docs/html/training/displaying-bitmaps/load-bitmap.jd

page.title=Loading Large Bitmaps Efficiently
parent.title=Displaying Bitmaps Efficiently
parent.link=index.html

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@jd:body

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<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#read-bitmap">Read Bitmap Dimensions and Type</a></li>
  <li><a href="#load-bitmap">Load a Scaled Down Version into Memory</a></li>
</ol>

<h2>Try it out</h2>

<div class="download-box">
  <a href="{@docRoot}downloads/samples/DisplayingBitmaps.zip" class="button">Download the sample</a>
  <p class="filename">DisplayingBitmaps.zip</p>
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</div>
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<p>Images come in all shapes and sizes. In many cases they are larger than required for a typical
application user interface (UI). For example, the system Gallery application displays photos taken
using your Android devices's camera which are typically much higher resolution than the screen
density of your device.</p>

<p>Given that you are working with limited memory, ideally you only want to load a lower resolution
version in memory. The lower resolution version should match the size of the UI component that
displays it. An image with a higher resolution does not provide any visible benefit, but still takes
up precious memory and incurs additional performance overhead due to additional on the fly
scaling.</p>

<p>This lesson walks you through decoding large bitmaps without exceeding the per application
memory limit by loading a smaller subsampled version in memory.</p>

<h2 id="read-bitmap">Read Bitmap Dimensions and Type</h2>

<p>The {@link android.graphics.BitmapFactory} class provides several decoding methods ({@link
android.graphics.BitmapFactory#decodeByteArray(byte[],int,int,android.graphics.BitmapFactory.Options)
decodeByteArray()}, {@link
android.graphics.BitmapFactory#decodeFile(java.lang.String,android.graphics.BitmapFactory.Options)
decodeFile()}, {@link
android.graphics.BitmapFactory#decodeResource(android.content.res.Resources,int,android.graphics.BitmapFactory.Options)
decodeResource()}, etc.) for creating a {@link android.graphics.Bitmap} from various sources. Choose
the most appropriate decode method based on your image data source. These methods attempt to
allocate memory for the constructed bitmap and therefore can easily result in an {@code OutOfMemory}
exception. Each type of decode method has additional signatures that let you specify decoding
options via the {@link android.graphics.BitmapFactory.Options} class. Setting the {@link
android.graphics.BitmapFactory.Options#inJustDecodeBounds} property to {@code true} while decoding
avoids memory allocation, returning {@code null} for the bitmap object but setting {@link
android.graphics.BitmapFactory.Options#outWidth}, {@link
android.graphics.BitmapFactory.Options#outHeight} and {@link
android.graphics.BitmapFactory.Options#outMimeType}. This technique allows you to read the
dimensions and type of the image data prior to construction (and memory allocation) of the
bitmap.</p>

<pre>
BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeResource(getResources(), R.id.myimage, options);
int imageHeight = options.outHeight;
int imageWidth = options.outWidth;
String imageType = options.outMimeType;
</pre>

<p>To avoid {@code java.lang.OutOfMemory} exceptions, check the dimensions of a bitmap before
decoding it, unless you absolutely trust the source to provide you with predictably sized image data
that comfortably fits within the available memory.</p>

<h2 id="load-bitmap">Load a Scaled Down Version into Memory</h2>

<p>Now that the image dimensions are known, they can be used to decide if the full image should be
loaded into memory or if a subsampled version should be loaded instead. Here are some factors to
consider:</p>

<ul>
  <li>Estimated memory usage of loading the full image in memory.</li>
  <li>Amount of memory you are willing to commit to loading this image given any other memory
  requirements of your application.</li>
  <li>Dimensions of the target {@link android.widget.ImageView} or UI component that the image
  is to be loaded into.</li>
  <li>Screen size and density of the current device.</li>
</ul>

<p>For example, it’s not worth loading a 1024x768 pixel image into memory if it will eventually be
displayed in a 128x96 pixel thumbnail in an {@link android.widget.ImageView}.</p>

<p>To tell the decoder to subsample the image, loading a smaller version into memory, set {@link
android.graphics.BitmapFactory.Options#inSampleSize} to {@code true} in your {@link
android.graphics.BitmapFactory.Options} object. For example, an image with resolution 2048x1536 that
is decoded with an {@link android.graphics.BitmapFactory.Options#inSampleSize} of 4 produces a
bitmap of approximately 512x384. Loading this into memory uses 0.75MB rather than 12MB for the full
image (assuming a bitmap configuration of {@link android.graphics.Bitmap.Config ARGB_8888}). Here’s
a method to calculate a sample size value that is a power of two based on a target width and
height:</p>

<pre>
public static int calculateInSampleSize(
            BitmapFactory.Options options, int reqWidth, int reqHeight) {
    // Raw height and width of image
    final int height = options.outHeight;
    final int width = options.outWidth;
    int inSampleSize = 1;

    if (height &gt; reqHeight || width &gt; reqWidth) {

        final int halfHeight = height / 2;
        final int halfWidth = width / 2;

        // Calculate the largest inSampleSize value that is a power of 2 and keeps both
        // height and width larger than the requested height and width.
        while ((halfHeight / inSampleSize) &gt; reqHeight
                && (halfWidth / inSampleSize) &gt; reqWidth) {
            inSampleSize *= 2;
        }
    }

    return inSampleSize;
}
</pre>

<p class="note"><strong>Note:</strong> A power of two value is calculated because the decoder uses
a final value by rounding down to the nearest power of two, as per the {@link
android.graphics.BitmapFactory.Options#inSampleSize} documentation.</p>

<p>To use this method, first decode with {@link
android.graphics.BitmapFactory.Options#inJustDecodeBounds} set to {@code true}, pass the options
through and then decode again using the new {@link
android.graphics.BitmapFactory.Options#inSampleSize} value and {@link
android.graphics.BitmapFactory.Options#inJustDecodeBounds} set to {@code false}:</p>

<a name="decodeSampledBitmapFromResource"></a>
<pre>
public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId,
        int reqWidth, int reqHeight) {

    // First decode with inJustDecodeBounds=true to check dimensions
    final BitmapFactory.Options options = new BitmapFactory.Options();
    options.inJustDecodeBounds = true;
    BitmapFactory.decodeResource(res, resId, options);

    // Calculate inSampleSize
    options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);

    // Decode bitmap with inSampleSize set
    options.inJustDecodeBounds = false;
    return BitmapFactory.decodeResource(res, resId, options);
}
</pre>

<p>This method makes it easy to load a bitmap of arbitrarily large size into an {@link
android.widget.ImageView} that displays a 100x100 pixel thumbnail, as shown in the following example
code:</p>

<pre>
mImageView.setImageBitmap(
    decodeSampledBitmapFromResource(getResources(), R.id.myimage, 100, 100));
</pre>

<p>You can follow a similar process to decode bitmaps from other sources, by substituting the
appropriate {@link
android.graphics.BitmapFactory#decodeByteArray(byte[],int,int,android.graphics.BitmapFactory.Options)
BitmapFactory.decode*} method as needed.</p>