Video Green Screen Chromakey Essentials

LED Lights, Camcorder, Green Screen

With these simple tips for creating and lighting a green screen, editors will have a far easier time pulling clean keys. The secret to success is to shoot with the edit in mind.

Shooting great green screen shots and pulling clean chromakeys may seem daunting to the uninitiated, but with the continual improvements the industry has made to camera and editing technology, going green is easier than ever. A producer with a high quality video camera and an editing application like Adobe Premiere, Apple Final Cut Pro, or Sony Vegas, already has the expensive parts covered. The rest is paint, light and technique. The good news is that the greatest difference between gorgeous green screen shots and crummy chromakeys isn’t about the cost of equipment. It’s about knowledge and execution; and those can be taught. So let’s take a closer look at the key components of setting up and shooting good green screen footage to help achieve optimally clean chromakeys in the edit suite. 

How it Works

While the technique is often called “green screening” in casual conversation, the technical term for the effect is chrominance keying, and the color green, though preferred by many producers, isn’t really a requirement. In laymen’s terms, chromakeying is a process of isolating and replacing any color value within a frame of video with another image. Any color you can shoot can be chromakeyed, so you can key out a blue sky, a purple bedroom wall or a yellow sticky note. However, television, video and film professionals generally prefer two colors: bright green and bright blue. The reason for these color preferences is a very practical one. In order to pop a person off of the background, the color behind them must be distinctly different than any color on them. Any part of the subject that is too similar in color value to the background risks becoming fully or partially transparent, and that will never work. The bright, hideously ugly green that is commonly used for chromakey effects is a favorite because it is vastly different than the color tones found in skin, hair and, in most cases, clothing. Therefore, for the purposes of this discussion we will base our technique on using the color green. 

Going Green

One of the first considerations before building out a green screen set is size. If the scripts calls for chromakeying a sock puppet into outer space, a few pieces of poster board may be plenty. However, if the goal is to shoot a full-body shot of a subject the wall must be much larger. For full head-to-toe chromakeys, the green screen will need to fill the entire frame behind the subject as well as the floor under his feet. For best results on this scale, look into one of the many professional green screen products on the market. These may be permanent fiberglass cycloramas or foldable fabric backdrops. In most cases, a combination of waist-up and head-and-shoulders shots will work well enough. Shots of this style can be accomplished with one of several low-cost, homemade green screen backgrounds that can be set up fairly quickly. Experiment by building your own green backdrop using fabric, paper or paint. Remember, the key is color, not molecular makeup. Head to a local hardware, hobby or fabric store and see what they have available. 

Fabric - Go to a fabric store and look for bright green cloth sold by the yard. Make sure the material is not too thin or too shiny. Thin, shiny fabric will not take light evenly. The more thick, opaque and matte the material the better. Also be aware of width. While you can purchase bulk fabric in any length you like, you may be limited by the width of the strip. You can certainly overcome this limitation by sewing a seam, but if that seems like too much work, consider using another approach.

Paper or Plastic - Affordable green screen backdrops made of paper rolls or foldable plastic are commercially available for purchase. 

C-Stand with Green Paper and Clamp
These work well when suspended on a frame or hung between C-Stands. You can also make your own paper greenscreen by piecing together several bright green poster boards that can be purchased at any department store or hobby shop. While not the ideal solution, a little bit of tape along the seams on the back side can net a workable hunk of green for just a couple dollars. They even make bright green gaffers tape if you want to get fancy.

Paint - With a little paint, any available section of wall can become a green screen set. Any home hardware store with a paint section will have a whole host of green paint swatches that can work for your chromakey color. The national chain hardware stores will mix up small samples for just a couple bucks. It might take two or three sample containers to get all the paint needed to paint a large enough area, but with a paint roller and $20, any empty wall can be transformed into a visual effects backdrop. Paint is still a viable option even if a wall cannot be dedicated. Instead of painting a wall, consider painting a couple 4’x8’ pieces of wall panel or insulation board. They take paint well, and can be hung for shooting, then taken down for storage. 

Lighting the wall

Once the paint has dried, it’s time to throw a little light on the wall. The secret to successful green screen keys is to light the green wall as evenly as possible. The wall must be lit up nice and bright for optimal chromakey results, so no one should expect to get good results by merely setting up the wall and shooting with available light. Because the computer needs to key out a narrow color range, the goal is to minimize any shadows or shifts in shade. Bright hot spots are as harmful as dark areas. There are a few ways to go about lighting a chromakey backdrop evenly. Which technique you should use depends largely on the lighting gear you have available.

Tubular Fluorescent Bulbs - Affordable tubular fluorescent light fixtures are a cheap and simple hardware store solution for casting even light on a green screen set. This can be achieved by hanging one or two lights along and slightly in front of the top of the set, and on the floor facing up. The number of fixtures will depend on the size of the wall.

Video Production Lighting Instruments - Small and medium sized walls can be lit with one or two video production lighting instruments. Start with a single light placed on a low stand directly behind the talent, pointing back at the wall.  

Two Kino Flo Celeb 200 Lights
Soft lights distribute the light more evenly, but you can also try using a hard light with an adjustable beam set to flood and place diffusion over it. For head and shoulders shots, this may be all the set lighting that’s necessary. For larger sets, dedicate two additional lights positioned in front of the backdrop and out to the sides. These can be physically moved closer to, or farther from the set to adjust where the beam falls and with what intensity. 

Separate the Subject

The last piece of the puzzle is lighting the subject. One of the most common mistakes that causes failed chromakey shoots is positioning the subject too close to the wall. It’s important to put as much distance as possible between the subject and the set. This is important because light bounces. Light reflecting off of the green screen can create a green halo around a subject’s hair, shoulders and skin that will wreak havoc when trying to pull the key in post production. A subject too close to the backdrop can also cast catastrophic shadows on the set. It is best to not try to light the subject with the lamps that are being used to light the wall. Instead, pull the subject away from the green background and light them with a separate set of lighting instruments. If you can, use a backlight with a minus green gel over it to help counteract any green spill from the wall.  Likewise, take care to make sure that the lights for your talent do not spill onto the background and wash out the color. 

Camera Considerations

There are a few camera considerations that chromakey shooters need to know before they roll. One important tech spec is color subsampling. Cameras and codecs that compress color at 4:2:0 or 4:1:1 color sampling lose chrominance information, and neither of these sample rates is an ideal option for pulling clean keys. For ideal results, the footage should be shot with a camera using 4:2:2 color space (or better, 4:4:4).

With a smooth, evenly lit green screen backdrop and a well-positioned subject, shooting high quality chromakey footage is a breeze. Most of the magic of chromakey happens before the shoot, in building and lighting the set, and in post production when the key is performed. With these simple tips for creating and lighting a green screen, editors will have a far easier time of pulling clean keys. The secret to success is to shoot with the edit in mind. 

Sidebar

Lock it Down

Lock it down
Because the subject will be composited over a new background, camera moves, whether in the form of handheld shots, lose tripod drags, pans, tilts, or zooms, are not good partners for chromakey shooting.  To get this right, you’ll have to mark your wall and track your camera in post production, which can be really tough to pull off correctly. Shooting chromakey with the camera solidly locked into position on a sturdy tripod is a much simpler process. So unless you’re adventurous, set your framing, hit record, and step away from the camera.  It can save you hours in the edit room.


Chuck Peters is a 3-time Emmy Award Winning writer and producer.

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Chuck
Peters
Mon, 05/12/2014 - 8:58pm