Draw a text string in OpenCV

Drawing text on an image can be useful in many scenarios like watermarking or labeling. OpenCV provides the putText() function to overlay text on images. Here's a tutorial on how to draw a text string using OpenCV:

Steps to Draw Text on an Image using OpenCV:

1. Setup:

Ensure you have OpenCV installed:

pip install opencv-python

Import necessary libraries:

import cv2
import numpy as np

2. Load or Create an Image:

You can either create a blank image or load an existing one:

# Create a blank image (white background)
image = np.ones((500, 500, 3), np.uint8) * 255

# Or, load an existing image:
# image = cv2.imread('path_to_image.jpg')

3. Add Text:

Use the cv2.putText() function:

cv2.putText(img, text, org, fontFace, fontScale, color[, thickness[, lineType[, bottomLeftOrigin]]])

Where:

img: The image on which you want to draw the text.
text: The text string to be drawn.
org: Bottom-left corner of the text string in the image (origin of the coordinates).
fontFace: Font type (e.g., cv2.FONT_HERSHEY_SIMPLEX).
fontScale: Font scale factor.
color: Text color in (B, G, R) format.
thickness (optional): Thickness of the lines used to draw text.
lineType (optional): Line type (default is cv2.LINE_AA for anti-aliased).
bottomLeftOrigin (optional): If true, the image data origin is at the bottom-left corner. Otherwise, it's at the top-left corner.

Example:

text = "Hello, OpenCV!"
org = (50, 250)  # x, y coordinates of the bottom-left corner of the text
fontFace = cv2.FONT_HERSHEY_SIMPLEX
fontScale = 1
color = (0, 0, 255)  # Red color in BGR format
thickness = 2

cv2.putText(image, text, org, fontFace, fontScale, color, thickness, cv2.LINE_AA)

4. Display the Image with Text:

cv2.imshow('Text', image)
cv2.waitKey(0)
cv2.destroyAllWindows()

Complete Code:

Here's a combined script for the entire process:

import cv2
import numpy as np

# Create a blank image
image = np.ones((500, 500, 3), np.uint8) * 255

# Add text to the image
text = "Hello, OpenCV!"
org = (50, 250)
fontFace = cv2.FONT_HERSHEY_SIMPLEX
fontScale = 1
color = (0, 0, 255)  # Red in BGR
thickness = 2

cv2.putText(image, text, org, fontFace, fontScale, color, thickness, cv2.LINE_AA)

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

This will display a white image with the text "Hello, OpenCV!" written in red. Adjust the text, coordinates, font, scale, color, and thickness as needed.

Python code for drawing text in OpenCV:

import cv2
import numpy as np

# Create a black image
img = np.zeros((300, 400, 3), dtype=np.uint8)

# Draw text on the image (position, text, font, size, color, thickness)
cv2.putText(img, 'Hello OpenCV!', (50, 150), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 255, 0), 2)

# Display the image with the drawn text
cv2.imshow('Text Drawing', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Text drawing functions and parameters in OpenCV:
```
# OpenCV putText function signature
cv2.putText(img, text, org, fontFace, fontScale, color, thickness, lineType)
```
- text: Text string to be drawn.
- org: Bottom-left corner of the text string.
- fontFace: Font type (e.g., cv2.FONT_HERSHEY_SIMPLEX).
- fontScale: Font scale factor.
- color: Text color.
- thickness: Thickness of the text.
- lineType: Type of line for text (e.g., cv2.LINE_AA for antialiased).

Drawing text with specified font, size, and color in OpenCV:

import cv2
import numpy as np

# Create a black image
img = np.zeros((300, 400, 3), dtype=np.uint8)

# Draw text with specified font, size, and color
cv2.putText(img, 'OpenCV Text', (50, 150), cv2.FONT_HERSHEY_COMPLEX, 1.5, (255, 0, 0), 2)

# Display the image with the drawn text
cv2.imshow('Styled Text Drawing', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Real-time text drawing in OpenCV:

import cv2
import numpy as np

# Create a black image
img = np.zeros((300, 400, 3), dtype=np.uint8)

# Callback function for keyboard events
def draw_text(event, x, y, flags, param):
    if event == cv2.EVENT_LBUTTONDOWN:
        text = 'Coordinates: ({}, {})'.format(x, y)
        cv2.putText(img, text, (50, 150), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 255, 0), 2)
        cv2.imshow('Real-time Text Drawing', img)

# Create a named window
cv2.namedWindow('Real-time Text Drawing')

# Set the callback function for mouse events
cv2.setMouseCallback('Real-time Text Drawing', draw_text)

while True:
    # Break the loop on 'Esc' key press
    if cv2.waitKey(1) & 0xFF == 27:
        break

cv2.destroyAllWindows()

Drawing text on images and videos in OpenCV:

import cv2

# Load an image
img = cv2.imread('image.jpg')

# Draw text on the image
cv2.putText(img, 'Hello OpenCV!', (50, 150), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 255, 0), 2)

# Display the image with the drawn text
cv2.imshow('Text on Image', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

For videos, use the same cv2.putText() function within a video processing loop.

Combining text drawing with other OpenCV functions:

import cv2
import numpy as np

# Load an image
img = cv2.imread('image.jpg')

# Draw text and a rectangle on the image
cv2.putText(img, 'OpenCV Text', (50, 150), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 255, 0), 2)
cv2.rectangle(img, (100, 100), (300, 200), (0, 0, 255), 2)

# Display the image with the drawn text and rectangle
cv2.imshow('Text and Rectangle Drawing', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Text annotations for image labeling in OpenCV:

import cv2
import numpy as np

# Load an image
img = cv2.imread('object_detection_image.jpg')

# Annotate object with text
cv2.putText(img, 'Object Detected', (100, 50), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 255, 0), 2)

# Display the image with the text annotation
cv2.imshow('Object Labeling with Text', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Drawing multiline text and text with line breaks in OpenCV:

import cv2
import numpy as np

# Create a black image
img = np.zeros((300, 400, 3), dtype=np.uint8)

# Draw multiline text with line breaks
text = 'Multiline Text\nLine 2\nLine 3'
cv2.putText(img, text, (50, 150), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 255, 0), 2)

# Display the image with the drawn text
cv2.imshow('Multiline Text Drawing', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Rotating and aligning text in OpenCV:

import cv2
import numpy as np

# Create a black image
img = np.zeros((300, 400, 3), dtype=np.uint8)

# Draw rotated text
text = 'Rotated Text'
center = (200, 150)
angle = 30
scale = 1
cv2.putText(img, text, center, cv2.FONT_HERSHEY_SIMPLEX, scale, (0, 255, 0), 2, cv2.LINE_AA, False)
rotated_img = cv2.warpAffine(img, cv2.getRotationMatrix2D(center, angle, scale), (img.shape[1], img.shape[0]))

# Display the image with the drawn rotated text
cv2.imshow('Rotated Text Drawing', rotated_img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Adding transparency to drawn text in OpenCV:

import cv2
import numpy as np

# Create a transparent image
img = np.zeros((300, 400, 4), dtype=np.uint8)
img[:, :, 3] = 255  # Set alpha channel to 255 (fully opaque)

# Draw transparent text
cv2.putText(img, 'Transparent Text', (50, 150), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 255, 0, 128), 2)

# Display the image with the drawn transparent text
cv2.imshow('Transparent Text Drawing', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Code examples for drawing text with different styles in OpenCV:

import cv2
import numpy as np

# Create a black image
img = np.zeros((300, 400, 3), dtype=np.uint8)

# Draw regular, italic, and bold text
cv2.putText(img, 'Regular', (50, 50), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 255, 0), 2)
cv2.putText(img, 'Italic', (50, 150), cv2.FONT_HERSHEY_SIMPLEX | cv2.FONT_ITALIC, 1, (0, 255, 0), 2)
cv2.putText(img, 'Bold', (50, 250), cv2.FONT_HERSHEY_SIMPLEX, 1, (0, 255, 0), 2, cv2.LINE_AA, False)

# Display the image with the drawn text
cv2.imshow('Text with Different Styles', img)
cv2.waitKey(0)
cv2.destroyAllWindows()