[FFmpeg-devel] [PATCH 2/2] doc/utils/eval: clarify meaning of random* seed value

[Stefano Sabatini]

On date Wednesday 2024-01-03 12:20:12 +0100, Michael Koch wrote:
> > Possible address trac issue:
> 
> > http://trac.ffmpeg.org/ticket/10763
> 

> I don't like the random generator as it is, because the first two random numbers
> are very close to zero, as can be shown with this command line:
> ffmpeg -loglevel repeat -f lavfi -i nullsrc=size=1x1,format=gray -vf "geq=lum='print(random(0));print(random(0));print(random(0));print(random(0))'"
> -frames 1 -y out.png 0.000000 0.000091 0.285346 0.929202 This behaviour can
> be improved by inizializing the generator with a large number as seed value.

OTOH this seems only to happen with 0, which is also the default seed
value used by the random generator (since all variables are set to 0).

$ echo "st(0, UINT64_MAX/3); st(1, 10); while(gt(ld(1), 0), print(random(0)); st(1, ld(1)-1))" | tools/ffeval 
0.666667
0.333374
0.616496
0.023515
0.931519
0.317050
0.393449
0.969531
0.842403
0.129183
=> 0.000000
$ echo "st(0, UINT64_MAX/2); st(1, 10); while(gt(ld(1), 0), print(random(0)); st(1, ld(1)-1))" | tools/ffeval 
0.500000
0.500091
0.785217
0.673490
0.103073
0.829142
0.144794
0.508226
0.735092
0.655693
=> 0.000000
$ echo "st(0, UINT64_MAX/7); st(1, 10); while(gt(ld(1), 0), print(random(0)); st(1, ld(1)-1))" | tools/ffeval 
0.285714
0.571498
0.855853
0.398648
0.009648
0.802972
0.608529
0.007854
0.405221
0.959683
=> 0.000000
$ echo "st(0, UINT64_MAX/13); st(1, 10); while(gt(ld(1), 0), print(random(0)); st(1, ld(1)-1))" | tools/ffeval 
0.384615
0.923180
0.593572
0.754748
0.415309
0.067556
0.905708
0.928599
0.448587
0.772728
=> 0.000000
$ echo "st(0, UINT64_MAX/6); st(1, 10); while(gt(ld(1), 0), print(random(0)); st(1, ld(1)-1))" | tools/ffeval 
0.833333
0.166733
0.451010
0.605203
0.240940
0.007228
0.778975
0.354595
0.413333
0.898240
=> 0.000000
$ echo "st(0, 0); st(1, 10); while(gt(ld(1), 0), print(random(0)); st(1, ld(1)-1))" | tools/ffeval 
0.000000
0.000091
0.285346
0.929202
0.239519
0.140744
0.686872
0.275489
0.537404
0.583466
=> 0.000000

(BTW it could be handy to have an inc and dec function as well to
simplify this kind of loops.)

> I'm not sure if it's a good idea to share the same variables for ld() and
> st() (as double) and random() (as unsigned int).

All functions work with double precision floating point numbers, but
they can be used also to represent integers, but with a few
limitations (e.g. you cannot represent many integers with a float
exactly).

> Wouldn't it be better to use separate variables for random()?
> Michael

One idea would be to use a random_seed(idx) function, but this would mean
that we need to use separate registers only for the random function,
and this would also break backward compatibility.