Draw a filled polygon using the OpenCV function fillPoly()

In this tutorial, we will learn how to draw a filled polygon using OpenCV's fillPoly() function. This function fills a polygon defined by a set of vertices with a specified color.

Syntax

cv2.fillPoly(image, pts, color)

Parameters

The fillPoly() function accepts the following parameters ?

• image ? The input image on which to draw the polygon
• pts ? Array of polygon vertices (points)
• color ? Fill color of the polygon in BGR format

Algorithm

Step 1: Import cv2 and numpy
Step 2: Define the polygon vertices (endpoints)
Step 3: Create a blank image using zeros
Step 4: Draw the filled polygon using fillPoly()
Step 5: Display the result

Example

Let's create a filled rectangle using fillPoly() ?

import cv2
import numpy as np

# Define polygon vertices (rectangle)
vertices = np.array([[50,50], [50,150], [150,150], [150,50]])

# Create blank image (height=200, width=200, 3-channel for color)
image = np.zeros((200, 200, 3), dtype=np.uint8)

# Fill the polygon with white color
cv2.fillPoly(image, pts=[vertices], color=(255, 255, 255))

# Display the image
cv2.imshow("Filled Polygon", image)
cv2.waitKey(0)
cv2.destroyAllWindows()

Creating Complex Polygons

You can create more complex shapes by defining different vertices ?

import cv2
import numpy as np

# Create a blank image
image = np.zeros((300, 300, 3), dtype=np.uint8)

# Define triangle vertices
triangle = np.array([[150, 50], [100, 150], [200, 150]])

# Define pentagon vertices
pentagon = np.array([[150, 180], [120, 200], [130, 240], [170, 240], [180, 200]])

# Fill triangle with red color
cv2.fillPoly(image, [triangle], color=(0, 0, 255))

# Fill pentagon with green color
cv2.fillPoly(image, [pentagon], color=(0, 255, 0))

cv2.imshow("Multiple Polygons", image)
cv2.waitKey(0)
cv2.destroyAllWindows()

Key Points

• Vertices must be defined as NumPy arrays
• Use 3-channel images (height, width, 3) for colored polygons
• Colors are specified in BGR format (Blue, Green, Red)
• Multiple polygons can be drawn by passing a list of vertex arrays

Conclusion

The fillPoly() function is useful for creating filled geometric shapes in computer vision applications. Define vertices as NumPy arrays and specify BGR colors to create various polygon shapes.