안드로이드 Supporting Multiple Screens *****

 

Quickview

  • Android runs on devices that have different screen sizes and resolutions.
  • The screen on which your application is displayed can affect its user interface.
  • The platform handles most of the work of adapting your app to the current screen.
  • You can create screen-specific resources for precise control of your UI, if needed.
  • Older applications run in a compatibility mode that provides best-effort rendering on the current screen.
  • It’s important to follow the best practices described in this document and test your application in all supported screens.

In this document

  1. Overview of Screen Support
    1. Range of screens supported
    2. How Android supports multiple screens
    3. Density independence
    4. Manifest attributes
    5. Resource qualifiers
  2. Best Practices for Screen Independence
  3. Strategies for Legacy Apps
  4. How to Test Your App

See Also

  1. <supports-screens>
  2. <uses-sdk>
  3. Alternative Resources
  4. Android Virtual Devices

Android is designed to run on a variety of devices that offer a range of screen sizes and resolutions. For applications, the platform provides a consistent environment across devices and handles much of the complexity of adapting an application’s UI to the screen on which it is being displayed. At the same time, the platform exposes APIs that give application developers precise control over their application’s UI when displayed on specific screen sizes and resolutions.

This document explains the screens-support features provided by the platform and how you use them in your application. By following the practices described here, you can easily create an application that displays properly on all supported device screens and that you can deploy to any device as a single .apk.

If you have already developed and published an application for Android 1.5 or earlier, you should read this document and consider how you may need to adapt your application for proper display on new devices that offer different screens and that are running Android 1.6 or later. In most cases, only minor adjustments are needed, however you should make sure to test your application on all supported screens.

In particular, if you have an existing application that you would like to make available for users of devices with small screens (such as QVGA), please see Strategies for Legacy Applications for more information about how to do that.

Overview of Screens Support

The sections below provide an overview of the Android platform’s support for multiple screens, including an introduction to terms and concepts used in this document and in the API, a summary of the screen configurations that the platform 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 collapses all actual screen sizes into three generalized sizes: large, normal, and small. Applications can provide custom layouts for each of these three sizes — the platform transparently handles the rendering of the layouts at the actual screen size.

Aspect ratio
The porportional relationship of the screen’s physical width to its height. Applications can provide layout resources for specific aspect ratios by using the resource qualifiers long and notlong.
Resolution
The total number of physical pixels on a screen. Note that, although resolution is often expressed as width x height, resolution does not imply a specific aspect ratio. In Android, applications do not work directly with resolution.
Density
Based on the screen resolution, the spread of pixels across the physical width and height of the screen.

A screen with lower density has fewer available pixels spread across the screen width and height, where a screen with higher density has more — sometimes significantly more — pixels spread across the same area. The density of a screen is important because, other things being equal, a UI element (such as a button) whose height and width are defined in terms of screen pixels will appear larger on the lower density screen and smaller on the higher density screen.

For simplicity, Android collapses all actual screen densities into three generalized densities: high, medium, and low. Applications can provide custom resources for each of these three densities — the platform handles the scaling of the resources up or down to meet the actual screen density.

Density-independent pixel (dip)
A virtual pixel unit that applications can use in defining their UI, 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, the baseline density assumed by the platform (as described later in this document). At run time, the platform transparently handles any scaling of the dip units needed, based on the actual density of the screen in use. The conversion of dip units to screen pixels is simple: pixels = dips * (density / 160). For example, on 240 dpi screen, 1 dip would equal 1.5 physical pixels. Using dip units to define your application’s UI is highly recommended, as a way of ensuring proper display of your UI on different screens.

Range of screens supported

Starting from Android 1.6, the platform provides support for multiple screen sizes and resolutions, reflecting the many new types and sizes of devices on which the platform runs. If you are developing an application that will run on Android 1.6 or later, you can use the compatibility features of the Android platform to ensure that your application UI renders properly across the range of supported screen sizes and resolutions.

To simplify the way that developers design their user interfaces for multiple devices and to allow more devices to participate without affecting applications, the platform divides the range of actual supported screen sizes and resolutions into:

  • A set of three generalized sizes: large, normal, and small, and
  • A set of three generalized densities: hdpi (high), mdpi (medium), and ldpi (low)

Applications can provide custom resources (primarily layouts) for any of the three generalized sizes and can provide resources (primarily drawables such as images) for any of the three generalized densities. Applications do not need to work with the actual physical size or density of the device screen. At run time, the platform handles the loading of the correct size or density resources, based on the generalized size or density of the current device screen, and adapts them to the actual pixel map of the screen.

The generalized size/density configurations are arranged around a baseline configuration that is assigned a size of normal and a density of mdpi (medium). All applications written for Android 1.5 or earlier are (by definition) designed for the baseline HVGA screen used on the T-Mobile G1 and similar devices, which is sizenormal and density mdpi.

Each generalized screen configuration spans a range of actual screen densities and physical sizes. For example, that means that multiple devices that report a screen size of normal might offer screens that differ slightly in actual size or aspect ratio. Similarly, devices that report a screen density of hdpi might offer screens with slightly different pixel densities. The platform makes these differences abstract, however — applications can offer UI designed for the generalized sizes and densities and let the system handle the actual rendering of the UI on the current device screen according to its characteristics.

Figure 1. Illustration of how the Android platform maps actual screen densities and sizes to generalized density and size configurations.

Although the platform lets your application provide layouts and resources for generalized size-density configurations, you do not necessarily need to do write custom code or provide custom resources for each of the nine supported configurations. The platform provides robust compatibility features, described in the sections below, that can handle most of the work of rendering your application on any device screen, provided that you’ve implemented your application UI properly. For more information about how to implement a UI that renders properly across device screens and platform versions, see Best Practices for Screen Independence.

To help you test your applications, the Android SDK includes emulator skins that replicate the sizes and densities of actual device screens on which your application is likely to run. You can also modify the default size and density of the emulator skins to replicate the characteristics of any specific screen.

Table 1. Screen sizes and densities of emulator skins included in the Android SDK.

Low density (120), ldpi
Medium density (160), mdpi
High density (240), hdpi

Small screen
QVGA (240×320)

Normal screen
WQVGA400 (240×400)
WQVGA432 (240×432)
HVGA (320×480)
WVGA800 (480×800)
WVGA854 (480×854)

Large screen
WVGA800* (480×800)
WVGA854* (480×854)

* To emulate this configuration, specify a custom density of 160 when creating an AVD that uses a WVGA800 or WVGA854 skin.

For an overview of the relative numbers of high (hdpi), medium (mdpi), and low (ldpi) density screens in Android-powered devices available now, see the Screen Sizes and Densities dashboard.

How Android supports multiple screens

Using the alternative resources framework

The platform’s support for loading screen size- and density-specific resources at run time is based on the alternative resources framework.

If you want to use size- or density-specific layouts or drawables in your application and you are not familiar with resource qualifiers or how the platform uses them, please read Alternative Resources.

The foundation of Android’s support for multiple screens is a set of built-in compatibility features that together manage the rendering of application resources in an appropriate way for the current device screen. The platform handles most of the work of rendering your application, but also gives you two key ways to control how your application is displayed, if you need or want to use them:

  • The platform supports a set of resource qualifiers that let you provide size- and density-specific resources, if needed. The qualifiers for size-specific resources are large, normal, and small, and those for density-specific resources are hdpi (high), mdpi (medium), and ldpi (low). The qualifiers correspond to the generalized densities described in Range of screens supported, above.
  • The platform also provides a <supports-screens> manifest element, whose attributesandroid:largeScreens, android:normalScreens, and android:smallScreens let you specify what generalized screen sizes your application supports. A fourth attribute, android:anyDensity, lets you indicate whether or not your application includes built-in support for multiple densities.

At run time, the platform provides three types of support to your application, to ensure the best possible display on the current device screen:

  1. Pre-scaling of resources (such as image assets)

    Based on the density of the current screen, the platform automatically loads any size- or density-specific resources from your application and displays them without scaling. If no matching resources are available, the platform loads the default resources and scales them up or down as needed to match the current screen’s generalized density. The platform assumes that default resources are designed for proper display at the baseline screen density of "medium" (160), unless they are loaded from a density-specific resource directory.

    For example, if the current screen’s density is "high", the platform loads resources that are tagged with the qualifier hdpi and uses them without scaling. If no such resources are available, the platform uses the default resources instead, scaling them from the baseline density ("medium") to "high".

    For more information about how to create size- and density-specific resources, see Resource qualifiers.

  2. Auto-scaling of pixel dimensions and coordinates

    If the application states that it does not support different screen densities, the platform auto-scales any absolute pixel coordinates, pixel dimension values, and pixel math used in the application (such as might be used for specifying the width or padding for a view). It does this to ensure that pixel-defined screen elements are displayed at approximately the same physical size as they would be at the baseline density of "medium" (160). The platform handles this scaling transparently to the application and also reports scaled overall pixel dimensions to the application, rather than physical pixel dimensions.

    For instance, suppose a given device is using a WVGA high-denisty screen, which is 480×800 and about the same size as a traditional HVGA screen, but it’s running an app that states that it does not support multiple densities. In this case, the system will "lie" to the application when it queries for screen dimensions, and report 320×533. Then, when the app does drawing operations, such as invalidating the rectangle from (10,10) to (100, 100), the system will likewise automatically transform the coordinates by scaling them the appropriate amount, and actually invalidate the region (15,15) to (150, 150). The same thing happens in the other direction, if the application is running on a lower-density screen, coordinates are scaled down.

    For more information, see the android:anyDensity attribute in Manifest attributes for screens support.

    Publishing to Small Screen Devices

    To ensure the best experience for users on small-screen devices, Android Market only shows applications that explicitly declare support for small screens. If you developed an application on Android 1.5 or earlier and published it on Android Market, you need to test your application on small screens and then upload an updated version that explicitly indicates support for small screens.

  3. Compatibility-mode display on larger screen-sizes

    If the current screen’s size is larger than your application supports, as specified in the supports-screens element, the platform displays the application at the baseline size ("normal") and density ("medium). For screens larger than baseline, the platform displays the application in a baseline-sized portion of the overall screen, against a black background.

    For instance, suppose a given device is using a WVGA medium density screen, classified as a "large" screen, but the application states that it does not support large screens; in this case, the system will again "lie" to the application when it queries for screen dimensions, and report 320×480. Instead of scaling the application, however, the application’s 320×480 interface will be placed as a "postage stamp" in the larger 480×800 screen.

    For more information, see the android:anyDensity attribute in Manifest elements for screens supportand the Screen-Compatibility Examples section.

In general, these compatibility features ensure that all applications, including those written against Android 1.5 and earlier platform versions, can display properly on most devices, especially when the device’s screen is at the baseline "normal" size or larger.

However, note that applications written for the baseline screen may need minor adjustments before they display properly on smaller screens such as QVGA. With the reduced screen area of small screens, there may be tradeoffs in design, content, and function that you, as the application developer, need to consider. For more information about how to prepare an existing application for display on small screens, see Strategies for Legacy Applications.

The sections below provide more information how to take advantage of the platform’s multiple-screens support.

Density independence

The goal of density independence is to preserve the physical size, from the user’s point of view, of user interface elements declared in an application, when the application is displayed on screens with different densities. Density independence applies to both layouts and drawables such as icons. Maintaining density-independence is important because, other things being equal, a UI element (such as a button) whose height and width are defined in terms of screen pixels will appear physically larger on the lower density screen and smaller on the higher density screen. Such density-related size changes can cause problems in application layout, usability, and consistency with other applications installed on the device.

The platform provides density independence to applications by default. It does this in three ways:

  • Through pre-scaling of drawable resources (scaled at resource loading time)
  • Through auto-scaling of density-independent pixel (dip) values used in layouts
  • Through auto-scaling of absolute pixel values used in the application (only needed if the application has set android:anyDensity="false" in its manifest)

The example screens below illustrate the density independence provided by the platform. Note that both the layouts and launcher icons are displayed at the same physical sizes, although screen sizes, aspect ratios, and densities are different.

Figure 2. Examples of density independence on WVGA high density (left), HVGA medium density (center), and QVGA low density (right).

In most cases, you can take advantage of density independence in your application simply by making sure that your layouts specify all dimension values in density-independent pixels (dip or dp) or scale-independent pixels (sip or sp, for text only). If you are using absolute pixel values in the application and manifest includesandroid:anyDensity="true", you will also need to scale the pixel values. See Converting from dips to pixels for more information.

Manifest attributes for screens support

Android 1.6 introduced a new manifest element, <supports-screens>, whose attributes you can use to control the display of your application on different classes of device screens, as listed below. The smallScreens, normalScreens, and largeScreens attributes correspond to the generalized screen sizes described in Range of screens supported, earlier in this document.

Attribute
Description
Default value,
Android 1.5 and Lower
Default value,
Android 1.6 and Higher

android:smallScreens
Whether or not the application UI is designed for use on small screens — "true" if it is, and "false" if not.

"false"
"true"

android:normalScreens
Whether or not the application UI is designed for use on normalscreens — "true" if it is, and "false" if not. The default value is always "true".
"true"
"true"

android:largeScreens
Whether or not the application UI is designed for use on large screens — "true" if it is, and "false" if not.
"false"
"true"

android:anyDensity

Whether or not the application is designed to manage its UI properly in different density environments — "true" if so, and "false" if not.

  • If set to "true", the platform disables its density-compatibility features for all screen densities — specifically, the auto-scaling of absolute pixel units (px) and math — and relies on the application to use density-independent pixel units (dp) and/or math to manage the adaptation of pixel values according to density of the current screen. That is, as long as your application uses density-independent units (dp) for screen layout sizes, then it will perform properly on different densities when this attribute is set to "true".
  • If set to "false", the platform enables its density-compatibility features for all screen densities. In this case, the platform provides a scaled, virtual screen pixel map to the application, against which it can layout and draw its UI as though against a medium-density screen (160). The platform then transparently auto-scales the application’s pixel units and math as needed to match the actual device screen density.

Note that the setting of this attribute affects density-compatibility only. It does not affect size-compatibility features such as display on a virtual baseline screen.

"false"
"true"

In general, when you declare a screen-size attribute (smallScreens, normalScreens, or largeScreens) as "true", you are signaling to the platform that your application is designed to render properly on that screen size. As a result, the platform does not apply any size-compatibility features (such as a virtual HVGA display area). If you declare a screen-size attribute as "false", you are signaling that your application is not designed for that screen size. In this case, the platform doesapply size-compatibility features, rendering the application in an HVGA baseline display area. If the current screen is larger than normal size, the platform renders the application in a virtual HVGA screen on the larger screen. See Screen-Compatibility Examples for an illustration of what an application looks like when displayed in a virtual HVGA screen.

In other words, setting a <supports-screens> attribute to "false" tells the platform to enable it’s compatibility features when displaying the application on a screen of that size or any larger size, if also disallowed. Otherwise, the platform gives the application a normal display area that can use the full device screen area, if appropriate.

Android Market also makes use of the <supports-screens> attributes. It uses them to filter the application from devices whose screens are not supported by the application. Specifically, Android Market considers an application compatible with a device if the application supports a screen that is the same or smaller than the current device screen. Android Market filters the application if it disallows the device’s screen size and does not support a smaller size. In general, Android does not provide downward size-compatibility features for applications.

Here are some examples:

  • Assume that you declare smallScreens="false" normalScreens="true" largeScreens="false" in your application’s manifest.

    Although the application is not designed for display on large screens, the platform can still run it successfully in size-compatibility mode. Android Market does not filter the application from devices normal and large size screens, but does filter it from small size screens, since the application provides no screen support atsmall size (and there is no smaller size).

  • Assume that you declare smallScreens="false" normalScreens="false" largeScreens="true" in your application’s manifest.

    Android Market filters the application from users of devices with small and normal size screens. In effect, this prevents such users from installing the application.

If you declare the android:anyDensity attribute as "true", you are signaling to the platform that your application is designed to display properly on any screen density. In this case, the application must ensure that it declares its UI dimensions using density-independent pixels (dp) and scales any absolute pixel values (px) or math by the scaling factor available from android.util.DisplayMetrics.density. See Converting from dips to pixels for an example.

Note that the setting of the android:anyDensity attribute does not affect the platform’s pre-scaling of drawable resources, such as bitmaps and nine-patch images, which always takes place by default.

The following example shows a manifest that declares support for large, normal, and small screens in any densities.

<manifest xmlns:android="http://schemas.android.com/apk/res/android">
  ...
  <supports-screens
          android:largeScreens="true"
          android:normalScreens="true"
          android:smallScreens="true"
          android:anyDensity="true" />
  ...
</manifest>
Default values for attributes

The default values for the <supports-screens> attributes differ, depending on the the value of the android:minSdkVersion attribute in the application’s manifest, as well as on the value of android:targetSdkVersion, if declared:

  • If android:minSdkVersion or android:targetSdkVersion is "4" (Android 1.6) or higher, the default value for everything is "true". If your application uses APIs introduced in Android 1.6 or higher, but does not support specific screen densities and/or screen sizes, you need to explicitly set the appropriate attributes to "false".
  • If android:minSdkVersion is declared with a value of "3" (Android 1.5) or lower and a android:targetSdkVersion attribute is not declared with a value of "4" or higher, the default value for all attributes except android:normalScreens is "false". If you are primarily targeting pre-Android 1.6 platforms but also want to support other densities/screen sizes, you need to explicitly set the appropriate attributes to "true".
  • Note that android:normalScreens always defaults to true.
Resource directory qualifiers for screen size and density

Android supports resource directory qualifiers for controlling the selection of resources based on the characteristics of the screen on which your application is running. You can use these qualifiers to provide size- and density-specific resources in your application. For more information about the generalized sizes and densities that correspond to the qualifiers, see Range of Screens Supported, earlier in this document.

Screen characteristic
Qualifier
Description

Size
small
Resources designed for small size screens.

normal
Resources designed for normal size screens.

large
Resources for large size screens.

Density
ldpi
Resources designed for low-density (ldpi) screens.

mdpi
Resources designed for medium-density (mdpi) screens.

hdpi
Resources designed for high-density (hdpi) screens.

nodpi
Density-independent resources. The platform does not auto-scale resources tagged with this qualifier, regardless of the current screen’s density.

Aspect ratio
long
Resources for screens of any size or density that have a significantly taller (in portrait mode) and wider (in landscape mode) aspect ratio than the baseline screen configuration.

notlong
Resources for use only on screens that have an aspect ratio that is similar to the baseline screen configuration.

Platform version
v<api-level>
Resources that are for use only on a specific API Level or higher. For example, if your application is designed to run on both Android 1.5 (API Level 3) and Android 1.6 (API Level 4 and higher), you can use the -v4 qualifier to tag any resources that should be excluded when your application is running on Android 1.5 (API Level 3).

Note that the density and the screen size are independent parameters and are interpreted by the system individually. For example, WVGA high density is considered a normal screen because its physical size is about the same as one of T-Mobile G1. On the other hand, a WVGA medium density screen is considered a large screen — it offers the same resolution but at lower pixel density, meaning that it is both physically larger than the baseline screen and can display significantly more information than a normal screen size.

Here is an example of the resource directory structure of an application that supports low and high density, and employs different layout schemes.

res/layout/my_layout.xml            // layout for normal screen size
res/layout-small/my_layout.xml      // layout for small screen size
res/layout-large/my_layout.xml      // layout for large screen size
res/layout-large-land/my_layout.xml // layout for large screen size in landscape mode

res/drawable-ldpi/my_icon.png       // icon image for low density
res/drawable-mdpi/dpi/my_icon.png   // icon for medium density
res/drawable-hdpi/my_icon.png       // icon image for high density

res/drawable-nodpi/composite.xml    // density independent resource

For more information about how to use resource qualifiers or how the platform selects them, please read Alternative Resources.

Best practices for Screen Independence

The objective of supporting multiple screens is to create an application that can run properly on any display and function properly on any of the generalized screen configurations supported by the platform.

You can easily ensure that your application will display properly on different screens. Here is a quick checklist:

  1. Prefer wrap_content, fill_parent and the dip unit to px in XML layout files
  2. Avoid AbsoluteLayout
  3. Do not use hard coded pixel values in your code
  4. Use density and/or resolution specific resources
1. Prefer wrap_content, fill_parent and the dip unit to absolute pixels

When defining the layout_width and layout_height of views in an XML layout file, using wrap_content, fill_parent or the dip will guarantee that the view is given an appropriate size on the current device screen. For instance, a view with a layout_width="100dip" will measure 100 pixels wide on an HVGA@160 density display and 150 pixels on a WVGA@240 density display, but the view will occupy approximately the same physical space.

Similarly, you should prefer the sp (scale-independent pixel, the scale factor depends on a user setting) or dip (if you don’t want to allow the user to scale the text) to define font sizes.

2. Avoid AbsoluteLayout

AbsoluteLayout is one of the layout containers offered by the Android UI toolkit. Unlike the other layouts however, AbsoluteLayout enforces the use of fixed positions which might easily lead to user interfaces that do not work well on different displays. Because of this, AbsoluteLayout was deprecated in Android 1.5 (API Level 3).

You can achieve much the same layout by using a FrameLayout instead, and setting layout_margin attributes of the children. This approach is more flexible and will yield better results on different screens.

3. Do not use hard-coded pixel values in your code

For performance reasons and to keep the code simpler, the Android framework API uses pixels as the standard unit for expressing dimension or coordinate values. That means that the dimensions of a View are always expressed in the code in pixels. For instance, if myView.getWidth() returns 10, the view is 10 pixels wide. In some cases, you may need to scale the pixel values that you use in your code. The sections below provide more information.

Converting from dips to pixels

In some cases, you will need to express dimensions in dip and then convert them to pixels. Imagine an application in which a scroll gesture is recognized after the user’s finger has moved by at least 16 pixels. On a baseline screen, the user will have to move his finger by 16 pixels / 160 dpi = 1/10th of an inch (or 2.5 mm) before the gesture is recognized. On a device with a high (240) density display, the user will move his finger by only 16 pixels / 240 dpi = 1/15th of an inch (or 1.7 mm.) The distance is much shorter and the application thus appears more sensitive to the user. To fix this issue, the gesture threshold must be expressed in the code in dipand then converted to actual pixels.

// The gesture threshold expressed in dip
private static final float GESTURE_THRESHOLD_DIP = 16.0f;

// Convert the dips to pixels
final float scale = getContext().getResources().getDisplayMetrics().density;
mGestureThreshold = (int) (GESTURE_THRESHOLD_DIP * scale + 0.5f);

// Use mGestureThreshold as a distance in pixels

The android.util.DisplayMetrics.density field specifies the the scale factor you must use to convert dips to pixels according to the current screen density. You can access the current screen’s metrics through a Context or Activity. On a medium (160) density screen, DisplayMetrics.density equals "1.0", whereas on a high (240) density screen it equals "1.5". You can refer to the documentation of the DisplayMetrics class for details.

Use pre-scaled configuration values

The ViewConfiguration class can be used to access the most common distances, speeds, and times used in the Android framework. For instance, the distance in pixels used by the framework as the scroll threshold can be obtained as follows:

ViewConfiguration.get(aContext).getScaledTouchSlop()

Methods starting with the getScaled prefix are guaranteed to return a value in pixels that will display properly regardless of the current screen density.

4. Use density and/or size-specific resources

Figure 3. Comparison of pre-scaled and auto-scaled bitmaps.

Even with the size- and density-compatibility features that the platform provides, you may still want to make adjustments to the UI of your application when it displayed on certain screen sizes or densities. You can do this by providing size- or density-specific resources — assets, layouts, strings, and so on. If you want, you can also take control over the scaling of images assets. The sections below provide more information.

Custom resources and directories

If you need to control exactly how your application will look on various displays, simply adjust your assets and layouts in configuration-specific resources directories. For example, consider an icon that you want to display on medium and high density screens. Simply create your icon at two different sizes (for instance 100×100 for medium density and 150×150 for high density) and put the two variations in the appropriate directories, using the proper qualifiers:

res/drawable-mdpi/icon.png   // for medium-density screens

res/drawable-hdpi/icon.png   // for high-density screens

If a density qualifier is not defined in a resource directory name, the platform assumes that the resources in that directory are designed for the baseline medium density. It is not recommended that you put density-specific resources such as images in the default directory.

For more information about valid resource qualifiers, see Resource directory qualifiers, earlier in this document.

Pre-scaling and auto-scaling of bitmaps and nine-patches

When a bitmap or nine-patch image is loaded from the application’s resources, the platform attempts to pre-scale it to match the display’s density. For instance, if you placed a 100×100 icon in the res/drawable/ directory and loaded that icon as a bitmap on a high-density screen, Android would automatically scale up the icon and produce a 150×150 bitmap.

This pre-scaling mechanism works independently of the source. For instance, an application targeted for a high-density screen may have bitmaps only in the res/drawable-hdpi/ directory. If one of the bitmaps is a 240×240 icon and is loaded on a medium-density screen, the resulting bitmap will measure 160×160.

The platform pre-scales resources as needed, whether the application is running with density-compatibility features enabled or not (as specified by the value of android:anyDensity). However, when running with density-compatibility enabled, the platform continues to report the size of pre-scaled bitmaps and other resources as if they were loaded in a medium-density environment. For example, when density-compatibility is enabled, if you load a 76×76 image from the default resources for display on a high-density screen, the platform will pre-scale the image to 114×114 internally. However, the API still reports the size of the image as 76×76. This discrepancy may cause unexpected behavior if your application somehow directly manipulates the scaled bitmap, but this was considered a reasonable trade-off to keep the performance of existing applications as good as possible.

This does not apply for the case that an application creates an in-memory bitmap internally and draws something on it, for later display on the screen. The platform auto-scales such bitmaps on the fly, at draw time. Other side effects of such a case might be that fonts drawn in such a bitmap will be scaled at the bitmap level, when the off-screen bitmap is finally rendered to the display, resulting in scaling artifacts.

There are situations in which you may not want Android to automatically scale a resource. The easiest way to accomplish this is to put it in a "nodpi" resource directory:

res/drawable-nodpi/icon.png

You can also take complete control of the scaling mechanism by using the BitmapFactory.Options class, which lets you define whether you want the bitmap to be pre-scaled and what the density of the bitmap should be. For instance, if you are loading a bitmap from a web server, you may want to force the bitmap’s density to be high density. When pre-scaling is disabled, the resulting bitmap is in auto-scaling mode. The bitmap is associated with a density (that you may or may not have specified through the BitmapFactory.Options) which will be used to scale the bitmap on screen at drawing time.

Using auto-scaling instead of pre-scaling is more CPU expensive than pre-scaling but uses less memory. You can refer to the documentation of BitmapFactory,Bitmap, and Canvas for more information on auto-scaling.

Figure 3, at right, demonstrates the results of the pre-scale and auto-scale mechanisms when loading low (120), medium (160) and high (240) density bitmaps on a baseline screen. The differences are subtle, because all of the bitmaps are being scaled to match the current screen density, however the scaled bitmaps have slightly different appearances depending on whether they are pre-scaled or auto-scaled at draw time.

Strategies for Legacy Applications

If you have already developed and published an Android application based on Android 1.5 or earlier platform version, you need to consider how you will adapt your application so that it is deployable to

  • Existing devices, which may be running Android 1.5 (or lower) platform version, as well as to
  • Newer devices that are running Android 1.6 (or higher) and offering various screen sizes and resolutions

To support the newer devices and the different screens they use, you might need to make some changes in your app, but at the same time your app may be very stable and so you want to minimize the changes. There are a variety of ways that you can extend your existing application to support new devices with multiple screens and existing devices running older platform versions. You should be able to make these changes to your application such that you can distribute a single .apk to any and all devices.

The recommended strategy is to develop against the most recent version of the platform you are targeting, and test on the minimum one you want to run on. Here’s how to do that:

  1. Maintain compatibility with existing devices by leaving your application’s android:minSdkVersion attribute as it is. You do not need to increment the value of the attribute to support new devices and multiple screens.
  2. Extend compatibility for Android 1.6 (and higher) devices by adding a new attribute — android:targetSdkVersion — to the uses-sdk element. Set the value of the attribute to "4". This allows your application to "inherit" the platform’s multiple screens support, even though it is technically using an earlier version of the API.
  3. Add an empty <supports-screens> element as a child of <manifest>. If you need to enable size or density attributes later, this is where you will add them.
  4. Change your application’s build properties, such that it compiles against the Android 1.6 (API Level 4) library, rather than against the Android 1.5 (or earlier) library. You will not be able to compile your application against the older platform because of the new manifest attribute.
  5. Set up AVDs for testing your application on Android 1.6 and higher releases. Create AVDs that use the screen sizes and densities that you want to support. When you create the AVDs, make sure to select the Android 1.6 or higher platform as the system image to run. For more information, see How to Test Your Application on Multiple Screens, below.
  6. Set up AVDs for testing your application on Android 1.5 (or earlier platform). You need AVDs running the older platforms you are targeting, so that you can test for compatibility and ensure that there are no functional regressions.
  7. Compile your application against the Android 1.6 library and run it on the AVDs you created. Observe the way your application looks and runs, and test all of the user interactions.
  8. Debug any display or functional issues. For issues that you resolve in your application code, make certain not to use any APIs introduced in API Level 4 or later. If you are in doubt, refer to SDK reference documentation and look for the API Level specifier for the API you want to use. Using an API introduced in API Level 4 or later will mean that your application will no longer be compatible with devices running Android 1.5 or earlier.
  9. For resource-related issues, you can try resolving them by:
    • Adding a anyDensity="false" attribute to <supports-screens>, to enable density-compatibility scaling.
    • Creating any size- or density-specific resources you need and placing them in directories tagged with the correct qualifiers. Qualifiers must be arranged in a proscribed order. See Alternative Resources for more information.
    • Note that if you add size- or density-specific resource directories tagged with any of the resource qualifiers listed in this document, you should make sure to also tag those directories with the v<api-level> qualifier (for example, -v4). This ensures that those resources will be ignored when the application is run on Android 1.5 or lower platform versions.
  10. If your application does not offer support (such as custom layouts) for large screens and you want the platform to display your application in screen-compatibility mode on larger screens, add a largeScreens="false" attribute to the <supports-screens> element in the manifest. See Screen-Compatibility Examples for illustrations of how the platform displays your application in this case.
  11. If your application does not offer support (such as custom layouts) for small screens (such as on a QVGA low-density screen) and you do not want Android Market to offer the application to users of small-screen devices, you must add a smallScreens="false" attribute to the <supports-screens> element.
  12. Continue testing and debugging until your application performs as expected on all of the platforms and screen sizes your application will support.
  13. Export, zipalign, and sign your application using the same private key you used when publishing the previous version, then publish the application to users as an update.

In particular, remember to test your application on an AVD that emulates a small-screen device. Devices that offer screens with QVGA resolution at low density are available now. Users of those devices may want to download your application, so you should understand how your application will look and function on a small-screen device. In many cases, the reduced screen area and density mean that you may need to make tradeoffs in design, content, and function on those devices.

How to Test Your Application on Multiple Screens

Before publishing an application that supports multiple screens, you should thoroughly test it in all of the targeted screen sizes and densities. You can test how it displays with the platform’s compatibility features enabled or with screen-specific UI resources included in your application. The Android SDK includes all the tools you need to test your application on any supported screen.

Figure 4. A typical set of AVDs for testing screens support.

As a test environment for your applications, set up a series of AVDs that emulate the screen sizes and densities you want to support. The Android SDK includes six emulator skins to get you started. You can use the Android AVD Manager or the android tool to create AVDs that use the various emulator skins and you can also set up custom AVDs to test densities other than the defaults. For general information about working with AVDs, see Android Virtual Devices.

The Android SDK provides a set of default emulator skins that you can use for testing. The skins are included as part of each Android platform that you can install in your SDK. The Android 1.6 platform offers these default skins:

  • QVGA (240×320, low density, small screen)
  • HVGA (320×480, medium density, normal screen)
  • WVGA800 (480×800, high density, normal screen)
  • WVGA854 (480×854 high density, normal screen)

The Android 2.0 platform offers all of the Android 1.6 default skins, above, plus:

  • WQVGA400 (240×400, low density, normal screen)
  • WQVGA432 (240×432, low density, normal screen)

If you are using the android tool command line to create your AVDs, here’s an example of how to specify the skin you want to use:

android create avd ... --skin WVGA800

We also recommend that you test your application in an emulator that is set up to run at a physical size that closely matches an actual device. This makes it a lot easier to compare the results at various resolutions and densities. To do so you will need to know the approximate density, in dpi, of your computer monitor (a 30" Dell monitor has for instance a density of about 96 dpi.). Use your monitor’s dpi as the value of the -scale option, when launching the emulator, for example:

emulator -avd <name> -scale 96dpi

If you are working in Eclipse with ADT, you can specify the -scale 96dpi option in the Target tab of run and debug configurations, under "Additional Emulator Command Line Options" field.

Note that starting the emulator with the -scale option will scale the entire emulator display, based on both the dpi of the skin and of your monitor. The default emulator skins included in the Android SDK are listed in Table 1, earlier in this document.

Figure 5. Resolution and density options that you can use, when creating an AVD using the AVD Manager.

You should also make sure to test your application on different physical screen sizes within a single size-density configuration. For example, to display this screen configuration on a 30" monitor you will need to adjust the value passed to -scale to 96*2.8/3.3 = 81dpi. You can also pass a float value to -scale to specify your own scaling factor:

emulator -avd <name> -scale 0.6

If you would like to test your application on a screen that uses a resolution or density not supported by the built-in skins, you can either adjust an existing skin, or create an AVD that uses a custom resolution or density.

In the AVD Manager, you can specify a custom skin resolution or density in the Create New AVD dialog, as shown in Figure 5, at right.

In the android tool, follow these steps to create an AVD with a custom resolution or density:

  1. Use the create avd command to create a new AVD, specifying the--skin option with a value that references either a default skin name (such as "WVGA800") or a custom skin resolution (such as 240×432). Here’s an example:
    android create avd -n <name> -t <targetID> --skin WVGA800
  2. To specify a custom density for the skin, answer "yes" when asked whether you want to create a custom hardware profile for the new AVD.
  3. Continue through the various profile settings until the tool asks you to specify "Abstracted LCD density" (hw.lcd.density). Enter an appropriate value, such as "120" for a low-density screen, "160" for a medium density screen, or "240" for a high-density screen.
  4. Set any other hardware options and complete the AVD creation.

In the example above (WVGA medium density), the new AVD will emulate a 5.8" WVGA screen.

As an alternative to adjusting the emulator skin configuration, you can use the emulator skin’s default density and add the -dpi-device option to the emulator command line when starting the AVD. For example,

emulator -avd WVGA800 -scale 96dpi -dpi-device 160

Screen-Compatibility Examples

This section provides examples of how the Android platform displays an application written for the baseline screen configuration — HVGA (320×480) resolution on a 3.2" screen — with all of the platform’s size- and density-compatibility features enabled. That is, the examples show how the platform displays an application that doesn’t provide built-in support for the screen on which it is being rendered, but which instead relies completely on the platform.

The platform’s screen-compatibility features are designed to provide such an application with a virtual baseline screen environment against which to run, while at the same time ensuring for the user a physical display that is approximately the same as the baseline screen size and density.

Legacy applications that have not been modified to support multiple screens would be typical examples of such applications. In most cases, you would want to add multiple-screens support to a legacy application and publish an updated version, as described in Strategies for Legacy Applications. However, if you did not do so, the platform still performs best-effort rendering of your application, as illustrated below.

Internally, these are the compatibility features that the platform provides, based on the current device screen:

  • If the device’s screen density is not medium, the application’s layout and drawing of its content is as if the screen is medium density, but the framework scales the layout and images (if the image for the target density is not available) to fit the target density. It scales 1.5 times if the target density is high density (160->240 virtual dpi), or 0.75 times if the target density is low density (160 -> 120 virtual dpi).
  • If the device’s screen size is small, there are few options options for making Android 1.5 applications work well on such a screen, so Android Market will filter applications that are not known to support these screens from the device.
  • If the device’s screen size is large, it limits the application’s screen to the normal size and draws a black background around the application. For example, if an application supports high density, but does not support large screens, it only uses a 480×720 area of the screen and the rest will be filled with a black background (see example below).

HVGA, normal size, normal density
[ emulator -skin HVGA ]

WVGA, normal size, high density
[emulator -skin WVGA854 -dpi-device 240]

The application occupies full screen as its considered to be normal size. (close to 480×720)

VGA, large size, medium density
[ emulator -skin 640×480 ]

The application occupies 320×480 of VGA.

SVGA, large size, high density
[ emulator -skin 800×600 -dpi-device 240]

The application occupies 480×720 (=1.5 x [320×480]) of 800×600.

Screen-compatibility limitations on small, low-density screens

Because these device has smaller state/resolution, there are known limitations when application runs in compatibility mode.

QVGA

Because QVGA (240×320) screens have less screen area available and lower density than normal, which is 240×360 in low density, some applications cannot render all their content properly on those screens.  As a result, on a QVGA device, Android Market will filter out all applications that do not declare they support small screens.

Examples:

The part of z value graph is chopped.
The lap time area is chopped.


Images with 1 pixel height/width.

If an image has 1 pixel height or width, it may not be shown on the screen due to rounding issue. This is inevitable as it just does not have enough pixels.

For example, in the screen below, the divider in the menu is invisible because the width of the image is trancated to 0. (This particular problem is solvable because menu is handled inside framework, but there is no generic solution as it just does not have enough pixels.)

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Android 2.2 r1 – 01 Nov 2010 16:46

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