Cycles: refactor rays to have start and end distance, fix precision issues

For transparency, volume and light intersection rays, adjust these distances
rather than the ray start position. This way we increment the start distance
by the smallest possible float increment to avoid self intersections, and be
sure it works as the distance compared to be will be exactly the same as
before, due to the ray start position and direction remaining the same.

Fix T98764, T96537, hair ray tracing precision issues.

Differential Revision: https://developer.blender.org/D15455

1 files changed, 11 insertions, 11 deletions

diff --git a/intern/cycles/kernel/integrator/mnee.h b/intern/cycles/kernel/integrator/mnee.h
index 70b009d3b48..7a6f866b1a0 100644
--- a/intern/cycles/kernel/integrator/mnee.h
+++ b/intern/cycles/kernel/integrator/mnee.h
@@ -442,6 +442,7 @@ ccl_device_forceinline bool mnee_newton_solver(KernelGlobals kg,
   projection_ray.self.light_prim = PRIM_NONE;
   projection_ray.dP = differential_make_compact(sd->dP);
   projection_ray.dD = differential_zero_compact();
+  projection_ray.tmin = 0.0f;
   projection_ray.time = sd->time;
   Intersection projection_isect;
 
@@ -505,8 +506,8 @@ ccl_device_forceinline bool mnee_newton_solver(KernelGlobals kg,
         projection_ray.self.prim = pv.prim;
         projection_ray.P = pv.p;
       }
-      projection_ray.D = normalize_len(tentative_p - projection_ray.P, &projection_ray.t);
-      projection_ray.t *= MNEE_PROJECTION_DISTANCE_MULTIPLIER;
+      projection_ray.D = normalize_len(tentative_p - projection_ray.P, &projection_ray.tmax);
+      projection_ray.tmax *= MNEE_PROJECTION_DISTANCE_MULTIPLIER;
 
       bool projection_success = false;
       for (int isect_count = 0; isect_count < MNEE_MAX_INTERSECTION_COUNT; isect_count++) {
@@ -525,8 +526,7 @@ ccl_device_forceinline bool mnee_newton_solver(KernelGlobals kg,
 
         projection_ray.self.object = projection_isect.object;
         projection_ray.self.prim = projection_isect.prim;
-        projection_ray.P += projection_isect.t * projection_ray.D;
-        projection_ray.t -= projection_isect.t;
+        projection_ray.tmin = intersection_t_offset(projection_isect.t);
       }
       if (!projection_success) {
         reduce_stepsize = true;
@@ -858,6 +858,7 @@ ccl_device_forceinline bool mnee_path_contribution(KernelGlobals kg,
   Ray probe_ray;
   probe_ray.self.light_object = ls->object;
   probe_ray.self.light_prim = ls->prim;
+  probe_ray.tmin = 0.0f;
   probe_ray.dP = differential_make_compact(sd->dP);
   probe_ray.dD = differential_zero_compact();
   probe_ray.time = sd->time;
@@ -873,13 +874,13 @@ ccl_device_forceinline bool mnee_path_contribution(KernelGlobals kg,
     ccl_private const ManifoldVertex &v = vertices[vi];
 
     /* Check visibility. */
-    probe_ray.D = normalize_len(v.p - probe_ray.P, &probe_ray.t);
+    probe_ray.D = normalize_len(v.p - probe_ray.P, &probe_ray.tmax);
     if (scene_intersect(kg, &probe_ray, PATH_RAY_TRANSMIT, &probe_isect)) {
       int hit_object = (probe_isect.object == OBJECT_NONE) ?
                            kernel_data_fetch(prim_object, probe_isect.prim) :
                            probe_isect.object;
       /* Test whether the ray hit the appropriate object at its intended location. */
-      if (hit_object != v.object || fabsf(probe_ray.t - probe_isect.t) > MNEE_MIN_DISTANCE)
+      if (hit_object != v.object || fabsf(probe_ray.tmax - probe_isect.t) > MNEE_MIN_DISTANCE)
         return false;
     }
     probe_ray.self.object = v.object;
@@ -958,15 +959,16 @@ ccl_device_forceinline int kernel_path_mnee_sample(KernelGlobals kg,
   probe_ray.self.light_object = ls->object;
   probe_ray.self.light_prim = ls->prim;
   probe_ray.P = sd->P;
+  probe_ray.tmin = 0.0f;
   if (ls->t == FLT_MAX) {
     /* Distant / env light. */
     probe_ray.D = ls->D;
-    probe_ray.t = ls->t;
+    probe_ray.tmax = ls->t;
   }
   else {
     /* Other lights, avoid self-intersection. */
     probe_ray.D = ls->P - probe_ray.P;
-    probe_ray.D = normalize_len(probe_ray.D, &probe_ray.t);
+    probe_ray.D = normalize_len(probe_ray.D, &probe_ray.tmax);
   }
   probe_ray.dP = differential_make_compact(sd->dP);
   probe_ray.dD = differential_zero_compact();
@@ -1048,9 +1050,7 @@ ccl_device_forceinline int kernel_path_mnee_sample(KernelGlobals kg,
 
     probe_ray.self.object = probe_isect.object;
     probe_ray.self.prim = probe_isect.prim;
-    probe_ray.P += probe_isect.t * probe_ray.D;
-    if (ls->t != FLT_MAX)
-      probe_ray.t -= probe_isect.t;
+    probe_ray.tmin = intersection_t_offset(probe_isect.t);
   };
 
   /* Mark the manifold walk invalid to keep mollification on by default. */