star-phor

commit caff45f6a6d99beeeff7c85b0ed9bbbeac7e1933
parent a3f743987d61709209ed359eba7b2c1cf7d55208
Author: Eduardo Fontana Lazzari <edufonlaz@gmail.com>
Date:   Fri, 27 Feb 2026 14:10:06 +0100

Implement LAMBERT BTDF and update path sampling

Implement an internal Lambertian bidirectional transmission distribution
function (BTDF), inspired by the Lambertian reflection model in the
star-sf.

Integrate the new function into the path sampling routine.

Diffstat:
M
src/sphor_ran_bsdf.c
 | 
107
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-

1 file changed, 106 insertions(+), 1 deletion(-)
diff --git a/src/sphor_ran_bsdf.c b/src/sphor_ran_bsdf.c
@@ -30,6 +30,104 @@
 #include <star/ssf.h>
 
 /*******************************************************************************
+ * Internal BSDFs
+ ******************************************************************************/
+
+/* BTDF LAMBERTIAN */
+struct lambertian_transmission {
+  double transmissivity;
+};
+
+static double
+lambertian_transmission_eval
+  (void* data,
+   const double wo[3],
+   const double N[3],
+   const double wi[3])
+{
+  struct lambertian_transmission* btdf = data;
+
+  ASSERT(NULL != data);
+  ASSERT(NULL != N);
+  ASSERT(NULL != wi);
+  ASSERT(d3_is_normalized(N) && d3_is_normalized(wi));
+  ASSERT(d3_dot(wi, N) < 0 && d3_dot(wo, N) > 0);
+  (void)wo, (void)N, (void)wi;
+
+  return btdf->transmissivity/ PI;
+}
+
+static double
+lambertian_transmission_sample
+  (void* data,
+   struct ssp_rng* rng,
+   const double wo[3],
+   const double N[3],
+   double wi[3],
+   int* type,
+   double* pdf)
+{
+  double sample[3];
+
+  ASSERT(NULL != data);
+  ASSERT(NULL != rng);
+  ASSERT(NULL != N);
+  ASSERT(NULL != wi);
+  ASSERT(d3_is_normalized(wo) && d3_is_normalized(N) && d3_dot(wo, N) > 0);
+  (void)wo;
+
+  ssp_ran_hemisphere_cos(rng, N, sample, pdf);
+  d3_muld(wi, sample, -1);
+  if (type) *type = SSF_TRANSMISSION | SSF_DIFFUSE;
+  return ((struct lambertian_transmission*)data)->transmissivity;
+}
+
+static double
+lambertian_transmission_pdf
+  (void* data,
+   const double wo[3],
+   const double N[3],
+   const double wi[3])
+{
+  double cos_wi_N;
+  ASSERT(NULL != data);
+  ASSERT(NULL != N);
+  ASSERT(NULL != wi);
+  ASSERT(d3_is_normalized(N) && d3_is_normalized(wi));
+  (void)data, (void)wo;
+
+  cos_wi_N = d3_dot(wi, N);
+  return cos_wi_N < 0.0 ? -cos_wi_N / PI : 0.0;
+}
+
+static res_T
+lambertian_transmission_setup
+  (struct ssf_bsdf* bsdf,
+   const double transmissivity)
+{
+  void* ptr = NULL;
+
+  ASSERT(NULL != bsdf);
+  ASSERT(0 <= transmissivity && transmissivity <=1);
+
+  ssf_bsdf_get_data(bsdf, &ptr);
+  struct lambertian_transmission* data = ptr;
+  data->transmissivity = transmissivity;
+
+  return RES_OK;
+}
+
+const struct ssf_bsdf_type lambertian_transmission = {
+  NULL,
+  NULL,
+  lambertian_transmission_sample,
+  lambertian_transmission_eval,
+  lambertian_transmission_pdf,
+  sizeof(struct lambertian_transmission),
+  ALIGNOF(struct lambertian_transmission)
+};
+
+/*******************************************************************************
  * Helper functions
  ******************************************************************************/
 static res_T
@@ -134,7 +232,6 @@ setup_bsdf_transmission
   ASSERT(NULL != intersection);
 
   (void)ray;
-  (void)transmissivity;
   (void)wavelength;
 
   primitive = (struct primitive*)&intersection->position.primitive;
@@ -148,6 +245,13 @@ setup_bsdf_transmission
 
   switch (btdf_dir) {
     case SPHIN_BTDF_DIRECTION_LAMBERT:
+      res = ssf_bsdf_create
+        (sphor->allocator, &lambertian_transmission, &bsdf);
+      if (RES_OK != res) { goto error; }
+
+      res = lambertian_transmission_setup(bsdf, transmissivity);
+      if (RES_OK != res) { goto error; }
+
       break;
 
     case SPHIN_BTDF_DIRECTION_KEEP_CURRENT_DIR:
@@ -214,6 +318,7 @@ exit:
 error:
   goto exit;
 }
+
 /*******************************************************************************
  * Local functions
  ******************************************************************************/