Sunday, May 23, 2010

Friday Flashback: Screencast 90s style

While digitizing some old home movies from VHS-C magnetic tapes, I came across a 5 minute screencast that I created sometime in the mid 1990's. The term "screencast" dates from 2004. While I am certainly not the first person in the world to produce one, I did do it about 10 years before the name was coined. The program1 used to generate these images was written about 10 years earlier still.

Three segments, edited from the one recorded screencast, are presented here. The first one (57 seconds) describes a particular one dimensional cellular automaton, which generates a fractal pattern over time.

The next one (38 seconds) describes a different one dimensional cellular automaton, which demonstrates what is called a "glider". Over time a configuration of cells moves, in this case to the right. A mirror image of this configuration would move to the left.

The final segment (90 seconds) uses the same cellular automaton rules2 as the previous one, and shows what is called a "glider gun". As time progresses, this configuration of cells emits gliders regularly, first one moving to the right, then another moving to the left, and so on. In a wrap-around world, like the one I used in this Apple ][ program, the glider moving to the right soon meets up with the next one moving to the left and the two annihilate each other.

Creating a screencast with an Apple ][ was not difficult, and did not require any software, since this personal computer used an ordinary television set as its monitor. So it was a simple matter to take the output of the TV, using an RCA video cable, and connecting it to the video-in of a camcorder. The voice over was captured by the built-in microphone of the camcorder, as I operated the keyboard and watched the screen.
  1. I wrote the program myself, in 6502 machine language, in the mid 1980's. Unfortunately, this program no longer exists, being a victim of digital decay.
  2. The cellular automata rules come from a series of articles in Scientific American written by Stephen Wolfram. The subscription to Scientific American was initially a gift from a former professor (who was also influential in my decision to major in Computer Science), Dr. Alan C. Ashton.

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