Green Screen Compositing
The green screen seems to be a major solution for many people who want to use visual effects and make scenes appear as though they were shot in unrealistic locations, while really just taking place in a local studio. The purpose of shooting on a green screen is for compositing, whether node or layer based, to combine images during the post-production phase. This process is a whole technical art on its own, but it can only be possible if the video captured in front of the green screen has been done properly. To do this, the green screen has to appear in a certain way, otherwise the canvas on which the images will be composited will not be able to actually support a clean image. Since most green screens are on stationary cycloramas, the main principle that has to be focused on is how to light it. Here are some of the top green screen lighting tips that will help you get the right image you need for proper compositing.
The most important of the top green screen lighting tips to follow is to get the light as even as possible. The green screen works because it is as solid, identifiable color. During the compositing phases the green color is replaced by another image, but it can only be replaced correctly if the entire green screen is the same color and lighting. This will not be able to occur if there are some parts that are darker than others, and when you try to chroma key you will find that the image does not come in clear and believable in all areas.
To do this you want to use soft lighting on all areas of the green screen. Oftentimes, the studio you have will have lights that are already positioned for the green screen, so make sure that they are lit correctly so that it appears even on the whole visible surface. If not, you can try to use things like large scale soft banks to light it, but this is going to take several lights and a lot more time to make even.
Once it is lit and you are doing light metering on it, you will want to use a spot meter on several different pieces of the screen, ranging from left to right, top to bottom. You will want to spot meter a representational sample of the green screen so that you can ensure that it is even.
One of the main problems that can happen with green screen lighting is that the green screen is very reflective, and the light it reflects is in the same green tone that you get on the screen itself. If this green color is reflected onto the subjects or objects that are in front of it, the compositing will be made difficult since you will have the image reflected onto the subjects.
To avoid this, you want to start by making sure that the lighting for the green screen and the subjects is completely different. Ensure that the subjects are at least eight to ten feet in front of the green screen, and have their own four point lighting set up that will cut them out from the background green screen. A strong edge light may be preferable in these situations.
The best of the top green screen lighting tips to help avoid this is to try to downplay bright colors in their clothing, or at least colors that may reflect the green screen itself. Try darker colors that blend well and will allow light to be absorbed.
It may go without saying, but an important green screen tip to follow is to ensure that the green screen is bright enough. It is common to find that in an effort to create a flat, even lighting set up, gaffers and directors of photography will allow for a somewhat darker image so that they can maintain the proper look. You will want the screen to be appropriately exposed without having to alter the image's color before compositing. The color needs to be bright enough so that the replacement image will be equally vibrant, and this cannot happen if it is a dismal green color.