KHR_lightmap draft

This implementation targets the KHR_lightmap proposal at bhouston/glTF c831545. The proposal is not a ratified glTF extension.

Rendering

Lightmaps contain linear RGB irradiance. The material intensity multiplies that irradiance before the diffuse BRDF and ambient occlusion are applied. Lightmaps do not contribute specular illumination. Unlit materials multiply their base color by irradiance divided by pi. Alpha is unchanged.

Material bindings support UV0, UV1, and KHR_texture_transform. A node binding replaces the entire selected primitive binding; omitted intensity and transform fields use the extension defaults, rather than inheriting the replaced material.

Instance atlas attributes _LIGHTMAP_OFFSET, _LIGHTMAP_SCALE, _LIGHTMAP_ROTATION, and _LIGHTMAP_INTENSITY compose after the selected binding transform. The current portable implementation expands these atlas batches into individual draws with independent material uniforms. It preserves I * B even when the composed matrix contains shear, but increases draw calls and resource usage compared with GPU batching. Ordinary instances without atlas attributes continue to use the existing GPU instancing path.

Supported profile

The renderer supports UV0/UV1 material bindings, complete node overrides, and the four instance atlas attributes. The node and instance atlas examples from the pinned proposal are tested with framebuffer readback on WebGL and WebGPU. The shared Cornell Box example loads the original source-hosted asset and isolates baked irradiance from extra punctual lights.

Node and instance bindings are preserved while switching between already lightmapped material variants. Changing the primitive's shader feature set through variants, additional UV sets, and the broader image-format/HDR combinations are outside this tested profile. The support registry retains loader-only status so strict required-extension loading does not claim complete draft conformance.

<GLTFFeatureExample feature="lightmap" />

The standalone shared viewer accepts ?feature=lightmap&backend=webgl or ?feature=lightmap&backend=webgpu to select one backend explicitly.