Android Supporting Multiple Screens

Supporting Multiple Screens

Quickview

• Android runs on devices that have different screen sizes and densities.


• The screen on which your application is displayed can affect its user interface.


• The system handles most of the work of adapting your app to the current screen.


• You should create screen-specific resources for precise control of your UI.

In this document

1. Overview of Screen Support
   
   
   
   1. Terms and concepts
   
   
   2. Range of screens supported
   
   
   3. Density independence
   
   
   
   


2. How to Support Multiple Screens
   
   
   
   1. Using configuration qualifiers
   
   
   2. Designing alternative layouts and drawables
   
   
   
   


3. Declaring Tablet Layouts for Android 3.2 new!
   
   
   
   1. Using new size qualifiers
   
   
   2. Configuration examples
   
   
   3. Declaring screen size support
   
   
   
   


4. Best Practices


5. Additional Density Considerations
   
   
   
   1. Scaling Bitmap objects created at runtime
   
   
   2. Converting dp units to pixel units
   
   
   
   


6. How to Test Your Application on Multiple Screens

Related samples

1. Multiple Resolutions

See also

1. Thinking Like a Web Designer


2. Providing Alternative Resources


3. Icon Design Guidelines


4. Managing Virtual Devices

Android runs on a variety of devices that offer different screen sizes and densities. For applications, the Android system provides a consistent development environment across devices and handles most of the work to adjust each application's user interface to the screen on which it is displayed. At the same time, the system provides APIs that allow you to control your application's UI for specific screen sizes and densities, in order to optimize your UI design for different screen configurations. For example, you might want a UI for tablets that's different from the UI for handsets.

Although the system performs scaling and resizing to make your application work on different screens, you should make the effort to optimize your application for different screen sizes and densities. In doing so, you maximize the user experience for all devices and your users believe that your application was actually designed for their devices—rather than simply stretched to fit the screen on their devices.

By following the practices described in this document, you can create an application that displays properly and provides an optimized user experience on all supported screen configurations, using a single .apk file.

Note: The information in this document assumes that your application is designed for Android 1.6 (API Level 4) or higher. If your application supports Android 1.5 or lower, please first read Strategies for Android 1.5.



Also, be aware that Android 3.2 has introduced new APIs that allow you to more precisely control the layout resources your application uses for different screen sizes. These new features are especially important if you're developing an application that's optimized for tablets. For details, see the section aboutDeclaring Tablet Layouts for Android 3.2.

Overview of Screens Support

This section provides an overview of Android's support for multiple screens, including: an introduction to the terms and concepts used in this document and in the API, a summary of the screen configurations that the system supports, and an overview of the API and underlying screen-compatibility features.

Terms and concepts

Screen size

Actual physical size, measured as the screen's diagonal.

For simplicity, Android groups all actual screen sizes into four generalized sizes: small, normal, large, and extra large.

Screen density

The quantity of pixels within a physical area of the screen; usually referred to as dpi (dots per inch). For example, a "low" density screen has fewer pixels within a given physical area, compared to a "normal" or "high" density screen.

For simplicity, Android groups all actual screen densities into four generalized densities: low, medium, high, and extra high.

Orientation

The orientation of the screen from the user's point of view. This is either landscape or portrait, meaning that the screen's aspect ratio is either wide or tall, respectively. Be aware that not only do different devices operate in different orientations by default, but the orientation can change at runtime when the user rotates the device.

Resolution

The total number of physical pixels on a screen. When adding support for multiple screens, applications do not work directly with resolution; applications should be concerned only with screen size and density, as specified by the generalized size and density groups.

Density-independent pixel (dp)

A virtual pixel unit that you should use when defining UI layout, to express layout dimensions or position in a density-independent way.

The density-independent pixel is equivalent to one physical pixel on a 160 dpi screen, which is the baseline density assumed by the system for a "medium" density screen. At runtime, the system transparently handles any scaling of the dp units, as necessary, based on the actual density of the screen in use. The conversion of dp units to screen pixels is simple: px = dp * (dpi / 160). For example, on a 240 dpi screen, 1 dp equals 1.5 physical pixels. You should always use dp units when defining your application's UI, to ensure proper display of your UI on screens with different densities.

Range of screens supported

Starting with Android 1.6 (API Level 4), Android provides support for multiple screen sizes and densities, reflecting the many different screen configurations that a device may have. You can use features of the Android system to optimize your application's user interface for each screen configuration and ensure that your application not only renders properly, but provides the best user experience possible on each screen.

To simplify the way that you design your user interfaces for multiple screens, Android divides the range of actual screen sizes and densities into:

• A set of four generalized sizes: small, normal, large, and xlarge
  

  Note: Beginning with Android 3.2 (API level 13), these size groups are deprecated in favor of a new technique for managing screen sizes based on the available screen width. If you're developing for Android 3.2 and greater, see Declaring Tablet Layouts for Android 3.2 for more information.

  
  


• A set of four generalized densities: ldpi (low), mdpi (medium), hdpi (high), and xhdpi (extra high)

The generalized sizes and densities are arranged around a baseline configuration that is a normal size and mdpi (medium) density. This baseline is based upon the screen configuration for the first Android-powered device, the T-Mobile G1, which has an HVGA screen (until Android 1.6, this was the only screen configuration that Android supported).

Each generalized size and density spans a range of actual screen sizes and densities. For example, two devices that both report a screen size of normal might have actual screen sizes and aspect ratios that are slightly different when measured by hand. Similarly, two devices that report a screen density of hdpi might have real pixel densities that are slightly different. Android makes these differences abstract to applications, so you can provide UI designed for the generalized sizes and densities and let the system handle any final adjustments as necessary. Figure 1 illustrates how different sizes and densities are roughly categorized into the different size and density groups.

Figure 1. Illustration of how Android roughly maps actual sizes and densities to generalized sizes and densities (figures are not exact).

As you design your UI for different screen sizes, you'll discover that each design requires a minimum amount of space. So, each generalized screen size above has an associated minimum resolution that's defined by the system. These minimum sizes are in "dp" units—the same units you should use when defining your layouts—which allows the system to avoid worrying about changes in screen density.

• 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

Note: These minimum screen sizes were not as well defined prior to Android 3.0, so you may encounter some devices that are mis-classified between normal and large. These are also based on the physical resolution of the screen, so may vary across devices—for example a 1024x720 tablet with a system bar actually has a bit less space available to the application due to it being used by the system bar.

To optimize your application's UI for the different screen sizes and densities, you can provide alternative resources for any of the generalized sizes and densities. Typically, you should provide alternative layouts for some of the different screen sizes and alternative bitmap images for different screen densities. At runtime, the system uses the appropriate resources for your application, based on the generalized size or density of the current device screen.

You do not need to provide alternative resources for every combination of screen size and density. The system provides robust compatibility features that can handle most of the work of rendering your application on any device screen, provided that you've implemented your UI using techniques that allow it to gracefully resize (as described in the Best Practices, below).

Note: The characteristics that define a device's generalized screen size and density are independent from each other. For example, a WVGA high-density screen is considered a normal size screen because its physical size is about the same as the T-Mobile G1 (Android's first device and baseline screen configuration). On the other hand, a WVGA medium-density screen is considered a large size screen. Although it offers the same resolution (the same number of pixels), the WVGA medium-density screen has a lower screen density, meaning that each pixel is physically larger and, thus, the entire screen is larger than the baseline (normal size) screen.

Density independence

Your application achieves "density independence" when it preserves the physical size (from the user's point of view) of user interface elements when displayed on screens with different densities.

Maintaining density independence is important because, without it, a UI element (such as a button) appears physically larger on a low density screen and smaller on a high density screen. Such density-related size changes can cause problems in your application layout and usability. Figures 2 and 3 show the difference between an application when it does not provide density independence and when it does, respectively.

Figure 2. Example application without support for different densities, as shown on low, medium, and high density screens.

Figure 3. Example application with good support for different densities (it's density independent), as shown on low, medium, and high density screens.

The Android system helps your application achieve density independence in two ways:

• The system scales dp units as appropriate for the current screen density


• The system scales drawable resources to the appropriate size, based on the current screen density, if necessary

In figure 2, the text view and bitmap drawable have dimensions specified in pixels (px units), so the views are physically larger on a low density screen and smaller on a high density screen. This is because although the actual screen sizes may be the same, the high density screen has more pixels per inch (the same amount of pixels fit in a smaller area). In figure 3, the layout dimensions are specified in density-independent pixels (dp units). Because the baseline for density-independent pixels is a medium-density screen, the device with a medium-density screen looks the same as it does in figure 2. For the low-density and high-density screens, however, the system scales the density-independent pixel values down and up, respectively, to fit the screen as appropriate.

In most cases, you can ensure density independence in your application simply by specifying all layout dimension values in density-independent pixels (dp units) or with"wrap_content", as appropriate. The system then scales bitmap drawables as appropriate in order to display at the appropriate size, based on the appropriate scaling factor for the current screen's density.

However, bitmap scaling can result in blurry or pixelated bitmaps, which you might notice in the above screenshots. To avoid these artifacts, you should provide alternative bitmap resources for different densities. For example, you should provide higher-resolution bitmaps for high-density screens and the system will use those instead of resizing the bitmap designed for medium-density screens. The following section describes more about how to supply alternative resources for different screen configurations.

How to Support Multiple Screens

The foundation of Android's support for multiple screens is its ability to manage the rendering of an application's layout and bitmap drawables in an appropriate way for the current screen configuration. The system handles most of the work to render your application properly on each screen configuration by scaling layouts to fit the screen size/density and scaling bitmap drawables for the screen density, as appropriate. To more gracefully handle different screen configurations, however, you should also:

• Explicitly declare in the manifest which screen sizes your application supports
  

  By declaring which screen sizes your application supports, you can ensure that only devices with the screens you support can download your application. Declaring support for different screen sizes can also affect how the system draws your application on larger screens—specifically, whether your application runs in screen compatibility mode.

  
  

  To declare the screen sizes your application supports, you should include the <supports-screens> element in your manifest file.

  
  


• Provide different layouts for different screen sizes
  

  By default, Android resizes your application layout to fit the current device screen. In most cases, this works fine. In other cases, your UI might not look as good and might need adjustments for different screen sizes. For example, on a larger screen, you might want to adjust the position and size of some elements to take advantage of the additional screen space, or on a smaller screen, you might need to adjust sizes so that everything can fit on the screen.

  
  

  The configuration qualifiers you can use to provide size-specific resources are small, normal, large, and xlarge. For example, layouts for an extra large screen should go in layout-xlarge/.

  
  

  Beginning with Android 3.2 (API level 13), the above size groups are deprecated and you should instead use the sw<N>dp configuration qualifier to define the smallest available width required by your layout resources. For example, if your multi-pane tablet layout requires at least 600dp of screen width, you should place it inlayout-sw600dp/. Using the new techniques for declaring layout resources is discussed further in the section about Declaring Tablet Layouts for Android 3.2.

  
  


• Provide different bitmap drawables for different screen densities
  

  By default, Android scales your bitmap drawables (.png, .jpg, and .gif files) and Nine-Patch drawables (.9.png files) so that they render at the appropriate physical size on each device. For example, if your application provides bitmap drawables only for the baseline, medium screen density (mdpi), then the system scales them up when on a high-density screen, and scales them down when on a low-density screen. This scaling can cause artifacts in the bitmaps. To ensure your bitmaps look their best, you should include alternative versions at different resolutions for different screen densities.

  
  

  The configuration qualifiers you can use for density-specific resources are