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, 17 deletions

diff --git a/intern/cycles/util/math_intersect.h b/intern/cycles/util/math_intersect.h
index b0de0b25a45..c5b1cd51030 100644
--- a/intern/cycles/util/math_intersect.h
+++ b/intern/cycles/util/math_intersect.h
@@ -10,7 +10,8 @@ CCL_NAMESPACE_BEGIN
 
 ccl_device bool ray_sphere_intersect(float3 ray_P,
                                      float3 ray_D,
-                                     float ray_t,
+                                     float ray_tmin,
+                                     float ray_tmax,
                                      float3 sphere_P,
                                      float sphere_radius,
                                      ccl_private float3 *isect_P,
@@ -33,7 +34,7 @@ ccl_device bool ray_sphere_intersect(float3 ray_P,
       return false;
     }
     const float t = tp - sqrtf(radiussq - dsq); /* pythagoras */
-    if (t < ray_t) {
+    if (t > ray_tmin && t < ray_tmax) {
       *isect_t = t;
       *isect_P = ray_P + ray_D * t;
       return true;
@@ -44,7 +45,8 @@ ccl_device bool ray_sphere_intersect(float3 ray_P,
 
 ccl_device bool ray_aligned_disk_intersect(float3 ray_P,
                                            float3 ray_D,
-                                           float ray_t,
+                                           float ray_tmin,
+                                           float ray_tmax,
                                            float3 disk_P,
                                            float disk_radius,
                                            ccl_private float3 *isect_P,
@@ -59,7 +61,7 @@ ccl_device bool ray_aligned_disk_intersect(float3 ray_P,
   }
   /* Compute t to intersection point. */
   const float t = -disk_t / div;
-  if (t < 0.0f || t > ray_t) {
+  if (!(t > ray_tmin && t < ray_tmax)) {
     return false;
   }
   /* Test if within radius. */
@@ -74,7 +76,8 @@ ccl_device bool ray_aligned_disk_intersect(float3 ray_P,
 
 ccl_device bool ray_disk_intersect(float3 ray_P,
                                    float3 ray_D,
-                                   float ray_t,
+                                   float ray_tmin,
+                                   float ray_tmax,
                                    float3 disk_P,
                                    float3 disk_N,
                                    float disk_radius,
@@ -92,7 +95,8 @@ ccl_device bool ray_disk_intersect(float3 ray_P,
     }
     float3 P = ray_P + t * ray_D;
     float3 T = P - disk_P;
-    if (dot(T, T) < sqr(disk_radius) /*&& t > 0.f*/ && t <= ray_t) {
+
+    if (dot(T, T) < sqr(disk_radius) && (t > ray_tmin && t < ray_tmax)) {
       *isect_P = ray_P + t * ray_D;
       *isect_t = t;
       return true;
@@ -103,7 +107,8 @@ ccl_device bool ray_disk_intersect(float3 ray_P,
 
 ccl_device_forceinline bool ray_triangle_intersect(float3 ray_P,
                                                    float3 ray_dir,
-                                                   float ray_t,
+                                                   float ray_tmin,
+                                                   float ray_tmax,
                                                    const float3 tri_a,
                                                    const float3 tri_b,
                                                    const float3 tri_c,
@@ -149,16 +154,14 @@ ccl_device_forceinline bool ray_triangle_intersect(float3 ray_P,
 
   /* Perform depth test. */
   const float T = dot3(v0, Ng);
-  const int sign_den = (__float_as_int(den) & 0x80000000);
-  const float sign_T = xor_signmask(T, sign_den);
-  if ((sign_T < 0.0f) || (sign_T > ray_t * xor_signmask(den, sign_den))) {
+  const float t = T / den;
+  if (!(t >= ray_tmin && t <= ray_tmax)) {
     return false;
   }
 
-  const float inv_den = 1.0f / den;
-  *isect_u = U * inv_den;
-  *isect_v = V * inv_den;
-  *isect_t = T * inv_den;
+  *isect_u = U / den;
+  *isect_v = V / den;
+  *isect_t = t;
   return true;
 
 #undef dot3
@@ -171,8 +174,8 @@ ccl_device_forceinline bool ray_triangle_intersect(float3 ray_P,
  */
 ccl_device bool ray_quad_intersect(float3 ray_P,
                                    float3 ray_D,
-                                   float ray_mint,
-                                   float ray_maxt,
+                                   float ray_tmin,
+                                   float ray_tmax,
                                    float3 quad_P,
                                    float3 quad_u,
                                    float3 quad_v,
@@ -185,7 +188,7 @@ ccl_device bool ray_quad_intersect(float3 ray_P,
 {
   /* Perform intersection test. */
   float t = -(dot(ray_P, quad_n) - dot(quad_P, quad_n)) / dot(ray_D, quad_n);
-  if (t < ray_mint || t > ray_maxt) {
+  if (!(t > ray_tmin && t < ray_tmax)) {
     return false;
   }
   const float3 hit = ray_P + t * ray_D;