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Improve Runtime of a Function

asked 2016-04-26 13:15:52 -0500

asa gravatar image

Hi, I am using a function to find difference between 2 images. It takes two 8-bit grayscale images, converts them to CV_32FC1, does a subtraction. Here is the function I am using:

cv::Mat calculateDiff(const cv::Mat &image_one, const cv::Mat &image_two)
    cv::Mat im1, im2, im_dest;
    im1 /= 2.f;
    im1 += 128.f;
    im2 /= 2.f;
    cv::subtract(im1, im2, im_dest);
    im_dest.convertTo(im_dest, CV_8UC1);

   return im_dest;

I have measured the run-time of each major step individually

  1. convertTo steps
  2. divide by 2, add 128
  3. subtract() function
  4. convertTo()

When I call this function with images of sizes: 9000 x 6000. I get a run-time of about 900 msec, but each individual step takes a lot less time. Here's one example:

  1. Step 1 time: 64 msec
  2. Step 2 time: 76 msec
  3. Step 3 time: 51 msec
  4. Step 4 time: 22 msec

When I called the function: I get the function's runtime: 905 msec

The function call looks like this:

cv::Mat diff_image;
diff_image = calculate_diff(input_one, input_two);

I measure the runtime using cv::getTickCount() and cv::getTickFrequency()

Why is the function's runtime so large where individual step do no take that long? How to improve the runtime? Kindly Help


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I think that your program use opencl. Opencl source code is compiled during first call. If you want to measure real time don't measure first call but second one

LBerger gravatar imageLBerger ( 2016-04-26 13:27:54 -0500 )edit
  • how do you measure execution time ?
  • can you try a cerr << cv::getBuildInformation() << endl; , to find out, which optimization was built in ?
berak gravatar imageberak ( 2016-04-26 13:38:32 -0500 )edit

remember that cv::getTickCount() only measures CPU ticks, not operations on gpu(if opencl or such enabled), io(ie, imread), or sleep()

berak gravatar imageberak ( 2016-04-26 13:50:31 -0500 )edit

Got it. So what should I use if I want to measure execution time of functions that involve io operations say imread() and/or inwrite() ? I have functions that may or may not have io operations and I need to find execution time.

asa gravatar imageasa ( 2016-04-26 13:57:12 -0500 )edit

You can speed up your program when you use multiplication instead of division when possible. Further more you can specify the type inside some operations.

cv::Mat calculateDiff(const cv::Mat &image_one, const cv::Mat &image_two)
    cv::Mat im1, im2, im_dest;
    image_one.convertTo(im1, CV_32FC1, 0.5, 128);
    image_two.convertTo(im2, CV_32FC1, 0.5, 0);
    cv::subtract(im1, im2, im_dest, cv::noArray(), CV_8U);  // Not sure if this works here as expected
    // im_dest.convertTo(im_dest, CV_8UC1);

   return im_dest;

Aside of this it seems, that in OpenCV 3 the OpenCL code is build on first function call, even if you use cv::Mat. To avoid this #include <opencv2/core/ocl.hpp> and set ocl::setUseOpenCL(false) before function calls.

matman gravatar imagematman ( 2016-04-26 14:22:28 -0500 )edit

what about absdiff()?

sturkmen gravatar imagesturkmen ( 2016-04-26 14:34:26 -0500 )edit

From the head: Perhabs this does the same job a bit faster if your input is 8bit:

cv::Mat calculateDiff(const cv::Mat &image_one, const cv::Mat &image_two)
    cv::Mat im1, im_dest;
    image_one.convertTo(im1, CV_16S, 1, 255);
    cv::subtract(im1, image_two, im_dest, cv::noArray(), CV_16S);
    im_dest.convertTo(im_dest, CV_8U, 0.5);

   return im_dest;
matman gravatar imagematman ( 2016-04-26 14:59:07 -0500 )edit

Thanks! I'll give them a try

asa gravatar imageasa ( 2016-04-26 15:03:06 -0500 )edit

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answered 2016-04-26 13:45:54 -0500

asa gravatar image

I use cv::getTickCount()


I figured out the problem. The function was being called inside another function and there was a conditional cv::imwrite() in the function. That's why I was getting the problem. The conditional part comes from another section of the program. I've fixed the part and it's working ok.

Thanks everyone!

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Asked: 2016-04-26 13:15:52 -0500

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Last updated: Apr 26 '16