Min/Max

The Min/Max Filter performs morphological operations on images. These are fundamental image processing operations useful for noise removal, edge detection, and mask/matte refinement.

Overview

Morphological operations use a "structuring element" (kernel) that slides across the image. At each position, the operation examines all pixels within the kernel and selects the minimum or maximum value.

Operations

Operation	Description	Use Case
Minimum (Erosion)	Selects darkest pixel in kernel. Shrinks bright areas, expands dark areas.	Remove small bright noise, shrink masks
Maximum (Dilation)	Selects brightest pixel in kernel. Expands bright areas, shrinks dark areas.	Fill small holes, expand masks
Open	Min then Max. Removes small bright spots while preserving overall shape.	Remove salt noise, smooth contours
Close	Max then Min. Removes small dark spots while preserving overall shape.	Remove pepper noise, fill small holes
Gradient	Max minus Min. Highlights edges/boundaries.	Edge detection, outline extraction

Settings

Operation

Setting	Description
Operation	The morphological operation to perform
Iterations	Number of times to apply the operation (1-10). More = stronger effect.

Kernel

Setting	Description
Radius X	Horizontal radius of the kernel (1-100 pixels)
Radius Y	Vertical radius of the kernel (1-100 pixels)
Link radii	When ON, Radius Y follows Radius X for uniform kernel
Shape	Shape of the structuring element

Kernel Shapes

Shape	Description	Best For
Box	Square/rectangular kernel	General purpose, fast
Ellipse	Circular/elliptical kernel	Isotropic operations, natural look
Cross	Plus-shaped kernel (+)	Directional operations
Diamond	Diamond-shaped kernel (◇)	Compromise between box and ellipse

Processing (Advanced)

Setting	Description
Channel mode	Per-Channel: process R, G, B independently. Luminance: select whole pixel by brightness.
Process channels	Which channels to process (see below)

Process Channels

Option	Description
RGB	Process color channels only, preserve original alpha
RGBA	Process all channels including alpha
Alpha Only	Process only the alpha channel, preserve RGB colors

The Alpha Only option is particularly useful for:

Refining mattes and masks
Choking/spreading alpha edges without affecting color
Cleaning up keyed footage alpha

Keyboard Shortcuts

Key	Function
F1	Cycle through operations
F2	Cycle through kernel shapes
F3	Toggle channel mode (Per-Channel/Luminance)
F4	Cycle process channels (RGB/RGBA/Alpha)
F5	Toggle link radii
R	Reset all settings to defaults

Reset

Click the Reset all button or press R to restore all settings to their default values:

Operation: Maximum
Iterations: 1
Radius X/Y: 3
Link radii: On
Shape: Ellipse
Channel mode: Per-Channel
Process channels: RGB

Use Cases

Noise Removal

Salt noise (bright spots):

Use Open operation (Min then Max)
Small radius (1-3)
1-2 iterations

Pepper noise (dark spots):

Use Close operation (Max then Min)
Small radius (1-3)
1-2 iterations

Mask/Matte Refinement

Shrink mask (choke):

Use Minimum operation
Adjust radius to desired shrink amount
Use Ellipse shape for smooth edges

Expand mask (spread):

Use Maximum operation
Adjust radius to desired expansion
Use Ellipse shape for smooth edges

Clean up mask edges:

Use Open then Close (or vice versa)
Small radius (1-2)
Removes small artifacts

Edge Detection

Use Gradient operation
Adjust radius for edge thickness
Works like a simple edge detector

Creating Outlines

Apply Maximum to expand the shape
Subtract original from expanded version
Result is an outline of the original shape

Channel Modes

Per-Channel Mode

Processes R, G, B channels independently. Each channel gets its own min/max calculation. This is the standard mode and works well for most cases.

Luminance Mode

Calculates brightness (luminance) for each pixel in the kernel and selects the entire pixel with the minimum/maximum luminance. This preserves color relationships better but may produce different results.

Performance Notes

Larger radii are slower (O(r²) complexity)
Box shape is slightly faster than Ellipse
Multiple iterations are slower than single iteration with larger radius, but may produce different results
Processing alpha adds minimal overhead

Comparison with Median Filter

Feature	Min/Max	Median
Noise removal	Good for impulse noise	Better for mixed noise
Edge preservation	Can shift edges	Better edge preservation
Speed	Faster	Slower
Compound ops	Open/Close/Gradient	N/A
Use case	Mask refinement, morphology	General denoising

Technical Details

The structuring element (kernel) determines which pixels are considered at each position:

Box 3x3:        Ellipse 3x3:    Cross 3x3:      Diamond 3x3:
■ ■ ■           · ■ ·           · ■ ·           · ■ ·
■ ■ ■           ■ ■ ■           ■ ■ ■           ■ ■ ■
■ ■ ■           · ■ ·           · ■ ·           · ■ ·

For non-square kernels (Radius X ≠ Radius Y), the shapes scale proportionally.