From Ohm

For each cell the extracted line segment of the iso line for a particular iso value v, works by classifying each corner value s as below ($s<v$) or above of the iso value ($s \ge v$).

Each edge of the cell, that has one corner value classified as below and one classified as above has an intersection with the iso line.

There are $2^4$ cases, many of which are symmetrical. There are three main principal cases:

• no line segment
• one line segment
• two line segments as part of a saddle point

The latter case is ambiguous, which can be resolved by interpolating the center point and classifying it as well.