what media file formats are free software?

I don't think a format by itself is software. It can be encumbered by patents of course, which makes writing software for encoding and decoding problematic.

If you want to store your media in formats that writing free software for is unproblematic, then I'd recommend FLAC, Vorbis and Opus for Audio files (depending on what kind of compression you want to achieve) and VP9 or VP8 for the video part. Theora is also ok, but its quality is a bit lacking.

WebM/MKV are only containers, not codecs, so you can't make a blanket statement about them. As of now in a webm only vp8, vp9, vorbis and opus are allowed, so it's definitely a container format you can trust.

As for a list, look at this wikipedia article: https://en.wikipedia.org/wiki/Open_format And I'd also recommend reading up on the work of the Xiph.Org foundation https://xiph.org/

Edit: FLAC actually stands for "Free Lossless Audio Codec", but if your only worry was about storing audio data somehow lossless, of course just storing raw audio data is always possible and isn't patented. This raw audio data you could also compress with xz-utils, not as optimal as using FLAC files but still always a possibility. Storing video lossless and uncompressed is very unpractical because of the huge file sizes, so a good lossy codec is definitely needed. VP8/VP9 are from Google but Google provided a patent promise so they're safe to use for us. Google seems also to be working on VP10, BUT Xiph.Org and Mozilla are also working on a new video codec called Daala. Both are supposed to be released later this year.

Calling a file format "free software" makes no sense. It's not software, just a specification for how data should be stored. Free/libre software is perfectly able to read MP3 files.

The problem with MP3, as well as other MPEG formats, is that it's patent-encumbered, so restrictions are placed on their implementation. Software idea patents like these are incredibly dangerous and shouldn't exist at all; proprietary software suffers from this problem, too.

FLAC, uncompressed WAV, the Ogg formats, Speex, and WebM are all unencumbered by patents; these are the formats you should use. (There are other audio and video formats that aren't patent-encumbered, of course, but they're not worth using.)

> WebM/MKV are only containers, not codecs, so you can't
> make a blanket statement about them.

The WebM container is no different, technically, from Matroska. The whole point of it is it only supports certain codecs which aren't patent-encumbered, specifically so that you can make general statements/assumptions about a WebM file.

Personally I take the following ideal. I am more than happy to decode a proprietary/patent encumbered format with Free software but I would never distribute in it as it only encourages use of non-free software.

As others have said, anything that is in the WebM container is fine.

name at domain wrote:
> What about flac? If flac is not free software, then there is no other
> file format that provides the same sound quality?

FLAC is fully implemented with free software and (as far as I know)
unencumbered by patents.

As a practical matter, I think you'll find more support for FLAC using
native tags than Ogg FLAC files. But Ogg FLAC files are easily converted
to Ogg-less FLAC as the flac reference command-line program can decode
the Ogg FLAC and re-encode as FLAC without the Ogg container.

FLAC and the MKV container are also going to be supported in Windows 10 by default, but Microsoft will not support WebM or the codecs (VP8, VP9, Vorbis, Opus) by themselves even if they are royalty free under a permissive license. I know people can use VLC on Windows, but having native support means that Internet Explorer will play the formats in the