MCP backend

Use when: Any export to a non-.blend format, packaging for game engines, web, AR, or 3D printing.

Before the first Blender call, read ../text-to-blender/references/mcp-backends.md. Prefer official MCP for shared capabilities; preserve the selected backend for the whole task. Tool names in recipes are client/legacy examples: map them through that reference and its agent-host guide.

Blender Export

Export to the right format with the right settings. Wrong format choice = days of debugging in the target platform.

Format decision tree

Where is this going?
├── Web (Three.js, Babylon.js, model-viewer, AR Quick Look) → glTF / GLB
├── Game engine (Unity, Unreal, Godot)
│   ├── Animated/rigged → FBX (or glTF for modern engines)
│   └── Static → OBJ or FBX or glTF
├── Apple AR (USDZ) → USDZ (special, see Recipe 6)
├── 3D printing → STL (geometry only, must be watertight)
├── VFX pipeline (Maya, Houdini, Nuke) → USD
└── DCC roundtrip → FBX (industry standard)

Quick rule for unknown target: glTF / GLB. Open standard, modern, universally supported.

Recipes

Recipe 1 — glTF / GLB export (web / AR / general)

import bpy

bpy.ops.export_scene.gltf(
    filepath='/tmp/output.glb',
    export_format='GLB',                 # single-file binary; preferred
    export_apply=True,                   # apply modifiers before export
    export_materials='EXPORT',
    export_image_format='AUTO',          # PNG; AUTO falls back to JPEG for opaque images
    export_yup=True,                     # Y-up convention (most engines / web expect this)
    export_animations=True,              # toggle off for static models
    export_morph=True,                   # shape keys
    export_skins=True,                   # armatures + weights
    export_normals=True,
    export_tangents=False,               # skip unless target uses tangent-space normals beyond standard
)

# Verify
import os
size_mb = os.path.getsize('/tmp/output.glb') / (1024 * 1024)
print(f"export:gltf {size_mb:.2f} MB")

glTF caveats:

• Only Principled BSDF materials export cleanly. Procedural shaders are dropped or simplified.
• Hard cap: 15 MB; soft target: 8 MB.
• No KTX2 / Draco compression (unless target supports those loaders).
• PNG textures only (max 1024×1024 typical).

Recipe 2 — Decimate before export (if too large)

import bpy

obj = bpy.data.objects['GEO-target']
if obj.type != 'MESH':
    raise ValueError('Decimation requires a mesh')
selected = list(bpy.context.selected_objects)
active = bpy.context.view_layer.objects.active
mode = active.mode if active else 'OBJECT'
try:
    if mode != 'OBJECT':
        bpy.ops.object.mode_set(mode='OBJECT')
    bpy.ops.object.select_all(action='DESELECT')
    obj.select_set(True)
    bpy.context.view_layer.objects.active = obj
    mod = obj.modifiers.new('Decimate', type='DECIMATE')
    mod.decimate_type = 'COLLAPSE'
    mod.ratio = 0.7    # keep 70% of faces; lower = more reduction
    bpy.ops.object.modifier_apply(modifier=mod.name)
finally:
    bpy.ops.object.select_all(action='DESELECT')
    for item in selected:
        item.select_set(True)
    bpy.context.view_layer.objects.active = active
    if mode != 'OBJECT':
        bpy.ops.object.mode_set(mode=mode)

print(f"decimated:{obj.name} verts:{len(obj.data.vertices)}")

Then re-export. Iterate ratio until file size fits target.

Recipe 3 — FBX export (game engines)

import bpy

bpy.ops.export_scene.fbx(
    filepath='/tmp/output.fbx',
    use_selection=False,
    apply_unit_scale=True,
    apply_scale_options='FBX_SCALE_ALL',
    bake_space_transform=True,        # critical: applies rotation to mesh
    object_types={'MESH', 'ARMATURE', 'EMPTY'},
    use_mesh_modifiers=True,
    mesh_smooth_type='FACE',
    use_armature_deform_only=True,
    bake_anim=True,
    bake_anim_use_all_bones=True,
    bake_anim_use_nla_strips=True,
    bake_anim_use_all_actions=True,
    bake_anim_force_startend_keying=True,
    embed_textures=True,              # critical: embed textures into FBX
    path_mode='COPY',
    axis_forward='-Z',
    axis_up='Y',
)
print('export:fbx')

Common FBX gotchas:

• Model rotated 90° in target → check axis_up='Y', axis_forward='-Z'
• Model 100× too large → Apply Transform on the object before export
• Textures missing → embed_textures=True, path_mode='COPY'
• Animation only plays one action → use NLA + bake all actions

Recipe 4 — OBJ export (simple / universal)

import bpy

bpy.ops.wm.obj_export(
    filepath='/tmp/output.obj',
    export_animation=False,
    apply_modifiers=True,
    export_eval_mode='DAG_EVAL_VIEWPORT',
    export_uv=True,
    export_normals=True,
    export_materials=True,
    export_triangulated_mesh=False,
    forward_axis='NEGATIVE_Z',
    up_axis='Y',
)
print('export:obj')

OBJ has no animation, no rigging, basic material support only. Use for simple geometry exchange.

Recipe 5 — STL export (3D printing)

import bpy

bpy.ops.wm.stl_export(
    filepath='/tmp/output.stl',
    ascii_format=False,    # binary STL (smaller, faster)
    apply_modifiers=True,
)
print('export:stl')

Critical for STL:

• Mesh must be watertight (no holes, no flipped normals, no internal faces).
• Pre-export: in Edit Mode, run Mesh → Clean Up → Make Manifold.
• STL has NO units — most slicers assume mm. Set Blender scene units to mm before modeling.
• STL has NO color/material — single-color grey only.

Recipe 6 — USD export (VFX pipeline)

import bpy

bpy.ops.wm.usd_export(
    filepath='/tmp/scene.usdc',
    export_animation=True,
    export_uvmaps=True,
    export_normals=True,
    export_materials=True,
    use_instancing=True,
    export_textures=True,
    overwrite_textures=True,
)
print('export:usd')

USD variants:

• .usd — text-based (debuggable, large)
• .usdc — binary (compact, fast — default choice)
• .usda — ASCII (human-readable, larger)
• .usdz — zipped USD with all assets (Apple AR / iOS)

Recipe 7 — Pre-export checklist (run before any export)

import bpy

# 1. Apply transforms (rotation + scale baked into geometry)
obj = bpy.data.objects['GEO-target']
bpy.context.view_layer.objects.active = obj
bpy.ops.object.transform_apply(location=False, rotation=True, scale=True)

# 2. Recompute normals
bpy.ops.object.mode_set(mode='EDIT')
bpy.ops.mesh.select_all(action='SELECT')
bpy.ops.mesh.normals_make_consistent(inside=False)
bpy.ops.mesh.remove_doubles(threshold=0.0001)
bpy.ops.object.mode_set(mode='OBJECT')

# 3. Apply modifiers (some exporters keep them, but for portability apply before export)
# Already done via export_apply=True / use_mesh_modifiers=True flags

# 4. Verify mesh stats
mesh = obj.data
print(f"export_check:{obj.name} verts:{len(mesh.vertices)} faces:{len(mesh.polygons)}")

Recipe 8 — Verify exported file

import os

filepath = '/tmp/output.glb'
if not os.path.exists(filepath):
    print(f"ERROR:not_found {filepath}")
else:
    size_mb = os.path.getsize(filepath) / (1024 * 1024)
    print(f"verified:{filepath} {size_mb:.2f}MB")

Always verify after export. Use the host's shell tool: ls -la /tmp/output.glb.

Recipe 9 — Unreal collision meshes

Export a separate collider with the render mesh in the same FBX. Its name must match the render mesh exactly: SM_Chair → UCX_SM_Chair_01, _02, and so on. For existing GEO- names, derive the collider name from that actual name; do not rename the render mesh just to fit an example.

Prefix	Required shape
UCX_	One closed, outward-wound convex hull
UBX_	Rectangular box, not deformed geometry
USP_	Sphere, not a stretched ellipsoid
UCP_	Cylinder with hemispherical ends

Use several simple hulls for concave props: one hull around a table blocks the space underneath. Derive positions from geometry, not hand-entered guesses. A face-plane convexity test alone accepts open or empty meshes; it is not an export gate. Use scripts/collision_preflight.py to inspect the evaluated collider, check names/closedness/winding/volume/convexity, and export only the intended objects. It rejects rather than repairs invalid geometry and preserves selection/mode.

import bpy, runpy
# Substitute the installed helper path; load inside Blender, not ordinary Python.
helpers = runpy.run_path('/absolute/path/to/blender-export/scripts/collision_preflight.py')
mesh = bpy.data.objects['SM_Chair']
colliders = [bpy.data.objects['UCX_SM_Chair_01']]
result = helpers['export_collision_fbx'](mesh, colliders, '/tmp/chair_collision.fbx')
print(result)

The primitive checks cover local-axis authored boxes/spheres/capsules; use UCX_ for shapes outside those constraints. Boxes/spheres are unsuitable for a non-uniformly scaled render object. Use a new output path rather than overwriting. Only one render mesh per collision FBX is supported by this helper.

Engine-side behavior is not yet validated. Epic documents these conventions, but an Unreal import must verify hull count, openings, scale, and collision before claiming engine acceptance. Missing custom collision may instead trigger automatic collision generation; do not assume every importer configuration behaves alike.

Recipe 10 — Multires: bake to the cage, then export

A live Multires modifier may export every evaluated subdivided face or omit sculpt geometry, depending on exporter settings. For a low-poly asset, bake the detailed surface to a normal map and export level 0. A 30-face cage at level 8 is approximately 2 million faces.

Use scripts/multires_bake_export.py for a single-material target with UVs, an active Image Texture holding a Non-Color bake image, and exactly one enabled Multires modifier last in the stack. The helper sets viewport level 0 before baking, keeps it there through FBX export, saves/connects the normal map, and then restores selection, active object, mode, bake settings, engine, and subdivision levels. Use a single-user material and bake image. It does not overwrite an existing normal connection or accepted output files.

import bpy, runpy
helpers = runpy.run_path('/absolute/path/to/blender-export/scripts/multires_bake_export.py')
result = helpers['bake_export_cage'](
    bpy.data.objects['GEO-target'], '/tmp/target_normal.png', '/tmp/target_cage.fbx')
print(result)

A saved normal map/material connection is the intended change. On failure, generated files remain failure evidence; the helper restores temporary state and removes its new normal connection. Use new paths for a retry. To include colliders, pass an export_fbx callback that calls export_collision_fbx inside this helper's level-0 context; see references/unreal-export.md. Do not re-export after subdivision levels have been restored, or add ordinary meshes to the bake selection.

The helper detects the installed 5.x/4.x bake-property layout rather than assuming a spelling. Live validation is on Blender 5.1.1; Blender 4.x and Unreal imports remain unverified. Normal maps preserve surface shading, not silhouette changes.

Polycount targets per platform

Platform	Target	Notes
Web (glTF)	≤ 30 000 tris	Mobile-safe
Hero web asset	≤ 60 000 tris	Desktop OK
Unity / Unreal hero	50 000–100 000 tris	High-end
Mobile game	≤ 10 000 tris	Per asset
AR USDZ	≤ 50 000 tris	iOS recommendation
3D print	unlimited	But export size matters

Common pitfalls

Symptom	Fix
FBX has no textures	Set embed_textures=True, path_mode='COPY'
Procedural material missing in glTF	Bake to image textures first; use only Principled BSDF
Game engine: model rotated 90°	FBX: axis_up='Y', axis_forward='-Z'; glTF: export_yup=True
Game engine: model 100× too large	Apply Transform; check unit scale
OBJ won't import elsewhere	Stick to ASCII filenames
STL won't print	Make Manifold; recompute normals
GLB > 15 MB	Apply Decimate (Recipe 2); reduce textures to 1024×1024
Bone count exceeded	Limit weights to 4 per vertex; reduce bone count
Animation didn't export	glTF: export_animations=True; FBX: bake_anim=True
Unreal: missing/wrong custom collision	Check names, convex closed hulls, and same-file selection (Recipe 9); inspect the import in Unreal
Multires bake fails with a misleading multi-resolution error	Keep Multires last and isolate the target selection (Recipe 10)
Baked normals disagree with exported cage	Set viewport level 0 before baking, not just before export
FBX unexpectedly contains millions of faces	Viewport levels controls evaluated Multires export, not total_levels
Unreal: no smoothing-group information	Use mesh_smooth_type='FACE'; then verify inside Unreal

When to load references/overview.md

Load when:

• Multi-format batch export needed
• Asset Browser / library override workflow
• USDZ for Apple AR specifics
• Game engine roundtrip troubleshooting (Unity/Unreal-specific quirks)
• LOD generation strategy

The reference covers: per-format pitfalls, asset browser workflow, library overrides for production, USD composition, polycount targets per platform.