How to Draw Rectangle on An Image using OpenCV in Python

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OpenCV (Open Source Computer Vision) is an open source computer vision library. To work with computer vision problems OpenCV is very helpful. We can perform many tasks using OpenCV like Recognition, Motion Analysis, Scene Reconstruction, image restoration.

We use  cv2.rectangle()  to draw rectangle on an image. This method is used to draw bounding boxes of the objects in object detection task.

draw rectangle on image using opencv in python

Table of Contents:

Prerequisites:

  • Python
  • OpenCV

Install OpenCV

If you have not already installed OpenCV on you system, you can install it using pip command as written below.

pip install opencv-python

Syntax

The syntax to draw rectangle on an image is as below. 


cv2.rectangle(image, start_point, end_point, color, thickness)

Parameters: 
image: image on that the rectangle to drawn
start_point:(width_min, height_min)= (xmin, ymin):start coordinate of rectangle. It is top left point on the image.
end_point: (width_max, height_max)= (xmax, yax):end coordinate of rectangle. It’s bottom right point on the image.
color: (B,G,R)-  Color of the line
thickness:  Thickness of the line

Steps

The steps to draw rectangle on an image is as follows
  1. Load the image
  2. define starting and ending point of the rectangle
  3. Define the color of the line of the rectangle
  4. Define the thickness of the line of the rectangle
  5. Draw the rectangle
  6. Display the image with drawn rectangle

Example 1.

Let's write complete Python 3 program for drawing rectangle on the image.


# Python 3 program to draw rectangle on an image

# import cv2
import cv2

# Read an image 
img = cv2.imread('Koala.jpg')

# Start coordinate (xmin, ymin), it represents the top left corner of rectangle
start_point = (320, 200)

# End coordinate (xmax, ymax), it represents the bottom right corner of rectangle
end_point = (800, 650)

# Green color in BGR
color = (0, 255, 0)

# Line thickness of 4 px
thickness = 4

# Using cv2.rectangle() 
# Draw a rectangle with green line of thickness of 4 px
img = cv2.rectangle(img, start_point, end_point, color, thickness)

# Display the image
cv2.imshow("Koala", img)
cv2.waitKey(0) 
cv2.destroyAllWindows() 

Output:

draw rectangle on image



Example 2.

If we use thickness as -1, it will fill whole rectangle with the chosen line color. Here we take green color to draw the rectangle but using thickness as -1, so the whole rectangle is filled with green color.


# Python 3 program to draw rectangle on an image

# import cv2
import cv2

# Read an image 
img = cv2.imread('Koala.jpg')

# Start coordinate (xmin, ymin), it represents the top left corner of rectangle
start_point = (320, 200)

# End coordinate (xmax, ymax), it represents the bottom right corner of rectangle
end_point = (800, 650)

# Green color in BGR
color = (0, 255, 0)

# Line thickness of -1 px
thickness = -1

# Using cv2.rectangle() 
# Draw a rectangle with green line of thickness of 4 px
img = cv2.rectangle(img, start_point, end_point, color, thickness)

# Display the image
cv2.imshow("Koala", img)
cv2.waitKey(0) 
cv2.destroyAllWindows() 

Output:

rectangle draw on image with line thickness-1

Further Readings:

Useful Resources:

Previous Post: Extracting and Saving Video Frames using OpenCV Python


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