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diff --git a/libdimension/dimension/math/ray.h b/libdimension/dimension/math/ray.h
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+/*************************************************************************
+ * Copyright (C) 2009-2014 Tavian Barnes <tavianator@tavianator.com>     *
+ *                                                                       *
+ * This file is part of The Dimension Library.                           *
+ *                                                                       *
+ * The Dimension Library is free software; you can redistribute it and/  *
+ * or modify it under the terms of the GNU Lesser General Public License *
+ * as published by the Free Software Foundation; either version 3 of the *
+ * License, or (at your option) any later version.                       *
+ *                                                                       *
+ * The Dimension Library is distributed in the hope that it will be      *
+ * useful, but WITHOUT ANY WARRANTY; without even the implied warranty   *
+ * of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU  *
+ * Lesser General Public License for more details.                       *
+ *                                                                       *
+ * You should have received a copy of the GNU Lesser General Public      *
+ * License along with this program.  If not, see                         *
+ * <http://www.gnu.org/licenses/>.                                       *
+ *************************************************************************/
+
+/**
+ * @file
+ * Lines in 3-D space.
+ */
+
+#ifndef DMNSN_MATH_H
+#error "Please include <dimension/math.h> instead of this header directly."
+#endif
+
+/** A line, or ray. */
+typedef struct dmnsn_ray {
+  dmnsn_vector x0; /**< The origin of the ray. */
+  dmnsn_vector n;  /**< The direction of the ray. */
+} dmnsn_ray;
+
+/** A standard format string for rays. */
+#define DMNSN_RAY_FORMAT "(<%g, %g, %g> + t*<%g, %g, %g>)"
+/** The appropriate arguements to printf() a ray. */
+#define DMNSN_RAY_PRINTF(l)                                     \
+  DMNSN_VECTOR_PRINTF((l).x0), DMNSN_VECTOR_PRINTF((l).n)
+
+/**
+ * Construct a new ray.
+ * @param[in] x0  The origin of the ray.
+ * @param[in] n   The direction of the ray.
+ * @return The new ray.
+ */
+DMNSN_INLINE dmnsn_ray
+dmnsn_new_ray(dmnsn_vector x0, dmnsn_vector n)
+{
+  dmnsn_ray l = { x0, n };
+  return l;
+}
+
+/**
+ * Return the point at \p t on a ray.
+ * The point is defined by \f$ l.\vec{x_0} + t \cdot l.\vec{n} \f$
+ */
+DMNSN_INLINE dmnsn_vector
+dmnsn_ray_point(dmnsn_ray l, double t)
+{
+  return dmnsn_vector_add(l.x0, dmnsn_vector_mul(t, l.n));
+}
+
+/** Add epsilon*l.n to l.x0, to avoid self-intersections. */
+DMNSN_INLINE dmnsn_ray
+dmnsn_ray_add_epsilon(dmnsn_ray l)
+{
+  return dmnsn_new_ray(
+    dmnsn_vector_add(
+      l.x0,
+      dmnsn_vector_mul(1.0e3*dmnsn_epsilon, l.n)
+    ),
+    l.n
+  );
+}
+
+/** Return whether a ray contains any NaN entries. */
+DMNSN_INLINE bool
+dmnsn_ray_isnan(dmnsn_ray l)
+{
+  return dmnsn_vector_isnan(l.x0) || dmnsn_vector_isnan(l.n);
+}