Create a rotated rectangle above a skew line using OpenCV in C++ - c++

Question

I am trying to draw a rectangle rotated suitable with the rotate of a line (this rectangle created by four points)
Basic rectangle
A white overlay in the image I created using a rectangle. I want to make it rotate and stand above the red rectangle.
Here are my red rectangle code:
std::vector<cv::Point> imagePoints;
imagePoints.push_back(it->rect_tl());
imagePoints.push_back(it->rect_tr());
imagePoints.push_back(it->rect_br());
imagePoints.push_back(it->rect_bl());
imagePoints.push_back(it->rect_tl());
polylines(cam_view, imagePoints, false, Scalar(0, 0, 255), 2);
Thanks for your help.

Answer 1

I assume you have the red rectangle already given. So I calculate the angle of the top line of the red rectangle and create a new rotated rectangle with the cv::RotatedRect function.
Here is the example code:
#include <iostream>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/imgproc/imgproc.hpp>
// Function to calculate the angle from 0 to 180° between two lines
float getClockwiseAngle0to180(cv::Point2f x_axis1, cv::Point2f x_axis2, cv::Point2f tl, cv::Point2f tr) {
float dot = (x_axis2.x - x_axis1.x) * (tr.x - tl.x) + (tr.y - tl.y) * (x_axis2.y - x_axis1.y);
float det = (x_axis2.x - x_axis1.x) * (tr.y - tl.y) - (x_axis2.y - x_axis1.y) * (tr.x - tl.x);
float angle = atan2(det, dot);
angle = angle * (180 / (float)CV_PI);
if (angle < 0) {
angle = angle + 360;
}
if (angle >= 180) {
angle = angle - 180;
}
return angle;
}
int main(int argc, char** argv) {
cv::Mat test_image(400, 400, CV_8UC3, cv::Scalar(0));
// You created the red rectangle with some detection algorithm and it seems that
// you already have topleft (tl), topright (tr)... coordinate of the red rectangle
std::vector<cv::Point2f> red_rect_points;
cv::Point2f tl(200.0, 200.0);
cv::Point2f tr(300.0, 150.0);
cv::Point2f br(350.0, 220.0);
cv::Point2f bl(250.0, 300.0);
red_rect_points.push_back(tl);
red_rect_points.push_back(tr);
red_rect_points.push_back(br);
red_rect_points.push_back(bl);
// Get the angle between the tl and tr point with the given function
float rotation = getClockwiseAngle0to180(cv::Point2f(0, 0), cv::Point2f(1, 0), tr, tl);
std::cout << rotation << std::endl;
// Create a new white rectangle with the same rotation angle
// Construct it using center, size and angle
cv::RotatedRect white_rectangle(cv::Point2f(200, 150), cv::Size2f(80, 50), rotation);
cv::Point2f white_vertices[4];
white_rectangle.points(white_vertices);
// Draw both rectangles
for (int i = 0; i < 4; ++i) {
line(test_image, red_rect_points[i], red_rect_points[(i+1)%4], cv::Scalar(0, 0, 255), 1, 8, 0);
line(test_image, white_vertices[i], white_vertices[(i+1)%4], cv::Scalar(255, 255, 255), 1, 8, 0);
}
cv::imshow("Rectangles", test_image);
cv::waitKey(0);
}