Facsimile and static presentation processing – Static presentation processing – Attribute control
Patent
1997-08-29
2000-07-18
Rogers, Scott
Facsimile and static presentation processing
Static presentation processing
Attribute control
358458, 382266, 382274, H04N 1405, H04N 1409, G06T 500
Patent
active
060915110
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to digital image processing in general and, more particularly, to enhancement of image resolution and contrast.
BACKGROUND OF THE INVENTION
An image to be printed or displayed is generally represented by an array of pixel elements, sometimes referred to as a bit-mapped array, which function as an array of control codes which control the operation of printing-elements or display-elements which, in turn, produce the printed or displayed image. The bit-mapped array is generally constructed based on an input, continuous tone, image using a halftoning process which may incorporate hardware or software or a combination of both. Each pixel element in the bit-mapped array is represented by data bits which define attributes of the printed or displayed pixel, such as pixel color and gray level. A monochrome image is generally defined by a single array of pixel elements, wherein the data bits of each pixel element represent the gray level of the printed or displayed pixel. A color image is generally defined by a plurality of arrays of pixels, wherein each array corresponds to a base-color, such as cyan, magenta, yellow and black.
Halftoning methods and apparatus are described, for example in U.S. Pat. No. 4,499,489, to Gall et al., and in U.S. Pat. No. 4,084,183, to Keller et al.
The resolution of an image is generally determined by the density of pixels used to represent the image. The range of available gray levels per pixel is generally determined by the number of data bits used for representing each pixel. For example, a system using 8 bits per pixel yields a range of 256 gray levels per pixel, in monochrome printing, or 64 gray levels per pixel in 4-color printing.
Due to limitations in processing speed and data storage capacity, the resolution and gray level separation capability of existing imaging systems is limited, particularly when images are processed "on the fly" in high speed printing systems. Therefore, in existing systems, image resolution must often be compromised in order to obtain a wider gray level range or vice versa.
SUMMARY OF THE INVENTION
The present invention seeks to provide a system and a method for enhancing both the resolution and the gray level range of a digitally reproduced image without increasing the amount of digital information required to define the image.
According to the present invention, a detection circuit or algorithm is applied to an input, continuous tone (CT), digital image which is to be printed, displayed or otherwise reproduced. The detection circuit or algorithm is utilized to distinguish between portions of the CT image to be reproduced at a high resolution but with a narrow gray-level range and portions of the CT image to be reproduced at a low resolution but with a wide gray-level range. The high-resolution, narrow gray level, portions are preferably represented by a relatively high pixel density, for example 800 dots per inch (DPI), but each pixel has a relatively narrow range of gray levels, for example one bit per pixel. The low resolution, wide gray level, portions are preferably represented by a lower pixel density, for example 200 DPI, but each pixel has a wider range of gray levels, for example 7 bits per pixel.
In a preferred embodiment of the invention, the detection circuit or algorithm includes edge detection circuitry or an edge-detection algorithm which analyzes sub-arrays of the digital image, for example 4 pixel by 4 pixel sub-arrays of the digital image. An "edge" sub-array is detected when the gray level contrast within the analyzed sub-array exceeds a predetermined threshold. A "non-edge" sub-array is detected when the gray level contrast within the analyzed sub-array is below the predetermined threshold. The gray level contrast may be defined, for example, as the difference or the ratio or any other suitable relation between the highest gray level and the lowest gray level within the analyzed sub-array. Subsequently, the "edge" sub-arrays are represented using the high resolution, low-g
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Carlsohn, M..F. "Proceedings From The SPIE. Conf: Advances In Image Processing", vol. 804, pp. 344-355. Mar. 31, 1987, The Hague," Adaptive Selection of Block Aperture in Transform Coding of Images.".
Peleg, S. IEEE Transactions on Patten Analysis and Machine Intelligence, "A Unified Approach to the Change of Resolution: Space and Gray-Level.", vol. 11, No. 7, pp. 739-742, Jul. 1989.
Ben Dror Yoav
Niv Yehuda
Indigo N.V.
Rogers Scott
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