Channel surface
（重定向自Canal surface）
![canalsurface: directrix is a helix, with its generating spheres](/Images/godic/202501/02/Canal-helix-s.svg2028.png")
![pipe surface: directrix is a helix, with generating spheres](/Images/godic/202501/02/Pipe-helix-spheres-s.svg2028.png")
![pipe surface: directrix is a helix](/Images/godic/202501/02/Pipe-helix-s.svg2028.png")
![pipe knot](/Images/godic/202501/02/Pipe-knot-s.svg2028.png")
A channel or canal surface is a surface formed as the envelope of a family of spheres whose centers lie on a space curve, its directrix. If the radii of the generating spheres are constant the canal surface is called pipe surface. Simple examples are: