KHR_materials_diffuse_transmission (#1825)

* Initial draft for KHR_materials_translucency.

* Added figure and code to describe BTDF.

* Update KHR_materials_translucency

- Move info in interaction with other extensions to translucency
- Add images
- Add more detailed explanation

* README.md edited

* Change summary images; Add review changes

* Change thin transmission image; Fix font

* Change summary images; Add review changes

* Add translucency color parameter and examples

* Optimize example image format/size

* Add example description; Update brdf notation

* Rename translucency -> diffuse_transmission

* Change example table layout, edit descriptions

* Update extensions/2.0/Khronos/KHR_materials_diffuse_transmission/schema/glTF.KHR_materials_diffuse_transmission.schema.json

Co-authored-by: abwood <[email protected]>

* Remove example for combined diffuse-/transmission

Effect not obvious in the image

* Remove all references to KHR_materials_sss

* Remove extensions combination overview for now

Needs better images or renderings

* Update image text

* Fix typo

* Revise spec

- Add more detailed parameter descriptions based on PR feedback + renderings
- Add dragon and candle renderings as example for diffuse transmission + volume
- Remove example section

* Fix image table layout. Add white bg to bsdf image for better visibility.

* Split up normative "material structure update" and non-normative implementation sections

* Fix Appendix B url

* Add exclusions; Add scene/texture attribution

* Use https uri

* Remove leaves image

---------

Co-authored-by: SDORRA Bastian <[email protected]>
Co-authored-by: Bastian Sdorra <[email protected]>
Co-authored-by: abwood <[email protected]>
Co-authored-by: Bastian SDORRA <[email protected]>

extensions/2.0/Khronos/KHR_materials_diffuse_transmission/schema/glTF.KHR_materials_diffuse_transmission.schema.json

@@ -0,0 +1,41 @@
+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_materials_diffuse_transmission glTF extension",
+    "type": "object",
+    "description": "glTF extension that defines the diffuse transmission of the material.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "diffuseTransmissionFactor": {
+            "type": "number",
+            "description": "The percentage of light that is diffusely transmitted through the surface.",
+            "default": 0.0,
+            "minimum": 0.0,
+            "maximum": 1.0,
+            "gltf_detailedDescription": "The percentage of non-specularly reflected light that is transmitted through the surface via a Lambertian diffuse transmission."
+        },
+        "diffuseTransmissionTexture": {
+            "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+            "description": "A texture that defines the percentage of light transmitted through the surface.",
+            "gltf_detailedDescription": "A texture that defines the strength of the diffuse transmission effect, stored in the alpha (A) channel. Will be multiplied by the diffuseTransmissionFactor."
+        },
+        "diffuseTransmissionColorFactor": {
+          "type": "array",
+          "items": {
+              "type": "number",
+              "minimum": 0.0
+          },
+          "description": "The color of the transmitted light.",
+          "default": [ 1.0, 1.0, 1.0 ],
+          "minItems": 3,
+          "maxItems": 3,
+          "gltf_detailedDescription": "The color of the transmitted light."
+      },
+      "diffuseTransmissionColorTexture": {
+          "allOf": [ { "$ref": "textureInfo.schema.json" } ],
+          "description": "A texture that defines the color of the transmitted light",
+          "gltf_detailedDescription": "A texture that defines the color of the transmitted light, stored in the RGB channels and encoded in sRGB. This texture will be multiplied by diffuseTransmissionColorFactor."
+      },
+        "extensions": { },
+        "extras": { }
+    }
+}