Isophote estimation by cubic-spline interpolation

We apply the cubic-spline interpolation to estimate isophotes from sparsely sampled digital images. For any non-pixel, we interpolate it by cubic spline, and by solving the yielding cubic function analytically, we find positions of pixels with the same intensity value. Experiment results are given and discussed. This spreads some important light on the nature of interpolation in images and why the well-known zigzag effects are obtained when images are interpolated.