What you should know about buying and shooting with Chroma Key Backdrops.
This article covers chroma key basics and reviews a few techniques that should minimize post-production headaches on your next keying project. We assume you have a basic knowledge of cameras and production equipment. Instead of talking about specific hardware or software, our goal is to explain general concepts so you can apply what you’ve learned to any situation.
Chroma Key, also known as “Color Keying” is a technique where a selected color hue from one source (image or video) is converted into a transparency mask and then composited on another source. Two of the most common materials used for color keying are paint and fabric. Lets talk about the pros and cons of each.
Paint - Paint is often used to cover floors, props, cyc walls, and wooden backdrop panels.
o Cover any shape object and almost any material.
o Easy way to cover large surface areas when fabric is not practical.
o Painted backdrop panels hold up well in outdoor conditions.
o Paint has a tendency to cause glare and is usually considered expendable.
o Extended setup times because surfaces must be constructed and painted before use.
o Painted panels are heavy and not very portable.
Fabric - Fabric is often used for backdrops, prop coverings, and bodysuits.
o Fabric is lightweight and portable.
o Quality keying fabrics are manufactured to minimize glare, wrinkling problems, and are machine washable.
o Setup time is minimal and fabric backdrops can be hung almost anywhere.
o Can be manufactured into clothing or prop covers.
o Requires fabrication to cover unique shaped objects and props.
o Requires rigging (stands, frames, trusses, etc).
o When used outdoors, fabric is susceptible to rippling under windy conditions.
Lights, Camera, Action! - Using Your Chroma Key Backdrop
Lighting is an important part of any chroma key production. When working with keying backdrops, you should not mix lighting temperatures. Work with 5600k daylight or 3200k tungsten and set your camera’s white balance with a white card. If you need to mix lighting temps use CTB gels on tungsten lights to make them daylight corrected.
How to Effectively Light Your Green Screen
Many factors determine the number of lights required to light your screen. In general, the more lights the better. Under-exposure causes grain, color, and compression artifacts and should be avoided. Lighting the backdrop is a separate process from lighting the subject. The lights on foreground elements may change based on the video they are composited on top of, but the technique for lighting keying backdrops stays the same in almost all situations.
Make sure you light your green screen evenly, minimize shadows, and set your white balance correctly. A good wayto minimize shadow problems on a fabric backdrop is to eliminate wrinkles and hang the backdrop tight. Your goal is to balance lights equally across the whole backdrop while keeping glare minimal. Place a group of lights off to one side and aim them towards the backdrop at a ~45 degree angle. Setup another group of lights that mirrors the first on the opposite side. Diffusers are not required because shadows created from one set of lights will be cancelled out by the other set. Figure 2 above shows a basic green screen lighting setup. Positioning your lights in this configuration also minimizes glare when the camera is positioned in the center. Figure 3 is a production frame grab taken from a project that utilized this type of lighting setup. For large setups, additional lights can be hung from above and placed evenly down the backdrop.
Test Your Lighting
If you have a light meter you should attempt to get the backdrop’s exposure within ¼ f-stops. If your camera displays zebra strips you can also use them to assist your lighting setup. Turn on zebra strips and begin increasing the camera’s exposure until the zebra strips begin to populate the backdrop. If the lighting is even, the zebra strips should populate most of the backdrop at the same time. If time permits you can take a sample video clip and test keying it inside your software package prior starting your production.
Color Bleed – What is it and How to Minimize it.
Color bleed is a term used when the chroma key color is reflected off the backdrop onto the subject (see Figure 4). When this happens, keying software can interpret the foreground as part of the background. There are a few simple things you can do to minimize this. For starters keep your subject as far away from the backdrop as possible and cover up areas that are off camera or not needed. You should stay away from shooting highly reflective, semi-transparent, or extremely bright surfaces. Using a back light or Rim Light on the subject is also helpful in minimizing the visibility of color bleed problems.
Focus – Keep the subject in focus and the background out of focus.
Focus is an important aspect of any successful green screen production. It can be difficult to preview focus on LCD viewfinders. A video might seem in focus on a viewfinder, but ends up looking soft when seen at full resolution. Whenever possible, focus should be previewed on a HD monitor. Most mid to pro grade cameras also have focus assist options that help. A good way to achieve proper focus is to set the camera where it will be when shooting, zoom in as far as possible, and then adjust focus. Once focus is achieved, zoom back out and frame the shot. Highlights in the eyes, hair and blemishes are all good things to look at when zooming in and adjusting focus.
Record at a higher resolution then your final output.
Whenever possible, it is a good idea to scale green screen footage down by 25-50% after compositing. Scaling the footage reduces visible artifacts. An example of this is to shoot 1080, but make your final output 720, or shoot 4k and make your final output 2k.
Shoot portrait instead of landscape.
When shooting full body shots on video, you can tilt the camera sideways (Portrait instead of Landscape). For example, if your camera is set at 1080p, rotating the camera 90 degrees almost doubles the height, giving you 1920 pixels instead of 1080. This is a good trick to utilize when shooting people on green screen.
Hopefully, what you have learned in this article will be helpful the next time you work with chroma key.