The Basics of Green Screen Effects

Green screen compositing today is ridiculously easy compared to days gone by. Years ago, when shooting and finishing on motion picture film, one had to shoot the subject against a green screen, then create film mattes, and combine it all using an optical printer. Generation loss, contrast shifts, and registration problems were the bane of the filmmaker. This doesn’t even begin to encompass all the other problems, such as matte lines, elements that drift out of alignment, transparency issues, and garbage mattes. The introduction of high-resolution film scans and electronic compositing made things better, but it is only in the past few years that one could say green screening has become available to pretty much anyone. There are even cell phone apps that let you composite in real time. I know—I used it during my daughter’s birthday party.

The Process

Creating a green screen composite can be broken down into a few steps: shooting, pulling the key, and compositing the foreground with the background material. Pulling the key refers to using your software to create the area of the image that is transparent. This is usually anything that isn’t your foreground subject, and the area to be made transparent is your green screen. So, if you want to composite your actor into deep space, you shoot that person against a green screen, pull a key—also known as creating a matte or an alpha channel (the alpha channel contains the transparency info). Now that your software knows what part of the image is transparent, you can key/composite your actor into a background. Everything that you can do in the shooting process to make pulling your key better will end up making for an easier and better composite. At a kid’s party, I’m not going to care too much about the quality of the matte, but on anything else, unless I’m purposefully going for a bad compositing look, I want to create the cleanest, most seamless composite I can. To aid you on this journey, here are a few things you should try to do, and things to avoid when shooting for composites.

Resolution

The higher the resolution of your original material, the better. Remember—your image is going to end up as pixels, and pixels are lined up in a grid, but most of the objects you are shooting for your foreground subject aren’t going to fit neatly into a grid, so the more resolution you start out working with, the finer edge detail you are able to capture. This will allow for creating mattes with smoother edges, which make for more realistic composites, as opposed to lower-resolution elements with their blocky edges.

Progressive or Interlaced

Progressive, always shoot progressive. This is a no-brainer. Interlaced is still available, but it is a horrible origination format, especially for any kind of compositing. Look at it this way: each interlaced frame is made up of two half-resolution fields that are captured slightly after each other. This can create what are known as temporal artifacts. It isn’t that big a deal when capturing still life subjects, but once there is movement involved, whether camera movement or the subject moving in frame, you get a drop in resolution and problems with your edges because there is movement between the odd and even fields that make up an interlaced frame. Generally, you are better off shooting a lower-resolution progressive video than a higher-resolution interlaced frame. For example, 720p is better than 1080i , especially when you consider the resolution of 1080i is closer to 540p than 720p. Even if you are delivering a finished project in interlaced, it is far better to start with progressive elements, then convert to interlaced after you’ve composited.

Get the Right Screen Color

Green screen is what is usually used because green is furthest away from most skin tones, and so should have the least effect on skin tones when keyed. Green is also the color channel that always has the most information, unless you are recording your footage at 4:4:4, and it tends to have the least noise, which makes for a cleaner key. However, there are times that green is not the right color screen to use. For example, if your subject is wearing clothing that has green in it, getting a clean key might be more problematic. Blue screens tend to be preferable for subjects with blonde hair, because generally, they pull a better key. The skin tone of your subject should be taken into consideration when choosing whether to go with green, blue, or some other color.

Distance Matters

You want to keep your subject as far away from the screen as possible, and keep your focus on the foreground subject.

Lighting

Light your foreground subject and your screen separately.

Lighting your Screen

It may seem counterintuitive, but when shooting a green screen, you want to saturate the screen—in other words, darker is better. There are a few reasons for this. One is that a well-saturated screen provides good color separation, while a bright screen can be harder to key. The rule of thumb is to have your screen be ¾ of a stop darker than your key light. In addition to pulling a better key, a darker screen will project less reflection, also known as spill, onto your subject. This spill can contaminate your foreground subject, making it more difficult to pull a matte/key with clean edges. Shadows are a definite no-no. Avoid casting shadows on your green screen. Shadows are going to cause you lots of headaches when you try to pull the matte, either forcing you to expand the color/chroma that you are keying, which could affect your foreground image, or you will have to start using garbage mattes or start rotoscoping your mattes (drawing your mattes by hand).

Lighting your Foreground

It is a good idea to consider the environment into which you are compositing your foreground subject when lighting them. If they are being composited into a night shot, it may not make sense to have them brightly lit, since they won’t necessarily fit into the scene.

Conclusion

Green screen work is used to great effect in many films, and can be quite unnoticeable, but it wasn’t always that way. I hope that this article has helped speed your way to creating believable mattes that engage your audience in your film. For more on the green screen process, check out our Beginner’s Buying Guide for Green Screen and Software Tools for Green Screen/Chroma Key. To watch video about green screen techniques, click here or here.

Thanks for reading, and if you have any questions, thoughts, or experience with green screen compositing, please feel free to leave them in the Comments section, below.