Unity shader: Bank BRDF

Back BRDF是一种 模拟 金属表面 的 各向异性 光泽的经验模型，具体公式参照 GPU 编程与CG 语言之阳春白雪下里巴人 中120页的公式（10-14）

Shader "Custom/Bank-BRDF" {
    Properties {
        _AmbiColor ("Main Color", Color) = (1, 1, 1, 1)
        _Ak ("Ambient Coef", float) = 1
        _DiffColor ("Diff Color", Color) = (1, 1, 1, 1)
        _Dk ("Diff Coef", float) = 1
        _SpecColor ("Spec Color", Color) = (1, 1, 1, 1)
        _Sk ("Sk", float) = 1
        _Sp ("Sp", Range(0, 5)) = 1
    }

    SubShader {
        Pass {
            CGPROGRAM
            #include "UnityCG.cginc"
            #pragma vertex vert
            #pragma fragment frag

            float4 _AmbiColor;
            float _Ak;
            float4 _DiffColor;
            float _Dk;
            float4 _SpecColor;
            float _Sk;
            float _Sp;

            struct v2f {
                float4 pos : POSITION;
                float3 normal : TEXCOORD0;
                float3 light : TEXCOORD1;
                float3 view : TEXCOORD2;
                float3 targent : TEXCOORD3;
            };

            v2f vert(appdata_base v) {
                v2f o;
                o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
                o.normal = v.normal;
                o.light = ObjSpaceLightDir(v.vertex);
                o.view = ObjSpaceViewDir(v.vertex);
                o.targent = cross(v.normal, o.view);

                return o;
            }

            float4 frag(v2f i) : COLOR {
                float3 l = normalize(i.light);
                float3 t = normalize(i.targent);
                float3 v = normalize(i.view);
                float3 n = normalize(i.normal);

                float lt = dot(l, t);
                float vt = dot(v, t);
                float nl = dot(n, l);
                float nv = dot(n, v);

                float4 ambi = _AmbiColor * _Ak;
                float4 diff = _Dk * _DiffColor * saturate(nl);

                if (nl <= 0 || nv <= 0)
                    return ambi + diff;

                float p = sqrt(1 - lt * lt) * sqrt(1 - vt * vt) - lt * vt;
                float f = _Sk * pow(p, _Sp);
                float spec = f * _SpecColor * saturate(dot(l, n));

                return ambi + diff + spec;
            }
            ENDCG
        }
    } 
    FallBack "Diffuse"
}

效果如下：