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KHR_materials_unlit
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| # KHR\_materials\_unlit | ||
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| ## Contributors | ||
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| * 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) | ||
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| ## Status | ||
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| Complete | ||
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| ## Dependencies | ||
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| Written against the glTF 2.0 spec. | ||
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| ## Overview | ||
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| 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: | ||
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| * 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. | ||
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| 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. | ||
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| ## Extending Materials | ||
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| 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. | ||
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| ```json | ||
| { | ||
| "materials": [ | ||
| { | ||
| "name": "MyUnlitMaterial", | ||
| "pbrMetallicRoughness": { | ||
| "baseColorFactor": [ 0.5, 0.8, 0.0, 1.0 ] | ||
| }, | ||
| "extensions": { | ||
| "KHR_materials_unlit": {} | ||
| } | ||
| } | ||
| ] | ||
| } | ||
| ``` | ||
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| ### Definition | ||
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| 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. | ||
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| Color is calculated as: | ||
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| ``` | ||
| color = <baseColorTerm> | ||
| ``` | ||
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| `<baseColorTerm>` is defined by the [core glTF material specification](https://github.com/KhronosGroup/glTF/blob/master/specification/2.0/README.md#metallic-roughness-material). | ||
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| ### Example | ||
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| The following example defines an unlit material with a constant red color. | ||
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| ```json | ||
| "materials": [ | ||
| { | ||
| "name": "red_unlit_material", | ||
| "pbrMetallicRoughness": { | ||
| "baseColorFactor": [ 1.0, 0.0, 0.0, 1.0 ] | ||
| }, | ||
| "extensions": { | ||
| "KHR_materials_unlit": {} | ||
| } | ||
| } | ||
| ] | ||
| ``` | ||
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| ### Extension compatibility and fallback materials | ||
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| 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. | ||
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| Defining a fallback to the default PBR shading model is achieved by setting | ||
| appropriate parameters on the default material. | ||
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| ```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": {} | ||
| } | ||
| } | ||
| ] | ||
| ``` | ||
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| > **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. |
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extensions/2.0/Khronos/KHR_materials_unlit/schema/glTF.KHR_materials_unlit.schema.json
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| { | ||
| "$schema": "http://json-schema.org/draft-04/schema", | ||
| "title": "KHR_materials_unlit glTF extension", | ||
| "type": "object", | ||
| "description": "glTF extension that defines the unlit material model.", | ||
| "allOf": [ { "$ref": "glTFProperty.schema.json" } ], | ||
| "properties": { | ||
| "extensions": { }, | ||
| "extras": { } | ||
| } | ||
| } |
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