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Chroma Keying Part 5: Mastering Advanced Despill operations and Color Corrections based on Spill Maps.
In this Insight, we continue with our Chroma Keying in Fusion series. This time we’re tackling the important topic of ‘spill’ – when the colored light of our chroma key wraps around the foreground and taints the natural colors of the foreground. If we don’t tackle the spill, our composite becomes less convincing and can pull the audience out of story.
You will step up our efforts from earlier in this series to learn advanced techniques for managing spill, particularly in challenging scenarios with reflective surfaces or complex edges. We will remove the spill while maintaining luminance and leverage a spill map for advanced color correction or adaptive despill to naturally blend the keyed image with the new background.
Understanding the nature of ‘spill’
Spill is caused by two different effects when shooting green/blue screen footage (or any other colored background).
First, we can have green light from the screen that falls onto the actor and leads to a color cast.
Sometimes, skin tones turn slightly towards green or yellow, or bright clothing shows green spill. We may simply want to remove this spill. However, depending on the new background, we may decide to preserve the luminance from the spill or even replace the spill light with a different-colored light to emulate the new environment.
Second, we have colored artifacts where pixels contain mixed information from the foreground and background.
Examples of this include soft edges, loose hair, or semitransparent material. Simply removing or neutralizing the spill can be successful; however, replacing the green spill with the actual colors present in the new background, which simulates how the new background would interact with those pixels, frequently yields better results.
Replacing the spill color with the new background color leads to an adaptive despill technique.
The order of operations for spill removal
No matter how we remove the spill, the first order of business is to find good methods of removing the excess color:
Building a ‘spill map’
After removing the spill, you can choose to analyze exactly what you have removed by computing the difference of the colored channels between the despilled image and the original. This difference gives you a ‘spill map’ – a greyscale image that indicates which areas contain the most spill.
This map can guide further color corrections to better integrate the image with a new background. For an adaptive despill based on the new background, you can blend a blurred version of the background into the foreground based on intensities prescribed by the spill map.
Key takeaways from this Insight
By the end of this Insight, you should understand how to:
Download the footage and follow along
In Part 1 of this series, I shared the download link for the footage I’m using so you can follow along and practice. After logging in, any active Mixing Light member has access to the download.
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– Bernd
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