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diff --git a/src/sphere.cpp b/src/sphere.cpp
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+/*
+
+ * Copyright © 2010 Ian D. Romanick
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+#include "glu3.h"
+#include "revolve.h"
+#include "mesh.h"
+
+/**
+ * \name Call-back functions for sphere generation routine
+ */
+/*@{*/
+static void sphere_revolve_cb(void *data,
+			      const GLUvec4 *position,
+			      const GLUvec4 *normal,
+			      const GLUvec4 *tangent,
+			      const GLUvec4 *uv);
+
+static void sphere_begin_cb(void *data, GLenum mode);
+
+static void sphere_index_cb(void *data, unsigned index);
+
+static void sphere_end_cb(void *data);
+/*@}*/
+
+/**
+ * GLUsphere decorator to implement call-backs
+ *
+ * To generate the sphere data, the \c GLUsphere class interfaces with various
+ * C functions that use a call-back mechanism.  These call-backs are analogous
+ * the \c emit_vertex, \c emit_begin, \c emit_index, and \c emit_end methods
+ * that \c GLUsphere subclasses will provide.
+ *
+ * However, these methods are all \c protected.  As a result the non-class
+ * call-back functions cannot call these methods unless they are \c friend
+ * functions.  It is undesireable to expose the implementation detail in the
+ * application-facing header file.  This can be worked around by creating a
+ * dummy subclass of \c GLUsphere that only contains the \c friend function
+ * declarations.  Pointers to \c GLUsphere objects can be cast to pointers to
+ * \c GLUsphereFriend objects without side-effect.
+ *
+ * This is arguably a mis-use of the "decorator" pattern, but it is the most
+ * efficient way to do this.
+ */
+class GLUsphereFriend : public GLUsphere {
+	friend void sphere_revolve_cb(void *data,
+				      const GLUvec4 *position,
+				      const GLUvec4 *normal,
+				      const GLUvec4 *tangent,
+				      const GLUvec4 *uv);
+
+	friend void sphere_begin_cb(void *data, GLenum mode);
+
+	friend void sphere_index_cb(void *data, unsigned index);
+
+	friend void sphere_end_cb(void *data);
+};
+
+
+GLUsphere::GLUsphere(GLdouble radius, GLint slices, GLint stacks)
+{
+	this->radius = radius;
+	this->slices = (slices < 4) ? 4 : (unsigned) slices;
+	this->stacks = (stacks < 4) ? 4 : (unsigned) stacks;
+}
+
+
+unsigned
+GLUsphere::vertex_count(void) const
+{
+	/* Each line of vertices from the north pole to the south pole consists
+	 * of (stacks+1) vertices.  There are a total of (slices+1) of these
+	 * lines of vertices.
+	 */
+	return (slices + 1) * (stacks + 1);
+}
+
+
+unsigned
+GLUsphere::primitive_count(void) const
+{
+	/* For each slice there is a triangle strip from the north pole to the
+	 * south pole.
+	 */
+	return slices;
+}
+
+
+unsigned
+GLUsphere::element_count(void) const
+{
+	/* Each slice is a triangle strip represented by 2 elements plus
+	 * 2 elements for each stack;
+	 */
+	return slices * (2 * (stacks + 1));
+}
+
+
+static void
+sphere_revolve_cb(void *data, const GLUvec4 *position, const GLUvec4 *normal,
+		  const GLUvec4 *tangent, const GLUvec4 *uv)
+{
+	GLUsphereFriend *s = (GLUsphereFriend *) data;
+
+	s->emit_vertex(*position, *normal, *tangent, *uv);
+}
+
+
+static void
+sphere_begin_cb(void *data, GLenum mode)
+{
+	GLUsphereFriend *s = (GLUsphereFriend *) data;
+
+	s->emit_begin(mode);
+}
+
+
+static void
+sphere_index_cb(void *data, unsigned index)
+{
+	GLUsphereFriend *s = (GLUsphereFriend *) data;
+
+	s->emit_index(index);
+}
+
+
+static void
+sphere_end_cb(void *data)
+{
+	GLUsphereFriend *s = (GLUsphereFriend *) data;
+
+	s->emit_end();
+}
+
+
+void
+GLUsphere::generate(void)
+{
+	generate_sphere(radius, slices, stacks,
+			normals_point_out,
+			sphere_revolve_cb, (void *) this);
+
+	generate_triangle_mesh(slices, stacks + 1, stacks + 1,
+			       sphere_begin_cb,
+			       sphere_index_cb,
+			       sphere_end_cb,
+			       (void *) this);
+}