<title>Fast algorithms for automatic moire fringe analysis: application to noncontact measurements for quality control of industrial components</title>

Abstract

Moire methods are optical neasurernent methods , that are based on the effect of superposition ofgrating lines and have been widely used in the context of industrial applications for shape analysis for non-contact measurements and for quality control of industrial components . In apf)hcations the following computations: image filtering fringe skeletonising and fringe numbering have to be performed for each test object , before comparison between the numerically reconstructed test object shape and its CAD. inodel . in oider to ieduce the computing time required by the preceeding computations , the inverse nioiré technique has been intoduced by Harthong. Instead of using a grating made of parallel straight lines , the inverse moire technique uses a pre-computed specific grating , that is formed of curved lines such that the moire pattern is composed of parallel straight fringes if the test object shape is conform to its CAD. model . Defects are then characterized by a deformation and a curvature of these parallel fringes . In this paper , we present examples showing tlia.t standard fringe extraction by automatic thresholding is not that. easy. ic) OV(ICOfl1( this (liihcultv, wc propose a four stage process a.lgorithrnical approach that. al lows fri nge det.ect.ioi i i i nverse n ot r i m ages with high sensitivity and sJ)ecificity. lirs we used the well-known image processing technique called urisharp masking , to enhaiice moire iiiage and to emphasize low contrasted fringes. The second SteJ) is to extract bright fringes by image segmentation and constrained contour modeling. After deletion of these bright fringes inside the zone of interest of the moire image, we get the thick skeleton of adjacent background and of dark fringes. The third step is to skeletonize this thick skeleton of adjacent background and of dark fringes, using morphological thinning of well-composed sets , that assures that each fringe skeleton will be one pixel thick ,at the difference of standard thinning techniques . The fourth step is to apply a graph technique to isolate the individual dark fringes. When all these four steps have been followed , one is left with a binary iniage showing the dark fringe pattern skeleton . The experimental results that have been obtained have shown the robustness of this algorithmical approach for the analysis of noisy iiiverse nioiré 111 iages 1 Introduction Moire methods are optical measurement metho(ls [2], [7] , [8] , [12], that are based oii the effect of superposition of grating lines . These techniques have 54 / SPIE Vol. 2786 0-8194-21 72-3/96/$6.00 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/21/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

<title>Optoelectronic hit/miss transform for real-time defect detection by moire image analysis</title>

“…The first moire image (figure 1) corresponds to a cylinder without any deformation and the other ones (2,3,4,5) are characterized by an increasing deformation of the observed cylinder.…”

“…These algorithms integrate unsharp masking for low-contrasted fringe enhancement , segmentation of enhanced moire images by automatic thresholding, morphological thinning of binary thick skeleton patterns and extraction of individual black fringes by a graph technique. 'i'hese algorithms [3] are robust and able to correctly extract low-contrasted fringes, by using a standard computer. Hut, they are rather time consuming on a standard nhi(rocomputer.…”

“…model . Defects in the test object shape are then characterized by a deformation and a curvature of these parallel fringes Recently, robust algorithms have been designed and evaluated [3] for the automatic analysis of fringes in inverse moire images . These algorithms integrate unsharp masking for low-contrasted fringe enhancement , segmentation of enhanced moire images by automatic thresholding, morphological thinning of binary thick skeleton patterns and extraction of individual black fringes by a graph technique.…”