blender-impl-automation

blender-impl-automation

Quick Reference

Critical Warnings

NEVER use bpy.ops.export_scene.obj() or bpy.ops.export_mesh.stl() in Blender 4.0+ — these Python exporters are REMOVED. Use bpy.ops.wm.obj_export() and bpy.ops.wm.stl_export().

NEVER call viewport operators (bpy.ops.view3d.*, bpy.ops.screen.*) in --background mode — no UI context exists. Check bpy.app.background before any UI-dependent operator.

NEVER use bpy.ops.wm.invoke_* in background mode — modal/interactive operators require a window event loop.

NEVER hardcode absolute file paths — use os.path, pathlib.Path, or command-line arguments.

NEVER omit sys.exit() in CI/CD scripts — Blender returns 0 by default even on script errors. Use --python-exit-code 1 (3.0+) or explicit sys.exit(1).

ALWAYS set apply_scale_options explicitly in FBX export — default scale causes unit mismatch.

ALWAYS verify axis conventions (axis_forward, axis_up) match the target application for FBX/OBJ.

ALWAYS set export_custom_properties=True for Alembic when preserving AEC metadata.

ALWAYS parse custom arguments AFTER the -- separator in command-line scripts.

Decision Tree: Automation Approach

What kind of automation?
├─ Render frames/animation?
│  ├─ Single frame → blender -b file.blend -o /path/frame_ -f FRAME
│  ├─ Animation   → blender -b file.blend -o /path/frame_ -a
│  └─ Custom settings → blender -b file.blend -P render_script.py
├─ Convert file formats?
│  ├─ Single file → blender -b -P convert.py -- --input f --output f
│  └─ Batch dir   → blender -b -P batch_convert.py -- --dir /path
├─ Assemble scene from sources?
│  ├─ Full copies → bpy.ops.wm.append() or bpy.data.libraries.load()
│  ├─ Linked refs → bpy.ops.wm.link()
│  └─ Overridable → Library Overrides (4.0+)
├─ Process data without rendering?
│  └─ blender -b [-file.blend] -P script.py
└─ CI/CD pipeline?
   └─ blender -b --python-exit-code 1 -P script.py

Decision Tree: Export Format

Target?
├─ Game engines → FBX (animation) or glTF/GLB (static)
├─ Web/WebGL   → glTF/GLB (prefer GLB binary)
├─ 3D printing → STL (bpy.ops.wm.stl_export, 4.0+)
├─ CAD interchange → OBJ (bpy.ops.wm.obj_export, 4.0+)
├─ VFX pipeline → USD or Alembic (animated geometry)
└─ Archival     → USD (.usdc/.usda)

I/O Operator Version Matrix

Format	Blender 3.x	Blender 4.0+
OBJ	bpy.ops.export_scene.obj()	REMOVED → bpy.ops.wm.obj_export()
STL	bpy.ops.export_mesh.stl()	REMOVED → bpy.ops.wm.stl_export()
PLY	bpy.ops.export_mesh.ply()	REMOVED → bpy.ops.wm.ply_export()
FBX	bpy.ops.export_scene.fbx()	Same (unchanged)
glTF	bpy.ops.export_scene.gltf()	Same (unchanged)
USD	bpy.ops.wm.usd_export()	Same (unchanged)
Alembic	bpy.ops.wm.alembic_export()	Same (unchanged)

---

Section 1: Command-Line Interface

# Background mode
blender -b -P script.py
blender -b scene.blend -P script.py

# Custom arguments (after -- separator)
blender -b scene.blend -P script.py -- --my-arg value

# Render single frame / animation / frame range
blender -b scene.blend -o /tmp/render_ -f 1
blender -b scene.blend -o /tmp/render_ -a
blender -b scene.blend -o /tmp/render_ -s 1 -e 250 -a

# Set engine / threads
blender -b scene.blend -E CYCLES -o /tmp/render_ -f 1
blender -b scene.blend -t 8 -o /tmp/render_ -a

# Python expression
blender -b --python-expr "import bpy; print(bpy.app.version)"

# CI/CD exit code propagation (Blender 3.0+)
blender -b --python-exit-code 1 -P script.py

Argument Parsing Pattern

# Blender 3.x/4.x/5.x: Parse arguments after '--' separator
import sys, argparse

def get_args():
    argv = sys.argv
    if "--" not in argv:
        return argparse.Namespace()
    parser = argparse.ArgumentParser()
    parser.add_argument("--input", required=True)
    parser.add_argument("--output", required=True)
    parser.add_argument("--scale", type=float, default=1.0)
    return parser.parse_args(argv[argv.index("--") + 1:])

---

Section 2: Background Mode

# Blender 3.x/4.x/5.x: Detect and guard
import bpy

if bpy.app.background:
    # SAFE: bpy.data, bpy.ops.wm.*, file I/O, render
    # UNSAFE: viewport ops, UI drawing, modal operators, GPU
    pass

Category	Background Mode
bpy.data.* access	YES
File I/O (import/export)	YES
bpy.ops.render.render()	YES
Non-UI operators	YES
Viewport ops (view3d.*)	NO
Modal operators	NO
context.area	None — guard access
GPU drawing	NO

bpy as a Module (Alternative)

# Outside Blender: pip install bpy (match Blender's Python version)
import bpy
bpy.ops.wm.read_homefile(use_empty=True)
# Same capabilities and limitations as --background

---

Section 3: File Format I/O

Version-Safe I/O Pattern

# Blender 3.x/4.x/5.x: Version-safe OBJ/STL
import bpy

def export_obj(filepath, **kwargs):
    if bpy.app.version >= (4, 0, 0):
        bpy.ops.wm.obj_export(filepath=filepath, **kwargs)
    else:
        bpy.ops.export_scene.obj(filepath=filepath, **kwargs)

def export_stl(filepath, **kwargs):
    if bpy.app.version >= (4, 0, 0):
        bpy.ops.wm.stl_export(filepath=filepath, **kwargs)
    else:
        bpy.ops.export_mesh.stl(filepath=filepath, **kwargs)

Key Parameters per Format

FBX (bpy.ops.export_scene.fbx() — all versions):

• apply_scale_options='FBX_SCALE_ALL' — ALWAYS set
• axis_forward='-Z', axis_up='Y' — verify against target
• use_mesh_modifiers=True, use_custom_props=True

glTF/GLB (bpy.ops.export_scene.gltf() — all versions):

• export_format='GLB' — prefer binary single-file
• export_apply=True, export_materials='EXPORT'

USD (bpy.ops.wm.usd_export() — 2.9+):

• selected_objects_only=True, export_materials=True
• use_instancing=True

Alembic (bpy.ops.wm.alembic_export() — 2.78+):

• export_custom_properties=True — REQUIRED for AEC metadata
• selected=True, use_instancing=True

OBJ 4.0+ (bpy.ops.wm.obj_export()):

• export_selected_objects=True, apply_modifiers=True
• forward_axis='NEGATIVE_Z', up_axis='Y'

STL 4.0+ (bpy.ops.wm.stl_export()):

• export_selected_objects=True, ascii_format=False
• apply_modifiers=True

---

Section 4: Batch Rendering

# Blender 3.x/4.x/5.x: Scripted render
import bpy

scene = bpy.context.scene
scene.render.engine = 'CYCLES'
scene.render.resolution_x = 1920
scene.render.resolution_y = 1080
scene.render.image_settings.file_format = 'PNG'

# Single frame
scene.render.filepath = "/tmp/render_output"
bpy.ops.render.render(write_still=True)

# Animation
scene.frame_start = 1
scene.frame_end = 250
scene.render.filepath = "/tmp/anim/frame_"
bpy.ops.render.render(animation=True)

Multi-Camera Render

# Blender 3.x/4.x/5.x
import bpy, os

output_dir = "/tmp/renders"
os.makedirs(output_dir, exist_ok=True)
scene = bpy.context.scene

for cam in [o for o in bpy.data.objects if o.type == 'CAMERA']:
    scene.camera = cam
    scene.render.filepath = os.path.join(output_dir, f"{cam.name}_")
    bpy.ops.render.render(write_still=True)

EEVEE Engine Version Detection

# Blender 3.x → 'BLENDER_EEVEE', Blender 4.0+ → 'BLENDER_EEVEE_NEXT'
if bpy.app.version >= (4, 0, 0):
    bpy.context.scene.render.engine = 'BLENDER_EEVEE_NEXT'
else:
    bpy.context.scene.render.engine = 'BLENDER_EEVEE'

---

Section 5: Scene Assembly

Append vs Link vs libraries.load()

	bpy.data.libraries.load()	bpy.ops.wm.append()	bpy.ops.wm.link()
Returns imported data	YES	NO	NO
Operator context needed	NO	YES	YES
Background mode safe	YES	YES	YES
Recommended for scripts	YES	Simple cases	Linked refs

bpy.data.libraries.load() (Preferred for Scripts)

# Blender 3.x/4.x/5.x: Batch append from .blend files
import bpy, os, glob

for blend_path in glob.glob("/path/to/assets/*.blend"):
    with bpy.data.libraries.load(blend_path, link=False) as (data_from, data_to):
        data_to.objects = data_from.objects  # Import all objects
    for obj in data_to.objects:
        if obj is not None:
            bpy.context.collection.objects.link(obj)

Append/Link via Operators

# Blender 3.x/4.x/5.x: Append single object
import bpy, os

source = "/path/to/library.blend"
bpy.ops.wm.append(
    filepath=os.path.join(source, "Object", "MyObject"),
    directory=os.path.join(source, "Object"),
    filename="MyObject",
    link=False,
)

---

Section 6: Pipeline Integration

Exit Code Pattern

# ALWAYS use in CI/CD scripts
import sys

try:
    do_work()
except Exception as e:
    print(f"FAILED: {e}", file=sys.stderr)
    sys.exit(1)

blender -b --python-exit-code 1 -P script.py
echo $?  # Non-zero on failure

Environment Variables

Variable	Purpose
BLENDER_USER_SCRIPTS	Custom script search path
BLENDER_SYSTEM_SCRIPTS	System-wide script path
BLENDER_USER_CONFIG	User config directory
OCIO	OpenColorIO config

---

Reference Links

• references/methods.md — Complete API signatures for I/O, render, and assembly operators
• references/examples.md — Full working automation scripts
• references/anti-patterns.md — Common automation mistakes with fixes

Official Sources

• https://docs.blender.org/api/current/bpy.ops.wm.html
• https://docs.blender.org/api/current/bpy.ops.export_scene.html
• https://docs.blender.org/api/current/bpy.ops.import_scene.html
• https://docs.blender.org/api/current/bpy.ops.render.html
• https://docs.blender.org/api/current/bpy.app.html
• https://docs.blender.org/manual/en/latest/advanced/command_line/arguments.html

Related Skills

• blender-impl-operators — Modal operators, batch processing, progress reporting
• blender-core-runtime — Background mode, threading, handlers
• blender-core-versions — Breaking changes across versions