[Digikam-users] Metadata and jpg quality?

[Jean-François Rabasse]

Hi Kim,

On Tue, 19 Feb 2013, Kim wrote:

> I was wondering if adding metadata or updating metadata to jpg files in 
> Digikam has any effect on the image quality?
> I've read many times that resaving a jpg image recompresses the image and so 
> it loses quality.  Because of this I normally only edit copies and so I love 
> digikam how saves in versions.
> I was watching a webinar today on the topic of metadata and the person said 
> that adding/editing metadata did have an effect on jpg quality.  When I tried 
> a websearch I couldn't find a straight answer, but it seemed to depend on 
> what TOOL you used to do the editing as to whether it had any effect on the 
> jpg quality or not.

The answer is no. (No effect on image quality)
JPEG files have a very simple structure made of a sequence of sections,
and sections content is specialized, one section for Exif metadata
(camera infos), one section for XMP metadata, possible other 
informations sections (Photoshop), and image data dedicated sections.

So, it's easy to update some sections, or the others, and left the rest
unchanged.
The problem you raise come when metadata edition is done with image
editing programs. These programs save both image data and metadata,
and right, saving image data often lead to recoding an adding artefacts.
This is the « what TOOL you used ... »

But Digikam is not an image editor. It provides an image edition 
function, to modify/update/rewrite image data.
Metadata edition is performed by a separate library, libexiv2, that
don't touch the image data section (and probably doesn't even
know what a JPEG scanline is...)


Regards,
Jean-François