Question

This actually works:

def tileshift(original, size, iterations):
    im = Image.open(original)
    format = im.format

    x, y = [float(v) for v in im.size]
    xr, yr = [float(v) for v in size]
    r = max(xr / x, yr / y)
    im = im.resize((int(round(x * r)), int(round(y * r))),
               resample=Image.ANTIALIAS)

    corners = _corners(im.size)
    lu = im.crop(corners[0])
    ru = im.crop(corners[1])
    ll = im.crop(corners[2])
    rl = im.crop(corners[3])

    # debugging each tile
    lu.save('tileshifted/lu.jpg')
    ru.save('tileshifted/ru.jpg')
    ll.save('tileshifted/ll.jpg')
    rl.save('tileshifted/rl.jpg')

    im.paste(lu, corners[1])
    im.paste(ru, corners[3])
    im.paste(ll, corners[0])
    im.paste(rl, corners[2])
    return (im, format)

def _corners(size):
    w, h = size
    return (
      (0, 0, w / 2, h / 2),
      (w / 2, 0, w, h / 2),
      (0, h / 2, w / 2, h),
      (w / 2, h / 2, w, h)
     )

Now, this actually works. I save the outputted picture and look at it. What the produced picture is is a picture with each quadrant is moved one tile clockwise.

To make my question clearer, I’ve uploaded all the pictures here:
http://www.peterbe.com/stackoverflowquestion/index.html

But! when I stopped the debugging of each corner. Ie. I comment out all the intermediate saves so it’s not like this:

    ...
    # debugging each tile
    #lu.save('tileshifted/lu.jpg')
    #ru.save('tileshifted/ru.jpg')
    #ll.save('tileshifted/ll.jpg')
    #rl.save('tileshifted/rl.jpg')
    ...

Now it stops working! And you get a composite picture where it appears that the upper left-hand corner tile has been repeated three times.

Obviously I don’t need the debugging but obviously those calls to instance.save() does something important.

UPDATE

Seems I might have found a solution. If run lu.load() right after the lu instance has been created, then it works!
Got inspiration from this one: https://stackoverflow.com/a/3838495/205832

Answer 1

Per the documentation for crop: “This is a lazy operation. Changes to the source image may or may not be reflected in the cropped image. To get a separate copy, call the load method on the cropped copy.”

So each of the corner crops, without the save, is a reference to a portion of the original image, rather than a copy, so you paste LU to RU, then try to paste from RU to RL but get LU again, and so on. The save apparently forces PIL to make a copy out of the crops.

(Also, you didn’t actually ask a question. Your question was implicit, but if you actually go all the way to formulating a clear and concise question, then as often as not, you’ll realize the answer, or realize you need to provide more information which may lead you to the answer.)