How it Works

The code uses two grayscale images, nicely antialiased so they’re not visibly “jaggy.” The first, 128 by 128 pixels (matching the HalloWing screen size) represents the outside boundary of the googy eye:

The second, 48 by 48 pixels, represents the pupil. It’s been drawn with a bit of a gleam because specular highlights make everything at least three times funnier:

The two images are embedded in the code (in the file graphics.h) as uint8_t (unsigned 8-bit) arrays. The conversion was done with some throwaway Python code. Each byte represents a brightness from 0 (black) to 255 (white).

Getting the pupil to “google” properly took some doing. Making something to bounce in a rectangular space is easy — just reverse horizontal or vertical motion where an edge collision occurs — but in this googly setting, a collision may occur at any angle.

The fact that the eye and pupil are both perfect circles makes a shortcut possible…

Rather than calculating the intersections between two circles (the 128-pixels-across outside boundary, and the 48-pixels-across pupil), we can more easily process this as a single point moving within an 80 pixel wide circle (the 128 pixel boundary minus the 48 pixel pupil).

You’ll see constants for these #defined in the code, though it’s using radii (half these measurements) rather than diameters (the aforementioned pixel dimensions). Using a Cartesian coordinate system with (0,0) at the screen center also helps.

As each frame of animation is processed, the pupil’s vector of motion is modified by input from the accelerometer, plus a tiny bit of drag or resistance to keep things from sliding forever.

If that vector of motion would have the pupil’s center point crossing outside the 80-pixel threshold, the path of motion is reflected back into the eye at a suitable angle, and both the position and velocity are scaled back slightly to represent an inelastic collision (where some kinetic energy is lost due to internal friction), else it would never stop bouncing.

The pull of gravity and these other coefficients can be changed near the top of the code:

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#define G_SCALE       40.0   // Accel scale; no science, just looks good
#define ELASTICITY     0.80  // Edge-bounce coefficient (MUST be <1.0!)
#define DRAG           0.996 // Dampens motion slightly

The G_SCALE value isn’t scientifically determined…it just seemed a reasonably “realistic” value with some trial and error. ELASTICITY and DRAG also aren’t based on any specific real-world material physical properties, again just trial and error with what felt “real.” You can fiddle around with these, but the latter two values should not be equal or greater than 1.0 (else the pupil will pick up energy rather than settling at the bottom).

Each frame, the code updates the minimum bounding rectangle of the pupil’s old and new positions. The whole rectangle is drawn in one pass, rather than erasing the old pupil and then drawing the new one, which would exhibit a lot of flicker and destroy the illusion.

Though the HalloWing’s TFT display supports only 5 bits for red and blue, and 6 bits for green, the boundary and pupil images are stored using 8 bits per pixel. We’re not pressed for space and the math for overlaying the two images is just easier that way. The conversion to “565” color is performed as each 8-bit value is processed.

Unlike the original “spooky eye” code, which allows two HalloWings to communicate and stay in sync, this project intentionally does not operate like that. The desynchronization of two eyes reacting to slightly different inputs adds to the hilarity — it’s exactly how googly eyes should be!

Color Eye

The principle behind the color eye’s operation is exactly the same, only the graphics are different. The code is a bit more complicated in this case because it has to blend two RGB images rather than grayscale, but the code shares a lot in common.

The color eye is enabled near the top of the code, where it normally looks like this:

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#include "graphics.h"
//#include "gritty.h"

Just switch out the comment characters (“//”) so the second line is enabled rather than the first:

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//#include "graphics.h"
#include "gritty.h"

This tells the compiler to use a different graphics file, where a flag is also set to enable the color drawing code.

There’s the same 128x128 size background, but now in color:

And the pupil is just slightly larger than the grayscale version, now 54 by 54 pixels:

This guide was first published on Nov 04, 2018. It was last updated on Nov 04, 2018. This page (How it Works) was last updated on May 09, 2019.