Interpolate colors in LAB space

scaleBreaks.js

'use strict'

// DEPENDENCIES ---------------------------------------------------------------

import { argv, exit } from 'node:process'

import { rgb } from 'd3-color'
import { interpolateLab, piecewise } from 'd3-interpolate'
import { range } from 'd3-array'

// ARGUMENTS ------------------------------------------------------------------

let [ minColor, midColor, maxColor, classCountRaw ] = argv.slice(2)
if ( classCountRaw == null ) {
  classCountRaw = maxColor
  maxColor = midColor
  midColor = null
}
const classCount = parseInt(classCountRaw, 10)

const anyRequiredArgIsNull = (
  minColor == null || maxColor == null || classCountRaw == null)
const classCountIsNotInt = classCount !== parseFloat(classCountRaw)
if ( anyRequiredArgIsNull || classCountIsNotInt ) {
  console.error(
    `Usage: ${argv[1]} MIN_COLOR [MID_COLOR] MAX_COLOR CLASS_COUNT`)
  exit(1)
}

// INTERPOLATION --------------------------------------------------------------

const interpolator = (function() {
  if ( midColor == null ) {
    return interpolateLab(minColor, maxColor)
  }
  return piecewise(interpolateLab, [minColor, midColor, maxColor])
})();
const colors = range(classCount).map(function(i) {
  return rgb(interpolator(i / (classCount - 1))).hex()
})

// OUTPUT ---------------------------------------------------------------------

console.log(JSON.stringify(colors, null, 2))