PDE Evolutions for M-Smoothers in One, Two, and Three Dimensions

Abstract: Local M-smoothers are interesting and important signal and image processing techniques with many connections to other methods. In our paper we derive a family of partial differential equations (PDEs) that result in one, two, and three dimensions as limiting processes from M-smoothers which are based on local order-$p$ means within a ball the radius of which tends to zero. The order $p$ may take any nonzero value $>-1$, allowing also negative values. In contrast to results from the literature, we show in the space-continuous case that mode filtering does not arise for $p \to 0$, but for $p \to -1$. Extending our filter class to $p$-values smaller than $-1$ allows to include e.g. the classical image sharpening flow of Gabor. The PDEs we derive in 1D, 2D, and 3D show large structural similarities. Since our PDE class is highly anisotropic and may contain backward parabolic operators, designing adequate numerical methods is difficult. We present an $L^\infty$-stable explicit finite difference scheme that satisfies a discrete maximum--minimum principle, offers excellent rotation invariance, and employs a splitting into four fractional steps to allow larger time step sizes. Although it approximates parabolic PDEs, it consequently benefits from stabilisation concepts from the numerics of hyperbolic PDEs. Our 2D experiments show that the PDEs for $p<1$ are of specific interest: Their backward parabolic term creates favourable sharpening properties, while they appear to maintain the strong shape simplification properties of mean curvature motion.