Function clearColor

addons/addon-webgl/src/TextureAtlas.ts:1083–1122

* Makes a particular rgb color and colors that are nearly the same in an ImageData completely * transparent. * @returns True if the result is "empty", meaning all pixels are fully transparent.

(imageData: ImageData, bg: IColor, fg: IColor, enableThresholdCheck: boolean)

 * @returns True if the result is "empty", meaning all pixels are fully transparent.
 */
function clearColor(imageData: ImageData, bg: IColor, fg: IColor, enableThresholdCheck: boolean): boolean {
  // Get color channels
  const r = bg.rgba >>> 24;
  const g = bg.rgba >>> 16 & 0xFF;
  const b = bg.rgba >>> 8 & 0xFF;
  const fgR = fg.rgba >>> 24;
  const fgG = fg.rgba >>> 16 & 0xFF;
  const fgB = fg.rgba >>> 8 & 0xFF;

  // Calculate a threshold that when below a color will be treated as transpart when the sum of
  // channel value differs. This helps improve rendering when glyphs overlap with others. This
  // threshold is calculated relative to the difference between the background and foreground to
  // ensure important details of the glyph are always shown, even when the contrast ratio is low.
  // The number 12 is largely arbitrary to ensure the pixels that escape the cell in the test case
  // were covered (fg=#8ae234, bg=#c4a000).
  const threshold = Math.floor((Math.abs(r - fgR) + Math.abs(g - fgG) + Math.abs(b - fgB)) / 12);

  // Set alpha channel of relevent pixels to 0
  let isEmpty = true;
  for (let offset = 0; offset < imageData.data.length; offset += 4) {
    // Check exact match
    if (imageData.data[offset] === r &&
        imageData.data[offset + 1] === g &&
        imageData.data[offset + 2] === b) {
      imageData.data[offset + 3] = 0;
    } else {
      // Check the threshold based difference
      if (enableThresholdCheck &&
          (Math.abs(imageData.data[offset] - r) +
          Math.abs(imageData.data[offset + 1] - g) +
          Math.abs(imageData.data[offset + 2] - b)) < threshold) {
        imageData.data[offset + 3] = 0;
      } else {
        isEmpty = false;
      }
    }
  }

  return isEmpty;
}

function checkCompletelyTransparent(imageData: ImageData): boolean {
  for (let offset = 0; offset < imageData.data.length; offset += 4) {