We study the problem of computing the L₁-distance between two piecewise-linear bivariate functions f and g, defined over a bounded polygonal domain 𝕄 ⊂ ℝ², that is, computing the quantity ‖f-g‖₁ = ∫_𝕄 |f(x,y)-g(x,y)| dx dy. If f and g are defined by linear interpolation over triangulations 𝐓_f and 𝐓_g, respectively, of 𝕄 with a total of n triangles, we show that ‖f-g‖₁ can be computed in Õ(n^α) time, where α = max{(ω+1)/2, 8/5}, ω is the matrix multiplication exponent, and Õ notation hides factors of the form n^ε for any ε > 0. This bound holds for the currently best known value of ω, which is approximately 2.37. More generally, if the complexity of the overlay of 𝐓_f and 𝐓_g is κ, then the runtime of our algorithm is Õ(κ^{α-1}n^{2-α}).
@InProceedings{agarwal_et_al:LIPIcs.SoCG.2025.4, author = {Agarwal, Pankaj K. and Aronov, Boris and Devillers, Olivier and Knauer, Christian and Moroz, Guillaume}, title = {{A Subquadratic Algorithm for Computing the L₁-Distance Between Two Terrains}}, booktitle = {41st International Symposium on Computational Geometry (SoCG 2025)}, pages = {4:1--4:17}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-370-6}, ISSN = {1868-8969}, year = {2025}, volume = {332}, editor = {Aichholzer, Oswin and Wang, Haitao}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.SoCG.2025.4}, URN = {urn:nbn:de:0030-drops-231561}, doi = {10.4230/LIPIcs.SoCG.2025.4}, annote = {Keywords: Terrain similarity, volume computation, polynomial interpolation, geometric cuttings, bivariate multipoint evaluation} }
Feedback for Dagstuhl Publishing