Metadata and jpg quality?

[Kim]

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 main reason I'm wondering is I work with a lot of heirloom family
photos.  I do only edit copies, but I would love to have the originals
tagged and info added to the captions/description.  This would help in
keeping track of the photos.

Thanks!
Kim
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[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

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[Simon Cropper-3]

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
>
>
> _______________________________________________
> Digikam-users mailing list
> [hidden email]
> https://mail.kde.org/mailman/listinfo/digikam-users
>

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

    Free and Open Source Software Workflow Guides
    ------------------------------------------------------------
    Introduction               http://www.fossworkflowguides.com
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[Kim]

Thank you Jean, that makes perfect sense!  I'm so glad to know that
Digikam saves the metadata in a way that doesn't hurt the photo
quality.  Now another reason that I love this software. :)

Kim

On 02/19/2013 04:54 PM, Jean-François Rabasse wrote:
>
> 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.
>

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[Kim]

Hi Simon,  Thank you for testing that out and sharing the results.

Many of the files I'm working with right now, I scanned about 10-12
years ago and no longer have access to the original photos. I did scan
them at 300dpi thankfully.  The older photos that I do own, I scan as
both TIFF and JPG.  I don't think the scanner I had 10 years ago offered
TIFF's in their software and I know the pc I had back then wouldn't have
had room on it's tiny hard drive for many of them either.

Kim

On 02/19/2013 05:15 PM, Simon Cropper wrote:

>
> 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.
>

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[Anders Lund]

Onsdag den 20. februar 2013 09:15:06 skrev Simon Cropper:
> 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.

You can change the settings for digikams jpeg filter. Setting to a high
quality and disabling subsampling will minimize the loss, but jpeg *is* a
lossy format. Save to png or another lossless format to avoid problems, and
export to jpeg when you want to share images on the web, that will ensure you
keeps quality.

--
Anders
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