create-pattern-data-url.js

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const patternSize = 4;

export default function createPatternDataURL() {

  const ctx = document.createElement('canvas').getContext('2d');

  ctx.canvas.width = patternSize;

  ctx.canvas.height = patternSize;

  const b = [0, 0, 0, 255];

  const t = [0, 0, 0, 0];

  const r = [255, 0, 0, 255];

  const g = [0, 255, 0, 255];

  const imageData = new ImageData(patternSize, patternSize);

  imageData.data.set([

    b, t, t, r,

    t, b, g, t,

    t, r, b, t,

    g, t, t, b,

  ].flat());

  ctx.putImageData(imageData, 0, 0);

  return {patternSize, dataURL: ctx.canvas.toDataURL()};