On 9/13/21 11:47 AM, Alain Martel wrote:
> Very interesting. Mostly useful for wooden objects that are viewed from 
> a relatively short distance.

Thanks. Mostly, Yes. :-)

> Is there any way to have undisturbed rings in the centre with the 
> disturbance increasing as you get farther ?

First, to be clear, the turbulence in the previously posted image was 
accomplished with two 'external' to f_wood() turbulence warps. One for 
fuzz in the rings and another for the log level. The f_wood() function 
doesn't have any inbuilt turbulence - unlike the traditional wood pattern.

In povr certainly a fade is possible, as the Turbulence(1) and vector 
turbulence (DTurbulence) functionality of POV-Ray is brought out as 
inbuilt functions.

If not using that sort of functionality... I think one could probably do 
something like an inward black hole warp(2), apply turbulence, then an 
outward black hole warp. Similar to scaling patterns up (or down) prior 
to applying turbulence and then reversing it.

While, the fade used is the opposite you asked about, attached is a test 
isosurface image based upon f_wood() where I'm using the ring count 
value to fade the rings to nothing in height while moving out from the 
center.

FWIW, the, povr only, core SDL for the isosurface is:

//---
//...
#declare Fn00 = function (x,y,z) {
      f_wood(0,f_hypot(x,z),Core,Width,Rings,Seed,Mean,Sigma)
}
#declare Fn01 = function (v) {
      f_distribution(7,// A triangle distribution. Wikipedia version.
          f_dec2x_f32(v,0),
          A,B,C,ReturnMode,nullArg
      )* ((f_dec2x_f32(v,1)*-0.0035)+0.059) // The fade to nothing.
}
#declare Iso99 = isosurface {
      function { abs(y)-Fn01(Fn00(x,y,z)) }
...
}
//---

Bill P.

(1) - The povr branch has fixed the forever standing Turbulence() 
octaves distribution mean drift issue.

(2) - The povr branch has a black hole type 1 without the clamping 
and/or reaching outside the black hole radius issues of POV-Ray's 
black_hole. The povr type 1 forms would likely be needed for other than 
linear fading / strength 1.

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