How Can I Resize Images with PIL While Maintaining Aspect Ratio?

Resizing Images with PIL while Preserving Aspect Ratio

The Python Imaging Library (PIL) offers a comprehensive set of image manipulation functions. One useful operation is resizing images, but many developers encounter challenges in maintaining the original aspect ratio. This article delves into this issue and provides solutions to ensure proportional image resizing.

Issue

While attempting to create thumbnails, a user stumbled upon the following challenge:

Is there an obvious way to do this that I'm missing? I'm just trying to make thumbnails.

Solution

To resize an image while preserving its aspect ratio, consider the following steps:

1. Determine Maximum Dimensions: Establish a maximum width and height for your thumbnail. These values define the desired output size.
2. Calculate Resize Ratio: Compute the resize ratio by dividing the maximum width by the original width and the maximum height by the original height. The smaller of these ratios will ensure proportional resizing.
3. Compute New Size: Determine the new size of the image by multiplying the original size by the calculated resize ratio.
4. PIL Library Method: PIL provides a simplified approach through the Image.thumbnail method. This method directly modifies the image, so it's recommended to make a copy if necessary:

import os, sys
from PIL import Image

size = 128, 128  # Define the maximum size of the thumbnail

for infile in sys.argv[1:]:
    outfile = os.path.splitext(infile)[0] + ".thumbnail"  # Generate the output filename
    if infile != outfile:
        try:
            im = Image.open(infile)
            im_copy = im.copy()  # Create a copy to avoid modifying the original image
            im_copy.thumbnail(size, Image.Resampling.LANCZOS)  # Resize the image
            im_copy.save(outfile, "JPEG")  # Save the resized image
        except IOError:
            print(f"cannot create thumbnail for '{infile}'")  # Handle any exceptions

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