Et me voila again, just to prove I'm no coward
To keep my word (and to oblige myself, of course
) I downloaded and tested three format converters.
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Wings-1.4.1 looks simple and honest enough, and does the job. As in AC3D the files must be
imported rather than
opened - in my case import an obj and export a 3ds. Takes some time and patience.
Freeware.
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3dconverter claims it can do the job but when it comes to exporting, the shareware version invites you to register first ($50), and I'd rather not invest any money in something before I'm sure it does what I expect it to.
Some remarks I run into, not all positive, persuaded me that what I needed was something else. As said,
shareware as long as all you want is playing with it.
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AccuTrans (at3d2-12-1) looks
very professional, does the job and promises (didn't test that feature yet) it can be done in batch too.
Freeware. I think I'll stick with that one.
* * *
As far as the original problem is concerned (group and materials info getting lost when importing an .obj), it seems to be a bug of AC3D: the relative info is actually exported (other programs read it correctly), but while re-importing an obj AC3D ignores it even if the file is of its own making.
By the way, Stiglr was absolutely right: by reimporting the converted .3ds files the problem disappears. Thanks again!