Compute the base dpi from X

chrome/browser/ui/libgtk2ui/gtk2_ui.cc

 // Copyright (c) 2012 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 "chrome/browser/ui/libgtk2ui/gtk2_ui.h"
 
 #include <math.h>
 #include <set>
 
 #include <pango/pango.h>
+#include <X11/Xlib.h>
 
 #include "base/command_line.h"
 #include "base/debug/leak_annotations.h"
 #include "base/environment.h"
 #include "base/i18n/rtl.h"
 #include "base/logging.h"
 #include "base/nix/mime_util_xdg.h"
 #include "base/nix/xdg_util.h"
 #include "base/stl_util.h"
 #include "base/strings/string_split.h"
@@ skipping 21 matching lines @@
 #include "third_party/skia/include/core/SkCanvas.h"
 #include "third_party/skia/include/core/SkColor.h"
 #include "third_party/skia/include/core/SkShader.h"
 #include "ui/base/resource/resource_bundle.h"
 #include "ui/gfx/canvas.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/geometry/size.h"
 #include "ui/gfx/image/image.h"
 #include "ui/gfx/skbitmap_operations.h"
 #include "ui/gfx/skia_util.h"
+#include "ui/gfx/x/x11_types.h"
 #include "ui/resources/grit/ui_resources.h"
 #include "ui/views/controls/button/label_button.h"
 #include "ui/views/controls/button/label_button_border.h"
 #include "ui/views/linux_ui/window_button_order_observer.h"
 
 #if defined(USE_GCONF)
 #include "chrome/browser/ui/libgtk2ui/gconf_listener.h"
 #endif
 
 // A minimized port of GtkThemeService into something that can provide colors
@@ skipping 309 matching lines @@
     LOG(WARNING) << "Unexpected gtk-xft-rgba \"" << rgba << "\"";
     params.subpixel_rendering = gfx::FontRenderParams::SUBPIXEL_RENDERING_NONE;
   }
 
   g_free(hint_style);
   g_free(rgba);
 
   return params;
 }
 
-double GetDPI() {
+double GetBaseDPI() {
+  XDisplay* xdisplay = gfx::GetXDisplay();
+  int xscreen = DefaultScreen(xdisplay);
+  return (static_cast<double>(DisplayWidth(xdisplay, xscreen)) * 25.4) /

[Evan Stade, 2015/05/13 22:09:27]

is this casting truly necessary? 25.4 is already a double, so I think everything
else will be converted to a double

[oshima, 2015/05/13 22:21:23]

I was lazy, sorry. (was cut&paste) Fixed.

+      static_cast<double>(DisplayWidthMM(xdisplay, xscreen));
+}
+
+double GetFontDPI() {
   GtkSettings* gtk_settings = gtk_settings_get_default();
   CHECK(gtk_settings);
   gint gtk_dpi = -1;
   g_object_get(gtk_settings, "gtk-xft-dpi", &gtk_dpi, NULL);
 
   // GTK multiplies the DPI by 1024 before storing it.
-  return (gtk_dpi > 0) ? gtk_dpi / 1024.0 : 96.0;
+  return (gtk_dpi > 0) ? gtk_dpi / 1024.0 : GetBaseDPI();
 }
 
 // Queries GTK for its font DPI setting and returns the number of pixels in a
 // point.
 double GetPixelsInPoint(float device_scale_factor) {
-  double dpi = GetDPI();
+  double dpi = GetFontDPI();
 
   // Take device_scale_factor into account — if Chrome already scales the
   // entire UI up by 2x, we should not also scale up.
   dpi /= device_scale_factor;
 
   // There are 72 points in an inch.
   return dpi / 72.0;
 }
 
 views::LinuxUI::NonClientMiddleClickAction GetDefaultMiddleClickAction() {
@@ skipping 1012 matching lines @@
   NativeThemeGtk2::instance()->NotifyObservers();
 }
 
 void Gtk2UI::UpdateDeviceScaleFactor(float device_scale_factor) {
   device_scale_factor_ = device_scale_factor;
   GtkStyle* label_style = gtk_rc_get_style(fake_label_.get());
   UpdateDefaultFont(label_style->font_desc);
 }
 
 float Gtk2UI::GetDeviceScaleFactor() const {
-  const int kCSSDefaultDPI = 96;
-  float scale = GetDPI() / kCSSDefaultDPI;
-  // Round to 2 decimals, e.g. to 1.33.
-  return roundf(scale * 100) / 100;
+  float scale = GetFontDPI() / GetBaseDPI();
+  // Round to 1 decimals, e.g. to 1.4.
+  return roundf(scale * 10) / 10;
 }
 
 }  // namespace libgtk2ui
 
 views::LinuxUI* BuildGtk2UI() {
   return new libgtk2ui::Gtk2UI;
 }