做android对大图片加载常会出现out of memory，其实在这么小的屏幕上可以不要这么高精度而大的图。下面引用google的android文档中的方法，也是最好的方法了
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public static int calculateInSampleSize(
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BitmapFactory.Options options, int reqWidth, int reqHeight) {
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// Raw height and width of image
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final int height = options.outHeight;
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final int width = options.outWidth;
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int inSampleSize = 1;
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if (height > reqHeight || width > reqWidth) {
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final int halfHeight = height / 2;
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final int halfWidth = width / 2;
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// Calculate the largest inSampleSize value that is a power of 2 and keeps both
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// height and width larger than the requested height and width.
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while ((halfHeight / inSampleSize) > reqHeight
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&& (halfWidth / inSampleSize) > reqWidth) {
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inSampleSize *= 2;
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}
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}
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return inSampleSize;
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}
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public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId,
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int reqWidth, int reqHeight) {
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// First decode with inJustDecodeBounds=true to check dimensions
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final BitmapFactory.Options options = new BitmapFactory.Options();
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options.inJustDecodeBounds = true;
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BitmapFactory.decodeResource(res, resId, options);
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// Calculate inSampleSize
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options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
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// Decode bitmap with inSampleSize set
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options.inJustDecodeBounds = false;
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return BitmapFactory.decodeResource(res, resId, options);
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}
当我们的显示面积很小时，其实应该用较大的sample size，加快速度和节约内存。
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