Improve Runtime of a Function
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
- convertTo steps
- divide by 2, add 128
- subtract() function
- 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:
- Step 1 time: 64 msec
- Step 2 time: 76 msec
- Step 3 time: 51 msec
- 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!
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
cerr << cv::getBuildInformation() << endl;, to find out, which optimization was built in ?remember that cv::getTickCount() only measures CPU ticks, not operations on gpu(if opencl or such enabled), io(ie, imread), or sleep()
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.
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.
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 setocl::setUseOpenCL(false)before function calls.what about absdiff()?
From the head: Perhabs this does the same job a bit faster if your input is 8bit:
Thanks! I'll give them a try