Table 1. Color difference checking.

	Data: Image Im, binary mask M, contours C, direction di, max distance kmax, color threshold Td = 20
1	Initialize track of background thickness TH for each point in C
2	for point p in C do
3	k = 2
4	while k ≤ kmax do
5	Find point q: distance p to q in direction di is k
6	if q not exist then
7	break
8	end
9	if M[q] == 1 then
10	d = | Im[p] – Im[q] |
11	if d > Td then
12	TH[p] = k
13	Break
14	end
15	k = k + 1
16	end
17	end
18	end