How to Draw on Image Using OpenCV in Computer Vision

To draw on an image in OpenCV, use functions like cv2.line(), cv2.rectangle(), cv2.circle(), and cv2.putText(). These functions let you add lines, shapes, and text directly onto the image matrix for visualization or annotation.

Syntax

OpenCV provides several drawing functions to add shapes and text on images. Each function requires the image, coordinates, color, thickness, and other optional parameters.

• cv2.line(img, pt1, pt2, color, thickness): Draws a line from pt1 to pt2.
• cv2.rectangle(img, pt1, pt2, color, thickness): Draws a rectangle with opposite corners pt1 and pt2.
• cv2.circle(img, center, radius, color, thickness): Draws a circle at center with given radius.
• cv2.putText(img, text, org, font, fontScale, color, thickness): Writes text starting at org point.

python

cv2.line(img, (x1, y1), (x2, y2), (B, G, R), thickness)
cv2.rectangle(img, (x1, y1), (x2, y2), (B, G, R), thickness)
cv2.circle(img, (x, y), radius, (B, G, R), thickness)
cv2.putText(img, 'text', (x, y), cv2.FONT_HERSHEY_SIMPLEX, fontScale, (B, G, R), thickness)

Example

This example loads a blank image and draws a blue line, a green rectangle, a red circle, and white text on it.

python

import cv2
import numpy as np

# Create a black image
img = np.zeros((400, 600, 3), dtype=np.uint8)

# Draw a blue line from top-left to bottom-right
cv2.line(img, (0, 0), (599, 399), (255, 0, 0), 5)

# Draw a green rectangle
cv2.rectangle(img, (50, 50), (200, 150), (0, 255, 0), 3)

# Draw a red circle
cv2.circle(img, (300, 200), 75, (0, 0, 255), -1)  # filled circle

# Put white text
cv2.putText(img, 'OpenCV Drawing', (50, 350), cv2.FONT_HERSHEY_SIMPLEX, 1, (255, 255, 255), 2)

# Show the image
cv2.imshow('Drawings', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Output

A window opens showing a black image with a blue diagonal line, green rectangle, filled red circle, and white text 'OpenCV Drawing'.

Common Pitfalls

• Using incorrect color format: OpenCV uses BGR, not RGB, so red is (0,0,255), not (255,0,0).
• For thickness, -1 fills the shape; positive values draw outlines.
• Coordinates must be integers within image bounds; otherwise, drawing may fail or cause errors.
• For cv2.putText, fontScale controls text size and must be positive.
• Remember to call cv2.imshow() and cv2.waitKey() to see the drawing.

python

import cv2
import numpy as np

img = np.zeros((100, 200, 3), dtype=np.uint8)

# Wrong color (RGB instead of BGR) - text will appear blue instead of red
cv2.putText(img, 'Wrong Color', (5, 50), cv2.FONT_HERSHEY_SIMPLEX, 1, (255, 0, 0), 2)

# Correct color (BGR) - text appears red
cv2.putText(img, 'Correct Color', (5, 90), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 0, 255), 2)

cv2.imshow('Color Pitfall', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Output

A window shows two texts: the first in blue (wrong color), the second in red (correct color).

Quick Reference

Remember these key points when drawing on images with OpenCV:

• Colors use BGR format, not RGB.
• Thickness -1 fills shapes; positive values draw outlines.
• Coordinates are (x, y) pixels, integers within image size.
• Use cv2.FONT_HERSHEY_SIMPLEX or other fonts for text.
• Always display image with cv2.imshow() and wait for key press.

Key Takeaways

Use OpenCV drawing functions like cv2.line, cv2.rectangle, cv2.circle, and cv2.putText to add shapes and text on images.

Colors in OpenCV are in BGR order, so specify colors accordingly.

Thickness -1 fills shapes; positive thickness draws outlines.

Coordinates must be integers within the image dimensions.

Always use cv2.imshow and cv2.waitKey to display the image with drawings.