Op 27/10/2021 om 22:44 schreef Samuel B.:
> Thomas de Groot <tho### [at] degrootorg> wrote:
>> Crystal replaced by a mesh2 version. Less grainy by not using
>> stochastic. Render time about 27 minutes. Collected photons, about 1 GB.
>>
>> Additional scene developments pending.
> 
> It's looking better and a little less noisy now. It also seems a bit more
> bluish. Looking online, this mineral has a range of hues spanning between
> colorless, pale yellow, and pale/bluish green. All seem to be valid.
> 
Yes, in between renders I added a bit more blue.

>>> How long did this take to render? The file name says 004d, so I really hope it
>>> wasn't 4 days ':/
>>
>> Oh no! That is the version number ;-) This render was pretty fast: less
>> than an hour iirc.
> 
> Thank goodness, lol. That makes more sense.
> 
Yes, it is rather fast.

In my latest experiment, I cranked photon spacing down to 0.001 (for the 
examples shown here, this was 0.004) but then POV-Ray stopped along the 
parsing way, with an 'out of memory' message. Even 0.004 might be 
overkill for all I know; I shall need to investigate.

>>> I'd like to discover a cheap and realistic way to add internal fractures to
>>> mineral renders. Apophyllite is one of those minerals prone to being found in a
>>> fractured state. I have two ideas in mind, but both are rather expensive: 1)
>>> height fields intersecting not only each other, but also the crystal shape
>>> (which is itself an intersection); or 2) isosurfaces. Both can be very, very
>>> slow. Sometimes I wish media had an ior block.
>>
>> Ah... yes indeed. I have not considered your first method, but
>> considered the isosurface one. However, I also cringe at the implied
>> render time.
> 
> Both methods are going to be slow, yeah. And care must be taken when choosing
> the fracture planes for a given mineral. Apophyllite appears to exhibit basal
> cleavage, so its fractures tend to be horizontal more often than not.
> 
Correct. This needs a good knowledge of crystallography indeed. I am a 
layman on this really...

> 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. (It seems to me that there
> was an experimental version of POV-Ray a long time ago which allowed one to
> specify the reflectivity of participating media. I wonder what happened to
> that?)
> 
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!

[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]

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...

>> Another thing I would like to do is model more asymmetric crystals, like
>> they occur in nature. The KrystalShaper models are too perfect for our
>> grubby little hands. :-) Maybe by carefully manipulating the set of
>> planes in the intersection, or working directly on a mesh2 model
>> converted back to .obj for instance, and load it up in our favourite
>> modeller...
> 
> Personally, before going that far, I'd just change the distance values of each
> crystal face in KrystalShaper, and not even mess with any other program save
> POV-Ray. KS is great like that; you can make entirely new minerals if you wish.
> And there's always doing what I did: copy the HKL+distance data to POV-readable
> arrays. Then you could take it a step further and add some randomization before
> the HKL+D data is used to make the intersection.
> 
Yes, I think you are right there.

> 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. My alum render, for instance, was not what you'd consider a typical
> specimen; you'd usually only see eight main faces in real life. And the
> apophyllite model is the same way: in nature you usually only see cuboids
> truncated with a steep pyramid. (The bevel between pyramid faces is atypical.)
> None of this is a bad thing, of course, and it helps to show us just how a
> mineral can form secondary & tertiary faces.
> 
Yes, I have been wondering about that. Not being an expert, I have been 
hesitating between the different models proposed.

-- 
Thomas

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