Hello-

This is my first posting (just (re) started using POV).  The enclosed image
is made from an isosurface for the towers and a height field for the
swimming thing.  

The isosurface is a repeating, with modifications at each repetition,
function using the mod (to cause repeats) and div (to discretely change
certain parameters for each copy); following ideas presented in some recent
discussion on POV news. I am quite happy with the result, except for a few
things:
1)it renders quite slowly (this took about 30 hours on a 1.4GHz PIII, of
course radiosity and focal blur are used.)
2)the wake needs work
3) there is an artifact (boxed in red) of a sort I have not seen before in
the isosurface. I imagine upping the accuracy 0.002 and max_gradient 40
(current settings) will help.

Interestingly POV reports a max_gradient of over 3000 when it renders.  I
suspect this is due to the DIV statements discontinuous nature; but it makes
it hard to know what the max_gradient of the continuous surface should be.

Comments?
Questions?
Suggestions?
Look under the hood?

Regards,
Leo

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leosti wrote:
> Hello-
> 
> This is my first posting (just (re) started using POV).  The enclosed image
> is made from an isosurface for the towers and a height field for the
> swimming thing.  
> 
> The isosurface is a repeating, with modifications at each repetition,
> function using the mod (to cause repeats) and div (to discretely change
> certain parameters for each copy); following ideas presented in some recent
> discussion on POV news. I am quite happy with the result, except for a few
> things:
> 1)it renders quite slowly (this took about 30 hours on a 1.4GHz PIII, of
> course radiosity and focal blur are used.)
> 2)the wake needs work
> 3) there is an artifact (boxed in red) of a sort I have not seen before in
> the isosurface. I imagine upping the accuracy 0.002 and max_gradient 40
> (current settings) will help.
> 
> Interestingly POV reports a max_gradient of over 3000 when it renders.  I
> suspect this is due to the DIV statements discontinuous nature; but it makes
> it hard to know what the max_gradient of the continuous surface should be.

The max_gradient does not have to do anything with the continuity of the 
*surface* but with the continuity of the *function*.  When you generate 
a repetitive function with mod() but vary the different repetitions it 
will be very hard to get a continuous function (to be precise you will 
have to make sure the variation drops to zero at the repetition 
borders).  Looking at your image - you'd probably have it much easier 
(and faster) by using a separate isosurface for each of these 'columns'.

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 25 Oct. 2003 _____./\/^>_*_<^\/\.______

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Christoph Hormann wrote:
>
>The max_gradient does not have to do anything with the continuity of the
>*surface* but with the continuity of the *function*.  When you generate
>a repetitive function with mod() but vary the different repetitions it
>will be very hard to get a continuous function (to be precise you will
>have to make sure the variation drops to zero at the repetition
>borders).  Looking at your image - you'd probably have it much easier
>(and faster) by using a separate isosurface for each of these 'columns'.
>
>Christoph
>
>POV-Ray tutorials, include files, Sim-POV,
>HCR-Edit and more: http://www.tu-bs.de/~y0013390/
>Last updated 25 Oct. 2003 _____./\/^>_*_<^\/\.______
>

Yes, it is the discontinuous nature of the DIV function itself that causes
the problem.  Computing just one 'column' gives a MAX_GRADIENT of 77.
Still quite high, but better than the 59000 reported for the whole.

My concern, really, as that the artifact is showing up in the continuous
section.  I am going to rerender with the MAX_GRADIENT set to 77 and see if
this fixes it.

Leo

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Leo wrote:
> [...]
> 
> Yes, it is the discontinuous nature of the DIV function itself that causes
> the problem.  Computing just one 'column' gives a MAX_GRADIENT of 77.
> Still quite high, but better than the 59000 reported for the whole.
> 
> My concern, really, as that the artifact is showing up in the continuous
> section.  I am going to rerender with the MAX_GRADIENT set to 77 and see if
> this fixes it.

To clear up two misconceptions here: the maximum gradient reported by 
POV is not necessarily the real maximum gradient occuring (which would 
be infinite in cases of discontinuities anyway).  And too low 
max_gradient settings can lead to artefacts in areas with lower 
gradients (which should be perfectly clear, otherwise only the grandient 
at the surface would be relevant).

And it is not that the use of a discontinuous function anywhere in the 
isosurface automatically leads to an infinite gradient, you just have to 
know what you are doing.

Christoph

-- 
POV-Ray tutorials, include files, Sim-POV,
HCR-Edit and more: http://www.tu-bs.de/~y0013390/
Last updated 25 Oct. 2003 _____./\/^>_*_<^\/\.______

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