The square artifacts in the dithered image are caused by the distribution not doing second passes over the pixels already with error distributed, this is a byproduct of the "custom" approach the OP uses, they've traded off (greater) individual colour error for general picture cohesion.

Me, I adjusted Atkinson a few years ago as I prefer the "blown out" effect: https://github.com/KodeMunkie/imagetozxspec/blob/master/src/...

A similar custom approach to prevent second pass diffusion is in the code too; it is slightly different implementation - processes the image in 8x8 pixel "attribute" blocks, where the error does not go out of these bounds. The same artifacts occur there too but are more distinct as a consequence. https://github.com/KodeMunkie/imagetozxspec/blob/3d41a99aa04...

Nb. 8x8 is not arbitrary, the ZX Spectrum computer this is used for only allowed 2 colours in every 8x8 block so this seeing the artifact on a real machine is less important as the whole image potentially had 8x8 artifacts anyway.

Great read and nice drawings!

I made some impractical dithering algorithms a while ago, such as distributing the error to far away pixels or distributing more than 100% of the error: https://burkhardt.dev/2024/bad-dithering-algorithms/

Playing around with the distribution matrices and exploring the resulting patterns is great fun.

The dithered images have the wrong brightness mapping.

The reason is that the described approach will estimate the error correction term wrong as the input RGB value is non-linear sRGB.

The article doesn't mention anything about this so I assume the author is oblivious to what color spaces are and that an 8bit/channel RGB value will most likely not represent linear color.

This is not bashing the article; most people who start doing anything with color in CG w/o reading up on the resp. theory first get this wrong.

And coming up with your own dither is always cool.

See e.g. [1] for an in-depth explanation why the linearization stuff matters.

[1] http://www.thetenthplanet.de/archives/5367

(Kudos on doing this in Racket. Besides being a great language to learn and use, using Racket (or another Scheme, or other less-popular language) is a sign that the work comes from genuine interest, not (potentially) just pursuit of keyword employability.)

Side note on Lisp formatting: The author is doing a mix of idiomatic cuddling of parenthesis, but also some more curly-brace-like formatting, and then a cuddling of a trailing small term such that it doesn't line up vertically (like people sometimes do in other languages, like, e.g., a numeric constant after a multi-line closure argument in a timer or event handler registration).

One thing some Lisp people like about the syntax is that parts of complex expression syntax can line up vertically, to expose the structure.

For example, here, you can clearly see that the `min` is between 255 and this big other expression:

    (define luminance
      (min (exact-round (+ (* 0.2126 (bytes-ref pixels-vec (+ pixel-pos 1)))   ; red
                           (* 0.7152 (bytes-ref pixels-vec (+ pixel-pos 2)))   ; green
                           (* 0.0722 (bytes-ref pixels-vec (+ pixel-pos 3))))) ; blue
           255))

Or, if you're running out of horizontal space, you might do this:

    (define luminance
      (min (exact-round
            (+ (* 0.2126 (bytes-ref pixels-vec (+ pixel-pos 1)))   ; red
               (* 0.7152 (bytes-ref pixels-vec (+ pixel-pos 2)))   ; green
               (* 0.0722 (bytes-ref pixels-vec (+ pixel-pos 3))))) ; blue
           255)))

Or you might decide those comments should be language, and do this:

    (define luminance
      (let ((red   (bytes-ref pixels-vec (+ pixel-pos 1)))
            (green (bytes-ref pixels-vec (+ pixel-pos 2)))
            (blue  (bytes-ref pixels-vec (+ pixel-pos 3))))
        (min (exact-round (+ (* red   0.2126)
                             (* green 0.7152)
                             (* blue  0.0722)))
             255)))

One of my teachers would still call those constants "magic numbers", even when their purpose is obvious in this very restricted context, and insist that you bind them to names in the language. Left as an exercise to the reader.

This reminds me a bit of the octree quantization implementation I hacked up to improve speed of generating Racket's animated gifs.

* https://github.com/racket/racket/commit/6b2e5f4014ed95c9b883...

* https://github.com/racket/racket/commit/f2a1773422feaa4ec112...