Handling large/high resolution images-优快云博客

本文介绍了如何提高Google TV设备上应用的图像质量，避免因使用低分辨率图片而导致的像素化、模糊等问题。通过按需加载图片、回收不再使用的Bitmap资源、使用弱引用存储Bitmap对象等方法来降低内存占用，并提供了从网络下载大尺寸图片时进行适当缩放的示例代码。

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Google TV devices have higher display resolution. If you provide low resolution or small images in your layouts, they will appear pixelated, fuzzy, or grainy. This is not a good experience for the user. Instead, use high resolution images. Remember, though, that downloading and storing high-resolution images may cause "out of memory" errors. To avoid these errors, follow these tips:

Load images only when they're displayed on the screen. For example, are displaying multiple images in a GridView, ListView or Gallery, only load an image when its getView() is called.
Call recycle() on Bitmaps that are no longer needed.
Use WeakReferences for storing references to Bitmap objects in a in memory collection.
If you fetch the images from the network, use AsyncTask to fetch them all at once or as needed. Never do network transactions on the main ("UI") thread. Instead, if possible, fetch all the images at once on a background thread, and store them on the sdcard.
Scale down very large images to a more appropriate size as you download them; otherwise, downloading the image may cause an "out of memory" error. Here is sample code that does this scaling while downloading:

// Get the source image's dimensions
   BitmapFactory.Options options = new BitmapFactory.Options();
   options.inJustDecodeBounds = true; // this does not download the actual image, just downloads headers.
   BitmapFactory.decodeFile(IMAGE_FILE_URL, options);

   int srcWidth = options.outWidth;  // actual width of the image.
   int srcHeight = options.outHeight;  // actual height of the image.

   // Only scale if the source is big enough. This code is just trying to fit a image into a certain width.
   if(desiredWidth > srcWidth)
     desiredWidth = srcWidth;

   // Calculate the correct inSampleSize/scale value. This helps reduce memory use. It should be a power of 2.
   int inSampleSize = 1;
   while(srcWidth / 2 > desiredWidth){
     srcWidth /= 2;
     srcHeight /= 2;
     inSampleSize *= 2;
   }

   float desiredScale = (float) desiredWidth / srcWidth;

   // Decode with inSampleSize
   options.inJustDecodeBounds = false;  // now download the actual image.
   options.inDither = false;
   options.inSampleSize = inSampleSize;
   options.inScaled = false;
   options.inPreferredConfig = Bitmap.Config.ARGB_8888;  // ensures the image stays as a 32-bit ARGB_8888 image. 
                                                         // This preserves image quality.
   Bitmap sampledSrcBitmap = BitmapFactory.decodeFile(IMAGE_FILE_URL, options);

   // Resize
   Matrix matrix = new Matrix();
   matrix.postScale(desiredScale, desiredScale);
   Bitmap scaledBitmap = Bitmap.createBitmap(sampledSrcBitmap, 0, 0,
       sampledSrcBitmap.getWidth(), sampledSrcBitmap.getHeight(), matrix, true);
   sampledSrcBitmap = null;

   // Save
   FileOutputStream out = new FileOutputStream(LOCAL_PATH_TO_STORE_IMAGE);
   scaledBitmap.compress(Bitmap.CompressFormat.JPEG, 100, out);
   scaledBitmap = null;