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Why is the first openCV API call so slow?

asked 2017-01-26 13:15:26 -0500

bottiger gravatar image

updated 2017-01-26 13:30:39 -0500

I am creating a mask from an image with the size: 2000x640, and I'm running this code on a modern powerfull laptop.

I have made a function which is suppose to create binary mask from my input image, the code is quite simple and shown below:

However, when running the code it seems like doing a simple threshold takes a very long time - almost half a second.

ms_allocation: 0.001

ms_converted: 0.001

ms_thresholding: 0.383

ms_median_blur: 0.001

ms_copy_mask: 0

ms_locate_contour: 0.067

This looked very odd, so I implemented my own thresholding which ran in a few ms. However, not suddenly the median_blur became equally slow.

I then completely removed the thresholding part of the code and got this performance result

ms_allocation: 0.0008

ms_converted: 0.002

ms_thresholding: 0.000

ms_median_blur: 0.424

ms_copy_mask: 0

ms_locate_contour: 0.106

Can someone help me understand what is going on here? Why is the first call to openCV so slow, and can I prevent it somehow?

I would really prefer is this code could run in less than 0.1 s, which I think it totally reasonable.

void calculateMask(Mat & pSrc, Mat & pDst)

    auto ms_alloc_start = getTickCount();

    // Allocate, just to be sure it's done in advance.
    Mat mask = Mat::zeros(pSrc.size(), CV_8U);
    Mat sharp = Mat::zeros(pSrc.size(), CV_8U);
    Mat filled_mask = Mat::zeros(pSrc.size(), CV_8U);;
    Mat masks = Mat::zeros(pSrc.size(), CV_8U);;

    auto ms_alloc_end = getTickCount();
    auto ms_convert_start = getTickCount();

    pSrc.convertTo(sharp, CV_8U);

    auto ms_convert_end = getTickCount();
    auto ms_threshold_start = getTickCount();

    //threshold(sharp, mask, 20, 255, THRESH_BINARY);

    auto ms_threshold_end = getTickCount();
    auto ms_median_start = getTickCount();

    medianBlur(mask, filled_mask, 5);

    auto ms_median_end = getTickCount();
    auto ms_copy_start = getTickCount();


    auto ms_copy_end = getTickCount();

    auto ms_find_contour_start = getTickCount();

    locateLargestContour(masks, pDst);

    auto ms_find_contour_end = getTickCount();

    cout << "\n\n\tms_allocation: " << to_string((ms_alloc_end - ms_alloc_start) / getTickFrequency()) << "\n";
    cout << "\tms_converted: " << to_string((ms_convert_end - ms_convert_start) / getTickFrequency()) << "\n";
    cout << "\tms_thresholding: " << to_string((ms_threshold_end - ms_threshold_start)/ getTickFrequency()) << "\n";
    cout << "\tms_median_blur: " << to_string((ms_median_end - ms_median_start) / getTickFrequency()) << "\n";
    cout << "\tms_copy_mask: " << to_string((ms_copy_end - ms_copy_start) / getTickFrequency()) << "\n";
    cout << "\tms_locate_contour: " << to_string((ms_find_contour_end - ms_find_contour_start) / getTickFrequency()) << "\n";
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answered 2017-01-26 13:30:54 -0500

LBerger gravatar image

"Why is the first call to openCV so slow"? because in first call opencl code must be compiled. If you really need speed for first call disable opencl :

#include <opencv2\core\ocl.hpp >

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Thanks! I was actually thinking something along those lines, but I thought I had disabled opencl - apparently not. However, a small follow up question.I do not need speed for the first call, but I need the code to be fast in general. With a single image opencl makes it slower, with a lot if probably makes it faster. However, my code lives in a dll - will multiple calls to the dll recompile it multiple times, or can it handle that?

bottiger gravatar imagebottiger ( 2017-01-26 13:41:21 -0500 )edit

Opencl use a context which is global in core module I think. When opencl is disable it is disable. But when you have multi thread application i'm not sure and it should be tested.

LBerger gravatar imageLBerger ( 2017-01-26 13:47:49 -0500 )edit

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Last updated: Jan 26 '17