Stanford University, Fall 2015, CS 448Z: Physically Based Animation and Sound

Assignment #0: Audiovisual code warm-up using openFrameworks 

Due: Sunday, Oct 4, 2015

Goal: In this preliminary assignment, you will implement an interactive audiovisual simulation of a simple particle system in openFrameworks. The goal of this assignment is to ensure that everyone has basic coding functionality before we start a nontrivial animation-sound assignment.

First, visit the openFrameworks website ( and download a version appropriate for your computing platform. Complete or follow along with their tutorials ( in order to gain basic understanding. In particular, you should look at the 2D particle system tutorial shown here:

In the following steps, you will implement a dynamic particle system of N balls. You can use 2D or 3D depending on your graphics preferences:
  1. Implement the basic graphics framework to draw N balls on the screen, and maintain the position/velocity state of each spherical object.
  2. Making the particles move: Implement a symplectic Euler integrator which (as discussed in class) updates the particle velocity using applied forces, then updates the particle position. You can use random initial velocities (of a suitably bounded magnitude) to get started. Include a downward gravitational force.
  3. Wall-particle collision detection and response (or "staying inside the box"): The particles are currently not constrained to stay inside the computational cell. Implement a scheme to detect collisions with the four walls, and modify the velocity to provide simple collision response, using a (particle-specific) restitution coefficient to attenuate normal velocity. Be careful how you update your particle velocity/position or you might find your particles sinking into the floor.
  4. Adding sound: Everytime a ball hits a wall, you should play a suitable "click" sound to indicate that an impact was made, with amplitude proportional to the collision impulse magnitude.  Next, to avoid all collisions during a time-step producing "clicks" at the same time-step-size dependent instant, you can estimate the time of ball-wall collision more accurately, and shift the "click" sound in the sound buffer.  As an implementation detail, you can use a circular sound buffer to additively composite sounds into, then regularly xetract the next output sound clip (of suitable "buffer size"). See the documentation on how to stream sounds from a real-time application. Your sound should play clearly and without noticeable audio artifacts.
  5. Make it interactive: You can pull particles around by applying forces to particles near your mouse position. You can also change, e.g., rotate, the direction of gravity using keyboard controls to make all the particles move.
For this first warm-up assignment, feel free to ask questions on Piazza about the openFrameworks software system if you cannot find answers in the documentation or on the website.

Submission: Please upload your implementation and a very brief note about your findings as a zip file to your HW0 entry in Canvas.