utils.js

function lerp(a, b, t) {
  return a + (b - a) * t;
}
function getIntersection(a, b, c, d) {
  const tTop = (d.x - c.x) * (a.y - c.y) - (d.y - c.y) * (a.x - c.x);
  const uTop = (c.y - a.y) * (a.x - b.x) - (c.x - a.x) * (a.y - b.y);
  const bottom = (d.y - c.y) * (b.x - a.x) - (d.x - c.x) * (b.y - a.y);
  if (bottom != 0) {
    const t = tTop / bottom;
    const u = uTop / bottom;
    if (t >= 0 && t <= 1 && u >= 0 && u <= 1) {
      return {
        x: lerp(a.x, b.x, t),
        y: lerp(a.y, b.y, t),
        offset: t,
      };
    }
  }
  return null;
}

function polysIntersect(poly1, poly2) {
  for (let i = 0; i < poly1.length; i++) {
    for (let j = 0; j < poly2.length; j++) {
      const touch = getIntersection(
        poly1[i],
        poly1[(i + 1) % poly1.length],
        poly2[j],
        poly2[(j + 1) % poly2.length]
      );
      if (touch) {
        return true;
      }
    }
  }
  return false;
}
function getRGBA(value) {
  const alpha = Math.abs(value);
  const R = value < 0 ? 0 : 255;
  const G = R;
  const B = value > 0 ? 0 : 255;
  return "rgba(" + R + "," + G + "," + B + "," + alpha + ")";
}