Last modified: 2014-11-19 12:54:07 UTC
EXIF allows 8 possible values for the Orientation field: 0°, 90°, 180°, 270°, and the same four for the mirror image of the original. The last four are not used often, but are supported to some degree by the browsers. (See http://www.daveperrett.com/articles/2012/07/28/exif-orientation-handling-is-a-ghetto/ for an overview of support.) MediaWiki ignores mirrored rotations when generating thumbnails - the EXIF information is stripped, and the physical orientation is left unchanged. See [[commons:File:EXIF rotation 90 mirrored.jpg]] for an example (depending on the browser, you might need to open the image directly to see the correct position, or you might not see it at all - Chrome positions it correctly when viewed directly but not when viewed on a page). Obviously this is Priority: No One Cares but filing it anyway for the sake of completeness.
