Color Spill Suppression in Chroma Keying

What Is Spill?

In Chroma Key compositing, spill refers to the unwanted reflection or bleeding of the background color (usually green or blue) onto parts of the subject standing in front of the screen. This typically occurs when colored light from the screen bounces off the edges or surfaces of the subject.

For example, when a person stands in front of a green screen, green light may reflect onto their hair, clothing, or skin. As a result, a greenish halo or background-colored glow can appear along the edges of the subject. This can interfere with the keying process and make it difficult to cleanly separate the subject from the background, often causing unnatural or rough edges.

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Spill refers to the key color that has reflected or bled from the background (such as a green screen) onto the foreground (such as a presenter or an object in front of the screen)

• When performing chroma keying, we usually remove a specific color — typically green.
• However, that green color can sometimes unintentionally reflect or spill onto the edges or even parts of the foreground.
• As a result, the edges of the subject — such as around the hair or clothing — may take on a slight green tint or key color.
• This is an undesirable but very common issue known as key color spill.

Where does spill typically appear in chroma keying?

Spill usually appears around the edges of the subject, especially in areas close to the background. However, when we refer to the “interior” of the subject, we mean the core or main body — the central region that hasn’t reached the edges yet. This area is relatively free from reflected background colors, and in the chroma keying process, it plays a crucial role in accurately detecting boundaries and correcting spill.

• Interior (core body): The inner areas of the subject where the key color has not bled through or caused any discoloration.
• Foreground edge: The outer edges or contours of the subject that are more likely to be affected by spill — the unwanted reflection or bleeding of the key color.

To properly remove the background, it’s important to understand the difference between the subject’s core and its edges, since spill typically contaminates the edges and requires careful correction or processing.

On-Set Solutions to Reduce Spill in Chroma Keying

• The farther the subject is from the green screen, the less green light will reflect onto them. The subject isn’t limited to people — it can also be a product or any object being filmed.
• By lighting the green screen separately and evenly, and using a distinct lighting setup for the subject, you can significantly reduce unwanted spill from the background.
• A soft backlight aimed at the subject from behind can help brighten the edges and neutralize any green spill that might appear around them.
• Avoid reflective or glossy materials. Shiny clothing or light-colored surfaces are more prone to catching and reflecting green light. Neutral, dark, and matte colors are better choices for wardrobe and props.
• Only use as much green screen as is necessary to frame the subject or scene. Using a backdrop that is unnecessarily large can increase the chances of spill and complicate the keying process.

Spill Removal in Post-Production

Production conditions are not always ideal. During filming, factors such as the subject being too close to the green/blue screen, improper lighting, or color reflections can cause spill — the undesired presence of the key color bleeding onto the edges of the subject.

In such cases, the responsibility of fixing these issues shifts to post-production.

During post-production, it’s important to preserve all the visual details of the shot — even the fine edges of the subject that may have been affected by green or blue light spill.

To achieve this, various editing and compositing software provide tools that help reduce or eliminate color spill. These tools are commonly referred to as Spill Suppression or Despill features.

Tools in Popular Software:

Adobe After Effects

• Advanced Spill Suppressor: A specialized effect for reducing color spill, offering fine control over the intensity and balance of the unwanted key color.

DaVinci Resolve (Fusion Page)

• Delta Keyer Node: A professional chroma keying tool with built-in despill options and fine edge color control.
• Despill Node (in newer versions): Dedicated node for removing spill with full control over the target color, affected range, and intensity.

Summary

Spill in chroma keying refers to the unwanted reflection of the green or blue screen color (the background) onto the foreground subject. This causes the edges of the subject, such as hair or bright clothing, to have a halo of green or blue.

As a result of spill, when the green or blue screen is removed in the software, the edges of the subject may also become transparent or disappear because the software mistakenly interprets these spill-affected areas as part of the background.