For this recitation, the workshop recitation, I decided to attend the one on Serial Communication by Mister Young. I feel it was quite important for my project, since I would need to both be communicating from Arduino to Processing and Processing to Arduino at the same time.
I learned how to make a sensor value from the Arduino influence Processing (for example, in my Final Project, an accelerometer influenced whether the game would continue in Processing).
In this simple example shown in the video below, I connect an Infrared Sensor to Arduino and map the values from 0-1023 to 0-50 and have them being read in Processing.
It ended up being useful (at the start of my project, before I decided to have the start of the game be a different way than with an infrared sensor) since when a person would walk in front of the sensor it would turn on the game in Processing, and start the audio from Processing.