per-pixel random problems?
LukasT
lukast.dev at gmail.com
Thu Dec 11 23:05:38 CET 2008
On Thursday 11 December 2008 22:58:28 Matthew Woehlke wrote:
> Cyrille Berger wrote:
> > it's not erand... We need a non recursive function to be able to compute
> > it's value at every pixel independentely of other values. Being integer
> > based (unlike the current one) would be a huge bonus, since I discovered
> > since that time that floating point units in CPU are buggy and give very
> > different results from one arch to an other...
>
> Honestly I'm not clear why erand48 doesn't work (it doesn't, I've been
> playing with it in a stand-alone program). You'd think with suitably
> large and semi-prime numbers, you could get a distribution where A*x % b
> results in a halfway decent f(x) for reasonable B. Alas, seems not.
>
> Well, what I did for my plasma stand-alone is run (x%5 + y%37)
> iterations. It seems to give an acceptable result, but it's relatively
> expensive :-).
>
> > On Thursday 11 December 2008, Matthew Woehlke wrote:
> >> The png is substantial (500+kb), so I dumped it at
> >> http://img124.imageshack.us/img124/9986/noisetesthq8.png. (Um. And
> >> ImageShack no longer works without scripting or View Source; guess I
> >> won't be using them any more.)
> >
> > Here is what I get: http://cyrille.diwi.org/tmp/noise.png
>
> It's not as bad, but I would say there are artifacts there also. Hmm...
> I wonder how much difference using long double makes, it may be that
> you're getting 80-bit calculations and I am not?
>
> Anyway, this simple change... well, looks at least as good as yours,
> maybe a *tiny* bit better. I'm wondering if some of it is just in my
> head ;-). Regardless, this makes things on /my/ end a lot better:
>
> Index: image/kis_random_generator.cc
> ===================================================================
> --- image/kis_random_generator.cc (revision 895863)
> +++ image/kis_random_generator.cc (working copy)
> @@ -46,8 +46,8 @@
> // To plot it in Octave :
> // t = 1:1:100000;
> //plot (t, sort(0.5*(cos( cos(cos(t.*t.*t.*t))) + 1)));
> - // This function has a near-gaussian distribtution
> - int n = x + (y + 1) * d->seed;
> + // This function has a near-gaussian distribution
> + quint64 n = (quint64(x) * 1103515249) + (quint64(y) * 91573) +
> d->seed; return 0.5*(cos(pow(n, 4)) + 1);
> }
>
> (note also the spelling correction ;-) )
>
> I may keep fiddling with a pure-integer random function.
I use random in Sumi-e brush. You can look how it is used in brush.cpp
But basiclly
#ifdef _WIN32
#define srand48 srand
#define drand48 rand
#endif
and I'm using drand48();
Works quite fast :)
>
> --
> Matthew
> Please do not quote my e-mail address unobfuscated in message bodies.
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