Line Collision Detection

Found this excellent explanation of line-to-line collision and many others.

Boolean function

boolean linesTouching(float x1, float y1, float x2, float y2, float x3, float y3, float x4, float y4) {
  float denominator = ((x2 - x1) * (y4 - y3)) - ((y2 - y1) * (x4 - x3));
  float numerator1 = ((y1 - y3) * (x4 - x3)) - ((x1 - x3) * (y4 - y3));
  float numerator2 = ((y1 - y3) * (x2 - x1)) - ((x1 - x3) * (y2 - y1));

  // Detect coincident lines (has a problem, read below)
  if (denominator == 0) return numerator1 == 0 && numerator2 == 0;

  float r = numerator1 / denominator;
  float s = numerator2 / denominator;

  return (r >= 0 && r <= 1) && (s >= 0 && s <= 1);
}

Returns point of contact

PVector lineIntersection(float x1, float y1, float x2, float y2, float x3, float y3, float x4, float y4) {

  // calculate the distance to intersection point
  float uA = ((x4-x3)*(y1-y3) - (y4-y3)*(x1-x3)) / ((y4-y3)*(x2-x1) - (x4-x3)*(y2-y1));
  float uB = ((x2-x1)*(y1-y3) - (y2-y1)*(x1-x3)) / ((y4-y3)*(x2-x1) - (x4-x3)*(y2-y1));

  // if uA and uB are between 0-1, lines are colliding
  if (uA >= 0 && uA <= 1 && uB >= 0 && uB <= 1) {
    return new PVector(x1 + (uA * (x2-x1)), y1 + (uA * (y2-y1)));
  }
  return null;
}