Image analysis – Histogram processing – For setting a threshold
Patent
1988-03-15
1989-05-23
Eisenzopf, Reinhard J.
Image analysis
Histogram processing
For setting a threshold
382 52, 36472401, G06K 938
Patent
active
048337234
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to image processing methods and apparatus for removing spot defects from an image reconstructed from processed image signals generated by scanning and sampling an original image.
BACKGROUND ART
Such a process takes the form of dividing up the original image into image elements or "pixels", generating an array of image values representative of the content of these pixels, processing these pixel image values so as to remove or reduce unwanted components therein and constructing an array of pixel image values corresponding to the desired processed image.
Images, in particular those scanned and displayed electronically, may be degraded by several forms of noise, including:
(1) Broadband noise affecting all pixels, for example photographic grain. A variety of image processing techniques have been proposed using filtering and transform methods for the suppression of broadband noise, e.g. in U.S. Pat. No. 4,442,454
(2) Noise consisting of randomly distributed modifications of image values corresponding to single or small groups of pixels. Such an effect has been referred to as "salt and pepper" noise because it may consist of random white and/or black spots. These may arise at various stages, including:
Methods to detect and remove isolated spot defects are described in Rosenfeld, A. and Kak, A. C., "Digital Picture Processing", Academic Press, 1976, and Pratt, W. K., "Digital Image Processing", Wiley, 1978. These procedures calculate the difference between the central pixel value and either the mean or the extreme of its neighbourhood pixels. The central pixel is replaced by a local average value if the difference exceeds an absolute threshold.
Such a technique inevitably results in loss of overall sharpness of the image as a large amount of high frequency information is lost especially in cases where the nature of the image and the spot defects necessitate the use of a low threshold.
SUMMARY OF THE INVENTION
The present invention provides a method which removes unwanted spot defects without unduly affecting overall sharpness for images even in cases in which the spot defects have image values less than the maximum or greater than the minimum values.
According to the present invention there is provided a method for processing an image for the removal of spot defects comprising scanning and sampling the image to generate an array of pixel image values and constructing a corresponding array of pixel image values for the processed image by carrying out the following steps:
(a) examining the image values of each pixel of a predetermined subset of pixels in a block surrounding a central pixel thereby determining the image value corresponding to the highest and lowest values in said predetermined subset of said block (A.sub.high and A.sub.low respectively),
(b) determining the image value corresponding to the central pixel of said block (A.sub.centre),
(c) calculating either the ratio: ##EQU1## or the ratio ##EQU2##
(d) if the ratio is positive and its value is above a predetermined threshold value, writing an average value calculated on image values in the block to the element of the processed image array corresponding to said central pixel or, if not, writing the existing value (A.sub.centre) to said element, and
(e) repeating steps (a) to (d) until every pixel has been processed as the central pixel.
By using the value R.sub.1 in step (c) low image value spot defects will be corrected. Alternatively by using R.sub.2 high image value spots will be corrected. If both R.sub.1 and R.sub.2 are calculated in step (c) and the corrected value is written to the processed image array if either of R.sub.1 and R.sub.2 is positive and above the threshold value both types of defect will be corrected. Only one of them, of course, can be positive for each block location.
It is preferred that the predetermined subset of pixels of step (a) comprises all the pixels lying on the periphery of the block. In a (3.times.3) pixel block there are, of course, 8 peripheral pixels and one ce
REFERENCES:
patent: 4442454 (1985-09-01), Powell
patent: 4541116 (1985-09-01), Lougheed
patent: 4571635 (1986-02-01), Mahmoodi
patent: 4630307 (1986-12-01), Cok
patent: 4644410 (1987-02-01), Schichtig
patent: 4747156 (1988-05-01), Wahl
patent: 4760541 (1988-07-01), Weygandt
Pratt; "Digital Image Processing"-Wiley-1978, p. 319.
Loveridge Jennifer C.
Powell Philip G.
Close Thomas H.
Eastman Kodak Company
Eisenzopf Reinhard J.
Solis Jose M.
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