How to Composite Videos Using Green Screens or Chroma Keying

The purpose of this Hub is to introduce the novice video editor to the potentially powerful, yet relatively easy, video editing technique known as chroma keying, sometimes called green screening or blue screening. The basic premise is to insert or overlay a chroma-keyed image or video on top of existing video footage in order to give the appearance that the two were shot together.

The terms green screen and blue screen come from the film industry's pervasive use of these two colors as backdrops when filming subjects for compositing. However, when the situation calls for it, any color may be used for the backdrop in order to obtain the desired result. I'll explain this further in the short video to follow. At the end of the instructional portion of the video, I've included a cute little sequence that makes use of those things we'll be covering.

The software we'll be using in the demonstration may not be the same as yours, but the concepts presented are universally applicable in all software applications used to employ this technique, and will almost certainly have a similar environment, including workspaces for capturing and importing video, for editing video and adding transitions and effects, and with options to output to various video file formats or media storage devices.

Essentials: Okay, well I'm sure you've already guessed that you're going to need some equipment and supplies to accompish your goal. Following is a list of essentials. We will talk a little about each as we go along.

• Coloured backdrop
• Lighting in various wattages to cover illumination of background and subject
• Diffusion materials to soften shadows and reduce contrast(optional)
• Reflection surfaces or umbrellas for indirect lighting when preferred (optional)
• Video camera with appropriate output port
• Computer and software for importing, editing, keying and outputting to various file formats or storage devices

Okay, so let's break it down just a bit here. You're definitely going to need a backdrop of some type to record your subject against. There are a lot of really inexpensive ways to go in order to accomplish this. Depending on the size you need, green screen backdrops can be purchased from ~$15 and up. You could possibly use a length of appropriately coloured material from a fabric shop. Flat green paint on a wall may work in some situations. What I'm trying to say here is, if budgeting is a big concern, be creative in your acquisition of equipment and supplies.

You will definitely need lighting. Professional lighting could cost you many thousands of dollars. If you're just feeling the waters of chroma keying and video editing, you may want to go a different route than purchasing professional lighting.

Anyway, you will need one, two, or perhaps even more lights ranging from 200 to 600 watts to illuminate your screen.

A backlight in the range of 100 watts and up, for illuminating the subject from behind to give some highlight and separation from the backdrop.

You will need a key light and a fill light of about 1000 watts each, with some type of diffusion layer, a translucent material placed in front of a light to soften highlights and shadows, and reduce contrast. If you are going to experiment with materials for diffusion, be aware of the heat generated by your light and the proximity of your diffusion material. We don't want to melt your material or start any fires.

Of course, once you have finished shooting, you will need a computer with an available port for importing your video. Once imported, you'll need software for editing and processing your background footage and key, as well as to save your file in various formats and/or for outputting to media storage devices.

Typical Green Screen Setup: The image above shows a typical layout for a green screen setup showing one possible scenario for placement of screen, subject and lighting. It should be mentioned here that it is ideal to set up your lighting separately for both your subject and your backdrop. This will help you maintain focus on each of the two elements separately, without being distracted by the other. This also makes sense as, in the end, the two will be separated permanently anyway, so they essentially have little to do with each other in the final cut.

General Lighting Considerations: Whether or not you are planning to take the whole concept of chroma keying to the next level, you should be well aware that lighting can be very expensive. My suggestion is to initially make do with whatever you can obtain from a local hardware store and try a few keys with whatever equipment you have. If you find that you like what you are doing, you can always expand and improve your collection of equipment. If your current task is a little more demanding, you may want to rent from a local lighting supply shop before deciding to purchase. You will have to experiment on the intensity, placement, colour, and total number of lights for any given situation.

Background Lighting: In the setup above, lights L1 and L2 are used to light your screen. These should be anywhere from 300 watts to about 600 watts. As stated above, you should start by lighting your screen first, and then go on to light your subject. To find the best angle and position for your setup, you will need to experiment a little by moving your lighting around. The whole idea is to have even lighting across your screen. You want to have as little variation across your backdrop as is possible with the lighting you have. You may also want to try using some type of diffusion to soften and spread the light a bit. This will help prevent relative hot spots on your backdrop. Once you are satisfied with the way you have things set up, turn these lights off and go on to setting up the lighting of your subject.

Key Light: The key light provides the main illumination of your subject. In this case it is L4 and is in the range of 1000 watts. The key light provides the simulated source in your key scene to match whatever lighting is in your background footage. You'll want this light positioned to provide shadowing from the appropriate side of your subject, and with the right type of diffusion, if any, to match the lighting in your background. Please don't be confused when I use the terms background and backdrop. The former refers to the background scene over which your key will be composited. Backdrop refers to your green screen.

Fill Light:The fill light in this scene is L5, and is used to soften or control the density of shadows on your subject. It will be placed on that side of your subject that is opposite the key light and faces your subject. You will almost certainly use diffusion for this light, or turn your light away and have it reflected back toward the subject by a reflective surface (not highy reflective like a mirror, but something with low reflectivity like a white surface of some sort). The fill light is to be placed near the camera to avoid it casting shadows on your subject.

Back Light: The back light, L3, is a relatively low wattage diffuse light that is used to illuminate the back of your subject in order to separate them from the backdrop. In most cases, these lights are used to define a bit of highlighting of your subject, but without causing any glare. This separation gives some depth to your scene.

Okay, so those are the fundamentals of setting up a chroma key. Please click on the video below to see a short demonstration using Pinnacle Studio video editing software to composite the background footage and key. And finally, at the end of the video is the short sequence that was created for this Hub.

Well I hope this short introduction was helpful to some of you. One more thing to keep in mind. There are many choices out there for video editing software today, ranging from under one hundred dollars to many thousands of dollars. So, before you buy, Google this stuff, take a good look around at what's available, read some specs, and consider what you'll be using yours for overall. Happy editing.