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Thresholding technique for graphics images using histogram analysis
| Details |
Inventors: Pennebaker, William B.;
Assignee: International Business Machines Corporation (Armonk, NY)
Primary Examiner: Boudreau; Leo H.
Assistant Examiner: Couso; Jose L.
Attorney, Agent or Firm: Dowd; Thomas P., Ilardi; Terry J.
A system is described for thresholding graphics images to reduce the information content for transmission and reproduction and which uses a particular image data thresholding technique that may be used to obtain display images with a number of intensity levels. Firstly, an histogram of the image to be encoded is generated and analyzed to determine essential parameters. In particular, the background color of the image is determined by summing the histogram elements to find the median of the distribution, that is, the point at which the half sum of total pixels is reached. The extrema of the histogram are calculated and the skew is determined from the position of the median with respect thereto. If the difference between the median and the black extreme is less than half the difference between the median and the white extreme then the background of the image is deemed to be black, otherwise, the image will be defined as having a white background. After the background color is determined, the width of the background region, that is, the difference between the median and the foreground color extreme, is used to define the edge between the background and foreground regions, which edge occurs at one-half the background width from the median in the direction of the background color extreme. A sequence of threshold levels may then be determined using this edge as a base, by setting them at points disposed at fractionally different distances between the edge and the background color extreme. |
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DETAILED DESCRIPTION The present invention involves a method and apparatus for thresholding image data by determining three threshold levels in the image histogram that are used for producing four intensity levels of the image.
Firstly, an histogram of the image is generated and the median of the distribution is calculated by summing the histogram elements until the half sum is reached.
The extrema are calculated by summing from each end of the intensity range until respective small but significant numbers are found, and then scanning back toward the respective intensity range ends until zero elements are found in the histogram.
To determine the color of the background the skew of the histogram is then determined from the position of the median relative to the extrema.
Unless the document background can be clearly classified as black, that is, the difference between the median and the black extreme is less than half the difference between the white extreme and the median, then the image is defined to have a white background.
A measure of the width of the background region is then determined from the difference between the median and the extreme for the background color, and the edge between the background and foreground regions is defined as being at a distance of one half the background width measured from the median toward the foreground color extreme.
This convenient edge determination is then used as a base for calculating the sequence of threshold levels which are set as fractions of the difference between the edge and the foreground extreme.
A nonlinear scale is preferably used, the three levels being determined at 1/8, 1/4 and 1/2 the distance between the edge and the foreground extreme, or at 3/16, 3/8 and 3/4, which differences define the threshold levels for a white background image as between white/gray-white, gray-white/gray-black, and gray-black/black.
This technique is particularly suitable for use in a teleconferencing system for thresholding of images to a two-bit/pixel form such as described in U
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