Combines a base and a detail normal map to a new normal map texture, for Unity3D.

CombineNormalMapsWizard.cs

using UnityEditor;
using UnityEngine;

using System.IO;

// PLACE IN EDITOR FOLDER

class CombineNormalMapsWizard : ScriptableWizard {

	public Texture2D baseNormal = null;
	public Texture2D detailNormal = null;
	public float detailStrength = 1.0f;

	[MenuItem ("Tools/Combine Normal Maps")]
	static void CreateWizard () {
		ScriptableWizard.DisplayWizard<CombineNormalMapsWizard>("Combine Normal Maps", "Combine");
	}

	void OnWizardCreate () {
		bool goodToGo = true;
		
		if ( ( baseNormal == null ) || ( detailNormal == null ) )
		{
			Debug.LogWarning ( "You must specify a base and a detail normal map.");
			goodToGo = false;
		}
		
		else if ( ( baseNormal.height != detailNormal.height ) || ( baseNormal.width != detailNormal.width ) )
		{
			Debug.LogWarning ( "Base and detail textures must be the same dimensions.");
			goodToGo = false;
		}
		
		if (goodToGo) {
			string path;
			TextureImporter textureImporter;
			
			bool baseNormalReadable;
			bool baseNormalNormalmap;
			TextureImporterFormat baseNormalFormat;
			
			bool detailNormalReadable;
			bool detailNormalNormalmap;
			TextureImporterFormat detailNormalFormat;
			
			// Set up textures as we want them.
			path = AssetDatabase.GetAssetPath(baseNormal);
			textureImporter = AssetImporter.GetAtPath(path) as TextureImporter;
			baseNormalReadable = textureImporter.isReadable;
			baseNormalNormalmap = textureImporter.normalmap;
			baseNormalFormat = textureImporter.textureFormat;
			textureImporter.isReadable = true;
			textureImporter.normalmap = false;
			textureImporter.textureFormat = TextureImporterFormat.RGBA32;
			AssetDatabase.ImportAsset(path);
			
			path = AssetDatabase.GetAssetPath(detailNormal);
			textureImporter = AssetImporter.GetAtPath(path) as TextureImporter;
			detailNormalReadable = textureImporter.isReadable;
			detailNormalNormalmap = textureImporter.normalmap;
			detailNormalFormat = textureImporter.textureFormat;
			textureImporter.isReadable = true;
			textureImporter.normalmap = false;
			textureImporter.textureFormat = TextureImporterFormat.RGBA32;
			AssetDatabase.ImportAsset(path);
			
			Color[] detailNormalPixels;
			Color[] combinedNormalPixels;

			int width = baseNormal.width;
			int height = baseNormal.height;

			combinedNormalPixels = baseNormal.GetPixels (0);
			detailNormalPixels = detailNormal.GetPixels (0);

			int numPixels = width * height;
			
			
			// Do the conversion.
			for (int i = 0; i < numPixels; ++i)
			{
				// Method from http://blog.selfshadow.com/publications/blending-in-detail/ used.
				// I find it gives the nicest results.
				Vector3 t = new Vector3 (combinedNormalPixels[i].r * 2.0f - 1.0f, combinedNormalPixels[i].g * 2.0f - 1.0f, combinedNormalPixels[i].b * 2.0f);
				Vector3 u = new Vector3 (detailNormalPixels[i].r * -2.0f + 1.0f, detailNormalPixels[i].g * -2.0f + 1.0f, detailNormalPixels[i].b * 2.0f - 1);
				
				if ( detailStrength != 1.0f )
				{
					u = new Vector3 ( u.x * detailStrength, u.y * detailStrength, u.z );
					u.Normalize();
				}
				
				Vector3 r = t * Vector3.Dot(t, u) - u * t.z;
				r.Normalize ();
				
				Color returnColor = new Color (r.x * 0.5f + 0.5f, r.y * 0.5f + 0.5f, r.z * 0.5f + 0.5f, combinedNormalPixels[i].a);
				combinedNormalPixels[i] = returnColor;
			}

			Texture2D normal = new Texture2D(width, height, TextureFormat.ARGB32, false);
			normal.SetPixels (combinedNormalPixels);
			normal.Apply();

			byte[] bytes = normal.EncodeToPNG();
			DestroyImmediate(normal);
			
			string assetPath = AssetDatabase.GetAssetPath(baseNormal);
			string assetDir = Path.GetDirectoryName(assetPath);
			string assetName = Path.GetFileNameWithoutExtension(assetPath) + "_combined.png";
			string newAsset = Path.Combine(assetDir, assetName);
			File.WriteAllBytes(newAsset, bytes);
			
			// Revert textures back to the way they were.
			path = AssetDatabase.GetAssetPath(baseNormal);
			textureImporter = AssetImporter.GetAtPath(path) as TextureImporter;
			textureImporter.isReadable = baseNormalReadable;
			textureImporter.normalmap = baseNormalNormalmap;
			textureImporter.textureFormat = baseNormalFormat;
			AssetDatabase.ImportAsset(path);
			
			path = AssetDatabase.GetAssetPath(detailNormal);
			textureImporter = AssetImporter.GetAtPath(path) as TextureImporter;
			textureImporter.isReadable = detailNormalReadable;
			textureImporter.normalmap = detailNormalNormalmap;
			textureImporter.textureFormat = detailNormalFormat;
			AssetDatabase.ImportAsset(path);
			
			// Import the new texture, set as a normal map.
			AssetDatabase.ImportAsset(newAsset);
			textureImporter = AssetImporter.GetAtPath(newAsset) as TextureImporter;
			textureImporter.textureType = TextureImporterType.Bump;
			textureImporter.normalmap = true;
			textureImporter.convertToNormalmap = false;
			AssetDatabase.ImportAsset(newAsset);
			
			Debug.Log ("Normal maps combined to " + newAsset);
		}
	}

	void OnWizardUpdate () {
		helpString = "Press Combine to combine normal maps.\nSaved to folder with base normal map.";
	}
}