Lee Slope

// composition: one great S-ridge enters at the left edge, crests near x=300, and dives to the lower-right
// corner; a second shorter ridge crosses it rising from the bottom edge. the top 40% of the sheet stays empty sky.
// layering: pale under-wash to seat the dunes, umber spray on the lee slopes, sparse ochre spray on the lit sides, rust crest lines last.
const PALETTE = [
  "#eadfc6", // pale sand paper / sky
  "#f2ead4", // lit-slope highlight sand
  "#c98d3f", // ochre mid sand
  "#4f3a28", // deep shadow umber — the mass
  "#b0562e", // rust crest accent
];

const RIDGE_A = [
  [-20, 432], [110, 392], [250, 352], [370, 356], [470, 398],
  [560, 452], [650, 520], [730, 590], [820, 648],
];
const RIDGE_B = [
  [235, 820], [300, 700], [380, 615], [470, 560], [575, 528], [690, 505],
];

function pathPoint(pts, t) {
  const ft = t * (pts.length - 1);
  const i = constrain(floor(ft), 0, pts.length - 2);
  const u = ft - i;
  return [lerp(pts[i][0], pts[i + 1][0], u), lerp(pts[i][1], pts[i + 1][1], u)];
}

function crest(pts, w1, w2) {
  brush.noFill();
  brush.set("2B", PALETTE[4], w1);
  brush.beginShape(0.4);
  for (let i = 0; i < pts.length; i++) {
    brush.vertex(pts[i][0] + random(-3, 3), pts[i][1] + random(-2, 2));
  }
  brush.endShape(false);
  brush.set("pen", PALETTE[4], w2);
  brush.beginShape(0.4);
  for (let i = 0; i < pts.length; i++) {
    brush.vertex(pts[i][0] + random(-2, 2), pts[i][1] - 2 + random(-1.5, 1.5));
  }
  brush.endShape(false);
}

function setup() {
  createCanvas(800, 800, WEBGL);
  brush.scaleBrushes(2.3);
  noLoop();
}

function draw() {
  translate(-width / 2, -height / 2);
  background(PALETTE[0]);

  // one broad pale under-wash below the main ridge, so the dunes sit in warmth
  brush.noStroke();
  brush.fillBleed(0.6, "out");
  brush.fillTexture(0.4, 0.5);
  brush.fill(PALETTE[2], 32);
  brush.beginShape(0.4);
  brush.vertex(-20, 440);
  brush.vertex(250, 366);
  brush.vertex(470, 408);
  brush.vertex(650, 530);
  brush.vertex(820, 656);
  brush.vertex(820, 820);
  brush.vertex(-20, 820);
  brush.endShape(true);

  // lee-slope shadow of the main ridge: hard light from the right throws grain down-left
  for (let i = 0; i < 240; i++) {
    const t = random(0.02, 0.98);
    const p = pathPoint(RIDGE_A, t);
    const d = abs(randomGaussian(0, 1));
    const len = 16 + d * 48;
    brush.set("spray", PALETTE[3], max(0.4, random(0.9, 2.1) - d * 0.28));
    brush.line(
      p[0] + random(-8, 8), p[1] + 2 + d * 5,
      p[0] - len * (0.7 + random(0.25)), p[1] + len * (0.5 + random(0.2))
    );
  }

  // crossing ridge: shorter, its shadow also falling left, denser near the crossing
  for (let i = 0; i < 150; i++) {
    const t = random(0.05, 0.95);
    const p = pathPoint(RIDGE_B, t);
    const d = abs(randomGaussian(0, 1));
    const len = 12 + d * 36;
    brush.set("spray", PALETTE[3], max(0.4, random(0.8, 1.8) - d * 0.25));
    brush.line(
      p[0] + random(-6, 6), p[1] + 2 + d * 4,
      p[0] - len * (0.8 + random(0.2)), p[1] + len * (0.35 + random(0.2))
    );
  }

  // lit slopes: sparse ochre grain rising to the right of each crest
  for (let i = 0; i < 85; i++) {
    const onA = random(1) < 0.62;
    const p = pathPoint(onA ? RIDGE_A : RIDGE_B, random(0.05, 0.95));
    const len = random(8, 34);
    brush.set("spray", PALETTE[2], random(0.6, 1.2));
    brush.line(p[0] + random(-5, 5), p[1] + random(1, 8), p[0] + len * 0.75, p[1] + len * 0.35);
  }
  for (let i = 0; i < 40; i++) {
    const p = pathPoint(RIDGE_A, random(0.05, 0.9));
    const len = random(6, 20);
    brush.set("spray", PALETTE[1], random(0.6, 1.1));
    brush.line(p[0] + random(-4, 4), p[1] + random(1, 5), p[0] + len * 0.7, p[1] + len * 0.3);
  }

  // faint wind ripples in the lower-left flat
  brush.noFill();
  for (let i = 0; i < 6; i++) {
    const rx = random(40, 210);
    const ry = random(600, 770);
    brush.set("2H", PALETTE[2], random(0.5, 0.9));
    brush.beginShape(0.6);
    brush.vertex(rx, ry);
    brush.vertex(rx + random(40, 80), ry - random(4, 12));
    brush.vertex(rx + random(100, 150), ry + random(-4, 8));
    brush.endShape(false);
  }

  // rust crest lines, drawn last so they cut both grain fields
  crest(RIDGE_A, 1.5, 0.7);
  crest(RIDGE_B, 1.1, 0.6);
}