How to Draw Rectangle in OpenCV for Computer Vision

Use the cv2.rectangle() function in OpenCV to draw a rectangle on an image by specifying the image, top-left and bottom-right corner points, color, and thickness. This function modifies the image in place and is commonly used for highlighting regions in computer vision tasks.

Syntax

The cv2.rectangle() function draws a rectangle on an image. It requires the image, two corner points, color, and thickness as inputs.

• img: The image where the rectangle will be drawn.
• pt1: Top-left corner of the rectangle (x, y).
• pt2: Bottom-right corner of the rectangle (x, y).
• color: Rectangle color in BGR format (e.g., (255, 0, 0) for blue).
• thickness: Thickness of the rectangle border in pixels. Use -1 to fill the rectangle.

python

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

Example

This example loads a blank image, draws a blue rectangle with a thickness of 3 pixels, and displays the result.

python

import cv2
import numpy as np

# Create a black image 400x400 pixels
img = np.zeros((400, 400, 3), dtype=np.uint8)

# Define top-left and bottom-right points
pt1 = (50, 50)
pt2 = (300, 300)

# Define color (Blue in BGR)
color = (255, 0, 0)

# Draw rectangle with thickness 3
cv2.rectangle(img, pt1, pt2, color, 3)

# Show the image in a window
cv2.imshow('Rectangle', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Output

A window opens displaying a black image with a blue rectangle drawn from (50, 50) to (300, 300) with a border thickness of 3 pixels.

Common Pitfalls

• Using incorrect point coordinates can draw the rectangle outside the image or inverted.
• For color, OpenCV uses BGR, not RGB, so colors may appear swapped if misunderstood.
• Setting thickness to 0 will not draw anything; use positive integers or -1 to fill.
• Modifying a read-only image or wrong image type can cause errors.

python

import cv2
import numpy as np

img = np.zeros((200, 200, 3), dtype=np.uint8)

# Wrong: Points inverted (bottom-right first)
# cv2.rectangle(img, (150, 150), (50, 50), (0, 255, 0), 2)  # This still works but is confusing

# Correct: Top-left then bottom-right
cv2.rectangle(img, (50, 50), (150, 150), (0, 255, 0), 2)

Quick Reference

Remember these tips when drawing rectangles with OpenCV:

• Points: Always specify top-left then bottom-right corners.
• Color: Use BGR format, not RGB.
• Thickness: Positive integer for border, -1 to fill.
• Image: Must be a valid OpenCV image (numpy array).

Key Takeaways

Use cv2.rectangle() with image, two corner points, color in BGR, and thickness to draw rectangles.

Specify points as top-left and bottom-right corners to avoid confusion.

Use thickness=-1 to fill the rectangle instead of just drawing the border.

OpenCV colors use BGR order, so blue is (255, 0, 0), not (0, 0, 255).

Always ensure the image is a valid numpy array before drawing.