How to draw in opencv with active camera stream? - opencv

Question

I am trying to make something similar to this: https://www.youtube.com/watch?v=D2Kb3ryfGNc
I succeeded in detecting laser position, but now I can't figure out how to paint where the laser has been?
Do I need to paint lines of where laser has been in one frame and add it to camera stream frame in order to make sure that lines are painted?

Answer 1

Here's the thing - When we stream a continuous video using openCV Mat object, it displays one frame after another, thus the info of the nth frame is lost when the (n+1)th frame is received.
What you need are 2 Mat objects- one to stream the camera (say Mat_cam), and the other to draw the laser trajectory (Mat_traj). Mat_cam will be used to track the laser position frame-by-frame, using any standard colour thresholding algo. Even the video says that the laser should be bright, meaning that jimez86 might be using white color threshold, followed by largest blob localization.
As a new laser position is received in nth frame, draw a corresponding circle on Mat_traj. When the next frame is received, Mat_cam will be updated and it'll have a new laser position, but Mat_traj will be the same, since it will not be cleared/refreshed with every 'for' loop iteration, hence it will contain the whole trajectory. Adding Mat_traj and Mat_cam using Weighted addition will give you the desired result. Follow the algo below:
Mat Mat_traj(640,480,CV_8UC1,Scalar(0)),Mat_cam,Mat_res;
VideoCapture cam(0);
for(;;)
{
cam>>Mat_cam;
Point laserCentre=getLaserCentre(Mat_cam);//you'll be defining this function;
drawCircle(Mat_traj,laserCentre);
addWeighted(Mat_cam,Mat_traj,other_params,Mat_res);
imshow("out",Mat_res);
waitKey(10);
}