diff --git a/rtiow/renderer/src/bvh_triangles.rs b/rtiow/renderer/src/bvh_triangles.rs
index 2d469d8..9e4f3fc 100644
--- a/rtiow/renderer/src/bvh_triangles.rs
+++ b/rtiow/renderer/src/bvh_triangles.rs
@@ -16,9 +16,10 @@ use crate::{
 #[derive(Debug, PartialEq)]
 struct BVHNode {
     aabb: AABB,
-    left_child: usize,
-    first_prim: usize,
-    prim_count: usize,
+    // When prim_count==0, left_first holds the left child's index in bvh_nodes. When >0 left_first
+    // holds the index for the first triangle in triangles.
+    left_first: u32,
+    prim_count: u32,
 }
 
 impl BVHNode {
@@ -79,11 +80,11 @@ where
             }
             let n = &self.bvh_nodes[i];
             if n.is_leaf() {
-                for t_idx in n.first_prim..(n.first_prim + n.prim_count) {
+                for t_idx in n.left_first..(n.left_first + n.prim_count) {
                     if f.alternate() {
                         write!(f, "\t")?;
                     }
-                    write!(f, "{:?} ", self.triangles[t_idx])?;
+                    write!(f, "{:?} ", self.triangles[t_idx as usize])?;
                     if f.alternate() {
                         writeln!(f)?;
                     }
@@ -100,6 +101,7 @@ where
     M: Material,
 {
     pub fn new(stl: &STL, material: M) -> BVHTriangles<M> {
+        assert_eq!(std::mem::size_of::<BVHNode>(), 32);
         let div3 = 1. / 3.;
         let triangles: Vec<_> = stl
             .triangles
@@ -131,9 +133,8 @@ where
         // assign all triangles to root node
         let root = BVHNode {
             aabb: AABB::default(),
-            left_child: 0,
-            first_prim: 0,
-            prim_count: self.triangles.len(),
+            left_first: 0,
+            prim_count: self.triangles.len() as u32,
         };
         self.bvh_nodes.push(root);
         self.update_node_bounds(ROOT_NODE_IDX);
@@ -144,8 +145,8 @@ where
         let node = &mut self.bvh_nodes[node_idx];
         let mut aabb_min: Vec3 = f32::MAX.into();
         let mut aabb_max: Vec3 = f32::MIN.into();
-        for i in node.first_prim..(node.first_prim + node.prim_count) {
-            let leaf_tri = &self.triangles[i];
+        for i in node.left_first..(node.left_first + node.prim_count) {
+            let leaf_tri = &self.triangles[i as usize];
             aabb_min = vec3::min(aabb_min, leaf_tri.verts[0]);
             aabb_min = vec3::min(aabb_min, leaf_tri.verts[1]);
             aabb_min = vec3::min(aabb_min, leaf_tri.verts[2]);
@@ -160,7 +161,7 @@ where
         if self.bvh_nodes[idx].prim_count <= 2 {
             return;
         }
-        let (first_prim, prim_count, left_count, i) = {
+        let (left_first, prim_count, left_count, i) = {
             let node = &self.bvh_nodes[idx];
 
             // Compute split plane and position.
@@ -169,7 +170,7 @@ where
             let split_pos = node.aabb.min()[axis] + extent[axis] * 0.5;
 
             // Split the group in two halves.
-            let mut i = node.first_prim as isize;
+            let mut i = node.left_first as isize;
             let mut j = i + node.prim_count as isize - 1;
             while i <= j {
                 if self.triangles[i as usize].centroid[axis] < split_pos {
@@ -181,43 +182,41 @@ where
             }
 
             // Create child nodes for each half.
-            let left_count = i as usize - node.first_prim;
+            let left_count = i as u32 - node.left_first;
             if left_count == 0 || left_count == node.prim_count {
                 return;
             }
-            (node.first_prim, node.prim_count, left_count, i)
+            (node.left_first, node.prim_count, left_count, i)
         };
 
         // create child nodes
-        let left_child_idx = self.bvh_nodes.len();
-        let right_child_idx = left_child_idx + 1;
+        let left_child_idx = self.bvh_nodes.len() as u32;
+        let right_child_idx = left_child_idx + 1 as u32;
         let left = BVHNode {
             aabb: AABB::default(),
-            left_child: 0,
-            first_prim: first_prim,
-            prim_count: left_count,
+            left_first,
+            prim_count: left_count as u32,
         };
         let right = BVHNode {
             aabb: AABB::default(),
-            left_child: 0,
-            first_prim: i as usize,
-            prim_count: prim_count - left_count,
+            left_first: i as u32,
+            prim_count: (prim_count - left_count) as u32,
         };
         self.bvh_nodes.push(left);
         self.bvh_nodes.push(right);
         let node = &mut self.bvh_nodes[idx];
-        node.left_child = left_child_idx;
+        node.left_first = left_child_idx;
         node.prim_count = 0;
 
         // Recurse
-        self.update_node_bounds(left_child_idx);
-        self.update_node_bounds(right_child_idx);
-        self.subdivide(left_child_idx);
-        self.subdivide(right_child_idx);
+        self.update_node_bounds(left_child_idx as usize);
+        self.update_node_bounds(right_child_idx as usize);
+        self.subdivide(left_child_idx as usize);
+        self.subdivide(right_child_idx as usize);
     }
 
-    fn intersect_bvh(&self, r: Ray, node_idx: usize, t_min: f32, t_max: f32) -> Option<HitRecord> {
-        let node = &self.bvh_nodes[node_idx];
+    fn intersect_bvh(&self, r: Ray, node_idx: u32, t_min: f32, t_max: f32) -> Option<HitRecord> {
+        let node = &self.bvh_nodes[node_idx as usize];
         if !node.aabb.hit(r, t_min, t_max) {
             return None;
         }
@@ -253,8 +252,8 @@ where
                 .filter_map(|hr| hr)
                 .min_by(|a, b| a.t.partial_cmp(&b.t).unwrap());
         } else {
-            let r1 = self.intersect_bvh(r, node.left_child, t_min, t_max);
-            let r2 = self.intersect_bvh(r, node.left_child + 1, t_min, t_max);
+            let r1 = self.intersect_bvh(r, node.left_first, t_min, t_max);
+            let r2 = self.intersect_bvh(r, node.left_first + 1, t_min, t_max);
             // Merge results, if both hit, take the one closest to the ray origin (smallest t
             // value).
             match (&r1, &r2) {