Keying is one of the core components of compositing. No one key is absolutely perfect, and the tools which allow us to pull out those ugly backgrounds are constantly changing.
In The Foundry’s Nuke, several top of the line keying packages are right there from your first launch, but their application is far more complicated than a simple drag, drop, and adjustment. To key well in Nuke you need an expert understanding of several concepts. From spill suppression to alpha math and formulas, these concepts are solidified through experience and practice.
For beginners, there are some less obvious points to consider when approaching an advanced key. More than the buttons you press, the way you approach a shot can mean the difference between a nice clean alpha, and a noisy swiss cheese of black and white. Here are three tips to remember as you key in Nuke.
Identify Problem Areas
Before you start your key you want to look at your footage… Really look at your footage. One of the most common mistakes with keying (and related compositing workflows) is jumping right in without a plan.
The reason this is especially important for keying is to identify problem areas. These problem areas are portions of your scene, which you can tell by scrubbing through, that are going to give you trouble. This is usually due to either poorly lit areas of your background causing an uneven color or materials on your actor/foreground which either reflect the color of the screen or have transparency like fur/hair.
The best way to look at your footage is through Nuke’s built-in channel viewers for Red, Blue, Green, and Luminance; these are enabled using the “r”, “b”, “g”, and “y” keys respectively. Flip through the channels and pay close attention to the differences you see between your foreground and background, especially in the color of your screen.
Once you’ve identified these areas, you can start to consider the best method for tackling them and sketch out an outline of your comp.
Prep your Plate
As Abraham Lincoln always taught us: preparation is key. Whether you’re actually chopping down a tree or pulling an alpha off a green screen (sometimes closer to grey or brown), the first step of the process can make or break your final success.
The most frustrating part of keying is working on bad footage. For independent and student productions especially, there isn’t always a VFX Supervisor on set and the cameras aren’t kind when it comes to noise and edge mingling.
Your first step to combatting this is denoise. The industry standard here is a plug-in called Neat Video, but in many cases, Nuke’s built-in denoise node will do the trick. Tweak your denoise sliders until you find a good amount of denoise while still retaining sharpness in your details. Once you’ve got a good look, you want to write out your file and use it as your new plate. This is crucial as the denoise node is extremely resource intensive and will bog down your script.
The next step to preparing your footage is performing basic color corrections. Like most things, coloring in Nuke is extremely complex and takes time to master, but the fundamental practices are universal. This step is about correcting the screen if there were issues while shooting it. Make sure your white balance is correct and your shot has correct contrast and exposure. While you make adjustments, continuously sample your footage and keep track of what you’re doing to the channel values. The goal is to create an even difference between your foreground and background elements.
Use Multiple Keys
This final tip is the most important for beginners in terms of setting the expectation for future shots. Keying takes a lot of time and is far more involved than what it may appear when transitioning to more difficult shots in Nuke.
Even with plates that are captured professionally, chances are you’ll need multiple keys. Once you start introducing elements such as hair, windows, glasses, and other high detail materials, you will find that a part of your key which works perfectly for your actors left arm doesn’t work at all for their right.
This tip goes hand in hand with identifying your problem areas. As you gain experience keying, you’ll start to know right away where you’ll need to use individual or specific keys for certain parts of your footage. With hair, for example, you’ll want to be much more careful with your edges and take more time preserving detail than you’d need to on the shoulder of a leather jacket.
If you have unwanted elements in your scene such as stands or tracking markers that are more complicated than a garbage matte can address, you can key these separately as well—anything to save yourself from roto work.
Once you have your keys finished and ready, you can use a Keymix node to combine all of your alphas into one master key. The keymix node is very powerful and offers several options for merging multiple keys, so keep track of the math it’s performing on your alphas and read through The Foundry’s site! First time here? ActionVFX creates action stock footage for VFX and filmmaking. (We also have some great free stuff!)
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