Source: src/gcode/README.md — this page is rendered directly from the file in the repository. Edit it there.

gcode — G-code generation

Converts Vec<SliceLayer> → a firmware-ready G-code String.

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Module layout

gcode/
├── mod.rs          re-exports; module-level docs
├── flavor.rs       GcodeFlavor enum (Marlin | Klipper)
├── dialect.rs      GcodeDialect trait + WarnFn
├── generator.rs    GcodeGenerator façade + generate_gcode()
├── simplify.rs     Ramer-Douglas-Peucker polyline simplification
├── source.rs       resolve_gcode_source() file/string resolver
└── dialects/
    ├── mod.rs      re-exports
    ├── marlin.rs   MarlinDialect  (M104/M109/M140/M190 etc.)
    └── klipper.rs  KlipperDialect (START_PRINT / END_PRINT macros)

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Call flow

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Dialect abstraction

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Extrusion math

For each XY segment of length L the required filament advance is:

E = L × (layer_height × nozzle_ø) / (π × (filament_ø/2)²)

This is the volumetric flow balance: the rectangular cross-section of the deposited bead (layer_height × nozzle_ø) must equal the volume of filament pushed through (π r² × E).

Both diameters are configurable via SlicingParams (fields nozzle_diameter_mm and filament_diameter_mm). Typical defaults: filament ø = 1.75 mm, nozzle ø = 0.40 mm.

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Surface generation

Top and bottom surfaces are generated by generate_top_bottom_surfaces() in src/core.rs. They use solid rectilinear infill at a configurable angle (SlicingParams::surface_infill_angle, default 45°).

Detection algorithm

A region of layer i is treated as a top surface when it is not covered by every one of the top_layers layers above it simultaneously. Formally, the surface region is computed by progressive intersection:

covered = perimeters[i]
for j in 1..=top_layers:
    if layer[i+j] does not exist or is empty:
        covered = ∅          ← model ends here; whole remaining region is exposed
        break
    covered = intersect(covered, perimeters[i+j])
    if covered = ∅: break

top_region[i] = diff(perimeters[i], covered)

top_region[i] is the area of layer i that is not enclosed by all top_layers consecutive layers above it. Bottom surfaces follow the same logic looking downward.

Key behaviours:

• Model top/bottom — when fewer than N layers exist above/below, the intersection yields ∅, so the entire current slice becomes a surface.
• Mid-model surfaces — ledges, internal floors, cabin roofs, etc. are detected because the layers above/below do not fully cover the current footprint.
• Non-monotonic shapes — a region that is exposed at any of the N successor layers (even if the N-th successor covers it) is correctly flagged as a top surface, because the progressive intersection narrows to the smallest coverage found in the window. The old single-comparison approach (diff(layer[i], layer[i+N])) could silently miss such regions.

Infill spacing

Line spacing for solid surface infill is derived from layer_height:

line_spacing = layer_height × SOLID_INFILL_EXTRUSION_WIDTH_MULTIPLIER  (= 1.2)

Configurable parameters

SlicingParams field	Default	Description
top_layers	3	Number of solid layers above a top face
bottom_layers	3	Number of solid layers below a bottom face
surface_infill_angle	45.0°	Angle of rectilinear infill lines

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Travel sequence per path

Every path (closed contour or infill line) is preceded by a travel move from the previous path's end. Whether that travel is wrapped in the retract / z-hop / travel / lower / un-retract guard depends on a smart retract policy that mirrors PrusaSlicer / OrcaSlicer / Cura:

Travel distance	Role change?	Retract?	Why
> 2.0 mm	any	yes	Long hops always ooze enough to need a retract
1.0 – 2.0 mm	yes	yes	Crossing role boundaries (e.g. infill → outer wall) shows seams without retract
1.0 – 2.0 mm	no	no	Same-role short hops oozing is invisible inside infill
< 1.0 mm	any	no	Retract ceremony costs more time than the hop itself

The cutoff is MIN_TRAVEL_FOR_RETRACT_MM = 1.0 (hard-coded in generator.rs). The role-aware branch eliminates the 99 %+ of pointless retracts that occurred on every wall-loop end on dense benchmarks, while still protecting the visible outer surface from oozing.

When a retract is emitted, the sequence is:

Otherwise a single bare G1 X… Y… F travel_speed_mm_min move is emitted.

All three distances and the travel speed are configurable via SlicingParams fields retract_mm (default 1.0 mm), z_hop_mm (default 0.2 mm), and travel_speed_mm_min (default 9000 mm/min = 150 mm/s).

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Path simplification (Ramer-Douglas-Peucker)

Sliced contours and infill paths often contain many near-collinear vertices. Streaming every one of them as a G1 move overwhelms firmware buffers (OctoPrint, Klipper) and bloats .gcode files. To address this, every path is thinned just before emission using the Ramer-Douglas-Peucker (RDP) algorithm.

Where it sits in the pipeline

Geometry calculations always use full mesh precision — simplification happens only at the output stage, so wall offsets, infill clipping, and surface detection are never degraded.

Algorithm

For each polyline, recursively find the vertex with the greatest perpendicular distance from the chord between the segment's endpoints. If that distance exceeds tolerance, split there and recurse on both halves; otherwise discard all interior vertices. The first and last point are always preserved.

Configuration — SlicingParams::path_tolerance

Value	Effect
0.0	Disabled — all vertices preserved
0.01	Conservative — high-quality printers, minimal visible impact
0.05 ⭐	Default — good balance of fidelity and move-count reduction
0.1+	Aggressive — best for slow firmware (legacy OctoPrint)

Future-feature checklist

When adding a new feature that emits paths through GcodeGenerator:

• ✅ Nothing to do. Simplification is applied inside the generator's per-path loop, so any new path source (new infill pattern, support, brim, ironing pass, …) automatically benefits.
• ⚠️ Bypass deliberately when curvature must be preserved point-for-point (e.g. arc-fitting / G2/G3 emission, exact-position commands). Either set path_tolerance = 0.0 for that pass or perform the special-case emission before the generic generator loop.
• ⚠️ Don't simplify upstream of geometry ops. Calling douglas_peucker on Clipper2 paths before offset/clip/intersect operations will cascade precision loss into walls and infill. Keep it strictly at the output layer.

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Lifecycle markers

When LifecycleMarkerConfig::enabled is true (default), each layer emits a structured block compatible with OrcaSlicer / PrusaSlicer post-processors:

;LAYER_CHANGE
;Z:{z}
;HEIGHT:{height}
;BEFORE_LAYER_CHANGE
;{z}            ← bare numeric marker for post-processing scripts
G92 E0          ← extruder reset (E tracking restarts each layer)
G1 Z{z} F9000
;AFTER_LAYER_CHANGE
;{z}

;TYPE:{role}    ← emitted once per extrusion-role transition
;WIDTH:{w}mm

All marker strings are templates: {z}, {height}, {type}, {width} are substituted at render time via render_marker(). Per-flavor overrides are stored in GlobalSettings::lifecycle_markers (keyed by flavor name).

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Script priority chain

CLI --start-gcode argument
        ↓  (overrides)
GlobalSettings.start_print_gcode
        ↓  (overrides)
GcodeDialect::start_script()  ← firmware default

resolve_gcode_source(input) auto-detects whether input is a file path or an inline G-code string (1 MiB file size limit enforced).