The Edge Detection operator is a real-time image processing filter that detects and highlights edges in video content. It identifies areas of rapid intensity change (boundaries between objects, textures, and shapes) using mathematical gradient algorithms. This effect is useful for creating stylized visuals, generating masks for compositing, or analyzing image structure.
.png)
When to Use Edge Detection
- Stylized video effects: Create sketch-like, neon outline, or technical blueprint aesthetics
- Mask generation: Generate alpha channels based on edge structure for compositing
- Motion graphics: Add dynamic edge overlays to footage for broadcast or music videos
- Technical analysis: Visualize object boundaries and image structure
- Rotoscoping assistance: Highlight edges to guide manual masking workflows
Parameters
Algorithm Settings
| Parameter | Default | Description |
|---|---|---|
| Algorithm | Sobel | Select the edge detection algorithm to use. See algorithm comparison table below. |
| Output mode | Edges Only | Select how the detected edges should be displayed. See output modes table below. |
Algorithms
Edge detection algorithms work by analyzing how quickly pixel brightness changes across an image. Areas with rapid change (high gradients) indicate edges. Different algorithms use different mathematical approaches to calculate these gradients, each with their own strengths and trade-offs between speed, accuracy, and noise sensitivity.
| Algorithm | Description | Best For |
|---|---|---|
| Sobel | Good balance of noise immunity and edge detection. Recommended for most uses. | General purpose |
| Prewitt | Similar to Sobel with simpler weights. Slightly faster but more noise-sensitive. | Fast processing |
| Laplacian | Detects edges in all directions simultaneously. Good for fine detail but sensitive to noise. | Fine detail |
| Roberts Cross | Fast computation, good for high-contrast edges. Best for clean source material. | High-contrast sources |
| Scharr | Enhanced Sobel with better rotational symmetry. Best accuracy for diagonal edges. | Precision work |
Output Modes
| Mode | Description |
|---|---|
| Edges Only | White edges on black background. Pure edge detection output. |
| Edges Colored | Colored edges on black background. Uses the edge color settings. |
| Overlay | Edges overlaid on original image. Combines detection with source footage. |
| Edge Alpha | Original image with edge strength as alpha. Useful for compositing workflows. |
| Edges as Alpha | Original RGB with edges controlling alpha. Keeps color while using edges for transparency. |
Edge Thresholds
| Parameter | Range | Default | Description |
|---|---|---|---|
| Minimum threshold (%) | 0-100 | 5 | Edges below this threshold will be discarded. Increase to reduce noise. |
| Maximum threshold (%) | 0-100 | 50 | Edges above this threshold will be fully visible. Creates soft edges between min and max. |
| Edge strength (%) | 0-500 | 100 | Multiplier for edge detection sensitivity. Values over 100 boost weak edges. |
Threshold Behavior Explained
The minimum and maximum thresholds work together to control edge visibility:
| Scenario | Result |
|---|---|
| Edge < Minimum | Completely hidden (black) |
| Edge between Min and Max | Gradual fade from hidden to visible |
| Edge > Maximum | Fully visible (white or colored) |
Tip: A larger gap between minimum and maximum creates softer, more gradual edges. A smaller gap creates harder, more defined edges.
Edge Color
These settings are available when Output mode is set to "Edges Colored" or "Overlay".
| Parameter | Range | Default | Description |
|---|---|---|---|
| Red | 0-255 | 255 | Red component of the edge color |
| Green | 0-255 | 255 | Green component of the edge color |
| Blue | 0-255 | 255 | Blue component of the edge color |
Reset to white: Button to reset edge color to white (255, 255, 255).
Overlay Settings (Advanced)
These settings are only available when Output mode is set to "Overlay".
| Parameter | Range | Default | Description |
|---|---|---|---|
| Overlay opacity (%) | 0-100 | 100 | Opacity of edges when overlaid on original image |
| Preserve original alpha | No/Yes | Yes | Keep the original image's alpha channel in overlay mode |
Additional Options (Advanced)
| Parameter | Range | Default | Description |
|---|---|---|---|
| Invert edges | No/Yes | No | Invert the edge detection result. Swaps black and white. |
| Edge glow | 0-100 | 0 | Adds a glow/expansion effect around detected edges |
| Show original (debug) | No/Yes | No | Bypass edge detection and show original image. Useful for comparison. |
Keyboard Shortcuts
| Key | Action |
|---|---|
| F1 | Set output mode to Edges Only |
| F2 | Set output mode to Edges Colored |
| F3 | Set output mode to Overlay |
| F4 | Set output mode to Edge Alpha |
| F5 | Set output mode to Edges as Alpha |
| F6 | Cycle through algorithms |
| F7 | Toggle Invert edges |
| F8 | Toggle Show original |
Workflow Tips
Starting Point
- Set algorithm to Sobel (best all-around choice)
- Set output mode to Edges Only to see raw detection
- Adjust minimum threshold to remove noise (start around 5-10%)
- Adjust maximum threshold to control edge intensity (start around 50%)
- Fine-tune edge strength if edges are too weak or too strong
For Clean Source Material
- Use Roberts Cross for fastest processing
- Lower minimum threshold (2-5%) to capture subtle edges
- Keep edge strength at 100%
For Noisy Source Material
- Use Sobel or Scharr for better noise immunity
- Increase minimum threshold (10-20%) to filter noise
- Consider reducing edge strength slightly
Creating Stylized Effects
Neon/Glow Effect:
- Output mode: Edges Colored
- Set edge color to bright neon (e.g., cyan: R=0, G=255, B=255)
- Increase edge glow (30-60%)
- Lower minimum threshold for more detail
Sketch/Line Art Effect:
- Output mode: Edges Only
- Invert edges: Yes (for white background with dark lines)
- Higher minimum threshold (15-25%) for cleaner lines
- Lower maximum threshold for harder edges
Subtle Edge Enhancement:
- Output mode: Overlay
- Lower overlay opacity (20-40%)
- Preserve original alpha: Yes
- Adjust edge color to complement footage
For Compositing/Masking
Generating Edge-Based Alpha:
- Output mode: Edge Alpha or Edges as Alpha
- Adjust thresholds to capture desired edge detail
- Use as input to other compositing operators
Isolating Specific Edges:
- Increase minimum threshold to keep only strong edges
- Use edge strength to fine-tune sensitivity
- Edge Alpha mode preserves original colors
Technical Notes
Algorithm Comparison
| Algorithm | Speed | Noise Immunity | Diagonal Accuracy | Best For |
|---|---|---|---|---|
| Sobel | Fast | Good | Good | General purpose |
| Prewitt | Fast | Moderate | Good | Clean sources |
| Laplacian | Fast | Low | Excellent | Fine detail |
| Roberts Cross | Fastest | Low | Moderate | High-contrast edges |
| Scharr | Fast | Good | Excellent | Precision work |
Algorithm Details
- Sobel/Prewitt/Scharr: Use 3×3 convolution kernels to compute horizontal and vertical gradients, then combine them
- Laplacian: Uses second-order derivatives to detect edges in all directions simultaneously
- Roberts Cross: Uses 2×2 diagonal kernels for fast edge approximation
Performance
- Single-pass GPU kernel execution
- GPU-accelerated via CUDA
- Real-time at 1080p/4K on modern GPUs
- Minimal memory overhead (single render buffer)
Requirements
- Input image with valid dimensions
- GPU with CUDA support
Troubleshooting
No edges visible:
- Check that edge strength is not set to 0%
- Lower the minimum threshold
- Ensure Show original (debug) is set to No
- Verify input has sufficient contrast
Too much noise in output:
- Increase minimum threshold to filter weak edges
- Try Sobel or Scharr algorithm instead of Laplacian or Roberts Cross
- Reduce edge strength if amplifying noise
Edges too thick or bloated:
- Reduce edge glow setting
- Increase minimum threshold
- Decrease edge strength
Edges too thin or broken:
- Decrease minimum threshold
- Increase edge strength
- Use a smaller gap between min and max thresholds
- Try Laplacian algorithm for finer detail
Edge color not changing:
- Ensure Output mode is set to "Edges Colored" or "Overlay"
- Edge color settings are hidden in other output modes
Overlay settings not visible:
- Overlay opacity and Preserve original alpha only appear when Output mode is "Overlay"
Edges look different than expected algorithm:
- Each algorithm has different characteristics
- Use F6 to cycle through and compare
- See algorithm comparison table for guidance
Original image showing through:
- Check that Show original (debug) is set to No
- In Overlay mode, reduce overlay opacity for more edge visibility
- Switch to Edges Only mode to see pure detection
Alpha channel issues:
- In Overlay mode, toggle Preserve original alpha based on your needs
- Edge Alpha and Edges as Alpha modes modify the alpha channel differently
- Verify downstream operators are respecting alpha