how to get the tag name for a tag ID in a group?

[dae65]

Here is a quick and dirty way to find out what tag name a tag ID was assigned in a (QuickTime) group:

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#!/usr/bin/perl -w
use strict;
my $group = shift // 'ItemList';
my $id = shift // "\xa9day";
say STDOUT ref ? $_->{Name} : $_ for do
{
  no strict 'refs';
  require Image::ExifTool::QuickTime;
  my $var = "Image::ExifTool::QuickTime::$group";
  ${$var}{$id};
};


Does the API provide a better way to get the tag name corresponding to a tag ID in a group?

Thanks

[Phil Harvey]

There is no way to do this in general.  The tag name may depend on other metadata in the file, so you won't know the actual tag name until a file is processed.

But for simple tags (ie. when ${$var}{$id} returns a HASH reference and not an ARRAY reference, then what you are doing is close, but instead requiring the table, you should do this:

my $table = Image::ExifTool::GetTagTable("Image::ExifTool::QuickTime::$group");

to prepare the table for access.

- Phil

[dae65]

Excellent. Thanks. The Image::ExifTool::GetTagTable function ensures that tag IDs map, never to scalars, but always to hash references with a Name key, doesn't it? (So, there is no need to test for ref.)

Code Select Expand


#!/usr/bin/perl -w
use strict;
use Image::ExifTool;
my $type  = shift // 'QuickTime';
my $group = shift // 'ItemList';
my $id    = shift // "\xa9day";
my $table = Image::ExifTool::GetTagTable ( "Image::ExifTool::${type}::${group}" ) or exit;
my $tag = $table->{$id} or exit;
say STDOUT $tag->{Name};


[Phil Harvey]

Yes.  After calling GetTagTable() the ID's will always map to a HASH ref or an ARRAY of HASH refs.  And Name is guaranteed to exist.  (Not that I recommend accessing ExifTool internals like this, but this is not likely to change.)

- Phil

[dae65]

(Not that I recommend accessing ExifTool internals like this, but this is not likely to change.)

I understand the risks. Let me explain the situation. Although the family 7 group names approach is working --- thanks for that --- family 7 group names are... well, group names; and so, an external app that only knows what IDs a file format uses, still needs to find out somehow what tag name an ID corresponds to according to Image::ExifTool, in order to be able use family 7 group names at all.

In other words, family 7 group names can disambiguate a tag, but can't specify a tag.

Ideally, I wish I could do without tag names, as I already have all the IDs. But I can't. What I can do now is either hard-code a table from IDs to names, or look up a table from GetTagTable() on the fly. Both ways, I'm making assumptions about ExifTool internals: after all, tag names may change just as much as GetTagTable().

For this reason, I'm writing an Image::ExifTool::ByID module with a ...ByID() family of wrapper methods that translate ID to names, ensure a group is specified, append :ID-<tag_id> to the group name, and then invoke standard methods such as SetNewValue().

[Phil Harvey]

an external app that only knows what IDs a file format uses, still needs to find out somehow what tag name an ID corresponds to according to Image::ExifTool, in order to be able use family 7 group names at all.

Knowing the ID, here is how you get the family 7 group name:

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sub GetIDGroup($)
{
    my $id = shift;
    $id =~ s/([^-_A-Za-z0-9])/sprintf('%.2x',ord $1)/ge;
    return "ID-$id";
}

- Phil

[dae65]

Thanks. I couldn't make myself understood. Knowing the ID (albm), and generating a 7 group name from it (ID-albm), is not enough. We need the ExifTool tag name (Album) also.

This doesn't work:

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$ exiftool -ItemList:ID-albm=ABCDEF file.m4a
Warning: Tag 'ItemList:ID-albm' is not defined or has a bad language code
Nothing to do.

It should be:

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$ exiftool -ItemList:ID-albm:Album=ABCDEF file.m4a
    1 image files updated


As it seems, family 7 names may well disambiguate a tag, but they can't specify a tag. That's why I'm looking for a reliable way of getting names (Album) from IDs (albm).

[Phil Harvey]

Right.  I understand now.  Upon re-reading your post, this should have been clear.

- Phil