Ski Instructing

I spent this past winter, 2014-15, as a full-time ski instructor at Breckenridge resort. I have gained lots of knowledge about myself, and about the sport of skiing. It was a lot of fun and I hope to continue teaching in a part-time capacity in the future.

Moby Trips

Launched a new app for Moby, Systems inc. called Moby Trips (download from the iTunes Store). This app builds upon Simple Location Sharing by adding features such as waypoints (pins), photos, and recording your path via GPS. The app uses the same, flexible, nodejs backend which powers Simple Location Sharing. I’m continuing to work on this app in my free time, so any feature suggestions are welcome!

Two interesting challenges I ran into were the manor in which iOS’s Asset Library class handles deleted files, and keeping the server and iOS client synchronized throughout areas of patchy cell service. Next time I may try to use Meteor’s Open DDP protocol to overcome this challenge, as REST wrangling became much more trouble than expected.

Future improvements include synchronization of the data across multiple devices, a more comprehensive HTML5 viewer, the ability to enable and disable live streaming, and exporting of your data to various formats such as GPX and Open Street Maps.

Relisten

I’ve been working on a side project, Relisten.  This is a web application for Facebook and Spotify users that aggregates listening history into weekly playlists that can be “relistened” through Spotify. The app came from my usage of Last.fm and the realization that no such tools exist for data on Facebook.

Relisten was an exercise in using the Facebook javascript API and HTML5 localStorage. The technical goal of Relisten is to achieve a fully functional web application without any server-side processing. Facebook data is retrieved from your own browser and stored locally, allowing for an intrinsically scalable app.

Rendering Trees with Meteor.js + Handlebars

Handlebars is currently the only template engine integrated with Meteor. Notably absent from Handlebar’s feature set is the ability to pass context between blocks. Furthermore, the scope of a parent block is absolutely inaccessible from the child’s context (no super, no this.parent).

One can get around any restriction imposed by Handlebar’s markup by writing a custom block helper. The theory behind block helpers is to keep logic separate from the design markup. Every block in Handlebars (if, each, with, list…) calls a block helper behind the scenes. The block helper is a function which takes the block’s HTML content as an argument, as well as any arguments listed in the block’s declaration, and uses these arguments to return a string which is rendered in place of the block. It’s sort of like a macro.

In the case of a tree, using a block helper is absolutely the way to go.

DSP Experiments

I spent the final days of the summer exploring the Fourier transform with Nicolas Avrutin by writing an image <-> audio converter in Python/Numpy. This was more an exercise in learning Numpy, but the result is pretty cool:

This is the spectrograph of an 18 second audio file generated by our script. The ghostly vertical lines are due to interpolation issues, translating an image sampled at a certain rate into an image with far fewer pixels columns than necessary. Color represents intensity of the sound.

We implemented a full color version by using a three channel WAV file (usually used for surround sound or secondary language purposes). We had to write a custom spectrograph generator, as SOX will not combine the channels into a color image. Here’s SOX’s spectrograph:

And our own spectrograph:

Work has been halted on this project while Nick and I deal with school, but this could be used to embed a faint image into music.

ORM Models for restful APIs

I wrote a nifty class which wraps URLLib behind an interface for RESTful web services. The result is a set of one-liners that can be used to import data from compatible APIs.

The goal of this project is to easily import and process data from disparate APIs without needing to read thousands of pages of poorly written documentation. Just import the models you need and hit the ground running.

Think of it as Yahoo Pipes, but local and in Python instead of a graphical programming language.

Single Rule L-System Fractal Generator in Python for Autocad

My first assignment at Cooper Union was to make something in AutoCad, a program used by engineers worldwide to draft architectural plans and machine parts. I decided to write a script in Python that generates the lines of an L-System fractal (as described in The Computational Beauty of Nature by Flake) I used the SDXF library for Python to write the AutoCad DXF file format. You can check out my source code here -> autol.py

Subversion Repository Index Page

Back when I was working on my FRC robotics team’s website, I wrote a simple page to display all of the subversion repositories related to my team. Realizing that there were no existing simple subversion repository index page generators, I decided to make one with human readable code and post it here.

See it in action!   http://svn.jperr.com

Download svnindex.py

The code was developed and tested using Python2.6, but I see no reason why it shouldn’t work in Python3.0. Likewise, I’ve developed and tested this using UNIX based machines (Mac 10.6 and Debian Linux), but it should work under Windows. The script requires the command line “svn” client to be installed and accessible by the user under which Python runs, and you must have the standard python “time” and “commands” modules installed. 99.9% of Python installations will have them pre-installed.

All you need to do to get svnindex.py running on your site:

  • Download svnindex.py (link above)
  • Modify the settings at the top of the file
  • Set up a daily cronjob to run the generation script. Something like `python /path/to/svnindex.py` works just fine.

Processing Chaos

After reading the wonderful Chaos (James Gleik), I was inspired to pick up a copy of The Computational Beauty of Nature (Gary Flake) and explore chaotic computation for myself! My tool of choice was Processing, a simplified yet powerful programming language built on Java.

I began this endeavor by exploring the Mandelbrot set with Zlata over the MIT Spark weekend. In the wee hours of Sunday morning, the first grainy images of the Mandelbrot set came into view. Uplifted by this success, we toiled tirelessly to create a full color, zoomable, and downright awesome fractal generator.

One of the best renderings from our Mandelbrot generator.

View Demo

Adding color was one of the toughest challenges we encountered. The standard RGB color notation is great for most purposes, but it really does fall short for generating the spectrum. In order to more easily generate a smooth color spectrum, we switched the project to HSL color and were able to simply scale the “H” (hue) property with each point’s iteration count.

Another interesting problem which still exists as a “bug” in the demo applet above is the lower limit imposed by the floating point datatype. As you zoom in farther and farther, the coordinates shrink down to absurdly small and precise values. Eventually, the measly floating point becomes imprecise, and the display seems pixelated. An easy fix would be switching all datatypes to doubles or bignums, but that would just take too much effort. It works!

Next I tackled the Lorenz Attractor. One of many such “strange attractors” that visualize the changes in a dynamic system by plotting the system’s state in “system space.” In the case of the Lorenz Attractor, fluid convection is represented in 3-space by the following set of equations:
From hypertextbook.com:
dx/dt = σ(y − x)
dy/dt = ρx − y − xz
dz/dt = xy − βz

As stated above, “x” is the convective flow, “y” is the horizontal temperature distribution, and “z” is the vertical temperature distribution of the system. Curiously, the orbit of point (x,y,z) over time will not “settle down” or form a repetitious cycle given certain critical values for σ, ρ, and β. Instead, the orbit forms a magnificent, infinitely complex object known as a strange attractor.

The Lorenz Attractor

View Demo