Box objects, parallel to the coordinate axes

This change implements Kay-Kajiya intersection (with slabs) too!

src/objectBox.cc

@@ -0,0 +1,152 @@
+/* objectBox.cc
+ * vim: set tw=80:
+ * Eryn Wells <eryn@erynwells.me>
+ */
+
+#include <limits>
+
+#include "objectBox.hh"
+
+
+namespace charles {
+
+Box::Box()
+      /* A unit box centered on the origin. */
+    : Box(Vector3(-0.5, -0.5, -0.5), Vector3(0.5, 0.5, 0.5))
+{ }
+
+
+Box::Box(const Vector3& near, const Vector3& far)
+    : Object(),
+      mNear(near),
+      mFar(far)
+{ }
+
+
+bool
+Box::DoesIntersect(const Ray& ray,
+                   TVector& t)
+    const
+{
+    /*
+     * XXX: For now, I'm assuming that all boxes are parallel to the coordinate
+     * axes. This is the Kay-Kajiya box intersection algorithm.
+     */
+
+    Double t0, t1;
+    Double tNear = -std::numeric_limits<Double>::infinity();
+    Double tFar = std::numeric_limits<Double>::infinity();
+
+    /*
+     * From the Ray Tracing book:
+     *
+     *   For a more efficient algorithm, unwrap the loop, expand the swap of t0
+     *   and t1 into two branches, and change the calculations to multiply by
+     *   the inverse of the ray's direction to avoid divisions. Unwrapping the
+     *   loop allows elimination of comparing t0 adn t1 to tNear and tFar [for
+     *   the X planes], as tNear will always be set to the smaller and tFar the
+     *   larger of t0 and t1.
+     *
+     * Initially there was a for loop iterating over each parallel pair of
+     * planes (X, Y, and Z planes).
+     */
+
+    /* Unrolling the loop, so we start with the X planes... */
+
+    if (ray.direction.x == 0.0) {
+        /* The ray is parallel to the X axis. */
+        if (ray.origin.x < mNear.x || ray.origin.x > mFar.x) {
+            /* The ray's origin is not between the slabs, so no intersection. */
+            return false;
+        }
+    } else {
+        Double invX = 1.0 / ray.direction.x;
+        t0 = (mNear.x - ray.origin.x) * invX;
+        t1 = (mFar.x - ray.origin.x) * invX;
+        if (t0 > t1) {
+            tNear = t1;
+            tFar = t0;
+        } else {
+            tNear = t0;
+            tFar = t1;
+        }
+
+        if (tNear > tFar) {
+            /* Box is missed. */
+            return false;
+        }
+        if (tFar < 0.0) {
+            /* Box is behind the ray. */
+            return false;
+        }
+    }
+
+    /* Now the Y planes. */
+
+    if (ray.direction.y == 0.0) {
+        /* The ray is parallel to the Y axis. */
+        if (ray.origin.y < mNear.y || ray.origin.y > mFar.y) {
+            /* The ray's origin is not between the slabs, so no intersection. */
+            return false;
+        }
+    } else {
+        /* The ray isn't parallel, so calculate the intersection points. */
+        Double invY = 1.0 / ray.direction.y;
+        t0 = (mNear.y - ray.origin.y) * invY;
+        t1 = (mFar.y - ray.origin.y) * invY;
+        if (t0 > t1) {
+            tNear = t1;
+            tFar = t0;
+        } else {
+            tNear = t0;
+            tFar = t1;
+        }
+
+        if (tNear > tFar) {
+            /* Box is missed. */
+            return false;
+        }
+        if (tFar < 0.0) {
+            /* Box is behind the ray. */
+            return false;
+        }
+    }
+
+    /* Finally, the Z planes. */
+
+    if (ray.direction.z == 0.0) {
+        /* The ray is parallel to the Z axis. */
+        if (ray.origin.z < mNear.z || ray.origin.z > mFar.z) {
+            /* The ray's origin is not between the slabs, so no intersection. */
+            return false;
+        }
+    } else {
+        Double invZ = 1.0 / ray.direction.z;
+        t0 = (mNear.z - ray.origin.z) * invZ;
+        t1 = (mFar.z - ray.origin.z) * invZ;
+        if (t0 > t1) {
+            tNear = t1;
+            tFar = t0;
+        } else {
+            tNear = t0;
+            tFar = t1;
+        }
+
+        if (tNear > tFar) {
+            /* Box is missed. */
+            return false;
+        }
+        if (tFar < 0.0) {
+            /* Box is behind the ray. */
+            return false;
+        }
+    }
+
+    /* We have an intersection! */
+    t.push_back(t0);
+    t.push_back(t1);
+    return true;
+}
+
+} /* namespace charles */
+