Is that intended that Xmpdatum::tagLabel() might throw an exception but print() won't?

[BLumia]

I'm not sure if it's a bug, I have an picture shot with Pixel 7 and it have a Xmp tag that I cannot use Xmpdatum::tagLabel() to get its tag value.

I'm not sure if it's a bug or I'm using the library in an incorrect way, but we can also test it via the exiv2 command.

$ exiv2 -p a PXL_20230214_045109312.MP.jpg
...
Xmp.xmpNote.HasExtendedXMP                   XmpText    32  3C4A84056291D00B0973809DD0D830F6
Xmp.Container.Directory                      XmpText     0  type="Seq"
Xmp.Container.Directory[1]                   XmpText     0  type="Struct"
Xmp.Container.Directory[1]/Container:Item    XmpText     0  type="Struct"
Xmp.Container.Directory[1]/Container:Item/Item:Mime XmpText    10  image/jpeg
Xmp.Container.Directory[1]/Container:Item/Item:Semantic XmpText     7  Primary
Xmp.Container.Directory[1]/Container:Item/Item:Length XmpText     1  0
Xmp.Container.Directory[1]/Container:Item/Item:Padding XmpText     1  0
Xmp.Container.Directory[2]                   XmpText     0  type="Struct"
Xmp.Container.Directory[2]/Container:Item    XmpText     0  type="Struct"
Xmp.Container.Directory[2]/Container:Item/Item:Mime XmpText     9  video/mp4
Xmp.Container.Directory[2]/Container:Item/Item:Semantic XmpText    11  MotionPhoto
Xmp.Container.Directory[2]/Container:Item/Item:Length XmpText     7  4745779
Xmp.Container.Directory[2]/Container:Item/Item:Padding XmpText     1  0

$ exiv2 -Pkyctl PXL_20230214_045109312.MP.jpg
...
Xmp.xmpNote.HasExtendedXMP                   HasExtendedXMP                 XmpText    32  3C4A84056291D00B0973809DD0D830F6
Xmp.Container.Directory                      Directory                      XmpText     0  type="Seq"
Xmp.Container.Directory[1]                   Directory[1]                   XmpText     0  type="Struct"
Xmp.Container.Directory[1]/Container:Item    Directory[1]/Container:Item    XmpText     0  type="Struct"
Xmp.Container.Directory[1]/Container:Item/Item:Mime Exiv2 exception in print action for file PXL_20230214_045109312.MP.jpg:
No namespace info available for XMP prefix `Item'

Since the image itself contain private content, so I try to trim the image but seems exiv2 no longer read all these Xmp tags (but it probably still exists since other application can read the content). I tries to take another photo with my Pixel 7 but exiv2 also no longer read the full Xmp tags. See this attachment for the two photos: PXL.zip

So my questions are:

1. Is that intended that Xmpdatum::tagLabel() might throw an exception but print() won't, or is it a bug?
2. What's the differences between tagLabel() and print()? I didn't really understand the documentation.
3. Also, by reading the source, we uses Params::prTrans for print(), what does Trans stands for? Does it mean the string might be translated into the user's language? If it is, can we assume it will always be UTF-8 encoded?
4. If we just want to display the value of a tag to user, then which one is preferred to be used? tagLabel() or print()?
5. How can I just dump the tag part of the image PXL_20230214_045109312.MP.jpg so I can provide the metadata/tag part of the image here for testing, without providing the content of the image?

Please let me know if you need any additional information.

Thanks!

[clanmills]

Lots of good questions here. I'll try my best to answer using a modified samples/xmpdump.cpp to which I've added a few lines of code as follows:

594 rmills@rmillsm1:~/gnu/github/exiv2/0.27-maintenance/build $ git diff ../samples/xmpdump.cpp 
diff --git a/samples/xmpdump.cpp b/samples/xmpdump.cpp
index 97803922c..9e2c1849b 100644
--- a/samples/xmpdump.cpp
+++ b/samples/xmpdump.cpp
@@ -51,6 +51,16 @@ int main(int argc, char* const argv[])
         }
         std::cout << xmpPacket << "\n";
 
+        Exiv2::XmpData &xmpData = image->xmpData();
+        Exiv2::XmpData::const_iterator end = xmpData.end();
+        for (Exiv2::XmpData::const_iterator i = xmpData.begin(); i != end; ++i) {
+            std::cout << " key: "         << i->key()
+                      << " print: "       << i->print()
+                      << " tagLabel: "    << i->tagLabel()
+                      << " value: "       << i->value()
+                      << "\n" ;
+        }
+
         return 0;
     } catch (Exiv2::AnyError& e) {
         std::cout << "Caught Exiv2 exception '" << e << "'\n";
595 rmills@rmillsm1:~/gnu/github/exiv2/0.27-maintenance/build $ 

Here's the output for Stonehenge.jpg:

596 rmills@rmillsm1:~/gnu/github/exiv2/0.27-maintenance/build $ curl -O https://clanmills.com/Stonehenge.jpg
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 6599k  100 6599k    0     0   643k      0  0:00:10  0:00:10 --:--:-- 1232k
597 rmills@rmillsm1:~/gnu/github/exiv2/0.27-maintenance/build $ env DYLD_LIBRARY_PATH=lib bin/xmpdump Stonehenge.jpg 
<?xpacket begin="" id="W5M0MpCehiHzreSzNTczkc9d"?> <x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="XMP Core 4.4.0-Exiv2"> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"> <rdf:Description rdf:about="" xmlns:xmp="http://ns.adobe.com/xap/1.0/" xmlns:cm2e="http://clanmills.com/exiv2/" xmlns:dc="http://purl.org/dc/elements/1.1/" xmp:Rating="0" xmp:ModifyDate="2015-07-16T20:25:28+01:00" cm2e:Father="Robin Mills"> <cm2e:Family> <rdf:Bag/> </cm2e:Family> <dc:description> <rdf:Alt> <rdf:li xml:lang="x-default">Classic View</rdf:li> </rdf:Alt> </dc:description> <dc:Family> <rdf:Bag> <rdf:li>Robin</rdf:li> </rdf:Bag> </dc:Family> </rdf:Description> </rdf:RDF> </x:xmpmeta>                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 <?xpacket end="w"?>
 key: Xmp.xmp.Rating print: 0 tagLabel: Rating value: 0
 key: Xmp.xmp.ModifyDate print: 2015-07-16T20:25:28+01:00 tagLabel: Modify Date value: 2015-07-16T20:25:28+01:00
 key: Xmp.cm2e.Father print: Robin Mills tagLabel: Father value: Robin Mills
 key: Xmp.cm2e.Family print:  tagLabel: Family value: 
 key: Xmp.dc.description print: lang="x-default" Classic View tagLabel: Description value: lang="x-default" Classic View
 key: Xmp.dc.Family print: Robin tagLabel: Family value: Robin
598 rmills@rmillsm1:~/gnu/github/exiv2/0.27-maintenance/build $ 

A typical tag is "Xmp.dc.description" obtained from the XMP:

      <dc:description>
        <rdf:Alt>
          <rdf:li xml:lang="x-default">Classic View</rdf:li>
        </rdf:Alt>
      </dc:description>

And presented by my modified xmpdump as:

key: Xmp.dc.description
print: lang="x-default" Classic View
tagLabel: Description
value: lang="x-default" Classic View

The term "translated" means "human readable". It does not imply any localisation. For XMP metadata, I'm not sure there's a difference between i->value() and i->print(). For Exif metadata tags such as Orientation are stored as an short. If you use the exiv2 terminal app, the options -pv are -pt different. The terminal app argument parser uses Params::prTrans

602 rmills@rmillsm1:~/gnu/github/exiv2/0.27-maintenance/build $ exiv2 -pv -g Orientation Stonehenge.jpg 
0x0112 Image        Orientation                 Short       1  1
603 rmills@rmillsm1:~/gnu/github/exiv2/0.27-maintenance/build $ exiv2 -pt -g Orientation Stonehenge.jpg 
Exif.Image.Orientation                       Short       1  top, left
604 rmills@rmillsm1:~/gnu/github/exiv2/0.27-maintenance/build $ 

To answer your specific questions:
1. Is that intended that Xmpdatum::tagLabel() might throw an exception but print() won't, or is it a bug?
I don't know if anybody can answer that question. I'm not sure that either will throw.

2. What's the differences between tagLabel() and print()? I didn't really understand the documentation.
Use print() to print the "translated" (human readable) value and value() to print the "raw" (untranslated) value.

tagLabel() returns the label. For example: Description.

3. Also, by reading the source, we uses Params::prTrans for print(), what does Trans stands for? Does it mean the string might be translated into the user's language? If it is, can we assume it will always be UTF-8 encoded?
I think I've answered that question.

4. If we just want to display the value of a tag to user, then which one is preferred to be used? tagLabel() or print()?
I think I've answered this. Use value().

5. How can I just dump the tag part of the image PXL_20230214_045109312.MP.jpg so I can provide the metadata/tag part of the image here for testing, without providing the content of the image?

Use exiv2 -ea to extract all metadata from your file:

610 rmills@rmillsm1:~/gnu/github/exiv2/0.27-maintenance/build $ env DYLD_LIBRARY_PATH=lib bin/exiv2 -ea --verbose Stonehenge.jpg 
File 1/1: Stonehenge.jpg
Writing Exif data from Stonehenge.jpg to ./Stonehenge.exv
Writing IPTC data from Stonehenge.jpg to ./Stonehenge.exv
Writing XMP data from Stonehenge.jpg to ./Stonehenge.exv
611 rmills@rmillsm1:~/gnu/github/exiv2/0.27-maintenance/build $ 

I retired from Exiv2 in 2021 (after working on the project since 2008). I'm surprised nobody from the team replied. It's a boring Sunday afternoon in England and your questions have caught my attention. If you are still interested in this matter, please email me directly as I do not monitor the project.

[BLumia]

Thanks! I actually also created an issue report and also got related replies from the issue:

• Is that intended that Xmpdatum::tagLabel() might throw an exception but print() won't? #2549

Hope it could also help others finding the related discussions :)

[clanmills]

Oh, that's fine. I enjoyed thinking about your questions. However, I'm not tempted to return to working on the project. And I'm really pleased to see that @postscript-dev worked with you.