Blender export (engine-ready files)

An export is done when the file has been re-imported and measured, not when the exporter returned FINISHED. Gate the asset first, export with a preset, then round-trip the file. This skill builds on scenario-blender-expert (execution channels, the delivery gate) and imports its scripts, so install the two side by side. If a sibling skill named here is missing from your available skills, ask the user to install it (npx skills add scenario-labs/skills --skill <name>); unattended, proceed from tool schemas and flag the gap.

Toolkit: scripts/bx_export.py. Per-engine facts with their sources: references/engines.md.

Quick reference

Target	Preset	File	Engine side
Unity, static mesh	unity	FBX	import defaults
Unity, character	unity_skeletal	FBX	leave Bake Axis Conversion off
Unreal Engine	unreal	FBX	Convert Scene Unit (to centimeters)
Unreal Engine, character	unreal_skeletal	FBX	one root bone
Godot 4	godot	GLB	import defaults; Generate LODs in the dock
Roblox	roblox	FBX	20,000 triangles per mesh
three.js and the web	web	GLB, meshopt, WebP	MeshoptDecoder; not for Godot
any glTF reader	gltf	GLB
USD pipelines	usd	USDC
Apple AR Quick Look	usdz	USDZ, 2048 textures
3D printing	print	STL in millimeters	slicer reads millimeters

The Unity and Godot presets were imported into Unity 6000.3 and Godot 4.7.2 and measured; unreal, unreal_skeletal and roblox follow Epic's and Roblox's documentation and pass the Blender round trip only. The measurements live in references/engines.md.

Procedure

1. Gate. Run bx_audit.deliverable(parts, profile) from scenario-blender-expert and fix every error. Scale left on an object travels into the engine as the node's transform.
   • Open borders and edges shared by three or more faces have no automatic fix. On a generated chest, hole filling closed 4 of 24 borders and added a back-to-back face.
   • Rebuild the surface with scenario-blender-retopology, bake the original's maps onto it, then gate again.
   • Without a rebuild, report the file as blocked for that profile (roblox requires watertight meshes). Never deliver it as ready.
2. Size and pivot. A generated mesh arrives at an arbitrary size with its origin at its center, so it sinks into the floor. bxe.ground(parts, width=0.9) (or height=, in meters) bakes the transforms, puts the center of the base at the origin and scales to the real size. Ask for the size when the brief does not give it; unattended, pick a plausible one and state it as an assumption. Static meshes only.
3. Pick the preset from the table. Pass every object the file needs: a skinned mesh AND its armature, LOD meshes and their group, collision meshes beside their render mesh.
4. Export: bxe.export(path, preset, objects, **overrides). Any exporter keyword overrides the preset, such as bake_anim=True on roblox for an animated file. The result records the settings used.
5. Round-trip: bxe.round_trip(path, objects, preset=...) re-imports the file in a background Blender, leaving the open scene alone. It compares size (1 mm), triangles, deforming bones, root bones (one expected), armature scale, shape keys, every action stored for the objects, and the image maps on each material (base color, metallic, roughness, normal, emission). Then it lists every difference. Fix each one, or say why it is expected.
6. glTF contents: bxe.glb_summary(path) lists extensions_required, morph targets, skins, clips and image types without Blender. A loader refuses a file that requires an extension it lacks.
7. Engine check: import into the engine and look. For Godot, the headless check prints each node's rotation and scale, each mesh's size and floor height, and the maps each material received. Full flows: scenario-unity-pipeline-automation, scenario-unreal-pipeline-automation, scenario-godot-pipeline-automation.

Scale, axes and units

• Blender works in meters, Z-up, with the asset's front facing -Y. glTF is meters, Y-up (export_yup). FBX stores its axes as metadata; the presets write -Z forward, Y up.
• Keep scene.unit_settings.scale_length at 1.0. Any other value rescales every export.
• Apply object scale before export: obj.data.transform(Matrix.Diagonal(obj.scale).to_4x4()), then obj.scale = (1, 1, 1), so the engine node imports at scale 1. bxe.ground() does it for every mesh at once.
• Engines read 1 unit as 1 meter and place an object by its origin. A prop's origin belongs at the center of its base, so it stands on the floor where it is placed (step 2).
• FBX scale: unity writes FBX_SCALE_ALL (Unity reads file scale 1). unreal keeps FBX_SCALE_NONE (Unity reads it as file scale 0.01, a centimeter file). roblox uses FBX_SCALE_UNITS, as Roblox's export requirements ask.
• STL has no unit and slicers read millimeters, so print multiplies by 1000.

Characters

• Export the armature with its meshes. use_armature_deform_only leaves control bones out, so keep every deforming bone in one hierarchy under one root bone. Unreal's importer reports "Found multiple roots. The importer only supports one root per mesh."
• add_leaf_bones is off in every preset. Left on, each chain gains an extra end bone that breaks retargeting.
• unity_skeletal and unreal_skeletal bake every action (bake_anim_use_all_actions), one clip per action. Unity names each clip Armature|Action (measured: Rig|Wave), so name actions as the clips should read. glTF exports each action as an animation under its own name.
• roblox exports no animation. For an animated file, pass bake_anim=True to export() and to round_trip(), which then compares the actions too.
• Shape keys travel in FBX (blend shapes), glTF (morph targets, names in extras.targetNames) and USD. Building ARKit or viseme sets is scenario-blender-rigging's work.
• Roblox: at most 4 influences per vertex, bones frozen (scale 1, rotation 0), root at the origin with no influences, one animation track per mesh. Check influences with max(len(v.groups) for v in obj.data.vertices).

LODs and collision

• bxe.lods(obj, ratios=(0.5, 0.25)) renames the mesh Name_LOD0 and adds decimated Name_LOD1..n under an empty called Name, which takes the mesh's place under its parent. Static meshes only: it refuses a skinned mesh. Unity builds the LOD Group from those names. Godot generates LODs at import (Generate LODs). Unreal documents "Import LODs", all LODs sharing one pivot. The group exports as a plain FBX Null, and Unreal's import of it is unmeasured. It welds the vertices a glTF import splits at UV seams before decimating; without the weld, the LODs open into cracks.
• bxe.collision(obj, engine, shape, max_verts=64) adds a collision mesh named for the importer:
  • Unreal: UCX_Name_00 (convex hull) or UBX_Name_00 (box), exported in the same FBX. Unreal skips Auto Generate Collision when the file carries one.
  • Godot: Name-convcolonly for convex (one hull) and box (the bounds, a box collider), or Name-colonly for mesh (the mesh as a trimesh, static bodies only).
  • Unity and Roblox: returns None. Colliders are set in the editor.
  • A convex hull keeps at most max_verts vertices, spread over the shape. A generated mesh's full hull runs to thousands of faces. 64 is this toolkit's budget, not an engine limit. None keeps every vertex, and a dense full hull can come out with open edges.
• Decimation can wreck UV seams and silhouettes. Review every LOD with bx_review before export.

Batch conversion

blender -b --factory-startup --python-exit-code 1 -P bx_export.py -- \
        --batch /abs/in /abs/out --preset godot --profile game

For every model in the folder (GLB, glTF, FBX, OBJ, USD, STL, PLY): import into an empty scene, run the delivery gate, export with the preset, re-import and compare. It writes /abs/out/report.json with each file's gate errors, size and round-trip differences. One failing file never stops the folder: it gets a failed entry. Two inputs sharing a name keep their format in the output name (hero_fbx.glb, hero_glb.glb). Open the report before calling the batch done.

Worked example: a generated GLB to Unity and Godot

import sys
sys.path.append("/abs/skills/scenario-blender-expert/scripts")
sys.path.append("/abs/skills/scenario-blender-export/scripts")
import bpy, bx_audit, bx_export as bxe

bpy.ops.wm.read_factory_settings(use_empty=True)
bpy.ops.import_scene.gltf(filepath="/abs/in/robot.glb")
parts = [o.name for o in bpy.context.scene.objects if o.type == "MESH"]
gate = bx_audit.deliverable(parts, profile="game")
print(gate["errors"])            # fix each one, then gate again until "pass" is True
bxe.ground(parts, height=1.2)    # the robot's real height in meters, base center at the origin

bxe.export("/abs/out/robot.fbx", "unity", parts)
print(bxe.round_trip("/abs/out/robot.fbx", parts, preset="unity")["diffs"])
# ["Robot: material Body lost its Metallic, Roughness maps"]: write them for the engine
print(bxe.extra_maps(parts, "/abs/out", "unity")["written"])
bxe.export("/abs/out/robot.glb", "godot", parts)
print(bxe.glb_summary("/abs/out/robot.glb")["extensions_required"])            # []

A generated GLB often holds one object per part, all sharing the asset's pivot. Keep them as separate objects when the engine should animate or swap parts. Join them (bpy.ops.object.join with the parts selected) for a single static prop, then gate again.

Common mistakes

• Calling an export done on FINISHED. The exporter succeeds on files the engine imports at 100x scale or without the skin.
• Exporting a skinned mesh without its armature, or the armature without its mesh.
• Setting Bake Axis Conversion or Apply Transform to remove the armature node's rotation: neither removes it, and the first turns the character around.
• The web preset for Godot, or any compressed GLB for a loader without its decoder. Check extensions_required first.
• FBX textures left as external paths: the presets copy and embed them (path_mode="COPY", embed_textures=True), so the file travels alone. export() gives packed images without an extension (as a GLB import leaves them) a .jpg or .png name first. An image never saved (a new bake target) or edited since travels as a packed copy of its pixels (packed_unsaved_images).
• A textured FBX that shows untextured in Unity: the default import keeps embedded textures inside the file. Extract Textures (Materials tab) into the model's folder assigns them.
• Trusting an FBX made from a glTF import for metal and roughness. The glTF importer routes one packed image through Separate Color, and the FBX exporter drops both maps: the file re-imports with Metallic 0 and Roughness 0.5. Base color, normal and emission travel. round_trip() reports the loss. bxe.extra_maps(parts, out_dir, engine) writes the two maps as files to assign:
  • Unity: <material>_MetallicSmoothness.png in the Metallic Map slot (Standard or URP Lit), Smoothness Source on Metallic Alpha, sRGB unchecked. Unity's FBX import assigned only the base color, even from a file carrying both maps (measured).
  • Roblox and Unreal: <material>_Metallic.png and <material>_Roughness.png, 8-bit grayscale. On Roblox, set them as MetalnessMap and RoughnessMap of a SurfaceAppearance. In Unreal, uncheck sRGB on both.
  • Godot: deliver the godot GLB, which keeps every map.
• An STL at Blender scale: a 10 cm part prints at 0.1 mm.
• LODs exported without a look: decimation can collapse thin parts.