Cycles: Implement watertight ray/triangle intersection

Using this paper: Sven Woop, Watertight Ray/Triangle Intersection

  http://jcgt.org/published/0002/01/05/paper.pdf

This change is expected to address quite reasonable amount of reports from the
bug tracker, plus it might help reducing the noise in some scenes.

Unfortunately, it's currently about 7% slower than the previous solution with
pre-computed triangle plane equations, but maybe with some smart tweaks to the
code (tests reshuffle, using SIMD in a nice way or so) we can avoid the speed
regression.

But perhaps smartest thing to do here would be to change single triangle / ray
intersection with multiple triangles / ray intersections. That's how Embree does
this and it's watertight single ray intersection is not any faster that this.

Currently only triangle intersection is modified accordingly to the paper, in
the future we would also want to modify the node / ray intersection.

Reviewers: brecht, juicyfruit

Subscribers: dingto, ton

Differential Revision: https://developer.blender.org/D819

1 files changed, 16 insertions, 0 deletions

diff --git a/intern/cycles/util/util_math.h b/intern/cycles/util/util_math.h
index 78005546a01..6898dc974c6 100644
--- a/intern/cycles/util/util_math.h
+++ b/intern/cycles/util/util_math.h
@@ -1452,6 +1452,22 @@ ccl_device bool map_to_sphere(float *r_u, float *r_v,
 	}
 }
 
+ccl_device_inline int util_max_axis(float3 vec)
+{
+	if(vec.x > vec.y) {
+		if(vec.x > vec.z)
+			return 0;
+		else
+			return 2;
+	}
+	else {
+		if(vec.y > vec.z)
+			return 1;
+		else
+			return 2;
+	}
+}
+
 CCL_NAMESPACE_END
 
 #endif /* __UTIL_MATH_H__ */