mylloon.fr/static/js/markdown.js

const Mode = {
  Light: 1,
  Dark: 2,
};

/**
 * Convert to grayscale
 * @param {{r: number, g: number, b: number}} color
 * @returns Number between 0 and 255
 */
const colorToGrayscale = (color) => {
  return 0.3 * color.r + 0.59 * color.g + 0.11 * color.b;
};

/**
 * Extract color using canvas2d
 * @param {HTMLImageElement} image Image source
 * @returns Colors represeting the image
 */
const extractColors = (image) => {
  const canvas = document.createElement("canvas");
  canvas.width = image.naturalWidth || image.width;
  canvas.height = image.naturalHeight || image.height;
  const ctx = canvas.getContext("2d");
  ctx.drawImage(image, 0, 0);

  const imageData = ctx.getImageData(
    0,
    0,
    Math.max(1, canvas.width),
    Math.max(1, canvas.height)
  );
  const pixelData = imageData.data;

  const colors = [];
  for (let i = 0; i < pixelData.length; i += 4) {
    if (pixelData[i + 3] > 0) {
      colors.push({
        r: pixelData[i],
        g: pixelData[i + 1],
        b: pixelData[i + 2],
      });
    }
  }

  return colors;
};

/**
 * Change the color theme based on the mode
 * @param {Mode} mode
 */
const changeTheme = (mode) => {
  for (const item of document.getElementsByTagName("svg")) {
    const image = document.createElement("img");
    image.src = `data:image/svg+xml;base64,${btoa(
      new XMLSerializer().serializeToString(item)
    )}`;

    image.width = item.viewBox.baseVal.width;
    image.height = item.viewBox.baseVal.height;

    image.onload = () => {
      applyFilter(mode, item, extractColors(image));
    };
  }

  // SVG embedded in images as base64
  for (const item of document.getElementsByTagName("img")) {
    if (!item.src.startsWith("data:image/svg+xml;base64,")) {
      // Exclude image who aren't SVG and base64 encoded
      continue;
    }

    applyFilter(mode, item, extractColors(item));
  }
};

/**
 * Apply the filter based on the selected mode
 * @param {Mode} mode Current theme
 * @param {HTMLImageElement | SVGSVGElement} item Element
 * @param {{r: number, g: number, b: number}} colors Array of the colors of the element
 * @returns
 */
const applyFilter = (mode, item, colors) => {
  // Calculate the average grayscale value
  const grayscaleValues = colors.map(colorToGrayscale);
  const totalGrayscale = grayscaleValues.reduce((acc, val) => acc + val, 0);
  const averageGrayscale = totalGrayscale / grayscaleValues.length;

  const treshold = 128;

  const style = "filter: ";
  const dim = "brightness(0.8) contrast(1.2)";

  if (averageGrayscale < treshold && mode === Mode.Dark) {
    item.style = style + dim + " invert(1);";
    return;
  }

  if (averageGrayscale > treshold && mode === Mode.Light) {
    item.style = style + "invert(1);";
    return;
  }

  if (mode === Mode.Dark) {
    item.style = style + `${dim};`;
    return;
  }

  item.style = "";
};

window.addEventListener("load", () => {
  // Fix SVG images
  changeTheme(
    window.matchMedia("(prefers-color-scheme: dark)").matches
      ? Mode.Dark
      : Mode.Light
  );
});

window
  .matchMedia("(prefers-color-scheme: dark)")
  .addEventListener("change", (event) =>
    changeTheme(event.matches ? Mode.Dark : Mode.Light)
  );