加载大图片到内存，进行缩放

    public class MainActivity extends Activity {

    private ImageView iv;
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        iv = (ImageView) findViewById(R.id.iv);

        BitmapFactory.Options opts = new BitmapFactory.Options();
        opts.inJustDecodeBounds = true ;
        BitmapFactory.decodeFile("/mnt/sdcard/a.jpg", opts);

        int bmWidth = opts.outWidth;
        int bmHeight = opts.outHeight;

        WindowManager wm = (WindowManager) getSystemService(WINDOW_SERVICE);
        Display display = wm.getDefaultDisplay();
        int screenWidth = display.getWidth();
        int screenHeight = display.getHeight();

        int scale = 1 ;
        int dx = bmWidth/screenWidth;
        int dy = bmHeight/screenHeight;

        if(dx>dy&&dy>0){
            scale = dx;
        }
        if(dy>dx&&dx>0){
            scale = dy;
        }

        opts.inJustDecodeBounds = false;
        opts.inSampleSize = scale;

        Bitmap bm = BitmapFactory.decodeFile("/mnt/sdcard/a.jpg", opts);
        iv.setImageBitmap(bm);
    }



}