Replace Rayleigh scattering implementation with that from SNOMAN. The

angular distribution is slightly different, and now fits with the
distribution given in the GEANT4 physics reference manual.

Unit test is now included to verify the correctness of the scattering.

1 files changed, 41 insertions, 19 deletions

diff --git a/src/photon.h b/src/photon.h
index 45ad79b..e781864 100644
--- a/src/photon.h
+++ b/src/photon.h
@@ -105,30 +105,52 @@ __device__ void fill_state(State &s, Photon &p)
 
 } // fill_state
 
-__device__ void rayleigh_scatter(Photon &p, curandState &rng)
+__device__ float3 pick_new_direction(float3 axis, float theta, float phi)
 {
-	float theta, y;
-
-	while (true)
-	{
-		y = curand_uniform(&rng);
-		theta = uniform(&rng, 0, 2*PI);
-
-		if (y < powf(cosf(theta),2))
-			break;
+	// Taken from SNOMAN rayscatter.for
+	float cos_theta = cosf(theta);
+	float sin_theta = sinf(theta);
+	float cos_phi   = cosf(phi);
+	float sin_phi   = sinf(phi);
+	
+	float sin_axis_theta = sqrt(1.0f - axis.z*axis.z);
+	float cos_axis_phi, sin_axis_phi;
+	
+	if (isnan(sin_axis_theta) || sin_axis_theta < 0.00001f) {
+		cos_axis_phi = 1.0f;
+		sin_axis_phi = 0.0f;
+	} else {
+		cos_axis_phi = axis.x / sin_axis_theta;
+		sin_axis_phi = axis.y / sin_axis_theta;
 	}
 
-	float phi = uniform(&rng, 0, 2*PI);
-
-	float3 b = cross(p.polarization, p.direction);
-	float3 c = p.polarization;
-
-	p.direction = rotate(p.direction, theta, b);
-	p.direction = rotate(p.direction, phi, c);
+	return make_float3(cos_theta*axis.x + sin_theta*(axis.z*cos_phi*cos_axis_phi - sin_phi*sin_axis_phi),
+			   cos_theta*axis.y + sin_theta*(cos_phi*axis.z*sin_axis_phi - sin_phi*cos_axis_phi),
+			   cos_theta*axis.z - sin_theta*cos_phi*sin_axis_theta);
+}
 
-	p.polarization = rotate(p.polarization, theta, b);
-	p.polarization = rotate(p.polarization, phi, c);
+__device__ void rayleigh_scatter(Photon &p, curandState &rng)
+{
+	float cos_theta = 2.0f*cosf((acosf(1.0f - 2.0f*curand_uniform(&rng))-2*PI)/3.0f);
+	if (cos_theta > 1.0f)
+	  cos_theta = 1.0f;
+	else if (cos_theta < -1.0f)
+	  cos_theta = -1.0f;
+
+	float theta = acosf(cos_theta);
+	float phi = uniform(&rng, 0.0f, 2.0f * PI);
+
+	p.direction = pick_new_direction(p.polarization, theta, phi);
+
+	if (1.0f - fabsf(cos_theta) < 1e-6f) {
+		p.polarization = pick_new_direction(p.polarization, PI/2.0f, phi);
+	} else {
+		// linear combination of old polarization and new direction
+		p.polarization = p.polarization - cos_theta * p.direction;
+	}
 
+	p.direction /= norm(p.direction);
+	p.polarization /= norm(p.polarization);
 } // scatter
 
 __device__ int propagate_to_boundary(Photon &p, State &s, curandState &rng)