[digiKam-users] Metadata in PNG files

[woenx]

I have noticed that there is something funny with metadata in PNG files.

I sometimes use the exiftool command to edit metadata (mainly add or correct
dates to pictures). I can write and read the metadata from PNG files using
that command, and the metadata seems to be saved correctly.

However... digikam does not see that metadata. It's like there is nothing in
it. BUT, it can write its own metadata to the same PNG file.

So, both digikam and exiftool can write and read metadata from PNG files,
but that metadata must be stored in two different places, because a picture
can contain both entries, one for the exiftool metadata, and another for
digikam metadata, and can only be read by each program respectively. And I
am not talking about Exif, XMP or IPTC. Both of them are Exif.

So, what's going on? Is this normal?



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[Gilles Caulier-4]

Hi,

You are right, PNG has 2 place to store metadata :

One older way, very powerfull and implemented first in ImageMagick, using special PNG chunck to store Exif/Itc/Xmp data bytearray.

digiKam and Exiv2 use this way (i implemented myself this code in Exiv2).

 Adobe few year ago has finally accepted to support PNG and as standardized the XMP only storage in PNG in another chunck. I supose that Exiftool support this Adobe and ImageMagick. 

I'm sure that Exiv2 do not support yet the Adobe solution, as i don't touch Exiv2 PNG code since a while.

This is why you see a difference between Exiv2 and ExifTool.

Best

Gilles Caulier

Le mer. 24 avr. 2019 à 21:11, woenx <[hidden email]> a écrit :

I have noticed that there is something funny with metadata in PNG files.

I sometimes use the exiftool command to edit metadata (mainly add or correct
dates to pictures). I can write and read the metadata from PNG files using
that command, and the metadata seems to be saved correctly.

However... digikam does not see that metadata. It's like there is nothing in
it. BUT, it can write its own metadata to the same PNG file.

So, both digikam and exiftool can write and read metadata from PNG files,
but that metadata must be stored in two different places, because a picture
can contain both entries, one for the exiftool metadata, and another for
digikam metadata, and can only be read by each program respectively. And I
am not talking about Exif, XMP or IPTC. Both of them are Exif.

So, what's going on? Is this normal?



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[woenx]

Nice. Thanks for the explanation!



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