Digikam-users Digest, Vol 241, Issue 4

[Remco Viëtor]

On dimanche 8 juin 2025 06:45:18 heure d’été d’Europe centrale Carol C. 
Kankelborg wrote:
> Steve,
>   Most of that makes sense. I noticed the height/width dimension switch. The
> smaller file appears to have been rotated by DigiKam 8.1.0. That is the one
> I put in the vacation album, so if it needed rotating, it makes sense I
> would have done that.
> 
>  I am presently using 8.6.0. Is 8.6.0 smart enough to note rotation when
> displaying the image but 8.1.0 was not?  The larger image has the same
> orientation when I look at it in DigiKam (Similarity tab).
> 
> For software that rotates images (in this case I assume it was DigiKam
> 8.1.0), is the quality (and size) of the rotated image diminished because
> it had to be uncompressed, rotated, then re-compressed? If so, is it a
> serious degradation or should I not worry about it for JPG photos of
> vacations? If not, why would one be smaller than the other? Is it just the
> way the JPG compression works that portrait images are larger than
> landscape?
> 
It all depends...

It is possible to rotate a jpeg image by 90° without extra loss, even without 
decompressing it. Iirc, that's what digikam does when rotating in the album 
view. That operation shouldn't change the file size (or not by much: there may 
be some changes in the metadata).

If you use the image editor, the situation is different: the image is decoded 
*when you start the image editor* and any operation is done on the decoded 
image. The edited image is then re-encoded, possibly with different settings, 
when you save the image. This can change the image size, possibly by quite a 
lot. *Repeated* decoding and re-encoding can also degrade the image quality.

The batch queue uses the image editor, afaik (and you don't need it when just 
rotating a batch of images in the album view).

I didn't notice you mentioning how you rotated the images.



Related to that is the auto-detection of rotation: there is a metadata tag to 
indicate image orientation. It's what the camera uses to record landscape or 
portrait. A program *can* mimic an image rotation over 90° by adjusting this 
tag. A viewing program may or may not use this tag.
Otoh, if you edit an image, that tag may be cleared and the image adjusted. 
But there is no way for a program to deduce a rotation only from the image, it 
needs to read the metadata to detect an intended rotation.
(simple, isn't it?)