update skpdiff visualization (image magnification with alpha mask)

tools/skpdiff/skpdiff_main.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #if SK_SUPPORT_OPENCL
 
 #define __NO_STD_VECTOR // Uses cl::vectpr instead of std::vectpr
@@ skipping 20 matching lines @@
 
 #include "SkForceLinking.h"
 __SK_FORCE_IMAGE_DECODER_LINKING;
 
 // Command line argument definitions go here
 DEFINE_bool2(list, l, false, "List out available differs");
 DEFINE_string2(differs, d, "", "The names of the differs to use or all of them by default");
 DEFINE_string2(folders, f, "", "Compare two folders with identical subfile names: <baseline folder> <test folder>");
 DEFINE_string2(patterns, p, "", "Use two patterns to compare images: <baseline> <test>");
 DEFINE_string2(output, o, "", "Writes the output of these diffs to output: <output>");
+DEFINE_string(alphaDir, "", "Writes the alpha mask of these diffs to output: <output>");
 DEFINE_bool(jsonp, true, "Output JSON with padding");
 DEFINE_string(csv, "", "Writes the output of these diffs to a csv file");
 DEFINE_int32(threads, -1, "run N threads in parallel [default is derived from CPUs available]");
 
 #if SK_SUPPORT_OPENCL
 /// A callback for any OpenCL errors
 static void CL_CALLBACK error_notify(const char* errorInfo, const void* privateInfoSize, ::size_t cb, void* userData) {
     SkDebugf("OpenCL error notify: %s\n", errorInfo);
     exit(1);
 }
@@ skipping 128 matching lines @@
         }
     }
 
     if (!FLAGS_csv.isEmpty()) {
         if (1 != FLAGS_csv.count()) {
             SkDebugf("csv flag expects one argument: <csv file>\n");
             return 1;
         }
     }
 
+    if (!FLAGS_alphaDir.isEmpty()) {
+        if (1 != FLAGS_alphaDir.count()) {
+            SkDebugf("alphaDir flag expects one argument: <directory>\n");
+            return 1;
+        }
+    }
+
     SkDiffContext ctx;
     ctx.setDiffers(chosenDiffers);
 
+    if (!FLAGS_alphaDir.isEmpty()) {
+        ctx.setDifferenceDir(SkString(FLAGS_alphaDir[0]));
+    }
+
     if (FLAGS_threads >= 0) {
         ctx.setThreadCount(FLAGS_threads);
     }
 
     // Perform a folder diff if one is requested
     if (!FLAGS_folders.isEmpty()) {
         ctx.diffDirectories(FLAGS_folders[0], FLAGS_folders[1]);
     }
 
     // Perform a pattern diff if one is requested
@@ skipping 13 matching lines @@
     }
 
     return 0;
 }
 
 #if !defined(SK_BUILD_FOR_IOS) && !defined(SK_BUILD_FOR_NACL)
 int main(int argc, char * argv[]) {
     return tool_main(argc, (char**) argv);
 }
 #endif