 The same technique was used a year after in some images in "The Beauty of Fractals" by Heinz-Otto Peitgen and Michael M. Richter.They provide a formula to estimate the distance from a point outside the Mandelbrot set to the boundary of the Mandelbrot set (and a similar formula for the Julia sets).Landscapes can, for example, be formed from the distance function for a family of iterations of the form formula_1.