Add wxDisplayImpl::GetScaleFactor() and implement it for wxGTK

We need to account for the scale factor under GTK+ (and, presumably,
under macOS) to compute the correct PPI value as it must use the number
of physical and not logical pixels.

include/wx/private/display.h

@@ -81,8 +81,11 @@ public:
     // return the depth or 0 if unknown
     virtual int GetDepth() const = 0;
 
-    // return the resolution of the display, uses GetSizeMM() by default but
-    // can be also overridden directly
+    // return the scale factor used to convert logical pixels to physical ones
+    virtual double GetScaleFactor() const { return 1.0; }
+
+    // return the resolution of the display, uses GetSize(), GetScaleFactor()
+    // and GetSizeMM() by default but can be also overridden directly
     virtual wxSize GetPPI() const;
 
     // return the physical size of the display or (0, 0) if unknown: this is

src/common/dpycmn.cpp

@@ -195,9 +195,7 @@ bool wxDisplay::ChangeMode(const wxVideoMode& mode)
 
 wxSize wxDisplayImpl::GetPPI() const
 {
-    const wxSize pixels = GetGeometry().GetSize();
     const wxSize mm = GetSizeMM();
-
     if ( !mm.x || !mm.y )
     {
         // Physical size is unknown, return a special value indicating that we
@@ -205,6 +203,10 @@ wxSize wxDisplayImpl::GetPPI() const
         return wxSize(0, 0);
     }
 
+    // We need physical pixels here, not logical ones returned by
+    // GetGeometry(), to compute the real DPI.
+    const wxSize pixels = GetGeometry().GetSize()*GetScaleFactor();
+
     return wxSize(wxRound((pixels.x * inches2mm) / mm.x),
                   wxRound((pixels.y * inches2mm) / mm.y));
 }

src/gtk/display.cpp

@@ -46,6 +46,7 @@ public:
     virtual wxRect GetGeometry() const wxOVERRIDE;
     virtual wxRect GetClientArea() const wxOVERRIDE;
     virtual int GetDepth() const wxOVERRIDE;
+    virtual double GetScaleFactor() const wxOVERRIDE;
     virtual wxSize GetSizeMM() const wxOVERRIDE;
 
 #if wxUSE_DISPLAY
@@ -121,6 +122,11 @@ int wxDisplayImplGTK::GetDepth() const
     return 24;
 }
 
+double wxDisplayImplGTK::GetScaleFactor() const
+{
+    return gdk_monitor_get_scale_factor(m_monitor);
+}
+
 wxSize wxDisplayImplGTK::GetSizeMM() const
 {
     return wxSize
@@ -222,6 +228,9 @@ public:
     virtual wxRect GetGeometry() const wxOVERRIDE;
     virtual wxRect GetClientArea() const wxOVERRIDE;
     virtual int GetDepth() const wxOVERRIDE;
+#if GTK_CHECK_VERSION(3,10,0)
+    virtual double GetScaleFactor() const wxOVERRIDE;
+#endif // GTK+ 3.10
     virtual wxSize GetSizeMM() const wxOVERRIDE;
 
 #if wxUSE_DISPLAY
@@ -293,6 +302,16 @@ int wxDisplayImplGTK::GetDepth() const
     return gdk_visual_get_depth(gdk_window_get_visual(wxGetTopLevelGDK()));
 }
 
+#if GTK_CHECK_VERSION(3,10,0)
+double wxDisplayImplGTK::GetScaleFactor() const
+{
+    if ( gtk_check_version(3,10,0) == NULL )
+        return gdk_screen_get_monitor_scale_factor(m_screen, m_index);
+
+    return 1.0;
+}
+#endif // GTK+ 3.10
+
 wxSize wxDisplayImplGTK::GetSizeMM() const
 {
     // At least in some configurations, gdk_screen_xxx_mm() functions return