2D line intersection

From Ogre Wiki

bool vec2LinesIntersect ( Vector2 l1s, Vector2 l1f, Vector2 l2s, Vector2 l2f ){

  Vector2 v1 = l1f - l1s;
  
  Vector2 v2 = l2s - l1s;
  Vector2 v3 = l2f - l1s;
  
  Real crossProd1 = v1.crossProduct(v2);
  Real crossProd2 = v1.crossProduct(v3);
  
  if ( (crossProd1 > 0 && crossProd2 < 0) || (crossProd1 < 0 && crossProd2 > 0) 
|| (crossProd1 == 0 && crossProd2 == 0) ){
    
	v1 = l2s - l2f;
	
	v2 = l1s - l2f;
	v3 = l1f - l2f;
	
	crossProd1 = v1.crossProduct(v2);
	crossProd2 = v1.crossProduct(v3);	
	
	if ( (crossProd1 > 0 && crossProd2 < 0) || (crossProd1 < 0 && crossProd2 > 0) 
|| (crossProd1 == 0 && crossProd2 == 0) ){
	  return true;
	}
  }
  return false;
}

l1s is line 1 start, l1f is line 1 finish similarly l2s l2f