Basic Drawing Examples in Image using OpenCV & Python

Goal

• Learn to draw different geometric shapes with OpenCV
• You will learn these functions : cv2.line(), cv2.circle() , cv2.rectangle(), cv2.ellipse() etc.

Drawing a line:

cv2.line(img, Point pt1, Point pt2, color[,thickness[,lineType[,shift]]]) –> img

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: LINE_8 (or omitted) – 8-connected line. LINE_4 – 4-connected line. LINE_AA – antialiased line. shift – Number of fractional bits in the point coordinates.

Example 1: Drawing a line:

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import numpy as np
import cv2

# Create a black image
img = np.zeros((512,512,3), np.uint8)

# Draw a blue line with thickness of 5 px
cv2.line(img,(15,20),(70,50),(255,0,0),5)

#Display the image
cv2.imshow("img",img)

cv2.waitKey(0)

Running the program you will see the blue line.

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Drawing a Circle:

cv2.circle(img, center, radius, color[, thickness[, lineType[, shift]]]) → img

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. shift – Number of fractional bits in the coordinates of the center and in the radius value.

Example 2: Drawing a Circle:

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import numpy as np
import cv2

# Create a black image
img = np.zeros((512,512,3), np.uint8)

# Draw a red closed circle
cv2.circle(img,(200,200), 40, (0,0,255), 3)

#Display the image
cv2.imshow("img",img)

cv2.waitKey(0)

Running the program you will see red color circle

 

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Drawing an Ellipse:

cv2.ellipse(img, center, axes, angle, startAngle, endAngle, color[, thickness[, lineType[, shift]]]) → img

cv2.ellipse(img, box, color[, thickness[, lineType]]) → img

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. box – Alternative ellipse representation via RotatedRect or CvBox2D. 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. shift – Number of fractional bits in the coordinates of the center and values of axes.

Example 3: Drawing an Ellipse:

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import numpy as np
import cv2

# Create a black image
img = np.zeros((512,512,3), np.uint8)

# Draw ellipse
cv2.ellipse(img,(200,200),(80,50),0,0,360,(0,255,0),-1)
cv2.ellipse(img,(200,200),(80,50),45,0,360,(0,0,255),1)

#Display the image
cv2.imshow("img",img)

cv2.waitKey(0)

Running the program you will see ellipse

 

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Drawing a Rectangle:

cv2.rectangle(img, pt1, pt2, color[, thickness[, lineType[, shift]]]) → img

Parameters:

img – Image. pt1 – Vertex of the rectangle. pt2 – Vertex of the rectangle opposite to pt1 . rec – Alternative specification of the drawn rectangle. 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.. shift – Number of fractional bits in the point coordinates.

Example 4: Drawing a Rectangle:

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import numpy as np
import cv2

# Create a black image
img = np.zeros((512,512,3), np.uint8)

# Draw rectangle(not filled)
cv2.rectangle(img,(15,20),(70,50),(0,255,0),3)

# Draw filled rectangle
cv2.rectangle(img,(115,120),(170,150),(255,0,0),-1)

#Display the image
cv2.imshow("img",img)

cv2.waitKey(0)

Running the program you will see rectangle

 

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Drawing a Polygon:

cv2.polylines(img, pts, isClosed, color[, thickness[, lineType[, shift]]]) → img

Parameters:

img – Image. pts – Array of polygonal curves. npts – Array of polygon vertex counters. ncontours – Number of 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. shift – Number of fractional bits in the vertex coordinates.

Example 4: Drawing a Polygon:

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import numpy as np
import cv2

# Create a black image
img = np.zeros((512,512,3), np.uint8)

# Draw a polygon
pts = np.array([[10,5],[20,30],[70,20],[50,10]], np.int32)
pts = pts.reshape((-1,1,2))
cv2.polylines(img,[pts],True,(0,255,255))

#Display the image
cv2.imshow("img",img)

cv2.waitKey(0)

Running the program you will see polygon