Method and apparatus for effecting spot/void filtering of image data

• US 5,029,226 A
• Filed: 10/10/1989
• Issued: 07/02/1991
• Est. Priority Date: 10/10/1989
• Status: Expired due to Term

• Current Assignee: Unisys Corporation
• Original Assignee: Unisys Corporation
• Inventors: Klein, Robert, Williams, David C.
• Primary Examiner(s): Boudreau, Leo H.
• Application Number:
  US07/419,595
• Time in Patent Office: 630 Days
• Field of Search: 382/50-54, 358/447, 358/455, 358/465, 358/466, 235/470
• US Class Current: 382/275
• CPC Class Codes: G06T 1/20 Processor architectures; Processor configuration, e.g. pipelining

Abstract

Apparatus for filtering digital image data which is initially presented in the form of an array of multi-bit pixels having plural scale values. Threshold means are provided for converting the scale value pixels to an array of binary pixels in which each pixel is associated with either background or image formation data. Patterns of neighboring pixels are examined and used to transform a center pixel in the window as a function of the detected pattern in a manner to remove isolated spots or voids from the image data.

Claims

1. Apparatus for filtering digital image data in the form of an array of multi-bit pixels each having one of a plurality of gray-scale values, said apparatus comprising:

threshold means for storing a predetermined threshold and for converting said gray-scale value of each of said pixels to an array of binary signals in which each pixel has a binary signal associated therewith, one value of said binary signal being associated with said gray-scale value being greater than said predetermined threshold and a second value of said binary signal being associated with said gray-scale value being less than said predetermined threshold;

means for detecting a pattern of neighboring binary pixels in a window containing a center pixel being analyzed and its neighbors; and

transforming means for changing the gray-scale value of the center pixel in the window as a function of the detected pattern, the transforming means comprising averager means for averaging the gray-scale values of the pixels in the window, and selector means having a first input connected to the averager means, a second input connected to a predetermined gray-scale value associated with background, a third input connected to a predetermined gray-scale value associated with image formation data, a fourth input providing a gray-scale value of the center pixel being analyzed, and selection input means coupled to the means for detecting, operative to couple one of the inputs to the selector means to an output of the selector means as a function of a detected pattern of neighboring binary pixel values.

2. The apparatus of claim 1 wherein spots in said image data are removed by instructing said selector means to select said second input as its output in the event that the center pixel in the window has a value normally associated with image formation data but is surrounded by neighbors with values associated with background.

3. The apparatus of claim 1 which further comprises:

means for detecting invalid data for the center pixel in the window which represents a void in the image data; and

wherein said transforming means assigns the center pixel with a gray-scale value as a function of gray-scale values of its neighbors.

4. The apparatus of claim 1 wherein the means for detecting comprises a look-up table coupled for receipt of a pattern of binary pixels in the window and operative to generate selection control signals at a look-up table output coupled to the selection input means.

5. Apparatus for filtering digital image data in the form of a first array of multi-bit pixels each having one of a plurality of gray-scale values, said apparatus comprising:

threshold means for storing a predetermined threshold and for converting the gray-scale value of each of the pixels to a binary signal and placing binary signals of each pixel in the first array into a second array of binary signals in which each gray-scale pixel has a binary signal associated therewith, one value of the binary signal being associated with a gray-scale value being greater than the predetermined threshold and a second value of the binary signal being associated with a gray-scale value being less than the predetermined threshold;

control signal generating means coupled for receipt of the second array of binary signals and operative to generate a control signal at an output of the control signal generating means as a function of a pattern of neighboring binary pixels in the second array surrounding a center pixel being analyzed; and

selection means having a control input coupled for receipt of the control signal and a plurality of gray-scale inputs, each having a different predetermined gray-scale value coupled thereto, the selection means operative in accordance with the control signal to substitute one of the plurality of predetermined gray-scale values for a gray scale value of the center pixel being analyzed.

6. A method of filtering digital image data in the form of a first array of pixels each having one of a plurality of gray-scale values, said method comprising:

converting each gray-scale value in the first array into one of two binary values in accordance with a predetermined threshold gray-scale value;

placing each binary signal value into a second array of binary pixels, each binary pixel corresponding to one of the gray-scale pixels in the first array;

generating a control signal as a function of a pattern of binary signals in the second array; and

selecting one of a plurality of predetermined gray-scale values in accordance with the control signal and replacing a gray-scale value of a preselected pixel in the first array with the selected one predetermined gray-scale value.