Whenever I see those 'blocky' artifacts on a low-quality image, I used to just think of it as 'bad tech.' After reading this, it's cool to realize you're actually seeing the 8x8 DCT grid itself. You're literally seeing the math break down because there wasn't enough bit-budget to describe those high-frequency sine waves. It’s like looking at the brushstrokes on a digital painting.

The default implementation of the decoding adds the artifacts.

This tool uses more clever math to replace what's missing: https://github.com/victorvde/jpeg2png