BitmapFactory.Options.inSampleSize 的用法

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，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 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) {
    }

综合上述，完整代码是：

	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;
	}

	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;
	    }
	}