Recognising portrait orientation in JPG files

You should be able to work it out via Image Events.

set theOriginal to (choose file)

tell application "Image Events"
set theImage to open theOriginal
set {x, y} to theImage's dimensions

if x > y then -- image is landscape...
rotate theImage to angle -90 -- so rotate it
save theImage in (theOriginal as text) & ".new.jpg" -- and save a new version
end if
end tell

Of course, it's hard to automatically work out whether to rotate clockwise or counter-clockwise. That's up to you to decide 🙂

With some JPEG pictures, if you open them in Preview.app and select Tools > Inspector, then More Info > General, you will get information about their orientation, for example: "1 (Normal)", "6 (Rotated 90° CW)", "8 (Rotated 90° CCW)", etc.

If no such information exists, maybe you can do as Camelot suggested and just compare (under the General Info tab) the image size: “800 x 600 pixels” is most likely a landscape, whereas “600 x 800 pixels” would be a portrait.

With AppleScript, you can use GUI Scripting to get these informations from Preview.app.

Here is just a first draft of a script that can get some useful information from Preview.app:

tell application "Preview" to activate
tell application "System Events" to tell process "Preview"
if title of window 1 is not in {"General Info", "More Info", "Keywords"} then keystroke "i" using command down -- to open the Inspector
click radio button 2 of radio group 1 of window 1 -- More Info
click radio button "General" of tab group 1 of window 1 -- General
if (row 1 of table 1 of scroll area 1 of group 1 of window "More Info" whose value of text field 1 is "Orientation") exists then
get value of attribute "AXValue" of text field 2 of (row 1 of table 1 of scroll area 1 of group 1 of window "More Info" whose value of text field 1 is "Orientation") --> "1 (Normal)", "6 (Rotated 90° CW)", "8 (Rotated 90° CW)", etc. for example
else
click radio button 1 of radio group 1 of window 1 -- General Info
return value of UI element 1 of window 1 whose value contains "pixels" and value does not contain "inch" --> "800 x 600 pixels" for example
end if
end tell

It must be possible to do much better.

Comparing photos store by my Canon iXus 60 with those from a friend's Casio camera, I see in the Inspector More Info data that the Canon seems to store more information about the photo, in particular it has (for a portrait photo) a field... [snip]

That's because the iXus has an orientations sensor within it that detects the orientation of the camera and can record that alongside the image. The Casio (like most cameras) doesn't have an orientation sensor, so it is unable to determine which way up the image was taken.

returns Orientation 1 (Normal) for both a normal landscape photo and one taken upside down.

The sensor is likely detecting offset from horizontal. Since right-side-up and upside-down are both, essentially, horizontal, they're both recorded the same way (few people take pictures upside down 😉 )

Holding he camera at about 45° brought Orientation 1, in one case, and Orientation 6 in another.

That's probably because you weren't quite at 45 degrees on the first shot.

As for the script, since that data is stored in the EXIF data, it should be possible to extract it via Image Events, so a slightly puffed-up version of my script should be able to orient it correctly.