An isosurface of a scalar function 
is a level set 
for a fixed value
. At points where the gradient
of 
is nonzero, the implicit
function theorem makes the isosurface a smooth surface
whose normal direction is 
.
Isosurface
See also
Gradient, Level Set, SurfaceExplore with Wolfram|Alpha
References
Lorensen, W. E. and Cline, H. E. "Marching Cubes: A High Resolution 3D Surface Construction Algorithm." Comput. Graphics 21, 163-169, 1987.Cite this as:
Weisstein, Eric W. "Isosurface." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/Isosurface.html