UIDeviceExt provide class methods to get information about pixels-per-centimeter/inch and points-per-centimeter/inch of an iPod, iPhone or iPad. This information can be used to render graphics that has dimensions of known size in centimeters/inches, etc.
A demo shows how this can be used to simulate 1 mm graph-paper on any iPod, iPhone or iPad (best on Retina displays).
The interface is simply:
@interface UIDeviceExt : NSObject
+ (float)pointsPerCentimeter;
+ (float)pixelsPerCentimeter;
+ (float)pointsPerInch;
+ (float)pixelsPerInch;
@end
For example this can be used like this:
#import "UIDeviceExt.h"
float pointsPerMillimeter = [UIDeviceExt pointsPerCentimeter] * 0.1f;
Whenever a new iPod, iPhone or iPad gets released this code has to be updated. If not new devices will fail to derive the information!
UIDeviceExt has only been tested on iOS 7.1 but I assume it should be fine on any iOS version. Please note that the demo program requires ARC.
In Xcode add the UIDeviceExt subdirectory including the two files UIDeviceExt.h and UIDeviceExt.m from this repo into your project.