用于半色调图像的一种层次增强方法

A Dithering Method for Halftone Image Generation

半色调图像的层次分辨率是衡量半色调图像生成算法的一个重要指标 .在高质量图像硬拷贝设备上 ,其产生的半色调图像的层次分辨率必须达到较高的水平 ,才能保证图像复制的质量 .文中提出了一个基于线性像素置换理论的阈值矩阵抖动方法 ,该方法把一维线性像素置换扩展到了二维的情形 ,并使其作用于阈值矩阵的生成 .这个方法能在提高阈值矩阵的层次分辨率的同时 ,有效地避免附带产生的斑纹效应 .通过计算机的模拟显示和硬拷贝设备的实际输出试验 ,证明了此方法的有效性 .

Amplitude modulation screening is the most widely used screening method for printing industry. From the computer graphics point of the view, amplitude modulation screening is a kind of clustered dot ordered dithering. If the clustered dot ordered dithering is directly used in the amplitude modulation screening, only a limited amount of different intensity levels can be duplicated in the produced halftone image. Usually this amount is equal to the number of pixels contained in a screen dot, which does not meet the needs of the printing industry.Threshold matrix method is a commonly accepted technique for the amplitude modulation screening. The threshold matrix is a pre-calculated matrix that is used to decide the shape and align angle of the screen dots and number of different element values contained in the matrix equals to the maximum number of different intensities the produced halftone image can duplicate. Usually a threshold matrix contains hundreds of screen dots, so it is possible for the threshold matrix method to achieve a higher intensity duplication capability.To achieve higher intensity duplication capability for a threshold matrix, a permutation on the number of screen dots contained in the matrix must be calculated. Each permutation value is then used to shift the value of matrix elements of each screen dots contained in the matrix. After this shifting, there will be no two or more than two different matrix elements holding the same value, and so much more different intensity levels can be obtained in the produced halftone image.When the permutation is being calculated, attention must be paid on the artifacts appeared in the produced halftone image. The values in the permutation must be carefully arranged so that space distribution of the artifacts is scattered evenly and can not be seen by a naked eye.In this paper, investigations are focused on the employing linear pixel shuffling (LPS) technique to the generating of the permutation. Mathematically, the LPS is a permutation generator that can generate numerically evenly scattered permutation. We extends the evenness of the numerical scattering from one-dimensional case (vector) to the two-dimensional case (matrix) and investigats the effects of applying the 1-D and 2-D LPSs to the permutation calculation used in the threshold matrix. The result obtained by the experiment shows that the combination of 1-D and 2-D LPSs can obtain the most favorable halftone image and the method can be applied in the high quality image printing.