The concept is to use HN usernames as the seed into a deterministic avatar generator. This generator is built from the famously simple xorshift32 PRNG, which both provides a random variable for the image generator steps, and "pseudo-hashes" the seed string to provide the initial PRNG state using a non-linear step (adding each codepoint - which is likely not very robust against collisions compared to proper hashing algorithms, but is simple and good enough).
The image generation part is a probability distribution with mirrored pixels... specifically: r>>>29 > X*X/3+Y/2 where the left side is 3 of the upper bits of the PRNG state (providing a random integer 0-7), and the right side is the squared distance from the centre for X + the linear distance from the top for Y. i.e the further from the top centre the pixel is, the less likely it will be filled, but linearly for Y and squared for X.
Un-golfed version:
for (const u of document.querySelectorAll('.hnuser')) {
const p=2;
const c=document.createElement('canvas');
const x=c.getContext('2d');
c.width=p*7, c.height=p*7;
u.parentElement.prepend(c);
for (let s=u.innerText, r=1, i=28+s.length; i--;) {
// xorshift32
r^=r<<13, r^=r>>>17, r^=r<<5;
const X=i&3, Y=i>>2;
if (i >= 28) {
// seed state
r+=s.charCodeAt(i-28);
x.fillStyle='#'+(r>>8&0xFFFFFF)
.toString(16).padStart(0, 6);
} else {
// draw pixel
if (r>>>29 > X*X/3+Y/2)
x.fillRect(p*3+p*X, p*Y, p, p),
x.fillRect(p*3-p*X, p*Y, p, p);
}
}
}
It's worth mentioning these were heavily inspired by three particular dweets by @KilledByAPixel and @FireFly:
[1] https://www.dwitter.net/d/3078
[2] https://www.dwitter.net/d/19786