add skpdiff tool to compare bitmaps

experimental/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;
+
+/**
+ * Encapsulates an image difference metric algorithm that can be potentially run asynchronously.
+ */
+class SkImageDiffer {
+public:
+    SkImageDiffer();
+    virtual ~SkImageDiffer();
+
+    /**
+     * 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; }
+
+    /**
+     * 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
+     * @param  test     The bitmap whose difference is being tested
+     * @return          An non-negative diff ID on success, a negative integer on failure.
+     */
+    virtual int queueDiff(SkBitmap* baseline, SkBitmap* test) = 0;
+
+    /**
+     * Gets whether a queued diff of the given id has finished
+     * @param  id The id of the queued diff to query
+     * @return    True if the queued diff is finished and has results, false otherwise
+     */
+    virtual bool isFinished(int id) = 0;
+
+    /**
+     * Gets the results of the queued 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
+     * @return    A score between of how different the compared images are, with lower numbers being
+     *            more different.
+     */
+    virtual double getResult(int id) = 0;
+
+protected:
+    bool fIsGood;
+};
+
+
+#endif