BitmapFactory.Options opt = new BitmapFactory.Options(); 
               //这个isjustdecodebounds很重要	
               opt.inJustDecodeBounds = true;
               bm = BitmapFactory.decodeFile(absolutePath, opt);

               //获取到这个图片的原始宽度和高度
int picWidth  = opt.outWidth;
int picHeight = opt.outHeight;

               //获取屏的宽度和高度
WindowManager windowManager = getWindowManager();
Display display = windowManager.getDefaultDisplay();
int screenWidth = display.getWidth();
int screenHeight = display.getHeight();

                //isSampleSize是表示对图片的缩放程度，比如值为2图片的宽度和高度都变为以前的1/2
opt.inSampleSize = 1;
                //根据屏的大小和图片大小计算出缩放比例
if(picWidth > picHeight){
if(picWidth > screenWidth)
opt.inSampleSize = picWidth/screenWidth;
}
else{
if(picHeight > screenHeight)

opt.inSampleSize = picHeight/screenHeight;
}

                //这次再真正地生成一个有像素的，经过缩放了的bitmap
opt.inJustDecodeBounds = false;
bm = BitmapFactory.decodeFile(absolutePath, opt);

                //用imageview显示出bitmap
iv.setImageBitmap(bm);