Use 1.25 DSF for 150~180 DPI

ash/display/display_change_observer_chromeos.cc

 // Copyright 2013 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 "ash/display/display_change_observer_chromeos.h"
 
 #include <algorithm>
 #include <map>
 #include <set>
 #include <vector>
@@ skipping 12 matching lines @@
 #include "ui/display/types/chromeos/display_snapshot.h"
 #include "ui/display/util/display_util.h"
 #include "ui/gfx/display.h"
 
 namespace ash {
 
 using ui::DisplayConfigurator;
 
 namespace {
 
-// The DPI threshold to detect high density screen.
-// Higher DPI than this will use device_scale_factor=2.
-const unsigned int kHighDensityDPIThreshold = 170;
+// The DPI threshold to determine the device scale factor.
+// DPI higher than |dpi| will use |device_scale_factor|.
+struct DeviceScaleFactorDPIThreshold {
+  float dpi;
+  float device_scale_factor;
+};
+
+const DeviceScaleFactorDPIThreshold kThresholdTable[] = {
+  {180.0f, 2.0f},
+  {150.0f, 1.25f},
+  {0.0f, 1.0f},
+};
 
 // 1 inch in mm.
 const float kInchInMm = 25.4f;
 
 // Display mode list is sorted by (in descending priority):
 //  * the area in pixels.
 //  * refresh rate.
 struct DisplayModeSorter {
   bool operator()(const DisplayMode& a, const DisplayMode& b) {
     if (a.size.GetArea() == b.size.GetArea())
@@ skipping 81 matching lines @@
     if (state.display->type() == ui::DISPLAY_CONNECTION_TYPE_INTERNAL &&
         gfx::Display::InternalDisplayId() == gfx::Display::kInvalidDisplayID) {
       gfx::Display::SetInternalDisplayId(state.display->display_id());
     }
 
     const ui::DisplayMode* mode_info = state.display->current_mode();
     if (!mode_info)
       continue;
 
     float device_scale_factor = 1.0f;
-    if (!ui::IsDisplaySizeBlackListed(state.display->physical_size()) &&
-        (kInchInMm * mode_info->size().width() /
-         state.display->physical_size().width()) > kHighDensityDPIThreshold) {
-      device_scale_factor = 2.0f;
+    if (!ui::IsDisplaySizeBlackListed(state.display->physical_size())) {
+      device_scale_factor =
+          FindDeviceScaleFactor((kInchInMm * mode_info->size().width() /
+                                 state.display->physical_size().width()));
     }
     gfx::Rect display_bounds(state.display->origin(), mode_info->size());
 
     std::vector<DisplayMode> display_modes = GetDisplayModeList(state);
 
     std::string name =
         state.display->type() == ui::DISPLAY_CONNECTION_TYPE_INTERNAL ?
             l10n_util::GetStringUTF8(IDS_ASH_INTERNAL_DISPLAY_NAME) :
             state.display->display_name();
     if (name.empty())
@@ skipping 26 matching lines @@
 }
 
 void DisplayChangeObserver::OnAppTerminating() {
 #if defined(USE_ASH)
   // Stop handling display configuration events once the shutdown
   // process starts. crbug.com/177014.
   Shell::GetInstance()->display_configurator()->PrepareForExit();
 #endif
 }
 
+// static
+float DisplayChangeObserver::FindDeviceScaleFactor(float dpi) {
+  for (size_t i = 0; i < arraysize(kThresholdTable); ++i) {
+    if (dpi > kThresholdTable[i].dpi)
+      return kThresholdTable[i].device_scale_factor;
+  }
+  return 1.0f;
+}
+
 }  // namespace ash