How to Resize an Image Using PIL While Maintaining Aspect Ratio

Resizing an image is a common task in image processing, whether you’re preparing images for a website, creating thumbnails, or simply reducing the file size. The Python Imaging Library (PIL), now maintained under the name Pillow, makes it easy to resize images while maintaining their aspect ratio. Here’s a step-by-step guide on how to do this.

Why Maintain Aspect Ratio?

Aspect ratio refers to the proportional relationship between the width and height of an image. When you resize an image without maintaining its aspect ratio, it can become stretched or squished, resulting in a distorted appearance. By preserving the aspect ratio, the image retains its original proportions, ensuring a natural look.

Installing Pillow

Before you start, ensure you have the Pillow library installed. If it’s not installed, you can add it using pip:

pip install pillow

Step-by-Step Guide to Resizing

Here’s how you can resize an image while maintaining its aspect ratio:

1. Import the Required Modules

   Begin by importing the Image module from the Pillow library:

2. Open the Image File

Use the Image.open() method to load the image you want to resize:

image = Image.open(“example.jpg”)

3. Define the New Dimensions

Decide the maximum width or height for the resized image. The aspect ratio will be maintained based on these dimensions.

max_width = 800
max_height = 600

4. Calculate the New Dimensions

Use the original dimensions to calculate the new width and height while maintaining the aspect ratio:

original_width, original_height = image.size

if original_width / original_height > max_width / max_height:
new_width = max_width
new_height = int(max_width * original_height / original_width)
else:
new_height = max_height
new_width = int(max_height * original_width / original_height)

5. Resize the Image

Use the Image.resize() method to resize the image to the calculated dimensions. Set the Image.ANTIALIAS filter for smoother results:

= image.resized_imagee((new_width, new_height), Image.ANTIALIAS)

6. Save the Resized Image

Save the resized image to a file using the save() method:

resized_image.save(“resized_example.jpg”)

Complete Code Example

Here’s the complete code wrapped together:

from PIL import Image

def resize_image(input_path, output_path, max_width, max_height):
with Image.open(input_path) as image:
original_width, original_height = image.size

if original_width / original_height > max_width / max_height:
new_width = max_width
new_height = int(max_width * original_height / original_width)
else:
new_height = max_height
new_width = int(max_height * original_width / original_height)

resized_image = image.resize((new_width, new_height), Image.ANTIALIAS)
resized_image.save(output_path)

# Example usage
resize_image(“example.jpg”, “resized_example.jpg”, 800, 600)

Tips and Best Practices

• Choose Appropriate Dimensions: Pick dimensions that suit your application, such as common screen sizes or thumbnail dimensions.
• Use Lossless Formats: If possible, save resized images in lossless formats like PNG to preserve quality.
• Optimize for Web: Compress resized images further using tools like TinyPNG or libraries like imageio.

With these steps, you can easily resize images while maintaining their aspect ratio using Python’s Pillow library. This ensures your images look great and retain their original proportions, no matter the size.