Add KHR_materials_unlit extension.

extensions/2.0/Khronos/KHR_materials_unlit/README.md

@@ -0,0 +1,144 @@
+# KHR\_materials\_unlit
+
+## Contributors
+
+* Don McCurdy, Google, [@donrmccurdy](https://twitter.com/donrmccurdy)
+* Norbert Nopper, UX3D, <mailto:[email protected]>
+* Gary Hsu, Microsoft, [@bghgary](https://twitter.com/bghgary)
+* Scott Nagy, Microsoft, [@visageofscott](https://twitter.com/visageofscott)
+* Pär Winzell, Facebook, [@zellski](https://twitter.com/zellski)
+* Michael Feldstein, Facebook, [@msfeldstein](https://twitter.com/msfeldstein)
+* Robert Long, Mozilla, [@arobertlong](https://twitter.com/arobertlong)
+
+## Status
+
+Complete
+
+## Dependencies
+
+Written against the glTF 2.0 spec.
+
+## Overview
+
+This extension defines an unlit shading model for use in glTF 2.0
+materials, as an alternative to the Physically Based Rendering (PBR) shading
+models provided by the core specification. Three motivating uses cases for
+unlit materials include:
+
+* Mobile devices with limited resources, where unlit materials offer a
+performant alternative to higher-quality shading models.
+* Photogrammetry, in which lighting information is already present and
+additional lighting should not be applied.
+* Stylized materials (such as "anime" or "hand-drawn" looks) in which lighting is
+undesirable for aesthetic reasons.
+
+These use cases are not mutually exclusive: artists may pick an unlit material
+for performance reasons, and make aesthetic decisions to complement that
+choice. As a result, client implementations capable of rendering PBR should not
+automatically "upgrade" to fully-shaded PBR. Any core PBR properties specified
+on an unlit material (except `baseColor`) are meant only as fallbacks for
+clients that do not support the `KHR_materials_unlit` extension. The extension,
+whether required or optional within the asset, indicates preference for an
+unlit visual style.
+
+## Extending Materials
+
+The common Unlit material is defined by adding the
+`KHR_materials_unlit` extension to any glTF material. When present, the
+extension indicates that a material should be unlit and use available
+`baseColor` values, alpha values, and vertex colors while ignoring all
+properties of the default PBR model related to lighting or color. Alpha
+coverage and doubleSided still apply to unlit materials.
+
+```json
+{
+    "materials": [
+        {
+            "name": "MyUnlitMaterial",
+            "pbrMetallicRoughness": {
+                "baseColorFactor": [ 0.5, 0.8, 0.0, 1.0 ]
+            },
+            "extensions": {
+                "KHR_materials_unlit": {}
+            }
+        }
+    ]
+}
+```
+
+### Definition
+
+The Unlit material model describes a constantly shaded surface that is
+independent of lighting. The material is defined only by properties already
+present in the [glTF 2.0 material specification](https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#material).
+No new properties are added by this extension — it is effectively a boolean
+flag indicating use of an unlit shading model. Additional properties on the
+extension object are allowed, but may lead to undefined behaviour in conforming
+viewers.
+
+Color is calculated as:
+
+```
+color = <baseColorTerm>
+```
+
+`<baseColorTerm>` is defined by the [core glTF material specification](https://github.com/KhronosGroup/glTF/blob/master/specification/2.0/README.md#metallic-roughness-material).
+
+### Example
+
+The following example defines an unlit material with a constant red color.
+
+```json
+"materials": [
+    {
+        "name": "red_unlit_material",
+        "pbrMetallicRoughness": {
+            "baseColorFactor": [ 1.0, 0.0, 0.0, 1.0 ]
+        },
+        "extensions": {
+            "KHR_materials_unlit": {}
+        }
+    }
+]
+```
+
+### Extension compatibility and fallback materials
+
+When possible, authoring tools should provide a fallback material definition
+and mark the `KHR_materials_unlit` extension as optional. Models
+including the extension optionally will still render in all clients that
+support the core glTF 2.0 specification, falling back to a PBR metal/rough
+material. In clients that do not support the extension, direct lighting will
+be applied and performance may be degraded.
+
+Defining a fallback to the default PBR shading model is achieved by setting
+appropriate parameters on the default material.
+
+```json
+"materials": [
+    {
+        "name": "unlit_with_fallback",
+        "pbrMetallicRoughness": {
+            "baseColorFactor": [ 1.0, 1.0, 1.0, 1.0 ],
+            "baseColorTexture": { "index": 0 },
+            "roughnessFactor": 0.9,
+            "metallicFactor": 0.0
+        },
+        "extensions": {
+            "KHR_materials_unlit": {}
+        }
+    }
+]
+```
+
+> **Implementation Note:** For best fallback behavior in clients that do not
+> implement the `KHR_materials_unlit` extension, authoring tools may use:
+>
+> * `metallicFactor` is `0` and `emissiveFactor` is `[0, 0, 0]`.
+> * `roughnessFactor` is greater than `0.5`.
+> * Omit `metallicRoughnessTexture`, `occlusionTexture`, `emissiveTexture`,
+> and `normalTexture`.
+
+> **Implementation Note:** When `KHR_materials_unlit` is included with another
+> extension specifying a shading model on the same material, the result is
+> undefined.

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

@@ -0,0 +1,11 @@
+{
+    "$schema": "http://json-schema.org/draft-04/schema",
+    "title": "KHR_materials_unlit glTF extension",
+    "type": "object",
+    "description": "glTF extension that defines the unlit material model.",
+    "allOf": [ { "$ref": "glTFProperty.schema.json" } ],
+    "properties": {
+        "extensions": { },
+        "extras": { }
+    }
+}

extensions/README.md

@@ -4,11 +4,11 @@
 
 #### Khronos extensions
 * [KHR_materials_pbrSpecularGlossiness](2.0/Khronos/KHR_materials_pbrSpecularGlossiness/README.md)
+* [KHR_materials_unlit](2.0/Khronos/KHR_materials_unlit/README.md)
 * [KHR_draco_mesh_compression](2.0/Khronos/KHR_draco_mesh_compression/README.md)
 
 #### Draft Khronos extensions
 _Draft Khronos extensions are not ratified yet._
-* KHR_materials_unlit *(in progress)*
 * KHR_lights *(in progress)*
 * KHR_technique_webgl *(in progress)*
 * KHR_texture_transform *(in progress)*
@@ -34,7 +34,7 @@ _Draft Khronos extensions are not ratified yet._
 
 # About glTF Extensions
 
-glTF extensions extend the base glTF model format.  Extensions can introduce new properties (including properties that reference external data, and the extension can define the format of those data), new parameter semantics, reserved IDs, and new container formats.  Extensions are written against a specific version of glTF and may be promoted to core glTF in a later glTF version. 
+glTF extensions extend the base glTF model format.  Extensions can introduce new properties (including properties that reference external data, and the extension can define the format of those data), new parameter semantics, reserved IDs, and new container formats.  Extensions are written against a specific version of glTF and may be promoted to core glTF in a later glTF version.
 
 ## Extension Mechanics
 
@@ -78,7 +78,7 @@ This allows an engine to quickly determine if it supports the extensions needed
 
 To create a new extension, use the [extension template](Template.md) and open a pull request into this repo.  Make sure to add the extension to the glTF Extension Registry (top of this file).
 
-If lack of extension support prevents proper geometry loading, extension specification _must_ state that (and such extension must be mentioned in `extensionsRequired` top-level glTF property).  
+If lack of extension support prevents proper geometry loading, extension specification _must_ state that (and such extension must be mentioned in `extensionsRequired` top-level glTF property).
 
 Extensions start as a vendor extension, then can become a multi-vendor extensions if there are multiple implementations, and can become a ratified Khronos extension (the multi-vendor extension is an optional step).