Improve documentation for random() expression

[Michael Koch]

Please add this to the documentation of the random() expression:

random(0) uses the variable 0 as a seed value. The memory of variable 0 is a union of int and double. When used by the expressions ld() and st(), it's double. When used by random(), then it's int. Important thing to remember: If you want to set the random generator to a seed value much different from 0, then you have to use rather big numbers, for example st(0, 2^41).

[Michael Koch]

Wiki formatting did change the text. I meant this:

... for example st(0, 2^41).

[Michael Koch]

After the recent changes in the random() expression, it's no longer required to use huge integer values as seed. I suggest to add this note to the documentation:

The default seed value is 0, because all internal variables are initialized with 0. If you want to use a different seed value, you must set the internal variable to an integer value, for example st(0,99). It doesn't work if you set it to a float value between 0 and 1.

[Michael Koch]

This note might also be helpful:

The random generator does always reproduce the same sequence of random numbers.
If you want a different sequence each time FFmpeg is started, you can randomize the generator by using the realtime clock as seed: st(0,1e6*time(0));
The factor 1e6 is required to make sure that multiple threads are initialized with different seed values. This factor can be omitted if the number of threads is limited to 1 with -filter_threads 1.

[Gyan]

You don't need to constrain the entire filtergraph to the same thread count. Append threads=1 as an option to the filter you wish to limit.

[Michael Koch]

Append threads=1 as an option to the filter you wish to limit.

Is this documented? I can't find it.

[Gyan]

It's a generic filter option, same as enable for timeline editing. I don't see it documented either.

[Stefano Sabatini]

Possibly fixed in commit:
​https://git.videolan.org/?p=ffmpeg.git;a=commitdiff;h=8ceb37d5ad730b93e4a277b74a5279c5d1f7864a

Please reopen if you have further suggestions with the wording.