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PrusaSlicer FFF slicing

schemaxis.prusaslicer/1 slices one closed STL part into fused-filament (FFF/FDM) G-code with an explicit printer, filament and print profile. sliceModel publishes the G-code, a per-layer table, a summary with PrusaSlicer's native print-time and filament estimates, and the exact configuration it used. An independent pure-Python replay of the G-code decides whether the result is accepted; a successful native exit alone is not.

This adaptor never controls a printer. It does not upload, stream or send G-code to any machine, and it does not certify print quality, adhesion, strength, dimensional accuracy or thermal safety. See assumptions and limits.

PYTHONPATH=src python3 -m schemaxis plan examples/schemaxis/prusaslicer-pla.json
# Replace model.file with an absolute STL path first.
PYTHONPATH=src python3 -m schemaxis run my-slice.json \
  --prusaslicer /absolute/path/to/prusa-slicer --output-directory evidence/my-slice
PYTHONPATH=src python3 tools/qualify_prusaslicer.py --native \
  --executable /absolute/path/to/prusa-slicer --output-directory evidence/my-prusaslicer-qualification

Target runtime#

The target is the latest stable upstream release, PrusaSlicer 2.9.6 (release notes). The 3.0.0 alphas are prereleases and are not targeted. The adaptor uses only the public command-line interface: --help for runtime identity, --info for native model readback, and --load, --scale, --center, --threads, --datadir and --export-gcode for slicing. Option names and meanings follow the 2.9.6 configuration definitions.

Configure an absolute path to a PrusaSlicer 2.9.6 executable that accepts these console options, using schemaxis configure, --prusaslicer or the PRUSASLICER_EXECUTABLE environment variable. On Linux, the official distribution channel is Flathub; a flatpak launcher is not a plain executable, so point Schemaxis at a console binary. The first line of --help must identify version 2.9.6. Discovery and planning never import or launch PrusaSlicer.

Each native stage runs in its own process with exactly one slicing thread, a private --datadir and a private home directory inside the attempt. No user, vendor or system presets are read, and nothing is uploaded.

Document and units#

All lengths are millimetres, speeds mm/s, accelerations mm/s², temperatures degrees Celsius, density g/cm³, filament mass grams and time seconds. STL files carry no unit, so model.units is required.

The planner rejects invalid combinations before any file is read or process is launched: INVALID_PROFILE for layer heights above 0.8 × nozzle, widths outside 1–2 nozzle diameters or not above the layer heights, and 100 % non-rectilinear infill; INVALID_RESOURCE for relative or non-STL paths; MODEL_OUT_OF_BOUNDS for a centre outside the bed; and INVALID_REQUIREMENT for duplicate names.

Model checks before slicing#

Before launching PrusaSlicer, the run reads the STL with an independent bounded parser. It requires one closed, consistently oriented, non-degenerate triangle mesh: every directed edge appears once and has an oppositely directed partner, all triangles form one connected shell, and the signed volume is positive. Triangle winding is authoritative; stored facet normals are ignored. Edge pairing does not detect self-intersections. ASCII files must contain exactly one solid, with nothing after endsolid except its name. Volume uses a mesh-local reference vertex so translation does not change it.

The scaled bounding box must fit the bed around the placement centre and the declared height, and each footprint side must be at least two extrusion widths. Failures are INVALID_MODEL, MODEL_OUT_OF_BOUNDS or INPUT_DIGEST_MISMATCH, and no attempt directory is created. The model is always dropped onto the bed; it is never rotated, arranged or repaired.

Fixed generation policy#

Schemaxis renders config.ini from the declared values plus a fixed policy; see src/schemaxis/adaptors/prusaslicer/profile.py. Authored text never becomes G-code, configuration keys, preset names or scripts. The policy includes:

• start G-code G21, G90, M83, G28, G92 E0 and end G-code M104 S0, M140 S0, M107, M84; PrusaSlicer inserts the declared temperature waits before the start G-code;
• relative extrusion, 0.8 mm retraction at 35 mm/s, no Z-hop, wipe or firmware retraction;
• classic perimeters, one extruder, no supports, brim, skirt, raft, ironing, wipe tower, spiral vase, arc fitting, binary G-code, object labels or thumbnails;
• empty custom, layer and post-processing G-code;
• machine_limits_usage = time_estimate_only: machine limits affect only the native time estimate and are not emitted. Z and E limits are fixed at 12 and 120 mm/s;
• cooling on, with slowdown below 5 s layer time down to 10 mm/s; this can only lower speeds.

Independent checks#

Verification does not trust native success flags. After slicing, and again on every schemaxis verify, the adaptor recomputes everything from retained bytes:

1. Runtime identity: --help must report PrusaSlicer 2.9.6 and the G-code header must name the same runtime.
2. Native model readback: --info on the retained STL must match the independent facet count, extents and minimum corner (1e-5 times the largest extent), and volume (relative 1e-4). Each numeric allowance additionally includes 0.5e-6 in raw STL length or cubic units for PrusaSlicer 2.9.6's fixed six-decimal text rounding, before the authored unit conversion. Values must be finite; the reader must report a manifold single part and no repairs.
3. Configuration readback: every rendered key must appear with the same value in the configuration block that PrusaSlicer embeds in the G-code.
4. G-code replay (gcode.py): only the commands the fixed policy can emit are accepted. Arcs, inch units, tool changes, extrusion while changing Z, a second homing or homing after motion are rejected. Homing is modelled at the bed origin.
5. Build volume: every move stays inside the bed and below the maximum height.
6. Layers: the first extrusion is on the first layer, every extrusion layer lies on the grid first-layer height + k × layer height, and none lies above the model top. Native ;Z: annotations must equal the replayed layers. The gap below the model top and any empty grid layers are reported. Slivers or tips narrower than one line can leave a layer empty, so exact counts are asserted only by the flat-topped acceptance cases. PrusaSlicer 2.9.6 stops at the last full layer at or below the model top, so a 10 mm box with 0.2 + k × 0.15 mm layers ends at 9.95 mm.
7. Envelope: on every layer, extrusion centrelines stay inside the model footprint inset by half the extrusion width. The deviation of the union envelope from that inset is reported for prismatic parts.
8. Setpoints: M109/M190 waits for the first-layer temperatures precede the first extrusion. Every first-layer extrusion runs at the first-layer nozzle and bed setpoints with the fan off, and every later extrusion at the later setpoints and the declared fan duty. No undeclared temperature is ever commanded.
9. Feedrates: first-layer extrusion never exceeds the first-layer speed; later extrusion never exceeds the larger of the perimeter and infill speeds; travel never exceeds the travel speed.
10. Filament arithmetic: the replayed extruded length must match the native filament used [mm]; volume (from the declared diameter) and mass (from the declared density) must match the native cm³ and gram figures.
11. Time bounds: the native normal-mode estimate must lie between a constant-speed lower bound and a rest-to-rest trapezoidal upper bound, both computed from the replayed moves and the declared feedrate and acceleration limits.
12. Plausibility: deposited filament volume is positive and at most 1.2 × the model volume.
13. Fresh reload: a separate native process loads only the configuration embedded in the published G-code and re-slices the retained STL. Its G-code must be identical apart from the ; generated by timestamp line.

Tolerances: coordinates 0.011 mm, because G-code prints three decimals. Feedrates 0.01 mm/s, printed in mm/min. Filament length 0.006 mm plus 5×10⁻⁶ mm per extrusion move, for five-decimal E values and two-decimal statistics, plus 10⁻⁴ relative. Native times print whole seconds, so the bounds allow ±1 s. Fan duty allows half a step of the 0–255 scale.

Publications and evidence#

verification.json binds the publication digests, the retained STL copy, every stage's command, submission, stdout and stderr, and the reloaded G-code. Tampering with any of them makes verification fail. Publications can be passed to workflows. For example, layers.csv is a numeric table; its units are in its column names.

Faults and recovery#

Assumptions and limits#

• The G-code targets a generic Marlin 2 FFF machine described only by the declared values. It has not been executed on, sent to or validated for any physical printer, firmware or material.
• Temperatures, speeds and fan values are authored setpoints. Schemaxis checks that they are applied as declared, not that they suit the material, machine or environment.
• Print time and filament figures are PrusaSlicer model estimates for that generic machine. They are checked only against kinematic bounds and filament arithmetic, not measured on hardware.
• One STL object and one extruder only. There are no supports, modifiers, multi-material or variable layer height. 3MF/STEP input, mesh repair, printer presets, network upload and physical printer control are separate capabilities and are not implemented.

Acceptance cases#

A verified geometry artifact can supply model.file through a workflow $artifact reference. It must still satisfy the STL and physical-size checks, and model.units remains required. The OpenSCAD-to-PrusaSlicer example constructs a two-hole plate and slices its retained STL with explicit millimetre units. The native OpenSCAD 2021.01 / PrusaSlicer 2.9.6 handoff, workflow verification and unchanged reuse of both operations passed. It produces reviewable files and does not connect to a printer.

The development cases slice original analytic fixtures. Their literal expectations are derived from geometry, not copied from native output:

• examples/schemaxis/prusaslicer-box.stl: a 24 × 16 × 10 mm box (3840 mm³). With the PLA profile (0.2 mm layers, 0.45 mm width, centre 125, 105) it gives 50 layers from 0.2 to 10.0 mm. The perimeter envelope is x 113.225–136.775 and y 97.225–112.775. With the PETG profile (0.4 mm first layer, then 0.3 mm layers, 0.68 mm width, centre 150, 150) it gives 1 + 9.6/0.3 = 33 layers. Its 100 % rectilinear infill should deposit about the solid volume (ratio 1 ± 10 %).
• examples/schemaxis/prusaslicer-box-inch.stl: a 1.0 × 0.6 × 0.4 inch box declared in inches, sliced as 25.4 × 15.24 × 10.16 mm (3932.89536 mm³) with 0.254 mm layers: 40 layers to 10.16 mm.

Run tools/qualify_prusaslicer.py for host conformance; add --native and an executable for bounded native acceptance. See the completion review for the latest results and remaining gaps.