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This XMP/IPTC data is a pain. It seems the camera and ViewNX2 stores it in the NEF and it's with or a part of the EXIF data. Then ON1 Perfect Browser stores this data in sidecar files as I recall. Now it appears that ViewNXi and NX Studio stores XMP/IPCT data in *.nksc side car files.
Do I understand that ON1 and Nikon software as well as most other software will find the XMP/IPCT data in the NEF but will be oblivious to XMP/IPCT data stored in side car files other than their own?
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Yes to the On1 and Nikon software question. Once the ITPC data is entered into the NEF, eg. via Nikon transfer(which still embeds this data into the NEF) or ViewNX2 ViewNXi, CNX-D .. and pretty much all other software can read that this info is in the relevant section of EXIF data and can use it.
if you use ViewNXi to add keyword info(ie. via this sidecar method of ITPC) no other software(other than CNX-D .. I think .. maybe Studio too, but not sure) will see it.
This is one my dislikes of the sidecar method. There's nothing inherently wrong with it, but just like a jpg is a jpg, why can't a keywording/tagging/edit sidecar file just work across any and every software program and Operating System platform.
If you choose to try Photo Viewer, be sure you only install PhotoViewer at the time of install .. and yes it still works in Win10.
Best way to use it too, is to point just one of your image/photo folders to it so it can reference only this one. If you allow it to use any and many folders, it will create annoying duplicates of images and so you inflate it's database for no real gain.
Also remember you need to have the Nikon NEF codec installed onto the Windows PC for it to work properly.
The way I use it: I have two main folders(actually separate drives on my PC, one where I only download the recent images(eg. for the entire year), I call this drive B:\ AND one where I keep a local store of my entire photo image collection, which is my W:\ drive. This entire collection on W drive, is also backed up to two other drives on a NAS box.
In PhotoViewer I only have it pointed to the W drive, which is my entire collection. Don't absolutely need a local drive to store all your images(but I have plenty of drives) and nice to have local access for when (say) the network goes down(stupid switch!) and a bit more speed to boot.
I only use keywording/searching for my own personal needs, ie. just easier to find specific images if I need too. And like I said earlier, Windows search also recognises that this image keyword data base is there, and can also use it .. and because you installed the Nikon NEF codec, you can also add/edit/remove keywords directly via Window Explorer too. That's tedious unless you find a spelling mistake or something like that(easy to do when doing large quantities of keywording. PhotoViewer is much easier to add keywording to images.
It also has a nice feature where it uses facial recognition and can help keyword for images of people. Works well. Not perfectly, just good. 90% accurate I'd guess.