Update KHR_materials_common for glTF 2.0 #1150
There are no files selected for viewing
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
| Original file line number | Diff line number | Diff line change |
|---|---|---|
| @@ -0,0 +1,250 @@ | ||
| # KHR\_materials\_common | ||
|
|
||
| ## Contributors | ||
|
|
||
| * Norbert Nopper, UX3D, <mailto:nopper@ux3d.io> | ||
| * Thomas Kress, UX3D, <mailto:kress@ux3d.io> | ||
| * Don McCurdy, Google, <mailto:donmccurdy@google.com> | ||
|
|
||
| ## Status | ||
|
|
||
| Draft (not ratified yet) | ||
|
|
||
| ## Dependencies | ||
|
|
||
| Written against the glTF 2.0 spec. | ||
|
|
||
| ## Overview | ||
|
|
||
| This extension defines a set of common materials for use with glTF 2.0, as | ||
| alternatives to the Physically Based Rendering (PBR) based materials provided | ||
| by the core specification. Other extensions may define additional materials. | ||
|
|
||
| The `KHR_materials_common` extension defines three material types: Blinn-Phong, | ||
| Lambert, and Constant. | ||
|
|
||
| ## Properties | ||
|
|
||
| The following table lists the allowed types and ranges for the common materials: | ||
|
|
||
| | |Type | Technique(s) | Description | Required | | ||
| |----------------------------|-------------|--------------|-------------|----------| | ||
| |**technique** | `BLINNPHONG`, `LAMBERT`, `CONSTANT` | — |The shading model of the material. | Yes | | ||
| |**diffuseFactor** | `number[4]` | `LAMBERT`, `BLINNPHONG` | The reflected diffuse RGBA factor of the material. | No, default: `[1.0,1.0,1.0,1.0]` | | ||
| |**diffuseTexture** | [`textureInfo`](/specification/2.0/README.md#reference-textureInfo) | `LAMBERT`, `BLINNPHONG` | The diffuse texture. | No | | ||
| |**specularFactor** | `number[3]` | `BLINNPHONG` | The specular RGB color of the material. | No, default: `[1.0,1.0,1.0]` | | ||
| |**shininessFactor** | `number` | `BLINNPHONG` | The shininess of the material. | No, default: `1.0` | | ||
| |**specularShininessTexture**| [`textureInfo`](/specification/2.0/README.md#reference-textureInfo) | `BLINNPHONG` | The specular shininess texture. | No | | ||
|
|
||
| Additional properties are allowed and may lead to undefined behaviour in | ||
| conforming viewers. Emissive and occlusion values are not defined | ||
| explicitly by this extension, but are inherited from the core material | ||
| definition. | ||
|
|
||
| ## Techniques | ||
|
|
||
| For all technique computations, the following definitions are assumed: | ||
| *(all vectors normalized)* | ||
|
|
||
| * `diffuseTerm` – diffuseFactor * diffuseTexture | ||
| * *Reflected diffuse color of the material.* | ||
| * `specularTerm` – specularFactor * specularShininessTexture.rgb | ||
| * *Specular color of the material.* | ||
| * `shineTerm` – shininessFactor * specularShininessTexture.a | ||
| * *Shininess component of the specular color.* | ||
| * `emissiveTerm` – emissiveFactor * emissiveTexture | ||
| * [*From core glTF material specification.*](https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#additional-maps) | ||
| * `occlusionTerm` – occlusionFactor * occlusionTexture | ||
| * [*From core glTF material specification.*](https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#additional-maps) | ||
| * `aL` – Ambient light | ||
| * *Constant sum of all ambient light contributions from the scene.* | ||
| * `N` – Normal vector | ||
| * `L` – Light vector | ||
| * `I` – Eye vector | ||
| * `H` – Half-angle vector | ||
| * *Calculated as halfway between the unit Eye and Light vectors, using the equation `H = normalize(I+L)`.* | ||
|
|
||
| ### Constant | ||
|
|
||
| The `CONSTANT` technique defines shading that is independent of lighting, or | ||
| unlit. No extension properties are included with Constant materials, but | ||
| emissive color (from the core glTF 2.0 specification) is used. | ||
|
|
||
| The displayed color is calculated as: | ||
|
|
||
| ``` | ||
| color = emissiveTerm + occlusionTerm | ||
| ``` | ||
|
|
||
| > **Implementation Note**: The constant shading model is visually identical to | ||
| > Lambert or Blinn-Phong models in which all unused factors are set to zero. | ||
| > Accordingly, engines may implement only the Blinn-Phong model and use it to | ||
| > emulate simpler shading models if preferred. | ||
|
|
||
| <details> | ||
| <summary>Example: Constant</summary> | ||
| The following example defines a Constant material with an emissive red color. | ||
|
|
||
| ```json | ||
| "materials": [ | ||
| { | ||
| "name": "red_constant_material", | ||
| "emissiveFactor": [ 1.0, 0.0, 0.0 ], | ||
| "extensions": { | ||
| "KHR_materials_cmnBlinnPhong": { | ||
| "technique": "CONSTANT" | ||
| } | ||
| } | ||
| } | ||
| ] | ||
| ``` | ||
| </details> | ||
|
|
||
| ### Lambert | ||
|
|
||
| The `LAMBERT` technique defines a material shaded using Lambert shading. The | ||
| result is based on Lambert’s Law, which states that when light hits a rough | ||
| surface, the light is reflected in all directions equally. The reflected | ||
| color is calculated as: | ||
|
|
||
| ``` | ||
| color = emissiveTerm + occlusionTerm * diffuseTerm * max(N * L, 0) | ||
|
There was a problem hiding this comment. If ambient is now calculated from diffuse, it should probably be: |
||
| ``` | ||
|
|
||
| > **Implementation Note**: The Lambert shading model is visually identical to a | ||
| > Blinn-Phong model in which all unused factors are set to zero. Accordingly, | ||
| > engines may implement only the Blinn-Phong model and use it to emulate | ||
| > simpler shading models if preferred. | ||
|
|
||
| <details> | ||
| <summary>Example: Lambert</summary> | ||
| The following example defines a Lambert shaded material with a diffuse texture, | ||
| moderate shininess and red specular highlights. | ||
|
|
||
|
There was a problem hiding this comment. Looks like a copy-and-paste error. Maybe just "The following example defines a Lambert shaded material with a diffuse texture." or some-such? |
||
| ```json | ||
| "materials": [ | ||
| { | ||
| "name": "lambert_textured_material", | ||
| "emissiveTexture" : { | ||
| "index" : 0 | ||
| }, | ||
| "extensions": { | ||
| "KHR_materials_common": { | ||
| "technique": "LAMBERT", | ||
| "diffuseTexture" : { | ||
| "index" : 1 | ||
| } | ||
| } | ||
| } | ||
| } | ||
| ] | ||
| ``` | ||
| </details> | ||
|
|
||
| ### Blinn-Phong | ||
|
|
||
| The `BLINNPHONG` technique is intended for shading according to the Blinn-Phong | ||
| lighting model or a close approximation. The underlying equation is detailed by | ||
| ACM: “Models of Light Reflection for Computer Synthesized Pictures,” SIGGRAPH | ||
| 77, pp 192-198 | ||
| [http://portal.acm.org/citation.cfm?id=563893](http://portal.acm.org/citation.cfm?id=563893). | ||
| In particular see the section, "Simple Highlight Model." | ||
|
|
||
| The following code illustrates the computation: | ||
|
|
||
| ``` | ||
| color = emissiveTerm + occlusionTerm * (diffuseTerm * max(N * L, 0) + specularTerm * max(H * N, 0)^shineTerm) | ||
|
There was a problem hiding this comment. modulo code style urges, with aL factor: |
||
| ``` | ||
|
|
||
| > **Implementation Note**: Writers should be aware of the range of the specular | ||
| > exponent (`shininess`), which is _not_ a normalized range. Concretely | ||
| > speaking, given the above equation, a `shininess` value of 1.0 corresponds to | ||
| > a very low shininess. For orientation: using the traditional OpenGL fixed | ||
| > function pipeline, the specular exponent was expected to be within [0, 128]. | ||
| > However, using glTF, larger `shininess` values are clearly possible. | ||
|
|
||
| > **TODO:** Mention caution with `shininess < 1`? | ||
|
|
||
| <details> | ||
| <summary>Example: Blinn-Phong</summary> | ||
| The following example defines a Blinn-Phong shaded material with a diffuse texture, moderate shininess and red specular highlights. | ||
|
|
||
| ```json | ||
| "materials": [ | ||
| { | ||
| "name": "shiny_blinnphong_material", | ||
| "extensions": { | ||
| "KHR_materials_common": { | ||
| "technique": "BLINNPHONG", | ||
| "diffuseFactor" : [ | ||
| 0.8, | ||
| 0.8, | ||
| 0.8, | ||
| 1.0 | ||
| ], | ||
| "diffuseTexture" : { | ||
| "index" : 0 | ||
| }, | ||
| "shininessFactor" : 10.0, | ||
| "specularFactor" : [ | ||
| 1, | ||
| 0, | ||
| 0 | ||
| ] | ||
| } | ||
| } | ||
| } | ||
| ] | ||
| ``` | ||
| </details> | ||
|
|
||
| ## Extending Materials | ||
|
|
||
| Materials are defined by adding the `KHR_materials_common` extension to any | ||
| glTF material. When the extension is provided alongside a core glTF material | ||
| or another material extension, client implementations may choose the most | ||
| appropriate material for performance or other constraints. | ||
|
|
||
|
There was a problem hiding this comment. I have been wondering if we should attempt to guide the way in which clients render situations where multiple material extensions are present, or this extension along with pbrMetallicRoughness. From one point of view, a filled-out PBR struct would always be primary, and e.g. BLINNPHONG might be sent along as a fallback for weak devices. From another point of view, a LAMBERT material would be the model creator's ideal aesthetic, but they also fill out the PBR struct with (very) approximate values for clients that don't accept the extension. Does each client guess? Do we guide the guesswork? Do we include a boolean flag or some-such in this extension to help suggest "this is a fallback" vs "this is the ideal rendering path"? (I think that's probably not a good idea, but who knows.) |
||
| For example, a Blinn-Phong material using all possible parameters: | ||
|
|
||
| ```json | ||
| { | ||
| "materials": [ | ||
| { | ||
| "name": "complete_blinnphong_material", | ||
| "emissiveFactor" : [ | ||
| 0.2, | ||
| 0.2, | ||
| 0.2 | ||
| ], | ||
| "emissiveTexture" : { | ||
| "index" : 0 | ||
| }, | ||
| "extensions": { | ||
| "KHR_materials_common": { | ||
| "technique": "BLINNPHONG", | ||
| "diffuseFactor" : [ | ||
| 0.8, | ||
| 0.8, | ||
| 0.8, | ||
| 1.0 | ||
| ], | ||
| "diffuseTexture" : { | ||
| "index" : 1 | ||
| }, | ||
| "shininessFactor" : 2.0, | ||
| "specularFactor" : [ | ||
| 0.1, | ||
| 0.1, | ||
| 0.1 | ||
| ], | ||
| "specularShininessTexture" : { | ||
| "index" : 2 | ||
| }, | ||
| } | ||
| } | ||
|
|
||
| } | ||
| ] | ||
| } | ||
| ``` | ||
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
I think the math here is incorrect; I don't think the occlusionTerm should be added by itself.
There was a problem hiding this comment.
Choose a reason for hiding this comment
The reason will be displayed to describe this comment to others. Learn more.
Does that look correct? I'm probably not the person to be writing these equations.