Lee of the Stone

// composition: one near-black boulder on the lower-left thirds crossing, long sparse drift lines combing past it
// and streaming off the upper right; behind it the wind shadow is reserved as a flat pale lens of bare paper
// layering: cold field washes first, the pale lee shape reserved back out of them, drift lines, then the rock's darks last
const PALETTE = [
  "#edf0f1", // ground: cold snow paper
  "#cfd7dc", // pale drift, the flat shape
  "#9dabb4", // mid grey-blue, the combing lines
  "#5e6b74", // deep cold shade, the heavy ridges
  "#1c2125", // mass: the rock, one dark accent
  "#a2603a", // accent: frozen grass at the lee base
];

// wind axis: out of the lower left, streaming to the upper right at about 24 degrees
const RX = 268; // rock centre — the lower-left thirds crossing
const RY = 534;
const DX = 0.917; // downwind unit vector
const DY = -0.399;
const NX = 0.399; // cross-wind unit vector, positive toward the lower right
const NY = 0.917;

const ROCK = [
  [178, 602], [166, 548], [194, 500], [236, 466], [272, 470],
  [292, 496], [328, 508], [352, 548], [358, 590], [300, 606], [230, 608],
];

// half-width of the calm lens at along-wind distance u: tight upwind, flaring in the lee, closing slowly
function wake(u) {
  if (u < 0) return 74 * pow(0.5, -u / 52);
  const grow = 74 + 0.26 * u;
  return u > 200 ? grow * pow(0.5, (u - 200) / 170) : grow;
}

// a streamline carrying cross-wind offset v0 far upwind, shouldered aside as it passes the rock
function windPoint(v0, u, wob) {
  const s = wake(u);
  const a = abs(v0);
  const sign = v0 < 0 ? -1 : 1;
  const v = sign * (a + (s * s) / (s + a));
  const meander = (noise(u * 0.0024 + v0 * 0.017, v0 * 0.006 + 11.4) - 0.5) * wob;
  const vv = v + meander;
  return [RX + DX * u + NX * vv, RY + DY * u + NY * vv];
}

function drift(v0, u0, u1, col, wt, bname, wob, curv) {
  const n = constrain(floor((u1 - u0) / 26), 5, 64);
  brush.noFill();
  brush.set(bname, col, wt);
  brush.beginShape(curv);
  for (let i = 0; i <= n; i++) {
    const u = u0 + ((u1 - u0) * i) / n;
    const p = windPoint(v0 + randomGaussian(0, 1.4), u, wob);
    brush.vertex(p[0] + randomGaussian(0, 1.1), p[1] + randomGaussian(0, 1.1));
  }
  brush.endShape(false);
}

function wash(x, y, r, col, op, bleed) {
  brush.noStroke();
  brush.fillBleed(bleed, "out");
  brush.fillTexture(0.44, 0.4);
  brush.fill(col, op);
  brush.circle(x, y, r, true);
}

function shape(pts, col, op, curv, bleed, tex) {
  brush.noStroke();
  brush.fillBleed(bleed, "out");
  brush.fillTexture(tex, 0.55);
  brush.fill(col, op);
  brush.beginShape(curv);
  for (let i = 0; i < pts.length; i++) {
    brush.vertex(pts[i][0] + randomGaussian(0, 2.2), pts[i][1] + randomGaussian(0, 2.2));
  }
  brush.endShape(true);
}

function rockBody(col, op, grow, bleed, tex, curv) {
  brush.noStroke();
  brush.fillBleed(bleed, "out");
  brush.fillTexture(tex, 0.72);
  brush.fill(col, op);
  brush.beginShape(curv);
  for (let i = 0; i < ROCK.length; i++) {
    const dx = ROCK[i][0] - RX;
    const dy = ROCK[i][1] - RY;
    brush.vertex(RX + dx * grow + randomGaussian(0, 2.4), RY + dy * grow + randomGaussian(0, 2.4));
  }
  brush.endShape(true);
}

function setup() {
  createCanvas(800, 800, WEBGL);
  angleMode(DEGREES);
  brush.scaleBrushes(2.4);
  noLoop();
}

function draw() {
  translate(-width / 2, -height / 2);
  background("#edf0f1");

  // --- flat light on the field: cold tone laid along the diagonal band, the upper-left corner left bare
  wash(198, 706, 262, "#cfd7dc", 104, 0.72);
  wash(392, 672, 232, "#cfd7dc", 86, 0.74);
  wash(84, 596, 196, "#cfd7dc", 96, 0.7);
  wash(614, 616, 216, "#cfd7dc", 56, 0.76);
  wash(772, 704, 186, "#cfd7dc", 40, 0.7);
  wash(300, 786, 204, "#9dabb4", 46, 0.68);
  wash(556, 754, 176, "#9dabb4", 28, 0.7);
  wash(716, 196, 132, "#9dabb4", 18, 0.78);
  wash(180, 418, 104, "#cfd7dc", 34, 0.74);
  wash(108, 496, 106, "#cfd7dc", 44, 0.7);

  // the scoured trough running up to the windward face, colder than the field
  shape([[-40, 700], [148, 646], [276, 606], [292, 682], [150, 758], [-30, 800]],
    "#9dabb4", 58, 0.45, 0.5, 0.5);
  shape([[-30, 762], [120, 706], [244, 664], [252, 706], [110, 782], [-30, 800]],
    "#9dabb4", 40, 0.45, 0.45, 0.55);

  // --- the wind shadow: reserved back out of the field as one flat pale shape, tapering off the upper right
  shape([[286, 574], [396, 546], [512, 512], [604, 448], [688, 390], [752, 324],
    [654, 324], [556, 350], [446, 376], [342, 438], [248, 498]],
    "#edf0f1", 210, 0.5, 0.5, 0.3);
  shape([[300, 560], [420, 528], [530, 486], [628, 420], [714, 356],
    [636, 352], [520, 386], [418, 420], [310, 500]],
    "#edf0f1", 160, 0.5, 0.55, 0.28);
  wash(742, 306, 66, "#edf0f1", 130, 0.6);

  // --- the drift field: long combing lines, banked below the rock, sparse and pale above it
  // above the rock: few, far apart, thinning into the empty corner
  drift(-252, -200, 660, "#cfd7dc", 1.3, "cpencil", 26, 0.7);
  drift(-214, -210, 690, "#9dabb4", 1.0, "HB", 24, 0.74);
  drift(-176, -240, 30, "#cfd7dc", 1.2, "cpencil", 22, 0.7);
  drift(-144, -260, 680, "#9dabb4", 1.15, "HB", 20, 0.72);
  drift(-113, -270, 700, "#5e6b74", 1.9, "charcoal", 18, 0.7); // heavy ridge, upper comb
  drift(-71, -300, 700, "#9dabb4", 1.2, "HB", 16, 0.72);
  drift(-52, 210, 690, "#cfd7dc", 1.1, "cpencil", 15, 0.7);
  drift(-38, -310, 150, "#5e6b74", 1.0, "2B", 14, 0.7);
  drift(-14, -330, 700, "#5e6b74", 1.5, "2B", 13, 0.68); // the upper lip of the lens

  // below the rock: closer, heavier, the snow piling into the trough
  drift(15, -340, 700, "#5e6b74", 1.6, "2B", 13, 0.68); // the lower lip of the lens
  drift(34, -320, 640, "#9dabb4", 1.15, "HB", 14, 0.7);
  drift(52, -300, 240, "#9dabb4", 0.95, "2H", 15, 0.7);
  drift(68, -300, 700, "#5e6b74", 2.1, "charcoal", 16, 0.7); // heavy ridge, lower comb
  drift(88, -280, 660, "#9dabb4", 1.25, "HB", 17, 0.72);
  drift(110, -260, 380, "#5e6b74", 1.0, "2B", 18, 0.7);
  drift(112, 430, 700, "#cfd7dc", 1.2, "cpencil", 18, 0.7);
  drift(140, -240, 690, "#5e6b74", 1.5, "2B", 19, 0.7);
  drift(168, -220, 620, "#9dabb4", 1.2, "HB", 20, 0.72);
  drift(196, -180, 340, "#9dabb4", 0.95, "cpencil", 21, 0.7);
  drift(238, -140, 640, "#9dabb4", 1.3, "HB", 22, 0.72);
  drift(272, -60, 600, "#5e6b74", 1.4, "charcoal", 23, 0.7);
  drift(310, 20, 560, "#9dabb4", 1.1, "cpencil", 24, 0.7);
  drift(386, 170, 470, "#cfd7dc", 1.2, "cpencil", 26, 0.7);

  // short broken fragments, drift only just written into the crust
  drift(-232, 250, 430, "#5e6b74", 1.0, "2B", 20, 0.66);
  drift(-160, 430, 590, "#5e6b74", 0.95, "2B", 18, 0.66);
  drift(46, 380, 520, "#5e6b74", 1.05, "2B", 14, 0.66);
  drift(124, -160, 40, "#5e6b74", 1.1, "2B", 18, 0.66);
  drift(252, 200, 380, "#5e6b74", 1.0, "2B", 22, 0.66);
  drift(300, -20, 120, "#9dabb4", 0.95, "cpencil", 24, 0.66);

  // --- dry snow lifting off the crest and running down the edges of the lens
  brush.set("2H", "#5e6b74", 0.75);
  for (let i = 0; i < 84; i++) {
    const u = randomGaussian(230, 150);
    if (u < 30) continue;
    const side = random() < 0.5 ? -1 : 1;
    const v = side * wake(u) * constrain(randomGaussian(1.04, 0.13), 0.86, 1.4);
    const x = RX + DX * u + NX * v + randomGaussian(0, 4);
    const y = RY + DY * u + NY * v + randomGaussian(0, 4);
    if (x < -10 || x > 810 || y < -10 || y > 810) continue;
    const len = max(3, randomGaussian(9, 5));
    brush.line(x, y, x + DX * len, y + DY * len);
  }

  // --- the rock: the one dark accent, laid down last so it holds the whole sheet
  wash(330, 588, 66, "#5e6b74", 46, 0.5); // snow banked cold against the lee flank
  wash(210, 604, 54, "#5e6b74", 34, 0.5);
  rockBody("#5e6b74", 132, 1.03, 0.24, 0.44, 0.12);
  rockBody("#1c2125", 250, 1.01, 0.14, 0.34, 0.06);
  rockBody("#1c2125", 250, 0.94, 0.1, 0.3, 0.05);
  rockBody("#1c2125", 250, 0.84, 0.1, 0.26, 0.05);
  rockBody("#1c2125", 235, 0.66, 0.12, 0.3, 0.08);
  for (let i = 0; i < 34; i++) {
    const a = random(0, 6.28);
    const r = 52 * abs(randomGaussian(0, 0.6));
    wash(RX + Math.cos(a) * r, RY + Math.sin(a) * r * 0.78, max(9, randomGaussian(17, 5)),
      "#1c2125", 200 + random(-30, 45), 0.16);
  }

  // charcoal dragged across the mass, each stroke its own angle and weight
  brush.noFill();
  brush.set("charcoal", "#1c2125", 5.4);
  brush.line(196, 566, 246, 502);
  brush.set("charcoal", "#1c2125", 6.2);
  brush.line(214, 588, 280, 496);
  brush.set("charcoal", "#1c2125", 4.6);
  brush.line(248, 594, 318, 522);
  brush.set("charcoal", "#1c2125", 5.8);
  brush.line(284, 596, 342, 550);
  brush.set("charcoal", "#1c2125", 4.2);
  brush.line(186, 584, 224, 512);
  brush.set("charcoal", "#1c2125", 3.4);
  brush.line(306, 588, 348, 560);
  brush.set("charcoal", "#1c2125", 4.8);
  brush.line(232, 540, 300, 512);

  // the windward edge cut hard, the crown left soft where snow rides on it
  brush.set("charcoal", "#1c2125", 2.2);
  brush.beginShape(0.15);
  brush.vertex(196, 500);
  brush.vertex(172, 546);
  brush.vertex(180, 598);
  brush.vertex(232, 608);
  brush.endShape(false);
  brush.set("2B", "#1c2125", 1.5);
  brush.beginShape(0.15);
  brush.vertex(330, 510);
  brush.vertex(354, 550);
  brush.vertex(358, 588);
  brush.endShape(false);

  // Broad mineral facets give the stone a crown, a face and a sheltered side.
  shape([[180, 520], [214, 486], [270, 480], [248, 540], [199, 569]],
    "#9dabb4", 205, 0.12, 0.24, 0.1);
  shape([[248, 540], [270, 480], [320, 513], [337, 564], [290, 585]],
    "#5e6b74", 224, 0.08, 0.2, 0.1);
  brush.noFill();
  brush.hatchStyle("cpencil", "#cfd7dc", 0.85);
  brush.hatch(4.2, 62, { rand: 0.15, continuous: false, gradient: 0.6 });
  brush.noStroke();
  brush.polygon([[191, 520], [216, 492], [261, 485], [242, 538], [203, 560]]);
  brush.noHatch();
  brush.set("cpencil", "#1c2125", 0.9);
  brush.line(270, 485, 250, 538);
  brush.line(250, 538, 289, 580);

  // one lee plane catching the flat light, and the crown under its cap of snow
  shape([[276, 476], [300, 498], [332, 512], [340, 540], [300, 524], [274, 500]],
    "#5e6b74", 128, 0.15, 0.24, 0.55);
  shape([[214, 486], [252, 468], [288, 472], [274, 492], [234, 500]],
    "#edf0f1", 205, 0.5, 0.36, 0.3);
  shape([[152, 586], [222, 596], [298, 600], [356, 584], [372, 610], [280, 624], [164, 616]],
    "#edf0f1", 215, 0.5, 0.42, 0.32);
  shape([[196, 578], [250, 590], [318, 592], [346, 580], [340, 596], [252, 604], [198, 594]],
    "#edf0f1", 150, 0.5, 0.4, 0.3);

  // the hollow the wind scours out of the windward foot
  wash(154, 636, 62, "#5e6b74", 78, 0.55);
  wash(204, 654, 42, "#5e6b74", 56, 0.55);
  wash(114, 612, 34, "#5e6b74", 36, 0.6);

  // --- three frozen stems in the lee, bent flat downwind: the only warm marks on the sheet
  brush.noFill();
  brush.set("pen", "#a2603a", 0.8);
  brush.beginShape(0.6);
  brush.vertex(378, 600);
  brush.vertex(412, 580);
  brush.vertex(454, 570);
  brush.endShape(false);
  brush.beginShape(0.6);
  brush.vertex(392, 612);
  brush.vertex(428, 598);
  brush.vertex(478, 594);
  brush.endShape(false);
  brush.set("pen", "#a2603a", 0.6);
  brush.beginShape(0.6);
  brush.vertex(372, 592);
  brush.vertex(396, 566);
  brush.vertex(424, 558);
  brush.endShape(false);
  brush.noStroke();
  brush.fill("#a2603a", 130);
  brush.circle(484, 592, 2.8, true);
  brush.fill("#a2603a", 96);
  brush.circle(458, 566, 2.2, true);
}