Not only do Android devices come in different screen sizes (handsets, tablets, TVs, and so on), but their screens also have different pixel sizes. That is, while one device has 160 pixels per square inch, another device fits 480 pixels in the same space. If you don't consider these variations in pixel density, the system might scale your images (resulting in blurry images) or the images might appear at the completely wrong size.
This page shows you how you can design your app to support different pixel densities by using resolution-independent units of measurements and providing alternative bitmap resources for each pixel density.
Watch the video below for an overview of these techniques.
For more information about designing the actual icons assets, see the material design icon guidelines.
Use density-independent pixels
The first pitfall you must avoid is using pixels to define distances or sizes. Defining dimensions with pixels is a problem because different screens have different pixel densities, so the same number of pixels may correspond to different physical sizes on different devices.
To preserve the visible size of your UI on screens with different densities, you must design your UI using density-independent pixels (dp) as your unit of measurement. One dp is a virtual pixel unit that's roughly equal to one pixel on a medium-density screen (160dpi; the "baseline" density). Android translates this value to the appropriate number of real pixels for each other density.
For example, consider the two devices in figure 2. If you were to define a view to be "100px" wide, it will appear much larger on the device on the left. So you must instead use "100dp" to ensure it appears the same size on both screens.
When defining text sizes, however, you should instead use scale-independent pixels (sp) as your units (but never use sp for layout sizes). The sp unit is the same size as dp, by default, but it resizes based on the user's preferred text size.
For example, when you specify spacing between two views, use
<Button android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="@string/clickme" android:layout_marginTop="20dp" />
When specifying text size, always use
<TextView android:layout_width="match_parent" android:layout_height="wrap_content" android:textSize="20sp" />
Convert dp units to pixel units
In some cases, you will need to express dimensions in
dp and then
convert them to pixels. The conversion of dp units to screen pixels is simple:
px = dp * (dpi / 160)
Imagine an app in which a scroll or fling gesture is recognized after the user's
finger has moved by at least 16 pixels. On a baseline screen, a user's must move by
/ 160 dpi, which equals 1/10th of an inch (or 2.5 mm) before the gesture is recognized. On a device
with a high-density display (240dpi), the user's must move by
16 pixels / 240 dpi, which
equals 1/15th of an inch (or 1.7 mm). The distance is much shorter and the app thus appears
more sensitive to the user.
To fix this issue, the gesture threshold must be expressed in code in
dp and then
converted to actual pixels. For example:
// The gesture threshold expressed in dp private static final float GESTURE_THRESHOLD_DP = 16.0f; // Get the screen's density scale final float scale =
getDisplayMetrics().density; // Convert the dps to pixels, based on density scale mGestureThreshold = (int) (GESTURE_THRESHOLD_DP * scale + 0.5f); // Use mGestureThreshold as a distance in pixels...
specifies the scale factor you must use to convert
dp units to
pixels, according to the current pixel density. On a medium-density screen,
1.0; on a high-density screen it equals 1.5; on an extra-high-density screen,
it equals 2.0; and on a low-density screen, it equals 0.75. This figure is
the factor by which you should multiply the
dp units in order to
get the actual pixel count for the current screen.
Use pre-scaled configuration values
You can use the
ViewConfiguration class to access common
distances, speeds, and times used by the Android system. For instance, the
distance in pixels used by the framework as the scroll threshold can be obtained
private static final int GESTURE_THRESHOLD_DP = ViewConfiguration.get(myContext).getScaledTouchSlop();
ViewConfiguration starting with the
are guaranteed to return a value in pixels that will display properly regardless of the current
Provide alternative bitmaps
To provide good graphical qualities on devices with different pixel densities, you should provide multiple versions of each bitmap in your app—one for each density bucket, at a corresponding resolution. Otherwise, Android must scale your bitmap so it occupies the same visible space on each screen, resulting in scaling artifacts such as blurring.
There are sevaral density buckets available for use in your apps. Table 1 describes the different configuration qualifiers available and what screen types they apply to.
||Resources for low-density (ldpi) screens (~120dpi).|
||Resources for medium-density (mdpi) screens (~160dpi). (This is the baseline density.)|
||Resources for high-density (hdpi) screens (~240dpi).|
||Resources for extra-high-density (xhdpi) screens (~320dpi).||
||Resources for extra-extra-high-density (xxhdpi) screens (~480dpi).||
||Resources for extra-extra-extra-high-density (xxxhdpi) uses (~640dpi).|
||Resources for all densities. These are density-independent resources. The system does not scale resources tagged with this qualifier, regardless of the current screen's density.|
||Resources for screens somewhere between mdpi and hdpi; approximately 213dpi. This is not considered a "primary" density group. It is mostly intended for televisions and most apps shouldn't need it—providing mdpi and hdpi resources is sufficient for most apps and the system will scale them as appropriate. If you find it necessary to provide tvdpi resources, you should size them at a factor of 1.33*mdpi. For example, a 100px x 100px image for mdpi screens should be 133px x 133px for tvdpi.|
To create alternative bitmap drawables for different densities, you should follow the 3:4:6:8:12:16 scaling ratio between the six primary densities. For example, if you have a bitmap drawable that's 48x48 pixels for medium-density screens, all the different sizes should be:
- 36x36 (0.75x) for low-density (ldpi)
- 48x48 (1.0x baseline) for medium-density (mdpi)
- 72x72 (1.5x) for high-density (hdpi)
- 96x96 (2.0x) for extra-high-density (xhdpi)
- 144x144 (3.0x) for extra-extra-high-density (xxhdpi)
- 192x192 (4.0x) for extra-extra-extra-high-density (xxxhdpi)
Then, place the generated image files in the appropriate subdirectory
res/ and the system will pick the correct one automatically
based on the pixel density of the device your app is running on:
res/ drawable-xxxhdpi/ awesome-image.png drawable-xxhdpi/ awesome-image.png drawable-xhdpi/ awesome-image.png drawable-hdpi/ awesome-image.png drawable-mdpi/ awesome-image.png
Then, any time you reference
the system selects the appropriate bitmap based on the screen's dpi. If you
don't provide a density-specific resource for that density, the system picks
the next best match and scales it to fit the screen.
Tip: If you have some drawable resources that
the system should never scale (perhaps because you perform some adjustments to
the image yourself at runtime), you should place them in a directory with the
nodpi configuration qualifier. Resources with this qualifier are
considered density-agnostic and the system will not scale them.
For more information about other configuration qualifiers and how Android selects the appropriate resources for the current screen configuration, see Providing Resources.
Put app icons in mipmap directories
Like all other bitmap assets, you need to provide density-specific versions of you app icon. However, some app launchers display your app icon as much as 25% larger than what's called for by the device's density bucket.
For example, if a device's density bucket is xxhdpi and the largest app icon you
provide is in
drawable-xxhdpi, the launcher app scales up this icon,
and that makes it appear less crisp. So you should provide an even higher
density launcher icon in the
mipmap-xxxhdpi directory. Now the launcher can
use the xxxhdpi asset instead.
Because your app icon might be scaled up like this, you should put all
your app icons in
mipmap directories instead of
drawable directories. Unlike
drawable directory, all
mipmap directories are retained in the APK even
if you build density-specific APKs. This allows launcher apps to pick the best
resolution icon to display on the home screen.
res/ mipmap-xxxhdpi/ launcher-icon.png mipmap-xxhdpi/ launcher-icon.png mipmap-xhdpi/ launcher-icon.png mipmap-hdpi/ launcher-icon.png mipmap-mdpi/ launcher-icon.png
For icon design guidelines, see the material guide for icons.
For help building app icons, see Create App Icons with Image Asset Studio.
Use vector graphics instead
An alternative to creating multiple density-specific versions of an image is to create just one vector graphic. Vector graphics create an image using XML to define paths and colors, instead of using pixel bitmaps. As such, vector graphics can scale to any size without scaling artifacts, though they're usually best for illustrations such as icons, not photographs.
Vector graphics are often provided as an SVG (Scalable Vector Graphics) file, but Android does not support this format so you must convert SVGs files to Android's vector drawable format.
You can easily convert an SVG to a vector drawable from Android Studio using Vector Asset Studio as follows:
- In the Project window, right-click on the res directory and select New > Vector Asset.
- Select Local file (SVG, PSD).
Locate the file you want to import and make any adjustments.
You might notice some errors appear in the Asset Studio window, indicating some properties of the file that vector drawables do not support. But this will not prevent you from importing—the unsupported properties are simply ignored.
On the next screen, confirm the source set where you want the file in your project and click Finish.
Because one vector drawable can be used on all pixel densities, this file goes in your default drawables directory (you don't need to use density-specific directories):
res/ drawable/ ic_android_launcher.xml
For more information about creating vector graphics, read the Vector Drawable documentation.
Advice for uncommon density issues
This section describes more about how Android performs scaling for bitmaps on different pixel densities and how you can further control how bitmaps are drawn on different densities. Unless your app manipulates graphics or you have encountered problems when running on different pixel densities, you can ignore this section.
To better understand how you can support multiple densities when manipulating graphics at runtime, you should understand that the system helps ensure the proper scale for bitmaps in the following ways:
- Pre-scaling of resources (such as bitmap drawables)
Based on the density of the current screen, the system uses any density-specific resources from your app. If resources are not available in the correct density, the system loads the default resources and scales them up or down as needed. The system assumes that default resources (those from a directory without configuration qualifiers) are designed for the baseline pixel density (mdpi) and will resize those bitmaps to the appropriate size for the current pixel density.
If you request the dimensions of a pre-scaled resource, the system returns values representing the dimensions after scaling. For example, a bitmap designed at 50x50 pixels for an mdpi screen is scaled to 75x75 pixels on an hdpi screen (if there is no alternative resource for hdpi) and the system reports the size as such.
There are some situations in which you might not want Android to pre-scale a resource. The easiest way to avoid pre-scaling is to put the resource in a resource directory with the
nodpiconfiguration qualifier. For example:
When the system uses the
icon.pngbitmap from this folder, it does not scale it based on the current device density.
- Auto-scaling of pixel dimensions and coordinates
You can disable pre-scaling dimensions and images by setting
"false"in the manifest or programmatically for a
"false". In this case, the system auto-scales any absolute pixel coordinates and pixel dimension values at draw time. It does this to ensure that pixel-defined screen elements are still displayed at approximately the same physical size as they would be at the baseline pixel density (mdpi). The system handles this scaling transparently to the app and reports the scaled pixel dimensions to the app, rather than physical pixel dimensions.
For instance, suppose a device has a WVGA high-density screen, which is 480x800 and about the same size as a traditional HVGA screen, but it's running an app that has disabled pre-scaling. In this case, the system will "lie" to the app when it queries for screen dimensions, and report 320x533 (the approximate mdpi translation for the pixel density). Then, when the app does drawing operations, such as invalidating the rectangle from (10,10) to (100, 100), the system transforms the coordinates by scaling them the appropriate amount, and actually invalidate the region (15,15) to (150, 150). This discrepancy may cause unexpected behavior if your app directly manipulates the scaled bitmap, but this is considered a reasonable trade-off to keep the performance of apps as good as possible. If you encounter this situation, read Converting dp units to pixel units.
Usually, you should not disable pre-scaling. The best way to support multiple screens is to follow the basic techniques described in this document.
If your app manipulates bitmaps or directly interacts with pixels on the screen in some other way, you might need to take additional steps to support different pixel densities. For example, if you respond to touch gestures by counting the number of pixels that a finger crosses, you need to use the appropriate density- independent pixel values, instead of actual pixels, but you can easily convert between dp and px values.
Test on all pixel densities
It's important to test your app on multiple devices with different pixel densities so you can ensure your UI scales correctly. Testing on a physical device is easy but you can also the Android Emulator if you don't have access to physical devices for all the different pixel densities.
If you would rather test on a physical device, but don't want to buy the devices, you can use Firebase Test Lab to access devices in a Google data center.