I'm trying to implement an anisotropic version of GGX in UDK. Here is what the shader looks like:

float3 H = normalize(L + V); float NdotL = saturate(dot(N, L)); float NdotV = dot(N, V); float NdotH = dot(N, H); float LdotH = dot(L, H); float HdotX = dot(H, X); float HdotY = dot(H, Y); float NdotH_2 = NdotH * NdotH; float HdotX_2 = HdotX * HdotX; float HdotY_2 = HdotY * HdotY; float ax_2 = ax * ax; float ay_2 = ay * ay; float GGX = (ax * ay * pow(HdotX_2 / ax_2 + HdotY_2 / ay_2 + NdotH_2, 2.0)); float3 F = Ks + (1.0 - Ks) * exp(-6 * LdotH); float3 Rs = F / (4 * GGX * LdotH * LdotH); float3 Rd = Kd * (1 - Ks); return (Rs + Rd) * NdotL;

The problem is I'm not sure what to use for vectors X and Y. Because this is UDK, vectors L and V are relative to the surface (tangent space), so would I simply use X = [1, 0, 0], Y = [0, 1, 0]?

**Edited by Chris_F, 12 February 2013 - 08:05 PM.**