Keying / Color Keying

Technical Details

Keying isolates objects based on color information (chroma), brightness (luminance), or manual masks.

Keying Types:

1. Chroma Keying (Color Keying):

• Based on isolating a specific color (green or blue)
• Algorithm: Uses color space transformation (HSL, HSV, or YCbCr) to isolate chroma values
• Tools: Keylight, Ultimatte, Cryptomatte
• Accuracy: 95%+ for optimal setups, 60-70% for suboptimal conditions

1. Luminance Keying (Brightness Keying):

• Based on brightness/luminance rather than color
• Used for scenes with white or black backgrounds
• Method: Threshold-based or Luma-range-based
• Faster than chroma key, fewer spill issues

1. Difference Keying:

• Subtracts a reference image (clean plate) from the footage image
• Highly precise when a clean plate is available
• Vulnerable to lighting changes

1. Deep Keying / Multi-Channel Keying:

• Uses multiple channels simultaneously (RGB + Alpha)
• For images with multiple colors in the background
• Cryptomatte uses ID-based keying

Keying Parameters in Typical Software:

• Screen Color: Defines which color should be "transparent"
• Threshold: How similar a color must be to the screen color to be transparent
• Despill: Removes color casts from the background onto foreground objects
• Edge Softness: How much the object edges are smoothed
• Output Alpha: Purity and whiteness of the alpha channel

History & Development

Chroma keying was invented in 1958 by Petro Vlahos. The first practical implementation was the Blue Screen Process in the 1960s.

Software Evolution:

• 1990s: Discreet Combustion and After Effects 3.0 introduce digital keying
• 1995: Ultimatte Corporation develops logarithmic keying algorithms
• 2000: Keylight (Red Giant / GenArts) becomes an industry standard
• 2005: After Effects receives native keying improvements
• 2015: Boris FX Silhouette FX offers GPU-accelerated keying
• 2020: Nuke's Cryptomatte enables ID-based keying for CG layers
• 2023: AI-powered keyers (based on deep learning) begin to become available

Keying Workflow in After Effects (Standard)

1. Import footage (ProRes 422 HQ or RAW)
2. Create a new composition
3. Add footage layer
4. Apply Effect > Keying > Keylight
5. Activate Screen Color Picker and click on pure greenscreen pixels
6. Adjust Screen Shrink/Grow (-5 to -15 pixels)
7. Increase Screen Pre-Blur (2-4 pixels for noise reduction)
8. Set Despill Bias to "Green"
9. Adjust Output Alpha slider to 75-85%
10. Optional: Rotoscoping refinement for hair/edges

Keying Workflow in Nuke (Professional)

1. Import footage in linear color space
2. Primary Keyer: Add node (e.g., Keyer)
3. Define screen color (manually or with picker)
4. Check Keyer output (shows binary mask)
5. Despill: Apply ColorCorrect node to R/G/B channels
6. Matte Erosion: Erode filter at +2 to +8 pixels
7. Matte Blur: Light Blur at 0.5-1.5 pixels for edge softness
8. Output: Merge node to combine original with alpha mask
9. Grade spill suppression using HSL Qualifier
10. Render to ProRes 444 with 10-bit Alpha

Practical Application

Low-Budget Streaming: YouTube creators use simple Keylight in After Effects. Typical setup time: 30 minutes per episode.

Cinematic Quality: Blockbusters like "Doctor Strange" (2016) use professional keying in Nuke with senior artists. Per shot: 2-8 hours, depending on complexity.

Live-Action VFX: In "Avengers: Endgame" (2019), many superhero fight scenes were filmed with greenscreen keying combined with motion tracking. 70% of the action scenes were greenscreen-based.

Spill Suppression (Despill Techniques)

When green light reflects onto talent's skin/hair:

1. Primary Despill in Keyer: Use Keylight's Despill parameter
2. Secondary Despill: Separate curve adjustments on the Green channel
3. HSL Qualifier Despill: Isolate pure green tones and reduce saturation
4. Manual Spill Removal: ColorCorrect node to specifically correct green hair

Modern keyers (like Keylight 1.3) can automate despill up to 70%; the final 30% requires manual fine-tuning.

Quality Checkpoints

A professional key is checked for:

• No Halos: Black or white edges around objects
• No Black Holes: Transparency in solid objects (e.g., in eyes)
• Consistent Alpha: No flickering or jumping across frames
• Clean Edges: Sub-pixel accuracy at 1080p+
• Color Preservation: Original colors of the object unchanged
• No Excessive Spill: Background color not visible on foreground