modification historical on a pictures

[Gilles Caulier]

Hi all,

First you must know that JPEG (and also other major file formats) are
structured as chunks. Depending on the format architecture, these
chunks are ordered by usage and priority, and generally metadata are
hosted in a chunk somewhere on the front of the file to be processed
quickly while cataloging. The image data is stored in other chunks,
after the metadata.

When you modify a metadata you don't touch the image data. In the
inverse, when you change the image data, metadata are always changed
(typically to host the image sizes for ex).

When digiKam changes something in the metadata (as for ex a keywords),
this touches the metadata chunk, not the image, somewhere on the front
of the file (first 1K of the file stream). This is processed by Exiv2
library or ExifTool, depending on the digiKam settings. Of course
changing metadata will change the file date as it is touched by the OS
file system module. In digiKam, there is an option from the metadata
settings panel to restore the file date when metadata is changed.

Another point to know is the registration of a file in the database
with a checksum algorithm, based on the first K bytes of the files.
calculating a checksum on the whole file takes a while and will reduce
the performances. This checksum allows you to follow the change in the
file for example when you touch contents with an external application,
or to identify files stored in the collection and already downloaded
from a camera.

To come to the original problem on this thread, to identify a
corrupted file is a complex and long task to process and will require
to compute a checksum on the whole files. This can be done with
scripts of course, and this will take a while on a large collection.
Processing this computation on the collection and on a backup will
help to identify the touched files.

A program as rsync used complex and fast algorithms to synchronize a
source folder and a destination folder. This is the best open source
to use on the command line under Linux to process these kinds of jobs.
Rsync can be used  in a "dry" mode to identify the differences between
contents.

https://superuser.com/questions/748069/how-do-i-compare-two-folders-recursively-and-generate-a-list-of-files-and-folder

My best regards

Gilles Caulier

Le ven. 7 nov. 2025 à 12:44, frederic chaume
<frederic.chaume at gmail.com> a écrit :
>
> Hi Frederic
>
> situation are different, in my case I didn't change or rename tags.
> I see there is an option set "detect faces in newly added images" , and
> I see it is running. I don't know in which version this option has been
> added, but I see it is effectively running. Now this concern new added
> images and in my case, problem is relative to old images.
> Regards
> Frederic
>
> Le 07/11/2025 à 12:28, Frédéric Da Vitoria a écrit :
> > This has happened to me too, for example if I change a tag (for
> > example changing an upper case to lower case, or editing an
> > accent...). This tag modification is of course propagated to all
> > concerned pictures. I found this annoying at first, because previous
> > did not change the date in this case, but I understand that the files
> > are indeed modified. And for backup purposes, the backup files indeed
> > need to be refreshed IMO.
> >
> > But maybe this is not the correct explanation in your situation?
> >
>