关于ios device dpi

不同的apple设备dpi不同，根据官网提供的数据

http://developer.apple.com/library/ios/#DOCUMENTATION/UIKit/Reference/UIScreen_Class/Reference/UIScreen.html

scale

The natural scale factor associated with the screen. (read-only)

@property(nonatomic, readonly)  CGFloat scale

Discussion

This value reflects the scale factor needed to convert from the default logical coordinate space into the device coordinate space of this screen. The default logical coordinate space is measured using points, where one point is approximately equal to 1/160th of an inch. If a device’s screen has a reasonably similar pixel density, the scale factor is typically set to 1.0 so that one point maps to one pixel. However, a screen with a significantly different pixel density may set this property to a higher value.

Availability

• Available in iOS 4.0 and later.

应用要在缩放显示等做到在不同设备间的兼容性，就必须做以下判断：

void calScreenDpi(float& dpi)

{

    float scale = 1;

    if ([[UIScreen mainScreen] respondsToSelector:@selector(scale)]) {

        scale = [[UIScreen mainScreen] scale]; //得到屏幕分辨率

    }

    if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad) {

        dpi = 132 * scale;

    } else if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone) {

        dpi = 163 * scale;

    } else {

        dpi = 160 * scale;

    }

}