#version 3.7;
global_settings { assumed_gamma 1 }
camera
{ location <0.0, 1.5, -8.5>
  look_at <0.0, 1.5, 0.0>
  angle 29
}
light_source { <-4.562, 10.625, -7.902>, rgb 1 }
box { <-10, 0, -10>, <10, 12, 10> hollow pigment { rgb 1 } }
text
{ ttf "cyrvetic.ttf" str(version,0,2) 0.001, 0
  scale 0.3 translate <-0.25, 1.4, 0>
  pigment { rgb 0 }
  no_shadow
}

#declare Marker = sphere
{ 0, 0.12
  pigment { rgbt <0, 0, 1, 0.7> }
  no_shadow
}

#declare R = 0.07;

union
{ sphere_sweep
  { cubic_spline 6,
    <1, -1>, R,
    <-1, -1>, R,
    <-1, 1>, R,
    <1, 1>, R,
    <1, -1>, R,
    <-1, -1>, R
//  sturm
    bounded_by { box { -<1.5, 1.5, R>, <1.5, 1.5, R> } }

//  clipped_by { box { -<0.5, 1.5, R>, <1.5, 1.5, R> } } // Hmm...
//  bounded_by { box { -<0.5, 1.5, R>, <1.5, 1.5, R> } } // Hmm...

    pigment { red 1 }
//  translate 1.5 * y                  // If translate done below looks like bounded_by not adjusted...
                                       // I think the real issue is initial bbox size not adjusted to bounded_by shape's bbox...
                                       // where it does get done if translate seen inside spheresweep, but a guess at the moment.
                                       // Note: Need -ur option to override default removal of user bounded_by objects.
                                       //   * Christoph newsgroup suggested perhaps nested bounded_by NOT translated in union.
                                       // ---
                                       // What about clipped_by alone?
                                       // ---
                                       // Is the issue specific to sphere_sweep only or a general 3.7 issue?
  }
  object { Marker translate <1, 1> }
  object { Marker translate <1, -1> }
  object { Marker translate <-1, -1> }
  object { Marker translate <-1, 1> }
  translate 1.5 * y                    // Moving up into sphere_sweep fixes bounding...
}