In consideration of the irregular outline of surface crack

a subsection method for width evaluation was proposed.Based on digital image processing

the surface image was acquired and then was divided into several lengthways subsections

the width of each of which was respectively evaluated by Ferret algorithm.For comparative evaluation of diverse sectioning methods

a conception of useful information ratio was put forward for choosing among them an optimal method

by which the quantitative evaluation of crack width was obtained.Experimental data indicate that by using the method proposed in this paper

the width of surface crack can be quantitatively evaluated.The result can provide effective criterion for mechanism surface inspection

and moreover

this method can also be employed in quantitative evaluation for geometric dimensions of irregular graphics.