On k-Enclosing Objects in a Coloured Point Set Luis Barba Stephane Durocher Robert Fraser Ferran Hurtado Saeed Mehrabi Debajyoti Mondal Jason Morrison.

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Presentation transcript:

On k-Enclosing Objects in a Coloured Point Set Luis Barba Stephane Durocher Robert Fraser Ferran Hurtado Saeed Mehrabi Debajyoti Mondal Jason Morrison Matthew Skala Mohammad Abdul Wahid Aug 9, 2013 CCCG

Problem Definition

Motivation Inspired by problems in string processing (jumbled pattern matching).

Motivation Inspired by problems in string processing (jumbled pattern matching). Given a string of characters from some set {1,…,t} and a query c=(c 1,…,c t ), does there exist any contiguous substring that is a permutation of the query? String: Query: (1,6)

Let’s generalize this to 2D! Most natural (CG) generalization may be to have coloured points enclosed by a rectangle. Wait, wait, wait! Surely this has been done! What has been done?

Related Problems Jumbled Pattern Matching - Find a permutation of a query in a string (the 1D version of our problem). k-Enclosing Objects - Find the smallest k- enclosing rectangle/square/disc. – Points are uncoloured. Smallest Colour-Spanning Object - Smallest object containing at least one point of each colour. Subarray Sum - Given a 2D array and a value v, find a subarray whose sum is v.

Results

The Rectangle Problem ……

1 st Insight: Break into Strips! Rather than looking at every combination brute force, bound the top and bottom and look at the 1D problem in the strip!

Strips are good

How to do this quickly?

Improvements

k-Enclosing Discs Compute the kth order Voronoi diagram. Recall that a cell in the diagram corresponds to the set of points with the same k closest points

kth Order Voronoi Diagram ntations/egert-kth_order_voronoi.pdf ntations/egert-kth_order_voronoi.pdf

k-Enclosing Discs

Results Thanks!

k-Enclosing Objects

Smallest Colour-Spanning Object

Subarray Sum

Tweaking the running time

Counting the m’s ≤5? ≥5?

k-Enclosing Squares

Two-Sided Dominating Regions

Smallest Exact Solution For rectangles, no extra time is required. Simply compute the area of the rectangle for any solution found. Discs and squares may be solved with an at most O(k) increase in running time.