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: