Slicer Project Compilers

Intro

Slicer project compilers turn prepared OpenVCAD designs into slicer-native .3mf project files for filament printing workflows. Use them when you want OpenVCAD to generate geometry regions and attribute-derived assignments, while the downstream slicer still owns slicing, preview, and G-code generation.

There are two separate workflows:

Workflow	Input attributes	Output project	Best fit
PrusaSlicer settings meshes	Scalar planning attributes such as INFILL_DENSITY, FUZZY_SKIN_POINT_DISTANCE, LAYER_HEIGHT, or PERIMETER_SPEED	PrusaSlicer-style .3mf	Spatially varying slicer planning settings
PrusaSlicer virtual extrusion	TEMPERATURE with optional companion FLOW_RATE	PrusaSlicer-style .3mf with logical tools and toolchange G-code	Machine-state changes at region boundaries
PrusaSlicer ColorMix color	COLOR_RGB	PrusaSlicer-style .3mf with ColorMix virtual extruders	Mixed-filament color approximation from design-space colors
PrusaSlicer ColorMix material fractions	VOLUME_FRACTIONS plus volume_fraction_materials	PrusaSlicer-style .3mf with ColorMix virtual extruders	Direct material-fraction to filament-slot recipes
FullSpectrum color	COLOR_RGB	Orca / Bambu-style Full Spectrum .3mf	Mixed-filament color approximation from design-space colors
FullSpectrum material fractions	VOLUME_FRACTIONS plus volume_fraction_materials	Orca / Bambu-style Full Spectrum .3mf	Direct material-fraction to filament-slot recipes

For slice-image workflows instead of slicer projects, see Material Inkjet, Color Inkjet, and VAT Photo.

Prerequisites

• OpenVCAD with pyvcad, pyvcad_compilers, and pyvcad_rendering installed.

• Familiarity with attributes on geometry. See Getting Started with OpenVCAD, especially Lesson 4, and the Functional Grading Guide.

• A matching slicer for the project you generate: PrusaSlicer for Prusa-style scalar projects, or the OrcaSlicer-FullSpectrum fork for the FullSpectrum examples in this guide.

Shared 3MF Concepts

A 3MF file is a compressed package, not a single flat mesh file. Conceptually it behaves like a ZIP / OPC-style container with relationship files, content-type metadata, and XML model parts. The core model part stores triangle meshes as XML vertices and triangles. Slicers extend the package by adding their own project settings and per-model metadata.

Typical package shapes:

generic_part.3mf
  [Content_Types].xml
  _rels/.rels
  3D/3dmodel.model

prusa_project.3mf
  3D/3dmodel.model
  Metadata/Slic3r_PE.config
  Metadata/Slic3r_PE_model.config

prusa_colormix_project.3mf
  3D/3dmodel.model
  Metadata/Slic3r_PE_model.config
  Metadata/Prusa_Slicer_full_spectrum.json

orca_full_spectrum_project.3mf
  3D/3dmodel.model
  Metadata/project_settings.config
  Metadata/model_settings.config

A very small 3MF mesh fragment looks like this:

<model unit="millimeter" xml:lang="en-US"
       xmlns="http://schemas.microsoft.com/3dmanufacturing/core/2015/02">
  <resources>
    <object id="1" type="model">
      <mesh>
        <vertices>
          <vertex x="0" y="0" z="0"/>
          <vertex x="20" y="0" z="0"/>
          <vertex x="0" y="20" z="0"/>
        </vertices>
        <triangles>
          <triangle v1="0" v2="1" v3="2"/>
        </triangles>
      </mesh>
    </object>
  </resources>
</model>

The extra value of slicer project 3MFs comes from the attachments around that core mesh. A PrusaSlicer project can carry Prusa project configuration and per-volume settings. An Orca / Bambu-style project can carry model settings, project settings, and component assignments that tell the slicer which extruder or virtual mixed filament belongs to each region.

For the 3MF container and core mesh format, see the 3MF Consortium specification page and the Core 3MF Specification.

PrusaSlicer

Use the PrusaSlicer compiler when your OpenVCAD design carries scalar process attributes and you want a PrusaSlicer-compatible project file. The compiler samples scalar attributes over the solid, partitions the sampled field into a finite number of regions, emits a mesh for each region, and writes a PrusaSlicer-style .3mf.

The important idea is that a continuous OpenVCAD field becomes piecewise constant slicer regions. Each generated region gets one representative value. More regions preserve more variation, but they also create more sub-volumes for PrusaSlicer to manage.

Opening The Project

The compiler only writes the .3mf. It does not launch PrusaSlicer.

1. Run your Python script so compile() finishes and the .3mf path exists.

2. Open PrusaSlicer, use File -> Import, and select the generated .3mf.

3. Inspect sub-volumes and per-volume settings in the Plater / Models UI, then slice and review the G-code preview.

Settings Applied To Meshes

The settings-mesh path is for attributes that directly affect slicer planning. These settings change how PrusaSlicer computes toolpaths: infill density, layer height, perimeter counts, line widths, and feature speeds. OpenVCAD emits sub-volumes, and each sub-volume carries Prusa-compatible metadata for the selected slicer setting.

This path is appropriate when the parameter should influence slicing decisions before G-code is generated. For example, an INFILL_DENSITY gradient changes how dense the internal infill toolpaths are in each generated region.

Settings-mesh attributes are scalar fields. The Python names below are available under pv.DefaultAttributes.

Attribute	PrusaSlicer setting key	Typical use
INFILL_DENSITY	fill_density	Spatial infill percentage
FUZZY_SKIN_POINT_DISTANCE	fuzzy_skin_point_dist	Spatial fuzzy skin roughness
BRIDGE_SPEED	bridge_speed	Bridge planning speed
EXTERNAL_PERIMETER_EXTRUSION_WIDTH	external_perimeter_extrusion_width	Outer wall line width
EXTERNAL_PERIMETER_SPEED	external_perimeter_speed	Outer wall speed
EXTRUSION_WIDTH	extrusion_width	General line width
GAP_FILL_SPEED	gap_fill_speed	Gap fill speed
INFILL_EXTRUSION_WIDTH	infill_extrusion_width	Infill line width
INFILL_SPEED	infill_speed	Infill speed
IRONING_FLOWRATE	ironing_flowrate	Ironing flow
IRONING_SPEED	ironing_speed	Ironing speed
LAYER_HEIGHT	layer_height	Region layer height
PERIMETER_EXTRUSION_WIDTH	perimeter_extrusion_width	Perimeter line width
PERIMETER_SPEED	perimeter_speed	Perimeter speed
PERIMETERS	perimeters	Perimeter count
SMALL_PERIMETER_SPEED	small_perimeter_speed	Small-feature perimeter speed
SOLID_INFILL_EXTRUSION_WIDTH	solid_infill_extrusion_width	Solid infill line width
SOLID_INFILL_SPEED	solid_infill_speed	Solid infill speed
SUPPORT_MATERIAL_EXTRUSION_WIDTH	support_material_extrusion_width	Support extrusion width
TOP_INFILL_EXTRUSION_WIDTH	top_infill_extrusion_width	Top infill line width
TOP_SOLID_INFILL_SPEED	top_solid_infill_speed	Top solid infill speed

Fuzzy skin cylinder (01_fuzzy_skin_cylinder.py): this example applies FUZZY_SKIN_POINT_DISTANCE along the cylinder height and writes output/fuzzy_skin_cylinder.3mf. The generated sub-volumes carry fuzzy_skin_point_dist overrides, so PrusaSlicer changes the planned surface texture by region.

cylinder.set_attribute(
    pv.DefaultAttributes.FUZZY_SKIN_POINT_DISTANCE,
    pv.FloatAttribute(fuzzy_expr),
)

compiler = pvc.PrusaSlicerProjectCompiler(
    root,
    pv.Vec3(0.25, 0.25, 0.25),
    out_3mf,
    num_regions=12,
)

OpenVCAD (fuzzy skin distance)

PrusaSlicer preview (model and fuzzy skin G-code texture)

Models / sub-volumes (fuzzy skin overrides)

Infill density bar (02_infill_density_bar.py): this example ramps INFILL_DENSITY along the bar and writes output/infill_density_bar.3mf. The generated regions carry fill_density metadata, so PrusaSlicer computes denser or sparser infill toolpaths in each region.

bar.set_attribute(
    pv.DefaultAttributes.INFILL_DENSITY,
    pv.FloatAttribute(infill_expr),
)

compiler = pvc.PrusaSlicerProjectCompiler(
    root,
    pv.Vec3(0.25, 0.25, 0.25),
    out_3mf,
    num_regions=12,
)

OpenVCAD (infill density)

PrusaSlicer G-code preview (infill gradient cutaway)

Virtual Extrusion

Virtual extrusion controls machine state by taking advantage of slicer toolchange events. OpenVCAD assigns synthetic extruder indices to regions, patches the project profile to declare those logical tools, and provides toolchange G-code that applies the desired process commands when the slicer switches between regions.

The logical toolhead does not need to correspond to a separate physical nozzle. On a single-tool printer, the slicer can still emit toolchange events between logical tools, and the toolchange script can update temperature, flow, or other machine state without changing filament. On a multi-tool printer, the same concept can coexist with physical tool changes if the printer profile and script are written that way.

This path is intentionally different from settings meshes:

Path	Controls	Slicer sees it before planning?	Best fit
Settings mesh	Toolpath planning settings	Yes	Infill, speeds, widths, layer height, perimeters
Virtual extrusion	Machine state commands	No, it runs as G-code	Temperature, flow, and process state that can be changed at region boundaries

In the current method, TEMPERATURE is the primary virtual-extrusion attribute and FLOW_RATE is its companion.

Attribute	Role	Effect
TEMPERATURE	Primary region attribute	Creates logical tools and emits M104 temperatures through toolchange G-code
FLOW_RATE	Companion attribute	Supplies M221 flow values for the same logical toolchange events

The toolchange script applies commands such as:

M104 S{temperature}
M221 S{flow_rate}

Temperature and flow example (03_temperature_compensation_demo.py): this example applies a TEMPERATURE field, derives FLOW_RATE from a lookup table, and writes output/temperature_compensation_demo.3mf. The compiler uses Prusa profile files from examples/applications/foaming_filaments/profiles/ so the generated project can define logical tools and toolchange G-code.

rect_prism.set_attribute(
    pv.DefaultAttributes.TEMPERATURE,
    pv.FloatAttribute("max(min((x/3+236),256),216)"),
)

compiler = pvc.PrusaSlicerProjectCompiler(
    root,
    pv.Vec3(0.25, 0.25, 0.25),
    out_3mf,
    num_regions=10,
    printer_profile_path=printer_profile_path,
    filament_profile_path=filament_profile_path,
)

OpenVCAD (temperature)

PrusaSlicer G-code preview (temperature)

ColorMix

PrusaSlicer ColorMix uses Prusa’s FullSpectrum project metadata and Prusa_Slicer_full_spectrum.json to define virtual extruders whose IDs start after the physical filament slots. OpenVCAD assigns generated color/material regions to those physical or virtual extruder IDs in the normal Prusa Slic3r_PE_model.config metadata.

ColorMix mode is mutually exclusive with the scalar settings-mesh and virtual-extrusion paths. It is selected automatically when the design contains COLOR_RGB or VOLUME_FRACTIONS, or explicitly with enable_color_mix=True.

For COLOR_RGB, OpenVCAD uses a device-aware recipe selector. It generates valid Prusa ColorMix recipes from the configured physical filaments, predicts each recipe’s apparent color with Prusa’s mixer model, and then selects up to max_palette_size printable recipes that best cover the sampled design colors. max_palette_size counts both direct physical filament colors and virtual mixed recipes. The generated 3MF also includes Metadata/OpenVCAD_prusa_colormix_report.json, which records the selected recipes and DeltaE fit summary for inspection.

Color gradient example (06_prusa_colormix_color_gradient.py): this example applies a COLOR_RGB ramp and writes output/prusa_colormix_color_gradient.3mf.

bar.set_attribute(
    pv.DefaultAttributes.COLOR_RGB,
    pv.Vec3Attribute(red_expr, green_expr, blue_expr),
)

compiler = pvc.PrusaSlicerProjectCompiler(
    root,
    pv.Vec3(0.25, 0.25, 0.25),
    out_3mf,
    enable_color_mix=True,
    color_mix_recipe_preset="expanded",
    max_palette_size=10,
    min_component_percent=15,
    max_recipe_components=3,
    region_overlap_mm=0.2,
)

Volume-fraction example (07_prusa_colormix_volume_fractions.py): this example maps OpenVCAD materials directly to physical filament slots and writes output/prusa_colormix_volume_fractions.3mf.

compiler = pvc.PrusaSlicerProjectCompiler(
    root,
    pv.Vec3(0.25, 0.25, 0.25),
    out_3mf,
    enable_color_mix=True,
    total_physical_extruders=5,
    color_mix_filaments=[
        {"slot": 1, "color_hex": "#FF0000"},
        {"slot": 2, "color_hex": "#0000FF"},
    ],
    volume_fraction_materials={"red": 1, "blue": 2},
    max_palette_size=10,
    min_component_percent=1,
    max_recipe_components=3,
    region_overlap_mm=0.0,
)

Prusa Constructor Options

compiler = pvc.PrusaSlicerProjectCompiler(
    root,
    pv.Vec3(0.25, 0.25, 0.25),
    out_3mf,
    num_regions=12,
    printer_profile_path=printer_profile_path,
    filament_profile_path=filament_profile_path,
    enable_color_mix=False,
    color_mix_filaments=None,
    total_physical_extruders=5,
    color_mix_recipe_preset="expanded",
    max_palette_size=32,
    min_component_percent=15,
    max_recipe_components=3,
    direct_physical_delta_e=1.0,
    region_overlap_mm=0.2,
    volume_fraction_materials={},
    material_defs_path="",
)
compiler.compile()

Option	Meaning
root	The OpenVCAD design tree to compile.
voxel_size	Sampling spacing in mm. Smaller values resolve smaller regions but cost more time and memory.
output_file_path	Destination .3mf project path.
num_regions	Number of value bands for each scalar attribute. More regions preserve more variation but create more sub-volumes.
printer_profile_path	PrusaSlicer printer .ini. Required when virtual extrusion attributes are present.
filament_profile_path	PrusaSlicer filament .ini. Required when virtual extrusion attributes are present.
enable_color_mix	Explicitly enables Prusa ColorMix mode. COLOR_RGB or VOLUME_FRACTIONS also selects it automatically.
color_mix_filaments	Physical filament slots and colors for ColorMix. Each dictionary may include slot, color_hex, and nozzle_diameter_mm. Empty uses Prusa CMYKW defaults for COLOR_RGB: #0082ad, #d10f4f, #efd00b, #3d3e3d, #e6eaef.
total_physical_extruders	Total physical tool slots in PrusaSlicer. Virtual ColorMix extruders start after this count, so unused physical tool IDs can be skipped. Defaults to 5.
color_mix_recipe_preset	Recipe vocabulary for Prusa COLOR_RGB ColorMix. conservative uses base colors plus 25/75, 50/50, and 75/25 pair blends. expanded also includes 50/25/25 three-filament rotations.
max_palette_size	Maximum generated ColorMix palette size or volume-fraction recipe-region count. For Prusa COLOR_RGB, this counts both physical base colors and virtual mixed recipes.
min_component_percent	Minimum component percentage kept in generated ColorMix recipes.
max_recipe_components	Maximum physical filaments allowed in one Prusa ColorMix recipe. Prusa ColorMix supports up to 3.
direct_physical_delta_e	Legacy setting retained for compatibility. Prusa COLOR_RGB ColorMix now chooses physical colors and virtual recipes from the same device-aware candidate set.
region_overlap_mm	Small overlap between generated ColorMix sub-parts to avoid visible cracks between separately meshed regions.
volume_fraction_materials	Material-name to physical-slot mapping for direct VOLUME_FRACTIONS export, such as {"red": 1, "blue": 2}.
material_defs_path	Optional MaterialDefs JSON path. Empty uses the bundled default material definitions.

FullSpectrum

Use the FullSpectrum compiler when your OpenVCAD design carries either COLOR_RGB or VOLUME_FRACTIONS and you want an Orca / Bambu-style project for the FullSpectrum workflow.

Full Spectrum is a slicer workflow for getting more apparent colors out of a limited set of loaded filaments. Instead of truly melting filaments together into a homogeneous mixed polymer, it creates virtual mixed-color filaments by alternating physical filaments through layer dithering or related patterns. Thin layers and partially translucent filament let light interact with multiple colors, so the eye sees an intermediate color.

The achievable colors depend strongly on the actual filaments loaded in the printer, their translucency, layer height, surface orientation, and slicer settings. A CMYKW physical set, cyan, magenta, yellow, black, and white, is the recommended starting point because it gives the color-matching algorithm a broad gamut to work with.

OpenVCAD targets the OrcaSlicer-FullSpectrum community fork for this project format. Treat this as an emerging ecosystem: the generated projects use Orca / Bambu-style metadata and FullSpectrum mixed-filament settings, and stock PrusaSlicer, stock OrcaSlicer, or stock Bambu Studio may not interpret those settings correctly.

COLOR_RGB Workflow

Use COLOR_RGB when the design field is an intended color. The compiler samples the RGB target field, reduces it to a bounded palette with max_palette_size, compares each palette color against the loaded physical filaments and possible mixed-filament recipes, and assigns each generated mesh region to a physical or virtual mixed filament.

The default physical palette for COLOR_RGB mode is CYMKW if you do not pass filaments. Pass explicit filament dictionaries when your loaded spool colors differ.

compiler = pvc.FullSpectrumSlicerProjectCompiler(
    root,
    pv.Vec3(0.25, 0.25, 0.25),
    out_3mf,
    [
        {"slot": 1, "color_hex": "#00FFFF"},
        {"slot": 2, "color_hex": "#FF00FF"},
        {"slot": 3, "color_hex": "#FFFF00"},
        {"slot": 4, "color_hex": "#000000"},
        {"slot": 5, "color_hex": "#FFFFFF"},
    ],
    max_palette_size=32,
    min_component_percent=15,
    max_recipe_components=5,
)

Color gradient example (04_full_spectrum_color_gradient.py): this example applies a COLOR_RGB ramp from cyan toward magenta and writes output/full_spectrum_color_gradient.3mf. The compiler samples the RGB target field, reduces it to a small palette, and assigns each palette color to a physical or virtual mixed filament recipe.

bar.set_attribute(
    pv.DefaultAttributes.COLOR_RGB,
    pv.Vec3Attribute(red_expr, green_expr, blue_expr),
)

compiler = pvc.FullSpectrumSlicerProjectCompiler(
    root,
    pv.Vec3(0.25, 0.25, 0.25),
    out_3mf,
    max_palette_size=10,
    min_component_percent=15,
    max_recipe_components=5,
)

OpenVCAD (COLOR_RGB target)

OrcaSlicer-FullSpectrum g-code screenshot

VOLUME_FRACTIONS Workflow

Use VOLUME_FRACTIONS when the OpenVCAD field already describes material proportions and you want those proportions to become FullSpectrum recipe weights. This is simpler than color matching because the design says which materials should participate.

The compiler maps OpenVCAD material names to physical filament slots with volume_fraction_materials, normalizes the mapped fractions for each sample, clusters fraction vectors into at most max_palette_size recipe regions, and emits physical or virtual mixed-filament recipes using representative weights for each region.

compiler = pvc.FullSpectrumSlicerProjectCompiler(
    root,
    pv.Vec3(0.25, 0.25, 0.25),
    out_3mf,
    volume_fraction_materials={"red": 1, "blue": 2},
    max_palette_size=10,
    min_component_percent=1,
    max_recipe_components=5,
)

Volume-fraction example (05_full_spectrum_volume_fractions.py): this example ramps red and blue material fractions across the bar and writes output/full_spectrum_volume_fractions.3mf. The volume_fraction_materials={"red": 1, "blue": 2} mapping sends those OpenVCAD material names directly to physical filament slots.

bar.set_attribute(
    pv.DefaultAttributes.VOLUME_FRACTIONS,
    pv.VolumeFractionsAttribute(
        [
            (red_fraction_expr, materials.id("red")),
            (blue_fraction_expr, materials.id("blue")),
        ]
    ),
)

compiler = pvc.FullSpectrumSlicerProjectCompiler(
    root,
    pv.Vec3(0.25, 0.25, 0.25),
    out_3mf,
    volume_fraction_materials={"red": 1, "blue": 2},
    max_palette_size=10,
    min_component_percent=1,
    max_recipe_components=5,
)

OpenVCAD (VOLUME_FRACTIONS target)

OrcaSlicer-FullSpectrum g-code screenshot

FullSpectrum Constructor Options

compiler = pvc.FullSpectrumSlicerProjectCompiler(
    root,
    voxel_size,
    output_file_path,
    filaments=None,
    max_palette_size=32,
    min_component_percent=15,
    max_recipe_components=5,
    direct_physical_delta_e=1.0,
    region_overlap_mm=0.2,
    base_project_settings_path="",
    orca_machine_profile_path="",
    orca_process_profile_path="",
    orca_default_filament_profile_path="",
    orca_profile_search_paths=[],
    volume_fraction_materials={},
    material_defs_path="",
)

Option	Meaning
filaments	Physical filament slots and colors. Each dictionary may include slot, color_hex, filament_settings_id, filament_id, filament_profile_path, and nozzle_diameter_mm.
max_palette_size	Maximum generated COLOR_RGB palette size or VOLUME_FRACTIONS recipe-region count.
min_component_percent	Minimum component percentage kept in generated mixed recipes. Tiny components are dropped and the remaining weights are normalized.
max_recipe_components	Maximum physical filaments allowed in one mixed recipe.
direct_physical_delta_e	DeltaE threshold for assigning a COLOR_RGB target directly to a physical filament instead of creating a mixed recipe.
region_overlap_mm	Small overlap between generated FullSpectrum sub-parts to avoid visible cracks between separately meshed regions.
base_project_settings_path	Optional Orca project settings JSON to merge before compiler-controlled FullSpectrum keys are written.
orca_machine_profile_path	Optional Orca machine profile path or resolvable profile name.
orca_process_profile_path	Optional Orca process profile path or resolvable profile name.
orca_default_filament_profile_path	Optional default filament profile path or resolvable profile name.
orca_profile_search_paths	Additional directories searched for Orca JSON profiles.
volume_fraction_materials	Material-name to physical-slot mapping for direct VOLUME_FRACTIONS export, such as {"red": 1, "blue": 2}.
material_defs_path	Optional MaterialDefs JSON path. Empty uses the bundled default material definitions.

FullSpectrum Limitations

• FullSpectrum is not true molten filament mixing. It is an optical and geometric approximation based on alternating physical filaments.

• The reachable color gamut depends on the loaded filaments, translucency, layer height, top/bottom shell behavior, and surface orientation.

• OpenVCAD emits a finite number of mesh regions. More regions can preserve more variation, but they also increase project complexity.

• Sloped surfaces, small details, top layers, and short regions can show different color behavior than tall vertical walls.

• Stock slicers may ignore or reject FullSpectrum metadata. Use the documented FullSpectrum fork unless you have verified another slicer version.

• max_recipe_components and min_component_percent intentionally bound recipe complexity. Some requested colors or material blends cannot be represented within those limits.