Draft of FB_materials_lightmap extension

extensions/2.0/Vendor/FB_materials_lightmap/README.md

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+# FB\_materials\_lightmap
+
+## Contributors
+
+* Alfred Young, Facebook, [@idorurez](https://github.com/idorurez)
+* Pär Winzell, Facebook, [@zellski](https://twitter.com/zellski)
+
+## Status
+
+Draft
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Overview
+
+This extension adds an additional annotation to a material, providing a means to introduce a lightmap to be applied in the same scope.
+
+Some proven real-world applications for this information:
+1. Having multiple variations of lighting scenarios without having to bake lighting into a single, original base color/texture.
+2. Specify a separate UV channel (texCoord), to vary the new texture texel resolution with respect to the texture it's being applied to.
+3. Modulate the intensity of a lightmap, dimming or increasing it's effect.
+
+## Color
+
+### RGB
+
+Color is stored in the RGB channels, with no accommodation for an alpha channel. No restriction is placed on the the values.
+
+### Color Space
+
+We'll refer to Unity's guidelines regarding Color Space: "When Linear Color Space is used, the lightmap texture is marked as sRGB and the final value used by the shaders (after sampling and decoding) will be in Linear Color Space. When Gamma Color Space is used, the final value will be in Gamma Color Space."
+
+## Specifying Material extension
+
+The data is defined as an optional extension to a glTF `materials`, by adding an `extensions` property and defining a `FB_materials_lightmap` property with a `lightmapTexture` node inside it:
+
+```javascript
+"materials" : [
+  	{
+  		"name" : "material_test",
+  		"pbrMetallicRoughness" : {
+  			"baseColorTexture" : {
+  				"index" : 0,
+  				"texCoord" : 0
+  			}
+  		},
+  		"extensions" : {
+  			"FB_materials_lightmap" : {
+  				"lightmapTexture" : {
+  					"index" : 1,
+  					"texCoord" : 1,    
+  				},
+				"lightmapFactor" : [
+  						1,
+  						1,
+  						1
+  				]
+  			}
+  		}
+  	}
+  ],
+```
+
+## Sample Implementation
+
+In it's simplemest form, Lightmap implementation predominantely involes a base, unlit texture or color in a material, and the supplied lightmap texture is sampled and multiplied against that unlit texture/color.
+
+```javascript
+    lowp vec4 lightmapColor = texture2D(LightmapTexture, oLightmapTexCoord);
+    color.rgb *= (LightmapFactor.rgb * lightmapColor.rgb);
+
+```
+
+## Limitations
+This does not define how the lightmap will be applied, just that one exists.
+
+## lightmapTexture properties
+| Property | Description | Required |
+|:-----------------------|:------------------------------------------| :--------------------------|
+| `index` | Texture descriptor index. | :white_check_mark: Yes |
+| `texCoord` | Texture coordinate index, aka UV set | No, Default: `0` |
+| `lightmapFactor` | A 3-component vector describing the attenuation of the lightmap before baseColor application. | No, Default: `[1.0, 1.0, 1.0]` |
+
+

extensions/2.0/Vendor/FB_materials_lightmap/schema/lightmap.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "lightmap properties",
+    "type": "object",
+    "allOf": [
+        {
+            "$ref": "glTFProperty.schema.json"
+        }
+    ],
+    "properties": {
+        "index": {
+            "type": "number",
+            "description": "The texture index of which this lightmap refers to",
+            "minimum": 0,
+            "minItems": 1,
+            "maxItems": 1
+        },
+        "texCoord": {
+            "description": "The texture coordinate (uv set) that this lightmap texture uses",
+            "type": "number",
+            "items": {
+                "type": "number"
+            },
+            "default": 0,
+            "minimum": 0,
+            "minItems": 1,
+            "maxItems": 1
+        },
+        "extensions": {},
+        "extras": {},
+        "required" : [
+            "index"
+        ]
+    },
+}

extensions/2.0/Vendor/FB_materials_lightmap/schema/material.extension.FB_materials_lightmap.schema.json

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+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "FB_materials_lightmap material extension",
+    "type": "object",
+    "allOf": [
+        {
+            "$ref": "glTFProperty.schema.json"
+        }
+    ],
+    "properties": {
+        "lightmapTexture": {
+            "description": "The texture information for the lightmap.",
+            "allOf": [
+                {
+                    "$ref": "lightmap.schema.json"
+                }
+            ]
+        },
+        "lightmapFactor": {
+            "description": "The rgb component-wise factor that should be applied to the lightmap before applying to the base color",
+            "type": "array",
+            "items": {
+                "type": "number"
+            },
+            "default": [1, 1, 1],
+            "minItems": 3,
+            "maxItems": 3
+        },
+        "extensions": {},
+        "extras": {}
+    },
+    "required": [
+        "lightmapTexture"
+    ]
+}