COMP 599  Winter 2010  Assignment 2 Robust Collision Processing
Due 23:39 pm Tuesday February 16
Getting Started
In this assignment you will implemnet robust collision
processing, similar to the cloth system in [Bridson et al. 2002].
For simplicity, your implementation will be in 2D, and will in
many respects extend the code you worked on in assignment 1 (see
the notes on provided code below). In addition to creating a
video of something interesting, you are invited to take part in
the "alphabet soup challenge" to see how many letters you can simulate
before edgeedge interpenetrations occur.
Sample Code
Download the provided code from WebCT (do not distribute). The
code has two main packages, similar to the previous assignment. Note
that the tools package has minor updates, so you will want to
make sure you use this one. The a2 package contains code
which is very similar to the first assignment except for a few
notable differences
 ParticleSystem.java now implements a symplectic Euler
step in update particles, and does not make use of the
Integrator interface from the previous assignment.
 TestSystems.java implements a variety of test systems
that will help you debug and stresstest your code. Feel
free to add your own test systems too, though note you can
also do this on the fly using mouse clicks.
 LeafSpring.java implements a joint spring between three
particles, which is useful for creating long 2D strings
which hold their shape (for instnace, letters).
 AlphabetSoupFactory.java is used by TestSystem to generate
mass spring systems from fonts.
 SanityCheck.java is used by the application to test for
edgeedge collisions, that is, a discrete collision detection
test. If collisions are found, the background turns red and
the simulation is stopped (i.e., game over).
You do not need to change
anything except RobustCCD.java and optionally
TestSystems.java if you want to hard code additional test cases
or special scenarios. Be sure to leave a comment in your readme.txt
on any additional test cases you add, or other modifications
outside of the RobustCCD.java code.
The setup for jars and libraries
is the same from the previous assignment. Consult the
A1 page for more information. Likewise, you may also
want to visit the previous assignment specification for
instructions on making videos.
Steps and Objectives (5/5) * 95%
This assignment contains two main parts. First is the continuous
collision detection for points and line segments. Second is the
response to collisions. You will need to modify the symplectice
Euler step in the updateParticles() method, which advances the
system by the given time step h. As discussed in class and
documented in the method, this method does the following:
 Symplectic Euler velocity update using forces from springs, gravity, etc.
 Velocitylevel collision response (iterative solve)
 Symplectic Euler position update (guaranteed intersection free)
Both main parts of the assignment are concerned with step two listed
above (i.e., the velocitylevel collision resolution).
Continuous Collision Detection
Perform robust intervalbased collision detection of all
particleedge pairs during the time interval (0,h] for the
given step size h. The edges defined by Spring objects
connecting particles. As discussed in class, you will do this
by doing the following
 Given the particles are moving on linear trajectories
specified by their position at the current time and their
velocity, find all times of colinearity on (0,h] by solving
the appropriate quadratic equation.
 determine the contact location on the line segment, for
instance, as a parameter alpha in [0,1].
Note that some roots will give you a situation where the
particle is not on the line segment. You should go through
the roots in the interval (0,h] in ascending order, and then
process only the earliest collision you find.
Note also that your root finding needs to be robust! You
may need to add epsilon checks for the alpha parameter you
compute, and it may also be beneficial to check roots that
happen just after h (i.e., h plus some small time based
epsilon).
A quick way of testing this part is to pin the three
particles involved in a collision each time you find a
collision in the interval (0,h]. You should make a very short
movie to demonstrate this objective and to show your progress.
Collision Impulses
When you find a collision on (0,t], you must apply an
appropriate impulse to the particles involved to resolve the
collision. As discussed in class, this involves the following.
 Computing a suitable unit normal for the collision (at
the time of collision),
 Computing a suitable impulse using a small
nearinelastic restitution coefficient (e.g., the
default value which is set for you in the interface).
 Updating the velocities of the particles given the
impulse, and using the assumption that pinned particles
have infinite mass (i.e., in computing the impulse and
distributing the impulse to the 3 particles).
You may find it useful to temporarily setting the
restitution value to 1 (or other values using the slider)
to help debug your velocity impulses.
Iterative Solve
Many particles and edges may be colliding in a given time
interval. You will want to sequentially check all
particleedge pairs, and immediately after each check you
should apply the collision impulses you compute in the
previous step. You should continue through all the pairs
in this manner, but note that there may still be collisions
on the interval (0,t] after processing all pairs as some
particles may be involved in multiple collisions. As such
you'll need to iterate over all pairs until you have resolved
all collisions on (0,t]. As discussed in class, this may be
slow to converge. It is a good idea to give up after a some
maximum number of iterations, and return false to report the
problem and halt the simulation.
Penalty Forces
Penalty forces can help keep objects separate and reduce
the number of difficult velocitylevel impulses that need to
be applied. The penalty forces will let you model edges as
if they had a thickness of 2H. A default thickness of 2
pixels is set for you in the user interface controls. So,
as discussed in class, if a particle is within a distance of
H of a line segment, then apply a simple spring force
impulse with stiffness identical to the stretch springs.
see also [Bridson et al. 2002] for more details.
For your simulation to run smoothly, you'll likely need
to address particleparticle penalty forces, i.e., forces
when alpha falls just outside of [0,1].
Interesting Video
Record a sequence of something interesting or amusing.
Note that you can create particles by clicking with the mouse,
and you can also pin and unpin particles by clicking them.
So experiment, have fun, and make a movie of something
creative and interesting.
Optional: Alphabet Soup Challenge
How many letters can you simulate before the soup goes bad?
Post your successes to WebCT, and the final results will be
also added to this page for posterity. Note that simulation
parameters must be reasonable (no extra bouncy
collisions).
Optional: Extensions
You might optionally like to think about the following issues.
 Frictional contact is generally a hard problem, but
there exist many good models for friction during impact.
For impacts, you may want to read about what Bridson
suggests as a straightforward way to apply impulses
opposing tangential motion.
 The iterative solver can fail to converge, even after
adding penalty forces. You can improve robustness and
find solutions in these difficult cases by using rigid
impact zones as described in [Bridson 2002]. This
effectively acts like a velocity filter and provides a
"fail safe" to avoid interpenetration when the iterative
solver fails. There are drawbacks, however, as rigid
zones can be large and will eliminate interesting
deformable dynamics.
 As particle numbers increase, the all particleedge
pair collision test becomes quite expensive. Space time
bounds can be used to cull many root finding tests, but you
could also consider a "broadphase" collision detection
scheme, such as a uniform grid subdivision of space.
 You may like to think about the complexities of moving
this simulation to 3D. For instance, the modifications
would include: both particle triangle tests and edgeedge
tests, robust solution of cubic polynomials, robustly
testing point in triangle. A nice aspect of [Bridson et
al. 2002] is also the subdivision surface refinement of
the 3D cloth, which gives very smooth results for
coarse cloth simulations.
Some tips for robust simulation. Watch out for all special cases as
they will happen once you are running large simulations. If you are
dividing, you can probably be sure that the denominator will be zero
in some cases (and you should deal with these cases differently).
Use double precision floating point. Use epsilon checks to make
your code robust to floating point error. Finally, strive for
correctness instead of speed.
References
 Robert Bridson,
Ronald P. Fedkiw, John Anderson, Robust Treatment of Collisions, Contact,
and Friction for Cloth Animation, ACM Transactions on
Graphics, 21(3), July 2002, pp. 594603.
 Witkin, A., and Baraff, D., Eds. 2001. Physically
Based Modeling: Principles and Practice. Course Notes. ACM SIGGRAPH
'01.
 Doug James,
CS5643: Physically Based Animation for Computer Graphics,
Assignment #2, Cornell University.
Written Questions (1/1) * 5%
The written question is available as a PDF
download.
Prepare a pdf file with your answer to this written question
(scanned hand written document, or generated
with latex, openoffice, or otherwise) and submit it along with your
assignment via webCT.
Finished?
Great! Submit your source code and two xvid encoded videos as a
zip file via webCT. Include a readme.txt file with any specific
comments. Also be sure to include a pdf with your written answer
(i.e., scans of handwritten work, or typeset). Your readme
should provide a list of people with which you discussed the
assignment, or state that you did discuss the assignment with
anyone.
