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diff --git a/docs/html/training/displaying-bitmaps/load-bitmap.jd b/docs/html/training/displaying-bitmaps/load-bitmap.jd new file mode 100644 index 0000000..c0a5709 --- /dev/null +++ b/docs/html/training/displaying-bitmaps/load-bitmap.jd @@ -0,0 +1,165 @@ +page.title=Loading Large Bitmaps Efficiently +parent.title=Displaying Bitmaps Efficiently +parent.link=index.html + +trainingnavtop=true +next.title=Processing Bitmaps Off the UI Thread +next.link=process-bitmap.html + +@jd:body + +<div id="tb-wrapper"> +<div id="tb"> + +<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}shareables/training/BitmapFun.zip" class="button">Download the sample</a> + <p class="filename">BitmapFun.zip</p> +</div> + +</div> +</div> + +<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 the sample size value 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 > reqHeight || width > reqWidth) { + if (width > height) { + inSampleSize = Math.round((float)height / (float)reqHeight); + } else { + inSampleSize = Math.round((float)width / (float)reqWidth); + } + } + return inSampleSize; +} +</pre> + +<p class="note"><strong>Note:</strong> Using powers of 2 for {@link +android.graphics.BitmapFactory.Options#inSampleSize} values is faster and more efficient for the +decoder. However, if you plan to cache the resized versions in memory or on disk, it’s usually still +worth decoding to the most appropriate image dimensions to save space.</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>
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