Feature Request: Add Plugin Slot for Top/Bottom Surface Pattern Generation

[privitera]

Feature Request: Add Plugin Slot for Top/Bottom Surface Pattern Generation

Summary

Add a new plugin slot (e.g., Slot 202: SKIN_GENERATE) to enable custom top/bottom surface patterns via plugins, similar to how Slot 200 (INFILL_GENERATE) works for body infill patterns.

Problem Statement

Currently, the CuraEngine plugin system only supports custom infill patterns for body infill through Slot 200 (INFILL_GENERATE). Top and bottom surface patterns (top_bottom_pattern, roofing_pattern) are hardcoded to three options: "lines", "concentric", and "zigzag".

This limitation prevents plugin developers from:

• Creating decorative surface patterns (Hilbert curves, Archimedean spirals, geometric tiles)
• Implementing space-filling curves for top/bottom surfaces
• Offering users more aesthetic and functional surface pattern options

Current Workaround

Users must set infill_density: 100% and top_layers: 0 to force the top surface to use custom infill patterns. This is inefficient and prevents proper infill/surface separation.

Proposed Solution

Add a new plugin slot for surface pattern generation:

Slot 202: SKIN_GENERATE

• Purpose: Generate custom patterns for top/bottom surfaces
• gRPC Service: SkinGenerateService (similar to existing proto file at skin/v0/skin.proto)
• Settings Integration: Apply to top_bottom_pattern, roofing_pattern, top_bottom_pattern_0, bottom_pattern_initial_layer

Implementation Approach

1. Define new slot ID in slot_id.proto:

   SKIN_GENERATE = 202;  // Generate skin (top/bottom surface) patterns

2. Route surface pattern requests to plugins when custom patterns are registered via extend_category()

3. Allow plugins to return empty responses to fall back to built-in patterns (same as Slot 200 behavior)

Use Cases

1. Decorative Top Surfaces

• Print Hilbert curves, Archimedean spirals, or geometric patterns on top surfaces
• Create visually appealing prints without post-processing
• Enable artistic/customized prints

2. Functional Surface Patterns

• Space-filling curves for better surface strength
• Custom patterns optimized for specific materials
• Pattern variations for different print requirements

3. Plugin Ecosystem Growth

• Enables community-developed surface pattern plugins
• Consistent with existing plugin architecture (Slot 200)
• Reuses proven gRPC infrastructure

Technical Details

Existing Infrastructure

• Proto file already exists: skin/v0/skin.proto defines SkinModifyService
• Pattern registration via Python: definition.extend_category(pattern_key, pattern_label)
• gRPC slot system: Well-established pattern with Slot 200

Required Changes

1. Add SKIN_GENERATE = 202 to slot_id enum
2. Create SkinGenerateService proto definition (or extend existing skin.proto)
3. Route top_bottom_pattern setting to plugin when custom pattern registered
4. Handle empty responses as fallback to built-in patterns

Backward Compatibility

• No breaking changes - existing hardcoded patterns continue to work
• Plugins opt-in by registering custom patterns
• Falls back gracefully if plugin doesn't support pattern

Example Plugin Implementation

A plugin could register patterns like:

for pattern_key in ["top_bottom_pattern", "roofing_pattern"]:
    for definition in container.findDefinitions(key=pattern_key):
        for pattern in self.getSpaceFillingPatterns():
            definition.extend_category(
                pattern[0],
                pattern[1],
                plugin_id=self.getPluginId(),
                plugin_version=self.getVersion()
            )

Pattern examples:

• Hilbert Curve
• Archimedean Chords
• Octagram Spiral
• Flower of Life (tiled geometric)
• Custom user-defined patterns

Benefits

1. User Experience: More creative control over print aesthetics
2. Plugin Ecosystem: Enables new plugin development opportunities
3. Consistency: Aligns with existing Slot 200 architecture
4. Flexibility: Community can experiment with surface patterns without forking CuraEngine

Related Work

• Slot 200 (INFILL_GENERATE) demonstrates successful plugin-based pattern generation
• Proto file skin/v0/skin.proto exists but not connected to any slot
• Community interest in decorative surface patterns (PrusaSlicer, SuperSlicer have similar features)

Alternatives Considered

1. Modify CuraEngine source directly - Requires forking and doesn't benefit community
2. Use 100% infill workaround - Inefficient and loses infill/surface separation
3. Post-process G-code - Complex, error-prone, loses CuraEngine optimizations

References

• Current plugin system: Slot 200 (INFILL_GENERATE)
• Existing proto: cura/plugins/slots/skin/v0/skin.proto
• Slot ID definitions: cura/plugins/v0/slot_id.proto
• Example plugin: CuraEngineTiledInfill (demonstrates pattern plugin architecture)