[Digikam-users] Metadata and jpg quality?

[Simon Cropper]

On 20/02/13 08:54, Jean-François Rabasse wrote:
>
> 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
>
>
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Kim,

Although I was aware that this is the correct answer, I decided to test 
the system - which is easy enough to do.

Actually editing (even if no changes are made) then saving a JPG image 5 
or more times (usually less) will show clear changes in image quality.

I updated a copy of the image's metadata 12 times yet was unable to see 
any introduced artifacts using the light table feature at x1200. So as 
Jean-François rightfully explained only the header section of the JPG 
file is being updated.

It is worth noting that you can set the preferences so that the metadata 
is saved to XMP files -- a good option if you work with RAW files (as 
the save metadata to RAW files is only an experimental feature).

I also have worked through my old heirloom photos. Are these scanned 
images or old digital files that were taken in JPG? If you are scanning 
your photos why are you saving as JPGs? Being loosy you are introducing 
artifacts from the get-go.

-- 
Cheers Simon

    Simon Cropper - Open Content Creator

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