Rotate an image

The detail most rotate tools get wrong is the canvas size. A quarter turn just swaps width and height, but at 45° a square needs a canvas about 1.41 times as wide or the corners are cut off — and because the clipping happens at the edges, it often is not noticed until later. The canvas here is sized from the rotated bounding box, so the whole image survives at any angle.

The cost is that an arbitrary angle resamples. Rotating by 90, 180 or 270 is a pure pixel remapping — every pixel lands exactly on another pixel, and the result is bit-for-bit as sharp as the original. Any other angle has to interpolate, so fine detail softens slightly and hard edges pick up a faint stair-step. Rotating back and forth repeatedly compounds it, so make the correction once from the original rather than nudging it several times.

A related thing worth knowing: a photo that looks rotated on one device and upright on another is usually not rotated at all. Cameras store the orientation as an EXIF tag rather than rotating the pixels, and software that ignores the tag shows it sideways. Drawing to a canvas bakes the current orientation in and drops the tag, which is why saving from here makes the rotation stick everywhere.