Switch to Lanczos scaling from Hamming to get acceptable fast downscaling.

In visual tests we see that Hamming-1 is not as good as
Lanczos-2, however it is about 40% faster, and Lanczos-2 itself is
about 30% faster than Lanczos-3. The use of Hamming-1 has been deemed
an unacceptable trade-off between quality and speed due to the limited
pixel space it operates in, so we pick Lanczos-2 here.
On modern hardware, Lanczos-2 doesn't have any noticeable impact
in normal use.

1 files changed, 10 insertions, 15 deletions

diff --git a/gfx/2d/image_operations.cpp b/gfx/2d/image_operations.cpp
index 62215f007e..79fa879d93 100644
--- a/gfx/2d/image_operations.cpp
+++ b/gfx/2d/image_operations.cpp
@@ -1,4 +1,5 @@
 // Copyright (c) 2006-2012 The Chromium Authors. All rights reserved.
+// Copyright (c) 2018 Mark Straver BASc. All rights reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions
@@ -107,7 +108,7 @@ void ComputeFilters(ImageOperations::ResizeMethod method,
 
     // Compute the unnormalized filter value at each location of the source
     // it covers.
-    float filter_sum = 0.0f;  // Sub of the filter values for normalizing.
+    float filter_sum = 0.0f;  // Sum of the filter values for normalizing.
     for (int cur_filter_pixel = src_begin; cur_filter_pixel <= src_end;
          cur_filter_pixel++) {
       // Distance from the center of the filter, this is the filter coordinate
@@ -158,28 +159,22 @@ void ComputeFilters(ImageOperations::ResizeMethod method,
 
 ImageOperations::ResizeMethod ResizeMethodToAlgorithmMethod(
     ImageOperations::ResizeMethod method) {
-  // Convert any "Quality Method" into an "Algorithm Method"
+  // If we already have an "Algorithm Method", just return that.
   if (method >= ImageOperations::RESIZE_FIRST_ALGORITHM_METHOD &&
       method <= ImageOperations::RESIZE_LAST_ALGORITHM_METHOD) {
     return method;
   }
-  // The call to ImageOperationsGtv::Resize() above took care of
-  // GPU-acceleration in the cases where it is possible. So now we just
-  // pick the appropriate software method for each resize quality.
+  // Convert any "Quality Method" into an "Algorithm Method"
   switch (method) {
-    // Users of RESIZE_GOOD are willing to trade a lot of quality to
-    // get speed, allowing the use of linear resampling to get hardware
-    // acceleration (SRB). Hence any of our "good" software filters
-    // will be acceptable, and we use the fastest one, Hamming-1.
     case ImageOperations::RESIZE_GOOD:
-      // Users of RESIZE_BETTER are willing to trade some quality in order
-      // to improve performance, but are guaranteed not to devolve to a linear
-      // resampling. In visual tests we see that Hamming-1 is not as good as
-      // Lanczos-2, however it is about 40% faster and Lanczos-2 itself is
+      // Users of RESIZE_GOOD are willing to trade quality to get speed.
+      // In visual tests we see that Hamming-1 is not as good as
+      // Lanczos-2, however it is about 40% faster, and Lanczos-2 itself is
       // about 30% faster than Lanczos-3. The use of Hamming-1 has been deemed
-      // an acceptable trade-off between quality and speed.
+      // an unacceptable trade-off between quality and speed due to the limited
+      // pixel space it operates in, so we pick Lanczos-2 here.
     case ImageOperations::RESIZE_BETTER:
-      return ImageOperations::RESIZE_HAMMING1;
+      return ImageOperations::RESIZE_LANCZOS2;
     default:
       return ImageOperations::RESIZE_LANCZOS3;
   }