update skpdiff visualization (image magnification with alpha mask)

tools/skpdiff/SkImageDiffer.h

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkImageDiffer_DEFINED
 #define SkImageDiffer_DEFINED
 
 class SkBitmap;
 struct SkIPoint;
 
 /**
  * Encapsulates an image difference metric algorithm that can be potentially run asynchronously.
  */
 class SkImageDiffer {
 public:
     SkImageDiffer();
     virtual ~SkImageDiffer();
 
+    static const double RESULT_CORRECT = 1.0f;
+    static const double RESULT_INCORRECT = 0.0f;
+
     /**
      * Gets a unique and descriptive name of this differ
      * @return A statically allocated null terminated string that is the name of this differ
      */
     virtual const char* getName() = 0;
 
     /**
      * Gets if this differ is in a usable state
      * @return True if this differ can be used, false otherwise
      */
     bool isGood() { return fIsGood; }
 
     /**
      * Gets if this differ needs to be initialized with and OpenCL device and context.
      */
     virtual bool requiresOpenCL() { return false; }
 
     /**
+     * Enables the generation of an alpha mask for all points of interest.
+     * @return True if the differ supports generating an alpha mask and false otherwise.
+     */
+    virtual bool enablePOIAlphaMask() { return false; }
+
+    /**
      * Wraps a call to queueDiff by loading the given filenames into SkBitmaps
      * @param  baseline The file path of the baseline image
      * @param  test     The file path of the test image
      * @return          The results of queueDiff with the loaded bitmaps
      */
     int queueDiffOfFile(const char baseline[], const char test[]);
 
     /**
      * Queues a diff on a pair of bitmaps to be done at some future time.
      * @param  baseline The correct bitmap
@@ skipping 30 matching lines @@
      */
     virtual int getPointsOfInterestCount(int id) = 0;
 
     /**
      * Gets an array of the points of interest for the diff of the given id. The results are only
      * meaningful after the queued diff has finished.
      * @param  id The id of the queued diff to query
      */
     virtual SkIPoint* getPointsOfInterest(int id) = 0;
 
+    /*
+     * Gets a bitmap containing an alpha mask containing transparent pixels at the points of
+     * interest for the diff of the given id. The results are only meaningful after the
+     * queued diff has finished.
+     * @param  id The id of the queued diff to query
+     */
+    virtual SkBitmap* getPointsOfInterestAlphaMask(int id) { return NULL; }
 
 
 protected:
     bool fIsGood;
 };
 
 
 #endif