Shading Model

<cat>Material.ShaderBasic</cat>
<title>Shading Model</title>

# What is a Shading Model

Shading Models determines how a surface will interact with light. 

They dictate how the Material's input data is utilized to produce the final visual output.

Not only can you use one of these pre-defined shading models, but you can also create your own shading model to shade your surface the way you want.

S&box currently ships with one shading model `ShadingModelStandard` which should be what most people would use.

# Using built-in shading models
```c++
PS
{
    #include "common/pixel.hlsl"

	float4 MainPs( PixelInput i ) : SV_Target0
	{
		Material m = Material::From( i );
		/* m.Metalness = 1.0f; // Forces the object to be metalic */
		return ShadingModelStandard::Shade( i, m );
	}
}
```
# Creating a shading model

## Structure

## Light Structure

We provide the following light types to be used from shader code:

* `DynamicLight`
* `StaticLight`
* `EnvironmentMapLight`
* `AmbientLight`

All can be created with `Light::From()` and all but `AmbientLight` can be iterated with `Light::Count()`, 
Light types already handle everything from tiled rendering to shadowing seamlessly, for example:

```c++
//
// Shade direct lighting for dynamic lights
//
for ( uint index = 0; index < DynamicLight::Count( material.ScreenPosition ); index++ )
{
     Light light = DynamicLight::From( material.ScreenPosition, material.WorldPosition, index );
     vResult += light.Color * dot( material.Normal, light.Direction );
}```

Each light structure gives you the following data to be consumed:

```c++
//-----------------------------------------------------------------------------
// Light structure
//-----------------------------------------------------------------------------
struct Light
{
    // The color is an RGB value in the linear sRGB color space.
    float3 Color;

    // The normalized light vector, in world space (direction from the
    // current fragment's position to the light).
	// Skipped by Ambient Light
    float3 Direction;

    // The position of the light in world space. This value is the same as
    // Direction for directional lights.
    float3 Position;

    // Attenuation of the light based on the distance from the current
    // fragment to the light in world space. This value between 0.0 and 1.0
    // is computed differently for each type of light (it's always 1.0 for
    // directional lights).
	// Skipped by Ambient Light
    float Attenuation;

    // Visibility factor computed from shadow maps or other occlusion data
    // specific to the light being evaluated. This value is between 0.0 and
    // 1.0. (It's always 1.0 for ambient lights)
    float Visibility;
};
```

Note that ambient lights skip some of this data.

## Applying Fog
You can apply fog to shaders by using `DoAtmospherics()`
```
vColor = DoAtmospherics( material.WorldPosition, material.ScreenPosition.xy, vColor );
return vColor;
```

## Usage

## Example ShadingModel
Here's an example of doing a simple Toon shader from the structure you get from lights

Shading Model

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