static Vector3 TrowbridgeReitzSample(Vector3 wi, float alpha_x,
float alpha_y, float U1, float U2)
{
// 1. stretch wi
Vector3 wiStretched = Vector3.Normalize(new Vector3(alpha_x * wi.X, alpha_y * wi.Y, wi.Z));
// 2. simulate P22_{wi}(x_slope, y_slope, 1, 1)
float slope_x, slope_y;
TrowbridgeReitzSample11(ShadingSpace.CosTheta(wiStretched), U1, U2, out slope_x, out slope_y);
// 3. rotate
float tmp = ShadingSpace.CosPhi(wiStretched) * slope_x - ShadingSpace.SinPhi(wiStretched) * slope_y;
slope_y = ShadingSpace.SinPhi(wiStretched) * slope_x + ShadingSpace.CosPhi(wiStretched) * slope_y;
slope_x = tmp;
// 4. unstretch
slope_x = alpha_x * slope_x;
slope_y = alpha_y * slope_y;
// 5. compute normal
return(Vector3.Normalize(new Vector3(-slope_x, -slope_y, 1)));
}
