Op 26/04/2021 om 12:47 schreef Bald Eagle:
> Thomas de Groot <tho### [at] degrootorg> wrote:
> 
> Thanks for making the render.  Looks better than my sphere - which looks like a
> cheap hard-rubber bowling ball, or the foam they use in sofa cushions.  :D
> 
> Needs 0.2-0.3 more turbulence. ;)
> 
>> The granite pattern is (almost) correct (without the turbulence).
> 
> I don't know what you mean.
> 
The original granite code contains turbulence. That is one of the 
reasons the original granite looks like a 'flow' pattern. The first 
thing I did was to comment out the turbulence; the granite became more 
'crispy' and closer to the real thing.

>> It also explains more comprehensively the original name: Mahogany. The
>> colour is similar to mahogany wood. Afaiac, I believe I should follow
>> that path in the first place and maybe disregard my own lighter version
>> (or not). Not sure yet. What do you think?
> 
> Well, there are myriad samples available at stonecontact-dot-com, all of which
> fall under the blanket/umbrella trade name of Dakota Mahogany and which have
> widely differing "color maps".
> 
I know. One of the problems is probably also that we do not know how 
much photoshopping has been applied to those photographs. However, I 
strongly believe that there is a link between the trade name and the 
deep hue of the mahogany wood. It would certainly help the sell.

> I think we need to alternate in our approach to this, and on the one hand try to
> formulaicly mimic the underlying patterns and sub-patterns, but on the other
> hand not forget that we can do some good ole' raytracing sleight-of-hand and do
> everything we can to fool the eye and mind into believing that they see
> something that is not there.
> 
Oh yes.

> One thing I thought of was to take a/some granite photos and run some
> edge-finding filters on them to see what pops out.
> 
> I'm curious about the rgb color mapping - the individual values - but also,
> given an individual grain region, what is the rgb variation across it?  How much
> secondary or tertiary pigment is patterned into the grain?
> 
That is a tedious exercise ;-) I did it years ago to get correct colour 
values for Mediterranean-style roof tiles (for which I used the cells 
pattern, btw). Can be done of course; I shall put it on my 
ToDo/ToExplore list.

> Is there a way to estimate the grain sizes from the photos?  Or maybe there is
> statistical data in the literature?
> 
That is more difficult. Literature tells me that grain sizes range from 
microscopic to about 10-15mm in most cases and within the same granite, 
with sometimes larger minerals (phenocrysts) in the more porphyric 
varieties. See my answer to Mr.

> I ran across a neat conformal mapping image with circle packing that looked like
> a good mathematical way to get discrete regions with good grain-size variation.
> 
> AFAIK, we're using straight Perlin noise right now for granite.   Doing floor()
> on each of the axes produces the cells {} pattern.  I'm thinking if we could
> split the difference.... maybe with the step trick and/or select ()
> 
> Also need to keep in mind that the base pattern might just be a guide, and the
> final "look" is going to likely be a result of layering textures/materials.
> Hard to visualize.
> 
> And lastly, there were the brick pattern and random hexagon color threads that
> employed methods we might be able to use here.
> 
All the above: yes; might need additional investigation. Concerning the 
hexagon pattern: many if not most of the granite minerals are hexagonal. 
That would mean it would be more appropriate for granites. However, I am 
afraid it would look 'circular' in practice.

>
http://news.povray.org/povray.general/thread/%3C5ae17c9c%40news.povray.org%3E/?mtop=422221&moff=10
> 
>
http://news.povray.org/povray.binaries.images/thread/%3Cweb.5ad633941828641ca47873e10%40news.povray.org%3E/?mtop=422589
> 

-- 
Thomas

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