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      1 page.title=Supporting Different Densities
      2 page.metaDescription=Providing sets of layouts and drawable resources for specific ranges of device screens.
      3 meta.tags="multiple screens"
      4 
      5 parent.title=Designing for Multiple Screens
      6 parent.link=index.html
      7 
      8 trainingnavtop=true
      9 previous.title=Supporting Different Screen Sizes
     10 previous.link=screensizes.html
     11 next.title=Implementing Adaptative UI Flows
     12 next.link=adaptui.html
     13 
     14 @jd:body
     15 
     16 
     17 <!-- This is the training bar -->
     18 <div id="tb-wrapper"> 
     19 <div id="tb"> 
     20 
     21 <h2>This lesson teaches you to</h2>
     22 <ol>
     23   <li><a href="#TaskUseDP">Use Density-independent Pixels</a></li>
     24   <li><a href="#TaskProvideAltBmp">Provide Alternative Bitmaps</a></li>
     25 </ol>
     26 
     27 <h2>You should also read</h2>
     28 
     29 <ul>
     30   <li><a href="{@docRoot}guide/practices/screens_support.html">Supporting Multiple Screens</a></li>
     31   <li><a href="{@docRoot}guide/practices/ui_guidelines/icon_design.html">Icon Design
     32 Guidelines</a></li>
     33 </ul>
     34 
     35 <h2>Try it out</h2>
     36  
     37 <div class="download-box"> 
     38 <a href="http://developer.android.com/shareables/training/NewsReader.zip" class="button">Download
     39   the sample app</a>
     40 <p class="filename">NewsReader.zip</p> 
     41 </div> 
     42  
     43  
     44 </div> 
     45 </div> 
     46 
     47 <p>This lesson shows you how to support different screen densities
     48 by providing different resources and using resolution-independent units of
     49 measurements.</p>
     50 
     51 <h2 id="TaskUseDP">Use Density-independent Pixels</h2>
     52 
     53 <p>One common pitfall you must avoid when designing your layouts is using
     54 absolute pixels to define distances or sizes. Defining layout dimensions with
     55 pixels is a problem because different screens have different pixel densities,
     56 so the same number of pixels may correspond to different physical sizes on
     57 different devices. Therefore, when specifying dimensions, always use either 
     58 <code>dp</code> or <code>sp</code> units. A <code>dp</code> is a density-independent pixel
     59 that corresponds to the physical size of a pixel at 160 dpi. An <code>sp</code> is the same
     60 base unit, but is scaled by the user's preferred text size (its a
     61 scale-independent pixel), so you should use this measurement unit when defining
     62 text size (but never for layout sizes).</p>
     63 
     64 <p>For example, when you specify spacing between two views, use <code>dp</code> 
     65 rather than <code>px</code>:</p>
     66 
     67 <pre>
     68 &lt;Button android:layout_width="wrap_content" 
     69     android:layout_height="wrap_content" 
     70     android:text="&#64;string/clickme"
     71     android:layout_marginTop="20dp" /&gt;
     72 </pre>
     73 
     74 <p>When specifying text size, always use <code>sp</code>:</p>
     75 
     76 <pre>
     77 &lt;TextView android:layout_width="match_parent" 
     78     android:layout_height="wrap_content" 
     79     android:textSize="20sp" /&gt;
     80 </pre>
     81 
     82 
     83 <h2 id="TaskProvideAltBmp">Provide Alternative Bitmaps</h2>
     84 
     85 <p>Since Android runs in devices with a wide variety of screen densities,
     86 you should always provide your bitmap resources tailored to each of
     87 the generalized density buckets: low, medium, high and extra-high density.
     88 This will help you achieve good graphical quality and performance on all
     89 screen densities.</p>
     90 
     91 <p>To generate these images, you should start with your raw resource in
     92 vector format and generate the images for each density using the following
     93 size scale:</p>
     94 
     95 <p><ul>
     96   <li><code>xhdpi</code>: 2.0
     97   <li><code>hdpi</code>: 1.5
     98   <li><code>mdpi</code>: 1.0 (baseline)
     99   <li><code>ldpi</code>: 0.75
    100 </ul></p>
    101 
    102 <p>This means that if you generate a 200x200 image for <code>xhdpi</code>
    103 devices, you should generate the same resource in 150x150 for <code>hdpi</code>,
    104 100x100 for <code>mdpi</code> and finally a 75x75 image for <code>ldpi</code>
    105 devices.</p>
    106 
    107 <p>Then, place the generated image files in the appropriate subdirectory
    108 under <code>res/</code> and the system will pick the correct one automatically
    109 based on the screen density of the device your application is running on:</p>
    110 
    111 <pre class="classic no-pretty-print">
    112 MyProject/
    113   res/
    114     drawable-xhdpi/
    115         awesomeimage.png
    116     drawable-hdpi/
    117         awesomeimage.png
    118     drawable-mdpi/
    119         awesomeimage.png
    120     drawable-ldpi/
    121         awesomeimage.png
    122 </pre>
    123 
    124 <p>Then, any time you reference <code>&#64;drawable/awesomeimage</code>, the system selects the
    125 appropriate bitmap based on the screen's dpi.</p>
    126 
    127 <p>For more tips and guidelines for creating icon assets for your application, see the <a
    128 href="{@docRoot}guide/practices/ui_guidelines/icon_design.html">Icon Design
    129 Guidelines</a>.</p>
    130 
    131