Andreas Vrabl

Fast real-time recognition and quality inspection of printed characters via point correlation

Some technical applications need a fast and reliable OCR for critical circumstances like low resolution and poor contrast. A concrete example is the real-time quality inspection system of Austrian banknotes. One requirement to the system is that it has to read two serial numbers on each banknote and to check if they are identical To solve the problem we have developed a novel method based an idea similar to pattern matching. However, instead of comparing entire images we use reduced sets of pixels, one for each different numeral. The detection is performed by matching these pixel sets with the corresponding pixels in the image being analyzed. We present an algorithm based on two cost functions that computes in a reasonable time the reduced pixel sets from a given set of image templates. The efficiency of our OCR has been increased considerably by introducing an appropriate set of image preprocessing operations. These are tailored especially to images with low resolution and poor contrast, bu they are simple enough to allow a fast real-time implementation. They can be seen as a normalization step that improves the image properties which are essential for pattern matching.

Region based matching for print process identification

For quality inspection of security printing systems it is necessary to measure the displacement between printing processes. We present a new approach for region based matching of color images. Maximally stable extremal regions are extracted from image color channels and are the basis for matching. Binary template matching is performed between pairs of regions taken from the corresponding color channels of different images and a displacement vector is derived for each matching pair of regions. Clustering of measured displacements taken from sequences of sample images allows the estimation of the accuracy of printing processes and the alignment of printing processes. Results of an experimental application to banknote printing process inspection are given.

SIS-Stamp: an integrated inspection system for sheet prints in stamp printing application

Postal stamps with print defects are valuable collectables for philatelists. However, stamp printers strive to emit exclusively defect-free stamps. In order to achieve high quality of the production process, each sheet is inspected by especially trained staff. This kind of inspection is very flexible but it is tedious and leads to unstable and irreproducible results. Based on the experience in quality inspection, the ARC Seibersdorf Research Team set the goal to design an automated sheet inspection system (SIS), in which the front and rear side of the sheet are quality-inspected in a fully automatic mode using image processing methods. The print inspection covers each single stamp in the sheet and includes, among others, misalignments of the individual print phases and the perforation, detecting defects like smears, splashes and missing parts of the print. The features to be inspected, as well as the defect sensitivity, can be defined by a special setup program. The patented mechanical sheet transportation system separates the sheets, transports them to the inspection stations, and finally, stacks them in two trays for good and defective sheets. The prototype of the SIS-Stamp is installed in the Austrian State Printing Office (Osterreichische Staatsdruckerei, OSD) for final inspection of postal stamps and vouchers.

Print process separation using interest regions

For quality inspection of printing systems it is necessary to measure the displacement between printing processes. Tie points are employed in correspondence and displacement estimation between individual print elements. We compare interest point and region descriptors for tie point detection in industrial inspection tasks. Clustering of measured displacements taken from sequences of sample images allows the estimation of the accuracy of printing processes and the alignment of printing processes. Results of an experimental application to banknote printing process inspection are given.