doSegmentsIntersect static method

bool doSegmentsIntersect(Offset p1, Offset p2, Offset q1, Offset q2,)

Return true if two segments intersect

Implementation

static bool doSegmentsIntersect(Offset p1, Offset p2, Offset q1, Offset q2) {
  double dx1 = p2.dx - p1.dx;
  double dy1 = p2.dy - p1.dy;
  double dx2 = q2.dx - q1.dx;
  double dy2 = q2.dy - q1.dy;

  /// crossProduct is the cross product of the vectors formed by the segments.
  double crossProduct = dx1 * dy2 - dy1 * dx2;
  if (crossProduct == 0) {
    // The segments are parallel, they do not intersect
    return false;
  }

  /// t represents the position along the segment p1-p2 where the segments intersect.
  /// If t is between 0 and 1, this means that the point of intersection is located on the segment p1-p2.
  double t1 = ((q1.dx - p1.dx) * dy2 - (q1.dy - p1.dy) * dx2) / crossProduct;

  /// u represents the position along segment q1-q2 where the segments intersect.
  /// If u is between 0 and 1, it means that the point of intersection is located on the segment q1-q2.
  double u1 = ((q1.dx - p1.dx) * dy1 - (q1.dy - p1.dy) * dx1) / crossProduct;

  /// The points p1 and q1 are inverted, this makes it possible to check whether the segment q1-q2 is
  /// in the opposite direction with respect to the segment p1-p2. This inversion makes it possible to correct
  /// the signs of the values of u and to avoid incorrect results in certain specific cases.
  double t2 = ((q1.dx - p1.dx) * dy2 - (q1.dy - p1.dy) * dx2) / crossProduct;
  double u2 = ((p1.dx - q1.dx) * dy1 - (p1.dy - q1.dy) * dx1) / crossProduct;

  if (t1 >= 0 && t1 <= 1 && u1 >= 0 && u1 <= 1 ||
      t2 >= 0 && t2 <= 1 && u2 >= 0 && u2 <= 1) {
    // Segments intersect at a point
    return true;
  }

  // Segments do not intersect
  return false;
}