Perfomance Issues while capturing and processing a video - c++

Question

I'm currently working on a project where I need to display a processed live video capture. Therefore, I'm using something similar to this:
cv::VideoCapture cap(0);
if (!cap.isOpened())
return -1;
cap.set(CV_CAP_PROP_FRAME_WIDTH, 1280);
cap.set(CV_CAP_PROP_FRAME_HEIGHT, 720);
cv::namedWindow("Current Capture");
for (;;)
{
cv::Mat frame;
cap >> frame;
cv::Mat mirrored;
cv::flip(frame, mirrored, 1);
cv::imshow("Current Capture", process_image(mirrored));
if (cv::waitKey(30) >= 0) break;
}
The problem I have is, that process_image, which perfomes a circle detection in the image, needs some time to finish and causes the displaying to be rather a slideshow then a video.
My Question is: How can I speed up the processing without manipulating the process_image function?
I thought about performing the image processing in another thread, but I'm not really sure how to start. Do you have any other idea than this?
PS.: I'm not expecting you to write code for me, I only need a point to start from ;)
EDIT:
Ok, if there is nothing i can do about the performance while capturing, I will need to change the process_image function.
cv::Mat process_image(cv::Mat img)
{
cv::Mat hsv;
cv::medianBlur(img, img, 7);
cv::cvtColor(img, hsv, cv::COLOR_BGR2HSV);
cv::Mat lower_hue_range; // lower and upper hue range in case of red color
cv::Mat upper_hue_range;
cv::inRange(hsv, cv::Scalar(LOWER_HUE1, 100, 100), cv::Scalar(UPPER_HUE1, 255, 255), lower_hue_range);
cv::inRange(hsv, cv::Scalar(LOWER_HUE2, 100, 100), cv::Scalar(UPPER_HUE1, 255, 255), upper_hue_range);
/// Combine the above two images
cv::Mat hue_image;
cv::addWeighted(lower_hue_range, 1.0, upper_hue_range, 1.0, 0.0, hue_image);
/// Reduce the noise so we avoid false circle detection
cv::GaussianBlur(hue_image, hue_image, cv::Size(13, 13), 2, 2);
/// store all found circles here
std::vector<cv::Vec3f> circles;
cv::HoughCircles(hue_image, circles, CV_HOUGH_GRADIENT, 1, hue_image.rows / 8, 100, 20, 0, 0);
for (size_t i = 0; i < circles.size(); i++)
{
/// circle center
cv::circle(hsv, cv::Point(circles[i][0], circles[i][1]), 3, cv::Scalar(0, 255, 0), -1, 8, 0);
/// circle outline
cv::circle(hsv, cv::Point(circles[i][0], circles[i][1]), circles[i][2], cv::Scalar(0, 0, 255), 3, 8, 0);
}
cv::Mat newI;
cv::cvtColor(hsv, newI, cv::COLOR_HSV2BGR);
return newI;
}
Is there a huge perfomance issue I can do anything about?

Answer 1

If you are sure that the process_image function is what is causing the bottle neck in your program, but you can't modify it, then there's not really a lot you can do. If that function takes longer to execute than the duration of a video frame then you will never get what you need.
How about reducing the quality of the video capture or reducing the size? At the moment I can see you have it set to 1280*720. If the process_image function has less data to work with it should execute faster.