CS 15-497/15-861 Computer Animation (original) (raw)

CS 15-497/15-861 Computer Animation
Assignment 3: Cloth Simulation
Due Date: April 9
Grade Value: 15%

Overview:
In this assignment, you'll implement a cloth simulator and use it to suit up a moving human figure. Cloth is modeled using a spring mass damper system, with springs and dampers for stretch, shear and bend. The human figure is represented using a hierarchy of cylinders, one for each link of the body. The motion of the cylinders is obtained from optical motion capture (ASF and AMC format). We provide code to display and animate the human figure, and a basic particle system. Our particle data structure allows you to store a system of particles in 3D and add a spring between any two points. Our starter code shows you how to add a stretch spring between two points using this data structure, you'll have to implement shear and bend using this framework. We provide code to incorporate gravity forces on the particles, you'll have to implement forces due to spring forces, namely:

• Stretch spring and damping forces
• Shear spring and damping forces
• Bend spring and damping forces

In addition, you'll have to handle collisions between the cloth and the human figure. For this, you'll have to first detect collisions between a particle and a cylinder, and then perform collision response using penalty functions. Our starter code gives you structures for positioning, displaying and animating cylinders from AMC files. We also give you functions to constrain (attach) a rectangular piece of cloth to the hands of a walking "cylinder man".

Starter Code and Description:

You'll be writing various functions within the following two classes: CCloth and CPhysEnv.
CCloth implements a cloth simulation system and has functions for creating and displaying the cloth. It has the function CreateClothPatch that creates an array of particles (stored in the structure t_Visual) arranged in either rectangular pattern or a skirt pattern. You'll have to modify this function to add springs between the array of particles. The function _RunSim_initializes and starts the particle simulator. This class also has functions for loading, displaying and setting the collision cylinders from the AMC files.
CPhysEnv implements the guts of the spring mass damper system. It has structures for springs, particles, collision cylinders, integrators, etc. You'll have to write the function ComputeForces that will compute all the forces acting on the particle system. The starter code shows you how to implement gravity forces, you'll have to implement all the other forces acting on the particle system (springs, dampers). Our starter code implements constraint forces (to attach a piece of cloth to the moving cylinders). You have to write functions to detect and resolve collisions between the cylindrical objects and the particle system. Fill up the necessary sections of the functions: DistPointCylinder, CheckforCollisions, ResolveCollisions and ResolvePenetrations. We recommend using penalty-based methods to handle penetrations (by using a stiff spring). Finally, you should write the function Simulate that steps the simulation forward in time by integrating the particle system that you create using your choice of integrator (we provide you with Euler, midpoint and RK-4). This function sums up all the forces on the particles (including ones due to collision) and integrates forward by one step.
Note: We have provided the function arguments with some functions (that you are supposed to fill) and not with others. Please feel free to modify them according to your style and convenience.

Interface:
Steps: First Load the actor (ASF file) , then load the motion (AMC file), load Cloth and then press the Simulate button. If everything works, the rectangular piece of cloth (white stuff next to the cylinder man) should swing around and should be attached to the hands of the cylinder man. As the human moves around (when you press the play button), the cloth should collide against the cylinders of the legs and should move in a physically correct way. We provide you **here**with an executable from our implementation, with all forces and collisions implemented (look for the executable clothSimFinal in the src directory). This should run on the cluster machines. Make sure that you set the PATH and LD_LIBRARY_PATH variables correctly (as explained in first asst) .Your result should look our implementation when you are done with this assignment.

Requirements:

Hand in:

• Create a directory asst3 under your login name. Create three subdirectories: src, report and movies
• Submit your code in the src directory. You should also submit printouts of the following functions in separate pages (with your report):
  • ComputeForces, DispPointCylinder, CheckforCollisions and Simulate
• Write a report addressing the following points. Submit an electronic copy in the report subdirectory and a printout in the drop box outside the graphics lab (NSH 4228).
  • Discuss the effect of the spring/damping parameters on the behaviour (or properties) of the cloth. In particular, generate parameters that result in three different types of cloth (e.g., leather, silk and knit).
  • Discuss the collision detection system that you implemented (in about a paragraph). What are its flaws? How can you improve the collision detection system?
  • What kind of clothing can you model with the particle system that you have implemented? (eg, skirt, jeans, etc.?) Why? What kinds can you not model with this?
  • How would you add friction to this model? Can you model air drag?
• In the movies subdirectory, submit the following movies:
  • Result of your implementation of the cloth interacting with the cylinder man in the walk_turn.AMC sequence. This should do all that our executable currently does, namely: stretch, shear and bend forces, collision detection and response with static and moving human figure.
  • Constrain the same piece of cloth on the shoulders of the character, so that it now looks like a cape. Submit a movie for the cape interacting with the cylinders in the walk_turn.AMC sequence. Look at the function updateSuspensionPoints in CCloth to see how we constrain the cloth to the hands.
  • We have provided a sampler function to create a skirt in the CreateClothPatch function. Attach this skirt to the character and simulate its motion for the same AMC sequence. Submit a movie with this sequence.
    • Note that the skirt implementation provided to you is not complete, it is not seamed at the edges. If time permits, create a closed skirt by seaming this edge using springs (for extra credit).

Extra Credit (Maximum of 10 points):

• Implement texture mapping on the cloth system. Apply an interesting texture, and change the current lighting settings to create interesting looking animations.
• Implement friction, air drag, self collisions or any other interesting aerodynamic effects (like wind, etc.).
• Other interesting animations with multiple pieces of cloth. Be creative!

Last Updated: 03-11-02