In the paper

an on|line ship detecting system based on the probability velocity field is presented. First all environment factors that ships go through the ship lock are analyzed

such as lighting

shading

waving and so on; Second

a Gaussian probability distribution model(probability field) is generated from gray|level histogram to diminish the affection of shades and speckles

since shades and speckles' gray|levels are distributed in both sides of gray|level histogram

after histogram is fitted with the Gaussian distribution function

the probability of them is smaller than that of other gray|levels. Therefore a probability velocity field is defined and derived based on the optical flow to detect ships in lock on|line. The velocity field is calculated by the traditional optical flow estimation method using the probability of gray|levels

not the gray|levels. Finally the affection of shading and lighting is reduced greatly. The method has succeeded in testing a larger number of image sequences that ships pass through the temporary ship lock of the three gorges project(TGP) and ship lock of the Gezhou dam.