fix(snippets): export Y-up, meter glTF for Godot and Unreal too (#417)

The Godot preset exported Z-up glTF (export_yup=False) and the Unreal
glTF preset baked a 100x scale. Both contradict the glTF 2.0 spec (+Y up,
meters) and the engines: Godot is Y-up and imports glTF per the spec, so
every Godot asset arrived rotated -90 deg about X; Unreal's glTF importer
already converts meters to centimeters (Interchange import_scale defaults
to 100), so a baked file landed 100x too large. The Z-up choice was
justified in the skill only as keeping the axis test from being
"untestable".

All three engine glTF presets are now the same spec-compliant export
(export_yup=True, meters). Engine differences are documented where they
really live: Godot's node-name import hints (-convcolonly, -colonly,
-rigid) and Unreal's FBX axis_forward/axis_up/global_scale, which is
unchanged. The pipeline template's --preset now selects the same export
for every engine.

examples/export-preset-axis keeps its checks and still image but stops
calling the lying mast "Godot": it now contrasts the engine preset with a
naive export_yup=False file, which is what any glTF engine would show.

Closes #346

Signed-off-by: TMHSDigital <154358121+TMHSDigital@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Author: TMHSDigital

CLAUDE.md

@@ -37,7 +37,7 @@ VERSION                        - Source of truth for the repo version
 | addon-scaffolding | Extensions Platform manifest, file layout, register/unregister symmetry |
 | ai-mesh-cleanup | Ordered cleanup for imported generated meshes: units, transform apply, origin, normals, budget, collider |
 | bake-high-to-low | Cycles cage-bake of high-poly detail onto a low-poly target as a tangent-space normal map |
-| engine-export-presets | Unity Y-up, Godot Z-up, and Unreal centimeter glTF/FBX presets; glTF uses export_yup, FBX uses axis_forward/axis_up |
+| engine-export-presets | Unity, Godot and Unreal presets: glTF is +Y up and meters for every engine (export_yup=True; Unreal converts to cm on import), FBX uses axis_forward/axis_up plus centimeter scale |
 | operators | `bpy.types.Operator` lifecycle, `bl_idname`, redo, defensive context handling |
 | ui-panels | `bpy.types.Panel` declarative `draw()`, layout primitives, conditional UI |
 | custom-properties | `bpy.props` annotations, PropertyGroup, PointerProperty, storage tradeoffs |

ROADMAP.md

@@ -98,7 +98,7 @@ Provider-agnostic GLB-in / engine-ready-out. This repo does not generate meshes.
 
 - **Post-generation cleanup skills.** Import and unit-scale normalization, transform apply and origin, poly-budget decimate, LOD chain, collision mesh. Phase 1 shipped in v0.54.0 as `ai-mesh-cleanup`, four snippets, two rules.
 - **High-to-low normal bake.** **Delivered.** Skill `bake-high-to-low`, snippets `setup_bake_target_image.py` / `bake_normal_high_to_low.py` / `save_baked_image.py`, witness `examples/bake-normal-high-to-low/`. Cycles CPU cage bake; statistical gates, not byte-identity. UV transfer and atlas packing remain a later phase. Template integration remains a later phase.
-- **Engine export presets.** **Delivered.** Unity (Y-up glTF), Godot (Z-up glTF, meters), Unreal (centimeter glTF bake and FBX `global_scale`). Skill `engine-export-presets`, three snippets, rule `use-correct-axis-rna-per-exporter`, witness `examples/export-preset-axis/`. Draco remains opt-in via `gltf_draco_export.py`.
+- **Engine export presets.** **Delivered.** Unity, Godot and Unreal glTF are one spec-compliant export (+Y up, meters; Unreal converts to cm on import, corrected in #346), and Unreal FBX carries `axis_forward`/`axis_up` plus `global_scale`. Skill `engine-export-presets`, three snippets, rule `use-correct-axis-rna-per-exporter`, witness `examples/export-preset-axis/`. Draco remains opt-in via `gltf_draco_export.py`.
 - **`ai-asset-pipeline-template/`.** **Delivered.** Third template. Headless: GLB path in; LOD set, convex or box collider, engine-preset export; explicit CI exit codes. Pattern: `templates/headless-batch-script-template/`.
 - **Live-session agent bridge.** Research spike, not a committed deliverable. MCP server or socket listener so an agent can execute against a running Blender instance instead of blind `--background` scripts. Built on `templates/extension-addon-template/`. Needs its own design pass. Unpinned.
 

docs/gallery/export-preset-axis/index.html

@@ -115,11 +115,11 @@ <h3><a class="stretch" href="../uv-layer-grid/">UV Layer Grid</a></h3>
         </article>
         <article class="card mini">
           <div class="card-media">
-            <img src="../assets/export-preset-axis-hero-640.webp" srcset="../assets/export-preset-axis-hero-640.webp 640w, ../assets/export-preset-axis-hero.webp 1280w" sizes="(min-width: 560px) 340px, 100vw" alt="Two re-imports of a red-and-white radio mast with axis gizmos: the Unity copy stands with blue Z up, the Godot copy lies on the floor with green Y up and blue Z along the mast." width="1280" height="720" loading="lazy" decoding="async" />
+            <img src="../assets/export-preset-axis-hero-640.webp" srcset="../assets/export-preset-axis-hero-640.webp 640w, ../assets/export-preset-axis-hero.webp 1280w" sizes="(min-width: 560px) 340px, 100vw" alt="Two re-imports of a red-and-white radio mast with axis gizmos: the engine-preset copy stands with blue Z up, the naive Z-up copy lies on the floor with green Y up and blue Z along the mast." width="1280" height="720" loading="lazy" decoding="async" />
           </div>
           <div class="card-body">
             <h3><a class="stretch" href="../export-preset-axis/">Export Preset Axis</a></h3>
-            <p>A radio mast exported under Unity and Godot glTF presets and re-imported, proving the two files have different vertex orientation</p>
+            <p>A radio mast exported once with the engine glTF preset (export_yup=True, shared by Unity, Godot and Unreal) and once naively Z-up, then re-imported: the Z-up file lies on its back</p>
           </div>
         </article>
       </div>

docs/gallery/gltf-export-roundtrip/index.html

@@ -449,15 +449,15 @@ <h2><a class="stretch" href="gltf-export-roundtrip/">glTF Export Roundtrip</a></
           <span class="card-link" aria-hidden="true">View example &rarr;</span>
         </div>
       </article>
-      <article class="card" data-tags="export" data-kind="examples" data-name="export-preset-axis" data-search="export preset axis export-preset-axis a radio mast exported under unity and godot gltf presets and re-imported, proving the two files have different vertex orientation export ">
+      <article class="card" data-tags="export" data-kind="examples" data-name="export-preset-axis" data-search="export preset axis export-preset-axis a radio mast exported once with the engine gltf preset (export_yup=true, shared by unity, godot and unreal) and once naively z-up, then re-imported: the z-up file lies on its back export ">
         <div class="card-media">
-          <img src="assets/export-preset-axis-hero-640.webp" srcset="assets/export-preset-axis-hero-640.webp 640w, assets/export-preset-axis-hero.webp 1280w" sizes="(min-width: 900px) 340px, (min-width: 560px) 50vw, 100vw" alt="Two re-imports of a red-and-white radio mast with axis gizmos: the Unity copy stands with blue Z up, the Godot copy lies on the floor with green Y up and blue Z along the mast." width="1280" height="720" loading="lazy" decoding="async" />
+          <img src="assets/export-preset-axis-hero-640.webp" srcset="assets/export-preset-axis-hero-640.webp 640w, assets/export-preset-axis-hero.webp 1280w" sizes="(min-width: 900px) 340px, (min-width: 560px) 50vw, 100vw" alt="Two re-imports of a red-and-white radio mast with axis gizmos: the engine-preset copy stands with blue Z up, the naive Z-up copy lies on the floor with green Y up and blue Z along the mast." width="1280" height="720" loading="lazy" decoding="async" />
         </div>
         <div class="card-body">
           <h2><a class="stretch" href="export-preset-axis/">Export Preset Axis</a></h2>
           <p class="slug">export-preset-axis</p>
-          <p class="teaches">A radio mast exported under Unity and Godot glTF presets and re-imported, proving the two files have different vertex orientation</p>
-          <p class="witnesses"><span class="tag">witnesses</span> Unity export_yup=True round-trips standing (x,y,z); Godot export_yup=False reimports lying along -Y as (x,-z,y); --same-axis exits 9; exporter RNA is guarded so a future kwarg rename fails loudly</p>
+          <p class="teaches">A radio mast exported once with the engine glTF preset (export_yup=True, shared by Unity, Godot and Unreal) and once naively Z-up, then re-imported: the Z-up file lies on its back</p>
+          <p class="witnesses"><span class="tag">witnesses</span> The engine preset (export_yup=True) round-trips standing (x,y,z); a naive export_yup=False file reimports lying along -Y as (x,-z,y), as it would in any glTF engine; --same-axis exits 9; exporter RNA is guarded so a future kwarg rename fails loudly</p>
           <span class="card-link" aria-hidden="true">View example &rarr;</span>
         </div>
       </article>

docs/gallery/index.html

@@ -1,30 +1,37 @@
 # Export Preset Axis
 
-A runnable example that exports the same radio-mast mesh under the Unity
-and Godot glTF presets from
-[`engine-export-presets`](../../skills/engine-export-presets/SKILL.md) and
-re-imports both files. The check is on coordinates, not a screenshot: Unity
-(`export_yup=True`) stands; Godot (`export_yup=False`) lies.
+A runnable example that exports the same radio-mast mesh twice: once with
+the engine glTF preset from
+[`engine-export-presets`](../../skills/engine-export-presets/SKILL.md)
+(`export_yup=True`, the call Unity, Godot and Unreal all share), and once
+naively with `export_yup=False`. It re-imports both files. The check is on
+coordinates, not a screenshot: the preset copy stands; the Z-up copy lies.
 
-**What it witnesses:** named engine presets are not comments on the same
-kwargs. glTF axis RNA is `export_yup`, not FBX `axis_forward` / `axis_up`.
+**What it witnesses:** glTF is +Y up by spec, so `export_yup=False` is not
+an engine preset, it is a bug every engine shows the same way. glTF axis RNA
+is `export_yup`, not FBX `axis_forward` / `axis_up`.
 
-- **Disk POSITION follows the closed form.** Unity bakes
-  `(x, y, z) -> (x, z, -y)` with no node rotation. Godot writes raw Z-up
-  `(x, y, z)`. The check reads accessor min/max from the `.gltf` JSON.
-- **Re-import proves the conversion.** Blender's importer always treats the
-  file as Y-up: `blender = (gltf.x, -gltf.z, gltf.y)`. Unity restores the
-  source. Godot permutes again, so the mast lies along `-Y`.
-- **The two reimports differ.** Unity `z_span` matches source height; Godot
-  `y_span` matches that height. `--same-axis` exports both with
-  `export_yup=True`; both stand and the differ check exits 9.
+- **Disk POSITION follows the closed form.** The preset bakes
+  `(x, y, z) -> (x, z, -y)` with no node rotation. The naive export writes
+  raw Z-up `(x, y, z)`. The check reads accessor min/max from the `.gltf`
+  JSON.
+- **Re-import proves the conversion.** Blender's importer, like Unity's,
+  Godot's and Unreal's, treats the file as Y-up:
+  `blender = (gltf.x, -gltf.z, gltf.y)`. The preset restores the source. The
+  Z-up file permutes again, so the mast lies along `-Y`.
+- **The two reimports differ.** The preset's `z_span` matches source
+  height; the Z-up file's `y_span` matches that height. `--same-axis`
+  exports both with `export_yup=True`; both stand and the differ check
+  exits 9.
 - **Exporter RNA is guarded.** Every kwarg passed must still exist on
   `bpy.ops.export_scene.gltf`.
 
 Neighbor of [`gltf-export-roundtrip`](../gltf-export-roundtrip/) (Y-up bake
 vs Z-up on disk for one file) and [`unapplied-scale-gltf`](../unapplied-scale-gltf/)
-(`export_apply` is modifiers, not object scale). This example names the
-Unity vs Godot presets and asserts the re-imported orientations diverge.
+(`export_apply` is modifiers, not object scale). Until 0.143.x this example
+called the Z-up export the "Godot preset". That was wrong: Godot is Y-up and
+imports glTF per the spec, so the lying mast is what a Godot user would have
+seen (#346).
 
 The subject is a radio mast chosen because "up" is unmistakable on it: a
 stepped concrete footing, a bolted base flange, a tapered mast in red and
@@ -33,26 +40,25 @@ shrouded microwave dish on a raked arm, an equipment cabinet, and a red
 obstruction lamp under a lightning rod. The rod's point is the witnessed tip
 vertex.
 
-The still stages the two re-imports side by side: the Unity copy standing on
-the left, the Godot copy lying along the floor on the right. Beside each sits
+The still stages the two re-imports side by side: the preset copy standing on
+the left, the Z-up copy lying along the floor on the right. Beside each sits
 a modelled X/Y/Z axis gizmo (red, green, blue) showing that re-import's
 frame. The gizmo is measured, not placed: the render path tries all 24
 axis-aligned rotations and keeps the one that maps the source vertices onto
 the re-imported vertices (worst nearest-vertex distance, printed as
-`fit_err`; exit 12 if it exceeds 1 mm). The Unity gizmo comes out as the
-identity, blue Z up. The Godot gizmo comes out as `Y -> +Z, Z -> -Y`: green
+`fit_err`; exit 12 if it exceeds 1 mm). The preset gizmo comes out as the
+identity, blue Z up. The Z-up gizmo comes out as `Y -> +Z, Z -> -Y`: green
 Y points up and blue Z runs along the lying mast. The mast didn't fall over.
-The Godot file stores Z-up coordinates, and the importer reads them as
-Y-up. If `export_yup` were the same on both, the two copies and both gizmos
-would match.
+The Z-up file stores Z-up coordinates, and the importer reads them as
+Y-up, as every glTF consumer does.
 
 ## Run
 
 ```bash
 # Cheap correctness check (no render) - the CI check:
 blender --background --python export_preset_axis.py --
 
-# Falsifier: both presets Y-up. Must exit non-zero.
+# Falsifier: both exports Y-up. Must exit non-zero.
 blender --background --python export_preset_axis.py -- --same-axis
 
 # Also render a still (EEVEE on a GPU host; use --engine cycles on GPU-less hosts):
@@ -73,10 +79,10 @@ the shared asset-quality helper.
 | 2 | argparse / usage; also exporter RNA missing expected glTF kwargs |
 | 3 | Source mast is not Z-dominant, or tip drifted |
 | 4 | glTF reimport produced no mesh |
-| 5 | Unity disk POSITION is not `(x, z, -y)`, or Unity node has rotation |
-| 6 | Godot disk POSITION is not raw Z-up; also `--output` produced no file |
-| 7 | Unity reimport is not standing |
-| 8 | Godot reimport is not lying along Y, or reimported tip mismatch |
+| 5 | Preset disk POSITION is not `(x, z, -y)`, or preset node has rotation |
+| 6 | Z-up disk POSITION is not raw Z-up; also `--output` produced no file |
+| 7 | Preset reimport is not standing |
+| 8 | Z-up reimport is not lying along Y, or reimported tip mismatch |
 | 9 | Reimported orientations did not differ (`--same-axis` lands here) |
 | 10 | Gallery framing violation |
 | 11 | `--same-axis` did not collapse the axis difference; also asset-quality floor violation (render path) |