1. Field of the Invention
This invention relates, in general, to pictorial communications and, more specifically, to signal processing techniques for use with the electronic copiers, printers, and like devices.
2. Description of the Prior Art
Digital halftoning can be used in devices or systems, such as copiers and printers, where the pixels of the image data are represented by digital values. Halftoning is frequently used when the rendering device, or print engine, does not have the same gray-level capability as the input information. In halftoning, halftone cells are created wherein the overall density of the halftone cell is expected to be equal to the density of a group of pixels in the original input data. This provides the appearance of the same gray-level density for the area without the need for a gray-level rendering device. The most frequent use of digital halftoning is associated with reproducing gray-level input data with a binary output device. Although the overall density of the cell appears to be near the desired density, some sharpness information is lost from the original pixel data with many conventional halftone processing systems.
Error diffusion is another technique which may be used to convert gray-level values to binary values, or to gray-level values with a smaller number of usable levels. However, if error diffusion is used alone, the process stability of a single pixel dot at high resolution is a usual concern. In some processing, error diffusion and digital halftoning can be used together to provide the proper data for rendering the image on a binary device. This can provide good process stability and more overall-area gray-levels. If error diffusion is applied to each pixel of the halftone cell, much of the sharpness of the original data is preserved. However, pixel-to-pixel error diffusion is computationally intensive and can present major speed and hardware restraints. Error diffusion between halftone cells is much more economical as far as processing time is concerned, but a degree of sharpness is sacrificed with this arrangement.
U.S. Pat. No. 4,680,645, issued on July 14, 1987, discloses a method for rendering gray-level image data by a binary device. According to this patent, pixel-to-pixel error diffusion is used in conjunction with a variable sized dot rendering system. The variable sizes are modulated by the error in the gray-levels diffused or propagated to other pixels. Halftone cells are not used in this system. The background section of the patent includes a general discussion on error diffusion at column 1, lines 42-64.
In order to obtain an improved digital halftoning system, it is desirable, and an object of this invention, to provide a system for converting gray-scale data into binary data which accurately represents the density and sharpness information in the original image with reduced computations.