Engine Export Presets

Trigger

Use this skill when the user:

• Needs a Unity, Godot, or Unreal export from Blender Python
• Mentions Y-up, Z-up, centimeter scale, export_yup, axis_forward, or axis_up
• Is about to pass FBX axis kwargs to bpy.ops.export_scene.gltf
• Wants a headless preset the later ai-asset-pipeline-template can call

This skill is the export layer. It composes ai-mesh-cleanup (apply transforms, units), depsgraph-and-evaluated-data (export_apply ships evaluated mesh), and the unapplied-scale-gltf witness (export_apply does not bake object scale). It does not generate meshes.

The core misunderstanding

glTF and FBX do not share axis RNA. bpy.ops.export_scene.gltf has export_yup (boolean, "+Y Up"). It does not have axis_forward or axis_up. bpy.ops.export_scene.fbx has axis_forward and axis_up (axis enums) and global_scale. It does not have export_yup. Passing the other exporter's kwargs is a TypeError or a silent no-op depending on how the call is built.

That split is the contract. Verified on current RNA:

• glTF: https://docs.blender.org/api/current/bpy.ops.export_scene.html#bpy.ops.export_scene.gltf (export_yup, no axis enums)
• FBX: https://docs.blender.org/api/current/bpy.ops.export_scene.html#bpy.ops.export_scene.fbx (axis_forward, axis_up, global_scale)

Shared prelude

Before any preset: meters in the scene (scale_length == 1.0), identity object scale via transform_apply through temp_override, export_apply=True / use_mesh_modifiers=True so modifiers ship. See ai-mesh-cleanup and unapplied-scale-gltf.

def apply_selected_mesh_transforms():
    for obj in list(bpy.context.selected_objects):
        if obj.type != "MESH":
            continue
        with bpy.context.temp_override(
            object=obj, active_object=obj, selected_objects=[obj]
        ):
            bpy.ops.object.transform_apply(
                location=False, rotation=True, scale=True
            )

use_selection=True on every preset. Draco is opt-in on glTF; do not copy gltf_draco_export.py wholesale.

Unity (Y-up, meters, glTF)

bpy.ops.export_scene.gltf(
    filepath=path,
    use_selection=True,
    export_yup=True,
    export_apply=True,
    export_draco_mesh_compression_enable=draco,
    export_animations=False,
)

export_yup=True bakes (x, y, z) -> (x, z, -y) into POSITION with no node rotation. Witness: examples/gltf-export-roundtrip/ and examples/export-preset-axis/.

Snippet: snippets/export_preset_unity.py.

Godot (Z-up glTF, meters)

bpy.ops.export_scene.gltf(
    filepath=path,
    use_selection=True,
    export_yup=False,
    export_apply=True,
    export_draco_mesh_compression_enable=draco,
    export_animations=False,
)

export_yup=False writes Blender Z-up POSITION. This preset is the Z-up interop path. It is not the Unity kwargs; if both used export_yup=True the files would match and the axis contract would be untestable.

Snippet: snippets/export_preset_godot.py.

Unreal (centimeters)

glTF has no global_scale. Bake 100x (1 m -> 100 cm) onto the selected meshes, apply, then export_yup=True. That mutates the objects; copy first if the source must stay in meters.

FBX keeps the meter mesh and scales on the way out:

bpy.ops.export_scene.fbx(
    filepath=path,
    use_selection=True,
    axis_forward="-Z",
    axis_up="Y",
    global_scale=100.0,
    apply_unit_scale=False,
    use_mesh_modifiers=True,
    bake_anim=False,
)

Do not pass export_yup to FBX. Do not pass axis_forward to glTF.

Snippet: snippets/export_preset_unreal.py.

Common AI mistakes

1. export_scene.gltf(..., axis_forward="-Z", axis_up="Y"). Those names are FBX. Rule use-correct-axis-rna-per-exporter.
2. export_scene.fbx(..., export_yup=True). Same rule, other direction.
3. Skipping transform_apply. export_apply is modifiers, not object scale. examples/unapplied-scale-gltf/.
4. Unity and Godot as the same kwargs. They differ on export_yup. examples/export-preset-axis/ asserts the re-imported orientations diverge.
5. Unreal glTF without the 100x bake. glTF has no global_scale.

Version correctness

Probed on 4.5 LTS, 5.1, and 5.2: export_yup on glTF and axis_forward / axis_up / global_scale on FBX are present on all three. No version branch for the axis kwargs. Guard by requiring those names in operator RNA so a future rename fails loudly, as examples/gltf-export-roundtrip/ does.

export_format defaults differ by call site; pass the filepath suffix (.glb / .gltf / .fbx) and let the operator infer, or set export_format explicitly on glTF.

Related

• Skill ai-mesh-cleanup
• Skill depsgraph-and-evaluated-data
• Rule use-correct-axis-rna-per-exporter
• Rule no-unapplied-modifiers-on-export
• Rule validate-imported-mesh-scale
• Snippet snippets/export_preset_unity.py
• Snippet snippets/export_preset_godot.py
• Snippet snippets/export_preset_unreal.py
• Snippet snippets/gltf_draco_export.py
• Example export-preset-axis
• Example gltf-export-roundtrip
• Example unapplied-scale-gltf

References

• bpy.ops.export_scene.gltf: https://docs.blender.org/api/current/bpy.ops.export_scene.html#bpy.ops.export_scene.gltf
• bpy.ops.export_scene.fbx: https://docs.blender.org/api/current/bpy.ops.export_scene.html#bpy.ops.export_scene.fbx
• glTF 5.1: https://docs.blender.org/api/5.1/bpy.ops.export_scene.html#bpy.ops.export_scene.gltf
• FBX 5.1: https://docs.blender.org/api/5.1/bpy.ops.export_scene.html#bpy.ops.export_scene.fbx