PolyVox and Thermite3D now support the use of a density component as well as a material component for each voxel.

library/PolyVoxCore/include/MaterialDensityPair.h

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+#pragma region License
+/*******************************************************************************
+Copyright (c) 2005-2009 David Williams
+
+This software is provided 'as-is', without any express or implied
+warranty. In no event will the authors be held liable for any damages
+arising from the use of this software.
+
+Permission is granted to anyone to use this software for any purpose,
+including commercial applications, and to alter it and redistribute it
+freely, subject to the following restrictions:
+
+    1. The origin of this software must not be misrepresented; you must not
+    claim that you wrote the original software. If you use this software
+    in a product, an acknowledgment in the product documentation would be
+    appreciated but is not required.
+
+    2. Altered source versions must be plainly marked as such, and must not be
+    misrepresented as being the original software.
+
+    3. This notice may not be removed or altered from any source
+    distribution. 	
+*******************************************************************************/
+#pragma endregion
+
+#ifndef __PolyVox_MaterialDensityPair_H__
+#define __PolyVox_MaterialDensityPair_H__
+
+#pragma region Headers
+#include "PolyVoxForwardDeclarations.h"
+#include "PolyVoxImpl/TypeDef.h"
+#pragma endregion
+
+namespace PolyVox
+{
+	template <typename Type, uint8_t NoOfMaterialBits, uint8_t NoOfDensityBits>
+	class MaterialDensityPair
+	{
+	public:
+		MaterialDensityPair();
+		MaterialDensityPair(Type uMaterial, Type uDensity);
+
+		//Why are both of these needed?!
+		bool operator==(const MaterialDensityPair& rhs) const throw();
+		bool operator!=(const MaterialDensityPair& rhs) const throw();
+
+		bool operator<(const MaterialDensityPair& rhs) const throw();
+
+		Type getDensity() const throw();
+		Type getMaterial() const throw();
+
+		void setDensity(Type uDensity);
+		void setMaterial(Type uMaterial);
+
+		static Type getMaxDensity() throw();
+		static Type getMinDensity() throw();
+		static Type getMidDensity() throw();
+
+	private:
+		Type m_uMaterial : NoOfMaterialBits;
+		Type m_uDensity : NoOfDensityBits;
+	};
+
+	typedef MaterialDensityPair<uint8_t, 4, 4> MaterialDensityPair44;
+}
+
+#include "MaterialDensityPair.inl"
+
+#endif