Simplification of Trajectory Streams

Abstract

While there are software systems that simplify trajectory streams on the fly, few curve simplification algorithms with quality guarantees fit the streaming requirements. We present streaming algorithms for two such problems under the Fréchet distance d_F in ℝ^d for some constant d ≥ 2. 
Consider a polygonal curve τ in ℝ^d in a stream. We present a streaming algorithm that, for any ε ∈ (0,1) and δ > 0, produces a curve σ such that d_F(σ,τ[v₁,v_i]) ≤ (1+ε)δ and |σ| ≤ 2 opt-2, where τ[v₁,v_i] is the prefix in the stream so far, and opt = min{|σ'|: d_F(σ',τ[v₁,v_i]) ≤ δ}. Let α = 2(d-1)⌊d/2⌋² + d. The working storage is O(ε^{-α}). Each vertex is processed in O(ε^{-α} log 1/ε) time for d ∈ {2,3} and O(ε^{-α}) time for d ≥ 4 . Thus, the whole τ can be simplified in O(ε^{-α}|τ| log 1/ε) time. Ignoring polynomial factors in 1/ε, this running time is a factor |τ| faster than the best static algorithm that offers the same guarantees.
We present another streaming algorithm that, for any integer k ≥ 2 and any ε ∈ (0,1/17), maintains a curve σ such that |σ| ≤ 2k-2 and d_F(σ,τ[v₁,v_i]) ≤ (1+ε) ⋅ min{d_F(σ',τ[v₁,v_i]): |σ'| ≤ k}, where τ[v₁,v_i] is the prefix in the stream so far. The working storage is O((kε^{-1}+ε^{-(α+1)})log 1/(ε)). Each vertex is processed in O(kε^{-(α+1)}log²1/(ε)) time for d ∈ {2,3} and O(kε^{-(α+1)} log 1/ε) time for d ≥ 4.