Question

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

Answer 1

Define a maximum size. Then, compute a resize ratio by taking min(maxwidth/width, maxheight/height).

The proper size is oldsize*ratio.

There is of course also a library method to do this: the method Image.thumbnail.
Below is an (edited) example from the PIL documentation.

import os, sys import Image  size = 128, 128  for infile in sys.argv[1:]:     outfile = os.path.splitext(infile)[0] + ".thumbnail"     if infile != outfile:         try:             im = Image.open(infile)             im.thumbnail(size, Image.Resampling.LANCZOS)             im.save(outfile, "JPEG")         except IOError:             print "cannot create thumbnail for '%s'" % infile