Image gradient detectors operating in a partitioned low-pass cha

Details Image gradient detectors operating in a partitioned low-pass cha Image gradient detectors operating in a partitioned low-pass cha

1981-04-16

1984-05-01

Martin, John C.

Facsimile and static presentation processing

Facsimile

Specific signal processing circuitry

358167, H04N 514, H04N 521

Patent

active

044464843

DESCRIPTION:

BRIEF SUMMARY
This invention relates to electronic image processing systems using image dissection, for example, by solid state devices or by scanning and sampling.
Electronic image processing systems are known in which signals from a predetermined array of picture elements, or pels, are summed and averaged using electronic circuits so as to generate a signal or signals displaying selected components of the spatial fluctuations in image brightness. Appropriate weighting or multiplying factors are applied to the signals from the individual pels constituting the array in order to produce the desired response. One such known circuit generates a signal displaying mainly the large scale spatial fluctuations of the image brightness, the circuit acting as a low pass spatial filter. Another known circuit acts as a high pass spatial filter and generates a signal displaying mainly the small scale fluctuations that characterize the fine detail of the image. In practice, a set of filters having a high pass characteristic are utilized to generate signals corresponding to respective selected components of the spatial fluctuations in image brightness.
These filter circuits can be used at the same time on the signals from one array of pells, and if the combined weighting on each of the pels due to the low pass circuit and the set of circuits having the high pass characteristic is zero on all but one pel of the array, then these filters are complementary. Under this condition, addition of the output signals from the filters regenerates the input signal unchanged, except for any change in gain.
The two dimensional spatial frequency response of such complementary filters may be illustrated diagrammatically as performing a partitioning of the spatial frequency plane into two areas, a central area surrounding and including the zero frequency origin and representing the pass band of the low pass filter, and the area outside this central region and up to the band limit of the system representing the pass band of the high pass filter. The use of such complementary low pass and high pass spatial filters at the same time enables the high frequency components of the final scanned image to be selectively modified by amplification or attenuation for the purposes of image enhancement or noise suppression.
Electronic circuits performing the function of spatial filters as described have also been employed to detect the occurrence of predetermined image features. Each particular set of pel weightings, sometimes called a mask, is chosen to match a desired image feature. Thus a one pel, or point, feature detector on a 3.times.3 pel array can be formed by the set of pel weightings:


______________________________________ -1 -1 -1 -1 8 -1 -1 -1 -1 ______________________________________ form a high pass spatial filter. The complementary low pass filter would be formed by the set of weightings:


______________________________________ 1 1 1 1 1 1 1 1 1 ______________________________________


______________________________________ 0 0 0 0 9 0 0 0 0 ______________________________________ gain.
Similarly, particular vertical and horizontal line detectors on a 3.times.3 pel array, known in the prior art, are formed respectively by the sets of weightings


______________________________________ -1 2 -1 -1 -1 -1 -1 2 -1 and 2 2 2 -1 2 -1 -1 -1 -1 ______________________________________ spatial filtering techniques for feature detection thus involves summing the signals from an array of pels after weighting the signals in accordance with the characteristics of the feature required. If the absolute value of such a weighted sum exceeds a predetermined threshold, then the particular image feature associated with that set of weightings is assumed to have been detected.
Among the most important constituents of images are edges or brightness transitions. In order to detect such transitions, particular vertical and horizontal edge detectors on a 3.times.3 pel array, known in the prior art are formed respectively by

REFERENCES:
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patent: 3980813 (1976-09-01), Shinkai
patent: 3987243 (1976-10-01), Schwartz
patent: 4003024 (1977-01-01), Riganati et al.
patent: 4058828 (1977-11-01), Ladd
patent: 4213150 (1980-07-01), Robinson et al.
patent: 4220972 (1980-09-01), Geokezas et al.
patent: 4259694 (1981-03-01), Liao
Rossi, "Digital Techniques for Reducing Television Noise", SMPTE Journal, Mar. 1978, vol. 87 pp. 134-140.
Robinson et al., "A Real-Time Edge Processing Unit", Workshop on Picture Data Description and Management, Apr. 21/22, 1977 (NY:IEEE) pp. 155-164.
Baker et al., "Multiple Bandpass Filters in Image Processing", IEEE Proceedings, vol. 127, Pt. E, No. 5, Sep. 1980, pp. 173-184.

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