How do opensource projects appear that are analogues of proprietary products?

Question

open source

How do opensource projects appear that are analogues of proprietary products?
For example:
1) LibbreOffice - allows you to work with docx
2) Wargus - free Warcraft engine, works with original multimedia packages from Warcraft
3) ReactOS - almost free Windows
4) NTFS-3G - allows you to work with NTFS
5) Many with the prefix Open
6) fheroes2 is a free engine hero of might and magic

These are the ones I remembered and others.

Do they violate copyrights, if so, how do they get around the restrictions?
If I liked some program and I want to write the same one of my own, how do I understand where I cross the threshold of copyright?

Answer 1

If you look through a spherical horse in a vacuum, then it is enough to write a code that does the same thing, but in other "words".
Look at the Oracle and Google case, especially the rangeCheck function.
According to the points:
1. docx "free" format
2. The engine is free, but the files of models and other things you must have your own and licensed ones
3. The same, but written in your own words
4. NTFS is also quite free as a format
6. Engine there is, but what would you need to play! Original! game files
They don't break anything. As they violate, they will sue :) The authors of all sorts of cool mods who, after years of their work, receive a call and ask to destroy everything. I remember at least 5 such cases in the last year.
For example, the global mod The Middle-Earth Roleplaying Project (2008) received a warning in 2012 from Warner Brothers for copyright infringement and had to close

Answer 2

I'll upset you - "analogue" implies the execution of approximately the same list of tasks as the original. LibreOffice, on the other hand, is a pathetic parody not even of an office suite, but of a regular text editor. About Reaktos could not resist and laughed - over the years of development there was not even a beta, but it only works in a virtual machine.
This is the answer to your question - everyone just doesn't care about these parodies.

Answer 3

The fact that Libra opens Word docks is its feature, not its main functionality. She has quite a standard document format - OASIS. Well, about where it came from:
- back in the late 90s, there was such a German office StarDivision, which wrote StarOffice
- this office was bought by Sun, and since the office did not rest on it, most of it was made publicly available and replicated under the name OpenOffice. I also put my cat's paw into distributing it under FreeBSD :D (FreeBSD version of "Cat's Paw"). At the same time, Sun StarOffice existed
- then Sun, in turn, bought Oracle and OpenOffice became Oracle OpenOffice
- since the office didn’t knock anywhere either, he got rid of it, giving it to the Apache Foundation
- due to licensing disagreements (yes, there are license wars in OSS and what else!) Some of the developers separated and created The Document Foundation, which releases LibreOffice, while the Apache one stupidly died.