Instruction file imported from jgphilpott/polyslice (.github/instructions/slicer/skin/overview.instructions.md). Copyright stays with the author.

Skin Generation Overview

The skin module generates solid infill for top and bottom surfaces. Located in src/slicer/skin/.

Purpose

• Create solid surfaces on exposed top/bottom layers
• Generate adaptive skin for overhangs and cavities (when exposure detection enabled)
• Produce clean exterior surfaces by covering infill patterns

Skin vs Infill

Skin Components

Skin Wall

A perimeter pass around the skin boundary:

skinWallInset = nozzleDiameter  # Full nozzle width offset
skinWallPath = paths.createInsetPath(boundaryPath, skinWallInset, offsetDirection)

Skin Infill

Diagonal lines at 45° angle, alternating direction per layer:

# Odd layers: -45° (y = -x + offset)
# Even layers: +45° (y = x + offset)
useNegativeSlope = (layerIndex % 2) is 1

Skin Layer Detection

Absolute Top/Bottom Layers

skinLayerCount = Math.floor((shellSkinThickness / layerHeight) + 0.0001)

# Bottom layers
if layerIndex < skinLayerCount
    needsSkin = true

# Top layers
if layerIndex >= totalLayers - skinLayerCount
    needsSkin = true

Adaptive Skin (Exposure Detection)

When exposureDetection is enabled, middle layers can also have skin for:

• Overhangs (exposed from below)
• Cavities (exposed from above)

Exposure Detection Module

Located in src/slicer/skin/exposure/exposure.coffee.

Algorithm

1. Check layer N ± skinLayerCount for covering geometry
2. Calculate which areas are exposed (not covered)
3. Generate skin only for exposed regions

calculateExposedAreasForLayer: (currentPath, layerIndex, skinLayerCount, totalLayers, allLayers, resolution) ->
    # Check layer ahead (above)
    checkIdxAbove = layerIndex + skinLayerCount

    # Check layer behind (below)
    checkIdxBelow = layerIndex - skinLayerCount

    return { exposedAreas, coveringRegionsAbove, coveringRegionsBelow }

Hole Exposure Detection

For holes on middle layers:

shouldGenerateHoleSkinWalls: (path, layerIndex, skinLayerCount, totalLayers, allLayers) ->
    holeExposedAbove = @isHoleExposedAbove(...)
    holeExposedBelow = @isHoleExposedBelow(...)
    return holeExposedAbove or holeExposedBelow

A hole is exposed if:

• Near top of model (within skinLayerCount)
• Near bottom of model (within skinLayerCount)
• No corresponding hole exists in the check layer

Cavity Detection

Located in src/slicer/skin/exposure/cavity.coffee.

Fully Covered Regions

Areas that have geometry both above AND below don't need skin infill:

identifyFullyCoveredRegions: (currentPath, coveringRegionsAbove, coveringRegionsBelow) ->
    # Find intersection of coverage from both directions
    # These areas need only skin wall, not skin infill

Mixed Layer Strategy

On layers with partial exposure:

1. Generate infill first (entire layer)
2. Generate skin second (exposed areas only)

This allows skin to cover infill pattern on exposed surfaces.

Skin Generation Parameters

Offset Directions

# For covered areas (isCoveredArea=true): inset inward
# For real holes (isHole=true): outset outward (shrink hole)
# For normal skin boundaries: inset inward
offsetDirection = if isCoveredArea then false else isHole

Gaps and Spacing

infillGap = nozzleDiameter / 2  # Gap between skin wall and infill
infillInset = skinWallInset + infillGap  # Total: 1.5 * nozzleDiameter from boundary
lineSpacing = nozzleDiameter  # 100% density for solid fill

Diagonal Infill Generation

Line Equations

if useNegativeSlope
    # y = -x + offset
    offset = minY + minX - diagonalSpan
    maxOffset = maxY + maxX
else
    # y = x + offset
    offset = minY - maxX - diagonalSpan
    maxOffset = maxY - minX

# Spacing for 45° lines
offsetStep = lineSpacing * Math.sqrt(2)

Bounding Box Intersection

Calculate intersections with all four edges, then clip to actual boundary:

clippedSegments = clipping.clipLineWithHoles(
    intersections[0],
    intersections[1],
    infillBoundary,
    allExclusionWalls  # Includes hole walls and covered area boundaries
)

Travel Optimization

Nearest-Neighbor Rendering

while allSkinLines.length > 0
    # Find line with endpoint closest to current position
    # Can flip line direction for efficiency
    # Use combing for travel moves

Combing for Skin

combingPath = combing.findCombingPath(
    lastEndPoint or startPoint,
    startPoint,
    holeOuterWalls,
    infillBoundary,
    nozzleDiameter
)

Hole Skin Walls

Holes on skin layers need additional skin walls:

if isHole and generateSkinWalls and currentPath.length >= 3
    # Calculate skin wall path (inset from innermost wall)
    skinWallPath = paths.createInsetPath(currentPath, nozzleDiameter, isHole)
    holeSkinWalls.push(skinWallPath)

Return Value

The generateSkinGCode function returns the last endpoint for tracking:

if lastEndPoint?
    return lastEndPoint
else if skinWallPath? and skinWallPath.length > 0
    return { x: skinWallPath[0].x, y: skinWallPath[0].y, z: z }
else
    return null

Important Conventions

1. Type annotation: Add ; TYPE: SKIN comment when verbose mode enabled
2. Layer alternation: Flip diagonal direction each layer for strength
3. Hole handling: Exclude hole skin walls with infill gap offset
4. Covered areas: Use boundaries as-is (no additional offset) for exclusion
5. Start point optimization: Rotate path to start closest to last position