The second method uses a general Hough transform approach that uses an edge map ... The geometrical transformation needed to carry out the registration.
A Study of Registration Methods for Ceramic Tile Inspection Purposes* F. López, F. Acebrón, J.M. Valiente, E. Pérez Universidad Politécnica de Valencia. Departamento de Informática de Sistemas y Computadores (DISCA) Camino de Vera s/n 46072 Valencia (SPAIN)
Abstract This work presents a study of two image registration methods for ceramic tile inspection purposes. Tile registration is the first step in detecting surface faults when a ‘compare with a reference’ technique is used. The presented methods are based on edge detection techniques, used to extract the bounding rectangle defined by the tiles. They differ in the approach used to obtain this rectangle. The first method only uses a reduced set of border tile points, obtained with the minimum cross entropy thresholding algorithm, and fits them to straight lines by the least-squares method. The second method uses a general Hough transform approach that uses an edge map of the tile image. The geometrical transformation needed to carry out the registration is directly obtained using a simple geometrical mapping. Several experiments have been used to compare and select the method that offers the best results from the point of view of registration quality and computer time cost. The study is restricted to tile models with fixed decorative surface patterns.
Keywords: image registration, ceramic tile inspection, surface inspection.
1 Introduction In the ceramic tile manufacturing industry, a reliable and automated quality control process has become a necessary tool for competitive purposes. In recent years these industries have made great efforts to automate all the processes in the production chain. This has been achieved in the majority of the processes and now automatic quality control is the aim, not only for the final product but also for the intermediate stages. *
Work funded by FEDER project 1FD97-0999.
The great majority of tile faults on the production lines are surface defects, therefore, surface inspection is one of the most important quality control tasks to be automated. But this is a complex task due to the diversity of surface faults and the large variety of decorative surface patterns that can be made on production lines (plain, grained, textured, ...etc). Furthermore, the patterns can be fixed, random or pseudo-random textured. The present work is restricted to fixed pattern models. The technique used to detect faults on this kind of tile model is based on direct comparison, on a pixel-by-pixel basis, between the tile image and a reference template pattern. Therefore, an accurate registration method of these two patterns is needed as a pre-processing stage. This paper is concerned with the study of several registration methods based on edge detection techniques. Using the straight lines that compose the bounding rectangle of the tile piece, the registration problem is translated into a problem of aligning two geometric figures, only differentiated by a geometrical transformation. Because tiles are rigid objects, no elastic, deformation, or scale transformations have been considered. Once the tiles have been aligned, is easy to obtain a map of errors by comparing both images pixel by pixel. The faults detected on the tile will be determined by studying this map of errors. This process should be achieved in the smallest possible time, in order to be able to adapt the inspection system to the production line ratios that work at 60 tiles per minute. Therefore, minimal computer time for the registration task, as well as the categorisation of quality, is one of the major goals.
2 Registration methods Image registration techniques, including template matching, image warping and object recognition, have been applied in a wide number of areas such as medical imaging, automatic inspection, remote sensing, and so on. A survey of image registration techniques can be found in [1]. However, there are very few works in the specific field of tile surface inspection. In a recent paper, Costa & Petrou [2] introduce an interesting approach based on a coarse-to-fine process. In an initial stage, they produce a coarse tile registration using an edge detector and the Hough transform for the identification of straight lines on the edge map. They then apply a geometrical transformation mapping of the test image onto the reference image. In a second stage, they refine the transformation parameters by using the more precise phase correlation method. The phase correlation method is a non-linear, Fourier-spectrum-based area correlation technique. It uses the phase contents of the cross-power spectrum to obtain the translation parameters between the reference and test images. The rotation parameters can also be obtained by using a polar transformation of the Fourier space.
In this work we proceed in a similar way, but trying to avoid the costly Fourier transform or area-based correlation processes. We assume that, in a tile inspection system, it is possible to produce a coarse alignment of tiles by using line-positioning devices, so we rely on edge detection techniques to obtain a precise computation of the bounding tile rectangle, as in [3]. Once the test and reference tile rectangles have been computed, they are registered using transformation mapping as follows:
[ U cos α = \ U − VLQ α
VLQ α [ G [ ⋅ + (1) cos α \ G \
2 S [ \ U U U
D
G[G\
where p(x,y) and pr(xr,yr) are the test and reference tile corners, dx and dy are the displacements between the top left points in the two tiles and α is the rotation angle. (O,X,Y) is the image reference coordinate system. This approach saves a great deal of computing time as it directly calculates the exact inverse transformation.
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