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CSCI-1200 Data Structures — Fall 2019
Homework 9 — Priority Queues for Mesh Simplification
In this assignment we will work with a 2D mesh or graph of vertices, edges, and triangles. At the vertices we
store colors loaded from an image in .ppm format. The goal in this assignment is to reduce the total size of
the mesh, but still provide a reasonable approximation of the original image. The images below show meshes
with approximately 80,000 triangles, 10,000 triangles, and 1,000 triangles (from left to right), displayed with
and without the edges between the triangles drawn with white lines. Be sure to read the entire handout
before beginning the assignment.
To perform this simplification, we will use a standard mesh processing operation, the edge collapse, which is
illustrated below. First, we identify an edge in the mesh that should be removed (drawn in blue). Next, we
locate the two triangles on either side of that edge (drawn in light blue), and remove these triangles from
the mesh. Finally, we locate all triangles using exactly one of the two endpoints of the edge and change
that endpoint coordinate to the midpoint of the collapsed edge. Note: We reuse one of the endpoints of the
collapsed edge, while the other is deleted. After the edge collapse is finished, the mesh has 1 fewer vertex, 2
fewer triangles, and 3 fewer edges (the blue edge and 2 green edges are removed). Note that if the edge to
be collapsed is on the boundary of the image/mesh, the collapse will result in a mesh with 1 fewer vertex, 1
fewer triangle, and 2 fewer edges.
We provide a significant amount of code for this assignment. Be sure to review all of the provided code before
starting your implementation. Search for the word “ASSIGNMENT” within these files to see the sections you
need to fill in. The provided code should compile and run on your machine with no edits. The program
has reasonable default values for all parameters, but you can override the defaults by providing any or all of
these command line arguments (in any order):
-image The image file must be a .ppm
-dimensions The size of the grid for the original mesh
-target The target number of triangles after simplification
-shortest Choose the shortest edge to collapse
-random Choose a random edge to collapse
-color Choose an edge collapse with least impact on the overall appearance
-preserve_area Disallow edge collapses that affect the total area of the mesh
-debug Save intermediate meshes & perform extra error checking
-linear Perform linear sweep over all edges to find the best edge collapse
-priority_queue Use a priority queue to find the best edge collapse
As the parameters above indicate, we have some choices when selecting the next edge to collapse. Always
choosing the shortest edge (which is our default) is known to be a very good algorithm for producing high
quality meshes. The resulting triangles in the final mesh are roughly all the same area and many are
approximately equilateral in shape.
Your first task is to complete the implementation of the edge collapse. The provided code (which
should be rewritten/replaced) simply deletes the two triangles and leaves a hole. This should work “ok” for
at least a few collapses, but if you enable debugging and the extra error checking the program will eventually
crash with an error. Make sure your completed implementation of the edge collapse works robustly, and can
aggressively simplify a large mesh down to a small number of triangles. The Mesh::Check function (and later
the PriorityQueue::check_heap function) are used to sanity check the state of your data as you implement
and debug. Use a memory debugger and make sure you have no errors or memory leaks.
Once collapse is working, you can also experiment with the -preserve area command line option. Certain
edges should not be collapsed because they change the shape of the boundary of the mesh (as shown below
left and middle), and decrease the overall area. The image below right visualizes these illegal edges in red.
Note that most of these edges are along the boundary of the mesh, but sometimes they appear in the interior.
Sometimes an edge collapse will cause triangles in the neighborhood to twist or flip upside down and overlap
other triangles. These situations can be detected by calculating the total area before & after the proposed
collapse or checking the clockwise/counter-clockwise orientation of the vertices. We provide finished code to
detect illegal edge collapses.
In performing an edge collapse, a small neighborhood of the mesh is modified. Triangles change area and
edges change length. In the diagrams above we see that the green edges (the edges that were touching one
of the endpoints of the collapsed edge) change length. The orange edges (which share a vertex with one or
more of the green edges) do not change length; however, their status as legal or illegal may change. After
performing an edge collapse, you should recalculate both the length and the legal/illegal status (stored as
Edge class member variables for efficiency) of the green & orange edges. Carefully study the Mesh, Triangle,
Edge, and Vertex classes for helper functions that will help you identify these edges efficiently (without doing
an expensive linear sweep through all edges in the mesh!).
Even if you are efficient about recalculating the edge length and legal/illegal status, the simplification process
using the default -linear algorithm for finding the next edge is slow. Your second major task for this
assignment is to modify the code to add a priority queue to the Mesh representation. This priority
queue will store all edges in the mesh, organized by priority for collapse. When the -shortest option is
chosen the priority value is simply the edge length – except edges with illegal status are assigned a very large
number to ensure they fall to the bottom of the heap.
Note: The use of a priority queue for this problem is somewhat tricky because the length and/or legal/illegal
status of an edge will change as the algorithm progresses. Therefore, we cannot use the STL Priority Queue
and we instead need a custom PriorityQueue implementation that allows fast access to elements in the
middle of the heap. You’ll need to complete the implementation of several functions in this file.
Performance Analysis
Let’s assume that the input mesh has v0 vertices, e0 edges, and t0 triangles. Further let’s define k to be the
number of edges connected to a vertex. For a triangular mesh in 2D, k = 6 averaged across the entire mesh.
We note that the relative counts of elements in the mesh is a well-studied problem in mathematics. You can
read more about the Euler characteristic here: https://en.wikipedia.org/wiki/Euler_characteristic.
If we use the -shortest criteria for selecting an edge, analyze the performance of the overall program using
the -linear vs. -priority_queue command line options. What is the running time of the program to reduce
the mesh to contain the target = t
final or fewer triangles? Does the command line argument -preserve_area
change the answer? Separately analyze the different key functions in the program and justify your answer
with a clear and concise writeup. Additionally, using the UNIX time command, test your program on
different size inputs and target output sizes for the two different command line arguments. Create a neat
table summarizing these timing results in your README.txt file. Does it match your theoretical analysis?
Viewing the Output Meshes
The program outputs .html files using SVG (Scalable Vector Graphics) format. You should be able to view
these files in a modern web browser on your laptop and use the small checkboxes at the top to toggle on &
off the visualizations of the edges.
Extra Credit: Prioritizing Edge Collapse by Color
Explore alternate edge collapse criteria that consider not just the length and legality of a collapse, but also
determine the relative impact the collapse will have on the overall appearance of the image by analyzing the
colors of the vertices. Implement this variation with the optional -color command line option. Discuss the
quality of your results in your README.txt and include screenshots of your more impressive results.
Submission
Use the provided template README.txt file for your algorithm analysis and any notes you want the grader
to read. You must do this assignment on your own, as described in the “Academic Integrity for
Homework” handout. If you did discuss the problem or error messages with anyone, please
list their names in your README.txt file.

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