Advanced Despill Operations When Chroma Keying In Resolve Fusion

October 23, 2024

VFX artist Bernd Klimm teaches advanced techniques for managing chroma spill. Plus, learn to execute the concept of 'adaptive spill removal'.


Series

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:

  • Fusion’s matte control node contains off-the-shelf despill algorithms.
  • Using the color curves tool, you can also try spill suppression within the color corrector or a Hue vs Sat curve.
  • You can mix and match these different techniques or apply other color correction operations to achieve the desired result.

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:

  • Manually suppress green spill to achieve cleaner keying results.
  • Combine different despill methods.
  • Use adaptive despill for more natural integration with the new background.
  • Fusion nodes discussed in depth include: MatteControl, Color Corrector, ChannelBooleans, Bitmap

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.

Questions or Comments? Leave a comment!

Is this Insight useful to you? Let us know! Mixing Light is all about community discussions and we’re curious if you found this helpful, if you have something to add, or if you have more questions you need answered?

– Bernd


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