Why shouldn't raw files be rotated?

Question

When I attempt to rotate a CR2 file in Photo Viewer, a message appears:

That didn't work, RAW files don't rotate. Select edit to rotate and save it as a different file type.

There are many tips to rotate a raw file (not just simply save it in a different file type), e.g. using an exif tool. Apparently it can be rotated. However, I wonder why it shouldn't be rotated at the first place.

ths · Accepted Answer

You can't actually rotate any file. You only can rotate images, then save those images to a file.
But it is usually not possible to write (in the sense of changing the sensor data) RAW files; it would be possible theoretically, of course (your camera does it), but no software i'm aware of does it, not even the proprietary ones of the camera manufacturers.
What might be possible, with tools like exiftool, is to change some of the other data (aka metadata) contained in the file, like the thumbnail or EXIF fields, and  set the flag in the EXIF data which indicates camera orientation to rendering software. Usually this isn't terribly useful since RAW files aren't practical for direct viewing anyway, you typically open them in some image conversion software and save them eg. as JPEG. Or you apply all edits you like "virtually" in software like Lightroom which applies those edits on the fly when viewing the image.

qrk · Answer

RAW image files aren't meant to be manipulated directly. If your photo viewer is viewing a raw image file, it is probably looking at the JPEG thumbnail embedded in the raw file. The photo viewer I use, JPEGview, allows you to view the embedded thumbnail in raw files and allows you to rotate the view. However, you can't save the rotation info in the raw file with JPEGview, however, it will allow you to save the edited thumbnail as a separate JPG image file.
Programs that are used to edit raw images don't manipulate the raw file directly. Instead, a script of your changes are recorded. That script is played back when you convert raw format to something viewable like JPEG or TIFF. Programs like Lightroom show a list of the edits you have made which is essentially the script.

Why shouldn't raw files be rotated?

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