Plotting Velocity Vectors in a Binary Image Using OpenCV - c++

Question

I have a binary (Black and White) image over which I want to plot velocity vectors of certain velocity. In MATLAB, we can use quiver to plot these vectors. I am looking for a solution to this problem in OpenCV using C++. I would be grateful if someone can share a solution to this. Although one of the solutions is provided in using the static image on SO (OpenCV How to Plot velocity vectors as arrows in using single static image), its not clear of how to implement it on a binary image. I would be grateful if someone can guide me.
Look forward to some suggestion towards implementation.

Answer 1

This is my solution to your problem: in this example I start with an rgb image coming from the webcam, then i convert it to grayscale and then to binary after applying a threshold.
The next step, when you have a binary image, is to convert it to RGB again (or BGR as the OpenCV convention) and draw whatever you want on it. The code for the arrow is a copypasta of what you have linked.
Hope it helps
cv::VideoCapture cam(n_source);
cam >> frame;
cv::Mat grey_image;
cv::Mat binary_image; // Your binary image
cv::cvtColor(frame, grey_image, CV_RGB2GRAY);
cv::threshold(grey_image, binary_image, 100, 255, 0);
// Convert the binary to RGB
cv::Mat dst_rgb;
cv::cvtColor(binary_image, dst_rgb, CV_GRAY2BGR);
// Draw the arrow on the RGB image
int x = 200;
int y = 200;
int u = 100;
int v = 100;
cv::Point pt1,pt2;
double Theta;
double PI = 3.1416;
cv::Scalar Color(255,0,0);
int size = 5;
int Thickness = 5;
if(u==0)
Theta=PI/2;
else
Theta=atan2(double(v),(double)(u));
pt1.x=x;
pt1.y=y;
pt2.x=x+u;
pt2.y=y+v;
cv::line(dst_rgb,pt1,pt2,Color,Thickness,8); //Draw Line
size=(int)(size*0.707);
if(Theta==PI/2 && pt1.y > pt2.y)
{
pt1.x=(int)(size*cos(Theta)-size*sin(Theta)+pt2.x);
pt1.y=(int)(size*sin(Theta)+size*cos(Theta)+pt2.y);
cv::line(dst_rgb,pt1,pt2,Color,Thickness,8); //Draw Line
pt1.x=(int)(size*cos(Theta)+size*sin(Theta)+pt2.x);
pt1.y=(int)(size*sin(Theta)-size*cos(Theta)+pt2.y);
cv::line(dst_rgb,pt1,pt2,Color,Thickness,8); //Draw Line
}
else{
pt1.x=(int)(-size*cos(Theta)-size*sin(Theta)+pt2.x);
pt1.y=(int)(-size*sin(Theta)+size*cos(Theta)+pt2.y);
cv::line(dst_rgb,pt1,pt2,Color,Thickness,8); //Draw Line
pt1.x=(int)(-size*cos(Theta)+size*sin(Theta)+pt2.x);
pt1.y=(int)(-size*sin(Theta)-size*cos(Theta)+pt2.y);
cv::line(dst_rgb,pt1,pt2,Color,Thickness,8); //Draw Line
}
// Plot
cv::namedWindow("test rgb");
cv::imshow("test rgb", dst_rgb);
cv::waitKey(0);
An example:

Answer 2

After some discussion I have come across cv::arrowedLine in OpenCV whose one of the usage is as follows:
#define _CRT_SECURE_NO_WARNINGS
#include <iostream>
#include <string>
#include <opencv2/opencv.hpp>
#include <opencv2/highgui.hpp>
using namespace std;
using namespace cv;
int main(int argc, char* argv[])
{
auto width = 320;
auto height = 320;
auto img = cv::Mat(cv::Size(width, height), CV_8UC3); // create background image
auto center = cv::Point(width / 2, height / 2); // center point
int lineType = 8;
int thickness = 1;
double tipLength = 0.1;
img.setTo(255); // clear image - set to black (0) or white (255)
for (int angle = 0; angle < 360; angle += 15)
{
auto angleRad = angle*CV_PI / 180.0; // convert angle to radians
auto length = 150;
auto direction = cv::Point(length * cos(angleRad), length * sin(angleRad)); // calculate direction
tipLength = .01 + 0.4 * (angle % 180) / 360;
cv::arrowedLine(img, center + direction*0.5, center + direction, CV_RGB(255, angle, 0), thickness, lineType, 0, tipLength); // draw arrow!
++thickness;
if (0 == angle % 45)
thickness = 0;
if (180 <= angle)
lineType = CV_AA;
}
imshow("Arrowed Image", img); // show image
waitKey();
return EXIT_SUCCESS;
}