star-phor

commit 94a5f9e25a3f1859ec2113a70bf5fe17cdcac3f9
parent bc2abf06454e245f12a300a471eb982b61e7e542
Author: Eduardo Fontana Lazzari <edufonlaz@gmail.com>
Date:   Fri, 23 Jan 2026 12:19:35 +0100

Fix absorption handling after BRDF ray-intersection

Previously, the code assumed that ssf_bsdf_sample would return a
TRANSMISSION flag based on the reflectivity of the BRDF. This assumption
was incorrect: rays that should have been absorbed could continue,
leading to incorrect path termination.

This commit corrects the behavior by explicitly performing a Bernoulli
trial using the BRDF reflectivity. The ray is either reflected or
absorbed according to this probability.

Diffstat:
M
src/sphor_compute_mvrea.c
 | 
20
+++++++++++---------

1 file changed, 11 insertions(+), 9 deletions(-)
diff --git a/src/sphor_compute_mvrea.c b/src/sphor_compute_mvrea.c
@@ -311,7 +311,7 @@ setup_MVREA_accums
   res = setup_MVREA_volumes_accum(sphor, accum2id, accums);
   if (RES_OK != res) { goto error; }
 
-  /* One accum per sensor volume; all components of the volume contribute 
+  /* One accum per sensor volume; all components of the volume contribute
    * to the accum */
   res = setup_MVREA_per_volume_accums(sphor, accum2id, accums);
   if (RES_OK != res) { goto error; }
@@ -570,6 +570,7 @@ sample_interface_ray_interaction
   double normal[3] = {0};
   double wo[3] = {0};
   double pdf = 0;
+  double reflectivity = 0;
   double sample[3] = {0};
   int flag = 0;
   struct primitive* primitive = NULL;
@@ -586,8 +587,8 @@ sample_interface_ray_interaction
   primitive = &intersection->position.primitive;
 
   /* Ensure that the ray direction and the geometry normal are compatible with
-   * th star-sf library. */
-  d3_muld(wo, ray->direction,-1);
+   * the star-sf library. */
+  d3_muld(wo, ray->direction, -1);
   d3_set(normal, primitive->normal);
   /* Ensure normal and direction point to the same hemisphere */
   invert_normal = (d3_dot(normal, wo) > 0 ) ? 1. : -1.;
@@ -617,15 +618,16 @@ sample_interface_ray_interaction
     if (RES_OK != res) { goto error; }
   }
 
-  ssf_bsdf_sample(bsdf, rng, wo, normal, sample, &flag, &pdf);
+  reflectivity = ssf_bsdf_sample(bsdf, rng, wo, normal, sample, &flag, &pdf);
 
-  if (flag & SSF_TRANSMISSION) {
-    if (NULL != brdf) {
-      *interaction_type = RAY_INTERFACE_INTERACTION_ABSORPTION;
-    }
+  if (NULL != brdf) {
+    if (ssp_rng_canonical(rng) < reflectivity) {
+      *interaction_type = RAY_INTERFACE_INTERACTION_REFLECTION; }
     else {
-      *interaction_type = RAY_INTERFACE_INTERACTION_TRANSMISSION; }
+      *interaction_type = RAY_INTERFACE_INTERACTION_ABSORPTION; }
   }
+  else if (flag & SSF_TRANSMISSION) {
+    *interaction_type = RAY_INTERFACE_INTERACTION_TRANSMISSION; }
   else if (flag & SSF_REFLECTION) {
     *interaction_type = RAY_INTERFACE_INTERACTION_REFLECTION; }
   else { res = RES_BAD_ARG; }