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前言

我们在这篇文章中，继续来继续解析BRDF中的高配置情况（ UNITY_PBS_USE_BRDF1）下，迪士尼的漫反射计算

• Unity中Shader的BRDF解析（一）

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一、回顾一下上一篇的结尾

• 在上篇文章中，我们解析到了BRDF的具体计算

二、我们来解析一下 UNITY_PBS_USE_BRDF1（高配）

1、迪士尼BRDF原则

我们在该函数的最后能找到与迪士尼BRDF原则对应的程序：

2、迪士尼的漫反射函数

// Specular term
// HACK: theoretically we should divide diffuseTerm by Pi and not multiply specularTerm!
// 理论上漫反射项中应该除以 PI，但是由于以下两个原因没有这样做
// BUT 1) that will make shader look significantly darker than Legacy ones
//原因一:这样会导致最终效果偏暗
// and 2) on engine side “Non-important” lights have to be divided by Pi too in cases when they are injected into ambient SH
//原因二:当引擎光照为 不重要光照 时，进行球谐光照计算，会再除以一个 PI。所以，在Unity计算迪士尼漫反射时，不除以PI

我们主要来看，他用到的参数是什么

3、参数：perceptualRoughness（感性的粗糙度）

//感性的粗糙的 = 1 - smoothness
float perceptualRoughness = SmoothnessToPerceptualRoughness (smoothness);

4、参数：halfDir （半角向量）

//半角向量（一般用 H 表示）: H = 光线向量 + 视线向量（此处的 光线向量 和 视线向量 为单位向量，根据向量相加的四边形法则得出半角向量）
float3 halfDir = Unity_SafeNormalize1 (float3(light.dir) + viewDir);

5、参数：nv(法线向量 点积 视线向量)

	//法线 与 视线的点积在可见像素上不应该出现负值，但是他有可能发生在 投影 与 法线 映射 时
    //所以，可以通过某些方式来修正，但是会产生额外的指令运算
    //替代方案采用abs的形式，同样可以工作只是正确性少一些    
    // NdotV should not be negative for visible pixels, but it can happen due to perspective projection and normal mapping
    // In this case normal should be modified to become valid (i.e facing camera) and not cause weird artifacts.
    // but this operation adds few ALU and users may not want it. Alternative is to simply take the abs of NdotV (less correct but works too).
    // Following define allow to control this. Set it to 0 if ALU is critical on your platform.
    // This correction is interesting for GGX with SmithJoint visibility function because artifacts are more visible in this case due to highlight edge of rough surface
    // Edit: Disable this code by default for now as it is not compatible with two sided lighting used in SpeedTree.
    #define UNITY_HANDLE_CORRECTLY_NEGATIVE_NDOTV 0

    #if UNITY_HANDLE_CORRECTLY_NEGATIVE_NDOTV
        // The amount we shift the normal toward the view vector is defined by the dot product.
        half shiftAmount = dot(normal, viewDir);
        normal = shiftAmount < 0.0f ? normal + viewDir * (-shiftAmount + 1e-5f) : normal;
        // A re-normalization should be applied here but as the shift is small we don't do it to save ALU.
        //normal = normalize(normal);

        float nv = saturate(dot(normal, viewDir)); // TODO: this saturate should no be necessary here
    #else
        half nv = abs(dot(normal, viewDir));    // This abs allow to limit artifact
    #endif

6、参数：nl、nh、lv、lh

//其他向量之间的点积
float nl = saturate(dot(normal, light.dir));//法线 点积 光线
float nh = saturate(dot(normal, halfDir));//法线 点积 半角
half lv = saturate(dot(light.dir, viewDir));//光线 点积 视线
half lh = saturate(dot(light.dir, halfDir));//光线 点积 半角