Android中的px,in,pt,dp,dip,sp

Historically, programmers always designed UI in terms of pixels. For example, you might make a field 300 pixels wide, allow 5 pixels of spacing between columns, and define icons 16-by-16 pixels in size. The problem is that if you run that program on new displays with more and more dots（pixels） per inch (dpi), the UI appears smaller and smaller.

Resolution-independent measurements help solve this problem. Android supports all the following units:
• px (pixels): Dots on the screen.
• in (inches): Size as measured by a ruler.
• pt (points): 1/72 of an inch.
• dp (density-independent pixels): An abstract unit based on the density of the screen. On a display with 160 dots per inch, 1dp = 1px.
• dip: Synonym for dp, used more often in Google examples.
• sp (scale-independent pixels): Similar to dp, but also scaled by the user’s font size preference.
To make your interface scalable to any current and future type of display, I recommend you always use the sp unit for text sizes and the dip unit for everything else. You should also consider using vector graphics instead of bitmaps

px：像素点

in：英寸

pt：磅，1/72 英寸

density：表示每英寸有多少个dots（pixels）

dp：一个基于density的抽象单位，如果一个160dpi的屏幕，1dp=1px

dip：等同于dp

sp：同dp相似，但还会根据用户选定的字体大小来缩放。

建议使用sp作为文本的单位，其它用dip。

dp与px的换算：(必须知道所用屏幕的density)

px = (density / 160) * dp

说白了，dp就是选定了160 pixel per inch的density为基准，可以根据实际的density自动scale到实际使用的pixel。

HVGA屏density=160；

QVGA屏density=120；

WVGA屏density=240；

WQVGA屏density=120

当屏幕density=240时，使用hdpi 标签的资源
当屏幕density=160时，使用mdpi标签的资源
当屏幕density=120时，使用ldpi标签的资源。
不加任何标签的资源是各种分辨率情况下共用的。

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