opencv转置第一次需要很长时间

Question

我使用opencv 3.1并在windows 10中启用intel TBB with visual studio 2015 update 3。第一个转置时间需要100 ms，另一个转置需要0.02-0.05 ms。任何知道为什么第一次转置需要这么多时间用于1 * 1矩阵。

double ts = time_measure("start", 0); 
Mat_<uchar> A = (Mat_<uchar>(1, 1) << 1); 
Mat at = A.t(); 
cout << "transpose Times needed : " << time_measure("end", ts) * 1000 << " ms " << endl; 

for (int i = 0; i < 10; i++) { 
    ts = time_measure("start", 0); 
    Mat_<uchar> B = (Mat_<uchar>(1, 1) << 1); 
    Mat bt = B.t(); 
    cout << "transpose Times needed : " << time_measure("end", ts) * 1000 << " ms " << endl; 
} 


double time_measure(const string mode, double ts) { 
    double t = 0.0; 
    if (mode == "start") { 
     t = (double)getTickCount(); 
    } 
    else { 
     t = ((double)getTickCount() - ts)/getTickFrequency(); 
    } 
    return t; 
} 

The output 

transpose A Times needed : 112.062 mstranspose B Times needed : 0.0337221 ms 
transpose B Times needed : 0.0205265 ms 
transpose B Times needed : 0.0195491 ms 
transpose B Times needed : 0.0283461 ms 
transpose B Times needed : 0.0234589 ms 
transpose B Times needed : 0.0298123 ms 
transpose B Times needed : 0.0249251 ms 
transpose B Times needed : 0.0283461 ms 
transpose B Times needed : 0.0273687 ms 
transpose B Times needed : 0.02688 ms 

Answer 1

我没有启用TBB，但问题似乎与你测量性能的方式：

1. 不包括的时间来创建矩阵
2. 使用矩阵足够大。无论如何转置一个1x1矩阵没有任何意义。
3. 不使用字符串布尔

你可以尝试这样的事情，然后请让我知道你的执行时间：

double time_measure(bool start, double ts) { 
    double t = 0.0; 
    if (start) { 
     t = (double)getTickCount(); 
    } 
    else { 
     t = ((double)getTickCount() - ts)/getTickFrequency(); 
    } 
    return t; 
} 

int main() 
{ 
    for (int i = 0; i < 10; i++) { 

     // 1000 x 1000 random matrix 
     Mat_<uchar> B(1000, 1000); 
     randu(B, 0, 256); 

     double ts = time_measure(true, 0); 
     Mat bt = B.t(); 
     cout << "transpose Times needed : " << time_measure(false, ts) * 1000 << " ms " << endl; 
    } 

    getchar(); 
    return 0; 
} 

Answer 2

感谢您的评论。我已经尝试了你的代码，并且矩阵创建时间是固定的，第一个转置时间也比其他转置耗时长。以下是测试结果。我修改代码以打印矩阵创建时间和转置时间。

int _tmain(int argc, _TCHAR* argv[]) { 
    for (int i = 0; i < 10; i++) { 
     double ts = time_measure(true, 0); 
     // 1000 x 1000 random matrix 
     Mat_<uchar> B(1000, 1000); 
     randu(B, 0, 256); 
     cout << "create matrix Times needed : " << time_measure(false, ts) * 1000 << " ms " << endl; 

     ts = time_measure(true, 0); 
     Mat bt = B.t(); 
     cout << "transpose Times needed : " << time_measure(false, ts) * 1000 << " ms " << endl; 
    } 
} 

double time_measure(bool start, double ts) { 
double t = 0.0; 
if (start) { 
    t = (double)getTickCount(); 
} 
else { 
    t = ((double)getTickCount() - ts)/getTickFrequency(); 
} 
return t; 
} 

的输出中的以下列表：

create matrix Times needed : 49.3267 ms 
transpose Times needed : 427.299 ms 
create matrix Times needed : 51.8431 ms 
transpose Times needed : 0.889971 ms 
create matrix Times needed : 51.8084 ms 
transpose Times needed : 0.718917 ms 
create matrix Times needed : 52.4946 ms 
transpose Times needed : 0.742376 ms 
create matrix Times needed : 45.5454 ms 
transpose Times needed : 0.705721 ms 
create matrix Times needed : 45.218 ms 
transpose Times needed : 0.70621 ms 
create matrix Times needed : 44.5748 ms 
transpose Times needed : 0.713541 ms 
create matrix Times needed : 46.2501 ms 
transpose Times needed : 0.68715 ms 
create matrix Times needed : 45.153 ms 
transpose Times needed : 0.663691 ms 
create matrix Times needed : 44.1892 ms 
transpose Times needed : 0.584028 ms