Store monitor DPI in document structure

zathura/document.c

@@ -37,6 +37,7 @@ struct zathura_document_s {
   double cell_height; /**< height of a page cell in the document (not transformed by scale and rotation) */
   unsigned int view_width; /**< width of current viewport */
   unsigned int view_height; /**< height of current viewport */
+  double view_dpi; /**<DPI of the current viewport */
   zathura_device_factors_t device_factors; /**< x and y device scale factors (for e.g. HiDPI) */
   unsigned int pages_per_row; /**< number of pages in a row */
   unsigned int first_page_column; /**< column of the first page */
@@ -134,6 +135,7 @@ zathura_document_open(zathura_t* zathura, const char* path, const char* uri,
   document->cell_height = 0.0;
   document->view_height = 0;
   document->view_width  = 0;
+  document->view_dpi    = 0.0;
   document->device_factors.x = 1.0;
   document->device_factors.y = 1.0;
   document->position_x  = 0.0;
@@ -495,6 +497,15 @@ zathura_document_set_viewport_height(zathura_document_t* document, unsigned int
   document->view_height = height;
 }
 
+void
+zathura_document_set_viewport_dpi(zathura_document_t* document, double dpi)
+{
+  if (document == NULL) {
+    return;
+  }
+  document->view_dpi = dpi;
+}
+
 void
 zathura_document_get_viewport_size(zathura_document_t* document,
                                    unsigned int *height, unsigned int* width)
@@ -504,6 +515,15 @@ zathura_document_get_viewport_size(zathura_document_t* document,
   *width = document->view_width;
 }
 
+double
+zathura_document_get_viewport_dpi(zathura_document_t* document)
+{
+  if (document == NULL) {
+    return 0.0;
+  }
+  return document->view_dpi;
+}
+
 void
 zathura_document_set_device_factors(zathura_document_t* document,
                                     double x_factor, double y_factor)

zathura/document.h

@@ -239,6 +239,15 @@ zathura_document_set_viewport_width(zathura_document_t* document, unsigned int w
 void
 zathura_document_set_viewport_height(zathura_document_t* document, unsigned int height);
 
+/**
+ * Sets the viewport DPI (value based on the physical resolution of the monitor).
+ *
+ * @param[in] document     The document instance
+ * @param[in] height       The viewport DPI
+ */
+void
+zathura_document_set_viewport_dpi(zathura_document_t* document, double dpi);
+
 /**
  * Return the size of the viewport in pixels.
  *
@@ -249,6 +258,15 @@ void
 zathura_document_get_viewport_size(zathura_document_t* document,
                                    unsigned int *height, unsigned int* width);
 
+/**
+ * Return the size of the viewport in pixels.
+ *
+ * @param[in] document     The document instance
+ * @return    The viewport DPI (value based on the physical resolution of the monitor)
+ */
+double
+zathura_document_get_viewport_dpi(zathura_document_t* document);
+
 /**
  * Set the device scale factors (e.g. for HiDPI). These are generally integers
  * and equal for x and y. These scaling factors are only used when rendering to

zathura/zathura.c

@@ -18,6 +18,10 @@
 #include <glib/gstdio.h>
 #include <glib/gi18n.h>
 
+#ifdef GDK_WINDOWING_WAYLAND
+#include <gdk/gdkwayland.h>
+#endif
+
 #ifdef G_OS_UNIX
 #include <glib-unix.h>
 #include <gio/gunixinputstream.h>
@@ -134,6 +138,63 @@ create_directories(zathura_t* zathura)
   }
 }
 
+void
+zathura_update_view_dpi(zathura_t* zathura)
+{
+  if (zathura == NULL) {
+    return;
+  }
+
+  /* get view widget GdkMonitor */
+  GdkWindow* window = gtk_widget_get_window (zathura->ui.session->gtk.view); // NULL if not realized
+  if (window == NULL) {
+    return;
+  }
+  GdkDisplay* display = gtk_widget_get_display(zathura->ui.session->gtk.view);
+  if (display == NULL) {
+    return;
+  }
+  GdkMonitor* monitor = gdk_display_get_monitor_at_window(display, window);
+  if (monitor == NULL) {
+    return;
+  }
+
+  /* physical width of monitor */
+  int width_mm = gdk_monitor_get_width_mm(monitor);
+
+  /* size of monitor in pixels */
+  GdkRectangle monitor_geom;
+  gdk_monitor_get_geometry(monitor, &monitor_geom);
+
+  /* calculate dpi, knowing that 1 inch = 25.4 mm */
+  double dpi = 0.0;
+  if (width_mm == 0) {
+    girara_debug("cannot calculate DPI: monitor has zero width");
+  } else {
+    dpi = monitor_geom.width * 25.4 / width_mm;
+  }
+
+#ifdef GDK_WINDOWING_WAYLAND
+    /* work around apparend bug in GDK: on Wayland, monitor geometry doesn't
+     * return values in application pixels as documented, but in device pixels.
+     * */
+    if (GDK_IS_WAYLAND_DISPLAY(display))
+    {
+      girara_debug("on Wayland, correcting DPI for device scale factor");
+      /* not using the cached value for the scale factor here to avoid issues
+       * if this function is called before the cached value is updated */
+      int device_factor = gtk_widget_get_scale_factor(zathura->ui.session->gtk.view);
+      if (device_factor != 0) {
+        dpi /= device_factor;
+      }
+    }
+#endif
+
+  girara_debug("monitor width: %d mm, pixels: %d, dpi: %f", width_mm, monitor_geom.width, dpi);
+
+  zathura_document_set_viewport_dpi(zathura->document, dpi);
+}
+
 static bool
 init_ui(zathura_t* zathura)
 {
@@ -962,6 +1023,8 @@ document_open(zathura_t* zathura, const char* path, const char* uri, const char*
   const unsigned int view_height = (unsigned int)floor(gtk_adjustment_get_page_size(vadjustment));
   zathura_document_set_viewport_height(zathura->document, view_height);
 
+  zathura_update_view_dpi(zathura);
+
   /* get initial device scale */
   int device_factor = gtk_widget_get_scale_factor(zathura->ui.session->gtk.view);
   zathura_document_set_device_factors(zathura->document, device_factor, device_factor);

zathura/zathura.h

@@ -287,6 +287,13 @@ void zathura_set_plugin_dir(zathura_t* zathura, const char* dir);
  */
 void zathura_set_argv(zathura_t* zathura, char** argv);
 
+/**
+ * Calculate and store the monitor DPI for the view widget
+ *
+ * @param zathura The zathura session
+ */
+void zathura_update_view_dpi(zathura_t* zathura);
+
 /**
  * Opens a file
  *