关于android缩放bitmap，BitmapFactory.Options.inSampleSize的优化算法

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在android官网网站上的 Develop > Training > Displaying Bitmaps Efficiently > Loading Large Bitmaps Efficiently 章节看到的计算

BitmapFactory.Options.inSampleSize的算法，没看懂这官方示例代码的计算逻辑（好吧，这其实怪我数学不好~~~ - -b）

然后根据这个值的要求，我写了一段更符合逻辑更容易看懂的代码~~ duang~duang~duang~ 如下所示（至于这个值怎么计算的，有什么用，如果你不造的话，可以去了解下，据说这部分内容在android开发过程中还挺重要的、挺基础的~~据说。。。）：

// 计算缩放比
// Calculate the largest inSampleSize value that is a power of 2 and keeps both
// height and width larger than the requested height and width.
int n = 0;
int inSampleSize = 1;
long startTime = System.nanoTime();
while (imageHeight / Math.pow(2, n) > screenHeight
&& imageWidth / Math.pow(2, n) > screenWidth) {
n++;
}
inSampleSize = (int) Math.pow(2, n);
long endTime = System.nanoTime();
Log.i("----scs", "计算 inSampleSize 耗时 " + (endTime - startTime) + " ns.");
Log.i("----scs", "inSampleSize: " + inSampleSize);

相对应的，官方的计算方法是：[/code]

long startTime1 = System.nanoTime();
if (imageHeight > screenHeight || imageWidth > screenWidth) {

final int halfHeight = imageHeight / 2;
final int halfWidth = imageWidth / 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) > screenHeight
&& (halfWidth / inSampleSize) > screenWidth) {
inSampleSize *= 2;
}
}
long endTime1 = System.nanoTime();
Log.i("----scs", "计算 inSampleSize 耗时 " + (endTime1 - startTime1) + " ns.");
Log.i("----scs", "inSampleSize: " + inSampleSize);

然后，对比了一下，耗时如下：[/code]

06-24 09:45:57.745    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ imageWidth:1920, imageHeight: 1200
06-24 09:45:57.746    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ screenWidth:800, screenHeight: 480
06-24 09:45:57.747    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ 计算 inSampleSize 耗时 211846 ns.
06-24 09:45:57.747    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ inSampleSize: 4
06-24 09:45:57.747    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ 计算 inSampleSize 耗时 3077 ns.
06-24 09:45:57.748    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ inSampleSize: 4

翻转了一下屏幕：
06-24 09:46:31.276    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ imageWidth:1920, imageHeight: 1200
06-24 09:46:31.278    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ screenWidth:480, screenHeight: 800
06-24 09:46:31.278    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ 计算 inSampleSize 耗时 12846 ns.
06-24 09:46:31.278    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ inSampleSize: 2
06-24 09:46:31.278    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ 计算 inSampleSize 耗时 3385 ns.
06-24 09:46:31.278    8801-8801/com.jumple.scs.myapp1.app I/----scs﹕ inSampleSize: 2

果然第一种写法的耗时比较长（估计是因为第一种计算过程中使用的是double 而官方的方法只使用了int值来计算吧），哔哔了这么多，其实只是想说......我需要去补习数学了......