Thomas de Groot <tho### [at] degrootorg> wrote:
> Op 27/10/2021 om 22:44 schreef Samuel B.:
> > Thomas de Groot <tho### [at] degrootorg> wrote:
> >> Ah... yes indeed. I have not considered your first method, but
> >> considered the isosurface one. However, I also cringe at the implied
> >> render time.
> >
> > (...) care must be taken when choosing the fracture planes for a given mineral
(...)
> >
> Correct. This needs a good knowledge of crystallography indeed. I am a
> layman on this really...

It doesn't really take any in-depth knowledge. Just a bit of time reading and
browsing through images of minerals. (Which might be too time-consuming,
depending...)

> > And then there are clouds... Similar to fractures, the clouds you see in
> > minerals are often reflective/refractive gas or liquid pockets, and so they
> > might not easily be replicated with scattering media. (...)
> >
> Inclusions, yes. Scattering media is a poor substitute. There are the
> little foreign grains that got included, and the fluid inclusions. Those
> are really interesting!

Yeah, there are many fascinating inclusions that can occur in minerals. I
remember seeing a photo of some white crystals occasionally found in obsidian.
IIRC, they formed in groups of four tetrahedral crystals meeting at their
corners. Pretty much like the first iteration of a Sierpinski tetrahedron. If I
come across the image again, I'll post it. (Image search isn't turning up
anything relevant, and I originally found it in a book.)

> [Aside] in a very distant past, my wife did a study on fluid inclusions
> in quartz crystals from Greece. She could, by cooling the crystals in
> liquid nitrogen and then slow heating, determine the original
> temperature of the liquid in which the crystal grew. Original pressure
> was somehow derived from other factors. I forgot. [/Aside]

Crazy! I haven't a clue how those environmental factors could possibly be sussed
out. That's hacking, as far as I'm concerned... Physical hacking, not digital,
which is even cooler. She sounds like an awesome woman :)

> But, those fluid inclusions are difficult to model. I was thinking about
> clouds of bubbles in liquid (there are a couple of scene files drifting
> around in the pov world; I think I have a couple of those somewhere)
> using a gaussian distribution scheme...

Inclusions are pretty much the main impediment to making better mineral renders.

And inclusions take on many forms. Not all exhibit crystalline features when
viewed with the naked eye (e.g. light reflecting at certain angles only). Some
do, though. I had (or still have) a Herkimer 'diamond' that apparently has an
inclusion in the shape if a tiny quartz crystal. The bubble really looks like a
tiny, doubly-terminated quartz crystal. I'll have to take a photo of that.

> > Besides the symmetry, another thing I noticed about the crystal libraries in KS
> > is that the secondary and tertiary faces (modifications) tend to be somewhat
> > exaggerated. (...)
> >
> Yes, I have been wondering about that. Not being an expert, I have been
> hesitating between the different models proposed.

Eh, nothing is written in stone. (Haha, except stones.) But there are always
outliers to these things. Depending on the conditions in which a chemical is
grown, modifications may become more or less prominent. You never know.

Sam

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