/**
* Resample bitmap.
* 这个方法有两个缺陷：
* 1、我们先要从磁盘上先将图片加载到内存，然后才能对图片进行缩放，在移动设备上对内存的要求比较高，这在一定程度上降级了性能。
* 2、我们使用Bitmap.createBitmap这个方法进行缩放，使用的是Java层面的方法来缩放，我们知道Java层面对图片，视频等进行处理是有性能损失的。
* @param bitmap Bitmap.
* @param scale Resample ratio.
* @return Get resampled bitmap.
*/
public static Bitmap scaleBitmap(Bitmap bitmap, float scale) {
Matrix matrix = new Matrix();
matrix.postScale(scale, scale); //长和宽放大缩小的比例
Bitmap resizeBmp = Bitmap.createBitmap(bitmap,0,0,bitmap.getWidth(),bitmap.getHeight(),matrix,true);
return resizeBmp;
}
/**
* 下面给出优化方案，优化我们也是基于上面两点问题给出的。一次性从磁盘上读取并缩放，而且这个缩放实在Native层，能够显著提高效率，代码如下：
* 这段代码是google给出的，我们无需对它进行修改，在上面这段代码中我们使用BitmapFactory.Options的inScaled，inDensity，inTargetDensity来变相对图片进行缩放，
* 但是这有一个局限，就是只能对图片进行等比例的放大或者缩小，不能按任意大小进行缩放 *
*/
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
try {
BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeStream(getAssets().open("g_b090.jpg"), null, options);
int targetDensity = getResources().getDisplayMetrics().densityDpi;
DisplayMetrics dm = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(dm);
int x = dm.widthPixels;
int y = dm.heightPixels;
options.inSampleSize = calculateInSampleSize(options, x, y);
double xSScale = ((double)options.outWidth) / ((double)x);
double ySScale = ((double)options.outHeight) / ((double)y);
double startScale = xSScale > ySScale ? xSScale : ySScale;
options.inScaled = true;
options.inDensity = (int) (targetDensity*startScale);
options.inTargetDensity = targetDensity;
options.inJustDecodeBounds = false;
Bitmap bitmap = BitmapFactory.decodeStream(getAssets().open("g_b090.jpg"), null, options);
} catch (IOException e) {
e.printStackTrace();
}
}
public 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;
}
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