diff --git a/example/darkroom/two_spheres.hpp b/example/darkroom/two_spheres.hpp
new file mode 100644
index 00000000..94011580
--- /dev/null
+++ b/example/darkroom/two_spheres.hpp
@@ -0,0 +1,94 @@
+/*=============================================================================
+  Copyright (c) 2011-2013 Bolero MURAKAMI
+  https://github.com/bolero-MURAKAMI/Sprout
+
+  Distributed under the Boost Software License, Version 1.0. (See accompanying
+  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+=============================================================================*/
+#ifndef EXAMPLE_DARKROOM_TWO_SPHERES_HPP
+#define EXAMPLE_DARKROOM_TWO_SPHERES_HPP
+
+#include <sprout/darkroom.hpp>
+#include <sprout/math/constants.hpp>
+
+namespace darkcult {
+	using namespace sprout::darkroom;
+	//
+	// object
+	//
+	SPROUT_STATIC_CONSTEXPR auto object = sprout::make_tuple(
+		objects::make_aa_plane(
+			sprout::darkroom::objects::aa_plane_direction::y,
+			-2.0,
+			materials::make_plaid_material_image(
+				colors::rgb_f(1.0, 0.0, 0.0),
+				colors::rgb_f(1.0, 1.0, 0.0),
+				0.0,
+				0.0
+				)
+			),
+		objects::make_sphere(
+			coords::vector3d(-1.0, 0.5, 7.5),
+			2.5,
+			materials::make_uniform_material_image(
+				colors::rgb_f(0.0, 0.0, 1.0),
+				0.2
+				)
+			),
+		objects::make_sphere(
+			coords::vector3d(1.0, -1.0, 4.0),
+			1.0,
+			materials::make_uniform_material_image(
+				colors::rgb_f(0.0, 1.0, 0.0),
+				0.2
+				)
+			)
+		);
+	//
+	// light
+	//
+	SPROUT_STATIC_CONSTEXPR auto light = lights::make_light_list(
+		lights::make_point_light(
+			coords::vector3d(-3.0, 5.0, 0.0),
+			colors::rgb_f(7.0, 7.0, 7.0)
+			),
+		lights::make_parallel_light(
+			coords::vector3d(0.0, 1.0, 0.0),
+			colors::rgb_f(0.1, 0.1, 0.1)
+			),
+		lights::make_parallel_light(
+			coords::vector3d(1.0, 0.5, 0.0),
+			colors::rgb_f(0.1, 0.1, 0.1)
+			),
+		lights::make_parallel_light(
+			coords::vector3d(-1.0, 0.5, 0.0),
+			colors::rgb_f(0.1, 0.1, 0.1)
+			),
+		lights::make_parallel_light(
+			coords::vector3d(0.0, 0.5, 1.0),
+			colors::rgb_f(0.1, 0.1, 0.1)
+			),
+		lights::make_parallel_light(
+			coords::vector3d(0.0, 0.5, -1.0),
+			colors::rgb_f(0.1, 0.1, 0.1)
+			)
+		);
+	//
+	// camera
+	//
+	SPROUT_STATIC_CONSTEXPR auto camera = cameras::make_simple_camera(
+		sprout::math::root_three<double>() / 2
+		);
+	//
+	// renderer
+	//
+	SPROUT_STATIC_CONSTEXPR auto renderer = renderers::make_whitted_style(
+		renderers::make_uniform_color(colors::rgb_f(0.0, 0.0, 0.0))
+		);
+	//
+	// raytracer
+	//
+	SPROUT_STATIC_CONSTEXPR auto raytracer = tracers::make_raytracer();
+}	// namespace darkcult
+
+#endif	// #ifndef EXAMPLE_DARKROOM_TWO_SPHERES_HPP
diff --git a/sprout/math/constants.hpp b/sprout/math/constants.hpp
index 04855d77..951f4b8f 100644
--- a/sprout/math/constants.hpp
+++ b/sprout/math/constants.hpp
@@ -120,6 +120,7 @@ namespace sprout {
 		}
 		//
 		// root_two
+		// root_three
 		// half_root_two
 		//
 		template<typename T>
@@ -127,6 +128,10 @@ namespace sprout {
 			return 1.41421356237309504880168872420969807856967187537694807317667973799073247846210703885038753432764157273501384623L;
 		}
 		template<typename T>
+		inline SPROUT_CONSTEXPR T root_three() {
+			return 1.73205080756887729352744634150587236694280525381038062805580697945193301690880003708114618675724857567562614142L;
+		}
+		template<typename T>
 		inline SPROUT_CONSTEXPR T half_root_two() {
 			return 0.707106781186547524400844362104849039284835937688474036588339868995366239231053519425193767163820786367506923115L;
 		}
diff --git a/tools/darkroom/darkcult.cpp b/tools/darkroom/darkcult.cpp
index a46c70b2..68774dc8 100644
--- a/tools/darkroom/darkcult.cpp
+++ b/tools/darkroom/darkcult.cpp
@@ -9,7 +9,7 @@
 // DARKROOM_SOURCE
 //
 #ifndef DARKROOM_SOURCE
-#	error DARKROOM_SOURCE not defined. for example: "scene.hpp"
+#	error DARKROOM_SOURCE not defined. for example: "../../example/darkroom/two_spheres.hpp"
 #else
 #	include DARKROOM_SOURCE
 #endif
diff --git a/tools/darkroom/darkcult.sh b/tools/darkroom/darkcult.sh
index 780f10d4..41809be3 100755
--- a/tools/darkroom/darkcult.sh
+++ b/tools/darkroom/darkcult.sh
@@ -9,7 +9,7 @@
 #
 # requires: ImageMagick (http://www.imagemagick.org/script/index.php)
 #
-src=""
+src="../../example/darkroom/two_spheres.hpp"
 stagedir="darkroom"
 width=16
 height=16
@@ -46,7 +46,7 @@ echo ":   tile-width = ${tile_width}"
 echo ":   tile-height = ${tile_height}"
 echo ":   force = ${force}"
 
-if [ ! -f "${src}" ]; then
+if [ ! -f "${src}" -a ! -f "$(cd $(dirname $0); pwd)/${src}" ]; then
 	echo >&2 -e ": \e[31msource(${src}) is not exist.\e[m"
 	exit 1
 fi
@@ -96,4 +96,4 @@ popd
 let elapsed=$SECONDS-$start
 echo ": elapsed = ${elapsed}s"
 
-echo ": finished"
+echo ": finished."