noise generators (forked from: Krita user community?)

[Matthew Woehlke]

Cyrille Berger wrote:
> On Sunday 24 February 2008, Matthew Woehlke wrote:
>> (Ok, erand48 will have to go, but that's completely
>> self-contained as I've assumed from the outset that my current
>> procedural PRN formula would be only temporary... not least because it
>> turns out rand48 isn't very *good*.)
> 
> The slight annoyance with rand48/erand48 is that those functions are not 
> thread safe, and as it is recursive noise generator, it doesn't work well if 
> we ask for an update of a rectangle of the generated images which is 
> different from the original rect.

Eh? I'm using the flavor that claims to be re-entrant (i.e. the one that 
takes the seed as an in/out). I actually do all my PRN-getting via a 
myRandom() function that takes the X,Y coordinates and random seed, so 
replacing mine with Krita's should be easy. (Or maybe Krita's would 
benefit from my approach?)

> For this we have a random number generator 
> in Krita, that takes as argument the current coordinate of the pixel. (The 
> formula is probably not very *good*, but I don't know any random number 
> generator formula that is very *good*).

I trust it also takes the random seed? I especially need that because I 
need several *different* random values per pixel (I want to say, as many 
as five); I handle this by simply adding a fixed value to the random 
seed, which should be "good enough".

I don't have the exact formula handy (ask tomorrow if you want it), but 
what I do is approximately:

- Run the rand() formula (a*s + b) on the X coordinate
- Initialize (unsigned short) s[3] = with the X (after "salt") and Y 
coordinates with the upper 16 bits XOR'd onto the lower 16.
- Call rand48 on s for Y % 37 iterations
- Call rand48 one last time, this time returning the result

What I found is that without the two flavors of 'salt', if I filled 
every pixel with the straight-up results of rand48 with the seed 
initialized as above, I got something that wasn't random in the 
slightest (diagonal saw waves, in fact). With only one salt, it was 
better, but still looked regular.

-- 
Matthew
(sorry, .sig file is on the other computer)