Here are some advanced techniques for removing spill from your keyed footage.
One of the quickest ways to ruin the production value of your compositing and VFX shots is by leaving green or blue spill on your keyed subject. We’ve all dealt with it, from seasoned professionals to beginners — the dreaded green spill is an obstacle for any keyed footage.
There are a few different options in After Effects for removing spill. Some are more obvious or are built in to existing plugins. In some cases, spill suppression is automatic. It’s important to know your options and how to use them to your advantage. Daniel Broadway from aetuts+ created this extremely helpful and informative two-part series that covers spill suppression in great detail.
The Starting Point
Here is an extreme example of an effects shot with a pretty ridiculous amount of green spill. In this case, the brightness of the screen itself is causing the spill. A quick fix on set is to just turn down the brightness, but — as we all know — we don’t always have this kind of control on set.
So, if someone handed us this shot, how would we take care of that nasty spill?
Keylight’s Built-In Spill Suppression
Referenced at 4:39 in Part 1 of Daniel’s Video.
The built-in keylight plugin in After Effects is very effective — it also defaults to built-in spill suppression once you select your key color. However, it can look pretty extreme.
You can see what the keylight spill suppression is doing by changing the selection in the view dropdown from final result to corrected source.
As you can tell in the above example, it does remove the green spill, but it turns it to a nasty, deep brown. This may work in some cases, but there are better alternatives.
After Effects Advanced Spill Suppressor
Referenced at beginning of Part 1 of Daniel’s Video.
After Effects comes with an extremely useful effect called Advanced Spill Suppressor that, in many cases, will remove spill very effectively.
In the above example, I dragged the effect onto my keyed clip, and it did all of the work for me. The effect is automatically set to chroma-key green, so if you have a blue key (or any other color), you’ll need to dive deeper into the settings.
In this case, it was a great tool for the job; however, I’ve seen it do some very strange things to skin tones. It sometimes turns skin a dull-gray color or adds strange noise to the footage. In some cases, it’s the perfect choice, and in some others it’s not.
When removing spill, sometimes you have to try out every tool you have. There is only one method that, in my opinion, almost always works.
Green Limited By Average of Red and Blue
Referenced at 4:06 in Part 2 of Daniel’s Video.
In my opinion, this approach is the holy grail of spill suppression.
It’s my favorite way of removing spill while still retaining great skin tones and not affecting as much of the color data in the rest of your scene. In the above example, you can see that the skin tones remain a nice, normal skin color, the green-ish tones of the rest of the scene are mostly still there, and everything has remained very natural-looking.
Under the hood, the process takes any values in the green color channel and limits them to an average of the red and blue channels’ values. In other words, it pulls the green out, only to an extent that the red and blue channels are exactly as present — which allows you to still maintain a reasonable amount of green color. It just works.
To use this method, duplicate your keyed footage layer. On the top layer, set the transfer mode to darken. Then add the channel mixer effect on the top layer. Then, you’ll set the Green-Red channel to 50, the Green-Blue channel to 50, and the Green-Green channel to 0.
As with any spill suppression method, it’s not a complete guarantee. There will, at some point, be an occasion when it won’t work. However, I haven’t encountered one yet myself.
In some cases, it will alter your color significantly, especially if your footage has a green cast to it. If that’s the case, you can always add in a bit of green in color correction to bring some of it back.