From 4b19dc9ffce638276a546ef17d51442a96377d6b Mon Sep 17 00:00:00 2001
From: Sterling Harper <smharper@mit.edu>
Date: Tue, 1 Oct 2019 11:41:18 -0400
Subject: [PATCH] Add TODOs to rotate_angle for a change in azimuth

---
 src/distribution_multi.cpp | 10 ++++++++--
 src/math_functions.cpp     |  6 ++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

diff --git a/src/distribution_multi.cpp b/src/distribution_multi.cpp
index 0c1ecddecb..c388254b6e 100644
--- a/src/distribution_multi.cpp
+++ b/src/distribution_multi.cpp
@@ -59,8 +59,14 @@ Direction PolarAzimuthal::sample() const
   double mu = mu_->sample();
   if (mu == 1.0) return u_ref_;
 
-  // Sample the azimuthal angle with an offset to match azimuth conventions
-  double phi = phi_->sample() + 0.5*PI;
+  // Sample azimuthal angle
+  double phi = phi_->sample();
+
+  // If the reference direction is along the z-axis, rotate the aziumthal angle
+  // to match spherical coordinate conventions.
+  // TODO: apply this change directly to rotate_angle
+  if (u_ref_.x == 0 && u_ref_.y == 0) phi += 0.5*PI;
+
   return rotate_angle(u_ref_, mu, &phi);
 }
 
diff --git a/src/math_functions.cpp b/src/math_functions.cpp
index ec5d64eac7..7525a7ddda 100644
--- a/src/math_functions.cpp
+++ b/src/math_functions.cpp
@@ -665,6 +665,12 @@ Direction rotate_angle(Direction u, double mu, const double* phi)
     return {mu*u.x + a*(u.x*u.y*cosphi + u.z*sinphi) / b,
             mu*u.y - a*b*cosphi,
             mu*u.z + a*(u.y*u.z*cosphi - u.x*sinphi) / b};
+    // TODO: use the following code to make PolarAzimuthal distributions match
+    // spherical coordinate conventions. Remove the related fixup code in
+    // PolarAzimuthal::sample.
+    //return {mu*u.x + a*(-u.x*u.y*sinphi + u.z*cosphi) / b,
+    //        mu*u.y + a*b*sinphi,
+    //        mu*u.z - a*(u.y*u.z*sinphi + u.x*cosphi) / b};
   }
 }