From 108bddd712ca977831bfff4e82c8b2b8b1288700 Mon Sep 17 00:00:00 2001
From: Nathan Vegdahl <cessen@cessen.com>
Date: Sat, 2 Jan 2016 13:24:41 -0800
Subject: [PATCH] Optimization for BVH traversal.

Before this the BVH traversal was always traversing into the
same child first regardless of the situation.  Now it checks
the direction of the first ray of the batch and compares it
to the split axis of the node, and traverses into the closest
node first.
---
 src/bvh.rs | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/src/bvh.rs b/src/bvh.rs
index d0e3225..6cfa685 100644
--- a/src/bvh.rs
+++ b/src/bvh.rs
@@ -16,6 +16,7 @@ enum BVHNode {
     Internal {
         bounds: BBox,
         second_child_index: usize,
+        split_axis: u8,
     },
 
     Leaf {
@@ -77,6 +78,7 @@ impl<'a, T> BVH<'a, T> {
             self.nodes.push(BVHNode::Internal {
                 bounds: BBox::new(),
                 second_child_index: 0,
+                split_axis: 0,
             });
 
             // Determine which axis to split on
@@ -131,6 +133,7 @@ impl<'a, T> BVH<'a, T> {
             self.nodes[me] = BVHNode::Internal {
                 bounds: bounds,
                 second_child_index: child2_index,
+                split_axis: split_axis as u8,
             };
         }
 
@@ -147,7 +150,7 @@ impl<'a, T> BVH<'a, T> {
 
         while stack_ptr > 0 {
             match self.nodes[i_stack[stack_ptr]] {
-                BVHNode::Internal { bounds, second_child_index } => {
+                BVHNode::Internal { bounds, second_child_index, split_axis } => {
                     let part = partition(&mut rays[..ray_i_stack[stack_ptr]],
                                          |r| bounds.intersect_ray(r));
                     if part > 0 {
@@ -155,6 +158,9 @@ impl<'a, T> BVH<'a, T> {
                         i_stack[stack_ptr + 1] = second_child_index;
                         ray_i_stack[stack_ptr] = part;
                         ray_i_stack[stack_ptr + 1] = part;
+                        if rays[0].dir[split_axis as usize] > 0.0 {
+                            i_stack.swap(stack_ptr, stack_ptr + 1);
+                        }
                         stack_ptr += 1;
                     } else {
                         stack_ptr -= 1;