Drawing Functions

Classes
class	cv::LineIterator
	Line iterator. More...

Functions
void	cv::arrowedLine (InputOutputArray img, Point pt1, Point pt2, const Scalar &color, int thickness=1, int line_type=8, int shift=0, double tipLength=0.1)
	Draws a arrow segment pointing from the first point to the second one. More...
void	cv::circle (InputOutputArray img, Point center, int radius, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
	Draws a circle. More...
bool	cv::clipLine (Size imgSize, Point &pt1, Point &pt2)
	Clips the line against the image rectangle. More...
bool	cv::clipLine (Rect imgRect, Point &pt1, Point &pt2)
void	cv::drawContours (InputOutputArray image, InputArrayOfArrays contours, int contourIdx, const Scalar &color, int thickness=1, int lineType=LINE_8, InputArray hierarchy=noArray(), int maxLevel=INT_MAX, Point offset=Point())
	Draws contours outlines or filled contours. More...
void	cv::ellipse (InputOutputArray img, Point center, Size axes, double angle, double startAngle, double endAngle, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
	Draws a simple or thick elliptic arc or fills an ellipse sector. More...
void	cv::ellipse (InputOutputArray img, const RotatedRect &box, const Scalar &color, int thickness=1, int lineType=LINE_8)
void	cv::ellipse2Poly (Point center, Size axes, int angle, int arcStart, int arcEnd, int delta, std::vector< Point > &pts)
	Approximates an elliptic arc with a polyline. More...
void	cv::fillConvexPoly (Mat &img, const Point *pts, int npts, const Scalar &color, int lineType=LINE_8, int shift=0)
void	cv::fillConvexPoly (InputOutputArray img, InputArray points, const Scalar &color, int lineType=LINE_8, int shift=0)
	Fills a convex polygon. More...
void	cv::fillPoly (Mat &img, const Point **pts, const int *npts, int ncontours, const Scalar &color, int lineType=LINE_8, int shift=0, Point offset=Point())
void	cv::fillPoly (InputOutputArray img, InputArrayOfArrays pts, const Scalar &color, int lineType=LINE_8, int shift=0, Point offset=Point())
	Fills the area bounded by one or more polygons. More...
Size	cv::getTextSize (const String &text, int fontFace, double fontScale, int thickness, int *baseLine)
	Calculates the width and height of a text string. More...
void	cv::line (InputOutputArray img, Point pt1, Point pt2, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
	Draws a line segment connecting two points. More...
void	cv::polylines (Mat &img, const Point *const *pts, const int *npts, int ncontours, bool isClosed, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
void	cv::polylines (InputOutputArray img, InputArrayOfArrays pts, bool isClosed, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
	Draws several polygonal curves. More...
void	cv::putText (InputOutputArray img, const String &text, Point org, int fontFace, double fontScale, Scalar color, int thickness=1, int lineType=LINE_8, bool bottomLeftOrigin=false)
	Draws a text string. More...
void	cv::rectangle (InputOutputArray img, Point pt1, Point pt2, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)
	Draws a simple, thick, or filled up-right rectangle. More...
void	cv::rectangle (Mat &img, Rect rec, const Scalar &color, int thickness=1, int lineType=LINE_8, int shift=0)

Detailed Description

Drawing functions work with matrices/images of arbitrary depth. The boundaries of the shapes can be rendered with antialiasing (implemented only for 8-bit images for now). All the functions include the parameter color that uses an RGB value (that may be constructed with the Scalar constructor ) for color images and brightness for grayscale images. For color images, the channel ordering is normally Blue, Green, Red. This is what imshow, imread, and imwrite expect. So, if you form a color using the Scalar constructor, it should look like:

\[\texttt{Scalar} (blue \_ component, green \_ component, red \_ component[, alpha \_ component])\]

If you are using your own image rendering and I/O functions, you can use any channel ordering. The drawing functions process each channel independently and do not depend on the channel order or even on the used color space. The whole image can be converted from BGR to RGB or to a different color space using cvtColor .

If a drawn figure is partially or completely outside the image, the drawing functions clip it. Also, many drawing functions can handle pixel coordinates specified with sub-pixel accuracy. This means that the coordinates can be passed as fixed-point numbers encoded as integers. The number of fractional bits is specified by the shift parameter and the real point coordinates are calculated as \(\texttt{Point}(x,y)\rightarrow\texttt{Point2f}(x*2^{-shift},y*2^{-shift})\) . This feature is especially effective when rendering antialiased shapes.

Note

The functions do not support alpha-transparency when the target image is 4-channel. In this case, the color[3] is simply copied to the repainted pixels. Thus, if you want to paint semi-transparent shapes, you can paint them in a separate buffer and then blend it with the main image.

Function Documentation

void cv::arrowedLine	(	InputOutputArray	img,
		Point	pt1,
		Point	pt2,
		const Scalar &	color,
		int	thickness = 1,
		int	line_type = 8,
		int	shift = 0,
		double	tipLength = 0.1
	)

Draws a arrow segment pointing from the first point to the second one.

The function arrowedLine draws an arrow between pt1 and pt2 points in the image. See also cv::line.

Parameters

img	Image.
pt1	The point the arrow starts from.
pt2	The point the arrow points to.
color	Line color.
thickness	Line thickness.
line_type	Type of the line, see cv::LineTypes
shift	Number of fractional bits in the point coordinates.
tipLength	The length of the arrow tip in relation to the arrow length

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

Draws a circle.

The function circle draws a simple or filled circle with a given center and radius.

Parameters

img	Image where the circle is drawn.
center	Center of the circle.
radius	Radius of the circle.
color	Circle color.
thickness	Thickness of the circle outline, if positive. Negative thickness means that a filled circle is to be drawn.
lineType	Type of the circle boundary. See the line description.
shift	Number of fractional bits in the coordinates of the center and in the radius value.

Examples:

convexhull.cpp, houghcircles.cpp, kmeans.cpp, and minarea.cpp.

bool cv::clipLine	(	Size	imgSize,
		Point &	pt1,
		Point &	pt2
	)

Clips the line against the image rectangle.

The functions clipLine calculate a part of the line segment that is entirely within the specified rectangle. They return false if the line segment is completely outside the rectangle. Otherwise, they return true .

Parameters

imgSize	Image size. The image rectangle is Rect(0, 0, imgSize.width, imgSize.height) .
pt1	First line point.
pt2	Second line point.

bool cv::clipLine	(	Rect	imgRect,
		Point &	pt1,
		Point &	pt2
	)

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Parameters

imgRect	Image rectangle.
pt1	First line point.
pt2	Second line point.

void cv::drawContours	(	InputOutputArray	image,
		InputArrayOfArrays	contours,
		int	contourIdx,
		const Scalar &	color,
		int	thickness = 1,
		int	lineType = LINE_8,
		InputArray	hierarchy = noArray(),
		int	maxLevel = INT_MAX,
		Point	offset = Point()
	)

Draws contours outlines or filled contours.

The function draws contour outlines in the image if \(\texttt{thickness} \ge 0\) or fills the area bounded by the contours if \(\texttt{thickness}<0\) . The example below shows how to retrieve connected components from the binary image and label them: :

#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"

using namespace cv;
using namespace std;

int main( int argc, char** argv )
{
    Mat src;
    // the first command-line parameter must be a filename of the binary
    // (black-n-white) image
    if( argc != 2 || !(src=imread(argv[1], 0)).data)
        return -1;

    Mat dst = Mat::zeros(src.rows, src.cols, CV_8UC3);

    src = src > 1;
    namedWindow( "Source", 1 );
    imshow( "Source", src );

    vector<vector<Point> > contours;
    vector<Vec4i> hierarchy;

    findContours( src, contours, hierarchy,
        RETR_CCOMP, CHAIN_APPROX_SIMPLE );

    // iterate through all the top-level contours,
    // draw each connected component with its own random color
    int idx = 0;
    for( ; idx >= 0; idx = hierarchy[idx][0] )
    {
        Scalar color( rand()&255, rand()&255, rand()&255 );
        drawContours( dst, contours, idx, color, FILLED, 8, hierarchy );
    }

    namedWindow( "Components", 1 );
    imshow( "Components", dst );
    waitKey(0);
}

Parameters

image	Destination image.
contours	All the input contours. Each contour is stored as a point vector.
contourIdx	Parameter indicating a contour to draw. If it is negative, all the contours are drawn.
color	Color of the contours.
thickness	Thickness of lines the contours are drawn with. If it is negative (for example, thickness=CV_FILLED ), the contour interiors are drawn.
lineType	Line connectivity. See cv::LineTypes.
hierarchy	Optional information about hierarchy. It is only needed if you want to draw only some of the contours (see maxLevel ).
maxLevel	Maximal level for drawn contours. If it is 0, only the specified contour is drawn. If it is 1, the function draws the contour(s) and all the nested contours. If it is 2, the function draws the contours, all the nested contours, all the nested-to-nested contours, and so on. This parameter is only taken into account when there is hierarchy available.
offset	Optional contour shift parameter. Shift all the drawn contours by the specified \(\texttt{offset}=(dx,dy)\) .

Examples:

contours2.cpp, fitellipse.cpp, segment_objects.cpp, and watershed.cpp.

void cv::ellipse	(	InputOutputArray	img,
		Point	center,
		Size	axes,
		double	angle,
		double	startAngle,
		double	endAngle,
		const Scalar &	color,
		int	thickness = 1,
		int	lineType = LINE_8,
		int	shift = 0
	)

Draws a simple or thick elliptic arc or fills an ellipse sector.

The functions ellipse with less parameters draw an ellipse outline, a filled ellipse, an elliptic arc, or a filled ellipse sector. A piecewise-linear curve is used to approximate the elliptic arc boundary. If you need more control of the ellipse rendering, you can retrieve the curve using ellipse2Poly and then render it with polylines or fill it with fillPoly . If you use the first variant of the function and want to draw the whole ellipse, not an arc, pass startAngle=0 and endAngle=360 . The figure below explains the meaning of the parameters.

Parameters of Elliptic Arc

Parameters

img	Image.
center	Center of the ellipse.
axes	Half of the size of the ellipse main axes.
angle	Ellipse rotation angle in degrees.
startAngle	Starting angle of the elliptic arc in degrees.
endAngle	Ending angle of the elliptic arc in degrees.
color	Ellipse color.
thickness	Thickness of the ellipse arc outline, if positive. Otherwise, this indicates that a filled ellipse sector is to be drawn.
lineType	Type of the ellipse boundary. See the line description.
shift	Number of fractional bits in the coordinates of the center and values of axes.

Examples:

contours2.cpp, and fitellipse.cpp.

void cv::ellipse	(	InputOutputArray	img,
		const RotatedRect &	box,
		const Scalar &	color,
		int	thickness = 1,
		int	lineType = LINE_8
	)

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

Parameters

img	Image.
box	Alternative ellipse representation via RotatedRect. This means that the function draws an ellipse inscribed in the rotated rectangle.
color	Ellipse color.
thickness	Thickness of the ellipse arc outline, if positive. Otherwise, this indicates that a filled ellipse sector is to be drawn.
lineType	Type of the ellipse boundary. See the line description.

void cv::ellipse2Poly	(	Point	center,
		Size	axes,
		int	angle,
		int	arcStart,
		int	arcEnd,
		int	delta,
		std::vector< Point > &	pts
	)

Approximates an elliptic arc with a polyline.

The function ellipse2Poly computes the vertices of a polyline that approximates the specified elliptic arc. It is used by cv::ellipse.

Parameters

center	Center of the arc.
axes	Half of the size of the ellipse main axes. See the ellipse for details.
angle	Rotation angle of the ellipse in degrees. See the ellipse for details.
arcStart	Starting angle of the elliptic arc in degrees.
arcEnd	Ending angle of the elliptic arc in degrees.
delta	Angle between the subsequent polyline vertices. It defines the approximation accuracy.
pts	Output vector of polyline vertices.

void cv::fillConvexPoly	(	Mat &	img,
		const Point *	pts,
		int	npts,
		const Scalar &	color,
		int	lineType = LINE_8,
		int	shift = 0
	)

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

void cv::fillConvexPoly	(	InputOutputArray	img,
		InputArray	points,
		const Scalar &	color,
		int	lineType = LINE_8,
		int	shift = 0
	)

Fills a convex polygon.

The function fillConvexPoly draws a filled convex polygon. This function is much faster than the function cv::fillPoly . It can fill not only convex polygons but any monotonic polygon without self-intersections, that is, a polygon whose contour intersects every horizontal line (scan line) twice at the most (though, its top-most and/or the bottom edge could be horizontal).

Parameters

img	Image.
points	Polygon vertices.
color	Polygon color.
lineType	Type of the polygon boundaries. See the line description.
shift	Number of fractional bits in the vertex coordinates.

void cv::fillPoly	(	Mat &	img,
		const Point **	pts,
		const int *	npts,
		int	ncontours,
		const Scalar &	color,
		int	lineType = LINE_8,
		int	shift = 0,
		Point	offset = Point()
	)

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

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

Fills the area bounded by one or more polygons.

The function fillPoly fills an area bounded by several polygonal contours. The function can fill complex areas, for example, areas with holes, contours with self-intersections (some of their parts), and so forth.

Parameters

img	Image.
pts	Array of polygons where each polygon is represented as an array of points.
color	Polygon color.
lineType	Type of the polygon boundaries. See the line description.
shift	Number of fractional bits in the vertex coordinates.
offset	Optional offset of all points of the contours.

Size cv::getTextSize	(	const String &	text,
		int	fontFace,
		double	fontScale,
		int	thickness,
		int *	baseLine
	)

Calculates the width and height of a text string.

The function getTextSize calculates and returns the size of a box that contains the specified text. That is, the following code renders some text, the tight box surrounding it, and the baseline: :

String text = "Funny text inside the box";
int fontFace = FONT_HERSHEY_SCRIPT_SIMPLEX;
double fontScale = 2;
int thickness = 3;

Mat img(600, 800, CV_8UC3, Scalar::all(0));

int baseline=0;
Size textSize = getTextSize(text, fontFace,
                            fontScale, thickness, &baseline);
baseline += thickness;

// center the text
Point textOrg((img.cols - textSize.width)/2,
              (img.rows + textSize.height)/2);

// draw the box
rectangle(img, textOrg + Point(0, baseline),
          textOrg + Point(textSize.width, -textSize.height),
          Scalar(0,0,255));
// ... and the baseline first
line(img, textOrg + Point(0, thickness),
     textOrg + Point(textSize.width, thickness),
     Scalar(0, 0, 255));

// then put the text itself
putText(img, text, textOrg, fontFace, fontScale,
        Scalar::all(255), thickness, 8);

Parameters

	text	Input text string.
	fontFace	Font to use, see cv::HersheyFonts.
	fontScale	Font scale factor that is multiplied by the font-specific base size.
	thickness	Thickness of lines used to render the text. See putText for details.
[out]	baseLine	y-coordinate of the baseline relative to the bottom-most text point.

Returns

The size of a box that contains the specified text.

See also

cv::putText

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

Draws a line segment connecting two points.

The function line draws the line segment between pt1 and pt2 points in the image. The line is clipped by the image boundaries. For non-antialiased lines with integer coordinates, the 8-connected or 4-connected Bresenham algorithm is used. Thick lines are drawn with rounding endings. Antialiased lines are drawn using Gaussian filtering.

Parameters

img	Image.
pt1	First point of the line segment.
pt2	Second point of the line segment.
color	Line color.
thickness	Line thickness.
lineType	Type of the line, see cv::LineTypes.
shift	Number of fractional bits in the point coordinates.

Examples:

contours2.cpp, convexhull.cpp, fitellipse.cpp, houghlines.cpp, minarea.cpp, pca.cpp, and watershed.cpp.

void cv::polylines	(	Mat &	img,
		const Point *const *	pts,
		const int *	npts,
		int	ncontours,
		bool	isClosed,
		const Scalar &	color,
		int	thickness = 1,
		int	lineType = LINE_8,
		int	shift = 0
	)

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

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

Draws several polygonal curves.

Parameters

img	Image.
pts	Array of polygonal curves.
isClosed	Flag indicating whether the drawn polylines are closed or not. If they are closed, the function draws a line from the last vertex of each curve to its first vertex.
color	Polyline color.
thickness	Thickness of the polyline edges.
lineType	Type of the line segments. See the line description.
shift	Number of fractional bits in the vertex coordinates.

The function polylines draws one or more polygonal curves.

void cv::putText	(	InputOutputArray	img,
		const String &	text,
		Point	org,
		int	fontFace,
		double	fontScale,
		Scalar	color,
		int	thickness = 1,
		int	lineType = LINE_8,
		bool	bottomLeftOrigin = false
	)

Draws a text string.

The function putText renders the specified text string in the image. Symbols that cannot be rendered using the specified font are replaced by question marks. See getTextSize for a text rendering code example.

Parameters

img	Image.
text	Text string to be drawn.
org	Bottom-left corner of the text string in the image.
fontFace	Font type, see cv::HersheyFonts.
fontScale	Font scale factor that is multiplied by the font-specific base size.
color	Text color.
thickness	Thickness of the lines used to draw a text.
lineType	Line type. See the line for details.
bottomLeftOrigin	When true, the image data origin is at the bottom-left corner. Otherwise, it is at the top-left corner.

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

Draws a simple, thick, or filled up-right rectangle.

The function rectangle draws a rectangle outline or a filled rectangle whose two opposite corners are pt1 and pt2.

Parameters

img	Image.
pt1	Vertex of the rectangle.
pt2	Vertex of the rectangle opposite to pt1 .
color	Rectangle color or brightness (grayscale image).
thickness	Thickness of lines that make up the rectangle. Negative values, like CV_FILLED , mean that the function has to draw a filled rectangle.
lineType	Type of the line. See the line description.
shift	Number of fractional bits in the point coordinates.

Examples:

demhist.cpp, and grabcut.cpp.

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

This is an overloaded member function, provided for convenience. It differs from the above function only in what argument(s) it accepts.

use rec parameter as alternative specification of the drawn rectangle: r.tl() and r.br()-Point(1,1) are opposite corners