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diff --git a/libdimension/model/finishes/lambertian.c b/libdimension/model/finishes/lambertian.c
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+++ b/libdimension/model/finishes/lambertian.c
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+/*************************************************************************
+ * Copyright (C) 2010-2014 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
+ * Diffuse finish.
+ */
+
+#include "dimension/model.h"
+#include <math.h>
+#include <stdlib.h>
+
+/// Lambertian diffuse type.
+typedef struct dmnsn_lambertian {
+  dmnsn_diffuse diffuse;
+  double coeff;
+} dmnsn_lambertian;
+
+/// Diffuse finish callback.
+static dmnsn_color
+dmnsn_lambertian_diffuse_fn(const dmnsn_diffuse *diffuse,
+                            dmnsn_color light, dmnsn_color color,
+                            dmnsn_vector ray, dmnsn_vector normal)
+{
+  const dmnsn_lambertian *lambertian = (const dmnsn_lambertian *)diffuse;
+  double diffuse_factor = fabs((lambertian->coeff)*dmnsn_vector_dot(ray, normal));
+  return dmnsn_color_mul(diffuse_factor, dmnsn_color_illuminate(light, color));
+}
+
+dmnsn_diffuse *
+dmnsn_new_lambertian(dmnsn_pool *pool, double coeff)
+{
+  dmnsn_lambertian *lambertian = DMNSN_PALLOC(pool, dmnsn_lambertian);
+  lambertian->coeff = coeff;
+
+  dmnsn_diffuse *diffuse = &lambertian->diffuse;
+  dmnsn_init_diffuse(diffuse);
+  diffuse->diffuse_fn = dmnsn_lambertian_diffuse_fn;
+  return diffuse;
+}