Specular Color

[Alundra]

I understand from the paper of unreal that it’s the combination of 3 factor D, F and G.

float3 L = normalize( VectorToLight );
float3 H = normalize(V + L);
float NoL = saturate( dot(N, L) );
float NoV = saturate( dot(N, V) );
float NoH = saturate( dot(N, H) );
float VoH = saturate( dot(V, H) );
float D = Distribution( Roughness, NoH ) / PI;
float G = GeometricVisibility( Roughness, NoV, NoL, VoH, L, V );
float3 F = Fresnel( InGBufferData.SpecularColor, VoH );
SpecularLighting += D * G * F;

That give the screenshot I gave, but the result is not correct.
Another question is, PI is really needed in lighting equation ? and why ?

DiffuseColor / PI;

I want to understand because physical material is the new way of doing, phong is out.

EDIT:
I’ve tried the Schlick equation using as specular color :

lerp( 0.08f * Specular, FinalBaseColor.rgb, Metallic ) * Light.Color;

but on this screen :

The white part is not visible, I don’t understand why.

EDIT2:
OK, looks ok about roughness I understand now, the Schlick equation is not easy but give very good result.
The only thing now I don’t understand is the calcule of specular on the lighting and metallic to give this special color.