def intersect(a1, b1, a2, b2):
	"""finds the intersection of two lines"""
	x = (b1 - b2)/(a2 - a1)
	y = a1*x + b1
	return x,y


	#  '''
	# returns list_tips, a list of spiral tips.  Returns [] for no spiral tips.
	# contours_raw is the list of contour points for cond1, contours_edge is the list of 
	# contour points with cond2''' 
	# # if not contours_raw.any():
	# #   return []  
	# #  if not contours_edge.any():
	# #   return []   
	# # if not contours_edge.any():
	# # return []   
	# #put contour sample points into a dataframe