display: Add a way to query system DPI scaling

See #426

doc/ReleaseNotes

@@ -10,6 +10,7 @@ Recommended maintenance release.
 * Fix sounds resuming on reactivation if stop() was called while inactive (#559)
 * Collision traverser now releases GIL during traversal (#1033)
 * Fix crash caused by some gamepad drivers on Linux (#1066)
+* Add GraphicsPipe::get_display_zoom() for querying system DPI scaling
 * Skinning-enabled shaders can now properly render unskinned models as well
 * BitMask, SparseArray, BitArray types can now be pickled (#886)
 * VFSImporter now properly detects source file encodings in Python 3

panda/src/display/config_display.cxx

@@ -478,6 +478,14 @@ ConfigVariableBool sync_video
           "cheesy estimate of scene complexity.  Some drivers may ignore "
           "this request."));
 
+ConfigVariableDouble display_zoom
+("display-zoom", 0.0,
+ PRC_DESC("If this is set to a value other than 0.0, it overrides the detected "
+          "system DPI scaling.  GraphicsPipe::get_display_zoom() will instead "
+          "return whatever was passed in here.  You should generally only "
+          "change this based on a user preference change or to test how the UI "
+          "will look on monitors with different pixel densities."));
+
 /**
  * Initializes the library.  This must be called at least once before any of
  * the functions or classes in this library can be used.  Normally it will be

panda/src/display/config_display.h

@@ -108,6 +108,7 @@ extern EXPCL_PANDA_DISPLAY ConfigVariableDouble pixel_zoom;
 
 extern EXPCL_PANDA_DISPLAY ConfigVariableColor background_color;
 extern EXPCL_PANDA_DISPLAY ConfigVariableBool sync_video;
+extern EXPCL_PANDA_DISPLAY ConfigVariableDouble display_zoom;
 
 extern EXPCL_PANDA_DISPLAY void init_libdisplay();
 

panda/src/display/graphicsPipe.cxx

@@ -99,6 +99,9 @@ static void update_memory_info(DisplayInformation *info) {
 }
 #endif
 
+// Temporarily declared as global float.
+static PN_stdfloat detected_display_zoom = 1.0;
+
 TypeHandle GraphicsPipe::_type_handle;
 
 /**
@@ -268,6 +271,35 @@ make_output(const std::string &name,
   return nullptr;
 }
 
+/**
+ * Returns the display zoom factor configured in the operating system.  If the
+ * operating system automatically scales windows to match the DPI (such as when
+ * dpi-aware is set to false), this will be 1.0.  Otherwise, this will be set to
+ * a value approximating the density of the monitor divided by the standard
+ * density of the operating system (usually 96), yielding a value like 1.5 or
+ * 2.0.
+ *
+ * @since 1.10.8
+ */
+PN_stdfloat GraphicsPipe::
+get_display_zoom() const {
+  if (display_zoom.get_num_words() > 0) {
+    double override = display_zoom.get_value();
+    if (override != 0.0) {
+      return override;
+    }
+  }
+  return detected_display_zoom;
+}
+
+/**
+ * Called by derived class to set the display zoom factor.
+ */
+void GraphicsPipe::
+set_detected_display_zoom(PN_stdfloat zoom) {
+  detected_display_zoom = zoom;
+}
+
 /**
  * Gets the pipe's DisplayInformation.
  */

panda/src/display/graphicsPipe.h

@@ -91,8 +91,10 @@ PUBLISHED:
 
   INLINE int get_display_width() const;
   INLINE int get_display_height() const;
+  PN_stdfloat get_display_zoom() const;
   MAKE_PROPERTY(display_width, get_display_width);
   MAKE_PROPERTY(display_height, get_display_height);
+  MAKE_PROPERTY(display_zoom, get_display_zoom);
 
   DisplayInformation *get_display_information();
   MAKE_PROPERTY(display_information, get_display_information);
@@ -115,6 +117,8 @@ public:
   virtual PT(GraphicsStateGuardian) make_callback_gsg(GraphicsEngine *engine);
 
 protected:
+  void set_detected_display_zoom(PN_stdfloat zoom);
+
   virtual void close_gsg(GraphicsStateGuardian *gsg);
 
   virtual PT(GraphicsOutput) make_output(const std::string &name,

panda/src/windisplay/winGraphicsPipe.cxx

@@ -239,6 +239,22 @@ WinGraphicsPipe() {
           windisplay_cat.debug() << "Calling SetProcessDpiAwareness().\n";
         }
         pfnSetProcessDpiAwareness(Process_Per_Monitor_DPI_Aware);
+
+        HDC dc = GetDC(nullptr);
+        if (dc) {
+          int dpi = GetDeviceCaps(dc, LOGPIXELSX);
+          if (dpi > 0) {
+            PN_stdfloat zoom = (double)dpi / 96.0;
+            set_detected_display_zoom(zoom);
+
+            if (windisplay_cat.is_debug()) {
+              windisplay_cat.debug()
+                << "Determined display zoom to be " << zoom
+                << " based on LOGPIXELSX " << dpi << "\n";
+            }
+          }
+          ReleaseDC(nullptr, dc);
+        }
       }
     }
   }

panda/src/x11display/x11GraphicsPipe.cxx

@@ -16,6 +16,7 @@
 #include "config_x11display.h"
 #include "frameBufferProperties.h"
 #include "displayInformation.h"
+#include "pstrtod.h"
 
 #include <dlfcn.h>
 
@@ -300,6 +301,29 @@ x11GraphicsPipe(const std::string &display) :
   XFree(im_supported_styles);
   */
 
+  const char *dpi = XGetDefault(_display, "Xft", "dpi");
+  if (dpi != nullptr) {
+    char *endptr = nullptr;
+    double result = pstrtod(dpi, &endptr);
+    if (result != 0 && !cnan(result) && endptr[0] == '\0') {
+      result /= 96;
+      set_detected_display_zoom(result);
+
+      if (x11display_cat.is_debug()) {
+        x11display_cat.debug()
+          << "Determined display zoom to be " << result
+          << " based on specified Xft.dpi " << dpi << "\n";
+      }
+    } else {
+      x11display_cat.warning()
+        << "Unable to determine display zoom because Xft.dpi is invalid: "
+        << dpi << "\n";
+    }
+  } else if (x11display_cat.is_debug()) {
+    x11display_cat.debug()
+      << "Unable to determine display zoom because Xft.dpi was not set.\n";
+  }
+
   // Get some X atom numbers.
   _wm_delete_window = XInternAtom(_display, "WM_DELETE_WINDOW", false);
   _net_wm_pid = XInternAtom(_display, "_NET_WM_PID", false);