I have a PNG created by Clip Studio Paint with the following tEXt chunk:
PNG tEXt (26 bytes):
[adding COMMENT]
COMMENT = Celsys Studio Tool
If I do exiftool -PNG:Comment file.png this block is displayed.
If I do exiftool -PNG:Comment="something" I end up with a PNG file with
PNG tEXt (26 bytes):
[adding COMMENT]
COMMENT = Celsys Studio Tool
PNG tEXt (29 bytes):
Comment = something
So for the purpose of reading, it seems to consider the (somewhat strange) initial tEXt chunk to match PNG:Comment, for the purpose of updating PNG:Comment, it does not.
If I do exiftool -PNG:Comment= it will remove my 'something' comment. If I do it again, it will not consider there to be any matches for PNG:Comment in the file, another asymmetry from just reading PNG:Comment.
How can I properly update/remove these comments? They are removed if I do -PNG:All= but removing all PNG tags isn't ideal.
Addendum: To clarify, I tried PNG:COMMENT as well. It doesn't seem case-sensitive.
It seems that Clip Studio Paint is using an incorrect uppercase tag ID. (The "adding" message indicates that ExifTool is adding this to the internal tag table because it isn't a standard tag.)
To write this non-standard tag would require creating a user-defined tag. See the sample config file (https://exiftool.org/config.html) for examples of how to do this for PNG TextualData tags. You should give this tag a different name so you can distinguish it from the standard "Comment" tag.
- Phil
Quote from: Phil Harvey on April 07, 2022, 09:33:42 AM
It seems that Clip Studio Paint is using an incorrect uppercase tag ID.
The relevant section of the specs on w3.org (https://www.w3.org/TR/PNG-Chunks.html#C.tEXt)
The following keywords are predefined and should be used where appropriate:
<snip full list>
Comment Miscellaneous comment; conversion from GIF comment
...
Keywords must be spelled exactly as registered
Thanks StarGeek, but you left out the crucial part of your quote:
In particular, keywords are considered case-sensitive.
- Phil
Heh, yep. I stopped reading at "spelled exactly" part. ;D