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Related works & Fundamental background
In this chapter, the commonly used printing technologies and their processes are briefly presented. Image degradation is described as well. We also review some proposed printing models taking into account image degradation and their specific applications. The advantages of each model and their limits are also exposed. In the latter, we describe the estimation methods that are used in the dissertation. Partic-ularly, frequentist methods such as maximum likelihood estimation are recalled, we also mention about the numerical methods for solving equations. Bayesian based Monte Carlo methods with a powerful approach like Monte Carlo Markov Chain is also recalled.
Printing Technologies and Document image degradation
Printing is a reproduction process which applies printing ink to a printing substrate (e.g. a sheet of paper) in a specific mean to store the information (graphics, text). In the experiment, we utilize offset printing technology which can be divided into two popular systems: conventional offset and waterless offset printing technology. We do an experiment on electrophotographic printer. Other commonly used printing technologie is inkjet.
Offset printing
This is a lithographic technology [29], based on the physical phenomena of the immiscible liquids. In this technology, the printing areas are ink-accepting whereas the non-printing parts are ink-repellent. The inked image is transferred from a printing plate, which contains the form of the image, to a rubber blanket, then to the printing surface. The principle process is shown in Fig. 2.1. There are two common systems to obtain an ink-repellent areas:
• Conventional offset printing: The image-carrier plate is dampened with damp-ening solution (water with additives). Only the non-printing areas are water-receptive (hydrophilic), whereas the image areas are almost totally unreceptive to solution. Afterward, the next rollers cover the printing parts with ink de-livered from the ink duct, the very fine film of dampening solution preventing the transfer of ink.
• Waterless offset printing: This process eliminates the water or dampening system used in conventional printing. Instead of using water-based solution, the printing plate is coated with a highly ink-repellent silicone on the ink-free areas. The printing ink is only accepted on the areas in which the silicone coating has been removed. Therefore, waterless offset offers really shortened make-ready times and simplifies the printing process. Fig. 2.2 shows the print-ing plates of conventional offset and waterless offset printing.
Electrophotography
This is also called laser printing technology. In this process, the laser beam is used to charge a suitable photoconductive surface on the drum with the image or text. Afterward, the charged drum collects the special inks, powder or liquid toners, which have the opposite charge with the photoconductor surface. This ink-carrier drum, then, transfers these toners to the printing substrate, these ink particles are transferred to the paper by the contact pressure between the drum and the paper surface. The paper sheet is then conveyed through two rollers; by heat and pressure, they create a stable image on the paper. Finally, individual particles of toner are cleaned by brush or/and suction, while residual charged areas are neutralized by homogenous illumination, then the drum is charged again for the new printing cycle. There are a number of patents linked to this technology
Inkjet
In this technology, the ink is transferred directly onto the paper. There are con-tinuous ink jet and drop on demand inkjet. In the continuous inkjet technology, a stream of ink is generated constantly, the mechanical phenomena by which a liquid stream disintegrates was studied by Lord Rayleigh [34], these droplets are charged according to the image. These charged droplets are deflected when they fly through the magnetic field. According to the imaging signal, the droplets are directed either onto the paper or back to the system. In this technique, the large part of the stream is driven back into the ink tank.
With the drop on demand technology, the ink is only released when the printing part comes. Thermal ink jet and piezo ink jet printing are the most important drop on demand technologies [29]. In the thermal ink jet, they use bubble which contains heat sensitive liquid; according to the imaging signal, the ink drops are squeezed out when the volume of the bubble is expanded by heat. With piezo ink jet, the ink drop is catapulted out of the nozzle when the imaging signal arrives and makes the jet chamber deform. These ink jet technologies are illustrated in Fig. 2.4 to human eyes. This loss of quality, even slightly, is mainly responsible for the inaccuracy of the text/image recognition systems. A great diversity of mechanical reasons degrading the text/image is reviewed in [35]:
• variations of paper positions (skew, translation, etc);
• flaking and weak adhesion of ink/toner to printed material; in particular, weak adhesion maybe a problem for laser printing;
• low print contrast, this phenomena is noticeable between conventional offset print and waterless offset print;
• smudging of the paper surface;
• spatter, scatter, and spreading of ink/toner;
• Dot gain includes mechanical and optical dot gain, detailed information about dot gain can be found in sec. A.2.
Printing modeling
There are a number of printing models proposed in years. Depending on the purposes of the applications such as in optical character recognition (OCR) or in print authen-tication, a specific model is considered in the scale of the experiments/applications. One of the main purpose of a printing model is to accurately simulate the printed document before the printing process, which may improve significantly the OCR systems or other document analysis issues. For instance, halftoning technique is a method that takes advantage of the optical illusion of human vision to display continuous-tone gray-scale image with only black or/and white dots [36, 37]. The fundamental idea is to induce variation in size, in shape or in spacing of the dots, which generate a gray like effect of the image (see Fig. 2.5). The scale of the model is the resolution of the printer. Model-based halftoning was developed to have richer gray tone as well as to obtain better quality images [16, 17, 18, 19, 20, 21, 22].
Table of contents :
1. Introduction
2. Related works & Fundamental background
2.1. Printing Technologies and Document image degradation
2.1.1. Offset printing
2.1.2. Electrophotography
2.1.3. Inkjet
2.1.4. Document image degradation
2.2. Printing modeling
2.3. Fundamental backgrounds
2.3.1. Methods of Estimation
2.4. Conclusion
3. Experimental analysis
3.1. Materials and set-up
3.1.1. Materials
3.1.2. Pattern analysis
3.1.3. Parameters of a 2D object
3.2. Analysis
3.2.1. Offset printing
3.2.2. Electrophotography printing
3.3. Conclusion
4. Microscopic printing Probabilistic-based model
4.1. Model construction
4.2. Ponctual parametric modeling
4.2.1. Theoretical analysis by moments and histogram
4.2.2. Histograms fitting
4.3. Conclusion
5. Spatial binary model
5.1. Binary response model
5.2. The Gaussian power kernel
5.3. Estimation methods
5.3.1. Maximum Likelihood estimation
5.3.2. Bayesian estimation method
5.4. Experimental results and performance
5.4.1. Maximum likelihood estimation
5.4.2. Bayesian estimation method
5.4.3. Estimation result from printed dots
5.5. Conclusion
6. Markov spatial binary model
6.1. Interaction model with Markov chain
6.2. Estimation
6.3. Experimental results and performance
6.3.1. Numerical performance with simulated data
6.3.2. Estimation result from printed dots
6.4. Conclusion
7. Conclusions
Bibliography
