On 14/03/2023 14:01, yesbird wrote:
> while working on C4D->POV converter, I've discovered a strange form
> deviation of prism, extruded from bezier spline (see attachment).

And I should say, that for cubic spline I have the same distortion:
> //----------------------------------------------------------------------------
camera{	perspective
	location  <0, 0, 0>
	angle 53.130102

   matrix
  <0.999944, -0.000000, -0.010610,
   0.010182, 0.281279, 0.959572,
   0.002984, -0.959626, 0.281263,
   -0.018327, 5.930451, -1.559761>

}

#declare Extrude = prism { linear_sweep cubic_spline 0, 1.000000, 7

   <0.000000, -1.000000>
   <1.500000, 0.000000>
   <0.000000, 1.000000>
   <-1.500000, 0.000000>
   <0.000000, -1.000000>
   <1.500000, 0.000000>
   <0.000000, 1.000000>
   matrix
  <1.000000, 0.000000, 0.000000,
   0.000000, 1.000000, 0.000000,
   0.000000, 0.000000, 1.000000,
   0.000000, 0.000000, 0.000000>

   material { mat_default }
}

object{ Extrude }
> //----------------------------------------------------------------------------

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