An asterisk after the result means that the value is extrapolated rather than interpolated, i.e. the specified point lies outside the triangle determined by the given points.
If the result is "colinear" than means that the given point lie on a straight line and so it is not possible to carry out the interpolation.
How it works: In a nutshell, the page finds the barycentric coordinates of the point where you want to interpolate, and uses those coordinates as weights in a weighted average of the function values at the vertices of the triangle.
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