paperInkSceneSamples function

List<int>? paperInkSceneSamples(BackgroundSpec bg)

Guardrail background samples of a paper sheet, in painter order (paper, grain, lines): the grain bases, then each line ink over each base (ruled / dots: the rule; grid: minor and major lines). These are ceilings: rasterising only lowers a texel's coverage.

Implementation

List<int>? paperInkSceneSamples(BackgroundSpec bg) {
  if (bg.colors.isEmpty) return null;
  final paper = bg.colors.first | 0xFF000000;
  final bases = paperGrainBases(paper, paperGrainOf(bg));
  if (bg.colors.length < 2) return bases;
  final rule = bg.colors[1] | 0xFF000000;
  final op = bg.patternOpacityPermille;
  List<int> over(int permille) => [
    for (final b in bases)
      compositeOver(withAlpha(rule, paperAlpha(permille)), b),
  ];
  return switch (bg.pattern) {
    BackgroundPattern.none => bases,
    BackgroundPattern.stripes ||
    BackgroundPattern.dots => [...bases, ...over(op)],
    BackgroundPattern.grid => [
      ...bases,
      ...over(op),
      ...over(_majorPermille(op)),
    ],
    // Not generated: the painter falls back to a plain gradient of the
    // two colours.
    _ => [paper, rule],
  };
}