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, 2 insertions, 13 deletions

diff --git a/intern/cycles/kernel/integrator/shade_light.h b/intern/cycles/kernel/integrator/shade_light.h
index 1a5ac3637f6..910e3383f51 100644
--- a/intern/cycles/kernel/integrator/shade_light.h
+++ b/intern/cycles/kernel/integrator/shade_light.h
@@ -22,19 +22,8 @@ ccl_device_inline void integrate_light(KernelGlobals kg,
   const float3 ray_D = INTEGRATOR_STATE(state, ray, D);
   const float ray_time = INTEGRATOR_STATE(state, ray, time);
 
-  /* Advance ray beyond light. */
-  /* TODO: can we make this more numerically robust to avoid reintersecting the
-   * same light in some cases? Ray should not intersect surface anymore as the
-   * object and prim ids will prevent self intersection. */
-  const float3 new_ray_P = ray_P + ray_D * isect.t;
-  INTEGRATOR_STATE_WRITE(state, ray, P) = new_ray_P;
-  INTEGRATOR_STATE_WRITE(state, ray, t) -= isect.t;
-
-  /* Set position to where the BSDF was sampled, for correct MIS PDF. */
-  const float mis_ray_t = INTEGRATOR_STATE(state, path, mis_ray_t);
-  ray_P -= ray_D * mis_ray_t;
-  isect.t += mis_ray_t;
-  INTEGRATOR_STATE_WRITE(state, path, mis_ray_t) = isect.t;
+  /* Advance ray to new start distance. */
+  INTEGRATOR_STATE_WRITE(state, ray, tmin) = intersection_t_offset(isect.t);
 
   LightSample ls ccl_optional_struct_init;
   const bool use_light_sample = light_sample_from_intersection(kg, &isect, ray_P, ray_D, &ls);