//Steiner's Cross-Cap Surface
//Projective Transformation animation
//by Adam Coffman

camera {
    location  <-6, 0, 0>
    look_at   <0, 0, 0>
      } 
#include "colors.inc"
#declare cost = cos(pi*clock);
#declare sint = sin(pi*clock);
background {rgb <1,1,1>} 
poly{4, < 1, 0, 0, 0, 1, 0 , 0, 1-sint*cost-sint*sint,
  1+2*cost*sint-2*sint*sint, cost*sint+sint*sint, 0 , 0 , 0, 0 , 0, 0 ,
  0, 0, 0, 0, 1/4, 0 , 0, 1/4-sint*sint/2, sint*cost, sint*sint/2-1/4,
  0, 0, 0, 0, 0 , 0, 0, 0 , 0 >
         sturm        
         texture {
      pigment{Yellow}
      finish {
         phong 0.1
         phong_size 80
         diffuse 0.45     
         }
      }
}
light_source{<0.1,20,0.1> White} 
light_source{<-40,0.1,0.1> White}
light_source{<-60,0.1,40> White}