What is Nearest-neighbor interpolation?

Nearest-neighbor interpolation is a simple method of multivariate interpolation in one or more dimensions.

Interpolation is the problem of approximating the value of a function for a non-given point in some space when given the value of that function in points around that point. The nearest neighbor algorithm selects the value of the nearest point and does not consider the values of neighboring points at all, yielding a piecewise-constant interpolant. The algorithm is very simple to implement and is commonly used in real-time 3D rendering to select color values for a textured surface.