In message , Frank Slootweg
writes:
"J. P. Gilliver (John)" wrote:
[...]

For example, I know there are lossless, for example, _rotations_ for
JPEG,

For any decent renderer, JPEG images do not need to be rotated,
because the orientation can be set by bits in the EXIF part of the JPEG
file.

IrfanView - or possibly one of the plugins for it; I tend
to think of the combination as just IrfanView - offers lossless JPEG
crop and rotation.

IrfanView is an example of such a renderer:

Options - Properties/Settings... - JPG / PCD / GIF - JPEG-Load: -
tick 'Auto-rotate image according to EXIF info (if available)'.

That "(if available)" is the significant point. There are JPEG images
around that don't have that flag - either they predate its definition,
or the camera they were taken on did not have an orientation sensor.

I don't know if IrfanView can 'rotate' a picture by changing just the
EXIF info and not touching the rest of the file. It probably can, but I
didn't find it straight-away (I use other software to 'rotate' JPEG
pictures).

I've always _assumed_ IV's "lossless JPEG rotate" actually rearranged
the pixel data. (Could easily be established by turning _off_ the
auto-rotate setting, then trying such a rotate.)
--
J. P. Gilliver. UMRA: 1960/1985 MB++G()AL-IS-Ch++(p)Ar@T+H+Sh0!:`)DNAf

Easy reading is damned hard writing. -Nathaniel Hawthorne, writer (1804-1864)