Deferred Coarse Pixel Shading

# introduction
- 之前有个coarse pixel shading,大概就是在变化不大的区域可以一块是一次计算。主要是省电
- 似乎这个在forward已经有较好的解决方案了
    - 在3个level来shading
        - coarse pixel 2x2 block
        - pixel
        - sample
- 有人提出过deferred corse pixel
    - 用computed shader，work well with msaa
    - gbuffer是msaa分辨率的，有derivative
    - 然后找出哪些要以什么分辨率来shade at sample rate

# 31.3 algorithm

## 31.3.1 G-buffer generation
- 除了不同gbuffer的东西外，还存了derivate of view-space Z

```
struct GBuffer
{
    float4 normal_specular; //normal and specular power/intensity
    float4 albedo;
    float4 biased_albedo; //albedo sampled with biased sampler
    float2 positionZGrad; //ddx ddy of view space z
    float positionZ; //view space Z
}
```

## 31.3.2 shading pass
- 使用comput