When you resize, rotate, or in any way transform an image; or more generally when you resample some discrete signal, the software you are using must interpolate between the discrete points to produce a result. Linear and cubic interpolation are easy to explain, but Lánczos interpolation, one of the most popular methods, is more interesting. In this post we will build up to how Lánczos interpolation works, pulling in various interesting topics on the way.