decodeSampledBitmap.java

public static Bitmap decodeSampledBitmapFromDescriptor(
        FileDescriptor fileDescriptor, int reqWidth, 
        int reqHeight, ImageCache cache) {

    // 设置inJustDecodeBounds=true来获取图片的尺寸
    final BitmapFactory.Options options = new BitmapFactory.Options();
    options.inJustDecodeBounds = true;
    BitmapFactory.decodeFileDescriptor(fileDescriptor, null, options);

    // 计算采样率
    options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);

    // 设置inJustDecodeBounds=false得到bitmap对象
    options.inJustDecodeBounds = false;

    return BitmapFactory.decodeFileDescriptor(fileDescriptor, null, options);
}