if I have a source of 100 fps, how I have to do to convert to 25 fps?

[hello_hello]

People can play all kinds of goofy variants in their own entertainment silos. But just try passing those along to others (who may not have the same setup as you) and see how far you get without problems (not counting the legal kind here, just technical). - That kind of thing happening on p2p sites is one of the main causes for the many VH threads aimed at fixing problems.

Well you can't play an MKV using a DVD player any more than you can play a Bluray disc, but people still play Bluray discs.

If I'm passing stuff along to others I invariably ask the same questions, usually in the same order..... do you have a player which plays MKVs, do you have a player which plays AVIs, do you have a Bluray player, do you have a DVD player...... which helps establish the ability to play various file/video types in roughly the order of most convenient to least convenient.

My sister, who is not very tech savvy, finally bought a HDMI cable for her laptop a few weeks ago, so I figured if she's now in the land of "play almost anything", these days there's probably Amazonian tribes, still untouched by western civilisation, who prefer their video on thumb drives rather than discs. Although I still know a couple of people with "not pass along friendly" home entertainment set-ups. My father's still in DVD-land. No 720p video for him, mod16, progressive or otherwise.

Torrents? I'm pretty sure that since nearly none of it is legally legit, very little of it is technically legit. That's an argument?

To a certain extent. Much of it conforms to non-legit standards. "Non-legit" meaning not industry technical standards, but standards just the same. Some of the non technically legit content seems to have as many rules and regulations as the technically legit stuff.

Maybe you should throw out mod2 as well?

Mod2 has been permitted for years.

[jagabo]

This is a pointless argument.

[pandy]

Once again - ANY valid from codec syntax perspective combination of parameters is legit or allowed - industry standards is to make our life easier by reducing number of combinations. There can be situations where 720i100 can be desired (to fit for example in particular profile and level).
For sure i will not recommend to use this widely however it is common that many broadcasters offers 720p25 which seem to be completely not legit when industry standard says 720p50.
So don't get me wrong: if something is supported by codec then it is legit or allowed however it can be not recommended - that's all - for sure i will avoid claims that 720ixx is not allowed.