LNXSDK/leenkx/Shaders/std/brdf.glsl

#ifndef _BRDF_GLSL_
#define _BRDF_GLSL_

#ifndef PI
#define PI 3.1415926535
#endif
#ifndef INV_PI
#define INV_PI 0.3183098861
#endif
#ifndef INV_TWO_PI
#define INV_TWO_PI 0.1591549430
#endif
#ifndef SCHLICK_A
#define SCHLICK_A -5.55473
#endif
#ifndef SCHLICK_B
#define SCHLICK_B -6.98316
#endif
#ifndef SRGB_GAMMA
#define SRGB_GAMMA 2.2
#endif
#define srgbToLinear(x) pow(x, vec3(SRGB_GAMMA))

// http://xlgames-inc.github.io/posts/improvedibl/
// http://blog.selfshadow.com/publications/s2013-shading-course/
vec3 f_schlick(const vec3 f0, const float vh) {
	return f0 + (1.0 - f0) * exp2((SCHLICK_A * vh + SCHLICK_B) * vh);
}

float v_smithschlick(const float nl, const float nv, const float a) {
	return 1.0 / ((nl * (1.0 - a) + a) * (nv * (1.0 - a) + a));
}

//Uncorrelated masking/shadowing (info below) function
//Because it is uncorrelated, G1(NdotL, a) gives us shadowing, and G1(NdotV, a) gives us masking function.
//Approximation from: https://ubm-twvideo01.s3.amazonaws.com/o1/vault/gdc2017/Presentations/Hammon_Earl_PBR_Diffuse_Lighting.pdf
float g1_approx(const float NdotX, const float alpha)
{
	return (2.0 * NdotX) * (1.0 / (NdotX * (2.0 - alpha) + alpha));
}

//Uncorrelated masking-shadowing function
//Approximation from: https://ubm-twvideo01.s3.amazonaws.com/o1/vault/gdc2017/Presentations/Hammon_Earl_PBR_Diffuse_Lighting.pdf
float g2_approx(const float NdotL, const float NdotV, const float alpha)
{
	vec2 helper = (2.0 * vec2(NdotL, NdotV)) * (1.0 / (vec2(NdotL, NdotV) * (2.0 - alpha) + alpha));
	return max(helper.x * helper.y, 0.0); //This can go negative, let's fix that
}

float d_ggx(const float nh, const float a) {
	float a2 = a * a;
	float denom = nh * nh * (a2 - 1.0) + 1.0;
	denom = max(denom * denom, 0.00006103515625 /* 2^-14 = smallest possible half float value, prevent div by zero */);
	return a2 * INV_PI / denom;
}

vec3 specularBRDF(const vec3 f0, const float roughness, const float nl, const float nh, const float nv, const float vh) {
	float a = roughness * roughness;
	vec3 result = d_ggx(nh, a) * g2_approx(nl, nv, a) * f_schlick(f0, vh) / max(4.0 * nv, 1e-5); //NdotL cancels out later
	return result;
}

// John Hable - Optimizing GGX Shaders
// http://filmicworlds.com/blog/optimizing-ggx-shaders-with-dotlh/
vec3 specularBRDFb(const vec3 f0, const float roughness, const float dotNL, const float dotNH, const float dotLH) {
	// D
	float alpha = roughness * roughness;
	float alphaSqr = alpha * alpha;
	float denom = dotNH * dotNH * (alphaSqr - 1.0) + 1.0;
	float D = alphaSqr / (PI * denom * denom);
	// F
	const float F_a = 1.0;
	float F_b = pow(1.0 - dotLH, 5.0);
	// V
	float vis;
	float k = alpha / 2.0;
	float k2 = k * k;
	float invK2 = 1.0 - k2;
	vis = 1.0 / (dotLH * dotLH * invK2 + k2);
	vec2 FV_helper = vec2((F_a - F_b) * vis, F_b * vis);

	vec3 FV = f0 * FV_helper.x + FV_helper.y;
	vec3 specular = clamp(dotNL, 0.0, 1.0) * D * FV;
	return specular / 4.0; // TODO: get rid of / 4.0
}

vec3 lambertDiffuseBRDF(const vec3 albedo, const float nl) {
	return albedo * INV_PI * nl;
}

vec3 surfaceAlbedo(const vec3 baseColor, const float metalness) {
	return mix(baseColor, vec3(0.0), metalness);
}

vec3 surfaceF0(const vec3 baseColor, const float metalness) {
	return mix(vec3(0.04), baseColor, metalness);
}

float getMipFromRoughness(const float roughness, const float numMipmaps) {
	// First mipmap level = roughness 0, last = roughness = 1
	return roughness * numMipmaps;
}

// https://www.unrealengine.com/en-US/blog/physically-based-shading-on-mobile
// vec3 EnvBRDFApprox(vec3 SpecularColor, float Roughness, float NoV) {
// 	const vec4 c0 = { -1, -0.0275, -0.572, 0.022 };
// 	const vec4 c1 = { 1, 0.0425, 1.04, -0.04 };
// 	vec4 r = Roughness * c0 + c1;
// 	float a004 = min( r.x * r.x, exp2( -9.28 * NoV ) ) * r.x + r.y;
// 	vec2 AB = vec2( -1.04, 1.04 ) * a004 + r.zw;
// 	return SpecularColor * AB.x + AB.y;
// }
// float EnvBRDFApproxNonmetal(float Roughness, float NoV) {
// 	// Same as EnvBRDFApprox( 0.04, Roughness, NoV )
// 	const vec2 c0 = { -1, -0.0275 };
// 	const vec2 c1 = { 1, 0.0425 };
// 	vec2 r = Roughness * c0 + c1;
// 	return min( r.x * r.x, exp2( -9.28 * NoV ) ) * r.x + r.y;
// }
float D_Approx(const float Roughness, const float RoL) {
	float a = Roughness * Roughness;
	float a2 = a * a;
	float rcp_a2 = 1.0 / a2;//rcp(a2);
	// 0.5 / ln(2), 0.275 / ln(2)
	float c = 0.72134752 * rcp_a2 + 0.39674113;
	return rcp_a2 * exp2( c * RoL - c );
}

#ifdef _ClearCoat
float brdf_coatF0;
vec3 clearcoatBRDF(const float clearcoat, const float clearcoat_rough,
	const float coat_ior, const vec3 coatN, const vec3 l, const vec3 v, const vec3 h) {
	if (clearcoat <= 0.0) return vec3(0.0);
	float cdotNL = max(0.0, dot(coatN, l));
	float cdotNH = max(0.0, dot(coatN, h));
	float cdotNV = max(0.0, dot(coatN, v));
	float cdotVH = max(0.0, dot(v, h));
	float a = clearcoat_rough * clearcoat_rough;
	float F = brdf_coatF0 + (1.0 - brdf_coatF0) * exp2((SCHLICK_A * cdotVH + SCHLICK_B) * cdotVH);
	float D = d_ggx(cdotNH, a);
	float G = g2_approx(cdotNL, cdotNV, a);
	return vec3(clearcoat * D * G * F / max(4.0 * cdotNV, 1e-5));
}

float coatAttenuation(const float clearcoat,
	const float coat_ior, const vec3 coatN, const vec3 v) {
	if (clearcoat <= 0.0) return 1.0;
	float cdotNV = max(0.0, dot(coatN, v));
	float F = brdf_coatF0 + (1.0 - brdf_coatF0) * exp2((SCHLICK_A * cdotNV + SCHLICK_B) * cdotNV);
	return max(1.0 - F * clearcoat, 0.0);
}

vec3 coatTintAttenuation(const float clearcoat, const vec3 coat_tint, const vec3 coatN, const vec3 v) {
	if (clearcoat <= 0.0) return vec3(1.0);
	float cdotNV = max(0.0, dot(coatN, v));
	float absorption = 1.0 / max(cdotNV, 0.3);
	return mix(vec3(1.0), clamp(coat_tint, 0.0, 1.0), clamp(absorption * 0.2, 0.0, 1.0));
}
#endif

#ifdef _Sheen
float brdf_sheenAlbedo;
// based on Blender sheen model/Frostbite PBR
vec3 sheenBRDF(const float sheen, const float sheen_rough,
	const vec3 sheen_tint, const float dotNL, const float dotNH, const float dotNV) {
	if (sheen <= 0.0) return vec3(0.0);
	float rough = clamp(sheen_rough, 1e-3, 1.0);
	float a = rough * rough;
	float sinNH2 = 1.0 - dotNH * dotNH;
	float a2 = a * a;
	float denom = 1.0 + a2 * sinNH2;
	float D = (2.0 + a2) * sinNH2 * INV_TWO_PI / (denom * denom);
	float V = 1.0 / (4.0 * dotNL * dotNV + 1e-5);
	return sheen_tint * sheen * D * V * dotNL * brdf_sheenAlbedo;
}

float sheenAttenuation(const float sheen, const float sheen_rough,
	const vec3 sheen_tint, const float dotNV) {
	if (sheen <= 0.0) return 1.0;
	float maxComp = sheen * max(max(sheen_tint.r, sheen_tint.g), sheen_tint.b) * brdf_sheenAlbedo;
	return max(1.0 - maxComp, 0.0);
}
#endif

#ifdef _Anisotropy
// anisotropic GGX Burley 2012
vec3 anisotropicBRDF(const vec3 f0, const float roughness, const float anisotropy,
	const float aniso_rot, const vec3 tangent, const vec3 bitangent,
	const vec3 n, const vec3 l, const vec3 v,
	const float dotNL, const float dotNV) {
	if (abs(anisotropy) <= 0.001) return vec3(0.0);
	float rot = aniso_rot * PI * 2.0;
	float cr = cos(rot);
	float sr = sin(rot);
	vec3 t = normalize(tangent * cr + bitangent * sr);
	vec3 b = normalize(bitangent * cr - tangent * sr);
	float aniso_abs = abs(anisotropy);
	float at = max(roughness * (1.0 + aniso_abs), 1e-5);
	float ab = max(roughness * (1.0 - aniso_abs), 1e-5);
	if (anisotropy < 0.0) { vec3 tmp = t; t = b; b = tmp; }
	float at2 = at * at;
	float ab2 = ab * ab;
	vec3 h = normalize(l + v);
	float dotTH = dot(t, h);
	float dotBH = dot(b, h);
	float dotTV = dot(t, v);
	float dotBV = dot(b, v);
	float dotTL = dot(t, l);
	float dotBL = dot(b, l);
	float denom = max(dotTH * dotTH / at2 + dotBH * dotBH / ab2, 1e-7);
	float D = INV_PI / (at * ab * denom * denom);
	float V = 1.0 / max(dotNL * (dotTL / at + dotBL / ab) * (dotTV / at + dotBV / ab), 1e-5);
	float dotVH = max(dot(v, h), 0.0);
	vec3 F = f_schlick(f0, dotVH);
	return D * V * F / max(4.0 * dotNV, 1e-5);
}
#endif

#ifdef _Transmission
float brdf_transmissionF0;
// Blenders microfacet glass/refraction model
vec3 transmissionBRDF(const vec3 albedo, const float transmission,
	const float trans_rough, const float ior, const float thin_wall,
	const float dotNL, const float dotNV, const float dotVH) {
	if (transmission <= 0.0) return vec3(0.0);
	float F = brdf_transmissionF0 + (1.0 - brdf_transmissionF0) * exp2((SCHLICK_A * dotVH + SCHLICK_B) * dotVH);
	float transmittance = 1.0 - F;
	if (thin_wall > 0.5) {
		return albedo * transmission * transmittance * dotNL;
	}
	float a = trans_rough * trans_rough;
	float rough_atten = min(mix(1.0, 1.0 / max(dotNV, 0.1), a), 4.0);
	return albedo * transmission * transmittance * rough_atten * dotNL;
}
#endif

#ifdef _ExtBRDF
float brdf_sheenWeight = 1.0;
float brdf_coatWeight = 1.0;
vec3 brdf_coatTintAbsorb = vec3(1.0);

vec3 applyExtBRDFLayers(
	const vec3 direct,
	const vec3 albedo,
	const vec3 f0,
	const float roughness,
	const float dotNL, const float dotNV, const float dotNH, const float dotVH,
	const vec3 n, const vec3 l, const vec3 v, const vec3 h,
	#ifdef _ClearCoat
	const float clearcoat, const float clearcoatRough, const float coatIOR,
	const vec3 coatTint, const vec3 coatN,
	#endif
	#ifdef _Sheen
	const float sheen, const float sheenRough, const vec3 sheenTint,
	#endif
	#ifdef _Transmission
	const float transmission, const float transRough, const float ior, const float thinWall,
	#endif
	out float layerWeight
) {
	float sheenWeight = brdf_sheenWeight;
	float coatWeight = brdf_coatWeight;
	#ifdef _Sheen
	vec3 sheenContrib = sheenBRDF(sheen, sheenRough, sheenTint, dotNL, dotNH, dotNV);
	#endif
	#ifdef _ClearCoat
	vec3 coatContrib = clearcoatBRDF(clearcoat, clearcoatRough, coatIOR, coatN, l, v, h);
	#endif
	layerWeight = sheenWeight * coatWeight;
	vec3 result = direct * layerWeight;
	#ifdef _Transmission
	result += transmissionBRDF(albedo, transmission, transRough, ior, thinWall, dotNL, dotNV, dotVH) * layerWeight;
	#endif
	#ifdef _ClearCoat
	result *= brdf_coatTintAbsorb;
	result += coatContrib * sheenWeight;
	#endif
	#ifdef _Sheen
	result += sheenContrib;
	#endif
	return result;
}
#endif

#ifdef _ClearCoat
float coatIBLFresnel(const float clearcoat, const float coat_ior,
	const float dotNV_coat) {
	if (clearcoat <= 0.0) return 0.0;
	float F = brdf_coatF0 + (1.0 - brdf_coatF0) * exp2((SCHLICK_A * dotNV_coat + SCHLICK_B) * dotNV_coat);
	return F * clearcoat;
}
#endif

#ifdef _Sheen
float sheenIBLAlbedo(const float sheen, const float sheen_rough,
	const float dotNV) {
	if (sheen <= 0.0) return 0.0;
	float rough = clamp(sheen_rough, 1e-3, 1.0);
	return sheen * (1.0 - 0.5 * rough) * mix(1.0, dotNV, 0.5);
}
#endif

#ifdef _Anisotropy
vec3 anisotropicIBLDirection(const vec3 n, const vec3 v, const vec3 tangent,
	const float anisotropy, const float roughness) {
	if (abs(anisotropy) <= 0.001 || dot(tangent, tangent) < 0.001)
		return reflect(-v, n);
	vec3 bitangent = normalize(cross(n, tangent));
	vec3 r = reflect(-v, n);
	float aniso_abs = abs(anisotropy);
	vec3 stretchDir = anisotropy > 0.0 ? tangent : bitangent;
	float stretchAmt = aniso_abs * roughness;
	return normalize(r + stretchDir * stretchAmt * dot(r, stretchDir) * 0.5);
}
#endif

#ifdef _Transmission
float transmissionIBLFresnel(const float ior, const float dotNV) {
	return brdf_transmissionF0 + (1.0 - brdf_transmissionF0) * exp2((SCHLICK_A * dotNV + SCHLICK_B) * dotNV);
}

vec3 transmissionIBLDirection(const vec3 n, const vec3 v, const float ior) {
	float eta = 1.0 / ior;
	vec3 refrDir = refract(-v, n, eta);
	if (dot(refrDir, refrDir) < 0.001) {
		refrDir = reflect(-v, n);
	}
	return refrDir;
}
#endif

#endif