Grain Against the Hand

Usage

Press two distinct places to write directional grain along their connecting corridor. Release to inspect the remaining ribs, then press the center to reveal the grain through a leaning shoulder. Each press replaces the temporary mound and updates grain using the preceding contact in the current phrase. Wait strength controls forgetting duration; wait coordinates are unused. Optional parameters: learning, default 0.8, and forgetting, default 0.24, both clamped to 0–1. Frame zero is the untouched surface; subsequent event_index values count completed events. Metrics report spatially averaged grain and cumulative changes in thousandths.

Source SHA-256: 3dc287f17d687f8ec6bceb133469521eca33ea08bcbaea91175a5230a3fa2d45

Published source

function simulate(input) {
  var W = 24, H = 18, N = W * H;
  var gx = new Array(N).fill(0), gy = new Array(N).fill(0);
  var dent = new Array(N).fill(0), base = [], xs = [], ys = [];
  var frames = [], previous = null, probe = null;
  var presses = 0, written = 0, erased = 0;
  var hash = 2166136261;
  for (var k = 0; k < input.seed.length; k++) {
    hash = Math.imul(hash ^ input.seed.charCodeAt(k), 16777619) >>> 0;
  }
  var phase = (hash % 10000) / 10000 * Math.PI * 2;
  var parameters = input.parameters || {};
  function parameter(name, fallback, lo, hi) {
    var v = parameters[name];
    return typeof v === 'number' && Number.isFinite(v) ? Math.max(lo, Math.min(hi, v)) : fallback;
  }
  var forgetting = parameter('forgetting', 0.24, 0, 1);
  var learning = parameter('learning', 0.8, 0, 1);
  function bell(dx, dy, radius) {
    return Math.exp(-(dx * dx + dy * dy) / (2 * radius * radius));
  }
  for (var j = 0; j < H; j++) {
    for (var i = 0; i < W; i++) {
      var x = i / (W - 1), y = j / (H - 1);
      xs.push(x); ys.push(y);
      base.push(350 + 28 * Math.sin(2 * Math.PI * x + phase) * Math.sin(Math.PI * y) + 16 * Math.cos(3 * Math.PI * y - phase));
    }
  }
  function snapshot(index, label) {
    var values = [];
    for (var n = 0; n < N; n++) {
      var rx = xs[n] - 0.5, ry = ys[n] - 0.5;
      var grain = gx[n] * rx + gy[n] * ry;
      var height = base[n] + 155 * grain + dent[n];
      if (probe !== null) {
        var dx = xs[n] - probe.x, dy = ys[n] - probe.y;
        var footprint = bell(dx, dy, 0.23);
        height += 950 * probe.s * footprint * (gx[n] * dx + gy[n] * dy);
      }
      values.push(Math.round(Math.max(0, Math.min(1000, height))));
    }
    frames.push({event_index:index, label:label, visual:{kind:'field', width:W, height:H, values:values, palette:['#071316','#24565E','#5FA69C','#D5CE92','#FFF1CB'], description:'Height and color encode surface displacement. Shallow directional ribs persist after release; a press raises a mound and amplifies the stored grain into an asymmetric shoulder.'}});
  }
  snapshot(0, 'Unwritten grain');
  for (var e = 0; e < input.events.length; e++) {
    var event = input.events[e];
    var px = event.x / 1000, py = event.y / 1000, s = event.strength / 1000;
    if (event.kind === 'press') {
      presses++;
      if (previous !== null) {
        var vx = px - previous.x, vy = py - previous.y;
        var length = Math.sqrt(vx * vx + vy * vy);
        if (length > 0) {
          var ux = vx / length, uy = vy / length;
          var reach = Math.min(1, length / 0.35);
          for (var a = 0; a < N; a++) {
            var ax = xs[a] - previous.x, ay = ys[a] - previous.y;
            var t = Math.max(0, Math.min(1, (ax * vx + ay * vy) / (length * length)));
            var ex = xs[a] - (previous.x + t * vx);
            var ey = ys[a] - (previous.y + t * vy);
            var amount = learning * s * reach * bell(ex, ey, 0.17);
            var nx = gx[a] * (1 - amount) + ux * amount;
            var ny = gy[a] * (1 - amount) + uy * amount;
            written += Math.sqrt((nx - gx[a]) * (nx - gx[a]) + (ny - gy[a]) * (ny - gy[a]));
            gx[a] = nx; gy[a] = ny;
          }
        }
      }
      for (var b = 0; b < N; b++) {
        dent[b] = 210 * s * bell(xs[b] - px, ys[b] - py, 0.16);
      }
      probe = {x:px, y:py, s:s};
      previous = {x:px, y:py};
      snapshot(e + 1, 'Press ' + presses + ': grain bends the shoulder');
    } else if (event.kind === 'release') {
      dent.fill(0); probe = null; previous = null;
      snapshot(e + 1, 'Released: direction remains');
    } else {
      var retention = Math.pow(1 - forgetting, 1 + 3 * s);
      for (var c = 0; c < N; c++) {
        erased += (1 - retention) * Math.sqrt(gx[c] * gx[c] + gy[c] * gy[c]);
        gx[c] *= retention; gy[c] *= retention;
        dent[c] *= retention;
      }
      if (probe !== null) probe = {x:probe.x, y:probe.y, s:probe.s * retention};
      snapshot(e + 1, 'Wait: grain fades');
    }
  }
  var magnitude = 0, horizontal = 0, vertical = 0;
  for (var m = 0; m < N; m++) {
    magnitude += Math.sqrt(gx[m] * gx[m] + gy[m] * gy[m]);
    horizontal += gx[m]; vertical += gy[m];
  }
  return {frames:frames, metrics:{press_count:presses, mean_grain_magnitude_milli:Math.round(1000 * magnitude / N), mean_horizontal_grain_milli:Math.round(1000 * horizontal / N), mean_vertical_grain_milli:Math.round(1000 * vertical / N), cumulative_grain_change_milli:Math.round(1000 * written / N), cumulative_grain_erased_milli:Math.round(1000 * erased / N)}};
}