star-sf

commit b695e6b6ce02976610a0532c485ed9aa9d72a1f0
parent d7de821f5287bd8cba8392849c5251ed27efa118
Author: Vincent Forest <vincent.forest@meso-star.com>
Date:   Thu, 19 Jul 2018 15:10:17 +0200

Fix the evaluation of the HG phase function

Diffstat:
M
src/ssf_phase_hg.c
 | 
8
+++++++-
M
src/test_ssf_phase_hg.c
 | 
9
+++++----

2 files changed, 12 insertions(+), 5 deletions(-)
diff --git a/src/ssf_phase_hg.c b/src/ssf_phase_hg.c
@@ -43,13 +43,19 @@ static double
 hg_eval(void* data, const double wo[3], const double wi[3])
 {
   const struct hg* hg = data;
+  double w[3];
   double g;
   double cos_theta;
   double denom;
   ASSERT(data && wo && wi);
   ASSERT(d3_is_normalized(wo) && d3_is_normalized(wi));
+
   g = hg->g;
-  cos_theta = d3_dot(wi, wo);
+  /* By convention wo point outward the scattering point. Revert it to point
+   * inward the scattering point in order to compute the cosine of the
+   * scattering angle */
+  d3_minus(w, wo);
+  cos_theta = d3_dot(w, wi);
   denom = 1 + g*g - 2*g*cos_theta;
   ASSERT(denom != 0);
   return 1.0/(4.0*PI) * (1 - g*g) / (denom*sqrt(denom));
diff --git a/src/test_ssf_phase_hg.c b/src/test_ssf_phase_hg.c
@@ -63,13 +63,14 @@ main(int argc, char** argv)
       ssf_phase_sample(phase, rng, wo, wi, &pdf);
       CHK(d3_is_normalized(wi));
       CHK(eq_eps(ssf_phase_pdf(phase, wo, wi), pdf, 1.e-6));
-      r = ssf_phase_eval(phase, wo, wi);
-      /* HG(theta) = 1/(4*PI) * (1 - g^2) / (1 + g^2 - 2*g*cos(theta))^3/2 */
-      ref = 1/(4*PI) * (1-g*g) / pow(1+g*g-2*g*d3_dot(wi,wo), 1.5);
-      CHK(eq_eps(r, ref, 1.e-6));
       w = d3_dot(wo, wi);
       sum_cos += w;
       sum_cos_sqr += w*w;
+
+      /* HG(theta) = 1/(4*PI) * (1 - g^2) / (1 + g^2 - 2*g*cos(theta))^3/2 */
+      r = ssf_phase_eval(phase, wo, wi);
+      ref = 1/(4*PI) * (1-g*g) / pow(1+g*g-2*g*d3_dot(wi,d3_minus(wo,wo)), 1.5);
+      CHK(eq_eps(r, ref, 1.e-6));
     }
     /* On average cos(wo, wi) should be g */
     E = sum_cos / NSAMPS;