[digiKam-users] DK inquiry

[Remco Viëtor]

On mercredi 20 novembre 2019 22:00:11 CET woenx wrote:
> digikam-2 wrote
> 
> > Thanks for correcting my English. English not my first, nor second
> > language...
> 
> Don't worry, neither is mine :)
> 
> digikam-2 wrote
> 
> > * Are the XMP sidecars "standard" (whatever standard means, I'm not
> >   sure)?
> > 
> > * Do all imaging software read the XMP sidecars?
> > * Can they all read the tags and the descriptions  from the XMP
> >   sidecars?
> 
> According to wikipedia
> https://en.wikipedia.org/wiki/Extensible_Metadata_Platform XMP seems to be a
> standard, and it can be written in a sidecar file, or directly in the
> picture file, next to the Exif and IPTC metadata. In the wiki article
> there's a list of software that supports XML, but it doesn't specify if
> they use sidecars, or they embed the XMP information in the picture. I know
> first-hand that Adobe Lightroom / Bridge, Darktable and ACDSee both are
> compatible with XMP sidecars. Not sure about others, but these are some of
> the main ones.
> 
> I think that, in theory, the EXIF metadata can also be saved in the XMP
> sidecar files. Although I am not sure how that works.
> 
> --
> Sent from: http://digikam.1695700.n4.nabble.com/digikam-users-f1735189.html

XMP is an ISO standard (a form of XML), but the tags are not all described in 
the standard. XMP tags are organised in namespaces, which can be defined by 
anyone, and used by anyone as long as the definition is accessible. Each 
program decides which namespaces to use. 

One of those namespaces is "exif" (xmlns:exif="http://ns.adobe.com/exif/
1.0/"), which records the exif metadata. In a similar way, IPTC metadata can 
also be included in XMP files.

Similarly, digikam, darktable, and adobe and others have defined namespaces 
(see the attributes starting with "xmlns:", which stands for XML NameSpace). 
You can safely open a sidecar with a text editor (Vi, notepad, ..., do *not* 
use a word processor!) to have a look. The use of third-party namespaces is 
why your digikam tags/keywords appear several times*.

Normally, any program that writes XMP sidecars can also read them (and vice 
versa). But reading the sidecar isn't enough, the program has to be able to 
use the data, so it must understand the namespaces used by the programs that 
wrote the sidecar. That's what causes the "missing data" when using several 
programs.
Normally, such metadata should be ignored, and kept unchanged in the sidecar 
file, but that can be delicate if the sidecar is rewritten.

Remco

*: I use digikam and darktable, and between the two I see 14 namespaces used 
in the XMP files, and the keywords I assigned appear 5 times (at least): 3x in 
a  hierachical form, once as a simple list, and once as encoded xml (for 
acdsee).