before starting the text, first know that the (0, 0) coordinate point in the screen is in the upper left corner and the maximum coordinate is in the lower right corner.

1, Drawing of basic graphics

1. Rectangle

   the function of drawing rectangle in OpenCV is rectangle().

void rectangle(InputOutputArray img, Rect rec, const Scalar& color, int thickness = 1, int lineType = LINE_8, int shift = 0);

parameter	effect
InputOutputArray img	Target image to draw
Rect rec	rect class object
const Scalar& color	Line color
int thickness = 1	A positive number indicates the line width, - 1 indicates the filled rectangle
int lineType = LINE_8	Type of line
int shift = 0	Decimal places of coordinate points

An example procedure for drawing a rectangle is as follows:

void MyDemo::drawing_Demo(Mat& image) {
	//draw rectangle
	Rect rect;
	rect.x = 250;	//Top left endpoint x coordinate
	rect.y = 170;	//Top left endpoint x coordinate
	rect.width = 100;	//width
	rect.height = 100;	//height
	rectangle(image, rect, Scalar(0, 0, 255), 2, 8, 0);
	imshow("Drawing", image);
}

2. Round

   the function of drawing a circle in OpenCV is rectangle().

void cv::circle (InputOutputArray img, Point center, int radius, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0);

parameter	effect
InputOutputArray img	Target image to draw
Point center	Center coordinates
int radius	Radius of circle
const Scalar &color	Color of circle
int thickness=1	A positive number indicates the line width, - 1 indicates the filled circle
int lineType = LINE_8	Type of line
int shift = 0	Decimal places of coordinate points

An example program for drawing a circle is as follows:

void MyDemo::drawing_Demo(Mat& image) {
	//Draw circle
	circle(image, Point(250, 170), 20, Scalar(255, 0, 0), -1, 8, 0);
	imshow("Drawing", image);
}

  

3. Straight line

   the function of drawing a line in OpenCV is rectangle().

void cv::line(InputOutputArray img, Point pt1, Point pt2, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0);

parameter	effect
InputOutputArray img	Target image to draw
Point pt1r	Coordinates of endpoint 1
Point pt2	Coordinates of endpoint 2
const Scalar &color	Line color
int thickness=1	Line width
int lineType = LINE_8	Type of line
int shift = 0	Decimal places of coordinate points

An example program for drawing a circle is as follows:

void MyDemo::drawing_Demo(Mat& image) {
	//draw a straight line
	line(image, Point(250, 170), Point(350, 270), Scalar(0, 255, 0), 2, LINE_AA, 0);
	imshow("Drawing", image);
}

  

4. Ellipse

  the function for drawing ellipses in OpenCV is ellipse().

void ellipse(Mat&img, const RotatedRect&box, const Scalar& color, int thickness=1, int lineType=8);

parameter	effect
Mat&img	Target image to draw
const RotatedRect&box	Elliptic class
const Scalar &color	Color of ellipse
int thickness=1	A positive number indicates the line width, - 1 indicates the filled circle
int lineType = LINE_8	Type of line

The properties of the ellipse class RotatedRect are as follows (example):

RotatedRect rrt;	//create object
rrt.center = Point(100, 200);	//Center point of ellipse
rrt.size = Size(100, 200);		//Ellipse size (horizontal axis, vertical axis length)
rrt.angle = 0;		//Rotation angle

An example program for drawing a circle is as follows:

void MyDemo::drawing_Demo(Mat& image) {
	//Draw ellipse
	RotatedRect rrt;
	rrt.center = Point(100, 200);
	rrt.size = Size(100, 200);
	rrt.angle = 0;
	ellipse(image, rrt, Scalar(0, 255, 255), 1, 8);
	imshow("Drawing", image);
}

  

2, Polygon drawing method

   the essence of polygon drawing is to define a point set composed of multiple points, so as to connect the points in the point set to form a polygon.

2.1 polygon border

    the function used to draw the border of the polygon is ploylines().

void cv::polylines(
		InputOutputArray	img,
		InputArrayOfArrays	pts,
		bool	isClosed,
		const Scalar & color,
		int		thickness = 1,
		int		lineType = LINE_8,
		int		shift = 0
)

parameter	effect
img	Target image to draw
pts	Pre constructed point set
isClosed	Is the polygon a closed shape
color	The color of the polygon border
thickness	Line width (can only be positive)
lineType	Type of line
shift	Decimal places of coordinate points

Example program: drawing a Pentagon

void MyDemo::polyDrawing_Demo() {
	//Create canvas
	Mat bg = Mat::zeros(Size(512, 512), CV_8UC3);

	//Create five points as the five vertices of the polygon
	Point p1(100, 100);
	Point p2(350, 100);
	Point p3(450, 300);
	Point p4(250, 450);
	Point p5(80, 200);

	//Add five points to the point set in turn
	std::vector<Point> pts;
	pts.push_back(p1);
	pts.push_back(p2);
	pts.push_back(p3);
	pts.push_back(p4);
	pts.push_back(p5);

	//Draw polygon border
	polylines(bg, pts, true, Scalar(0, 255, 255), 3, LINE_AA, 0);
	imshow("Poly Drawing!", bg);
}

Where PTS push_ Back is used to add points to the point set array. It is used when the number of point sets is unknown. This program knows a total of five points, so you can also use the following code to create a point set:

	std::vector<Point> pts(5);
	pts[0] = p1;
	pts[1] = p2;
	pts[2] = p3;
	pts[3] = p4;
	pts[4] = p5;

2.2 polygon filling

   the function used to draw the filled polygon is fillPoly().

void cv::fillPoly(
		InputOutputArray	img,
		InputArrayOfArrays	pts,
		const Scalar & color,
		int		lineType = LINE_8,
		int		shift = 0
)

parameter	effect
img	Target image to draw
pts	Pre constructed point set
color	The color of the polygon border
lineType	Type of line
shift	Decimal places of coordinate points

Example program: drawing a filled Pentagon

void MyDemo::polyDrawing_Demo() {
	//Create canvas
	Mat bg = Mat::zeros(Size(512, 512), CV_8UC3);

	//Create five points as the five vertices of the polygon
	Point p1(100, 100);
	Point p2(350, 100);
	Point p3(450, 300);
	Point p4(250, 450);
	Point p5(80, 200);

	//Add five points to the point set in turn
	std::vector<Point> pts;
	pts.push_back(p1);
	pts.push_back(p2);
	pts.push_back(p3);
	pts.push_back(p4);
	pts.push_back(p5);

	//Draws a closed Pentagon
	fillPoly(bg, pts, Scalar(255, 255, 0), 8, 0);
	imshow("Poly Drawing!", bg);
}

2.3 drawing multiple polygons

  the function used to draw multiple polygons is drawContours().

void drawContours(
	InputOutputArray 	image,
	InputArrayOfArrays 	contours,
	int 	contourIdx,
	const Scalar& color,
	int 	thickness=1,
	int 	lineType=8,
	InputArray 	hierarchy=noArray(),
	int 		maxLevel=INT_MAX,
	Point 		offset=Point() )

parameter	effect
img	Target image to draw
contours	Input contour groups, each composed of point vector s
contourIdx	Draw several contours. If the parameter is negative, draw all contours
color	Line color
thickness	Line width, negative value or CV_FILLED indicates the interior of the fill profile
lineType	Type of line

Sample program: draw a Pentagon

void MyDemo::polyDrawing_Demo() {
	//Create canvas
	Mat bg = Mat::zeros(Size(512, 512), CV_8UC3);

	//Create five points as the five vertices of the polygon
	Point p1(100, 100);
	Point p2(350, 100);
	Point p3(450, 300);
	Point p4(250, 450);
	Point p5(80, 200);

	//Add five points to the point set in turn
	std::vector<Point> pts;
	pts.push_back(p1);
	pts.push_back(p2);
	pts.push_back(p3);
	pts.push_back(p4);
	pts.push_back(p5);

	//Add point set to contours
	//(equivalent to that contours is a collection of point sets)
	std::vector<std::vector<Point>> contours;
	contours.push_back(pts);

	//draw a polygon
	drawContours(bg, contours, -1, Scalar(255, 0, 255), -1);
	imshow("Poly Drawing!", bg);
}