 
 #elif defined(SPECULAR_SCHLICK_GGX)
 		// shlick+ggx as default
-		float alpha = roughness * roughness;
 
 		vec3 H = normalize(V + L);
 

		float ay = ry*ry;
		float XdotH = dot( T, H );
		float YdotH = dot( B, H );
		float denom = XdotH*XdotH / (ax*ax) + YdotH*YdotH / (ay*ay) + cNdotH*cNdotH;
		float D = 1.0 / ( M_PI * ax*ay * denom*denom );

#else
		float alphaSqr = alpha * alpha;
		float denom = cNdotH * cNdotH * (alphaSqr - 1.0) + 1.0;
		float D = alphaSqr / (M_PI * denom * denom);
#endif
		// F
		float F0 = 1.0; // FIXME
		float cLdotH5 = SchlickFresnel(cLdotH);
		float F = mix(cLdotH5, 1.0, F0);

		// V
		float k = alpha / 2.0f;
		float vis = G1V(cNdotL, k) * G1V(cNdotV, k);

		float speci = cNdotL * D * F * vis;

		specular_light += speci * light_color * specular_blob_intensity * attenuation;
#endif

#if defined(LIGHT_USE_CLEARCOAT)


#endif


// This returns the G_GGX function divided by 2 cos_theta_m, where in practice cos_theta_m is either N.L or N.V.
// We're dividing this factor off because the overall term we'll end up looks like
// (see, for example, the first unnumbered equation in B. Burley, "Physically Based Shading at Disney", SIGGRAPH 2012):
//
//   F(L.V) D(N.H) G(N.L) G(N.V) / (4 N.L N.V)
//
// We're basically regouping this as
//
//   F(L.V) D(N.H) [G(N.L)/(2 N.L)] [G(N.V) / (2 N.V)]
//
// and thus, this function implements the [G(N.m)/(2 N.m)] part with m = L or V.
//
// The contents of the D and G (G1) functions (GGX) are taken from
// E. Heitz, "Understanding the Masking-Shadowing Function in Microfacet-Based BRDFs", J. Comp. Graph. Tech. 3 (2) (2014).
// Eqns 71-72 and 85-86 (see also Eqns 43 and 80).

float G_GGX_2cos(float cos_theta_m, float alpha) {
	// Schlick's approximation
	// C. Schlick, "An Inexpensive BRDF Model for Physically-based Rendering", Computer Graphics Forum. 13 (3): 233 (1994)
	// Eq. (19), although see Heitz (2014) the about the problems with his derivation.
	// It nevertheless approximates GGX well with k = alpha/2.
	float k = 0.5*alpha;
	return 0.5 / (cos_theta_m * (1.0 - k) + k);

	// float cos2 = cos_theta_m*cos_theta_m;
	// float sin2 = (1.0-cos2);
	// return 1.0 /( cos_theta_m + sqrt(cos2 + alpha*alpha*sin2) );
}

float D_GXX(float cos_theta_m, float alpha) {
	float alpha2 = alpha*alpha;
	float d = 1.0 + (alpha2-1.0)*cos_theta_m*cos_theta_m;
	return alpha2/(M_PI * d * d);
}

float G_GGX_anisotropic_2cos(float cos_theta_m, float alpha_x, float alpha_y, float cos_phi, float sin_phi) {
	float cos2 = cos_theta_m * cos_theta_m;
	float sin2 = (1.0-cos2);
	float s_x = alpha_x * cos_phi;
	float s_y = alpha_y * sin_phi;
	return 1.0  / (cos_theta_m + sqrt(cos2 + (s_x*s_x + s_y*s_y)*sin2 ));
}

float D_GXX_anisotropic(float cos_theta_m, float alpha_x, float alpha_y, float cos_phi, float sin_phi) {
	float cos2 = cos_theta_m * cos_theta_m;
	float sin2 = (1.0-cos2);
	float r_x = cos_phi/alpha_x;
	float r_y = sin_phi/alpha_y;
	float d = cos2 + sin2*(r_x * r_x + r_y * r_y);
	return 1.0 / (M_PI * alpha_x * alpha_y * d * d );
}


