Extensions

May 5, 2026 ยท View on GitHub

AssetKit handles glTF extensions in three ways:

  • Typed support for extensions that affect AssetKit's runtime model.
  • Optional side decoder libraries for compressed payloads.
  • Preserved JSON payload through ak_extra() for app/vendor-specific data.

Required Extensions

extensionsRequired is strict. If an extension is required for correct geometry, animation, texture, material, or splat data, AssetKit must implement it or reject the asset.

extensionsUsed can be more permissive. If an optional extension is not needed for correct loading, AssetKit may preserve the payload in ak_extra() so a viewer/tool can inspect it.

Optional Decoder Libraries

Compression and heavy decoders live outside the main C library. AssetKit loads these side libraries only when needed:

  • libassetkit_draco
  • libassetkit_meshoptimizer
  • libassetkit_spz
  • libassetkit_ktx2

On Windows they are assetkit_*.dll; on macOS libassetkit_*.dylib; on Linux libassetkit_*.so. AssetKit first searches next to the loaded libassetkit binary, then falls back to the platform loader search path.

CMake builds the side libraries by default when dependencies are available or can be fetched:

cmake -S . -B build -DCMAKE_BUILD_TYPE=Release
cmake --build build

Useful options:

-DAK_FETCH_DEPS=OFF
-DAK_BUILD_GLTF_DRACO_DECODER=OFF
-DAK_BUILD_GLTF_MESHOPT_DECODER=OFF
-DAK_BUILD_GLTF_SPZ_DECODER=OFF
-DAK_BUILD_GLTF_KTX2_DECODER=OFF

Typed Extension Data

Some extensions are represented as normal AssetKit fields:

  • KHR_materials_variants

    • AkDoc.materialVariants
    • AkMeshPrimitive.variantMappings
    • ak_materialVariantByName()

  • KHR_gaussian_splatting

    • AkMeshPrimitive.gsplat
    • splat attributes stay in the primitive input chain

  • KHR_animation_pointer

    • maps supported JSON pointer targets to AssetKit animation targets

SPZ is only a decoder format. Public Gaussian splat data is generic and lives in AkGaussianSplat.

More detail: docs/source/extensions.rst