When 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, timeconsuming 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.

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.)

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 above (Fig. 1) is our first 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.

Video scopes make these problems even clearer. Since all major video editing software has video scopes built-in, we can easily check levels on set before beginning production for the day.

There are two relevant scopes:

¢ Waveform Monitor (Fig. 2), which measures gray-scale (light and dark) values

¢ Vectorscope (Fig. 3), which measures color values We use the two scopes in tandem to evaluate an image, because, for a key to work, we need to be able to isolate a SPECIFIC background color from all other colors in the image. Since a “color” consists of three values: hue, saturation and gray-scale these two scopes help us make sure we have a clearly isolated color.

Here’s that image displayed on the Waveform Monitor. See how the green values range from about 12% up to 50%? In fact, some of the green is darker than the performer. This huge difference in value will cause major problems in editing, because we won’t be able to isolate the talent from the background; all the colors are blending together.

Here’s the same shot, this time using the Vectorscope. Look how the colors are desaturated (close to the center) and range in value from close to gray to kinda-sorta-leaning-toward green. Maybe.

There’s no clear-cut color to grab. This causes edges to tear, strands of hair will be lost, and, in general, the fi nal key will look awful.

KEY RULE: The more of the actor’s body you need to see, the better your background and lighting need to be... or, plan to spend a LOT of money in post cleaning this up.

A better way to work

Compare that first image (Fig. 1) with the one above:

¢ Perfectly smooth background

¢ Evenly lit background

¢ Rich color

¢ And note that the actor is lit dramatically, while the background is lit evenly.

This will look magnificent when the key is pulled, even down to individual strands of hair.

Here’s the Waveform Monitor for that second image (Fig. 4). Look how even and tightly controlled the background is - almost exactly at 45% and dead smooth. This is a near-perfect example of how to light a background.

Here’s the same image on the Vectorscope (Fig. 5). Look at how clearly defi ned that green is, and the color separation between the background and the talent. This is exactly what you should expect to see when monitoring your effects on set.

Summary

I know that production never has enough time, but when it comes to chroma-keys a few extra minutes properly setting and lighting the studio for a chroma-key shot can save bundles of time and dollars later in post.

Whether you are a studio-owner or a producer, keep these rules in mind when considering a green-screen shoot. Pick facilities that can make your life easier and less stressful by giving you the room, surface and lights to make your keys look great.

And don’t be afraid to record a quick test clip and load it into the editing software of your choice. Check the shot on the scopes. It is far easier to fi x lighting and set issues in production, than work around them in post.