public override (SurfaceInteraction, double) Sample(SurfaceInteraction _si)
{
SurfaceInteraction si = WorldToObject.Apply(_si);
double dc2 = si.Point.LengthSquared();
if (dc2 <= Radius * Radius)
{
// Point inside sphere
return(base.Sample(_si));
}
// Point outside sphere
double dc = Math.Sqrt(dc2);
double sinThetaMax = Radius / dc;
double cosThetaMax = Utils.SinToCos(sinThetaMax);
// Determine theta and phi for uniform cone sampling
double cosTheta = (cosThetaMax - 1) * Samplers.ThreadSafeRandom.NextDouble() + 1;
double sinTheta = Utils.CosToSin(cosTheta);
double phi = Samplers.ThreadSafeRandom.NextDouble() * 2.0 * Math.PI;
// Distance between reference point and sample point on sphere
double ds = dc * cosTheta - Math.Sqrt(Math.Max(0, Radius * Radius - dc2 * sinTheta * sinTheta));
// Kosinusni zakon
double cosAlpha = (dc2 + Radius * Radius - ds * ds) / (2 * dc * Radius);
double sinAlpha = Utils.CosToSin(cosAlpha);
// Construct coordinate system and use phi and theta as spherical coordinates to get point on sphere
Vector3 wcZ = si.Point.Clone().Normalize();
(Vector3 wcX, Vector3 wcY) = Utils.CoordinateSystem(wcZ);
Vector3 nObj = Utils.SphericalDirection(sinAlpha, cosAlpha, phi, wcX, wcY, wcZ);
Vector3 pObj = nObj * Radius;
// Surface interaction
Vector3 dpdu = new Vector3(-nObj.y, nObj.x, 0.0);
SurfaceInteraction siSample = new SurfaceInteraction(pObj, nObj, Vector3.ZeroVector, dpdu, this);
// Uniform cone PDF
double pdf = Samplers.UniformConePdf(cosThetaMax);
return(ObjectToWorld.Apply(siSample), pdf);
}