Cycles / Wavelength to RGB node:

* Added a node to convert wavelength (in nanometers, from 380nm to 780nm) to RGB values. This can be useful to match real world colors easier.

* Code cleanup:
** Moved color functions (xyz and hsv) into dedicated utility files.
** Remove svm_lerp(), use interp() instead. 

Documentation:
http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/Nodes/More#Wavelength

Example render:
http://www.pasteall.org/pic/show.php?id=53202

This is part of my GSoC 2013. (revisions 57322, 57326, 57335 and 57367 from soc-2013-dingto).

intern/cycles/render/nodes.cpp

@@ -2953,6 +2953,30 @@ void WireframeNode::compile(OSLCompiler& compiler)
 	compiler.add(this, "node_wireframe");
 }
 
+/* Wavelength */
+
+WavelengthNode::WavelengthNode()
+: ShaderNode("Wavelength")
+{
+	add_input("Wavelength", SHADER_SOCKET_FLOAT, 500.0f);
+	add_output("Color", SHADER_SOCKET_COLOR);
+}
+
+void WavelengthNode::compile(SVMCompiler& compiler)
+{
+	ShaderInput *wavelength_in = input("Wavelength");
+	ShaderOutput *color_out = output("Color");
+
+	compiler.stack_assign(wavelength_in);
+	compiler.stack_assign(color_out);
+	compiler.add_node(NODE_WAVELENGTH, wavelength_in->stack_offset, color_out->stack_offset, NULL);
+}
+
+void WavelengthNode::compile(OSLCompiler& compiler)
+{
+	compiler.add(this, "node_wavelength");
+}
+
 /* Output */
 
 OutputNode::OutputNode()

intern/cycles/render/nodes.h

@@ -457,6 +457,11 @@ public:
 	bool use_pixel_size;
 };
 
+class WavelengthNode : public ShaderNode {
+public:
+	SHADER_NODE_CLASS(WavelengthNode)
+};
+
 class MathNode : public ShaderNode {
 public:
 	SHADER_NODE_CLASS(MathNode)