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Assignment 1: Simplification and Subdivision Surfaces
The goal of this assignment is to try your hand at various mesh
processing tasks. A halfedge mesh adjacency data structure is
provided, along with a very simple .obj mesh parser. As you modify
the triangle mesh, it is critical that the manifold properties of the
input surface are maintained, and that the adjacency data structure
remains consistent. Throughout this assignment you are asked to
consider the efficiency of various mesh operations: is it quadratic,
linear, logarithmic, constant, etc? Since this assignment is rather
long, it's ok if you don't always implement the most efficient
strategy (which may require additional data structures), but please
discuss the tradeoffs in your README.txt file. Please use
the provided README.txt template.
Warning: This assignment has one small component (Gouraud shading)
& two large components (simplification and subdivision). Start early,
and just get the basics working first. Spend a reasonable
amount of time coding and write about what you've learned in your
README.txt file. Also, to streamline grading, please
indicate which portions of the assignment are finished & bug free
(full credit), attempted (part credit) or not started (no credit) by
filling in the provided hw1_gradesheet.txt and submitting it with
your assignment. The instructor will then check, edit as needed, and
finalize your assessment.
Tasks
 Download the provided source code and compile it as
before. You should be able to load simple .obj files and turn
wireframe visualization on & off by pressing "w". Try these command
lines:
./mesher input cube.obj size 300
./mesher input cube.obj size 300 wireframe
./mesher input open_box.obj size 300
./mesher input open_box.obj size 300 wireframe
Notice that the adjacency data structure detects boundaries in the
mesh and draws them with thicker red lines in the wireframe visualization:
 Now you're ready to start coding. First make use of the
adjacency data structure provided to implement gouraud
shading (we'll talk about this in Lecture 3). In gouraud
shading, average normals are computed for each vertex, these normals
are used for lighting, and the shaded values automatically
interpolated in GL. You can change the color and/or normal between
each specified vertex. This shading option should be enabled by
specifying gouraud on the command line, or pressing "g".
./mesher input bunny_1k.obj size 500
./mesher input bunny_1k.obj size 500 gouraud
Note that the average normals for each vertex can be efficiently
computed without any adjacency information at all, but that wouldn't
give you practice with the data structure! For extra credit, don't
average the normals across "sharp crease" edges (whose faces differ in
normal by more than a userdefined threshhold angle).
Comment on the efficiency & order notation of your implementation
in your README.txt.
 Next, let's experiment with edge collapses, the key mesh
manipulation operation from the Progressive Mesh framework. When you
press the "d" key (for decimate, a.k.a. simplification) the number of
triangles in your mesh should be reduced by approximately 10 percent.
First implement the topology of an edge collapse. Identify a
target edge to collapse. For now, just pick a random edge (or any
edge) in the mesh. Using the adjacency data structure, identify which
two triangles will be removed permanently from the model, and which
other triangles will change. Choose a simple location for the
remaining vertex (e.g., it's old position, or a position averaged with
the deleted vertex). Note: in the provided adjacency data structure
it is recommended that you will delete and then recreate triangles
rather than modify them and the related edges. Initially you may want
to just collapse one edge each time the "d" key is pressed.
./mesher input bunny_1k.obj size 500 wireframe
Even though a single random collapse works great, you will
probably quickly run into problems: 1) the mesh looks bad with small
triangles and big triangles that don't represent the highresolution
model very well, 2) it creates self intersections of the surface (it
turns itself inside out and you can see the blue side), and 3)
(worstofall) it sometimes crashes with an error like:
assertion "opposite == NULL" failed: file "edge.h", line XXX
Think about what that error might indicate is wrong with your
implementation. (If you get a different error, of course, try to
explain that one.) Certain edges should not be collapsed
because it would cause the surface to become nonmanifold. For
example, the "ring" of vertices surrounding the target edge should be
unique. In the diagram below, if vertices 1 and 4 refer to the same
vertex, then after the collapse of the solid black edge, four
triangles will meet at the same edge. This proposed edge collapse
will alter the topology of the model and yield a nonmanifold
condition. Write code to check for these conditions. NOTE: This code
can be tricky. It's ok if you only just catch some of these
conditions and your program still crashes occasionally. Don't spend
too long trying to make it perfect. Discuss this in your
README.txt file.
Now implement something to make a smarter choice for which edge to
collapse (this might also minimize the occurance of lingering bugs
from the previous part). Collapsing the shortest edge first (or
actually make that the shortest legal edge) will usually do a
very good job at preserving overall surface shape. Done naively,
repeatedly selecting the shortest edge for collapse can be an
expensive operation. That's fine for now, you can implement something
very inefficient. Discuss these performance issues in your
README.txt file and what common data structures would help.
 Finally, implement the Loop Subdivision Surface scheme. When
the "s" key is pressed, your implementation should perform one
iteration of subdivision. First you want to get the topology correct.
Add a vertex at the center of each edge, then replace each triangle
with 4 smaller triangles. Make sure you share the new vertices across
the edges and don't create multiple copies of the vertex. Consider
how to do this efficiently and discuss various options in your
README.txt file. The provided hash table structure for
tracking the "parents" of each newly added vertex should be very
helpful for this task. You can verify correct vertex sharing by
visualizing the wireframe and looking for the red boundary edges. If
you have extra red lines on your mesh (not at the boundaries) you have
not properly shared the vertices.
./mesher input open_box.obj size 300 wireframe
Once you have the correct topology and vertex sharing, you can
adjust vertex positions according to the Loop Subdivision rules. To
handle general shapes you must implement the rules for regular
vertices (valence = 6, i.e., 6 edges meeting at a vertex),
extraordinary vertices (valence != 6), and boundary conditions. Here
are the Loop
Subdivision Rules from the SIGGRAPH
2000 course notes  Subdivision for Modeling and Animation (page 70).
Finish off your subdivision surface code by implementing the simple
binary (infinitely sharp or infinitely smooth) crease method described
in: "Piecewise Smooth Surface Reconstruction."
H. Hoppe, T. DeRose, T. Duchamp, M. Halstead, H. Jin, J. McDonald, J. Schweitzer, W. Stuetzle.
ACM SIGGRAPH 1994.
./mesher input creased_cube.obj size 300 wireframe
Ideas for Extra Credit
Include a short paragraph in your README.txt file
describing your extensions and sample command lines to demo them.
 Implement gouraud shading with a proper average weighted by
angle for the triangles meeting at each vertex (if you haven't done
this already) and a crease angle threshhold. Find or create a mesh
which illustrates these differences.
 Implement a "better" edge weight selection method. First define
your goal for simplification and construct an edge weight function
that matches that goal. E.g., if your goal is to create equilateral
triangles than the shortest edge collapse is a very good choice.
Discuss in your README.txt.
 Implement a better positioning method for the remaining vertex in
an edge collapse. Discuss in your README.txt.
 Implement the geomorph option to smoothly transition
between two different resolutions within a progressive mesh.
 Improve the efficiency of your implementation by using the right
data structures for various operations (hash tables, priority queues,
etc.)
 Implement the integer crease weight control described by "Subdivision
Surfaces in Character Animation", DeRose, Kass & Truong, SIGGRAPH
1998 to perform n iterations of crease subdivision
rules followed by infinite iterations of smooth subdivision.
 Implement the floating point weight control by interpolating
between the nearest integer iterations.
 Implement the butterfly interpolating subdivision surface scheme.
 Basic Code (vectors.h, matrix.h, matrix.cpp, argparser.h, camera.h, camera.cpp,
glCanvas.h, glCanvas.cpp, main.cpp, Makefile, MersenneTwister.h)
Similar to Homework 0.
 Mesh Library (boundingbox.h,
boundingbox.cpp, vertex.h, triangle.h, edge.h, edge.cpp, mesh.h,
mesh.cpp)
An implementation of the halfedge data structure. The load
function will parse simple .obj files.
 hash.h
For efficient, constant time access of parts of the data
structure, we store edges and vertex ancestry in a hashtable. It is
used by the Mesh code to efficiently find pairings between opposite
edges at load time. Unfortunately, the hash map or
unordered map is not quite standard in STL, and depending on
your platform, you may need to tweak this file.
 .obj files (cube.obj, creased_cube.obj, open_box.obj,
bunny_200.obj, bunny_1k.obj, bunny_40k.obj, cactus.obj,
knot.obj, and hypersheet.obj)
Some models from: "Piecewise Smooth Surface Reconstruction.", Hoppe et al, 1994
Please read the Homework information page again before submitting.
