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Homework 2: Line Segment Intersections & Adjacency Data Structures

Part 1: Book Problems

Prepare a PDF file named hw2_intersection_and_adjacency.pdf with your answers to the book problems below. You may use Latex, Google Docs (export/save as PDF), MS Word (export/save as PDF), and/or legibly handwrite on paper and scan to PDF.

From "Computational Geometry: Algorithms and Applications", de Berg, Cheong, van Kreveld, and Overmars.

You may discuss these problems with your classmates, but you must write up your solutions individually. Note the names of anyone you talked to or collaborated with in your hw2_intersection_and_adjacency.pdf writeup.

Please do not search for solutions or notes published by the authors or any instructor or notes or solutions shared by other students on the internet. Any use of these types of materials is considered a violation of Academic Integrity for this course.

2.1
Let S be a set of n disjoint line segments whose upper endpoints lie on the line y = 1 and whose lower endpoints lie on the line y = 0. These segments partition the horizontal strip [ ∞ : - ∞ ] X [ 0 : 1 ] into n+1 regions. Give an O(n log n) time algorithm to build a binary search tree on the segments in S such that the region containing a query point can be determined in O(log n) time. Also describe the query algorithm in detail.

2.8
Give pseudocode for an algorithm that lists all vertices adjacent to a given vertex v in a doubly-connected edge list. Also, give pseudocode for an algorithm that lists all edges that bound a face in a not necessarily connected subdivision.

2.11
Let S be a set of n circles in the plane. Describe a plane sweep algorithm to compute all intersection points between the circles. (Because we deal with circles, not discs, two circles do not intersect if one lies entirely inside the other.) Your algorithm should run in O((n + k) log n) time, where k is the number of intersection points.

Part 2: CGAL Programming Task

  • Your task is implement and debug the algorithm requested for CGAA Book Problem 2.9 using the CGAL Surface Mesh data structure, an implementation of a Half Edge Data Structure.

    2.9
    Suppose that a doubly-connected edge list of a connected subdivision is given. Give pseudocode for an algorithm that lists all faces with vertices that appear on the outer boundary.


  • We recommend you start coding from the "Draw a Surface Mesh" example, which is part of the CGAL demos & examples you downloaded for Homework 1: CGAL-5.3.1/examples/Surface_mesh

    Here is the finished CMakeList.txt file for the instructor's solution.


  • Your program will take as an input a 2D or 3D polygonal mesh stored in the "Object File Format" .off. This file format stores vertices and faces. Each face refers to 3 or more vertices by index, ordered in a counter-clockwise loop. Below is a sample .off file that stores 17 vertices (a 2D mesh with all vertices in the z=0 plane), 11 faces (5 triangles, 4 quads, 1 pentagon and 1 hexagon). This file with the left image below.

    sample.off 
    OFF
17 11 0
4 0 0
7 0 0
2 2 0
4 3 0
6 3 0
8 2 0
0 3 0
0 5 0
2 6 0
4 5 0
6 5 0
8 6 0
9 4 0
3 9 0
5 8 0
7 9 0
5 10 0
3 0 1 4
3 0 3 2
3 0 4 3
3 1 5 4
6 3 9 8 7 6 2
4 10 9 3 4
5 11 10 4 5 12
4 13 8 9 14
4 13 14 15 16
4 10 11 15 14
3 10 14 9

    The image on the above right shows only the faces that have a vertex that is on the outer boundary.


  • Your program can be run in two different ways. First, it may be run 3 arguments, the input mesh .off file, and two output files. The first output, the edges text file, will list all boundary edges on the boundary in a clockwise traversal (starting at any edge). If there multiple cycles of edges (if the mesh has an interior hole) the cycles will be listed separately. The second output is a new mesh .off file storing only the faces that have at least one vertex on the boundary of the mesh. 

    ./identify_boundary_faces ../src/sample.off output_edges.off output_sample.off

    Second, it may be run with 5 arguments, the input .off file, two output files, and two integers that index into the vertex list. These vertices specify an edge on the boundary of the mesh, in this case, the leftmost edge of the hexagon face. The two output files are the same as above -- except it will only include the edges and faces on the boundary cycle that is specified by the input edge. 

    ./identify_boundary_faces ../src/sample.off output_edges.txt output_sample.off 7 6

    For the simple example above, with a single cycle of boundary edges, both command lines will produce the same output. Note that the running time for the second version of the program should be output-sensitive; that is, faster for complex meshes when the specified boundary is a small fraction of the entire mesh.


  • IMPORTANT NOTE: You are expected to write the loops that walk over the the mesh traversing "next" and "opposite" half edge pointers. Even if you find a built-in function that loops and gathers the necessary information for you... we encourage you to re-write the traversal algorithm on your own for practice.


  • The output_edges.txt file will contain the number of boundary edge cycles detected, and then the edges in each cycle, ordered in a clockwise walk along the boundary, starting at any edge in the cycle.

    output_edges.txt 
    Found 1 cycle(s)
CYCLE with 12 edges:
v7 -> v8
v8 -> v13
v13 -> v16
v16 -> v15
v15 -> v11
v11 -> v12
v12 -> v5
v5 -> v1
v1 -> v0
v0 -> v2
v2 -> v6
v6 -> v7

  • The output_sample.off file contains the boundary faces in the "Object File Format" .off. This file is shown in the image above right. It is the same as the sample.off input file, except it is missing the two interior faces.

    output_sample.off 
    OFF
17 9 0
4 0 0
7 0 0
2 2 0
4 3 0
6 3 0
8 2 0
0 3 0
0 5 0
2 6 0
4 5 0
6 5 0
8 6 0
9 4 0
3 9 0
5 8 0
7 9 0
5 10 0
3 4 0 1
3 2 0 3
3 3 0 4
3 4 1 5
6 2 3 9 8 7 6
5 12 11 10 4 5
4 14 13 8 9
4 16 13 14 15
4 14 10 11 15

  • Additional input files:
    closed_box.off
    open_box.off
    open_tube.off
    hypersheet.off

    Additional sample command lines:

    ./identify_boundary_faces ../src/open_tube.off output_all_tube_edges.txt output_tube.off
./identify_boundary_faces ../src/open_tube.off output_select_tube_edges.txt output_selected_tube.off 12 14

    Additional sample output:
    output_all_tube_edges.txt
    output_tube.off
    output_select_tube_edges.txt
    output_selected_tube.off

  • After your program has finished saving the output files, your program should launch a CGAL Qt window that draws the surface mesh containing only the desired boundary faces. Use the CGAL::draw function call, which is used in the draw_surface_mesh example. Note that this function is blocking, so make sure you have finished writing the output files before calling this function.

Part 3: Analysis of Implementation

Analyze the running time and additional memory used to run the two versions of your algorithm to collect and save the faces on the boundary. Do not include the file to parse/load the input file or the memory used to store the mesh data. Write your answer in terms of n, the number of edges in the input mesh; f, the number of faces in the input mesh; b, the number of edges on the selected boundary; and o, the number of faces on the boundary that are output.

Add this analysis to your PDF file named hw2_intersection_and_adjacency.pdf.