Add comment functions

[rmnvgr]

GExiv2 has three functions to manipulate comments:

• gexiv2_metadata_get_comment()
• gexiv2_metadata_set_comment()
• gexiv2_metadata_clear_comment()

It would be nice to have them as, contrary to what the gexiv2_metadata_get_comment() doc says, it can return metadata not accessible otherwise.

[felixc]

Thanks for the suggestion! It looks like they are already exposed via the underlying gexiv2-sys FFI wrapper, which makes exposing them here simpler:

https://github.com/felixc/gexiv2-sys/blob/f53a6da4a6bd1d3a35d207c8d72cd500b1c7009c/src/lib.rs#L156-L158

(It also means that if you need this right now, a temporary workaround might be to call these functions directly)

I think (but don't really remember) that I originally didn't expose them because I saw the comment you mentioned that suggests these are just wrappers for bulk-operating on fields that are already individually accessible… but you note that

it can return metadata not accessible otherwise.

Can you elaborate on that? What is not possible to do currently that only these wrappers provide?

[rmnvgr]

For example, if you take the dirty.jpg test file from mat2, it contains a "Created with GIMP" comment.

If you try each of the fields mentioned in the docs, none of them return the comment. However, gexiv2_metadata_get_comment() returns the comment correctly.

[jonas-hagen]

There is a JPEG comment, which is stored in the JPEG meta data along with width, height, depth information. This has nothing to do with Exif, XMP and IPTC metadata and is thus no accessible by get_tag_* functions.

It would be nice to have these functions wrapped to access the JPEG comment as well. Not all jpeg decoders for rust support reading the comment.

[felixc]

Thanks for the clarification of where that extra metadata was coming from. In principle I agree it would be very desirable to be able to set/unset JPEG comments through this library as well, so I'm on board with the general goal here. Do you happen to know if there's any other JPEG metadata other than these comments that we should generalize this to?

What I'm worried about is that exposing gexiv2_metadata_get/set/clear_comment() may be a sort of roundabout side-effect-based of accomplishing the real goal, and I don't know if we want to encourage its use when upstream gexiv2 seem to be deprecating it. If we're using it to get at the JPEG comment, it seems undesirable that it also does stuff to a somewhat arbitrary set of other tags...

I wonder if upstream gexiv2 would be open to a PR implementing a set of functions for JPEG comments specifically (and/or any other JPEG metadata, if any such exists)... How would you feel about that approach?

[rmnvgr]

Well I don't see that gexiv2 is deprecating the use of gexiv2_metadata_get/set/clear_comment(). I only read that they are recommending to "use Exiv2 tags directly rather than this method, which is more useful for quick or casual use".

It may be nice indeed to have specific functions for accessing only the comment, but if they are accepted, it will take time before downstreams are updated. Having the already existing functions available in rexiv2 would be great.