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MX_lightmap ​

Contributors ​

  • Robert Long, The Matrix.org Foundation

Status ​

Draft

Dependencies ​

Written against the glTF 2.0 spec.

Overview ​

This extension allows for adding lightmaps to nodes with meshes.

glTF Schema Updates ​

Usage of the extension must be listed in the extensionsUsed array:

json
"extensionsUsed": [
  "MX_lightmap"
]

The MX_lightmap extension can be added to any node property with a mesh.

json
"nodes": [
  {
    "mesh": 0,
    "extensions": {
      "MX_lightmap": {
        "lightMapTexture": {
          "index": 0,
          "texCoord": 1
        },
        "scale": [0.5, 0.5],
        "offset": [0.5, 0.5],
        "intensity": 1.0
      }
    }
  }
]

lightMapTexture is a TextureInfo object. The texCoord should always be set to 1 (the second UV set). If it is not provided, it will be automatically set to 1, however for forwards compatibility, it should always be set. The lightmap should be RGBM encoded and use a format that supports alpha (PNG/BASISU).

The lightmap texture's color is decoded with the following shader code:

glsl
vec4 lightMapTexel = texture2D( lightMap, vUv2 );
lightMapTexel.rgb = 34.49 * pow(lightMapTexel.a, 2.2) * lightMapTexel.rgb;
lightMapTexel.a = 1.0;

scale and offset are the meshes' UV scale and offset into the provided lightMapTexture if they are not defined they are assumed to be (1, 1) and (0, 0) respectively.

intensity is a multiplier which determines how emissive the lightmap is.

When used with the EXT_mesh_gpu_instancing extension:

json
"nodes": [
  {
    "mesh": 0,
    "extensions": {
      "MX_lightmap": {
        "lightMapTexture": {
          "index": 0,
          "texCoord": 1
        },
        "intensity": 1.0
      },
      "EXT_mesh_gpu_instancing": {
        "attributes": {
          "TRANSLATION": 0,
          "ROTATION": 1,
          "SCALE": 2,
          "_LIGHTMAP_SCALE": 3,
          "_LIGHTMAP_OFFSET": 4
        }
      }
    }
  }
]

_LIGHTMAP_SCALE and _LIGHTMAP_OFFSET are references to accessors containing Vec2 elements for each instance's UV scale / offset into the referenced lightMapTexture. The scale and offset properties on the MX_lightmap object will be ignored if provided.

JSON Schema ​

node.MX_lightmap.schema.json

Known Implementations ​