Modifies the threshold for hidpi displays.

ui/display/display_util.cc

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/display/display_util.h"
 
 #include "base/logging.h"
 
 namespace ui {
 
 namespace {
 
 // A list of bogus sizes in mm that should be ignored.
 // See crbug.com/136533. The first element maintains the minimum
 // size required to be valid size.
 const int kInvalidDisplaySizeList[][2] = {
   {40, 30},
   {50, 40},
   {160, 90},
   {160, 100},
 };
 
+// The DPI threshold to detect high density screen.
+// Higher DPI than this will use device_scale_factor=2.
+const unsigned int kHighDensityDPIThresholdSmall = 170;
+
+// The HiDPI threshold for large (usually external) monitors. Lower threshold
+// makes sense for large monitors, because such monitors should be located
+// farther from the user's face usually. See http://crbug.com/348279
+const unsigned int kHighDensityDPIThresholdLarge = 150;
+
+// The width threshold in mm for "large" monitors.
+const int kLargeDisplayWidthThresholdMM = 500;
+
+// 1 inch in mm.
+const float kInchInMm = 25.4f;
+
 }  // namespace
 
 bool IsDisplaySizeBlackListed(const gfx::Size& physical_size) {
   // Ignore if the reported display is smaller than minimum size.
   if (physical_size.width() <= kInvalidDisplaySizeList[0][0] ||
       physical_size.height() <= kInvalidDisplaySizeList[0][1]) {
     LOG(WARNING) << "Smaller than minimum display size";
     return true;
   }
   for (size_t i = 1; i < arraysize(kInvalidDisplaySizeList); ++i) {
     const gfx::Size size(kInvalidDisplaySizeList[i][0],
                          kInvalidDisplaySizeList[i][1]);
     if (physical_size == size) {
       LOG(WARNING) << "Black listed display size detected:" << size.ToString();
       return true;
     }
   }
   return false;
 }
 
+float GetScaleFactor(const gfx::Size& physical_size_in_mm,
+                     const gfx::Size& screen_size_in_pixels) {
+  if (IsDisplaySizeBlackListed(physical_size_in_mm))
+    return 1.0f;
+
+  const unsigned int dpi = (kInchInMm * screen_size_in_pixels.width() /
+                            physical_size_in_mm.width());
+  const unsigned int threshold =
+      (physical_size_in_mm.width() >= kLargeDisplayWidthThresholdMM) ?
+      kHighDensityDPIThresholdLarge : kHighDensityDPIThresholdSmall;
+  return (dpi > threshold) ? 2.0f : 1.0f;
+}
+
 }  // namespace ui