标签：style blog http io ar color sp for on
这几天在看文章的时候，看到这么一段话：
“First, we normalize the grayscale intensity in the eye region for each of the aligned facial images to zero mean and unit variance.”
意思就是第一步对眼睛的区域进行零均值单位方差。第一次知道还有这玩意。。
经过查阅matlab prestd函数就代表了这个意思：
试验一下：
>> a=[1 2 3 4]
a =
1 2 3 4
>> [pn m std]=prestd(a)
Warning: PRESTD is an obsolete function.
> In obs_fcn at 18
In prestd at 10
Use MAPSTD instead.
pn =
-1.1619 -0.3873 0.3873 1.1619
m =
2.5000
std =
1.2910
>>
其中pn代表已经零均值单位方差的矩阵，m代表a的均值 std代表a的方差
转成C++代码就应该是这样：
int a = 2; int b = 2; double gray[2 * 2] = { 1,2,3,4 }; double total = 0.0; for (int i = 0; i < b; i++) { for (int j = 0; j < a; j++) { total += (double)gray[i*a + j]; } } double meanvalue = total / (a*b); //零均值单位方差 double stdp = 0.0; for (int i = 0; i < b; i++) { for (int j = 0; j < a; j++) { double temp = gray[i*a + j]; stdp += ((temp - meanvalue)*(temp - meanvalue)); } } stdp = sqrt(stdp / (a*b));
qDebug() << meanvalue << " " << stdp;
for (int i = 0; i < b; i++)
{
for (int j = 0; j < a; j++)
{
double temp = gray[i*a + j];
qDebug()<<i<<" "<<j<<" "<<(temp - meanvalue) / stdp;
}
}
咱们看一下结果输出：
发现在方差的值就不一样了。为什么呢，
原来方差还存在偏差的概念；
double meanvalue = total / (a*b);//有偏差为n
double meanvalue = total / (a*b-1);//无偏差的方差n-1
而matlab默认是无偏差的方差，所以我们只要用无偏差替换到有偏差就和metlab相对应了。
标签：style blog http io ar color sp for on
原文地址：http://www.cnblogs.com/lanye/p/4146095.html
