Turning and mirroring
Rotating by ninety degrees or mirroring an image rearranges whole pixels rather than blending them, so the operation itself loses nothing — every pixel of the original survives in a new position. The file is re-encoded afterwards, which for a lossy format is where a small amount of quality goes.
A photograph that appears rotated in one program and upright in another usually has an orientation tag rather than rotated pixels. Rotating here writes the change into the pixels themselves, so every program agrees from then on.
- Quarter turns only
- Ninety, a hundred and eighty and two hundred and seventy. An arbitrary angle would leave empty corners and require resampling the whole image.
- Mirroring is not rotation
- A flip produces the mirror image — text in the picture reads backwards, which no amount of rotation would do.