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Drawing Shapes

Introduction​

This chapter teaches how to use OpenCV to draw shapes on images — a feature that will be frequently used later, such as annotating a region on an image or drawing a rectangle around a face.

Experiment Objective​

Use the OpenCV library to draw various shapes.

Experiment Explanation​

The OpenCV Python library provides various drawing functions that we can use directly.

Drawing Line Segments​

Use the line() function to draw line segments. Usage is as follows:

img = cv2.line(img, pt1, pt2, color, thickness)

To draw a line segment:

  • img: Image.
  • pt1: Starting point coordinates.
  • pt2: Ending point coordinates.
  • color: Color (in BGR order).
  • thickness: Line width (thickness).

Drawing Rectangles​

Use the rectangle() function to draw rectangles. Usage is as follows:

img = cv2.rectangle(img, pt1, pt2, color, thickness)

To draw a rectangle:

  • img: Image.
  • pt1: Upper-left corner coordinates.
  • pt2: Lower-right corner coordinates.
  • color: Color (in BGR order).
  • thickness: Line width (thickness). When the value is -1, it means filled.

Drawing Circles​

Use the circle() function to draw circles. Usage is as follows:

img = cv2.circle(img, center, radius, color, thickness)

To draw a circle:

  • img: Image.
  • center: Center coordinates.
  • radius: Radius.
  • color: Color (in BGR order).
  • thickness: Line width (thickness). When the value is -1, it means filled.

Arbitrary Polygons​

Use the polylines() function to draw arbitrary irregular polygons; you only need to know the coordinates of each vertex. Usage is as follows:

img = cv2.polylines(img, pts, isClosed, color, thickness)

To draw an arbitrary polygon:

  • img: Image.
  • pts: Coordinates of each vertex. A NumPy array.
  • isClosed: Whether to close the polygon.
    • True: Closed polygon.
    • False: Open polygon.
  • color: Color (in BGR order).
  • thickness: Line width (thickness).

After becoming familiar with the common drawing methods, we will use code to create a new canvas (image), draw the above shapes on it, and display them. The code flow is as follows:


The reference code is as follows. The code uses NumPy's array creation functionality. NumPy is not heavily used in this chapter, so it will not be expanded upon here; you can search and learn about it online.

'''
Experiment Name: Drawing Shapes
Experiment Platform: WalnutPi 1B
'''

import cv2
import numpy as np

# Create a new 500×500 pixel RGB888 pure black image
img = np.zeros((500,500,3),np.uint8)

# Draw a line segment from (50,50) to (450,50), red color, line width 5 pixels.
img = cv2.line(img, (50,50), (450,50), (0,0,255), 5)

 # Draw rectangle 1 from (50,80) to (200,200), green, line width 5 pixels.
img = cv2.rectangle(img, (50,80), (200,200), (0,255,0), 5)

# Draw rectangle 2 from (250,80) to (450,200), green, filled.
img = cv2.rectangle(img, (250,80), (450,200), (0,255,0), -1)

# Draw a circle, center (150,300), radius 50, blue, line width 3
img = cv2.circle(img, (150, 300), 50, (255,0,0), 5)

# Draw arbitrary polygon 1, closed
pts = np.array([[50,400],[200,400],[150,480],[50,480]],np.int32)
img = cv2.polylines(img, [pts], True, (255,0,255), 5)

# Draw arbitrary polygon 2, open
pts = np.array([[250,400],[400,400],[350,480],[250,480]],np.int32)
img = cv2.polylines(img, [pts], False, (255,0,255), 5)

cv2.imshow('draw', img) # Display the image

cv2.waitKey() # Wait for any keyboard key to be pressed
cv2.destroyAllWindows() # Close the window

Experiment Results​

Run the code on the WalnutPi; the experiment results are as shown below:

shape