Hello, 
I was playing a little bit with the paragraph in the official
documentation explaining quadratic splines for lathe objects, and I have
encountered was seems (to me) to be a bug with the orthographic camera
(see scene source below).I'm using PovRay 3.1g (official) and MacMegaPov
0.6 on a Macintosh, and the pb appears in both.

Can anybody else confirm this on other platforms? Or is it a well known
problem? Or is it me who is doing something stupid ?

Cheers,
Stephane.


 // this file seems to demonstrate a bug involving orthographic cameras
 // and lathe objects with quadratic splines
 // Bug shows up both in 3.1g official and MegaPov 0.6 unofficial
 
 
  #include "colors.inc"


// the offending camera
  camera {
    orthographic
    up <0, 5, 0>
   right <5, 0, 0>
   
  // the following line was "location <2.5, 2.5, -10>" in the
documentation
  // changing the "2.51" a little bit changes the appearance of the bug.
  location <2.5, 2.51, -10>
  
   look_at <2.5, 2.5, 0>
  }

// use the following camera to see how the lathe object is suppose to be
/*
 camera {
   angle 10
   location <1, 9, -50>
   look_at <0, 2, 0>
 }
*/

 light_source {
   <20, 20, -20> color White
 }


 background{Gray50}

 #declare rayon_angle_plateau = 1.1;

  /* set the control points to be used */
  #declare Brown_Point = <0,0,0>
  #declare Red_Point    = <rayon_angle_plateau, 0.50, 0>;
  #declare Orange_Point = <1.75, 1.00, 0>;
  #declare Yellow_Point = <2.50, 2.00, 0>;
  #declare Green_Point  = <2.00, 3.00, 0>;
  #declare Blue_Point   = <1.50, 4.00, 0>;
  #declare Tan_Point = <0.0, 5.00, 0>

  /* make the control points visible */

  cylinder { Brown_Point , Brown_Point - 20*z, .05
    pigment { Brown }
    finish { ambient 1 }
  }
  cylinder { Red_Point, Red_Point - 20*z, .05
    pigment { Red }
    finish { ambient 1 }
  }
  cylinder { Orange_Point, Orange_Point - 20*z, .05
    pigment { Orange }
    finish { ambient 1 }
  }
  cylinder { Yellow_Point, Yellow_Point - 20*z, .05
    pigment { Yellow }
    finish { ambient 1 }
  }
  cylinder { Green_Point, Green_Point - 20*z, .05
    pigment { Green }
    finish { ambient 1 }
  }
  cylinder { Blue_Point, Blue_Point- 20*z, .05
    pigment { Blue }
    finish { ambient 1 }
  }
 cylinder { Tan_Point, Tan_Point- 20*z, .05
    pigment { Tan }
    finish { ambient 1 }
  }
  /* something to make the curve show up */
  lathe {
    quadratic_spline
    7,
    Brown_Point,
    Red_Point,
    Orange_Point,
    Yellow_Point,
    Green_Point,
    Blue_Point,
    Tan_Point
    pigment { Blue }
    finish {
      ambient .3
      phong .75
    }
  }

Post a reply to this message

As far as I remember the control points for 
a lathe should be 2 dimensional. I.e. not 
3 dimensional as you have made them.

And are you sure about your selection of 
the first and last points and the in your 
quadratic_spline ?

Are you aware that these have special meanings ?

And be careful with points in the lathe that 
will "force" the "curve" to move into the 
negative side of the v-axis (i.e. "the left side").

I don't have time now to check your code or 
to explain more. Maybe tomorrow...


-- 
Best regards,

Tor Olav

mailto:tor### [at] hotmailcom
http://www.crosswinds.net/~tok/tokrays.html


Stephane Nicolet wrote:
> 
> Hello,
> I was playing a little bit with the paragraph in the official
> documentation explaining quadratic splines for lathe objects, and I have
> encountered was seems (to me) to be a bug with the orthographic camera
> (see scene source below).I'm using PovRay 3.1g (official) and MacMegaPov
> 0.6 on a Macintosh, and the pb appears in both.
> 
> Can anybody else confirm this on other platforms? Or is it a well known
> problem? Or is it me who is doing something stupid ?
> 
> Cheers,
> Stephane.
> 
>  // this file seems to demonstrate a bug involving orthographic cameras
>  // and lathe objects with quadratic splines
>  // Bug shows up both in 3.1g official and MegaPov 0.6 unofficial
> 

... 8< ... Snip ... 8< ...

Post a reply to this message

In article <1env1l1.azsa4n1mhno7iN%Stephane.Nicolet@ens.fr> , 
Ste### [at] ensfr (Stephane Nicolet) wrote:

> I was playing a little bit with the paragraph in the official
> documentation explaining quadratic splines for lathe objects, and I have
> encountered was seems (to me) to be a bug with the orthographic camera
> (see scene source below).I'm using PovRay 3.1g (official) and MacMegaPov
> 0.6 on a Macintosh, and the pb appears in both.
>
> Can anybody else confirm this on other platforms? Or is it a well known
> problem? Or is it me who is doing something stupid ?

Try adding the "sturm" keyword to the lathe object.


      Thorsten

Post a reply to this message

Thorsten Froehlich <tho### [at] trfde> wrote:

> > I was playing a little bit with the paragraph in the official
> > documentation explaining quadratic splines for lathe objects, and I have
> > encountered was seems (to me) to be a bug with the orthographic camera
> > (see scene source below).I'm using PovRay 3.1g (official) and MacMegaPov
> > 0.6 on a Macintosh, and the pb appears in both.
> > Can anybody else confirm this on other platforms? Or is it a well known
> > problem? Or is it me who is doing something stupid ?
> 
> Try adding the "sturm" keyword to the lathe object.
>       Thorsten

Thanks, it was indeed a problem with the accuracy of the PovRay solver,
apparently, and the Sturmian solver works OK without being really slower
(rendering time is 30 sec vs. 27 sec). Now the fact that the results are
so unexpected suggests that 
a) PovRay *should* maybe switch automatically to the Sturmian solver for
quadratic spline lathes
b) or (at least) we could add a note in the beginning of the "advanced
shapes" of the documentation about the dangers of the default
intersection solver, and a pointer to the sturm keyword paragraph.

Stephane.

Post a reply to this message