Lossless JPG rotation

Help & Support
#1

Im very sorry, but after all this time, it's still not clear to me.

My questions are as follows:

  1. If I use the standard rotation buttons in the lister menu or right click menu, will DOpus always use lossless rotation?
  2. Is there a way to make sure that DOpus will always use lossless rotation for JPG files? (or not rotate at all if that's not possible)

I've done some testing:

  • Copy file1 to file2
  • Rotate file2 90 degrees
  • Rotate file2 270 degrees
  • I would expect the resulting file2 to be identical to file1, but it isn't.
  • Using exiftool (-all=) I removed all EXIF information from both files, but the files still differ.

Thanks!

#2

Lossless JPEG rotation is done by default when it is possible.

From the manual:

the width and height of the image must be an exactly multiple of the "block size" the image has been saved in (usually 8x8 or 16x16)

You'd need to examine the differences to tell if it was in the image data or something else.

e.g. Convert both to BMP format and then binary compare the two files. That will tell you if the image data changed, I think.

#3

Leo,

Thank you for your answer. I understand what you're saying about converting and binary comparison. I just didn't go that far yet.

Is there a way to stop DOpus from rotating AT ALL if the the image is not a multiple of its block size? I know, most images probably will use the "correct" resolution, but still, it bothers me :slight_smile:

#4

There isn't. If you ask Opus to rotate the image, it will rotate the image. Lossless if it can (and NOLOSSLESS isn't specified), but it'll fall back on lossy if it needs to.

#5

Thanks, everything's crystal clear now :slight_smile: