I've run into an interesting and totally unexpected situation in v3.6.1 while
animating, trying to produce some random values from frame to frame. I'm seeing
repeating patterns in the values; nothing that I try eliminates them.

The problem arose by using seed(clock) or seed(frame_number) in my SDL scene to
generate different random seed streams for each frame (a standard method?) then
pulling only one rand() value from that per frame--which made the patterns very
apparent. I *thought* that by simply throwing some multiplier value 'M'
at seed(M*clock) or seed(M*frame_number)--just one time in my POV scene, to use
for the entire animation--would give me 'obvious' random numbers from frame to
frame. Unfortunately, not so. There are, to my great surprise, clear
*almost*-repeating rand() patterns, no matter what seed values I use. And it
shows up quite quickly in animation--usually over 5 to 10 frames. I've included
some small test code. A few newsgroup posts mention that the current
random-number-generator in POV may need to be improved; my animation indicates
that as well.

The near-repetition of rand() is more apparent with certain seed values than
with others, but there are--in my limited experimentation--always patterns, if
you're patient enough to discern them.  It's especially obvious when using
strict integer multiples for M--3,6,9,12, for example. Actually, any multiple
of any integer. So simply using clock or frame_number to change the seed value
from frame to frame presents a problem, as they both count up linearly.

I had always thought (perhaps wrongly) that any particular value for seed would
produce a totally different rand() sequence...different enough to 'appear'
random, in any case, with no discernible pattern. Instead, it's as if a great
many seed() values are falling across or interacting with some kind of built-in
seed 'frequency.' My own analogy is light waves interfering--two waves of
different frequencies (one of them non-varying), interacting to form crests and
troughs. Or pure sine-wave sound tones causing 'beats.'

So, practically speaking: How do I get *obviously* random values in animation,
across frames? I'm stuck. Trying different schemes to vary seed hasn't worked so
far (although I haven't exhausted the idea yet.) Perhaps this:

#declare temp_seed = seed(frame_number);
#declare real_seed = seed(M*frame_number*sqrt(rand(temp_seed)));

Or this..
#declare temp_seed = seed(frame_number);
#declare real_seed = M*sqrt(clock)*rand(temp_seed);

Haven't tried these yet. (Such ideas have endless permutations!) Of course, they
could simply be substituting one pattern for another.

Here's the test code, to show the situation. Run a 100-frame animation, with
clock 0 to 1. It gives seed() a different integer for each frame, counting up
in integer multiples. The resulting visual pattern becomes apparent quite
quickly. (For fun, set multiplier M to 1000 to see an extreme situation; or 12,
for yet another clear pattern):

#declare M = 1; // or any integer--or *any* value >= 1
#declare P = seed(M*frame_number)
sphere{0,1
 texture{
  pigment{rgb 1}
  finish{
   ambient 1
   diffuse 0
   }
  }
 translate 20*rand(P)*y
 }

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Not that I have any idea about the inner working of POV's rand() generator - but
did you also try *dividing* clock or frame_number?

Maybe the integer part of the seed() argument is thrown away and only the
fraction is used?

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Kenneth wrote:
> I've run into an interesting and totally unexpected situation in v3.6.1 while
> animating, trying to produce some random values from frame to frame. I'm seeing
> repeating patterns in the values; nothing that I try eliminates them.

No, because so is the nature of a simple random number generator.

	Thorsten

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"Kenneth" <kdw### [at] earthlinknet> wrote in message 
news:web.497a1e5215dcf5ebf50167bc0@news.povray.org...
> I've run into an interesting and totally unexpected situation in v3.6.1 
> while
> animating, trying to produce some random values from frame to frame. I'm 
> seeing
> repeating patterns in the values; nothing that I try eliminates them.
> ... snip ...
> So, practically speaking: How do I get *obviously* random values in 
> animation,
> across frames? I'm stuck. Trying different schemes to vary seed hasn't 
> worked so
> far (although I haven't exhausted the idea yet.)

You could try using the same seed in each frame, using the frame_number or a 
multiple of it to step through to a fresh part of the pseudo random sequence 
. The following example throws away one for each frame that's gone before. 
If you use 10 in a frame you could loop 10 times as far through the stream.

#declare P = seed(1);
#local I = 0;
#while (I<frame_number)
  #local ThrowAway = rand(P);
#end

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clipka wrote:
> Maybe the integer part of the seed() argument is thrown away and only the
> fraction is used?

Other way around, as far as I know. The seed value is truncated.

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Kenneth <kdw### [at] earthlinknet> wrote:
> I've run into an interesting and totally unexpected situation in v3.6.1 while
> animating, trying to produce some random values from frame to frame. I'm seeing
> repeating patterns in the values; nothing that I try eliminates them.

  POV-Ray uses a linear congruential generator as RNG, and those tend to
produce patterns in certain situations.

  What you can do to get a more random sequence is this:

1) On the first frame seed the RNG with whatever value (eg. 0).
2) Then pull a value with rand(), multiply it by a large value and
   #write it to a file. Then use the value in your scene in any way
   you want.
3) In subsequent frames #read that value from the file and give it to seed()
4) Repeat the steps 2 and 3.

-- 
                                                          - Warp

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"clipka" <nomail@nomail> wrote:
> Not that I have any idea about the inner working of POV's rand() generator - but
> did you also try *dividing* clock or frame_number?
>
> Maybe the integer part of the seed() argument is thrown away and only the
> fraction is used?

My own experience is that seed() values less than 1 result in *almost*
NON-varying rand()values--in my animation test, anyway, when extracting only
one rand() number per frame--so I'm guessing that POV rounds the seed value up
or down to an integer before pulling a rand()sequence. Using something like
seed(pi*frame_number) produces a pattern as well.

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Thorsten Froehlich <tho### [at] trfde> wrote:
> Kenneth wrote:
> > I've run into an interesting and totally unexpected situation in v3.6.1 while
> > animating, trying to produce some random values from frame to frame. I'm seeing
> > repeating patterns in the values; nothing that I try eliminates them.
>
> No, because so is the nature of a simple random number generator.
>

Alas, so it seems. It's interesting that, when using only one seed() value--as
in a still image--rand() has no problem producing endlessly varying values from
it. (I've never *noticed* a pattern.)  So rand() itself isn't the problem,
AFAIK.

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"Chris B" <nom### [at] nomailcom> wrote:

>
> You could try using the same seed in each frame, using the frame_number or a
> multiple of it to step through to a fresh part of the pseudo random sequence...
>
> #declare P = seed(1);
> #local I = 0;
> #while (I<frame_number)
> #local ThrowAway = rand(P);
> #declare I = I + 1; // added by Kenneth  ;-)
> #end

That's quite interesting, and it makes great sense--it pulls a nice random value
for each frame. I'll try it!  It's one of those solutions where I smack my
forehead and say, "Of course!" I had a funny feeling that some kind of #while
loop might be a solution, though how and where had me baffled. I'll report back
with the results.

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Warp <war### [at] tagpovrayorg> wrote:
> Kenneth <kdw### [at] earthlinknet> wrote:
> > I've run into an interesting and totally unexpected situation in v3.6.1 while
> > animating, trying to produce some random values from frame to frame. I'm seeing
> > repeating patterns in the values; nothing that I try eliminates them.
>
>   POV-Ray uses a linear congruential generator as RNG, and those tend to
> produce patterns in certain situations.
>
>   What you can do to get a more random sequence is this:
>
> 1) On the first frame seed the RNG with whatever value (eg. 0).
> 2) Then pull a value with rand(), multiply it by a large value and
>    #write it to a file. Then use the value in your scene in any way
>    you want.
> 3) In subsequent frames #read that value from the file and give it to seed()
> 4) Repeat the steps 2 and 3.
>
> --
>                                                           - Warp

Another interesting idea; thanks. I see that it would produce what's needed:
Each subsequent frame's seed is using the rand() value from the previous frame
for ITS value; so even though there are patterns, each new frame sees what
amounts to a different pattern (if I'm thinking correctly.)  Nice.

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