editing mat is so slow

asked 2019-06-12 21:49:11 -0500

```
void test1(Mat& img, Mat& out)
{
  int element_nums = img.rows * img.cols;
  float *ptr_img = img.ptr<float>(0);
  float *ptr_out = out.ptr<float>(0);
  for (int i = 0; i < element_nums; ++i){
    ptr_out[i] = std::exp(ptr_img[i]);
    }
};

void test2(Mat& img, Mat& out)
{
  int element_nums = img.rows * img.cols;
  float *ptr_img = img.ptr<float>(0);
  float *ptr_out = out.ptr<float>(0);
  for (int i = 0; i < element_nums; ++i){
    ptr_out[i] = ptr_img[i];
    }
};

void test3(Mat& img, Mat& out){
  int element_nums = img.rows * img.cols;
  float *ptr_img = img.ptr<float>(0);
  float *ptr_out = out.ptr<float>(0);
  float* temp = new float[img.rows * img.cols];
  for (int i = 0; i < element_nums; ++i){
    temp[i] = std::exp(ptr_img[i]*(-i));
  }
};

int main(int argc, char* argv[])
{
    clock_t start, stop;
    Mat testInput = Mat::ones(4000,4000, CV_32FC1);
  Mat out = Mat(testInput.size(), CV_32FC1);
    start = clock();
    test1(testInput, out);
    stop = clock();
    cout<<"test1:  "<<(double)(stop - start)/CLOCKS_PER_SEC*1000<<"ms"<<endl;

  start = clock();
    test2(testInput, out);
    stop = clock();
    cout<<"test2:  "<<(double)(stop - start)/CLOCKS_PER_SEC*1000<<"ms"<<endl;

  start = clock();
    test3(testInput, out);
    stop = clock();
    cout<<"test3:  "<<(double)(stop - start)/CLOCKS_PER_SEC*1000<<"ms"<<endl;
}
test1:  221.924ms
test2:  17.146ms
test3:  0.001ms
open4.1  mac 1.4 GHz Intel Core i5

why is editing mat so low?

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Comments

First, don't use clock_t. It measures the CPU ticks (operations), which is not the same than the elapsed time (as modern CPUs make several operations per clock cycle). Use the functions provided by the standard chrono library.

kbarni gravatar imagekbarni ( 2019-06-13 04:23:30 -0500 )edit

Thanks for your suggestion. But editing mat is still so slow.

sundrops gravatar imagesundrops ( 2019-06-13 04:48:34 -0500 )edit