From c79c49f69d3c683e8f0eaf088fac3017d79e29c1 Mon Sep 17 00:00:00 2001
From: Michael Stapelberg <michael@stapelberg.de>
Date: Sat, 26 Apr 2014 17:17:37 +0200
Subject: [PATCH] libi3: add logical_px() for Retina display support

---
 include/libi3.h |  8 ++++++++
 libi3/dpi.c     | 16 ++++++++++++++++
 2 files changed, 24 insertions(+)
 create mode 100644 libi3/dpi.c

diff --git a/include/libi3.h b/include/libi3.h
index 3815777b..dc5d537f 100644
--- a/include/libi3.h
+++ b/include/libi3.h
@@ -381,3 +381,11 @@ char *get_process_filename(const char *prefix);
  * Returned value must be freed by the caller.
  */
 char *get_exe_path(const char *argv0);
+
+/**
+ * Convert a logical amount of pixels (e.g. 2 pixels on a “standard” 96 DPI
+ * screen) to a corresponding amount of physical pixels on a standard or retina
+ * screen, e.g. 5 pixels on a 227 DPI MacBook Pro 13" Retina screen.
+ *
+ */
+int logical_px(const int logical);
diff --git a/libi3/dpi.c b/libi3/dpi.c
new file mode 100644
index 00000000..c015065b
--- /dev/null
+++ b/libi3/dpi.c
@@ -0,0 +1,16 @@
+#include "libi3.h"
+#include <math.h>
+
+extern xcb_screen_t *root_screen;
+
+/*
+ * Convert a logical amount of pixels (e.g. 2 pixels on a “standard” 96 DPI
+ * screen) to a corresponding amount of physical pixels on a standard or retina
+ * screen, e.g. 5 pixels on a 227 DPI MacBook Pro 13" Retina screen.
+ *
+ */
+int logical_px(const int logical) {
+    const double dpi = (double)root_screen->height_in_pixels * 25.4 /
+                       (double)root_screen->height_in_millimeters;
+	return ceil((dpi / 96.0) * logical);
+}