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, 4 insertions, 5 deletions

diff --git a/intern/cycles/kernel/osl/services.cpp b/intern/cycles/kernel/osl/services.cpp
index 78c23b858c4..6b7981b7f3a 100644
--- a/intern/cycles/kernel/osl/services.cpp
+++ b/intern/cycles/kernel/osl/services.cpp
@@ -1094,10 +1094,8 @@ bool OSLRenderServices::get_background_attribute(const KernelGlobalsCPU *kg,
       ndc[0] = camera_world_to_ndc(kg, sd, sd->ray_P);
 
       if (derivatives) {
-        ndc[1] = camera_world_to_ndc(kg, sd, sd->ray_P + make_float3(sd->ray_dP, 0.0f, 0.0f)) -
-                 ndc[0];
-        ndc[2] = camera_world_to_ndc(kg, sd, sd->ray_P + make_float3(0.0f, sd->ray_dP, 0.0f)) -
-                 ndc[0];
+        ndc[1] = zero_float3();
+        ndc[2] = zero_float3();
       }
     }
     else {
@@ -1671,7 +1669,8 @@ bool OSLRenderServices::trace(TraceOpt &options,
 
   ray.P = TO_FLOAT3(P);
   ray.D = TO_FLOAT3(R);
-  ray.t = (options.maxdist == 1.0e30f) ? FLT_MAX : options.maxdist - options.mindist;
+  ray.tmin = 0.0f;
+  ray.tmax = (options.maxdist == 1.0e30f) ? FLT_MAX : options.maxdist - options.mindist;
   ray.time = sd->time;
   ray.self.object = OBJECT_NONE;
   ray.self.prim = PRIM_NONE;