Python OpenCV cv2.line() Guide

In this guide, we will explore how to use the cv2.line() function in Python OpenCV. This function is used to draw lines on images. It is a fundamental tool in image processing and computer vision tasks.

What is cv2.line()?

The cv2.line() function is part of the OpenCV library. It allows you to draw a line between two points on an image. This is useful for creating visual markers, annotations, or highlighting specific areas in an image.

Syntax of cv2.line()

The syntax for cv2.line() is straightforward. Here is the basic structure:

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

Let's break down the parameters:

• image: The image on which the line will be drawn.
• start_point: The starting point of the line (x1, y1).
• end_point: The ending point of the line (x2, y2).
• color: The color of the line in BGR format (e.g., (255, 0, 0) for blue).
• thickness: The thickness of the line in pixels.

Example: Drawing a Line on an Image

Let's see how to use cv2.line() in practice. We'll start by loading an image and then drawing a line on it.

import cv2
import numpy as np

# Create a blank image
image = np.zeros((500, 500, 3), dtype=np.uint8)

# Draw a line from (100, 100) to (400, 400)
cv2.line(image, (100, 100), (400, 400), (0, 255, 0), 5)

# Display the image
cv2.imshow('Line', image)
cv2.waitKey(0)
cv2.destroyAllWindows()

In this example, we create a blank image using np.zeros(). Then, we draw a green line from point (100, 100) to point (400, 400) with a thickness of 5 pixels. Finally, we display the image using cv2.imshow().

Combining cv2.line() with Other Functions

You can combine cv2.line() with other OpenCV functions to create more complex visualizations. For example, you can use cv2.rectangle() to draw rectangles and cv2.circle() to draw circles on the same image.

Here's an example that combines cv2.line() with cv2.rectangle():

import cv2
import numpy as np

# Create a blank image
image = np.zeros((500, 500, 3), dtype=np.uint8)

# Draw a line
cv2.line(image, (100, 100), (400, 400), (0, 255, 0), 5)

# Draw a rectangle
cv2.rectangle(image, (200, 200), (300, 300), (255, 0, 0), 3)

# Display the image
cv2.imshow('Line and Rectangle', image)
cv2.waitKey(0)
cv2.destroyAllWindows()

In this example, we draw a green line and a blue rectangle on the same image. This demonstrates how you can use multiple drawing functions together.

Conclusion

The cv2.line() function is a powerful tool for drawing lines on images in Python OpenCV. It is easy to use and can be combined with other functions like cv2.rectangle() and cv2.circle() to create complex visualizations. Whether you're annotating images or creating visual markers, cv2.line() is an essential function to know.

For more information on related functions, check out our guides on cv2.rectangle() and cv2.circle().