SpriteBackgroundRemover.cs

// original script : http://answers.unity3d.com/answers/252528/view.html

// Usage: Place this script in Editor/ folder
// Start the tool from menu, Window/Tools/Alpha-fy Images

using UnityEngine;
using UnityEditor;
using System.IO;
namespace UnityLibrary
{
    public class SpriteBackgroundRemover : EditorWindow
    {
        Texture2D img;
        Texture2D newImg;
        Color colorToRemove = Color.magenta;
        public static SpriteBackgroundRemover win;

        [MenuItem("Window/Tools/Alpha-fy Images")]
        static void Init()
        {
            win = ScriptableObject.CreateInstance(typeof(SpriteBackgroundRemover)) as SpriteBackgroundRemover;
            win.minSize = new Vector2(300, 350);
            win.ShowUtility();
        }

        void OnGUI()
        {
            GUILayout.BeginHorizontal();

            /** Toolbar **/
            GUILayout.BeginVertical();
            img = (Texture2D)EditorGUILayout.ObjectField(img, typeof(Texture2D), false, GUILayout.MinWidth(128), GUILayout.MinHeight(128), GUILayout.MaxWidth(128), GUILayout.MaxHeight(128));

            colorToRemove = EditorGUILayout.ColorField(colorToRemove, GUILayout.MaxWidth(128));

            if (GUILayout.Button("Preview", GUILayout.MinWidth(128), GUILayout.MinHeight(32), GUILayout.MaxWidth(128), GUILayout.MaxHeight(128)))
                newImg = RemoveColor(colorToRemove, img);

            if (GUILayout.Button("Alpha-fy All", GUILayout.MinWidth(128), GUILayout.MinHeight(32), GUILayout.MaxWidth(128), GUILayout.MaxHeight(128)))
                RemoveColor(colorToRemove, (UnityEngine.Object[])Selection.GetFiltered(typeof(Texture2D), SelectionMode.Assets));

            GUILayout.EndVertical();

            GUILayout.BeginVertical();
            GUILayout.Label("Selected Files", EditorStyles.boldLabel);
            foreach (Texture2D selected in Selection.GetFiltered(typeof(Texture2D), SelectionMode.Assets))
            {
                GUILayout.Label(selected.name);
            }
            GUILayout.EndVertical();

            /** Image Display **/
            GUILayout.BeginVertical();
            GUILayout.Label("Preview", EditorStyles.boldLabel);
            if (newImg)
            {
                GUILayout.Label(newImg);
            }
            GUILayout.EndVertical();

            GUILayout.EndHorizontal();

        }

        // for multiple images
        void RemoveColor(Color c, UnityEngine.Object[] imgs)
        {
            if (!Directory.Exists("Assets/AlphaImages/"))
            {
                Directory.CreateDirectory("Assets/AlphaImages/");
            }

            float inc = 0f;
            foreach (Texture2D i in imgs)
            {
                inc++;
                if (inc % 512 == 0 && EditorUtility.DisplayCancelableProgressBar("Playin' With Pixels", "Seaching for Color Matches", ((float)inc / (float)imgs.Length)))
                {
                    Debug.LogError("Cancelled..");
                    break;
                }

                CheckTextureSettings(i);

                Color[] pixels = i.GetPixels(0, 0, i.width, i.height, 0);
                var clear = new Color(0, 0, 0, 0);

                for (int p = 0; p < pixels.Length; p++)
                {
                    if (pixels[p] == c)
                    {
                        pixels[p] = clear;
                    }
                }

                Texture2D n = new Texture2D(i.width, i.height);
                n.SetPixels(0, 0, i.width, i.height, pixels, 0);
                n.Apply();

                byte[] bytes = n.EncodeToPNG();
                File.WriteAllBytes("Assets/AlphaImages/" + i.name + "_alpha.png", bytes);
            }

            EditorUtility.ClearProgressBar();

            AssetDatabase.SaveAssets();
            AssetDatabase.Refresh();
        }

        // for single image
        Texture2D RemoveColor(Color c, Texture2D i)
        {
            CheckTextureSettings(i);

            Color[] pixels = i.GetPixels(0, 0, i.width, i.height, 0);

            var clear = new Color(0, 0, 0, 0);

            for (int p = 0; p < pixels.Length; p++)
            {
                if (p % 512 == 0 && EditorUtility.DisplayCancelableProgressBar("Playin' With Pixels", "Seaching for Color Matches", ((float)p / pixels.Length)))
                {
                    Debug.LogError("Cancelled..");
                    break;
                }

                if (pixels[p] == c)
                {
                    pixels[p] = clear;
                }

            }

            Texture2D n = new Texture2D(i.width, i.height);
            n.SetPixels(0, 0, i.width, i.height, pixels, 0);
            n.Apply();
            EditorUtility.ClearProgressBar();
            return (n);
        }

        public void CheckTextureSettings(Texture2D texture)
        {
            if (texture == null) { Debug.LogError("CheckTextureSettings Failed - Texture is null"); return; }

            string path = AssetDatabase.GetAssetPath(texture);
            if (string.IsNullOrEmpty(path)) { Debug.LogError("CheckTextureSettings Failed - Texture path is null"); return; }

            TextureImporter textureImporter = AssetImporter.GetAtPath(path) as TextureImporter;

            if (!textureImporter.isReadable)
            {
                Debug.Log("Enabling read/write for image " + path);
                //            textureImporter.mipmapEnabled = false;
                textureImporter.isReadable = true;
                AssetDatabase.ImportAsset(path, ImportAssetOptions.ForceUpdate);
            }

        }

    }
}