Digikam-users Digest, Vol 241, Issue 4

[Carol C. Kankelborg]

Thanks for your answers. See below.

> On Jun 7, 2025, at 23:53, Remco Viëtor <remco.vietor at wanadoo.fr> wrote:
> 
> 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).


Do you happen to know if this was the case in DigiKam 8.1.0?


> 
> 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.
> 
> 

I appreciate knowing that just starting the image editor decodes the image. I will have to look into what my settings are.

I didn’t say because it was a few years ago that I would have done it, so I don’t remember. Given that the images are 99% similar and they look identical (to my eye, looking at the screen and switching between them rapidly), I would guess that I didn’t make any other changes. If so, I doubt I would have opened the editor for each photo that needed to be rotated, when DigiKam offered a quick option from the album view. 

The rotated files are about 2 MiB smaller according to the Properties tab. This difference seems significant to me. Is there metadata or are there tests I can conduct to see if one is more degraded compared with the other due to multiple decodings/re-encodings?

Does this refer to the image or the whole file with metadata included? Is there a way to tell how much space the metadata takes up vs. the image itself?


> 
> 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?)
> 

Do you happen to know if  DigiKam 8.1.0 did not use that metadata tag but 8.6.0 does? 

The smaller, moved to an album from its import location and rotated, image was imported in late 2023. The larger, still in the import album, was imported in August 2024, probably because it was still on the camera SD card and I wasn’t paying attention to make sure I didn’t import images I’d already imported.  


Carol