android 优化Bitmap.doc

Android 优化Bitmap避免OutOfMemoryError使用android提供的BitmapFactory解码图片时，往往会因为图片过大而遇到OutOfMemoryError的异常。要想正常使用，一种简便的方式是分配更少的内存空间来存储，即在载入图片的时候以牺牲图片质量为代价，将图片进行放缩，这是一种避免OOM所采用的解决方法。但是，这种方法是得不偿失的，牺牲了图片质量。在BitmapFactory中有一个内部类BitmapFactory.Options，其中值得我们注意的是inSampleSize和inJustDecodeBounds两个属性：inSampleSize是以2的指数的倒数被进行放缩If set to a value 1, requests the decoder to subsample the original image, returning a smaller image to save memory. (1 - decodes full size; 2 - decodes 1/4th size; 4 - decode 1/16th size). Because you rarely need to show and have full size bitmap images on your phone. For manipulations smaller sizes are usually enough.inJustDecodeBounds为Boolean型设置inJustDecodeBounds为true后，decodeFile并不分配空间，但可计算出原始图片的长度和宽度，即options.outWidth和options.outHeight。要对图片进行缩放，最大的问题就是怎么在运行时动态的改变inSampleSize的值，通过上面的inJustDecodeBounds可以知道图片原始的大小，那么这样以来就可以通过算法来得到一个恰当的inSampleSize值。其动态算法可参考下面的，网上也很多，大体都一样：/* compute Sample Size* param options* param minSideLength* param maxNumOfPixels* return*/ public static int computeSampleSize(BitmapFactory.Options options,int minSideLength,int maxNumOfPixels) int initialSize = computeInitialSampleSize(options, minSideLength,maxNumOfPixels);int roundedSize;if (initialSize =8) roundedSize = 1;while (roundedSize initialSize) roundedSize = 1; else roundedSize = (initialSize + 7) /8 *8;return roundedSize;/* compute Initial Sample Size* param options* param minSideLength* param maxNumOfPixels* return*/ private static int computeInitialSampleSize(BitmapFactory.Options options,int minSideLength,int maxNumOfPixels) double w = options.outWidth;double h = options.outHeight;/ 上下限范围 int lowerBound = (maxNumOfPixels = -1) ?1 : (int) Math.ceil(Math.sqrt(w * h / maxNumOfPixels);int upperBound = (minSideLength = -1) ?128 : (int) Math.min(Math.floor(w / minSideLength), Math.floor(h / minSideLength);if (upperBound lowerBound) / return the larger one when there is no overlapping zone. return lowerBound;if (maxNumOfPixels = -1) & (minSideLength = -1) return 1; else if (minSideLength = -1) return lowerBound; else return upperBound;有了上面的算法，我们就可以轻易的get到Bitmap了：/* get Bitmap* param imgFile* param minSideLength* param maxNumOfPixels* return*/ public static Bitmap tryGetBitmap(String imgFile,int minSideLength,int maxNumOfPixels) if (imgFile =null | imgFile.length() =0)return null;try FileDescriptor fd = new FileInputStream(imgFile).getFD();BitmapFactory.Options options = new BitmapFactory.Options();options.inJustDecodeBounds = true;/ BitmapFactory.decodeFile(imgFile, options); BitmapFactory.decodeFileDescriptor(fd, null, options);options.inSampleSize = computeSampleSize(options, minSideLength,maxNumOfPixels);try / 这里一定要将其设置回false，因为之前我们将其设置成了true / 设置inJustDecodeBounds为true后，decodeFile并不分配空间，即，BitmapFactory解码出来的Bitmap为Null,但可计算出原始图片的长度和宽度 options.inJustDecodeBounds = false;Bitmap bmp =