Curves

Accessible region

A

C 0

{ u 1 = u 2 } × [ v 1 , v 2 ]

S

C 1

{ u 1 = u 2 } { u 3 = u 4 } × [ v 1 , v 2 ]

2S

C 0

[ u 1 , u 2 ] × { v 1 = v 2 }

S

C 1

[ u 1 , u 2 ] × { v 1 = v 2 } { v 3 = v 4 }

2S