Op 12-4-2021 om 16:06 schreef Mr:
> Thomas de Groot <tho### [at] degrootorg> wrote:
>> Op 11/04/2021 om 14:48 schreef Mr:
>>> The polished one however really shows the repetition of procedural texturing too
>>> visibly, to the point that it breaks everything else, which would otherwise of
>>> course work fine for a polished material...
>>
>> I have a nagging question: Is the granite pattern a /repetitive/ pattern?
>>
>> Because I do not understand your comment about procedural texturing
>> here. In addition, I checked the result against a Real World example and
>> they are close. You will read in the header of granites21.inc that these
>> granites are based on Real World examples by the original author, and so
>> they are.
>>
>> --
>> Thomas
> 
> Since I'm invited to develop, I will, but please bear with some convoluted
> stream of gut feeling rather than thought process:
> 
> Is the pattern really mathematically repetitive? What matters more is, does it
> just even vaguely look like it is?
> 
> One of the key to overcome the "repetitive look" could be to break the scale
> invariance. what I call scale invariance would be a strong inherent component of
> computer generated fractal imagery: what is bigger looks quite like what is
> smaller. so we add a depth pass to refine detail adding say one voronoi cell
> inside another voronoi cell now it has depth of 2. this may look fascinating as
> it reminds the way nature does (a tree has branches over branches). It is even
> more so (fascinating) thanks to the computers ability to randomize that cell big
> or small, say with a rotation or whatever (e.g. tree's species branching could
> be described as generally between this and that angle for generation of branches
> out of trunk and these other angles for generation 2 of branches out of branches
> 1) so let's have the computer pick randomly a rotation value between these max
> and min and even state that it should never pick twice exactly the same
> number... sounds good,
> 

Ok. I follow you.

> BUT
> 
> Nature has kind of two margins for this randomness... The one that makes one
> recognize the represented object's characteristics, here they are perfect.
> AND the one that is over these boundaries but still occurs from times to times.
> The important thing is to really allow the additional level of detail (big or
> small) to go there, but still control this probability to a small amount and not
> have it constantly embedded in occurring variations, even if it's
> programmatically made to mathematically never be that exact one. Or else, a) it
> would look like repetition and b)more importantly,  it would break resemblance
> to the represented object, since we're not talking of the characteristic
> frequent values but rather the extremes.
> 

Understood.

> What the **** am I talking about HERE ? :-)
> 
> For this granite, here is a CC0 image that could come out as a pretty standard
> search result, rather consistent across the four or five first results:
> 
> https://en.wikipedia.org/wiki/Granite#/media/File:Fj%C3%A6regranitt3.JPG
> 

Yes, this is a typical granite. What we should understand however, is 
that granites can show a wide variety of grain-size differences, from 
fine-grained to rather coarse-grained and within this range, with fairly 
equal grain-sizes to more different grain-sizes either of the feldspars 
(like the image shown above) or the quartz grains (the white/translucent 
grains).

> If this was taken from say 1m away from surface with a 50/80mm focal length. And
> we tried to roughly approach the pov scene to cover the same field of view.
> Then if both in the photograph and rendered image one circled the salmon colored
> areas. One difference I would expect in resulting circle clouds to weight for my
> point in the balance: the radius would be almost constant in the pov result
> while the photograph shows isolated much bigger circles sometimes occurring. My
> gut feeling was just that the lack of these extreme occurrences gives the image
> the look of a fractal image from the nineties. Now that was a nice period, and I
> do value data preservation and archeology, I only fear the newer looking results
> are currently just not included along at all with POV package despite its being
> perfectly capable of it, as great images in these newsgroups frequently prove.
> I fear people tend to shy away from pov these days because of somewhat hidden
> modernity.
> 
I agree with you. In more "interesting" granite textures some random 
grain-size differences should be provided. Not yet sure how to do this 
presently, but I can imagine a couple of solutions. Something to 
investigate indeed and make available within future versions of 
granites, maybe through the macro structure proposed by Bald Eagle above.

> Maybe the first image looks much more natural to me because the additional black
> dimples and accurate roughness provide enough additional detail and variation
> especially at a lower frequency concerning the roughness because its shading is
> graded over all of the object.
> 

Quite probable indeed. :-)

-- 
Thomas

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