无题

Anisotropy

前提:

• 必须定义切线空间
• normalTex 和 anisotropyTex必须使用同一组uv

参数:

• anisotropyStrength(float): [0,1] 表示沿指定方向增加粗糙度,默认方向是切线方向
• anisotropyRoataion(float):
• anisotropyTexture(Texture)
  • r & g : anisotropy Dir in [-1,1] tangent, bitangent
  • b : anisotropy strength

映射

rg: [0,1] -> [-1,1]

directonalAlphaRoughness = mix(materialAlphaRoughness, 1, anisotropyStrength^2)

其中:

materialAlphaRoughness = materialRoughness^2

• materialRoughness: 设计师填入的粗糙度
• anisotropyStrength: 设计师填入的各向异性强度
• directionalAlphaRoughness: 各向异性计算后需要的粗糙度

D_AnisotropyGGX

定义:

• $a_t$: 沿各向异性方向上的粗糙度, 即directionalAlphaRoughness
• $a_b$: 垂直方向上的粗糙度, 即materialAlphaRoughness

UE4 实现:

// Anisotropic GGX
// [Burley 2012, "Physically-Based Shading at Disney"]
float D_GGXaniso( float ax, float ay, float NoH, float3 H, float3 X, float3 Y )
{
	float XoH = dot( X, H );
	float YoH = dot( Y, H );
	float d = XoH*XoH / (ax*ax) + YoH*YoH / (ay*ay) + NoH*NoH;
	return 1 / ( PI * ax*ay * d*d );
}

glTF实现案例:

float D_GGX_anisotropic(float NdotH, float TdotH, float BdotH, float at, float ab)
{
    float a2 = at * ab;
    vec3 f = vec3(ab * TdotH, at * BdotH, a2 * NdotH);
    float w2 = a2 / dot(f, f);
    return a2 * w2 * w2 / M_PI;
}

参考

glTF Specification

PBR White Paper