Draw an ellipse in OpenCV

Drawing an ellipse in OpenCV is straightforward using the ellipse() function. Let's go through a step-by-step tutorial on how to do this.

Steps to Draw an Ellipse using OpenCV:

1. Setup:

First, ensure you have OpenCV installed:

pip install opencv-python

Next, import necessary libraries:

import cv2
import numpy as np

2. Load or Create an Image:

For 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 an Ellipse:

You can use cv2.ellipse() to draw an ellipse:

cv2.ellipse(img, center, axes, angle, startAngle, endAngle, color[, thickness[, lineType[, shift]]])

Where:

img: The image on which the ellipse will be drawn.
center: Center of the ellipse.
axes: Length of the major and minor axes.
angle: Ellipse rotation angle in degrees.
startAngle: Starting angle of the elliptic arc (also in degrees).
endAngle: Ending angle of the elliptic arc.
color: Ellipse color.
thickness (optional): Thickness of the ellipse boundary. If this is set to a negative value (e.g., -1), it will fill the ellipse.
lineType (optional): Type of the ellipse boundary (like anti-aliased, etc.).
shift (optional): Number of fractional bits in the coordinates of the center and the axes values.

Here's an example:

center = (250, 250)
axes = (150, 100)  # major axis = 150, minor axis = 100
angle = 45  # Ellipse rotation angle
start_angle = 0
end_angle = 360  # For a complete ellipse
color = (0, 0, 255)  # Red color in BGR format
thickness = 2  # Set this to -1 if you want to fill the ellipse

cv2.ellipse(image, center, axes, angle, start_angle, end_angle, color, thickness)

4. Display the Image:

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

When you run the code, you'll see a window displaying an image with a red ellipse. You can adjust the center, axes, angle, color, and thickness as desired.

That concludes our tutorial! The ellipse() function in OpenCV is powerful and versatile, allowing for various ellipse styles and orientations by adjusting its parameters.

Python code for drawing ellipses in OpenCV:

import cv2
import numpy as np

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

# Draw an ellipse (center, axes, angle)
cv2.ellipse(img, (200, 150), (50, 30), 30, 0, 360, (0, 255, 0), 2)

# Display the image
cv2.imshow('Ellipse Drawing', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Drawing filled and unfilled ellipses in OpenCV:

import cv2
import numpy as np

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

# Draw a filled ellipse
cv2.ellipse(img, (200, 150), (50, 30), 30, 0, 360, (0, 255, 0), -1)

# Draw an unfilled ellipse
cv2.ellipse(img, (300, 150), (50, 30), 45, 0, 360, (0, 0, 255), 2)

# Display the image
cv2.imshow('Filled and Unfilled Ellipses', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Customizing ellipse appearance in OpenCV:

import cv2
import numpy as np

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

# Draw an ellipse with customized appearance
cv2.ellipse(img, (200, 150), (50, 30), 45, 0, 360, (255, 0, 0), 2, cv2.LINE_AA)

# Display the image
cv2.imshow('Customized Ellipse', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Ellipse drawing functions and parameters in OpenCV:
```
# OpenCV ellipse function signature
cv2.ellipse(img, center, axes, angle, startAngle, endAngle, color, thickness, lineType)
```
- center: Center coordinates of the ellipse.
- axes: Tuple containing major and minor axis lengths.
- angle: Rotation angle of the ellipse.
- startAngle: Starting angle of the elliptical arc.
- endAngle: Ending angle of the elliptical arc.
- color: Color of the ellipse.
- thickness: Thickness of the ellipse boundary (-1 for filled).
- lineType: Type of line used for drawing.

Drawing ellipses 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 an ellipse with specified color and thickness
cv2.ellipse(img, (200, 150), (50, 30), 45, 0, 360, (0, 0, 255), 3)

# Display the image
cv2.imshow('Ellipse with Color and Thickness', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Real-time ellipse drawing in OpenCV:

import cv2
import numpy as np

# Callback function for mouse events
def draw_ellipse(event, x, y, flags, param):
    if event == cv2.EVENT_LBUTTONDOWN:
        cv2.ellipse(img, (x, y), (30, 20), 0, 0, 360, (0, 255, 0), 2)
        cv2.imshow('Real-time Ellipse Drawing', img)

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

# Set the callback function for mouse events
cv2.namedWindow('Real-time Ellipse Drawing')
cv2.setMouseCallback('Real-time Ellipse Drawing', draw_ellipse)

while True:
    cv2.imshow('Real-time Ellipse Drawing', img)
    if cv2.waitKey(1) & 0xFF == 27:  # Press 'Esc' to exit
        break

cv2.destroyAllWindows()

Drawing ellipses on feature points in OpenCV:

import cv2
import numpy as np

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

# Detect features (example: using Harris corner detection)
gray = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
corners = cv2.cornerHarris(gray, 2, 3, 0.04)

# Draw ellipses on detected feature points
corners = np.int0(corners)
for i in corners:
    x, y = i.ravel()
    cv2.ellipse(img, (x, y), (10, 10), 0, 0, 360, (0, 255, 0), 2)

# Display the image with ellipses on feature points
cv2.imshow('Ellipses on Feature Points', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Drawing ellipses with specified center, axes, and angle in OpenCV:

import cv2
import numpy as np

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

# Draw an ellipse with specified center, axes, and angle
center = (200, 150)
axes = (50, 30)
angle = 45
cv2.ellipse(img, center, axes, angle, 0, 360, (0, 255, 0), 2)

# Display the image
cv2.imshow('Ellipse with Specified Parameters', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Drawing concentric ellipses in OpenCV:

import cv2
import numpy as np

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

# Draw concentric ellipses
for i in range(5):
    cv2.ellipse(img, (200, 150), (i * 20, i * 10), 0, 0, 360, (0, 255, 0), 2)

# Display the image
cv2.imshow('Concentric Ellipses', img)
cv2.waitKey(0)
cv2.destroyAllWindows()

Adding transparency to drawn ellipses 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 ellipse
cv2.ellipse(img, (200, 150), (50, 30), 45, 0, 360, (0, 255, 0), 2)

# Display the image with transparency
cv2.imshow('Transparent Ellipse', img)
cv2.waitKey(0)
cv2.destroyAllWindows()