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, 20 insertions, 27 deletions

diff --git a/intern/cycles/kernel/integrator/shade_surface.h b/intern/cycles/kernel/integrator/shade_surface.h
index 91e34968148..1514b3956ad 100644
--- a/intern/cycles/kernel/integrator/shade_surface.h
+++ b/intern/cycles/kernel/integrator/shade_surface.h
@@ -77,7 +77,7 @@ ccl_device_forceinline void integrate_surface_emission(KernelGlobals kg,
 #  endif
   {
     const float bsdf_pdf = INTEGRATOR_STATE(state, path, mis_ray_pdf);
-    const float t = sd->ray_length + INTEGRATOR_STATE(state, path, mis_ray_t);
+    const float t = sd->ray_length;
 
     /* Multiple importance sampling, get triangle light pdf,
      * and compute weight with respect to BSDF pdf. */
@@ -323,16 +323,21 @@ ccl_device_forceinline int integrate_surface_bsdf_bssrdf_bounce(
     return LABEL_NONE;
   }
 
-  /* Setup ray. Note that clipping works through transparent bounces. */
-  INTEGRATOR_STATE_WRITE(state, ray, P) = sd->P;
-  INTEGRATOR_STATE_WRITE(state, ray, D) = normalize(bsdf_omega_in);
-  INTEGRATOR_STATE_WRITE(state, ray, t) = (label & LABEL_TRANSPARENT) ?
-                                              INTEGRATOR_STATE(state, ray, t) - sd->ray_length :
-                                              FLT_MAX;
+  if (label & LABEL_TRANSPARENT) {
+    /* Only need to modify start distance for transparent. */
+    INTEGRATOR_STATE_WRITE(state, ray, tmin) = intersection_t_offset(sd->ray_length);
+  }
+  else {
+    /* Setup ray with changed origin and direction. */
+    INTEGRATOR_STATE_WRITE(state, ray, P) = sd->P;
+    INTEGRATOR_STATE_WRITE(state, ray, D) = normalize(bsdf_omega_in);
+    INTEGRATOR_STATE_WRITE(state, ray, tmin) = 0.0f;
+    INTEGRATOR_STATE_WRITE(state, ray, tmax) = FLT_MAX;
 #ifdef __RAY_DIFFERENTIALS__
-  INTEGRATOR_STATE_WRITE(state, ray, dP) = differential_make_compact(sd->dP);
-  INTEGRATOR_STATE_WRITE(state, ray, dD) = differential_make_compact(bsdf_domega_in);
+    INTEGRATOR_STATE_WRITE(state, ray, dP) = differential_make_compact(sd->dP);
+    INTEGRATOR_STATE_WRITE(state, ray, dD) = differential_make_compact(bsdf_domega_in);
 #endif
+  }
 
   /* Update throughput. */
   float3 throughput = INTEGRATOR_STATE(state, path, throughput);
@@ -349,12 +354,8 @@ ccl_device_forceinline int integrate_surface_bsdf_bssrdf_bounce(
   }
 
   /* Update path state */
-  if (label & LABEL_TRANSPARENT) {
-    INTEGRATOR_STATE_WRITE(state, path, mis_ray_t) += sd->ray_length;
-  }
-  else {
+  if (!(label & LABEL_TRANSPARENT)) {
     INTEGRATOR_STATE_WRITE(state, path, mis_ray_pdf) = bsdf_pdf;
-    INTEGRATOR_STATE_WRITE(state, path, mis_ray_t) = 0.0f;
     INTEGRATOR_STATE_WRITE(state, path, min_ray_pdf) = fminf(
         bsdf_pdf, INTEGRATOR_STATE(state, path, min_ray_pdf));
   }
@@ -371,17 +372,8 @@ ccl_device_forceinline int integrate_surface_volume_only_bounce(IntegratorState
     return LABEL_NONE;
   }
 
-  /* Setup ray position, direction stays unchanged. */
-  INTEGRATOR_STATE_WRITE(state, ray, P) = sd->P;
-
-  /* Clipping works through transparent. */
-  INTEGRATOR_STATE_WRITE(state, ray, t) -= sd->ray_length;
-
-#  ifdef __RAY_DIFFERENTIALS__
-  INTEGRATOR_STATE_WRITE(state, ray, dP) = differential_make_compact(sd->dP);
-#  endif
-
-  INTEGRATOR_STATE_WRITE(state, path, mis_ray_t) += sd->ray_length;
+  /* Only modify start distance. */
+  INTEGRATOR_STATE_WRITE(state, ray, tmin) = intersection_t_offset(sd->ray_length);
 
   return LABEL_TRANSMIT | LABEL_TRANSPARENT;
 }
@@ -432,7 +424,8 @@ ccl_device_forceinline void integrate_surface_ao(KernelGlobals kg,
   Ray ray ccl_optional_struct_init;
   ray.P = shadow_ray_offset(kg, sd, ao_D, &skip_self);
   ray.D = ao_D;
-  ray.t = kernel_data.integrator.ao_bounces_distance;
+  ray.tmin = 0.0f;
+  ray.tmax = kernel_data.integrator.ao_bounces_distance;
   ray.time = sd->time;
   ray.self.object = (skip_self) ? sd->object : OBJECT_NONE;
   ray.self.prim = (skip_self) ? sd->prim : PRIM_NONE;
@@ -616,7 +609,7 @@ ccl_device_forceinline void integrator_shade_surface(KernelGlobals kg,
           kg, state, current_kernel, DEVICE_KERNEL_INTEGRATOR_INTERSECT_SUBSURFACE);
     }
     else {
-      kernel_assert(INTEGRATOR_STATE(state, ray, t) != 0.0f);
+      kernel_assert(INTEGRATOR_STATE(state, ray, tmax) != 0.0f);
       integrator_path_next(kg, state, current_kernel, DEVICE_KERNEL_INTEGRATOR_INTERSECT_CLOSEST);
     }
   }