From 53c9ff95347505f7abb5d7e299bae3c8fd4af5b4 Mon Sep 17 00:00:00 2001
From: Nathan Vegdahl <cessen@cessen.com>
Date: Thu, 31 Dec 2015 11:15:35 -0800
Subject: [PATCH] Made a BVHTraverser iterator that traverses a BVH with a ray.

It yields the objects that the ray needs to be tested against.
Thus it is the responsibility of the code using the iterator
to actually do the object-level ray tests and update the ray's
max_t etc. accordingly.

This keeps all of the BVH-related code generic with respect to
what kind of object/data the BVH actually contains, which means
the same BVH code can be used for both scene-level and
triangle-level data.
---
 src/bvh.rs  | 74 +++++++++++++++++++++++++++--------------------------
 src/main.rs | 24 ++++++++++++++---
 2 files changed, 59 insertions(+), 39 deletions(-)

diff --git a/src/bvh.rs b/src/bvh.rs
index 512372b..584bc22 100644
--- a/src/bvh.rs
+++ b/src/bvh.rs
@@ -1,9 +1,9 @@
 #![allow(dead_code)]
 
+use std::marker;
+
 use bbox::BBox;
-use math::Point;
 use ray::Ray;
-use triangle;
 use algorithm::partition;
 
 #[derive(Debug)]
@@ -129,48 +129,50 @@ impl<'a, T> BVH<'a, T> {
 }
 
 
-pub fn intersect_bvh(bvh: &BVH<(Point, Point, Point)>, ray: &mut Ray) -> Option<(f32, f32, f32)> {
-    if bvh.nodes.len() == 0 {
-        return None;
+pub struct BVHTraverser<'a, T: 'a> {
+    bvh: &'a BVH<'a, T>,
+    ray: *mut Ray,
+    _ray_marker: marker::PhantomData<&'a mut Ray>,
+    i_stack: [usize; 65],
+    stack_ptr: usize,
+}
+
+impl<'a, T> BVHTraverser<'a, T> {
+    pub fn from_bvh_and_ray(bvh: &'a BVH<'a, T>, ray: &'a mut Ray) -> BVHTraverser<'a, T> {
+        BVHTraverser {
+            bvh: bvh,
+            ray: ray as *mut Ray,
+            _ray_marker: marker::PhantomData,
+            i_stack: [0; 65],
+            stack_ptr: 1,
+        }
     }
+}
 
-    let mut i_stack = [0; 65];
-    let mut stack_ptr: usize = 1;
-    let mut hit = false;
-    let mut u = 0.0;
-    let mut v = 0.0;
-
-    while stack_ptr > 0 {
-        match bvh.nodes[i_stack[stack_ptr]] {
-            BVHNode::Internal { bounds, second_child_index } => {
-                if bounds.intersect_ray(ray) {
-                    i_stack[stack_ptr] += 1;
-                    i_stack[stack_ptr + 1] = second_child_index;
-                    stack_ptr += 1;
-                } else {
-                    stack_ptr -= 1;
-                }
-            }
-
-            BVHNode::Leaf { bounds: _, object_index } => {
-                if let Some((t, tri_u, tri_v)) =
-                       triangle::intersect_ray(ray, bvh.objects[object_index]) {
-                    if t < ray.max_t {
-                        hit = true;
-                        ray.max_t = t;
-                        u = tri_u;
-                        v = tri_v;
+impl<'a, T> Iterator for BVHTraverser<'a, T> {
+    type Item = (&'a T, &'a mut Ray);
+    fn next(&mut self) -> Option<(&'a T, &'a mut Ray)> {
+        while self.stack_ptr > 0 {
+            match self.bvh.nodes[self.i_stack[self.stack_ptr]] {
+                BVHNode::Internal { bounds, second_child_index } => {
+                    if bounds.intersect_ray(&(unsafe { *self.ray })) {
+                        self.i_stack[self.stack_ptr] += 1;
+                        self.i_stack[self.stack_ptr + 1] = second_child_index;
+                        self.stack_ptr += 1;
+                    } else {
+                        self.stack_ptr -= 1;
                     }
                 }
 
-                stack_ptr -= 1;
+                BVHNode::Leaf { bounds: _, object_index } => {
+                    self.stack_ptr -= 1;
+                    unsafe {
+                        return Some((&self.bvh.objects[object_index], &mut (*self.ray)));
+                    }
+                }
             }
         }
-    }
 
-    if hit {
-        return Some((ray.max_t, u, v));
-    } else {
         return None;
     }
 }
diff --git a/src/main.rs b/src/main.rs
index fc4ee07..b6f3735 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -75,8 +75,8 @@ fn main() {
     // Generate a scene of triangles
     let mut triangles = {
         let mut triangles = Vec::new();
-        let xres = 512;
-        let yres = 512;
+        let xres = 16;
+        let yres = 16;
         let xinc = 512.0 / (xres as f32);
         let yinc = 512.0 / (yres as f32);
         for x in 0..xres {
@@ -113,6 +113,7 @@ fn main() {
             let offset = hash_u32(((x as u32) << 16) ^ (y as u32), 0);
             const SAMPLES: usize = 16;
             for si in 0..SAMPLES {
+                // Generate ray
                 let mut ray = Ray::new(Point::new(x as f32 +
                                                   fast_logit(halton::sample(0,
                                                                             offset + si as u32),
@@ -123,7 +124,23 @@ fn main() {
                                                              1.5),
                                                   0.0),
                                        Vector::new(0.0, 0.0, 1.0));
-                if let Some((_, u, v)) = bvh::intersect_bvh(&scene, &mut ray) {
+
+                // Test ray against scene
+                let (mut u, mut v) = (0.0, 0.0);
+                let mut hit = false;
+                for (tri, r) in bvh::BVHTraverser::from_bvh_and_ray(&scene, &mut ray) {
+                    if let Some((t, tri_u, tri_v)) = triangle::intersect_ray(r, *tri) {
+                        if t < r.max_t {
+                            hit = true;
+                            r.max_t = t;
+                            u = tri_u;
+                            v = tri_v;
+                        }
+                    }
+                }
+
+                // Update color based on ray hit
+                if hit {
                     r += u;
                     g += v;
                     b += (1.0 - u - v).max(0.0);
@@ -136,6 +153,7 @@ fn main() {
                     b += 0.1;
                 }
             }
+
             r *= 255.0 / SAMPLES as f32;
             g *= 255.0 / SAMPLES as f32;
             b *= 255.0 / SAMPLES as f32;