Geometric Audio 4: Taxi-Cab Distance Bounds on Integer Lattice Maps

Recall from the introductory post, the taxi-cab distance metric on the integer lattice map relates the max-order of image-sources to the lattice volume or the total number of image-sources contained within a k radius L_1 “ball”. This quantity is useful for determining the total costs of processing individual image-sources in cases where k can be known or estimated beforehand (e.g. k selected according to an estimate of a room’s RT60 using Sabine’s Equation).

In D dimensional space, the L_1 norm is given by ||\nu||_1 = \sum_{d=1}^D |\nu_d| where the lattice volume is defined by the number of integer lattice coordinates ||\nu||_1 \leq k. The visual equal-distance curve resembles a diamond in D=2 and the growth rate of the lattice volume seems obvious if we introduce a lattice-shell term given by the number of integer coordinates that satisfies ||\nu||_1 = k (see the animation below). Such ease is not the case for dimensions D>2.

Lattice shell adds to growth of the lattice volume with respect to image-source order k

We start with a counting argument that relates the lattice shell and lattice volumes across dimensions via a recurrence relation:


The lattice shell relation C(k,D) follows from observation that all lattice coordinates on the boundary can be formed by augmenting   ||\hat{\nu} ||_1 \leq k, \hat{\nu} \in Z^{D-1} with \tilde{\nu}_D =  k-||\hat{\nu}||_1 and  ||\tilde{\nu} ||_1 \leq k-1, \tilde{\nu} \in Z^{D-1} with \tilde{\nu}_D =  -(k-||\tilde{\nu}||_1). The lattice volume relation S(k,D) follows from the definition of a lattice shell which through substitution can be re-written as self-referential difference equation between successive dimensions. The counting solution is practical for small k and D  but can be improved by the following ansatz.

Ansatz: Suppose that the lattice volume can be expressed as a polynomial equation given by S(k, D) = 1 + \sum_{d=1}^D P_{d,D} k^d. Substitution into the difference equation gives

where P_{d,D} are unknown polynomial coefficients of the powers of radius k. Using the binomial theorem, the powers of k rearranged as to form a generalized square matrix-system given by


where upper triangular matrices B, \bar{B} contain the binomial coefficients and P_D is the vector of unknown polynomial coefficients to be solved for in terms of the polynomial coefficients  P_{D-1} in the preceding dimension.  Carrying the recursion out from the base case S(k, D=1) = 1+2k, the higher-dimensional lattice volumes are given by


where by inspection, the highest order coefficients decrease towards 0 as D increases. This is expected as the majority of the volume moves towards the origin as D grows. For some context, we can compare the lattice volumes bounded under larger p-norms such as L_2 or Euclidean (see post on Gauss circle problem, volume appx. hypersphere) and the L_{\infty} norm given by ||\nu||_{\infty} = \max \{ |\nu_1|, \hdots, |\nu_D|\} (lattice volume trivially (2k+1)^D). By inspection, the lattice volume gap between L_1 and L_2 grows wider as the leading polynomial terms of L_1 decay to 0 (see plot below).



This concludes our four-part foray into image-source models within high-dimensional integer lattice maps. If any new or interesting results come up or any errors found, I will update accordingly.

Notes: Animations generated in GeoGebra, equations and figures were from my draft paper.

2 thoughts on “Geometric Audio 4: Taxi-Cab Distance Bounds on Integer Lattice Maps

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