13.02.2024 – Green Screen

Week 16 – Green Screen in Nuke

Understanding the intricacies of green screen compositing is crucial for achieving seamless visual effects.

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HSV Color Scale:

• The HSV (Hue, Saturation, Value) scale provides a numerical readout corresponding to the color names in your image.
• Using the Colorspace node, you can convert RGB channels from Linear color space to HSV color space, aiding in color analysis of the plate.

HueCorrect Node:

• Mute, suppress, or desaturate specific colors using the HueCorrect node.
• Adjusting the green (mute) shifts the green color to another hue. Adjusting _sup (suppress) removes color and luminance. Adjusting sat (desaturate) removes only color while keeping luminance.

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Keyer Node:

• Create alpha from the channel set on the operation, such as luminance, red, green, etc.
• Settings in the Keyer node yield different results, allowing flexibility based on the plate you’re working with.

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Saturation Color Key:

• Convert the image to HSV color space and shuffle green (saturation) data to alpha.
• Color grading with this alpha produces more natural results than grading without a mask.

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Texture Key:

• Blurring and merging with the ‘minus’ operation can help select areas more efficiently than the Keyer alone.

Denoising for Analysis:

• When analyzing the video with luminance, denoise the plate for clarity and smoothness.

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Adding Channels:

• Use the Roto (paint) node to add a new channel by selecting output-new. This new channel can serve as a mask in other nodes.
• Alternatively, use the Add (Channel) node to create a new channel.

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Types of Keys:

• IBK Color/Gizmo:
  • Operates with a subtractive or difference methodology, ideal for fine hair and motion-blurred edges.
  • Create a clean plate using the IBKColor node and extract areas using the IBKGizmo node.
• ChromaKeyer:
  • Works well with evenly lit screens of saturated color.
  • Takes advantage of GPU devices for efficient processing.
• Keylight:
  • Similar to After Effects, automatically provides green despill.
• Primatte:
  • A 3D keyer that uses a special algorithm in 3D color space.
  • Offers an Auto-Compute feature for step-by-step alpha data extraction.
• Ultimatte:
  • An older node but excels in obtaining detailed keys, including fine details, shadows, and transparency from the same image.