Draw a line in OpenCV

Drawing a line on an image using OpenCV is a straightforward process. In this tutorial, I'll guide you through the steps to draw a line on an image using the line() function provided by OpenCV.

Steps to Draw a Line 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:

For the purpose of this tutorial, let's create a blank image. You can also load an existing image if you prefer:

# 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. Draw a Line:

Use the cv2.line() function:

cv2.line(img, pt1, pt2, color[, thickness[, lineType[, shift]]])

Where:

img: The image on which you want to draw the line.
pt1: Starting point of the line.
pt2: Ending point of the line.
color: Color of the line in (B, G, R) format.
thickness (optional): Thickness of the line (default is 1).
lineType (optional): Type of the line, e.g., 8-connected, anti-aliased line etc.
shift (optional): Number of fractional bits in the point coordinates.

Here's an example:

start_point = (100, 100)
end_point = (400, 400)
color = (0, 0, 255)  # Red color in BGR format
thickness = 2

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

4. Display the Image with the Line:

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

When you run the code, you'll see an image with a red line drawn from the point (100, 100) to the point (400, 400).

Complete Code:

Combining all the steps, here's the complete script to draw a line:

import cv2
import numpy as np

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

# Specify the start and end points, color, and thickness of the line
start_point = (100, 100)
end_point = (400, 400)
color = (0, 0, 255)  # Red in BGR
thickness = 2

# Draw the line on the image
cv2.line(image, start_point, end_point, color, thickness)

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

This will show a white image with a diagonal red line. Adjust the start and end points, color, and thickness as needed to draw different lines.

Python code for drawing lines in OpenCV:

import cv2
import numpy as np

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

# Draw a line (start_point, end_point, color, thickness)
cv2.line(img, (50, 50), (350, 250), (0, 255, 0), 2)

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

Line drawing functions and parameters in OpenCV:
```
# OpenCV line function signature
cv2.line(img, pt1, pt2, color, thickness, lineType)
```
- pt1: Starting point of the line.
- pt2: Ending point of the line.
- color: Color of the line.
- thickness: Thickness of the line.
- lineType: Type of line (e.g., cv2.LINE_8, cv2.LINE_AA).

Drawing lines with specified color and thickness in OpenCV:

import cv2
import numpy as np

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

# Draw a blue line with thickness 3
cv2.line(img, (50, 50), (350, 250), (255, 0, 0), 3)

# Display the image with the drawn line
cv2.imshow('Colored Line Drawing', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Real-time line 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 mouse events
def draw_line(event, x, y, flags, param):
    if event == cv2.EVENT_LBUTTONDOWN:
        cv2.line(img, (x, y), (x + 50, y + 50), (0, 255, 0), 2)
        cv2.imshow('Real-time Line Drawing', img)

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

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

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

cv2.destroyAllWindows()

Drawing lines on images and videos in OpenCV:

import cv2

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

# Draw a line on the image
cv2.line(img, (50, 50), (350, 250), (0, 255, 0), 2)

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

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

Combining line drawing with other OpenCV functions:

import cv2
import numpy as np

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

# Draw a line and a rectangle on the image
cv2.line(img, (50, 50), (350, 250), (0, 255, 0), 2)
cv2.rectangle(img, (100, 100), (300, 200), (0, 0, 255), 2)

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

Line annotations for object detection in OpenCV:

import cv2
import numpy as np

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

# Annotate object with a line
cv2.line(img, (100, 50), (250, 150), (0, 255, 0), 2)

# Display the image with the line annotation
cv2.imshow('Object Detection with Line Annotation', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Drawing lines with specified endpoints in OpenCV:

import cv2
import numpy as np

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

# Draw a line with specified endpoints
pt1 = (50, 50)
pt2 = (350, 250)
cv2.line(img, pt1, pt2, (0, 255, 0), 2)

# Display the image with the drawn line
cv2.imshow('Line with Specified Endpoints', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Drawing dashed and dotted lines in OpenCV:

import cv2
import numpy as np

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

# Draw a dashed line
cv2.line(img, (50, 50), (350, 250), (0, 255, 0), 2, lineType=cv2.LINE_AA)

# Draw a dotted line
cv2.line(img, (50, 150), (350, 350), (0, 0, 255), 2, lineType=cv2.LINE_AA)

# Display the image with the drawn lines
cv2.imshow('Dashed and Dotted Lines', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Adding transparency to drawn lines 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 a transparent line
cv2.line(img, (50, 50), (350, 250), (0, 255, 0, 128), 2)

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

Code examples for drawing lines 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 a solid line
cv2.line(img, (50, 50), (350, 250), (0, 255, 0), 2)

# Draw a dashed line
cv2.line(img, (50, 150), (350, 350), (0, 0, 255), 2, lineType=cv2.LINE_AA)

# Draw a dotted line
cv2.line(img, (50, 250), (350, 450), (255, 0, 0), 2, lineType=cv2.LINE_AA)

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