DiffImagesOptions

interface DiffImagesOptions extends DiffImageOptions {
  baseline?: number; // 0
}

class DiffImagesOptions extends DiffImageOptions {
  const DiffImagesOptions({
    this.baseline = 0,
    super.tolerance,
    super.ignoreAntialiasing,
    super.align,
    super.alignRadius,
    super.blockSize,
    super.maxRegions,
  });

  final int baseline;
}

Every option DiffImageOptions has, with the same defaults, and one more.

A comparison of several asks one of two questions, and baseline decides which. With a baseline it is "how does each of these differ from that one", which is what a saved version against four runs is. Without one it is "where do these disagree at all", and the answer to that is the same whichever of them the counting starts from: a pixel they all agree about has no bit set whoever is the baseline.