. Computing Contour Maps & Answering Contour Queries Pankaj K. Agarwal Joint work with Lars Arge ThomasMolhave Thomas Molhave Bardia Sadri

The TerraStream Modules

What is a Terrain

Representation: Triangulated Irregular Network (TIN)

Level Sets, Contours, and Contour Maps

Computing Contours Maps

Answering Contour Queries Preprocess Terrain  into a data structure Given h, compute contour at height h.

Contour Maps

Usage of contour lines (also called iso-contours, isogons, etc) goes back to at least 17 th century Philosophical Transactions of Royal Society of London, 1779

Find a seed point on each contour and traverse the triangulation to trace each contour Use a simple data structure to compute seed points Query time: O(log N + T) T: #contour edges Contour map: O(Nlog N +T) T: #contour map edges For massive terrains I/O efficiency is bad: O(N+T) instead of O((N+T)/B) Internal Memory Algorithm

I/O-Efficient Algorithms Answering a contour query: Preprocessing O(Nlog B N), Space: O(N/B) blocks Query: O(log B N+T/B)

Our results Computing contour maps: O(Sort(N)+T/B) I/Os Answering contour queries Preprocessing Time: O(Sort(N)) I/Os Space: O(N/B) disk blocks Query : O(log B N+T/B)[][][][] Ordering Theorem: C-ordering Ordering Theorem: A total ordering, called C-ordering, of triangles can be computed in O(Sort(N)) I/Os s.t. the subsequence of triangles intersecting a contour appears along the contour and contours in a level set are broken in nested order. Individual contours can be retrieved in O(T/B)I/Os from this ordering

The Algorithm 1. 1.Sort the vertices in the order of increasing height Compute the C-ordering of the triangles 3. 3.Determine the rank of each triangle in C-ordreing Scan the triangles in the order of increasing height of their lowest vertices: 5. 5.If a triangle intersects some level-set of interest add it to a buffer-tree using its rank for the key When the scan line reaches a height of interest, 1. 1.flush the buffer tree use the stack-based algorithm to extract individual contours Delete any triangle that does not intersect the next level-set. Use persistent buffer trees to store C-ordering at all heights!

Computing the C-Ordering

Height Graph

Critical Points maximum saddle minimum regular

Simple Terrains

Positive and Negative Saddles

Positive and Negative Cut-Trees

Simplifying Terrains by Surgery

What does simplification do to contours? [][][][]

Extensions (We believe) Our approach extends to higher genus 2- manifolds, i.e., contour queries in a fixed direction

Future Directions Computing iso-surface maps or answering iso-surface queries in the I/O model Preprocess a given 2-manifold M, represented as a triangulation, in a linear-size data structure so that For a query plane h, report the contours of M∩h quickly Time: O(n 2/3 +T) in RAM Model

Idea: grow contours contiguously but in parallel