Production: What Is Chroma Keying and How To Do It

Chroma keying is a commonly used tool in filmmaking today. Learning about and how to use it could make a big difference in how you choose to shoot certain scenes from here on out.

Step 1: Understanding Chroma Keying

Everything in our world has a unique color tone. With today's technology, we can select any one tone from an image made out of a conglomeration of colors. So, if a person dressed in red is standing in the middle of a green field, you could tell the computer to select only the color red. However, if this field has some red flowers in it, the computer will select both the red clothing and the red flowers. For an instance like this, you can narrow the spectrum to a specific kind of red to select only the clothing. The same concept applies in chroma keying. Chroma green has been established as a seldom occurring color in our world. Therefore, this is what has been used to create greenscreens. A less used color is chroma blue. Theoretically, any color can be used as long as it is unique in your frame.

Step 2: When to Use Chroma Key

You have a script that requires a person to walk around on the moon. Unfortunately, this is a highly unpractical location. Alternately, you can build a moon set but this can be very costly. So, you decide that you want to digitally create the moon in post production. However, you still need to shoot your actor walking on the moon. Eventually you will want to paste the character into the environment that you are drawing. So, you should shoot your actor against a greenscreen so that you can easily key out the background and replace it with your own.

Step 3: Monochromatic

The only way that this will work is if the greenscreen comes out completely monochromatic across its expanse. So you will need to get it as taught as possible with no wrinkles or waves. You will then need to light it evenly. Use multiple soft lights to get an even exposure across your frame. There is some margin for error, but being diligent now will avoid more work in post-production.

Step 4: Avoiding Spill

Now you have a brightly lit greenscreen. Unfortunately, this has now created a giant green bounced light source. Light is reflecting off of the greenscreen forward. If your subject is standing three feet from the screen, they are being hit by the green spill. This compromises the effect because now the person will also become a part of the chroma key. And if the case is not quite so severe, they will still look green. So, be sure to place the subject at least 15 feet from the greenscreen. Add a hair light to fully separate them from the background.

Chroma keying could make your life a lot easier if you have taken on an ambitious project. However, if you do not do it properly, you will create more problems than you were avoiding.

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