 A 3-D MixeD Cone filter can be designed using 2-D DHT and is shown below.An M-D signal, is considered to be spatially-planar(SP) if it is constant on all surfaces, i.e.formula_440formula_41 + formula_442formula_42 +formula_444 + formula_445formula_446 = d, for formula_31d formula_448R where R is the set of real numbers.