault! Pulling (“creating”) a high-quality key starts with a bit of planning before the shooting even starts. Since this is a “Studio Issue,” let’s talk keys and production. Background What a chroma-key does is select a specifi c color, generally green but it could be any color, and make it transparent. When shot and lit properly, this allows us to replace a green background behind an actor with an entirely different background. Piece of cake, right? Sigh... Nope. The key phrase is “select a specifi c color.” The problem is that many production crews think close enough is good enough. And it isn’t. Planning your production The best thing you can do to improve the quality of your keys is to improve how you shoot and light them. Here are nine rules for green-screen production: 1. The green screen background should be as smooth as possible; ideally, painted on a smooth surface. Never paint green on a textured background. If you are using fabric, iron out folds or ripples and stretch it tight. 3. Light the green screen evenly from top to bottom and from left to right. Ideally, the green should land between 40-50% on a Waveform Monitor. 4. There is NO relationship between how the background is lit and how your actors are lit. Light your background for smoothness. Light your actors for drama. Never light actors with the same lights you are using to light the background. 5. Actors should be at least 10 feet in front of the green screen. This avoids light from the background “spilling” around their body or shoulders, creating a blurry edge. 6. In general, don’t have actors cast shadows on the green screen. Be very careful shooting feet. If you need to deal with shadows and feet, expect to pay MUCH more for software to key them cleanly. 7. The green background does not need to fi ll the frame, but it DOES need to completely surround the edges of your actors. Editors use garbage mattes to get rid of the junk. 8. NEVER shoot interlaced video for your green-screen (foreground) shot. Always shoot progressive. Interlaced edges are very, very hard to key. (And, don’t worry. It is easy to convert a progressive clip to interlaced after your effects work is complete.) 9. It is often better, but not required, to shoot a higher-resolution image as the green-screen foreground shot, then reduce the image size during editing. For example, shoot 1080p, then edit in a 720p sequence. This can often improve edge detail, chroma saturation, and the ability to create a cleaner key. Catastrophe in practice Let’s compare two chroma- key shoots: one where it was more important to “get it done” than to get it done right, and the second where a bit more time was taken to make the key look as good as it could on set. The image below is our fi rst example. (Sorry, but I don’t have permission to show the performer’s face.) Look at how many of our “Key Rules of Production” this violates: • There are multiple shades of green. • The green is very unevenly lit; ESPECIALLY around the performer • There is almost no color in the frame; it is very desaturated • There is motion blur on the performer, making edges hard to determine • The performer is casting shadows on the green fl oor • There is all kinds of junk in the frame • The actor is lit using the same lights that are attempting to light the background The only good news is that the green screen behind the performer doesn’t have any folds or ripples. Um, yay. 2. The exact shade of green does not matter. What DOES matter is that all the green be the same color. (And it goes without saying that your actor should not wear any colors even close to that shade of green.) 40 | TV-BAY MAGAZINE: ISSUE 92 AUGUST 2014 TV-BAY092AUG14.indd 40 08/08/2014 15:28