Adding a method for cropping based on fractional coords instead of pixel coords

[ancri]

Greetings! I'd find it very useful to have an Image method similar to crop but which takes fractions rather than pixel coordinates, so as to play more nicely (and predictably!) with images of varying sizes, but a known aspect ratio. The proposed method would just convert the fractions to pixels and then call crop under the hood.

Would anyone else find this useful? Is this something the devs would be open to adding?

Example:
if img.size == (1920, 1080) then
img.cropf((0.1, 0.65, 0.2, 0.75)) == img.crop((192, 702, 384, 810)).

(where cropf is the name of the new proposed method)

[radarhere]

I feel like this function would be an odd exception to the general co-ordinate system of Pillow.

If it made it in, then why stop with crop()? Users might also want to resize() to fractions (although admittedly, reduce() does partly fill this role), or thumbnail() to fractions. My opinion is that it is better to stick to pixels.

[ancri]

Good point, it would indeed seem like a bizarre exception to have this only apply to cropping but nothing else (though, consistency aside, in my experience whenever I'm resizing I generally have a specific absolute size I'm targeting, whereas when I'm cropping I'm usually thinking in relative terms e.g. "this part of the image" irrespective of its pixel size... but still I agree it would feel bizarre).

What if this were implemented as a new argument that can be passed to all of the normally pixel-based methods (crop, resize, and so on), let's call it fractional_coordinates, defaulting to False, but if passed as True would interpret the coordinates as fractional?

[radarhere]

I don't personally see the need for this to be bundled into Pillow. It's a piece of functionality that should be straightforward for users to implement themselves, but that would complicate many methods if implemented properly within this library. Pillow has a co-ordinate system, and I'm reluctant to complicate it.

[ancri]

That's fair thing to be reluctant about. It's very unfortunate though, as this means that I have to continue to include that piece of functionality as an additional section of code separately in every application.