issue cocos2d#1712: Getting DPI support for Linux.

Author: James Chen

cocos2dx/platform/linux/CCDevice.cpp

@@ -1,10 +1,31 @@
 #include "platform/CCDevice.h"
+#include <X11/Xlib.h>
+#include <stdio.h>
 
 NS_CC_BEGIN
 
 float CCDevice::getDPI()
 {
-
+	static int dpi = -1;
+	if (dpi == -1)
+	{
+		Display *dpy;
+	    char *displayname = NULL;
+	    int scr = 0; /* Screen number */
+	    dpy = XOpenDisplay (displayname);
+	    /*
+	     * there are 2.54 centimeters to an inch; so there are 25.4 millimeters.
+	     *
+	     *     dpi = N pixels / (M millimeters / (25.4 millimeters / 1 inch))
+	     *         = N pixels / (M inch / 25.4)
+	     *         = N * 25.4 pixels / M inch
+	     */
+	    double xres = ((((double) DisplayWidth(dpy,scr)) * 25.4) / 
+	        ((double) DisplayWidthMM(dpy,scr)));
+	    dpi = (int) (xres + 0.5);
+	    printf("dpi = %d\n", dpi);
+	}
+	return dpi;
 }
 
 NS_CC_END

cocos2dx/proj.linux/Makefile

@@ -96,6 +96,7 @@ OBJECTS = ../actions/CCAction.o \
 ../platform/linux/CCApplication.o \
 ../platform/linux/CCEGLView.o \
 ../platform/linux/CCImage.o \
+../platform/linux/CCDevice.o \
 ../script_support/CCScriptSupport.o \
 ../sprite_nodes/CCAnimation.o \
 ../sprite_nodes/CCAnimationCache.o \