"Lanczos resampling is a multivariate interpolation method used to compute new values for any digitally sampled data. It is often used to resize digital images, but could be used for any other digital signal. In the case of digital image resizing, the Lanczos function indicates which pixels in the original image, and in what proportion, make up each pixel of the final image. Lanczos filtering gives very high quality results compared to more commonly used but faster techniques such as linear or cubic interpolation because it more closely approximates the optimal resampling filter, the sinc function. While the sinc function is infinite, making it very computationally intensive, the Lanczos function defines an approximation over a given range (the "window"), allowing the implementor the ability to improve the approximation by increasing the size of the window."
Just in case you were wondering.