glTF/GLB Asset Optimization

Large .glb/.ktx2 assets in assets/ are tracked by Git LFS. Every clone pulls the LFS objects at HEAD, and GitHub bills that bandwidth to the repo owner (including forks and anonymous clones). Shrinking the assets that ship at HEAD is the most direct way to lower the recurring bill.

Hard constraint: stay Bevy-0.18-safe

The editor loads models through Bevy 0.18's glTF loader, which does not support these extensions (see https://docs.rs/bevy_gltf/0.18.0/bevy_gltf/):

• KHR_draco_mesh_compression, EXT_meshopt_compression, KHR_mesh_quantization
• KHR_texture_basisu, EXT_texture_webp, EXT_mesh_gpu_instancing

So the usual 5–10× geometry-compression tools (gltfpack, gltf-transform optimize defaults, Draco) produce files that fail to render in the editor. Output must be plain glTF 2.0 core. Always verify extensionsRequired is empty before committing.

Step 1: Diagnose where the bytes are

npx @gltf-transform/cli inspect path/to/model.glb

• Geometry-dominated (many meshes, large vertex/index counts, few/no textures): the only Bevy-safe lever is reducing triangle count + dropping unused attributes. Common in CAD exports (over-tessellated, f32 positions, unused TEXCOORD_0).
• Texture-dominated (large embedded PNG/JPEG): resize/re-encode textures, but keep them as PNG/JPEG (Bevy can't read basisu/webp here).

Step 2: Optimize (use the bundled script)

# keep-ratio = fraction of triangles to KEEP (default 0.25)
scripts/optimize-glb.sh assets/<model>.glb 0.25

The script runs a lossless cleanup (drop unused UVs, weld, join, prune; no compression/instancing) then decimates triangles, and verifies the result needs no extensions. By default it overwrites the input; the original is recoverable with git checkout -- assets/<model>.glb (LFS-tracked).

Equivalent manual pipeline:

npx @gltf-transform/cli optimize in.glb tmp.glb \
  --compress false --texture-compress false --instance false --palette false --simplify false
npx @gltf-transform/cli simplify tmp.glb out.glb --ratio 0.25 --error 0.01

Choosing a ratio

Size scales ~linearly with triangle ratio for uncompressed geometry. Reference numbers from a 119 MB CAD drone (edu-450-v2-drone.glb, geometry-only):

Tune with: lower --ratio for more reduction; if curved surfaces look faceted, raise it. If gaps open between separate parts, add --lock-border true to the simplify step (reduces achievable savings on multi-shell models). GLB_SIMPLIFY_ERROR env var caps the simplification error (default 0.01).

Step 3: Verify before committing

1. Confirm Bevy-safe: the script aborts unless extensionsRequired is empty.
2. Visual check: open the output in the editor (drag into a schematic) or https://gltf.report. Watch for faceting on curved surfaces and gaps between parts.
3. Do not commit (the developer commits). Note the asset is LFS-tracked.

Prefer not to ship it at all

If an asset is optional or only used by one example, consider removing it from LFS and loading it on demand (the editor supports remote glb path="https://…" via the web_asset plugin). Eliminating a HEAD asset saves its full size on every clone. Note: a dead/remote URL must point at a reachable host — otherwise use a local path.