[jcynix]

> What has thus far remained a mystery to me is going from a bag of noisy pixels [...] to array of booleans.

Ah, OK. You can use software like ImageMagick to partition images into levels of gray or just black and white. I have some examples somewhere I played with some time ago, but not accessible online right now, sorry. If the contrast of the original image is high enough, just the qrcode would remain to be parsed.

[jcynix]

Here's an example to start with: ImageMagick's -threshold or -adaptive-threshold (depending on the image's lighting) are what you want to look at, e.g. try something like

    magick input.jpg \
      -colorspace Gray \
      -adaptive-threshold 8x8+10% \
      candidate.png

[edoceo]

And if you wanna go even lower, you'll "raw" read the image pixel by pixel to normalise colour to black/white and then read that matrix for the QR pattern.

And then, cause there are some inverted colour QRs, flip and scan again.