apcaContrastOfApcaY function

double apcaContrastOfApcaY(double textApcaY, double backgroundApcaY)

Implementation

double apcaContrastOfApcaY(double textApcaY, double backgroundApcaY) {
  const inputClampMin = 0.0;
  const inputClampMax = 1.1;
  final smallest = math.min(textApcaY, backgroundApcaY);
  final largest = math.max(textApcaY, backgroundApcaY);
  assert(smallest >= inputClampMin);
  assert(largest <= inputClampMax);

  var sapc = 0.0; // For raw SAPC value
  var outputContrast = 0.0;
  // Rational for ignore; crucial property of algorithm, present in original
  // implementation, and signals the code was ported correctly.
  // ignore: unused_local_variable
  var polarity = _ApcaPolarity.blackOnWhite;

  // Soft clamps Y for either color if it is near black.
  textApcaY = _fixupForBlackThreshold(textApcaY);
  backgroundApcaY = _fixupForBlackThreshold(backgroundApcaY);

  // Return 0 Early for extremely low ∆Y
  if ((backgroundApcaY - textApcaY).abs() < deltaYMin) {
    return 0.0;
  }

  // APCA/SAPC CONTRAST - LOW CLIP (W3 LICENSE)
  if (backgroundApcaY > textApcaY) {
    // For normal polarity, black text on white (BoW)

    // Calculate the SAPC contrast value and scale
    sapc =
        (math.pow(backgroundApcaY, normBg) - math.pow(textApcaY, normText)) *
        scaleBoW;

    // Low Contrast smooth rollout to prevent polarity reversal and also a
    // low-clip for very low contrasts
    outputContrast = (sapc < loClip) ? 0.0 : sapc - loBoWOffset;
  } else {
    // background < text
    polarity = _ApcaPolarity.whiteOnBlack;
    sapc =
        (math.pow(backgroundApcaY, revBg) - math.pow(textApcaY, revText)) *
        scaleWoB;
    outputContrast = (sapc > (-1.0 * loClip)) ? 0.0 : sapc + loWoBOffset;
  }

  // return Lc (lightness contrast) as a signed numeric value
  //
  // Note: Original implementation could conditionally return a string based
  // on the value of a `places` argument.
  return outputContrast * 100.0;
}