the implmention follow Digital Image Processing_3ed_Gonzalez step by step
myrgb2gray.m ---> rgb to gray
mygradient.m ---> get gradient
myblur.m ---> image blur
mywatershed.m ---> watershed function body
sepcor.m ---> special dilate function used by watershed function
regrow.m ---> region growing algorithm
get_waterline.m ---> save result as .mat
show_water_line.m ---> show result as image