i searched google "draw a line c++" and found this
the adaptation to your case is below
vector<Point> linePoints(int x0, int y0, int x1, int y1)
{
vector<Point> pointsOfLine;
int dx = abs(x1-x0), sx = x0<x1 ? 1 : -1;
int dy = abs(y1-y0), sy = y0<y1 ? 1 : -1;
int err = (dx>dy ? dx : -dy)/2, e2;
for(;;)
{
pointsOfLine.push_back(Point(x0,y0));
if (x0==x1 && y0==y1) break;
e2 = err;
if (e2 >-dx)
{
err -= dy;
x0 += sx;
}
if (e2 < dy)
{
err += dx;
y0 += sy;
}
}
return pointsOfLine;
}
to test it you can try the code below
//modified from https://github.com/Itseez/opencv/blob/master/apps/annotation/opencv_annotation.cpp
// move the mouse on the window and try left click and right click
#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/imgproc.hpp>
using namespace std;
using namespace cv;
// Function prototypes
void on_mouse(int, int, int, int, void*);
vector<Point> linePoints(int x0, int y0, int x1, int y1)
{
vector<Point> pointsOfLine;
int dx = abs(x1-x0), sx = x0<x1 ? 1 : -1;
int dy = abs(y1-y0), sy = y0<y1 ? 1 : -1;
int err = (dx>dy ? dx : -dy)/2, e2;
for(;;)
{
pointsOfLine.push_back(Point(x0,y0));
if (x0==x1 && y0==y1) break;
e2 = err;
if (e2 >-dx)
{
err -= dy;
x0 += sx;
}
if (e2 < dy)
{
err += dx;
y0 += sy;
}
}
return pointsOfLine;
}
// Public parameters
Mat image(600, 800, CV_8UC3, Scalar(220, 220, 220));
vector<Point> points;
int roi_x0 = 0, roi_y0 = 0, roi_x1 = 0, roi_y1 = 0;
bool start_draw = false;
// Window name for visualisation purposes
const string window_name = "OpenCV Mouse Event Demo";
// FUNCTION : Mouse response for selecting objects in images
// If left button is clicked, start drawing a rectangle and circle as long as mouse moves
//If right button is clicked, look what will happen !
// Stop drawing once a new left click is detected by the on_mouse function
void on_mouse(int event, int x, int y, int, void *)
{
// Action when left button is clicked
if (event == EVENT_LBUTTONDOWN)
{
if (!start_draw)
{
roi_x0 = x;
roi_y0 = y;
start_draw = true;
}
else
{
roi_x1 = x;
roi_y1 = y;
start_draw = false;
vector<vector<Point> > contours;
contours.push_back(linePoints(roi_x0,roi_y0,roi_x1,roi_y1));
drawContours( image, contours, 0, Scalar(0, 0, 0), FILLED, 8 );
imshow(window_name, image);
}
}
// Action when mouse is moving
if ((event == EVENT_MOUSEMOVE) && start_draw)
{
// Redraw bounding box and rectangle
Mat current_view;
image.copyTo(current_view);
line(current_view, Point(roi_x0, roi_y0), Point(x, y), Scalar(0, 0, 255));
imshow(window_name, current_view);
}
}
int main(int argc, const char** argv)
{
// Init window interface and couple mouse actions
namedWindow(window_name, WINDOW_AUTOSIZE);
setMouseCallback(window_name, on_mouse);
imshow(window_name, image);
int key_pressed = 0;
do
{
// Keys for processing
// Based on the universal ASCII code of the keystroke: http://www.asciitable.com/
// c = 99 add rectangle to current image
// <ESC> = 27 exit program
key_pressed = 0xFF & waitKey(0);
if (key_pressed==99)
{
image=Scalar(220&rand(), 220, 220);
imshow(window_name, image);
}
}
// Continue as long as the <ESC> key has not been pressed
while (key_pressed != 27);
// Close down the window
destroyWindow(window_name);
return 0;
}