Self-conjugate watermarking technique for halftone images

A novel method for hiding visual patterns in a single error diffused halftone image is presented called self-conjugate error diffusion (SCED). The hidden patterns appear on the halftone image when the halftone image is folded or is overlapped with its own rotated version. Simulation results show that the halftone images have good visual quality and the hidden pattern is clearly visible.     

Inverse halftoning using binary permutation filters

The problem of reconstructing a continuous-tone image given its ordered dithered halftone or its error-diffused halftone image is considered. We develop a modular class of nonlinear filters that can reconstruct the continuous-tone information preserving image details and edges that provide important visual cues. The proposed nonlinear reconstruction algorithms, denoted as binary permutation filters, are based on the space and rank orderings of the halftone samples provided by the multiset permutation of the "on" pixels in a halftone observation window. For a given window size, we obtain a wide range of filters by varying the amount of space-rank ordering information utilized in the estimate. For image reconstructions from ordered dithered halftones, we develop periodically space-varying filters that can account for the periodical nature of the underlying screening process. A class of suboptimal but simpler space-invariant reconstruction filters are also proposed and tested. Constrained LMS type algorithms are employed for the design of reconstruction filters that minimize the reconstruction mean squared error. We present simulations showing that binary permutation filters are modular, robust to image source characteristics, and that they produce high visual quality image reconstruction.     

Robust Watermarking with Kernels-Alternated Error Diffusion and Weighted Lookup Table in Halftone Images

A halftone watermarking method of high quality, robustness, and capacity flexibility is presented in this paper. An objective halftone image quality evaluation method based on the human visual system obtained by a least-mean-square algorithm is also introduced. In the encoder, the kernels-alternated error diffusion (KAEDF) is applied. It is able to maintain the computational complexity at the same level as ordinary error diffusion. Compared with Hel-Or using ordered dithering, the proposed KAEDF yields a better image quality through using error diffusion. We also propose a weighted lookup table (WLUT) in the decoder instead of lookup table (LUT), as proposed by Pei and Guo, so as to achieve a higher decoded rate. As the experimental results demonstrate, this technique is able to guard against degradation due to tampering, cropping, rotation, and print-and-scan processes in error-diffused halftone images.     

Modeling and quality assessment of halftoning by error diffusion

Digital halftoning quantizes a graylevel image to one bit per pixel. Halftoning by error diffusion reduces local quantization error by filtering the quantization error in a feedback loop. In this paper, we linearize error diffusion algorithms by modeling the quantizer as a linear gain plus additive noise. We confirm the accuracy of the linear model in three independent ways. Using the linear model, we quantify the two primary effects of error diffusion: edge sharpening and noise shaping. For each effect, we develop an objective measure of its impact on the subjective quality of the halftone. Edge sharpening is proportional to the linear gain, and we give a formula to estimate the gain from a given error filter. In quantifying the noise, we modify the input image to compensate for the sharpening distortion and apply a perceptually weighted signal-to-noise ratio to the residual of the halftone and modified input image. We compute the correlation between the residual and the original image to show when the residual can be considered signal independent. We also compute a tonality measure similar to total harmonic distortion. We use the proposed measures for edge sharpening, noise shaping, and tonality to evaluate the quality of error diffusion algorithms.     

基于色调处理技术的图像认证算法

基于色调处理技术,该文给出了一种有效可行且具有自修复能力的图像认证算法。首先,基于误差扩散色调处理技术将水印图像4bit色调量化,井依据混沌置乱算子,将色调结果置乱,然后构造平均误差最小的特征集合C,最后依据误差扩散数据隐藏算法将置乱后水印图像隐藏于原始图像中;在认证端,从接收到的图像提取其中所隐藏水印信息并进行逆置乱,比较接收到的图像和反置乱后的隐藏信息,判断内容发生变化的位置,并依据所提取的水印信息修复被篡改图像。实验结果表明,该算法对删除、替换、篡改等破坏图像内容的恶意操作有精确的检测和定位,以及自修复能力。     

Adaptive error diffusion and its application in multiresolutionrendering

Error diffusion is a procedure for generating high quality bilevel images from continuous-tone images so that both the continuous and halftone images appear similar when observed from a distance. It is well known that certain objectionable patterning artifacts can occur in error-diffused images. Here, we consider a method for adjusting the error-diffusion filter concurrently with the error-diffusion process so that an error criterion is minimized. The minimization is performed using the least mean squares (LMS) algorithm in adaptive signal processing. Using both raster and serpentine scanning, we show that such an algorithm produces better halftone image quality compared to traditional error diffusion with a fixed filter. Based on the adaptive error-diffusion algorithm, we propose a method for constructing a halftone image that can be rendered at multiple resolutions. Specifically, the method generates a halftone from a continuous tone image such that if the halftone is down-sampled, a binary image would result that is also a high quality rendition of the continuous-tone image at a reduced resolution. Such a halftone image is suitable for progressive transmission, and for cases where rendition at several resolutions is required. Cases for noninteger scaling factors are also considered.     

Joint halftoning and watermarking

A framework to jointly halftone and watermark a grayscale images is presented. The framework needs the definition of three components: a human visual system (HVS)-based error metric between the continuous-tone image and a halftone, a watermarking scheme with a corresponding watermark detection measure, and a search strategy to traverse the space of halftones. We employ the HVS-based error metric used in the direct binary search (DBS) halftoning algorithm, and we use a block-based spread spectrum watermarking scheme and the toggle and swap search strategy of DBS. The halftone is printed on a desktop printer and scanned using a flatbed scanner. The watermark is detected from the scanned image and a number of post-processed versions of the scanned image, including one restored in Adobe PhotoShop. The results show that the watermark is extremely resilient to printing, scanning, and post-processing; for a given baseline image quality, joint optimization is better than watermarking and halftoning independently. For this particular algorithm, the original continuous-tone image is required to detect the watermark.     

Memory efficient error diffusion

Because of its good image quality and moderate computational requirements, error diffusion has become a popular halftoning solution for desktop printers, especially inkjet printers. By making the weights and thresholds tone-dependent and using a predesigned halftone bitmap for tone-dependent threshold modulation, it is possible to achieve image quality very close to that obtained with far more computationally complex iterative methods. However, the ability to implement error diffusion in very low cost or large format products is hampered by the requirement to store the tone-dependent parameters and halftone bitmap, and also the need to store error information for an entire row of the image at any given point during the halftoning process. For the first problem, we replace the halftone bitmap by deterministic bit flipping, which has been previously applied to halftoning, and we linearly interpolate the tone-dependent weights and thresholds from a small set of knot points. We call this implementation a reduced lookup table. For the second problem, we introduce a new serial block-based approach to error diffusion. This approach depends on a novel intrablock scan path and the use of different parameter sets at different points along that path. We show that serial block-based error diffusion reduces off-chip memory access by a factor equal to the block height. With both these solutions, satisfactory image quality can only be obtained with new cost functions that we have developed for the training process. With these new cost functions and moderate block size, we can obtain image quality that is very close to that of the original tone-dependent error diffusion algorithm.     

Entropy-constrained halftoning using multipath tree coding

We suggest an optimization-based method for halftoning that involves looking ahead before a decision for each binary output pixel is made. We first define a mixture distortion criterion that is a combination of a frequency-weighted mean square error (MSE) and a measure depending on the distances between minority pixels in the halftone. A tree-coding approach with the ML-algorithm is used for minimizing the distortion criterion to generate a halftone. While this approach generates halftones of high quality, these halftones are not very amenable to lossless compression. We introduce an entropy constraint into the cost function of the tree-coding algorithm that optimally trades off between image quality and compression performance in the output halftones.     

Data hiding watermarking for halftone images

In many printer and publishing applications, it is desirable to embed data in halftone images. We proposed some novel data hiding methods for halftone images. For the situation in which only the halftone image is available, we propose data hiding smart pair toggling (DHSPT) to hide data by forced complementary toggling at pseudo-random locations within a halftone image. The complementary pixels are chosen to minimize the chance of forming visually undesirable clusters. Our experimental results suggest that DHSPT can hide a large amount of hidden data while maintaining good visual quality. For the situation in which the original multitone image is available and the halftoning method is error diffusion, we propose the modified data hiding error diffusion (MDHED) that integrates the data hiding operation into the error diffusion process. In MDHED, the error due to the data hiding is diffused effectively to both past and future pixels. Our experimental results suggest that MDHED can give better visual quality than DHSPT. Both DHSPT and MDHED are computationally inexpensive.     

A new framework for characterization of halftone textures.

Characterization of halftone texture is important for quantitative assessment of halftone quality. In this paper, we develop a new framework based on directional local sequency analysis and a filter bank structure. We decompose a halftone image into subband images, from which we can easily reconstruct the original halftone. Based on these subband images, we define the directional sequency spectrum which is analogous to the two-dimensional Fourier spectrum, and formulate several texture measures. Two test image sets are used to justify these measures.     

A Survey of Electronic Techniques for Pictorial Image Reproduction

This paper is a tradeoff study of image processing algorithms that can be used to transform continuous tone and halftone pictorial image input into spatially encoded representations compatible with binary output processes. A large percentage of the electronic output marking processes utilize a binary mode of operation. The history and rationale for this are reviewed and thus the economic justification for the tradeoff is presented. A set of image quality and processing complexity metrics are then defined. Next, a set of algorithms including fixed and adaptive thresholding, orthographic pictorial fonts, electronic screening, ordered dither, and error diffusion are defined and evaluated relative to their ability to reproduce continuous tone input. Finally, these algorithms, along with random nucleated halftoning, the alias reducing image enhancement system (ARIES), and a new algorithm, selective halftone rescreening (SHARE), are defined and evaluated as to their ability to reproduce halftone pictorial input.     

自适应反馈视觉感知差的误差扩散半色调算法

文章基于人眼对亮度的视觉感受特性。提出了一种自适应反馈视觉感知差的误差扩散算法（PEF Error Diffusion算法）。首先提出视觉感知差的概念，然后根据原图像区域的灰度特征，自适应地计算反馈系数，将视觉感知差反馈给原连续调图像，以补偿误差扩散所引起的不同区域的灰度损失。实验结果表明，由于视觉感知差的引入及反馈系数的自适应变化。PEF Error Diffusion算法能够明显增强图像的整体对比度．减弱点增益现象的不良影响，并准确再现更多的图像细节．因而表现出比传统算法更好的主观视觉效果。     

Hierarchical Error Diffusion

This paper develops a distinctive class of color error diffusion algorithm, called hierarchical error diffusion (HED). It aims to achieve perceptually pleasing color halftone through neither conventional joint quantization nor interchannel error diffusion. Instead, it explicitly controls three critical factors sequentially to yield high-quality color halftone: dot-overlapping control, dot-positioning control, and dot-coloring control. A specific implementation of HED is presented with the objective of minimum brightness variation rendering (MBVR). First, an optimal color transform is derived for dot-overlapping control to achieve minimum brightness variation color density (MBVCD). Then, the embedded monochrome error diffusion is employed in dot-positioning control. By sequentially thresholding the elements in partial density sum vector, better dot-positioning is encouraged for more visible color dots. The ldquoblue noiserdquo characteristics of dot-positioning from the monochrome error diffusion are inherited by the color halftone. The simple density priority strategy is applied in dot-coloring control. The pixel color error is diffused channel-independently with a single error filter in halftone dot color space. A comparison with the state-of-the-art color error diffusion algorithms demonstrates excellent halftone quality of HED, while without the typical artifacts of vector error diffusion. Evidence also shows that HED is closer to achieve MBVR than the minimum brightness variation quantization (MBVQ) color diffusion algorithm proposed in.     

A fast, high-quality inverse halftoning algorithm for errordiffused halftones

Halftones and other binary images are difficult to process with causing several degradation. Degradation is greatly reduced if the halftone is inverse halftoned (converted to grayscale) before scaling, sharpening, rotating, or other processing. For error diffused halftones, we present (1) a fast inverse halftoning algorithm and (2) a new multiscale gradient estimator. The inverse halftoning algorithm is based on anisotropic diffusion. It uses the new multiscale gradient estimator to vary the tradeoff between spatial resolution and grayscale resolution at each pixel to obtain a sharp image with a low perceived noise level. Because the algorithm requires fewer than 300 arithmetic operations per pixel and processes 7x7 neighborhoods of halftone pixels, it is well suited for implementation in VLSI and embedded software. We compare the implementation cost, peak signal to noise ratio, and visual quality with other inverse halftoning algorithms.     

Tree-structured method for LUT inverse halftoning and for imagehalftoning

The authors previously proposed a look up table (LUT) based method for inverse halftoning of images. The LUT for inverse halftoning is obtained from the histogram gathered from a few sample halftone images and corresponding original images. Many of the entries in the LUT are unused because the corresponding binary patterns hardly occur in commonly encountered halftones. These are called nonexistent patterns. In this paper, we propose a tree structure which will reduce the storage requirements of an LUT by avoiding nonexistent patterns. We demonstrate the performance on error diffused images and ordered dither images. Then, we introduce LUT based halftoning and tree-structured LUT (TLUT) halftoning. Even though the TLUT method is more complex than LUT halftoning, it produces better halftones and requires much less storage than LUT halftoning. We demonstrate how the error diffusion characteristics can be achieved with this method. Afterwards, our algorithm is trained on halftones obtained by direct binary search (DBS). The complexity of TLUT halftoning is higher than the error diffusion algorithm but much lower than the DBS algorithm. Also, the halftone quality of TLUT halftoning increases if the size of the TLUT gets bigger. Thus, the halftone image quality between error diffusion and DBS will be achieved depending on the size of the tree-structure in the TLUT algorithm     

基于矢量量化的误差扩散半色调图像有损压缩算法

随着误差扩散半色调图像在书刊、杂志、打印输出和传真文件中广泛应用和大量传播,有必要对这类特殊的二值图像进行压缩以利于节省存储空间且加快传输速度。提出一种基于矢量量化思想并结合人眼视觉特征的误差扩散半色调图像有损压缩方法。首先,原始图像被分成若干个4×4的像素块,将这些块分别与一个模拟人眼视觉特性的5×5高斯滤波器做卷积。然后,将得到的8×8的卷积结果作为输入矢量,经过LBG算法训练得到一个码书。之后,对每个码字,找到与其最相似的4×4的像素块作为最终的码字。这样就建立了用来压缩原始图像的码书。最后一步就是利用该码书用传统的矢量量化思想压缩原始图像并得到最终的码字索引。仿真实验结果表明通过该方法得到的压缩图像的视觉质量得到进一步的提高。该方法在压缩比和保持图像质量取得了较好的折中。     

Model-based color halftoning using direct binary search.

In this paper, we develop a model-based color halftoning method using the direct binary search (DBS) algorithm. Our method strives to minimize the perceived error between the continuous tone original color image and the color halftone image. We exploit the differences in how the human viewers respond to luminance and chrominance information and use the total squared error in a luminance/chrominance based space as our metric. Starting with an initial halftone, we minimize this error metric using the DBS algorithm. Our method also incorporates a measurement based color printer dot interaction model to prevent the artifacts due to dot overlap and to improve color texture quality. We calibrate our halftoning algorithm to ensure accurate colorant distributions in resulting halftones. We present the color halftones which demonstrate the efficacy of our method.     

Embedded multilevel error diffusion

We present an algorithm for image browsing systems that embeds the output of binary Floyd-Steinberg (1975) error diffusion, or a low bit-depth gray-scale or color error diffused image into higher bit-depth gray-scale or color error diffused images. The benefits of this algorithm are that a low bit-depth halftoned image can be directly obtained from a higher bit-depth halftone for printing or progressive transmission simply by masking one or more bits off of the higher bit-depth image. The embedding can be done in any bits of the output, although the most significant or the least significant bits are most convenient. Due to constraints on the palette introduced by embedding, the image quality for the higher bit-depth halftone may be reduced. To preserve the image quality, we present algorithms for color palette organization, or binary index assignment, to be used as a preprocessing step to the embedding algorithm.     

基于离散Sibson插值的反半色调算法

本文提出了一种基于离散Sibson插值的盲反半色调方法.算法建立在统一的离散Voronoi图框架之上,通过自适应Voronoi区域估计得到种子点的灰度值,然后通过离散Sibson插值将传统的反半色调问题转化为插值问题并计算获取连续色调图像.算法不需要知道任何先验知识,因此算法具有更广的适用性,可以被应用于任意方法生成的...