diff --git a/src/voxelburst/core.clj b/src/voxelburst/core.clj
index 82ad0cf..d137659 100644
--- a/src/voxelburst/core.clj
+++ b/src/voxelburst/core.clj
@@ -2,6 +2,7 @@
   (:require
    [clojure.java.io :as io]
    [clojure.string :as string]
+   [clojure.tools.trace :refer :all]
    ;;[thi.ng.geom.aabb :as a]
    [thi.ng.geom.core :as g]
    [thi.ng.geom.core.vector :refer [vec3]]
@@ -64,8 +65,53 @@
                (vec3 (+ x border) (+ y border) (+ z border)))]
     (svo/apply-voxels svo/set-at tree coll)))
 
-(defn make-tree []
-  (let [tree (svo/voxeltree 8 (double 1/2))]
+;; {:pos v :vel v :rad r}
+
+(def gravity [1 1 1.2])
+
+(defn next-pos
+  "Position at t+1"
+  [pos vel]
+  (mapv + pos vel))
+
+(defn next-vel
+  "Velocity at t+1"
+  [vel]
+  (mapv * vel gravity))
+
+(defn single-in-bounds?
+  "Is a single value in bounds?"
+  [v [lo hi]]
+  (and (>= v lo) (<= v hi)))
+
+(defn in-bounds?
+  "Is point inside the domain bounds?"
+  [p bounds]
+  (every? true? (map #(single-in-bounds? %1 %2) p bounds)))
+
+(deftrace next-point
+  "Return the next position and velocity, or nil if it falls outside
+  the domain bounds."
+  [pos vel bounds]
+  (let [next-pos (next-pos pos vel)
+        next-vel (next-vel vel)]
+    (when (in-bounds? next-pos bounds)
+      [next-pos next-vel])))
+
+(defn trajectory
+  "Return the points of the trajectory from the initial point until
+  the point exits the domain."
+  [pos vel bounds]
+  (loop [pts []
+         p pos
+         v vel]
+    (let [[next-pos next-vel] (next-point p v bounds)]
+      (if (not next-pos)
+        pts
+        (recur (conj pts next-pos) next-pos next-vel)))))
+
+(defn make-tree [size count]
+  (let [tree (svo/voxeltree size (double 1/2))]
     (svo/set-at tree (vec3 4 4 4))))
 
 (defn write-ply
@@ -77,5 +123,4 @@
 
 (defn -main
   [& [name count grid-size resolution]]
-  (let [tree (make-tree)]
-    (write-ply name tree resolution)))
+  (write-ply name (make-tree) resolution))