About those things PX, PT, PPI, DPI, DP

First introduced to the respective definitions:

The most basic units of a picture or photograph on the pixel, pixels, electronic screen: px

pt : Point, point, the printing industry common units, equal to 1/72 of an inch
ppi : Pixel per inch, the number of pixels per inch, the higher the value, the more delicate screen
dpi : DOT per inch, dots per inch, the more value is high, the more delicate images
DP : DIP, pixel density-Independent, Andrews development unit of length is used, the screen indicates 1DP pixel density of 160ppi length when 1px

SP : Scale-work of the Independent Pixel, Android developers font size of the unit used.

Wherein px, pt, dp units of length, ppi and dpi is a unit density

density	ldpi	mdpi	hdpi	xhdpi	xxhdpi
Density	120	160	240	320	480
Resolution	240*320	320*480	480*800	720*1280	1080*1920
Icon Size	36*36	48*48	72*72	96*96	144*144

Equation:

= The PPI √ (Y * X * X + Y) / Z formula length X is the number of pixels, Y is the number of pixels width, Z is the screen size.   

For example: Iphone 6s 4.7 inch a density of 2000 × 1125 pixels √ (2000 + 2000 * 1125. 1125. *) / 4.7 = 488ppi

Common methods:

1. Gets the screen height

public int getRealHeight(Activity activity) {  
 int heightPixels = 0;  
 Display display = activity.getWindowManager().getDefaultDisplay();  
 final int VERSION = Build.VERSION.SDK_INT;  

 if(VERSION < 13) {  
     display.getHeight();  
 }else if (VERSION == 13) {  
     try {  
         heightPixels = (Integer) Display.class.getMethod("getRawHeight").invoke(display);  
     } catch (Exception e) {  
     }  
 } else {  
     Point realSize = new Point();  
     try {  
         Display.class.getMethod("getRealSize", Point.class).invoke(display, realSize);  
         heightPixels = realSize.y;  
     } catch (Exception e) {  
     }  
 }  
 return heightPixels;  
}

2. take a screen density

public static float getScreenDensity(Activity activity) {  
 DisplayMetrics displayMetrics = new DisplayMetrics();  
 activity.getWindowManager().getDefaultDisplay().getMetrics(displayMetrics);  
 return displayMetrics.density;  
}

3. Get the width of the screen real number dp

public static float getScreenWidthDp(Activity activity) {  
 return getScreenWidth(activity) / getScreenDensity(activity);  
}

4. Get the number of the height of the screen is operable region dp

public static float getScreenHeightDp(Activity activity) {  
 return getScreenHeight(activity) / getScreenDensity(activity);  
}

5. Get the number of screens true width dp

public static float getRealWidthDp(Activity activity) {  
 return getRealWidth(activity) / getScreenDensity(activity);  
}

6. Gets the height of the number of screen real dp

public static float getRealHeightDp(Activity activity) {  
 return getRealHeight(activity) / getScreenDensity(activity);  
}

7. determined whether a flat plate

public static boolean isTablet(Activity activity) {  
 return isMoreThan6Inch(activity) && isScreenSizeLarge(activity);  
}
/** 
* 判断是否大于6英寸 
* @param activity 
* @return 
*/  
public static boolean isMoreThan6Inch(Activity activity) {  
 return getScreenInch(activity) >= 6.0;  
}
/** 
* 判断设备是否为大尺寸屏幕 
* 
* @param context 
* @return 
*/  
public static boolean isScreenSizeLarge(Context context) {  
 return (context.getResources().getConfiguration().screenLayout & Configuration.SCREENLAYOUT_SIZE_MASK) >= Configuration.SCREENLAYOUT_SIZE_LARGE;  
}