Writing metadata to file or not?

[ChicagoGhost]

I have searched through the help but am still unclear about this.
1. I don't know whether my understanding is right: After modifying the metadata of the file, the metadata is not written to the original file automatically. It is stored in the database. So if one chooses not to change the original files, one could just work with the metadata in the database and don't need to do anything. If one would like to change the original files' metadata, one should do the "write back".
2. Which method is better, to write to the original files or to maintain the metadata in the database? I see some software claims that they never change the original files.
3. I just did facial recognition. I see that almost all files have pending metadata write-back, even though many of them don't have people in them. The XMP::iptcExt\PersonInImage is for the recognized face. But some other metadata is also changed, e.g. XMP::exif\DateTimeOriginal. Why is that?

[Mario]

the metadata is not written to the original file automatically. It is stored in the database.

Correct. The reason is speed and easy of use. Full details are in the Metadata Write-back help topic.

2. Which method is better, to write to the original files or to maintain the metadata in the database?

Both. IMatch always caches metadata in the database for performance reasons.

Storing metadata in images makes it persistent and accessible for other applications. And it makes your metadata independent from IMatch, because you can see and use it in any other application which supports common metadata formats.

See Metadata for Beginners for a good introduction.

I see some software claims that they never change the original files.

What should that be good for? Metadata was invented to record data about the image in the image file. Standards like EXIF, GPS and XMP are designed to store metadata inside the image. Your camera stores metadata in the image.

If you later add or correct metadata in IMatch, for example adding a description or correcting a wrong time stamp, you have two sources of truth unless you write back the modified metadata to the original image (and the XMP sidecar file for RAW formats).

If an application only stores metadata you add and maintain in some proprietary "catalog", the data is hidden, invisible for other applications and you are locked into the application, unless you are prepared to loose data. Which is not good.

3. I just did facial recognition. I see that almost all files have pending metadata write-back,

I doubt this has something to do with face recognition. Usually, when IMatch imports metadata from an image, it produces a much richer set of metadata in XMP, applying industry and other standards to map existing metadata from EXIF, legacy IPTC, GPS etc. into the XMP record. Since this record is now much richer and holds more data, it should be written back to the image at some time.

metadata is also changed, e.g. XMP::exif\DateTimeOriginal. Why is that?

IMatch imports timestamps from a variety of sources when it imports an image. For example, the two create date and date subject created timestamps from the EXIF record in your image, plus, if available, time zone offsets and more. If the file had no XMP metadata record before, the now available time stamps in XMP will of course show as modified. Like many other freshly created tags, time stamps, digests (checksums) and more.

See How IMatch uses Date and Time Information for more information about how IMatch creates time stamps for your files. And the two other linked help topics for some background info on metadata and write-back.