Predicting Thumbnail Sizes from PIL

[Fredrik Lundh]

Roger wrote:

> I need to calculate the thumbnail sizes PILL will produce from an image.
> In most cases I can divide and round by adding .5, but PIL seems to
> round differently on odd sized images.
>
> For example, I want to reduce an 800x816 image to have a maximum size of
> 697.  (697/816) * 800 = 683.33, so my rounding results in 683.  But PIL
> produces an image of 684x697.

the algorithm is:

    x, y = im.size
    if x > size[0]: y = max(y * size[0] / x, 1); x = size[0]
    if y > size[1]: x = max(x * size[1] / y, 1); y = size[1]
    size = x, y

that is, it shrinks the image horizontally first, and it then shrinks the
resulting image vertically:

    >>> 816 * 697 / 800
    710
    >>> 697 * 697 / 710
    684

</F>