I want to move to GraphicsMagick because I'm only resizing and rotating photos, and I heard it was a little faster. But it doesn't take the -auto-orient option directly. Is there another way to do this efficiently? The Imagemagick convert -auto-orient option will read the EXIF orientation tag, rotate accordingly, then RESET the EXIF tag to orientation=1.
Not at this time.
As you said, you can guess the operation to accomplish with a simple switch case. This is taken from an interesting resource page: http://sylvana.net/jpegcrop/exif_orientation.html
switch EXIF:Orientation:
1) transform="";;
2) transform="-flip horizontal";;
3) transform="-rotate 180";;
4) transform="-flip vertical";;
5) transform="-transpose";;
6) transform="-rotate 90";;
7) transform="-transverse";;
8) transform="-rotate 270";;
*) transform="";;