It seems more like a modified version of jpg compression which also uses DCT.
Why not just downsample the original image to 10x10 jpg and encode it as base64 so it can be directly used in an <img> without blocking your main JS thread to decode the image? Apply css blur if you like the soft gradient effect.
Both formats are trying to encode the parameters of cosine functions except jpg is implemented natively in web browsers, likely with SIMD instructions. Is the ~200 bytes in savings per image really worth the extra computation cost you're passing onto your users?
It's unlikely since the smallest possible jpg 10x10 jpg is just under 300 bytes. You might be able to squeeze out a few dozen bytes if you tried multiple browser supported formats (e.g. jpg, webp, png, gif) and pick the smallest one since they might be better at encoding blurhash's gradient effect.
I'm sorry, I wasn't clear about what I meant. The idea was to convert the hash to JPEG once it reached the client, so that the browser's native JPEG support could be used to display and cache the image.
In this case it looks like a key consideration is the way each data point fits into exactly 2 characters, no tricky bit manipulation going on. I'd say it's pretty clever.
