High!


patterns (see attached image) on the surface looking like coincident 
surfaces - but that cannot be the case as I used only one isosurface.

Then I tried different max_trace_level values (3, 20 and 100), but the 
result remained the same. It is also not because of some interaction 
with the media, as I commented out the atmosphere shell, I still got 


Could the small dimensions of the landscape (1 POV unit = 1 km, the 
camera sits only 1.7 metres above the surce) be the cause?

See you in Khyberspace!

Yadgar

Now playing: Saint (Elton John)

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Download '2010-05-08 ghurghusht, 65 degrees west of substellar point from 1.7 metres,looking east, take 4 (max_trace_level 100).jp' (24 KB)

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=?ISO-8859-1?Q?J=F6rg_=27Yadgar=27_Bleimann?= <yaz### [at] gmxde> wrote:
> High!
>

> patterns (see attached image) on the surface looking like coincident
> surfaces - but that cannot be the case as I used only one isosurface.
>
> Then I tried different max_trace_level values (3, 20 and 100), but the
> result remained the same. It is also not because of some interaction
> with the media, as I commented out the atmosphere shell, I still got

>
> Could the small dimensions of the landscape (1 POV unit = 1 km, the
> camera sits only 1.7 metres above the surce) be the cause?
>
> See you in Khyberspace!
>
> Yadgar
>
> Now playing: Saint (Elton John)

Those could be related to the accuracy or max_gradient of the isosurface.  What
are the values for accuracy and max_gradient?

-Reactor

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High!

Reactor wrote:

> Those could be related to the accuracy 

Accuracy was originally set to 0.0001; I also tried values several 
powers of 10 smaller, as small as, 0.0000001, but instead I just got 
strange grainy shadows. Currently, I render at 0.001 (default)... the 
two images below show what I got with (first) and without (second) 
double_illuminate for the isosurface.

The camera looks eastward, the sun is 25 degrees above the eastern 
horizon, so the version without double_illuminate looks more believable 
to me...

See you in Khyberspace!

Yadgar

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Attachments:
Download '2010-05-08 ghurghusht, 65 degrees west of substellar point from 1.7 metres,looking east, take 5 (accuracy 0.001, double_' (14 KB) Download '2010-05-08 ghurghusht, 65 degrees west of substellar point from 1.7 metres,looking east, take 5 (accuracy 0.001, no doub' (5 KB)

Preview of image '2010-05-08 ghurghusht, 65 degrees west of substellar point from 1.7 metres,looking east, take 5 (accuracy 0.001, double_'

Preview of image '2010-05-08 ghurghusht, 65 degrees west of substellar point from 1.7 metres,looking east, take 5 (accuracy 0.001, no doub'

To me these images look like you definitely have /some/ kind of 
precision problem there. Looks to me like POV-Ray doesn't get enough 
"potential difference" for normals computation. Maybe making the 
isosurface function "sharper" might help somehow (if you know what I mean).

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High!

clipka wrote:
> To me these images look like you definitely have /some/ kind of 
> precision problem there. Looks to me like POV-Ray doesn't get enough 
> "potential difference" for normals computation. Maybe making the 
> isosurface function "sharper" might help somehow (if you know what I mean).

Normals? The texture of the isosurface doesn't contain any normal 
statement... and what do you mean with "sharper"? Adding more 
ruggedness, steeper ridges?

See you in Khyberspace!

Yadgar

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> High!
>
> clipka wrote:
>> To me these images look like you definitely have /some/ kind of
>> precision problem there. Looks to me like POV-Ray doesn't get enough
>> "potential difference" for normals computation. Maybe making the
>> isosurface function "sharper" might help somehow (if you know what I
>> mean).
>
> Normals? The texture of the isosurface doesn't contain any normal
> statement... and what do you mean with "sharper"? Adding more
> ruggedness, steeper ridges?

Obviously, with isosurfaces POV-Ray needs to have /some/ way of 
determining which direction the surface is facing at any intersection 
point it finds (the so-called "[unpertubed] surface normal vector").

To do this, AFAIK POV-Ray samples the function value of a few other 
points nearby, and from the difference in the function value computes 
the normal vector.

Now imagine what happens if the nearby points return values like:

  0.00000000000[01235]
  0.00000000000[42260]
  0.00000000001[73450]
-0.00000000000[03345]

(assuming your threshold is 0.0), with the digits in square brackets 
being lost due to rounding issues or some such.

With the function values of the nearby points varying only between 0, 
1e-11 and -1e-11, you can probably imagine that the normals computed 
from these values will be confined to a very small set of directions (I 
guess mainly parallel to the coordinate axes or tilted by 45 degrees), 
and subject to a lot of random noise.

If this should be the cause of your artifacts, it might help to modify 
the functions so that the function values increase in absolute 
magnitude, e.g.:

  0.00000000001[235]
  0.00000000042[260]
  0.00000000073[450]
-0.00000000003[345]

Then again, maybe the rounding happens somewhere else in the function. 
In that case you might need to get a clear picture of where exactly you 
lose precision in there.

(The most "precision-eating" operations in floating-point math are sums 
(or differences) of values with very different orders of magnitude.)

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> High!
>

> patterns (see attached image) on the surface looking like coincident
> surfaces - but that cannot be the case as I used only one isosurface.
>
> Then I tried different max_trace_level values (3, 20 and 100), but the
> result remained the same. It is also not because of some interaction
> with the media, as I commented out the atmosphere shell, I still got

>
> Could the small dimensions of the landscape (1 POV unit = 1 km, the
> camera sits only 1.7 metres above the surce) be the cause?
>
> See you in Khyberspace!
>
> Yadgar
>
> Now playing: Saint (Elton John)

Looks like floating point precision noise to me.


Alain

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High!

On 05/08/2010 08:19 AM, clipka wrote:

> If this should be the cause of your artifacts, it might help to modify
> the functions so that the function values increase in absolute
> magnitude, e.g.:
>
> 0.00000000001[235]
> 0.00000000042[260]
> 0.00000000073[450]
> -0.00000000003[345]
>
> Then again, maybe the rounding happens somewhere else in the function.
> In that case you might need to get a clear picture of where exactly you
> lose precision in there.
>
> (The most "precision-eating" operations in floating-point math are sums
> (or differences) of values with very different orders of magnitude.)

So, scaling the whole isosurface one or two orders of magnitude larger 
would solve the problem? Or do I have to add some "micro-noise" to the 
function?

See you in Khyberspace!

Yadgar

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> So, scaling the whole isosurface one or two orders of magnitude larger
> would solve the problem? Or do I have to add some "micro-noise" to the
> function?

I don't really know - I'm guessing here, so take it as hints for what to 
experiment with.

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