Rendered from src/adhesion/README.md in the repository — edit it there.

Adhesion — Skirt, Brim & Raft Generation

This module exists to answer one question: how do we help the first layer stick to the bed? It turns the AdhesionType selection (plus its sub-parameters) into extra geometry, and it obeys a single rule: it runs last and never touches the object's own toolpaths. Skirt/brim loops are prepended to the first layer(s); a raft prepends whole layers and lifts the object in Z. Nothing the object already generated (walls, surfaces, infill, ordering, flow) is reshaped.

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Why a separate, last-stage module

Bed adhesion is additive scaffolding, not part of the part. Slicing it inside the wall/infill passes would entangle it with the very geometry it must stay clear of (separation gaps, wall-footprint clips, seam ordering). Mature slicers (PrusaSlicer, OrcaSlicer, CuraEngine) all generate adhesion from the finished first-layer outline as a distinct step for exactly this reason. Running after path ordering and flow compensation means the loops are appended cleanly and the object is provably unperturbed.

The contract

• Footprint from OuterWall centerlines. The wall generator emits centerlines inset d/2 from the true surface (d = nozzle diameter), so the object outline is recovered by unioning the layer-0 OuterWall paths and inflating outward by d/2. Winding is preserved (CCW solids, CW holes) so Clipper2 treats holes as voids — see ../core/README.md.
• Loops print first. Skirt/brim paths are prepended so the nozzle primes on them before the part. They carry the Skirt role (a closed-loop role in the G-code generator, Orca-compatible ;TYPE:Skirt).
• Raft is a base, not a part. Raft layers carry the Support role, sit on the bed (Z from layer_height/2), and the object is shifted up by raft_layers · layer_height + raft_air_gap so it lands on the raft top.

The catalog

Type	Geometry	Role	Key params
Skirt	skirt_loops outward loops from the outline, over the first skirt_height layers	Skirt	skirt_loops, skirt_distance, skirt_height
Brim	apron loops hugging the object on layer 0	Skirt	brim_width, brim_type, brim_separation
Raft	coarse base + finer interface layers under the object	Support	raft_layers, raft_air_gap

brim_type (BrimType): outer_only offsets outward; inner_only steps into each hole; outer_and_inner does both; ears stamps concentric discs only at sharp convex corners (interior angle < 120°) where warping actually starts — detected on a miter footprint so the round offset doesn't bevel the corners away first.

Geometry primitives

• offset / offset_join — the polygon-offset workhorse. Round for skirt/brim/raft outlines; Miter only for ears corner detection.
• outward_loops / hole_loops — concentric loops stepping out from the object or into each hole. Loop k sits at gap + d/2 + k·d.
• concentric_loops — fills an arbitrary region (the ears disc-minus-object) with d-spaced loops from its boundary inward.
• convex_corners — signed exterior turn via atan2(cross, dot); positive turn on a CCW contour is a convex corner.

Lifecycle

apply_adhesion is called once at the tail of process_mesh, gated on adhesion_type != None. It mutates the layer vector in place (skirt/brim) or replaces it with [raft…, object…] (raft). There is no persistence and no per-layer state — it is a pure function of the finished layers plus params.

Non-goals

• No brim-to-part fusion tuning beyond brim_separation. The brim abuts the wall at the configured gap; it does not model inter-bead flow welding.
• No variable raft layer heights. Raft layers use layer_height because the G-code generator computes extrusion E from the global layer_height; a thicker base would mis-estimate flow. This is a deliberate, honest limitation.
• No bed-bounds clamping. Skirt/raft expansion is not clipped to the build plate here; that belongs to a bed-aware validation pass, not adhesion.
• No support generation. Rafts reuse the Support role for annotation only; actual support structures are a separate, unimplemented feature.

See also

• mod.rs — the implementation.
• ../settings/params.rs — AdhesionType, BrimType, and the sub-parameters.
• ../core/pipeline.rs — where apply_adhesion is called.
• ../core/README.md — the pipeline order this pass runs last in, and which Clipper2 fill rule to use where.