BitmapFactory.Options.inSampleSize(避免OOM)

> inSampleSize的源码

》 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opts.width和opts.height。有了这两个参数，再通过一定的算法，即可得到一个恰当的inSampleSize。

查看Android源码，我们得知，为了得到恰当的inSampleSize，Android提供了一种动态计算的方法。

public static Bitmap createImageThumbnail(String filePath){

Bitmap bitmap = null;

BitmapFactory.Options opts = new BitmapFactory.Options();

opts.inJustDecodeBounds = true;

BitmapFactory.decodeFile(filePath, opts);

opts.inSampleSize = computeSampleSize(opts, -1, 128*128);

opts.inJustDecodeBounds = false;

try {

bitmap = BitmapFactory.decodeFile(filePath, opts);

}catch (Exception e) {

// TODO: handle exception

}

return bitmap;

}

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;

}

//使用该算法，就可动态计算出图片的inSampleSize。

private static int computeInitialSampleSize(BitmapFactory.Options options,int minSideLength, int maxNumOfPixels) {

double w = options.outWidth;

double h = options.outHeight;

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;

}

}