Modifies the threshold for hidpi displays.

ash/display/display_change_observer_chromeos.cc

Index: ash/display/display_change_observer_chromeos.cc
diff --git a/ash/display/display_change_observer_chromeos.cc b/ash/display/display_change_observer_chromeos.cc
index cfc23017e3d9124f96d97cd41b50ccacf8fd6828..e85f8f0bfc7c8e72f850a43ff8791f99c3645ea2 100644
--- a/ash/display/display_change_observer_chromeos.cc
+++ b/ash/display/display_change_observer_chromeos.cc
@@ -32,7 +32,15 @@ namespace {
 
 // The DPI threshold to detect high density screen.
 // Higher DPI than this will use device_scale_factor=2.
-const unsigned int kHighDensityDPIThreshold = 170;
+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;
@@ -88,6 +96,21 @@ std::vector<DisplayMode> DisplayChangeObserver::GetDisplayModeList(
   return display_mode_list;
 }
 
+// static
+float DisplayChangeObserver::GetScaleFactor(

[oshima, 2014/04/09 23:07:37]

can you move this (plus constants) to ui/display/display_util.h ?

[Jun Mukai, 2014/04/09 23:16:00]

Done.

+    const gfx::Size& physical_size,
+    const gfx::Size& screen_size) {
+  if (ui::IsDisplaySizeBlackListed(physical_size))
+    return 1.0f;
+
+  const unsigned int dpi = (kInchInMm * screen_size.width() /
+                            physical_size.width());
+  const unsigned int threshold =
+      (physical_size.width() >= kLargeDisplayWidthThresholdMM) ?
+      kHighDensityDPIThresholdLarge : kHighDensityDPIThresholdSmall;
+  return (dpi > threshold) ? 2.0f : 1.0f;
+}
+
 DisplayChangeObserver::DisplayChangeObserver() {
   Shell::GetInstance()->AddShellObserver(this);
 }
@@ -133,12 +156,8 @@ void DisplayChangeObserver::OnDisplayModeChanged(
     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;
-    }
+    float device_scale_factor = GetScaleFactor(
+        state.display->physical_size(), mode_info->size());
     gfx::Rect display_bounds(state.display->origin(), mode_info->size());
 
     std::vector<DisplayMode> display_modes = GetDisplayModeList(state);