In the scene below I get a bicubic patch object where many triangles are
missing. Scaling the patch bigger makes the problem disappear while scaling
the patch smaller makes the problem worse. Increasing u_steps and v_steps
also makes the problem worse, so the problem apparently is dependent on the
sizes of the individual triangles in the bicubic patch object.

In that regard it looks like the well-known problem with precision errors
that happen with very small objects. But the difference here is that the
bicubic patch object is really not very small, nor are the individual
triangles in it. So it might be a bug though I'm not entirely sure.

I get the problem in both POV-Ray 3.1 and in POV-Ray 3.5 beta 8.
My computer is a P150 with 16 MB RAM, Windows 95.

Warp has reproduced the problem, as can be seen in povray.general.

// start of scene

#declare M = 0.4; // 0.4=broken, 0.6=ok

camera {
   location <0.9,0.3,-1.5>*M
   angle 32
   look_at 0
   translate <-0.54,-0.36,0>*M
}
light_source {<0,1,-2>*10*M, color 1.2}
background {color rgb 1}

bicubic_patch {
   type 1 u_steps 4 v_steps 4
   <  0.2204, -0.0904, -1.0329>
   <  0.2125, -0.1580, -1.0308>
   <  0.1594, -0.1118, -1.0336>
   <  0.1447, -0.1788, -1.0142>
   <  0.1659, -0.0932, -1.0648>
   <  0.1600, -0.1595, -1.0614>
   <  0.1177, -0.1174, -1.0527>
   <  0.1068, -0.1819, -1.0314>
   <  0.0600, -0.0967, -1.1058>
   <  0.0581, -0.1598, -1.1010>
   <  0.0366, -0.1248, -1.0771>
   <  0.0400, -0.1858, -1.0537>
   <  0.0000, -0.0967, -1.1058>
   <  0.0000, -0.1607, -1.1009>
   <  0.0000, -0.1255, -1.0768>
   <  0.0000, -0.1858, -1.0537>
   texture {pigment {color rgb 1}}
   scale M
}

// end of scene

Rune
--
3D images and anims, include files, tutorials and more:
Rune's World:    http://rsj.mobilixnet.dk (updated Nov 5)
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