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, 13 insertions, 7 deletions

diff --git a/intern/cycles/kernel/camera/camera.h b/intern/cycles/kernel/camera/camera.h
index 7e1b1c037e9..926ccf7b86f 100644
--- a/intern/cycles/kernel/camera/camera.h
+++ b/intern/cycles/kernel/camera/camera.h
@@ -165,9 +165,11 @@ ccl_device void camera_sample_perspective(KernelGlobals kg,
   float nearclip = kernel_data.cam.nearclip * z_inv;
   ray->P += nearclip * ray->D;
   ray->dP += nearclip * ray->dD;
-  ray->t = kernel_data.cam.cliplength * z_inv;
+  ray->tmin = 0.0f;
+  ray->tmax = kernel_data.cam.cliplength * z_inv;
 #else
-  ray->t = FLT_MAX;
+  ray->tmin = 0.0f;
+  ray->tmax = FLT_MAX;
 #endif
 }
 
@@ -231,9 +233,11 @@ ccl_device void camera_sample_orthographic(KernelGlobals kg,
 
 #ifdef __CAMERA_CLIPPING__
   /* clipping */
-  ray->t = kernel_data.cam.cliplength;
+  ray->tmin = 0.0f;
+  ray->tmax = kernel_data.cam.cliplength;
 #else
-  ray->t = FLT_MAX;
+  ray->tmin = 0.0f;
+  ray->tmax = FLT_MAX;
 #endif
 }
 
@@ -258,7 +262,7 @@ ccl_device_inline void camera_sample_panorama(ccl_constant KernelCamera *cam,
 
   /* indicates ray should not receive any light, outside of the lens */
   if (is_zero(D)) {
-    ray->t = 0.0f;
+    ray->tmax = 0.0f;
     return;
   }
 
@@ -349,9 +353,11 @@ ccl_device_inline void camera_sample_panorama(ccl_constant KernelCamera *cam,
   float nearclip = cam->nearclip;
   ray->P += nearclip * ray->D;
   ray->dP += nearclip * ray->dD;
-  ray->t = cam->cliplength;
+  ray->tmin = 0.0f;
+  ray->tmax = cam->cliplength;
 #else
-  ray->t = FLT_MAX;
+  ray->tmin = 0.0f;
+  ray->tmax = FLT_MAX;
 #endif
 }