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Skin Generation

The skin module generates solid fill patterns for top and bottom surfaces of a print.

Overview

Skin layers are the fully-filled surfaces at the top and bottom of a print. Unlike infill (which uses patterns like grid or hexagons), skin is printed as solid diagonal lines to create a smooth, continuous surface.

Features

  • Solid diagonal fill - 45° alternating lines for strength
  • Skin wall perimeter - Single perimeter around skin area
  • Hole avoidance - Proper handling of holes in skin regions
  • Combing integration - Optimized travel paths between lines
  • Adaptive skin detection - See EXPOSURE_DETECTION.md

Usage

Skin generation is automatic during slicing:

const { Polyslice } = require("@jgphilpott/polyslice");

const slicer = new Polyslice({
    shellSkinThickness: 0.8, // Skin thickness in mm (converted to layer count internally)
    infillSpeed: 60,         // Speed for skin infill lines
    perimeterSpeed: 45,      // Speed for skin perimeter
    nozzleDiameter: 0.4,     // Line spacing equals nozzle diameter
    nozzleTemperature: 200,
    bedTemperature: 60
});

const gcode = slicer.slice(mesh);

Configuration Options

shellSkinThickness

Type: Number | Default: 0.8 | Unit: mm

Thickness of solid layers at top and bottom surfaces. The actual number of skin layers is calculated internally based on layer height.

  • 0.4mm (1-2 layers) - Thin skin, may show infill pattern through
  • 0.6-0.8mm (3 layers at 0.2mm height) - Good balance (default)
  • 1.0mm+ (5+ layers) - Very solid, better for functional parts
slicer.setShellSkinThickness(1.0); // ~5 layers at 0.2mm layer height
const thickness = slicer.getShellSkinThickness(); // 1.0

Skin Generation Process

1. Skin Wall (Perimeter)

A single perimeter is drawn around the skin boundary first:

// Create inset path for skin wall
skinWallPath = createInsetPath(boundaryPath, nozzleDiameter);

// Generate perimeter around skin area
for (point in skinWallPath) {
    // Calculate extrusion and generate G-code
}

2. Diagonal Infill

Solid diagonal lines fill the skin area:

// Generate 45° lines with nozzle-diameter spacing
// Alternates between +45° and -45° per layer
useNegativeSlope = (layerIndex % 2) === 1;

// Generate lines across the skin area
// Clip to boundary and avoid holes

3. Line Pattern

  • Even layers - +45° diagonal lines (y = x + offset)
  • Odd layers - -45° diagonal lines (y = -x + offset)
  • Spacing - Equal to nozzle diameter (solid fill)

Function Reference

generateSkinGCode(slicer, boundaryPath, z, centerOffsetX, centerOffsetY, layerIndex, lastWallPoint, isHole, generateInfill, holeSkinWalls, holeOuterWalls, coveredAreaSkinWalls, isCoveredArea)

Main skin generation function.

Parameters:

Parameter Type Description
slicer Object Polyslice slicer instance
boundaryPath Array Skin boundary points
z Number Z-coordinate for layer
centerOffsetX Number X offset for centering
centerOffsetY Number Y offset for centering
layerIndex Number Current layer index
lastWallPoint Object Last position for combing
isHole Boolean Is this a hole boundary?
generateInfill Boolean Generate diagonal infill?
holeSkinWalls Array Hole boundaries to exclude
holeOuterWalls Array Holes for travel avoidance
coveredAreaSkinWalls Array Covered areas to exclude
isCoveredArea Boolean Is this a covered area?

Returns: Object - Last endpoint for combing tracking

G-code Output

Skin sections are marked in the G-code:

; TYPE: SKIN
G0 X15.00 Y10.00 Z0.60 F9000  ; Move to skin wall
G1 X45.00 Y10.00 E1.20 F2700  ; Skin wall segment
G1 X45.00 Y40.00 E2.40 F2700  ; Skin wall segment
G1 X15.00 Y40.00 E3.60 F2700  ; Skin wall segment
G1 X15.00 Y10.00 E4.80 F2700  ; Close skin wall

; Moving to skin infill line
G0 X16.00 Y11.00 Z0.60 F9000
G1 X44.00 Y39.00 E5.90 F3600  ; Diagonal infill line
; Moving to skin infill line
G0 X16.40 Y11.40 Z0.60 F9000
G1 X43.60 Y38.60 E6.98 F3600  ; Next diagonal line
...

Hole Handling

The skin module properly handles holes in skin regions:

Hole Skin Walls

When a hole is detected in a skin layer, a skin wall is generated around it:

// Hole detected in skin region
// Generate skin wall around hole perimeter
generateSkinGCode(slicer, holePath, z, ..., isHole=true, generateInfill=false);

Hole Infill Exclusion

Skin infill lines are clipped to exclude hole areas:

// Create gap-adjusted hole boundaries
holeSkinWallsWithGap = [];
for (holeWall in holeSkinWalls) {
    holeWithGap = createInsetPath(holeWall, infillGap, true);
    holeSkinWallsWithGap.push(holeWithGap);
}

// Clip infill lines against boundaries
clippedSegments = clipLineWithHoles(lineStart, lineEnd, boundary, holeSkinWallsWithGap);

Covered Area Detection

Skin can detect and handle fully covered regions (cavities):

What Are Covered Areas?

Areas that have solid geometry both above and below, like:

  • Interior of a closed box
  • Between layers of a thick section
  • Areas that shouldn't get skin treatment

Exclusion from Skin

Covered areas are excluded from skin infill to avoid over-extrusion:

// Covered areas used directly as exclusion zones
coveredAreaSkinWallsWithGap = coveredAreaSkinWalls;  // No additional offset

// Combine with hole walls for clipping
allExclusionWalls = holeSkinWallsWithGap.concat(coveredAreaSkinWallsWithGap);
clippedSegments = clipLineWithHoles(start, end, boundary, allExclusionWalls);

Travel Optimization

Combing

Travel moves use combing to avoid crossing holes:

combingPath = findCombingPath(lastWallPoint, targetPoint, holeOuterWalls, boundaryPath, nozzleDiameter);

Nearest Neighbor

Skin lines are printed in optimal order:

  1. Start from current position
  2. Find nearest unprinted line endpoint
  3. Print that line (possibly reversed)
  4. Repeat until all lines printed

Alternating Pattern

The diagonal pattern alternates per layer for strength:

Layer Pattern Equation
Even (0, 2, 4...) +45° y = x + offset
Odd (1, 3, 5...) -45° y = -x + offset

This creates a crosshatch pattern over multiple layers.

File Structure

src/slicer/skin/
├── skin.coffee           # Main skin generation
├── skin.test.coffee      # Skin tests
└── exposure/             # Exposure detection
    ├── exposure.coffee   # Exposure detection module
    ├── exposure.test.coffee
    ├── cavity.coffee     # Cavity detection
    └── cavity.test.coffee

Related Documentation

Best Practices

Skin Thickness Recommendations

Use Case Recommended Thickness ~Layers (0.2mm height)
Fast prototypes 0.4mm 2 layers
General prints 0.6mm 3 layers
High quality 0.8-1.0mm 4-5 layers
Waterproof 1.0mm+ 5+ layers

Quality Tips

  1. Proper overlap - Skin should overlap slightly with walls
  2. Consistent flow - Calibrate extrusion for solid fill
  3. Temperature - Ensure proper layer adhesion
  4. Speed - Slower speeds for better surface finish