Method for converting a multiple-density image into a binary den

Details Method for converting a multiple-density image into a binary den Method for converting a multiple-density image into a binary den

1990-08-24

1991-12-24

Harkcom, Gary V.

Image analysis

Histogram processing

For setting a threshold

382 51, 358465, G06K 938

Patent

active

050758721

ABSTRACT:
An apparatus converts a multi-density image into a binary value image by converting pixels with densities above a threshold to a first value and pixels below a threshold to a second value. The apparatus generates a histogram relating image densities with the numbers of pixels having those densities. A first memory holds the image pixel densities. A second memory has address inputs attached to the data outputs of the first memory. Addresses of the second memory correspond to image densities. As a pixel is read out of the first memory, its density value serves as an address for a location in the second memory, and a count stored in that address is incremented. After reading all of the pixels from the first memory and incrementing counts, the second memory holds a histogram of the image. The apparatus evaluates the histogram to identify a density for which the number of pixels in the image is a local minimum. The local minimum is selected as the threshold. The apparatus begins with a density having a maximum number of pixels, and evaluates densities in increasing or decreasing order. Densities are evaluated rapidly under a first condition until the numbers of pixels approach a local minimum. Then a second condition is used to evaluate densities until selecting a threshold.

REFERENCES:
patent: 4601057 (1986-07-01), Tsuji et al.
patent: 4656665 (1987-04-01), Pennebaker
patent: 4710822 (1987-12-01), Matsunawa
patent: 4742557 (1988-05-01), Ma
patent: 4791678 (1988-12-01), Iwase et al.
patent: 4847786 (1989-07-01), Wang et al.
Campbell and Dierker, Calculus with Analytic Geometry, 2nd Edition, 1978, pp. 122-127.

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