Light Wrap

The Light Wrap operator is a compositing filter that simulates the natural phenomenon where background light wraps around the edges of foreground subjects in real photography. When a subject stands in front of a bright or colorful background, light naturally bleeds around their edges creating a subtle rim glow. This effect is essential for making keyed footage look naturally integrated with its background.

When to Use Light Wrap

• After chroma keying: Apply to keyed foreground to blend naturally with background
• Green/blue screen composites: Helps eliminate the "cut out" look
• Bright background scenarios: Subject in front of sunset, sky, LED walls, etc.
• Colorful environments: When background colors should influence foreground edges

Setup

1. Layer Structure:

   • Background layer (below): Your replacement background
   • Foreground layer (above): Your keyed subject with alpha channel

2. Apply Light Wrap to the foreground layer

3. Select Background Layer:

   • Choose your background layer from the "Background layer" dropdown
   • A warning note will appear if no layer is selected

Parameters

Background Source

Wrap Settings

Blend Modes

Fine Tuning (Advanced)

Core Penetration Explained

By default (0%), the light wrap only affects semi-transparent edge pixels where alpha transitions from 0 to 1. This works well for soft-edged keys but may look insufficient for hard-edged composites.

Core Penetration allows the wrap to spill from edges into fully opaque foreground areas:

How it works technically:

• The foreground is blurred to calculate "edge proximity"
• Pixels near edges have lower blurred alpha values
• Core penetration uses this to apply wrap that naturally fades as you move away from edges
• The blur radius affects how far the spill extends into the subject

Output Modes

Use output modes to visualize components for fine-tuning:

Keyboard Shortcuts

Workflow Tips

Starting Point

1. Select your background layer from the dropdown
2. Set blur radius to roughly match the edge softness of your keyed footage
3. Start with 50% wrap width and intensity at 100%
4. Use Screen blend mode as default
5. Adjust luminance bias to favor bright background areas

For Hard-Edged Keys

1. Increase core penetration to 30-50%
2. This allows wrap to spill into opaque areas
3. The spill naturally falls off as distance from edge increases
4. Adjust blur radius to control spill distance

Fine-Tuning

1. Use "Wrap Mask" output to see where effect applies
2. Adjust wrap width if effect goes too far into foreground
3. Increase edge softness for more gradual falloff
4. Use "Wrap Only" to see the isolated contribution

Common Scenarios

Bright Sky/Sunset Background:

• Higher luminance bias (70-100%)
• Screen or Add blend mode
• Moderate blur (20-40)
• Core penetration 20-40% for hard keys

Colorful LED Wall:

• Higher saturation (100-150%)
• Lower luminance bias (20-40%)
• Screen blend mode
• Core penetration as needed

Subtle Integration:

• Lower intensity (30-60%)
• Soft Light blend mode
• Higher edge softness
• Low or no core penetration

Technical Notes

Algorithm

1. Background layer is sampled and blurred using separable Gaussian blur
2. Foreground is also blurred to calculate edge proximity (for core penetration)
3. Edge mask is computed from foreground alpha (strongest at semi-transparent edges)
4. Core penetration extends mask into opaque areas based on edge distance
5. Luminance bias weights the mask toward brighter background areas
6. Blurred background is blended with foreground using selected blend mode
7. Result is masked to affect only edge areas (and nearby opaque areas with core penetration)

Performance

• Two-pass separable Gaussian blur for background
• Two-pass separable Gaussian blur for foreground alpha (edge detection)
• Single composite pass
• GPU-accelerated via CUDA
• Real-time at 1080p/4K on modern GPUs

Requirements

• Foreground must have valid alpha channel (from keying)
• Background layer must exist and be visible
• Both layers should be same resolution for best results

Troubleshooting

"You must select a visible background layer" warning:

• Select a background layer from the dropdown
• Ensure the background layer is visible and has rendered content

No effect visible:

• Verify background layer is selected and visible
• Check that foreground has alpha (view Alpha output from keyer)
• Increase intensity if effect is too subtle

Effect too strong/weak:

• Adjust intensity parameter
• Check wrap width isn't too high/low
• Try different blend modes

Wrap not reaching into solid areas:

• Increase core penetration
• Increase blur radius for wider spill
• Check wrap mask output to verify

Wrap color looks wrong:

• Adjust saturation parameter
• Check luminance bias setting
• Verify correct background layer is selected

Harsh edges on wrap:

• Increase edge softness
• Increase blur radius
• Lower wrap width

Background layer not found:

• The selected layer may have been deleted or hidden
• Select a new background layer from the dropdown