Novel methods in image halftoning

Ankara : Department of Electrical and Electronics Engineering and Institute of Engineering and Science, Bilkent Univ., 1998.Thesis (Master's) -- Bilkent University, 1998.Includes bibliographical references leaves 97-101Halftoning refers to the problem of rendering continuous-tone (contone) images on display and printing devices which are capable of reproducing only a limited number of colors. A new adaptive halftoning method using the adaptive QR- RLS algorithm is developed for error diffusion which is one of the halftoning techniques. Also, a diagonal scanning strategy to exploit the human visual system properties in processing the image is proposed. Simulation results on color images demonstrate the superior quality of the new method compared to the existing methods. Another problem studied in this thesis is inverse halftoning which is the problem of recovering a contone image from a given halftoned image. A novel inverse halftoning method is developed for restoring a contone image from the halftoned image. A set theoretic formulation is used where sets are defined using the prior information about the problem. A new space domain projection is introduced assuming the halftoning is performed ,with error diffusion, and the error diffusion filter kernel is known. The space domain, frequency domain, and space-scale domain projections are used alternately to obtain a feasible solution for the inverse halftoning problem which does not have a unique solution. Simulation results for both grayscale and color images give good results, and demonstrate the effectiveness of the proposed inverse halftoning method.Bozkurt, GözdeM.S

Restoration of error-diffused images using projection onto convex sets

Cataloged from PDF version of article.In this paper, a novel inverse halftoning method is proposed to restore a continuous tone image from a given half-tone image. A set theoretic formulation is used where three sets are defined using the prior information about the problem. A new spacedomain projection is introduced assuming the halftoning is performed using error diffusion, and the error diffusion filter kernel is known. The space-domain, frequency-domain, and space-scale domain projections are used alternately to obtain a feasible solution for the inverse halftoning problem which does not have a unique solution

Descreening of Color Halftone Images in the Frequency Domain

Scanning a halftone image introduces halftone artifacts, known as Moiré patterns, which significantly degrade the image quality. Printers that use amplitude modulation (AM) screening for halftone printing position dots in a periodic pattern. Therefore, frequencies relating halftoning are easily identifiable in the frequency domain. This paper proposes a method for descreening scanned color halftone images using a custom band reject filter designed to isolate and remove only the frequencies related to halftoning while leaving image edges sharp without image segmentation or edge detection. To enable hardware acceleration, the image is processed in small overlapped windows. The windows are filtered individually in the frequency domain, then pieced back together in a method that does not show blocking artifacts

Watermarking-Based Inpainting Under Data Transmition Environment

[[abstract]]This studyb proposes a novel image inpainting technique based on watermarking and halftoning. This technique use LSB method to embed error diffusion halftone image into original image for protecting the image. In image repair process, we use LSB method to extract the halftone information, and the reference image is achieved from LUT inverse halftone. Finally we use the reference imageto finish the image inpainting work. Experiment shows the performance of our method is very excellent in image inpainting.[[conferencetype]]國際[[conferencedate]]20101206~20101208[[booktype]]電子版[[iscallforpapers]]Y[[conferencelocation]]Chengdu, Chin

Clustered-dot periodic halftone screen design and ICC profile color table compression

This dissertation studies image quality problems associated with rendering images in devices like printing or displaying. It mainly includes two parts: clustered-dot periodic halftone screen design, and color table compression. Screening is a widely used halftoning method. As a consequence of the lower resolution of digital presses and printers, the number of printer-addressable dots or holes in each microcell may be too few to provide the requisite number of tone lev- els between paper white and full colorant coverage. To address this limitation, the microcells can be grouped into supercells. The challenge is then to determine the desired supercell shape and the order in which dots are added to the microcell. Using DBS to determine this order results in a very homogeneous halftone pattern. To simplify the design and implementation of supercell halftone screens, it is common to repeat the supercell to yield a periodically repeating rectangular block called the basic screen block (BSB). While applying DBS to design a dot-cluster growth order- ing for the entire BSB is simpler to implement than is the application of DBS to the single non-rectangular supercell, it is computationally very inefficient. To achieve a more efficient way to apply DBS to determine the microcell sequence, we describe a procedure for design of high-quality regular screens using the non-rectangular super- cell. A novel concept the Elementary Periodicity Set is proposed to characterize how a supercell is developed. After a supercell is set, we use DBS to determine the micro-cell sequence within the supercell. We derive the DBS equations for this situation, and show that it is more efficient to implement. Then, we mainly focus on the regular and irregular screen design. With digital printing systems, the achievable screen angles and frequencies are limited by the finite addressability of the marking engine. In order for such screens to generate dot clusters in which each cluster is identical, the elements of the periodicity matrix must be integer-valued, when expressed in units of printer-addressable pixels. Good approximation of the screen sets result in better printing quality. So to achieve a better approximation to the screen sets used for commercial offset printing, irregular screens can be used. With an irregular screen, the elements of the periodicity matrix are rational numbers. In this section, first we propose a procedure to generate regular screens starting from midtone level. And then we describe a procedure for design of high-quality irregular screens based on the regular screen design method. We then propose an algorithm to determine how to add dots from midtone to shadow and how to remove dots from midtone to highlight. We present experimental results illustrating the quality of the halftones resulting from our design procedure by comparing images halftoned with irregular screens using our approach and a template-based approach. We also present the evaluation of the smoothness and improvement of the proposed methods. In the next part, we study another quality problem: ICC profile color table compression. ICC profiles are widely used to provide transformations between different color spaces in different devices. The color look-up tables (CLUTs) in the profiles will increase the file sizes when embedded in color documents. In this chapter, we discuss compression methods that decrease the storage cost of the CLUTs. For DCT compression method, a compressed color table includes quantized DCT coefficients for the color table, the additional nodes with large color difference, and the coefficients bit assignment table. For wavelet-based compression method, a compressed color table includes output of the wavelet encoding method, and the additional nodes with large color difference. These methods support lossy table compression to minimize the network traffic and delay, and also achieves relatively small maximum color difference

Wavelet Methods in Steganography

This paper is devoted to graphic information protection by means of computer steganography using wavelet techniques. The methods of the image processing using digital watermarking are classified. Some research of the structure of the digital image is investigated. In the first solution the steganographic system, which usesa discrete wavelet transform for embedding of grayscale watermark into wavelet coefficients of grayscale container is mоdeled and realized. Stego image is achieved by the inverse discrete wavelet transform. Binarization using halftoning algorithm is considered as a kind of powerful attack to stego image. After binarization attack the extracted watermark was found to have acceptable visual quality. The degradation ofthe embedded digital watermark after binarization is estimated using human visual system. Corresponding conclusions on steganographic system stability are drawn. In the second solution we propose the steganographic scheme which uses the proposed wavelet basis images

Minimization of Halftone Noise in FLAT Regions for Improved Print Quality

The work in this thesis proposes a novel algorithm for enhancing the quality of flat regions in printed color image documents. The algorithm is designed to identify the flat regions based on certain criteria and filter these regions to minimize the noise prior and post Halftoning so as to make the hard copy look visibly pleasing. Noise prior to halftone process is removed using a spatial Gaussian filter together with a Hamming window, concluded from results after implementing various filtering techniques. A clustered dithering is applied in each channel of the image as Halftoning process. Furthermore, to minimize the post halftone noise, the halftone structure of the image is manipulated according to the neighboring sub-cells in their respective channels. This is done to reduce the brightness variation (a cause for noise) between the neighboring subcells. Experimental results show that the proposed algorithm efficiently minimizes noise in flat regions of mirumal gradient change in color images

Optimising Spatial and Tonal Data for PDE-based Inpainting

Some recent methods for lossy signal and image compression store only a few selected pixels and fill in the missing structures by inpainting with a partial differential equation (PDE). Suitable operators include the Laplacian, the biharmonic operator, and edge-enhancing anisotropic diffusion (EED). The quality of such approaches depends substantially on the selection of the data that is kept. Optimising this data in the domain and codomain gives rise to challenging mathematical problems that shall be addressed in our work. In the 1D case, we prove results that provide insights into the difficulty of this problem, and we give evidence that a splitting into spatial and tonal (i.e. function value) optimisation does hardly deteriorate the results. In the 2D setting, we present generic algorithms that achieve a high reconstruction quality even if the specified data is very sparse. To optimise the spatial data, we use a probabilistic sparsification, followed by a nonlocal pixel exchange that avoids getting trapped in bad local optima. After this spatial optimisation we perform a tonal optimisation that modifies the function values in order to reduce the global reconstruction error. For homogeneous diffusion inpainting, this comes down to a least squares problem for which we prove that it has a unique solution. We demonstrate that it can be found efficiently with a gradient descent approach that is accelerated with fast explicit diffusion (FED) cycles. Our framework allows to specify the desired density of the inpainting mask a priori. Moreover, is more generic than other data optimisation approaches for the sparse inpainting problem, since it can also be extended to nonlinear inpainting operators such as EED. This is exploited to achieve reconstructions with state-of-the-art quality. We also give an extensive literature survey on PDE-based image compression methods