Supporting Multiple Screens
The next article is from http://android-developers.blogspot.co.il/2011/07/new-tools-for-managing-screen-sizes.html
nice tool
http://coh.io/adpi/
Understanding Screen Densities and the “dp”
Resolution is the actual number of pixels available in the display, density is how many pixels appear within a constant area of the display, and size is the amount of physical space available for displaying your interface. These are interrelated: increase the resolution and density together, and size stays about the same. This is why the 320x480 screen on a G1 and 480x800 screen on a Droid are both the same screen size: the 480x800 screen has more pixels, but it is also higher density.
To remove the size/density calculations from the picture, the Android framework works wherever possible in terms of "dp" units, which are corrected for density. In medium-density ("mdpi") screens, which correspond to the original Android phones, physical pixels are identical to dp's; the devices’ dimensions are 320x480 in either scale. A more recent phone might have physical-pixel dimensions of 480x800 but be a high-density device. The conversion factor from hdpi to mdpi in this case is 1.5, so for a developer's purposes, the device is 320x533 in dp's.
Screen-size Buckets
Android has included support for three screen-size “buckets” since 1.6, based on these “dp” units: “normal” is currently the most popular device format (originally 320x480, more recently higher-density 480x800); “small” is for smaller screens, and “large” is for “substantially larger” screens. Devices that fall in the “large” bucket include the Dell Streak and original 7” Samsung Galaxy Tab. Android 2.3 introduced a new bucket size “xlarge”, in preparation for the approximately-10” tablets (such as the Motorola Xoom) that Android 3.0 was designed to support.
The definitions are:
- xlarge screens are at least 960dp x 720dp.
- large screens are at least 640dp x 480dp.
- normal screens are at least 470dp x 320dp.
- small screens are at least 426dp x 320dp. (Android does not currently support screens smaller than this.)
Here are some more examples of how this works with real screens:
- A QVGA screen is 320x240 ldpi. Converting to mdpi (a 4/3 scaling factor) gives us 426dp x 320dp; this matches the minimum size above for the small screen bucket.
- The Xoom is a typical 10” tablet with a 1280x800 mdpi screen. This places it into the xlarge screen bucket.
- The Dell Streak is a 800x480 mdpi screen. This places it into the bottom of the large size bucket.
- A typical 7” tablet has a 1024x600 mdpi screen. This also counts as a large screen.
- The original Samsung Galaxy Tab is an interesting case. Physically it is a 1024x600 7” screen and thus classified as “large”. However the device configures its screen as hdpi, which means after applying the appropriate ⅔ scaling factor the actual space on the screen is 682dp x 400dp. This actually moves it out of the “large” bucket and into a “normal” screen size. The Tab actually reports that it is “large”; this was a mistake in the framework’s computation of the size for that device that we made. Today no devices should ship like this.
