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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;
    image_one.convertTo(im1,CV_32FC1);
    image_two.convertTo(im2,CV_32FC1);
    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

Thanks!