Green screen can be handy for situations where you’re not able to shoot talent in a particular location.
Have you ever shot green screen? I have. A lot. Hundreds of different projects over the years. First of all, let’s get this out of the way, “green screen” has become a catch-all term that describes a process of shooting a subject against a colored backdrop that will electronically be replaced, usually through a postproduction process called compositing or keying, with another background. I’ve shot blue screen, red screen and white screen for luminance keying, but the color green is the most commonly used, so the process has come to be known as such. However, that term is a bit like Kleenex being used in place of tissue or “Coke or Pepsi” being used in place of soft drink.
The Colors Of The Compositing Rainbow
Why would you want to use green or blue or red or any other color to key a subject over? Green screen is the most popular because it tends to work well with people’s skin tones since most people aren’t green unless they’re seasick or nauseated. Blue screen is more commonly used for scenes where there may be foliage or green wardrobe. Back in the days of film, blue screen tended to work better when film composites were brought into the compositing workflow pipeline. Why would you ever shoot red screen? I’ve shot red screen when shooting tropical plants that had a ton of green and blue in their natural coloring, but they had little to no red, so using a red backdrop allowed for easier keying.
There are also camera-related technical reasons why one might choose a particular color to key over. In RGB color space with most cameras, blue tends to have more noise than the green or red channels, so in that process, amplifying the blue channel can result in an increase in noise in the image. In the end, choosing the right color backdrop should be a collaboration between the cinematographer and the compositor based on what’s being shot and where, what’s being keyed, the colors of all of the objects in the scene that are shot live and several other factors.
Green Screen Gets A Bad Rap
If we go back 10 or 20 years in production, shooting green/blue screen used to be much more difficult than it is in 2020. You had to really light any green or blue screen perfectly, making sure the lighting was even all of the way across the screen and at the right contrast ratio on the talent and subject versus the green/blue screen. You also had to make sure that there were no wrinkles or bunching on the physical screen itself. Fifteen years ago, keying software was good, but it wasn’t nearly as good as it is today, so generally, you just had to be much more careful about all of the details.
Your screen had to be lit nearly perfectly, and you had to mitigate the green or blue spill onto your talent from the green or blue screen itself, which required enough room on your set to physically separate the talent from the backdrop to cut down on the spill or eliminate it. Shooting and keying things like eyeglasses, where you could see the background through the edges and corners of the glasses, keying wispy blonde hair, cigarette smoke or fog effects were very difficult to pull off well. For these reasons, a good amount of shooters would end up with lousy keys with ragged edges, green edges, aliasing and other artifacts.
Cameras were often 8-bit recording in 4:2:0 color space, which meant that it was difficult to push a key or clean up a key because the bit space and color space were truncated compared to 10-bit 4:2:2. At the time, there was a huge price gap between low dollar SLRs and high dollar broadcast or digital cinema cameras that had 10-bit 4:2:2 recording capability. Today, there are fewer 8-bit 4:2:0 cameras on the market, and even low dollar cameras can record 10-bit 4:2:2 footage or even 10 or 12-bit RAW, making the recording much more robust for keying. Also, shooting at a higher resolution like 4K UHD for a 1080 project allows for more detail and increases the perceived color and detail resolution when downsampling a higher resolution image. There are so many factors in today’s cameras that allow for easier and better quality keys.
Why Shoot Green Screen?
The decision to shoot or not shoot green screen is an important one. I’ve shot a lot of green interviews where when speaking with the producer or director, the logical question is, “What will the background be?” Most of the time, the answer is, “We don’t know yet.” Not knowing what the background will be presents several challenges. What colors should the talent wear so they work with the color palette in the background? Which angle should they be shot from? Straight to camera or at an angle? What size should they be in the frame? How should they be lit?
That last point, in particular, has stung me in the past. If I’m lighting and shooting a green screen interview or host segment and I’m not told what the background will be and what the overall scene will look like, how can I know how to light the scene? The default is soft, frontal, flat light. If the background that the person will be composited into is lit in the same way, great—it should match. But what if the scene is high contrast with highly directional late afternoon light? In that case, the person that we shot with the flat, soft, even lighting will always look artificial and pasted into the scene because the lighting between the foreground and background doesn’t match in the least. The exception to this rule is when shooting talent that will be composited against graphics. Then, at least the graphics can be created to match the lighting of the person.
Some reasons I’ve been told why producers want to shoot green screen:
- Placing talent into an environment that doesn’t exist in reality (virtual sets, outer space story, etc.).
- Placing talent into an environment that isn’t accessible because of travel costs.
- When shooting talent in multiple locations that look and feel diverse, green screen can allow a unified and repeatable setting.
- We want to place graphics, animation or stylized elements into a virtual background.
- Bad weather in a location we want to shoot in, we can shoot clean plates later.
- Political strife/war/factors where it would be too risky to bring crew and talent.
- We want to place talent into archival footage shot in the past.
One of the best uses of green screen is for photorealistic composites. This is where great care is taken by the production to insert the green screen talent into a background in a way that’s seamless, realistic and carefully planned and executed. The results when this approach is taken can be breathtakingly realistic; you’d swear that the talent is in the location in real life. This can be accomplished with interviews, presentations, narrative, music videos—really in any format. In my experience, this is the most difficult green screen to pull off, but when you do, it’s the most gratifying too because the shot doesn’t scream “Green screen composite!” when viewed by the audience. It looks natural and doesn’t call attention to itself.
The reason why few productions try to tackle photorealistic compositing is that it takes a lot more planning, technique, skill and resources. You work backward in this process, shooting or gathering the background plates before shooting the talent. Then when you shoot the talent on set, you live composite the talent in front the of background so that the director, DP, gaffer, props, wardrobe and every other department can see the finished composite—at least a rough version of it. This way, all of the parameters—camera to subject distance, angles used, lens selection, focal length, exposure—can all be tweaked to perfection to make sure that the green screenshot matches the background perfectly.
This is even more important in shots where the background plate shot moves and the shot of the talent has to move accordingly. If you can sync the movement between the two elements perfectly as you shoot the talent shots, it really helps to sell the illusion. You are then getting into shot perspective matching, parallax correction and ensuring that the angles are in perfect sync. In my work, photorealism is always the goal when shooting green screen. It’s rarely realized because most of the projects I shoot on simply lack the budget and resources to do so. But on those rare occasions when I get to do this, it’s always a lot of fun and very gratifying when you nail it.
To me, as a DP, the alternative to green screen is always real locations or high-quality sets. Shooting the real-deal talent in real physical locations, well lit and nicely composed is always the ultimate. As DPs, directors and videographers, I’d always encourage you to push your clients and projects toward shooting talent in real locations whenever possible. Green screen can be an incredibly useful tool, but it can also be a compromise and a logistical and post-production challenge.
If you do have to shoot green screen, your best practice should also always be to push for pre-production time to shoot tests and, whenever possible, if the background plates are real locations, push to shoot those plates well before you shoot talent. Record your camera used, raster size, codec, frame rate, camera to subject distance, lens used, focal length, ƒ-stop and exactly where your lighting sources were positioned in frame. The more data you have from your background plate shoot, the more you can apply that data to replicate the exact same lighting and perspective on your talent so that whoever does the composites finds that they have two perfectly matching puzzle pieces.