BitmapFactory。Options.inSampleSize用法

http://www.cnblogs.com/lost-in-code/archive/2012/04/06/2435325.html

BitmapFactory.decodeFile(imageFile);

用BitmapFactory解码一张图片时，有时会遇到该错误。这往往是由于图片过大造成的。要想正常使用，则需要分配更少的内存空间来存储。

BitmapFactory.Options.inSampleSize

设置恰当的inSampleSize可以使BitmapFactory分配更少的空间以消除该错误。inSampleSize的具体含义请参考SDK文档。例如：

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

opts.inSampleSize = 4;

Bitmap bitmap = BitmapFactory.decodeFile(imageFile, opts);

设置恰当的inSampleSize是解决该问题的关键之一。BitmapFactory.Options提供了另一个成员inJustDecodeBounds。

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

opts.inJustDecodeBounds = true;

Bitmap bitmap = BitmapFactory.decodeFile(imageFile, opts);

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，Android提供了一种动态计算的方法。

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;

}



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;

    }

} 

使用该算法，就可动态计算出图片的inSampleSize。

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

opts.inJustDecodeBounds = true;

BitmapFactory.decodeFile(imageFile, opts);

   

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

opts.inJustDecodeBounds = false;

try {

 Bitmap bmp = BitmapFactory.decodeFile(imageFile, opts);

 imageView.setImageBitmap(bmp);

    } catch (OutOfMemoryError err) {

    }