Lance Birch nous apporta ses lumieres en ce 2007/10/16 04:11:
> Woody wrote:
>> I was testing out some of the Isosurfaces from the documentation and came
>> across some thing which I'm not sure if its a bug, a limitation, or so 
>> I'm
>> just not doing it right.
>>
>> The code below renders to the attached image. There seems to be some 
>> weird
>> normals, shading, or with the general shape. But if I change the viewing
>> coordinates to something else like <3,2,5> everything looks fine.
>>
>> Can anybody tell me what I'm doing wrong or make any suggestions?
>>
>> Image at <web.4712c21a4144865430d4452f0@news.povray.org>
>>
>>
>> #include "colors.inc"
>>
>> camera {
>>  location <2,2,2>
>>  look_at <0,0,0>
>> }
>>
>> isosurface{
>>         function{ abs(x)+abs(y)-1}
>>         contained_by {box{-1,1}}
>>         pigment{color White}
>> }
>>
>>
>> light_source {
>>  <2,2,2>
>>  color White
>> }
> 
> Woody, I received your bug report to the povray.bugreports group, 
> however this is *not* a bug.  The messages output by POV-Ray at the time 
> of rendering should provide the clue:
> 
> "Shutdown Warning: The maximum gradient found was 1.375, but the 
> max_gradient of the isosurface was set to 1.100.  The isosurface may 
> contain holes!  Adjust max_gradient to get a proper rendering of the 
> isosurface."
> 
> Therefore, to resolve the issue you're seeing (and as Mark also noticed) 
> you have to include a higher max_gradient value in your isosurface, 
> which can be done as follows:
> 
> isosurface{
>   function{ abs(x)+abs(y)-1}
>   contained_by {box{-1,1}}
>   max_gradient 1.375
>   pigment{color White}
> }
> 
> I'd recommend checking the isosurfaces area in the documentation to read 
> more about max_gradient and why this happens as it's something that 
> needs to be watched out for.  Note that the higher the max_gradient 
> setting is, the slower the isosurface takes to render, which is why the 
> default value of max_gradient is set so low.
> 
> As an aside, please ensure in the future when submitting possible bugs 
> to the povray.bugreports group that you check that the provided SDL is 
> syntactically correct, as the SDL you sent with your post contained a 
> syntax error that made it difficult to determine whether the problem you 
> were having was with the syntax or with something else.
> 
> Kind regards,
> 
> Lance Birch.
> 
> thezone - thezone.firewave.com.au
A hi max_gradient don't always mean slow render, and a small gradient don't 
equate to fast render. A gradient of 1.4 is not large, and in this case, setting 
max_gradient to 1.3 should be enough.
If your function have area of small gradient (close to 1 or less close to the 
tresshold) and area of large gradient (over 1000), THEN you get slow rendering, 
as the low gradient areas get hugely oversampled. A possible solution for that 
can be to use the log of the function as it could compress the hi gradient areas.

-- 
Alain
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