Your imagination is the limit with chroma-keying!

By Miri-Joan de Wet

Are you shooting a scene that requires your character inside a volcano? What about in the middle of the deep, dark sea? Or perhaps even in space? Well, good luck getting all your filming equipment and crew there.

Of course, this is a ridiculous scenario. Not only will this be incredibly dangerous, costly and in most cases impossible, but it also isn’t necessary. Not with the technology of chroma keying.

When done right, this will leave your character with clean, crisp edges. In other words, them jumping into the volcano look incredibly realistic. However, when done incorrectly it can cause jagged edges around your character, give them a strangely green tint or even a green halo. We spoke to Red Dot Films’ Art Director, Yousif Al Hamadi, to get some of his insights about chroma keying.

So, let’s start with the obvious: what is it? Chroma keying is a visual effects technique that allows filmmakers to layer two images on top of each other based on their colour hue and chroma range. When keying you basically remove the green screen background in post-production editing and when keyed correctly the background will be completely transparent. This allows you to then fill that transparent area with whatever image or background you need for your scene.

The most common background used for chroma keying is green. However, as Al Hamadi explains it does not always have to be. Al Hamadi says, “It can be any colour, but the foreground.” Some filmmakers use blue or even red sometimes. Most often, a bright green is used because it is seen as the colour furthest away from human skin tones. The most important thing here is being aware of what you’re filming, this will determine the colour of your background screen.

If you are using a green screen background, make sure that nothing in the foreground is green. Al Hamadi explains that, “when you key, all green things will be removed once keyed out.” This can result in very awkward situations if your character is, for example, wearing green pants. In these cases, consider using a blue screen to avoid a shouting editor in post-production. 

The difficulty with this also comes to when anything reflective is needed in the scene, such as reflective metals or jewellery. The green reflection in these items will also be keyed out which could make those sword-fighting scenes a bit complicated to edit.

Using chroma keying is an amazing way to transport your actors into unimaginable worlds, but it does not come without its headaches. There is a lot of work and time that needs to be put in to assure clean keying before you’ve even started filming. We’ve already mentioned that choosing the colour of the background screen is vitally important, another major factor to consider is your lighting.

An unevenly lit screen will make the keying process difficult. Many people try to light the actors and the screen at the same time, but this is not recommended. Firstly, you’ll want lights pointed at your screen so that it creates a nice even surface on your screen and secondly, it’s vital that the lighting on your talent matches the light of the scene that you want to place them in. In other words, if you’re going to place them in a dark and dingy ditch, you probably should not be lighting them the same way your screen is being lit.

If you’re considering using a green screen Al Hamadi stresses the importance of hiring or consulting with a knowledgeable gaffer who knows how to ideally light your screen and talent. Lucky for you, Red Dot Films has roster of talented crew suggestions for your next production.

Some of the other issues that may hinder a clean and crisp key include motion blur and shadows. When using a green screen, the motion blur blends with the screen making it difficult to key without losing some of your subject in the process. “To avoid this, have your subject move slowly. But if you require fast movement, use a higher shutter speed to avoid blur,” Al Hamadi explains.

Shadows are also something which can complicate your keying process because the dark areas on the screen caused by the shadow are too similar to the dark areas that the shadow cast on your subject, making it difficult to key them out perfectly.

So, what is the alternative to the chroma keying process? In recent years the use of large LED screens has become increasingly useful because of how they’ve developed technologically and the accuracy they provide. When used in the background, they can supply good film material that appears real and this shortens the time spent on chroma keying.

The LED screens shorten your post-production process because it takes care of many of the issues we’ve previously discussed. Such as providing realistic and accurate reflections, realistic ambient lighting and creating an immersive on-set experience. However, Al Hamadi explains that although these screens are useful and have their uses they cannot always be used. “Numerous cases force production to use chroma or green screens.”

The good news is that if you do all your on-set work right; chose the right screen colour, lit everything correctly and looked out for those blurs there is some incredible software available that keys your footage with high quality and crisp, clean finishes.

So, avoid the old adage of ‘fix-it-in-post’ and your chroma keying process should transport your audience into unimaginable worlds. And remember, as our art director says, “it doesn’t look very natural for any human to have a green tint to their skin.”

Sources:

Yousif Al Hamadi, interview, (March, 2021)

Blankenship, T., 2019. How to Shoot a Green Screen for Perfect Chroma Keying. [online] The Beat: A Blog by PremiumBeat. Available at: https://www.premiumbeat.com/blog/tips-shooting-blue-green-screen/

Deighton, C., 2020. THE PROS AND CONS OF LED SCREENS AS GREEN SCREEN REPLACEMENT – Brompton Technology. [online] Brompton Technology. Available at: https://www.bromptontech.com/the-pros-and-cons-of-led-screens-as-green-screen-replacement/

Renée, V., 2015. 5 Secrets to Pulling Off a Hollywood-Level Chroma Key (Besides Good Lighting). [online] No Film School. Available at: https://nofilmschool.com/2015/09/5-secrets-pulling-hollywood-level-chroma-key-besides-good-lighting

Yeager, C., 2019. Everything You Need to Know About Chroma Key and Green Screen Footage – The Beat: A Blog by PremiumBeat. [online] The Beat: A Blog by PremiumBeat. Available at: https://www.premiumbeat.com/blog/chroma-key-green-screen-guide/

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