Wipe inward — High Level Design

Purpose and scope

Wipe inward reduces reheating of fresh plastic and visible seam artifacts by moving the hot nozzle toward adjacent printed material during the external-wall wipe. Wipe marks are especially visible at layer heights below 0.1 mm. The option applies only to wipes after external walls, including walls around holes. It does not offset wipes after inner walls, infill or supports. For an outer contour the move is inward; for a hole it is away from the hole, toward the surrounding material. The path must remain supported by material that is already present when the wipe executes.

The operation belongs to G-code generation. It uses extrusion paths, their actual widths and their print order. Changing its settings invalidates G-code export while preserving the sliced geometry.

Settings and eligibility

wipe_inward defaults to disabled and requires Wipe while retracting to be enabled for the active filament. wipe_inward_distance defaults to 50% of the actual external-wall extrusion width; it also accepts an absolute distance in millimeters. Using the path width makes Auto width and Arachne's variable widths meaningful. The effective offset is limited by that width and the spacing to the adjacent wall. A zero distance disables the offset.

Only external perimeters with a suitable, previously printed inner perimeter are eligible. A configured wall count alone cannot establish eligibility: the local geometry may contain fewer walls, and walls scheduled later do not provide support. Outer/Inner wall order therefore normally retains the regular wipe path.

Retraction and pressure advance calibrations disable inward wiping so it cannot mask the behavior being measured. The calibration settings turn it off, and G-code generation enforces this even if a profile or object override enables it.

Path selection and support

The planner identifies an adjacent inner perimeter on the material side of the outgoing wall. Contour winding and the distinction between outer contours and holes establish a preferred direction; local printed geometry resolves ambiguous or self-touching contours.

Candidate paths offset or translate the portion needed for the configured wipe distance. A wide seam gap can prevent a supported forward path; following the incoming printed wall backwards is also a candidate. If translating that wall cannot provide a complete wipe around a curve, the planner tries an offset of the reversed wall. Direction checks allow coordinate-rounding error at a perpendicular entry, while rejecting actual backtracking. The planner checks the complete executable path, including its connector from the nozzle position, against the current and earlier printed perimeters. Nearby endpoints alone do not establish support across a gap.

Each region accumulates its printed perimeter prefix once, in extrusion order. Every entity contributes its geometry only after it is printed, and the prefix is discarded when the region ends. This collection is skipped when inward wiping is disabled or its configured distance is zero. A mixed inner-wall loop remains an eligible target even when its first path is an overhang: ordinary inner-wall paths elsewhere in the loop identify it. Likewise, an external loop with an overhanging start remains eligible when other segments identify the external wall. It is available for support checks but is not an inner-wall target. Candidate-specific support filtering and AABB trees are built only for eligible external loops, then reused across their candidate paths.

Material-side validation applies with or without a seam gap. Along each candidate, local wall normals point toward the adjacent printed inner wall; samples on the opposite side are rejected even when they remain close enough to the external wall to pass the support check. This uses the open wall geometry without treating it as a closed polygon. Full paths at a zero-gap seam also retain clearance from the external wall after their initial connector. At a clipped corner, another branch can be closer than the requested offset, so material-side and support checks apply without that additional clearance rule.

An accepted candidate replaces the stored wipe path as a whole. A short direct inward move is also eligible when longer candidates fail validation. It may waive full wall clearance, but must pass the material-side check. Its initial direction is checked from the actual nozzle position after any loop pre-move; the original wall endpoint is retained separately for intersection checks. It takes priority over the alternate offset when the preferred and translated paths are unusable. A longer reversed path may replace the selected candidate only when its distance to the target inner wall is no worse within tolerance.

Fallback to the regular wipe

The original wipe path is retained when:

No suitable adjacent inner wall has already been printed near the seam. This includes single-wall areas, locally missing inner walls and normally Outer/Inner wall order. A distant wall or a wall on the air side does not qualify.
The requested or available offset, or the configured wipe distance, is zero or too small at the geometry's coordinate precision.
Degenerate geometry prevents construction of a usable candidate, or all candidates fail the checks for printed support, direction, wall clearance or the connector from the actual nozzle position. This can occur at tight corners, narrow features or seam gaps.

Corners and seam gaps do not automatically trigger fallback: an offset, translated, reversed or short direct inward path may still be valid. The regular wipe is retained only when no candidate is accepted.

Fallback uses the path and retraction rules for wipe_inward disabled. Wipe while retracting must still be enabled for a wipe to occur; wipe_on_loops remains controlled by its own setting.

Interaction with Wipe on loop

wipe_on_loops is an independent option that makes a short move before leaving an external loop. It can operate with wipe_inward disabled. When both options are enabled, its destination is the starting position for the inward wipe.

The loop move samples the outgoing and incoming paths by distance across path boundaries. The sampling distance is bounded by the nozzle diameter and one quarter of the total path length. It samples the outgoing path at up to 20% of the nozzle diameter and rotates that point around the seam through one third of the material-side corner angle. For a closed square outer contour, this produces a move of 20% of the nozzle diameter at 30 degrees into the corner. Coincident samples or degenerate angles suppress the move.

The nozzle position stored by G-code generation must match the emitted loop move. Both travel planning and wipe execution depend on this position, including when Wipe inward is disabled.

With a seam gap, a loop move may advance past the inward offset's original entry. If that alone makes the connector backtrack, the entry advances to the nozzle's projection on the offset. The planner extends the source as needed to preserve the configured wipe length and validates the new connector and complete path. Joins that already backtrack across the seam gap are not adjusted this way.

Execution and retraction

The stored wipe path uses a sentinel first point. Execution starts from the actual nozzle position and proceeds to the second stored point. Path selection, support validation and wipe-length calculation must all use this same executable geometry, especially after a Wipe on loop move.

An accepted inward path executes at the end of the external loop, after any Wipe on loop move, without retracting filament. It consumes the stored path and updates the nozzle position before travel planning. A short travel to the next wall cannot discard this wipe or force a retraction or Z-hop. Subsequent travel uses the normal minimum-travel threshold and retraction/lift settings from the new position. The regular wipe, including fallback, remains deferred until a normal retraction uses it.

Retraction is divided into portions before, during and after wiping. The amount that can be retracted during the wipe depends on its executable length, wipe speed and the active filament's retraction speed. Fractional retraction speeds are retained in this calculation. For a 2 mm wipe at 100 mm/s and a retraction speed of 25.5 mm/s, the wipe can retract 0.51 mm. With a total retraction of 0.8 mm and both before/after percentages set to zero, the remaining 0.29 mm is retracted before wiping. This split applies to regular deferred wipes, including fallback; an accepted inward wipe executes separately without retraction.

Implementation and verification

GCode.cpp integrates path selection, nozzle position and retraction; Print.cpp controls invalidation, and PrintConfig.cpp defines the settings.
WipePathHelpers implements path sampling, offset selection and support checks.
Geometry tests cover support, degenerate paths, contour and hole orientations, and exact loop-move geometry across path subdivisions.
FFF tests cover emitted trajectories, fallback, minimum-travel retraction and Z-hop rules, and export invalidation. With Wipe inward disabled, they check the loop move's direction and magnitude for Classic and Arachne, the subsequent wipe's start and length, and fractional retraction splitting in absolute and relative E modes. Loop-move checks use reserved role/wipe markers and extrusion state, and run with human-readable G-code comments both enabled and disabled.

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