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Expand Up @@ -28,7 +28,9 @@ The [conformance](#conformance) section specifies what an implementation must to

### Extending Materials

Common materials are defined by adding an `extensions` property to any glTF material, and defining its `KHR_materials_common` property. For example, the following defines a material with a Lambert shader with 50% gray diffuse color:
Common materials are defined by adding an `extensions` property to any glTF material, and defining its `KHR_materials_common` property. Each material has two properties: `technique`, which specifies the shading technique used e.g. `BLINN`, and `values` which contains a set of technique-specific values.

For example, the following defines a material with a Lambert shader with 50% gray diffuse color:

```javascript
"materials": {
Expand All @@ -53,26 +55,25 @@ Common materials are defined by adding an `extensions` property to any glTF mate

#### Common Material Shared Properties

Table 1 lists properties that are shared among several of the common material types.
Table 1 lists properties that can appear in the `values` property of a common material, and that are shared among several of the common material types.

Table 1. Common Material Shared Properties

| Property | Type | Description | Default Value | Applies To |
|:----------------------------:|:------------:|:-----------:|:-------------:|:----------:|
| `ambient` | `FLOAT_VEC4` | RGBA value for ambient light reflected from the surface of the object.|[0,0,0,1] | `BLINN`, `PHONG`, `LAMBERT` |
| `diffuse` | `FLOAT_VEC4` or `SAMPLER_2D` | RGBA value or texture sampler defining the amount of light diffusely reflected from the surface of the object. | [0,0,0,1] | `BLINN`, `PHONG`, `LAMBERT` |
| `emission` | `FLOAT_VEC4` | RGBA value for light emitted by the surface of the object. | [0,0,0,1] | `BLINN`, `PHONG`, `LAMBERT`, `CONSTANT` |
| `index_of_refraction` | `FLOAT` | Declares the index of refraction for perfectly refracted light as a single scalar index between 0.0 and 1.0. | 1.0 | `BLINN`, `PHONG`, `LAMBERT`, `CONSTANT` |
| `reflective` | `FLOAT_VEC4` or `SAMPLER_2D` | RGBA value or texture sampler defining the color of a perfect mirror reflection. | [0,0,0,1] | `BLINN`, `PHONG`, `LAMBERT`, `CONSTANT` |
| `diffuse` | `FLOAT_VEC4` or string | RGBA value or texture id defining the amount of light diffusely reflected from the surface of the object. | [0,0,0,1] | `BLINN`, `PHONG`, `LAMBERT` |
| `emission` | `FLOAT_VEC4` or string | RGBA value or texture id for light emitted by the surface of the object. | [0,0,0,1] | `BLINN`, `PHONG`, `LAMBERT`, `CONSTANT` |
| `indexOfRefraction` | `FLOAT` | Declares the index of refraction for perfectly refracted light as a single scalar index between 0.0 and 1.0. | 1.0 | `BLINN`, `PHONG`, `LAMBERT` |
| `reflective` | `FLOAT_VEC4` or string | RGBA value or texture id defining the color of a perfect mirror reflection. | [0,0,0,1] | `BLINN`, `PHONG`, `LAMBERT` |
| `reflectivity` | `FLOAT` | Declares the amount of perfect mirror reflection to be added to the reflected light as a value between 0.0 and 1.0. | 0.0 | `BLINN`, `PHONG`, `LAMBERT`, `CONSTANT` |
| `specular` | `FLOAT_VEC4` or `SAMPLER_2D` | RGBA value or texture sampler defining the color of light specularly reflected from the surface of the object *[need to explain the units here k-factor right?]*. | [0,0,0,1] | `BLINN`, `PHONG` |
| `specular` | `FLOAT_VEC4` or string | RGBA value or texture id defining the color of light specularly reflected from the surface of the object. | [0,0,0,1] | `BLINN`, `PHONG` |
| `shininess` | `FLOAT` | Defines the specularity or roughness of the specular reflection lobe of the object. | 0.0 | `BLINN`, `PHONG` |
| `technique` | string | Declares the lighting technique used. Must be one of `BLINN`, `PHONG`, `LAMBERT`, or `CONSTANT` | "" | `BLINN`, `PHONG`, `LAMBERT`, `CONSTANT` |
| `transparency` | `FLOAT` | Declares the amount of transparency as an opacity value between 0.0 and 1.0. | 1.0 | `BLINN`, `PHONG`, `LAMBERT`, `CONSTANT` |

#### Blinn-Phong

Blinn-Phong approximation. Uses all of the common material properties defined in Table 1. The following example defines a Blinn-Phong lit material with a diffuse texture, moderate shininess and red specular highlights.
When the value of `technique` is `BLINN`, this defines a material lit with a Blinn-Phong approximation. Blinn-Phong lighting uses all of the common material properties defined in Table 1. The following example defines a Blinn-Phong lit material with a diffuse texture, moderate shininess and red specular highlights.

```javascript
"materials": {
Expand All @@ -98,7 +99,7 @@ Blinn-Phong approximation. Uses all of the common material properties defined in

#### Phong

Phong lighting. Uses all of the common material properties defined in Table 1. The following example defines a Phong lit material with a white diffuse color, environment map texture and high reflectivity.
When the value of `technique` is `PHONG`, this defines a material lit with a Phong lighting equation. Phong lighting uses all of the common material properties defined in Table 1. The following example defines a Phong lit material with a white diffuse color, environment map texture and high reflectivity.

```javascript
"materials": {
Expand All @@ -124,9 +125,9 @@ Phong lighting. Uses all of the common material properties defined in Table 1. T

#### Lambert

Lambert shading does not reflect specular highlights; therefore the common material properties `specular` and `shininess` are not used. All other properties from Table 1 apply.
When the value of `technique` is `LAMBERT`, this defines a material shaded using Lambert shading. Lambert shading does not reflect specular highlights; therefore the common material properties `specular` and `shininess` are not used. All other properties from Table 1 apply.

The following example defines a Lambert lit material with a 50% gray emissive color and a diffuse texture map.
The following example defines a Lambert shaded material with a 50% gray emissive color and a diffuse texture map.

```javascript
"materials": {
Expand All @@ -151,7 +152,7 @@ The following example defines a Lambert lit material with a 50% gray emissive co

#### Constant

Constant shading does not reflect light sources in the scene; therefore the common material properties `ambient`, `diffuse`, `specular` and `shininess` are not used. All other properties from Table 1 apply.
When the value of `technique` is `CONSTANT`, this defines a material that uses constant shading. Constant shading does not reflect light sources in the scene; therefore the common material properties `ambient`, `diffuse`, `reflective`, `reflectivity`, `specular` and `shininess` are not used. All other properties from Table 1 apply.

The following example defines a Constant lit material with a 50% gray emissive color and 50% opacity.

Expand All @@ -176,13 +177,22 @@ The following example defines a Constant lit material with a 50% gray emissive c
},
```

### Interaction Between Attribute Semantics and Common Materials

The base specification defines attribute semantics for mesh primitives. The common materials in this extension shall support the semantics `POSITION`, `NORMAL`, `TEXCOORD`, `COLOR`, `JOINT`, `JOINTMATRIX`, and `WEIGHT`. For array semantics such as texture coordinates, the implemention is only required to support the 0th set of coordinates i.e. `TEXCOORD_0`.

If a conforming implementation of this extension supports skinned animations, then the common materials described in this extension must also support hardware skinning in its vertex and fragment shader programs.


<a name="lights"></a>
## Lights

Lights define light sources in the scene.

Lights are contained in nodes and thus can be transformed. Their world-space positions can be used in shader calculations.

A conforming implementation of this extension must be able to load light data defined in the asset, but it is not obligated to render the asset using those lights.

### Defining Lights

Lights are defined within a dictionary property in the file, by adding an `extensions` property to the top-level glTF object and defining its `KHR_materials_common` property with a `lights` dictionary object inside it.
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