options.inJustDecodeBounds = true; //true表示在解码的时候避免内存的分配，它会返回一个null的Bitmap，但是可以获取到 outWidth, outHeight 与 outMimeType。
BitmapFactory.decodeResource(getResources(), R.id.myimage, options);//解码的方法，根据数据源不同，有decodeByteArray(), decodeFile(), decodeResource()等几种
int imageHeight = options.outHeight;
int imageWidth = options.outWidth;
String imageType = options.outMimeType;

//计算需要缩放的比例的函数
public static int calculateInSampleSize(

BitmapFactory.Options options, int reqWidth, int reqHeight) {
// Raw height and width of image
final int height = options.outHeight;
final int width = options.outWidth;
int inSampleSize = 1;

if (height > reqHeight || width > reqWidth) {

final int halfHeight = height / 2;
final int halfWidth = width / 2;

// Calculate the largest inSampleSize value that is a power of 2 and keeps both
// height and width larger than the requested height and width.
while ((halfHeight / inSampleSize) > reqHeight
        && (halfWidth / inSampleSize) > reqWidth) {
    inSampleSize *= 2;
}
}

return inSampleSize;
}

//加载位图
public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId,

    int reqWidth, int reqHeight) {

// First decode with inJustDecodeBounds=true to check dimensions
final BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeResource(res, resId, options);

// Calculate inSampleSize
options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);//根据图片的大小和屏幕宽高的商中的大的值得出

// Decode bitmap with inSampleSize set
options.inJustDecodeBounds = false;
return BitmapFactory.decodeResource(res, resId, options);
}