Visualizing Magnetometer Data with Three.js

I’m happy that my simple exploratory web app was able to obtain data from my phone’s integrated magnetometer. I recognize there are some downsides to how easy it was, but right now I’m happy I can move forward. Ten times a second (10Hz is the maximum supported update rate) my callback receives x, y, and z values in addition to auxiliary data like a timestamp. That’s great, but the underlying meaning is not very intuitive for me to grasp. What I want next is a visualization of that three-axis magnetometer data.

I turned to the JavaScript 3D graphics library Three.js. The last time I used Three.js was to visualize the RGB332 color space, using a 2D projection to help me make sense of data along three dimensions of color: a cylinder representing HSV color space and a rectangular solid representing RGB. Now I want to visualize a single vector in three-dimensional space representing the local magnetic field as reported by my phone’s magnetometer. I was a little intimidated by the math for calculation 3D transforms. I tried to make my RGB332 color app transition between HSV and RGB color space but it never looked right because I didn’t understand the 3D transform math.

Fortunately, this time I didn’t have to do any of my own math at all. Three.js has a built-in function that accepts the x, y, and z components of a target coordinate and calculates the rotation required to have a 3D project look at that point. My responsibility is to create an object that will convey this information. I chose to follow the precedence of an analog compass which is built out of a small magnetic needle. Shaped like a narrow diamond with one half painted red and the other half painted white. For this 3D visualization I created a shape out of two cones, one red and one white. When this shape looks at the magnetometer vector, it functions very similarly to the sliver of magnet inside a compass.

As a precaution, I added a check for WebGL before firing up Three.js code. I was pretty confident any Android Chrome that supported the magnetometer API would support WebGL as well, but it was good practice to confirm.

One thing I’m not doing (but should) is to account for screen orientation. Chrome developers have added a feature to automatically adjust for screen orientation but right now I’m just going to deactivate auto-rotate on my phone (or… phones!)

Source code for my exploratory project is publicly available on GitHub

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