Originally Posted by buckethead 
Originally Posted by Quension I think the answer to your question is going to depend on what you're using to tag the files.
That I am not sure on yet, been trying out iTag and XnView.
iTag only alters the metadata; it will never degrade pictures.
XnView depends on exactly what steps you take. For the tags, you open the image and then Edit->Metadata->Edit IPTC data, Keywords tab. If you alter the keywords and then use the Write button on that dialog, it only alters the metadata, and there will be no degradation.
If you merely Apply and then close that dialog, followed by any form of File->Save/Export, it will rewrite the image data as well, and degradation will occur.
Originally Posted by Marc Sabatella But given that it's *more* work to go to all that trouble versus simply updating the EXIF, it's not just a question of quality - it's a matter of someone having to be deliberately stupid or mean to write a program that went out of its way to degrade an image when the simpler solution would *not*.
It's not more work, just different work. XnView is actually a good example; if the primary purpose of the app is viewing and conversion, then the image data and relevant metadata will be converted to in-memory formats on load and completely rewritten from scratch on save, according to the export/conversion format chosen.
Selectively modifying file metadata in-place for a specific format is a different task entirely, and is iTag's primary purpose. XnView accomplishes this by confining the task to the IPTC dialog itself. (IrfanView is similar.)