Roughly a year ago I re-worked the wood pattern and implemented an 
inbuilt function called f_wood() which could mimic it using the same 
basic approach long used for wood. I wasn't very satisfied with the 
result. My thinking was what we really needed was functionality which 
could create growth rings in a more realistic way.

One of the distribution types recently added to f_distribtion() was a 
normal/gaussian. With this in hand, I took another run at an f_wood() 
supporting more realistic growth rings. I left all turbulence and 
displacement to outside actors.

Attached is an image where in the top row the variation is accomplished 
with a random number stream adjusting a nominal ring width for each 
ring. In the second row the random values conform to a normal 
distribution - less often are extreme values seen. The columns represent 
the three return types.

Of note, the color_map for middle and right columns use the blend_mode 3 
feature of v3.8 to get the ring 'steps' to better show.

Bill P.


The documentation text from functions.inc:

Returns 0-1 ramp value within a ring and ring number. Optionally, the 
float portion of the ring number can represent the what fraction of the 
total rings in the tree a particular ring represents.

Values are returned as two 32 bit values encoded in the double space 
where the individual values are accessed via the f_dec2x_f32() function. 
The first 32 bit float is the 0-1 pattern value within the particular 
ring.  The second value is a 32 bit float representing this ring's count 
from the core by the integer portion of the float and optionally a 
[0..1) value representing thisRingCnt/treeRingsCount via the decimal 
portion.

The aim with f_wood is not to mimic the wood pattern, but rather provide 
a function returning growth rings which vary in width, a definable wood 
core and the ring number and fractional position in the wood log. The 
hope is upon this base, more natural algorithmic-ally generated 'wood 
patterns' can be created.

TODO. There are only 6-7 digits of accuracy in that second float! For 
typical ring counts OK, but maybe the user should be able to select the 
second return type?

Example:

#declare Core  = 0.02;
#declare Width = Core/2.0;
#declare Rings = int(((1.0-Core)/Width)+10);
#declare Seed  = 9871.123456;
#declare Mean  = 0.0;  // Used only if type is 1
#declare Sigma = 1/3;  // ditto

#declare Pigm00 = pigment {
     function {
         f_dec2x_f32(
             f_wood(1,f_hypot(x,y),Core,Width,Rings,Seed,Mean,Sigma),
         0)
     }
     color_map {...}
}

Though aimed at creating wood patterns, the function can be used with 
any gradient of values.

Eight parameters required:

1. Type. If 0, a random value stream is used to vary each ring width. If 
1, the random value stream conforms to a more nature like normal 
distribution. Option 1 is slower.

2. Input value. Usually a distance from a center, but any gradient works.

3. The core / starting radius. If positioning wood shapes in the pattern 
away from the core, this can be used to improve performance by skipping 
the ring calculations prior to those needed.

4. The nominal width of each ring in the 'tree log.'

5. The count of the rings in the tree. Used to calculated a stepped 
gradient of each ring from the core to the bark.

6. Seed for random number stream(s). It should be a float other than 0.

7. Mean value of the normal distribution. Used only if the type is 1.

8. Sigma value for the normal distribution. Used only if the type is 1.

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