I’ll preface this with the context that I’m not a photographer or photo editor and am likely working with a bit of an unusual use case. I did quite a bit of looking around and reading documentation, but apologies in advance if this is covered somewhere else.
Ultimately what I’m trying to do is use Raw Therapee to crop and sometimes slightly rotate RAW document scans without any modification or loss other than what is outside of the crop area. The changes are fairly minor, so ideally I’d like to overwrite the original scans and only keep one RAW file. I get image outputs with matching file hashes, so from what I can tell this seems to be what is happening with my current settings and cropping only.
However, when I rotate an image, save it, un rotate it, then compare a section of it to the same section of the larger base image, it no longer matches. My understanding is a non 90 degree rotation, is destructive, even when not using sharpening. I’ve been doing this to do a little correction to scans that aren’t completely straight, but have now realized that I am likely altering the image by doing so.
Is there anything that I’m overlooking that would be making modifications to the original image if just cropping and not rotating and is there any different way to rotate that is non destructive?
I attached a screenshot of the few settings I have been changing. Thanks so much for any input or insight in advance!
Not to my knowledge. Rotation by anything other 90, 180, 270 deg necessarily involves interpolation and re-sampling which can not be undone by rotating back by the same amount.
On the other hand cropping is non-destructive except perhaps at the edges but again can not be undone, i.e. cropped, saved, opened and resized.
I am willing to be corrected here if I am wrong, but I don’t see how any software such as RawTherapee, Darktable or other image software can rotate and crop and give you a RAW file as the output. These programs would need to export the edit as file format such as Tiff, PNG or another format.
Could you share a file so we can see what you are trying to achieve? What file format is your current RAW at the moment?
Sorry, I’m probably using incorrect terminology here. My scanner outputs an LZW compressed Tiff. Not truly a RAW file, but as close as I can get it to produce. Then after cropping it in RT, I save it as an uncompressed Tiff.
+1 to @cedric’s answer. There’s one other source of loss to account for, probably not an issue with documents with margins.
Rotation to one of the non-major compass points puts the image at odds with the row-column organization of the image format. A fully-preserved image rotated thusly would look like this:
Note in the parameters pane (bottom-left) the yellow rectangle identifies the crop, and the image segments outside of it are, cropped out. That would be a loss…
Ah, this was something that I was accounting for but forgot to mention in my first post. As you mentioned though, the scans do have some extra around the edges that I’m not concerned with preserving that is lost.
Thank you all so much for the thoughtful replies, I really appreciate it!
It sounds like other than the rotation, everything should be preserved. Fortunately I had only done a few documents before realizing my mistake!
