GGX Specualr

http://www.gamedev.net/topic/642501-metal-shader-correct/

 

 

 

 

float NdotH_2 = NdotH * NdotH;

float a_2 = a * a;

float D = a_2 / pow(NdotH_2 * (a_2 - 1.0) + 1.0, 2.0);

http://www.filmicworlds.com/2014/04/21/optimizing-ggx-shaders-with-dotlh/

 
Hey guys, I've been trying to implement a phsically based metal kind of shader and I was wondering if what I'm doing is correct.
 
Have a look at my shader:
	// Compute half-way-vector
	float3 H = (V + L);
	H = normalize(H); // normalize

	// Specular Fresnel Term approximation by Schlick
	float LdotH = saturate(dot(L, H));
	float3 Fresnel_spec_multiplier = 1.0f - FresnelReflectance;
	float3 Fresnel_spec = FresnelReflectance + exp2(log2(1.0f - LdotH) * 5 + log2(Fresnel_spec_multiplier));

	// Normalized Blinn-Phong NDF (D)
	float D = 0.0f;
	float NdotH = saturate(dot(N, H));
	float NormalizationFactor = ((glossiness + 2) / 2);
	D = exp2(log2(NdotH) * glossiness + log2(NormalizationFactor)); // NormalizationFactor * pow(NdotH, glossiness)

	// Cook-Torrance geometry term approximation by Kelemen & Szirmay-Kalos
	float G = 1.0f / (LdotH * LdotH + 0.0000001f);

	// Microfacet BRDF f(l,v)
	float3 f_microfacet = (Fresnel_spec * G * D) / 4.0f;

	Radiance += f_microfacet * Li * Cosine;	
	Radiance += ReflectionColor * Fresnel_spec;	
 
As you can see I'm already using a custom microfacet brdf using an approximation of the cook-torrance geometry term and the fresnel factor but for the NDF just a regular blinn-phong model. Is the cook-torrance NDF actually the beckmann distribution function or is it something different ? If yes then should I rather use a precomputed distribution texture for this like it's found on here: http://http.developer.nvidia.com/GPUGems3/gpugems3_ch14.html ?
 
I've found this code segment for the cook-torrance NDF on pixars renderman page:
float alpha = acos(NdotH);
float D = gaussConstant*exp(-(alpha*alpha)/(m*m));
 
 
alpha in this case would be NdotH and m is the microfacet H ? And the guassian constant would be something like 0.8346...? (according to what I found on wikipedia).