在OpenCV中将某些行从一个矩阵复制到另一矩阵的最快方法 [英] Fastest way to copy some rows from one matrix to another in OpenCV

问题描述

我有一个[32678 x 10]矩阵(w2c)，我想将其24700行复制到另一个矩阵(out).我有要在vector(index)中复制的行的索引.为此，我这样做:

I have a [32678 x 10] matrix (w2c) and I want to copy 24700 rows of it to another matrix(out). I have the index of the rows to be copied in a vector(index). For doing this in matlab I do:

out = w2c(index_im,:);

大约需要0.002622秒.

It takes approximately 0.002622 seconds.

在OpenCV中:

Mat out(index.cols, w2c.cols, w2c.type());
for (int i = 0; i < index.cols; ++i) {
    w2c.row(index.at<int>(i) - 1).copyTo(out.row(i));
}

大约需要0.015121秒.

It takes approximately 0.015121 seconds.

如您所见，Matlab的速度提高了6倍.如何提高OpenCV代码的效率?

As you can see Matlab is 6 times faster. How can I make the OpenCV code efficient?

我正在使用cmake-2.9，g ++-4.8，opencv-2.4.9，ubuntu 14.04

I am using cmake-2.9, g++-4.8, opencv-2.4.9, ubuntu 14.04

更新:

我在发布模式下运行了代码，结果如下(它仍然比Matlab慢得多)

I ran my code in release mode, here is the result ( It is still significantly slower than Matlab )

RELEASE     DEBUG       MATLAB
0.008183    0.010070    0.001604    
0.009630    0.010050    0.001679
0.009120    0.009890    0.001566
0.007534    0.009567    0.001635
0.007886    0.009886    0.001840

推荐答案

因此，我尝试了不同的方法来解决此问题，并且我能获得比Matlab更好的性能的唯一方法是使用memcpy并直接直接复制数据.

So I tried different methods for this problem and the only way I could achieve a better performance than Matlab was using memcpy and directly copying the data myself.

    Mat out( index.cols, w2c.cols, w2c.type() );
    for ( int i=0;i<index.cols;++i ){
        int ind = index.at<int>(i)-1;
        const float *src = w2c.ptr<float> (ind);
        float *des = out.ptr<float> (i);
        memcpy(des,src,w2c.cols*sizeof(float));
    }

这样，整个过程花费了大约0.001063，这比Matlab快一点.

this way the whole thing took approximately 0.001063 that is a little bit faster than Matlab.

我也发现以这种方式复制数据:

Also I found out that copying data this way:

    Mat out;
    Mat out( index.cols, w2c.cols, w2c.type() );
    for ( int i=0;i<index.cols;++i ){
        int ind = index.at<int>(i)-1;
        out.push_back(w2c.row(ind)); 
    }

比这样复制它快:

    Mat out( index.cols, w2c.cols, w2c.type() );
    for ( int i=0;i<index.cols;++i ){
        int ind = index.at<int>(i)-1;
        w2c.row(ind).copyTo(out.row(i));
    }

但是我不为什么.无论如何，它们都比Matlab慢.

but I don't why. Anyway both of them are slower than Matlab.

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