Green screening (also called chroma keying) is a very useful skill to have when shooting video. Even if you’re not using an actual grey screen, it can be handy to know how to easily mask out a particular colour, and composite something else in its place. In this video, Jordy at Cinecom walks through the top five things he’s learned when it comes to getting a good key.
Keying works on a principle of contrast. This is why most chroma keying is done on either a green or blue background, because one of those colours will likely contrast with everything in your scene, and both of them are nowhere near skin tones on the colour wheel. But it’s not always as easy as it sounds, so Jordy’s been experimenting.
To test his theories, Jordy throws Yannick into a green suit with a blond wig in front of the green screen. Certainly a challenging key. He wants to see if he can still extract him from the green background despite his outfit. His goal was to be able to do it with just a single click in the keying software and some minor tweaking of the sliders.
Tip 1 – The Basics
Make sure your green screen is tight – That means no wrinkles. Wrinkles create shadows, which means more different shades of green for you to try to remove in post.Create even lighting – Uneven lighting can cause even more of a problem than harsh hard shadows. At least with those shadows, it’s easy to use the pen tool to mask out those tiny bits. But for soft gradients, it is often much more challenging. Waveforms or the histogram can help with this.Separate your subject from the background – This is to help minimise spill. Even though the light on your green screen is white, if your subject is too close to it, then green light can bounce back onto them, making it difficult to get a good key.Speed up your shutter – Motion blur, while not impossible, can be very difficult to key out easily.
Tip 2 – Use your waveform
Every colour has a specific luminance value. This means how bright they appear to the camera. On waveforms on cameras and monitors, these are easy to spot at the time of shooting. If you’re lighting your background separately form your subject, this means you can get your green screen exactly the brightness it needs to be without it affecting your subject. Separating the background brightness from the subject brightness can also help when you need to key out a green subject on a green background.
Tip 3 – Use your vectorscope
A green screen should be green. I know, it sounds obvious, right? But it’s easy to think your camera is seeing it as green when it’s not. It might be leaning a little more towards yellow or cyan due to an incorrect white balance or tint, but look right to your eye on the monitor. So, make sure to turn on your vectorscope to make sure these are set correctly. If you don’t have an external monitor with a vectorscope, you can always create a quick clip, throw it into After Effects or Premiere Pro, and look at the vectorscope in there, instead. Then adjust the camera to compensate, and shoot another test clip. It’s a slower way of getting your settings dialled in, but it works.
Tip 4 – It matters where you click
Green is green, right? Click somewhere, and it all disappears, yeah? Well, that’s the perfect theory. But in the real world, that’s often not the case. No matter how evenly you try to light your green screen, there will always be some very slight variation, and where you tell your keying software or plugin to look for a colour to use as a base is important. You might have to click around different parts of the green screen in order to find just the right spot to cover the most amount of green possible, and then you can just clean it up by tweaking a few sliders.
Tip 5 – Tracking Markers
One of the drawbacks of green screen is that it’s essentially a featureless wasteland. That’s kinda the whole point. You want it to be as featureless as possible so that your plugin can more easily remove it all. But, that can present problems if you’re using a handheld or moving camera. While you’re following or spinning around your subject, the green environment around them always looks the same. So, it’s impossible for software to know how the camera was moved. In Hollywood, they often use red or orange markers. Jordy says he’s not entirely sure why they tend to use that colour, but I think I’ve figured it out. When you look at the markers in behind the scenes footage, like this shot from Avengers Infinity War, you can see the orange markers on the green screen. The contrast doesn’t look that great between them, right? They’re both about the same level of overall brightness.
But what about when we look at the red channel alone? Here the contrast becomes immediately obvious. The tracking software can look at only a single colour channel and very easily get a more accurate track because of the increased contrast between the markers and the green screen.
I could be wrong, but that seems like the most obvious reason to me. Of course, you’ll need to mask out those tracking markers yourself, because they’re not going to be picked up by your keying software. Jordy uses green markers in his video above, and he is able to not only get decent tracking but also easily key out the markers along with the background, too. Which will work best for you is going to require some experimentation. But those tips should help to get you started. Another tip, to make life a little easier, and a process I’ve been using for years is Garbage Mattes. The Keylight plugin for After Effects is awesome, but it can be a little hardware intensive. You can make life a little easier on your processor & GPU by pre-keying some of the background out using other means, to minimise the amount of data that Keylight has to look at. This video from Aharon Rabinowitz is from way back in 2012, but it’s still relevant today, and fantastic for keeping your machine running quick when you have a lot of shots you need to key out.
What are your favourite green screening tips?
