http://wiki.unity3d.com/index.php?title=HeightmapFromTexture converted to c#

HeightmapFromTexture.cs

using UnityEngine;
using UnityEditor;
public class HeightmapFromTexture{

	[MenuItem ("Terrain/Heightmap From Texture")]
     
    public static void ApplyHeightmap () {
    	var heightmap = Selection.activeObject as Texture2D;
    	if (heightmap == null) { 
    		EditorUtility.DisplayDialog("No texture selected", "Please select a texture.", "Cancel"); 
    		return; 
    	}
    	Undo.RegisterCompleteObjectUndo(Terrain.activeTerrain.terrainData, "Heightmap From Texture");
     
    	TerrainData terrain = Terrain.activeTerrain.terrainData;
    	int w = heightmap.width;
    	int h = heightmap.height;
    	int w2 = terrain.heightmapWidth;
    	float[,] heightmapData = terrain.GetHeights(0, 0, w2, w2);
    	Color[] mapColors = heightmap.GetPixels();
    	Color[] map = new Color[w2 * w2];
     
    	if (w2 != w || h != w) {
    		// Resize using nearest-neighbor scaling if texture has no filtering
    		if (heightmap.filterMode == FilterMode.Point) {
    			float dx = w/w2;
    			float dy = h/w2;
    			for (int y = 0; y < w2; y++) {
    				if (y%20 == 0) {
    					EditorUtility.DisplayProgressBar("Resize", "Calculating texture", Mathf.InverseLerp(0.0f, w2, y));
    				}
    				float thisY = (dy*y)*w;
    				int yw = y*w2;
    				for (int x = 0; x < w2; x++) {
    					map[yw + x] = mapColors[(int) (thisY + dx*x)];
    				}
    			}
    		}
    		// Otherwise resize using bilinear filtering
    		else {
			    float ratioX = 1.0f/((float)w2/(w-1));
			    float ratioY = 1.0f/((float)w2/(h-1));
    			for (int y = 0; y < w2; y++) {
    				if (y%20 == 0) {
    					EditorUtility.DisplayProgressBar("Resize", "Calculating texture", Mathf.InverseLerp(0.0f, w2, y));
    				}
    				float yy = Mathf.Floor((float) (y*ratioY));
    				float y1 = yy*w;
    				float y2 = (yy+1)*w;
    				float yw = y*w2;
    				for (int x = 0; x < w2; x++) {
    					float xx = Mathf.Floor((float) (x*ratioX));
     
    					Color bl = mapColors[(int) (y1 + xx)];
    					Color br = mapColors[(int) (y1 + xx+1)]; 
    					Color tl = mapColors[(int) (y2 + xx)];
    					Color tr = mapColors[(int) (y2 + xx+1)];
     
    					float xLerp = x*ratioX-xx;
    					map[(int) (yw + x)] = Color.Lerp(Color.Lerp(bl, br, xLerp), Color.Lerp(tl, tr, xLerp), y*ratioY-yy);
    				}
    			}
    		}
    		EditorUtility.ClearProgressBar();
    	}
    	else {
    		// Use original if no resize is needed
    		map = mapColors;
    	}
     
    	// Assign texture data to heightmap
    	for (int y = 0; y < w2; y++) {
    		for (int x = 0; x < w2; x++) {
    			heightmapData[y,x] = map[y*w2+x].grayscale;
    		}
    	}
    	terrain.SetHeights(0, 0, heightmapData);
    }
}