[Sidefx-houdini-list] Rand() glitch

Mark Elendt mark at sidefx.com
Tue Oct 9 17:07:22 EDT 2007

On Tuesday Oct 09 at 21:23, Jordi Bares wrote:
> Thanks all for the comment, I though I was mad.
> BTW, this is something SideFX could wrap as a different function, don't you
> think?
> Like pirand()

The issue is that you want to be able to generate the same random
numbers (when you play your animation again, you want it to be the

So, rand(seed) needs to return the same value, for the same seed.

Consider two expressions: rand($F) and rand($F+1)
For frame 1, you have: rand(1) and rand(2)

But then, for frame 2, you have: rand(2) and rand(3)

This means that the first expression will give you the same random
number that you had in the second expression the previous frame.

Even if you did: rand($PT + $F*$PI) and rand($PT + $F*$PI + 1), you'd
still end up with the same problem.

Point 1/Frame 1:  rand(4.141596), rand(5.141596)
Point 2/Frame 1:  rand(5.141596), rand(6.141596)

So, again, the first expression will be giving you the same random
number you had the previous frame for the second expression.

I hope this makes sense...

Personally, when I need a random expression, I end up typing some
random digits after the decimal:
    rand($PT + .319591)
    rand($PT + .9381)

The only way to "fix" this, would be to have a unique ID for each
parameter which would be re-generated identically each time you loaded
the .hip file.  We don't have this right now.

Of course, if you wanted something that wouldn't be the same every
time you evaluated it, then, that's a whole different issue and is
relatively easy to solve.

I hope this makes sense.

> On 9/10/07 16:00, "jlait at sidefx.com" <jlait at sidefx.com> wrote:
> > On Tue, Oct 09, 2007 at 03:06:30PM +0100, Jordi Bares wrote:
> >> Hi, I am using rand() function to select randomly textures and I feed a
> >> different value every time (incremented by 1) and I see the rand() function
> >> is generating the same exact set of numbers shifted by one.
> >> 
> >> At the moment my solution is to feed the number^2 so I shift more than 1
> >> pictures and avoid the repetition but it is very ugly.
> >> 
> >> My question is... What is the best way to generate pseudo-random numbers
> >> based on integers?
> > 
> >     rand($PT + $F*$PI)
> > 
> >     I also recommend $E as a multiplier.

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