dithSerp palette issue

If dithSerp is set to true it seems to introduce colors that are not in the palette for small palettes, repro:

const createCanvas = require('canvas');
const canvas = createCanvas(204, 204);
const ctx = canvas.getContext('2d');
ctx.fillStyle = 'white';
ctx.fillRect(0, 0, canvas.width, canvas.height);

ctx.font = '30px Impact';
ctx.rotate(0.1);
ctx.fillStyle = 'black';
ctx.fillText('Awesome!', 50, 100);

// quantitize
const RgbQuant = require('rgbquant');
const q = new RgbQuant({colors: 16, dithKern: 'FloydSteinberg', dithSerp: true});
q.sample(canvas);
const pal = q.palette(true);
const out = q.reduce(canvas);

The palette now contains:

[ [ 0, 0, 0 ],
  [ 17, 17, 17 ],
  [ 27, 27, 27 ],
  [ 38, 38, 38 ],
  [ 54, 54, 54 ],
  [ 75, 75, 75 ],
  [ 86, 86, 86 ],
  [ 103, 103, 103 ],
  [ 126, 126, 126 ],
  [ 137, 137, 137 ],
  [ 154, 154, 154 ],
  [ 169, 169, 169 ],
  [ 194, 194, 194 ],
  [ 218, 218, 218 ],
  [ 231, 231, 231 ],
  [ 255, 255, 255 ] ]

the colors of out are (unordered):

[ [ 255, 255, 255, 255 ],
  [ 231, 231, 231, 255 ],
  [ 0, 0, 0, 255 ],
  [ 17, 17, 17, 255 ],
  [ 38, 38, 38, 255 ],
  [ 54, 54, 54, 255 ],
  [ 86, 86, 86, 255 ],
  [ 103, 103, 103, 255 ],
  [ 137, 137, 137, 255 ],
  [ 169, 169, 169, 255 ],
  [ 194, 194, 194, 255 ],
  [ 126, 126, 126, 255 ],
  [ 218, 218, 218, 255 ],
  [ 75, 75, 75, 255 ],
  [ 154, 154, 154, 255 ],
  [ 27, 27, 27, 255 ],
  [ 253, 253, 253, 255 ] ]

The last entry is not in the palette. If you add more colors or gradients to the original image, more spurious colors will pop up. Is this a missing/misplaced nearest color matching in reduce?

[leeoniya]

it does feel like serpentine may not provide the best “improvement” since it’s pretty directional. you may want to look into Riemersma Dither, particularly one that follows a Hilbert space-filling curve. this might give you less directional dithering.

https://www.compuphase.com/riemer.htm https://www.compuphase.com/hilbert.htm

i think igor implemented this in his RgbQuant offshoot: https://github.com/ibezkrovnyi/image-quantization

also, here is way too much additional dithering info: https://bisqwit.iki.fi/story/howto/dither/jy/

[makeworld-the-better-one]

Yep, I’m aware of those methods and plan on implementing them later. I just also wanted to have serpentine available.