In Python, Python Image Library 1.1.6, how can I expand the canvas without resizing?

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I am probably looking for the wrong thing in the handbook, but I am looking to take an image object and expand it without resizing (stretching/squishing) the original image.

Toy example: imagine a blue rectangle, 200 x 100, then I perform some operation and I have a new image object, 400 x 300, consisting of a white background upon which a 200 x 100 blue rectangle rests. Bonus if I can control in which direction this expands, or the new background color, etc.

Essentially, I have an image to which I will be adding iteratively, and I do not know what size it will be at the outset.

I suppose it would be possible for me to grab the original object, make a new, slightly larger object, paste the original on there, draw a little more, then repeat. It seems like it might be computationally expensive. However, I thought there would be a function for this, as I assume it is a common operation. Perhaps I assumed wrong.

python-imaging-library python
4 Answers

This code will enlarge a smaller image, preserving aspect ratio, then center it on a standard sized canvas. Also preserves transparency, or defaults to gray background.

Tested with P mode PNG files.

Coded debug final.show() and break for testing. Remove lines and hashtag on final.save(...) to loop and save.

Could parameterize canvas ratio and improve flexibility, but it served my purpose.

"""
Resize ... and reconfigures. images in a specified directory

Use case:  Images of varying size, need to be enlarged to exaxtly 1200 x 1200
"""
import os
import glob

from PIL import Image

# Source directory plus Glob file reference (Windows)
source_path = os.path.join('C:', os.sep, 'path', 'to', 'source', '*.png')

# List of UNC Image File paths
images = glob.glob(source_path)

# Destination directory of modified image (Windows)
destination_path = os.path.join('C:', os.sep, 'path', 'to', 'destination')

for image in images:
    
    original = Image.open(image)

    # Retain original attributes (ancillary chunks)
    info = original.info
    
    # Retain original mode
    mode = original.mode

    # Retain original palette
    if original.palette is not None:
        palette = original.palette.getdata()[1]
    else:
        palette = False

    # Match original aspect ratio
    dimensions = original.getbbox()

    # Identify destination image background color
    if 'transparency' in info.keys():
        background = original.info['transparency']
    else:
        # Image does not have transparency set
        print(image)
        background = (64)

    # Get base filename and extension for destination
    filename, extension = os.path.basename(image).split('.')
    
    # Calculate matched aspect ratio
    if dimensions[2] > dimensions[3]:
        width = int(1200)
        modifier = width / dimensions[2]
        length = int(dimensions[3] * modifier)
    elif dimensions[3] > dimensions[2]:
        length = int(1200)
        modifier = length / dimensions[3]
        width = int(dimensions[2] * modifier)
    else:
        width, length = (1200, 1200)
    
    size = (width, length)

    # Set desired final image size
    canvas = (1200, 1200)
    
    # Calculate center position
    position = (
        int((1200 - width)/2),
        int((1200 - length)/2),
        int((1200 - width)/2) + width,
        int((1200 - length)/2) + length
    )

    # Enlarge original image proportionally
    resized = original.resize(size, Image.LANCZOS)

    # Then create sized canvas
    final = Image.new(mode, canvas, background)

    # Replicate original properties
    final.info = info

    # Replicate original palatte
    if palette:
        final.putpalette(palette)

     # Cemter paste resized image to final canvas
    final.paste(resized, position)

    # Save final image to destination directory
    final.show()

    #final.save("{}\\{}.{}".format(destination_path, filename, extension))

    break
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