An apparatus for binarizing image signals

Abstract: An apparatus for converting image signals into binary signals is disclosed. The apparatus comprises: a binarizing section for binarizing image signals (31,41); a connectivity distribution section (51,61) for obtaining connectivities between pixels from binary signals and for producing a connectivity distribution; and a judging section (7) for, from connectivity distributions obtained with different binary levels, judging the level at which the frequencies of maximum and minimum connectivities are highest and the sum of the frequencies of intermediate connectivities is minimal.

Description: This invention relates to an apparatus for binarizing image signals, and more particularly to an apparatus for converting image signals generated from a television camera or the like into binary signals, as referred to in claim 1. Figure 7 shows a conventional apparatus of such a kind. The apparatus of Figure 7 comprises an A/D converter 55, a density distribution memory 56, and a control unit 57. The image signals of an object 53 and a background 53a obtained by a television camera 54 are supplied to the A/D converter 55, and converted into digital signals. The digital image signals are stored in the density distribution memory 56. The control unit 57 determines a threshold level for binarizing the object53 (hereinafter, such a threshold level is referred to as "a binary level"), from the density distritution of the image signals as described below. Figure 8 shows one example of the density distribution of the image signals stored in the memory 56. As shown in Figure 8, the density distribution of the image signals obtained from the object 53 and background 53a has two peaks or maximal values 58 and 59 and one valley or minimal value 60. The control unit 57 judges the minimal value 60 sandwiched between the two maximal values 58 and 59 as the binary level, and then converts the image signals into binary signals by using the thus-determined binary level. When the density of the object 53 is not uniform, however, the density distribution will have two or more valleys as shown in Figure 9. In such a case, it is impossible to determine the binary level uniquely. When there is little difference in density between the object 53 and the background 53a moreover, the contour of the image may be obscure, with the result that the contour of the obtainend binary image becomes unstable. Further apparatus for controlling grey levels or analyzing an image of a pattern are disclosed in US-A-4,742,551, in US-A-4,601,0576 and in GB-A-2 129 635 as well as in the magazine "Systems & Computers in Japan", vol. 18, no. 11, November 1987, pages 91-100. The apparatus for binarizing image signals of this invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, comprises a binarizing means for binarizing image singals; a connectivity distribution means for obtaining connectivities between pixels from binary signals and for producing a connectivity distribution where the temporary binary level, at which frequencies of the minimum connectivity and the maximum connectivity are high and the sum of the frequencies of intermediate connectivities are minimal in the connectivity distribution, is judged as the optimum binary level; and a judging means for, from connectivity distributions obtained with different binary levels, judging the level at which the frequencies of maximum and minimum connectivities are highest and the sum of the frequencies of intermediate connectivities is minimal. Preferred embodiments are defined by the features in the subclaims. The apparatus for binarizing image signals of this invention further comprises a smoothing means for smoothing binary image signals before arriving at said connectivity distribution means. According to another preferred embodiment the apparatus for binarizing image signals of this invention further comprises a filtering means for filtering image signals before arriving at said binarizing means. According to a further preferred embodiment the apparatus for binarizing image signals of this invention further comprises a plurality of said binarizing means each for temporarily binarizing the same set of image signals with a different binary level; a plurality of said connectivity distribution means provided correspondingly for each of said binarizing means and each for obtaining connectivites between pixels from binary image signals obtained by the corresponding binarizing means and for obtaining a frequency distribution of connectivities; and said judging means is for judging one of said different binary levels as an optimum binary level for said set of image signals, the frequency distribution obtained by using said one of said different binary levels showing that the frequencies of maximum and minimum connectivities are highest and the sum of the frequencies of the intermediate connectivities is minimal. In another preferred embodiment of the apparatus for binarizing image signals according to this invention a plurality of said smoothing means are provided correspondingly for each of said binarizing means and each for smoothing said binary image signals obtained by the corresponding binarizing means for each of said plurality of connectivity distribution means. According to a further preferred embodiment, the apparatus for binarizing image signals of this invention further comprises a filtering means for filtering a set of image signals to be directed to said plurality of binarizing means. In a preferred embodiment, each of the connectivity distribution means obtains connectivities of 4-neighbors. In a preferred embodiment, each of the connectivity distribution means obtains connectivities of 8-neighbors. In a preferred embodiment, each of the connectivity distribution means obtains connectivities between pixels separated from each other by one or more pixels. Thus, the invention described herein makes possible the objectives of (1) providing an apparatus for binarizing image signals which can obtain uniquely the binary level even when the density of the object is not uniform; and (2) providing an apparatus for binarizing image signals which can obtain stably binary images even when there is little difference in density between the object and the background. This invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follows: Figure 1 is a block diagram showing an apparatus according to the invention. Figure 2 is a diagram illustrating connectivities of 4-neighbors obtained in the apparatus of Figure 1. Figure 3A is a diagram illustrating an image of binary signals. Figure 3B is a diagram illustrating connectivities obtained from the binary image of Figure 3A. Figure 4 shows histograms of connectivity distributions obtained with different binary levels. Figure 5 is a block diagram showing another apparatus according to the invention. Figure 6 is a block diagram showing a further apparatus according to the invention. Figure 7 is a block diagram showing a conventional apparatus. Figure 8 is a graph of density distribution obtained in the apparatus of Figure 7. Figure 9 is another graph of density distribution obtained in the apparatus of Figure 7. Figure 1 shows an apparatus according to the invention. The apparatus shown in Figure 1 converts image signals of an object 1 and background la obtained by a television camera 2 into binary image signals, and comprises binarization units B₁ - B N , and a control unit 7. The binarization units B₁ - B N include binarizing sections 3₁ - 3 N , binary signal memory sections 4₁ - 4 N , connectlvity distribution sections 5₁ - 5 N , and connectivity memory sections 6₁ - 6 N , respectively. The binarization units B₁ - B N are controlled by the control unit 7. Actually, the binarization units B₁ - B N and control unit 7 are realized by a computer system having a CPU, ROM and RAM. Image signals representing pixels of the object 1 and background 1a are generated by the television camera 2, and supplied to the binarizing sections 3₁ - 3 N . In each of the binarizing sections 3₁ - 3 N , the image signals are temporarily converted into binary signals with a predetermined binary level. The binary levels of the binarizing sections 3₁ - 3 N are different from each other, and determined so that they distribute uniformly in a predetermined range. The binary signals output from the binarizing sections 3₁ - 3 N are stored respectively in the binary signal memory sections 4₁ - 4 N . Figure 3A shows one example of images of binary signals. In Figure 3A, the hatched squares indicate pixels of "1", and the blank squares indicate pixels of "0". The connectivity distribution sections 5₁ - 5 N obtain 4-neighbor connectivities of each pixels, from the stored binary signals. The manner of obtaining the connectivities will be described. When the function of connectivity distribution is h(n) and the binary signal at coordinate (x, y) is S(x, y), the distribution of the connectivity group n is obtained by calculating the following expressions: n = S(x,y) + S(x-1,y) + S(x+1,y) + S(x,y-1) + S(x,y+1) h(n) = h(n) + 1 over the entire area of the object 1 and background 1a to be recognized. The relationship of expression (1) is illustrated in Figure 2. When all of an object pixel and its 4-neighbors are "0", the connectivity of the pixel is "0" ((a) of Figure 2). When only one of an object pixel and its 4-neighbors is "0", the connectivity is "1" ((b) of Figure 2). Similarly, connectivities "2" to "5" are obtained ((c) to (f) of Figure 2). The connectivities of the image signals of Figure 3A which are calculated by the expression (1) are shown in Figure 3B. The thus-calculated connectivities are arranged in the form of histograms as shown in Figure 4 which indicate the distribution of connectivities, and stored in the connectivity memory sections 6₁ - 6 N , respectively. The control unit 7 reads the connectivity distributions from the connectivity memory sections 6₁ - 6 N , and judges one of the temporary binary levels at which the frequencies of the maximum and minimum connectivities are high and the sum of the frequencies of the intermediate connectivities is minimal, as the binary level. More specifically, the temporary binary level at which the frequencies of the minimum connectivity "0" and the maximum connectivity "5" are high and the sum of the frequencies of the intermediate connectivities "1" to "4" are minimal in the connectivity distribution is judged as the optimum binary level. In a stable binary image, the frequencies of the maximal and minimal connectivities are high, and those of the intermediate connectivities are low. If the temporary binary level is too low, the frequencies in 4-neighbor connectivity distribution concentrate around connectivity "5" as shown in (a) of Figure 4. If the temporary binary level is too high, in contrast, the frequencies concentrate around connectivity "0" as shown in (c) of Figure 4. At the optimum binary level, the object 1 and background 1a are clearly separated, and since the binary noise disappears the frequencies of connectivity concentrate at connectivity "0" for the background 1a and at connectivity "5" for the object 1, and the frequencies of the intermediate connectivities "1" - "4" become very low. In this way, connectivity distributions obtained at different temporary binary levels are examined, and the temporary binary level at which a connectivity distribution is obtained wherein the frequencies of the maximum and minimum connectivities are high and the sum of the frequencies of the intermediate connectivities is low is judged as the optimum binary level, whereby the optimum binary level can be uniquely determined even when the density of the object is not uniform. Thereafter, the control unit 7 converts the image signals output from the camera 2 into binary signals, by using the thus-determined binary level. Alternatively, the control unit 7 recognizes as the proper binary signals the binary signals stored in one of memory section 4₁ - 4 N which belongs to the binarization unit producing the optimum binary level. The connectivity distribution sections 5₁ - 5 N may obtain 8-neighbor connectivities of each pixels instead of 4-neighbor connectivities. Also, it is not necessary to obtain connectivities with respect to all pixels, but it is possible to obtain connectivities with respect to pixels which are arbitrarily selected and are separated by intervals. In the apparatus of Figure 1, the operation of temporarily converting the image signals into binary signals is concurrently conducted in all of the binarization units B₁ - B N . The apparatus may be provided with only one binarization unit, and designed so that plurality of such operations with different binary levels are conducted in series by the sole binarization unit. Figure 5 shows another apparatus according to the invention. In the apparatus of Figure 5, smoothing sections 8₁ - 8 N are disposed respectively between the binarizing sections 3₁ - 3 N and the binary signal memory sections 4₁ - 4 N . Other components may be the same as those of the apparatus of Figure 1. The smoothing sections 8₁ - 8 N performs one of known smoothing methods (e.g., by magnifying and reducing the images) to smooth the binary image signals output from the binarizing sections 3₁ - 3 N . The smoothed binary signals are stored in the binary signal memory sections 4₁ - 4 N . According to the above-described configuration, the signal converted to a binary signal is smoothed, and the connectivity distributions of the smoothed image signals obtained at different temporary binary levels are examined, whereby the optimum binary level at which a stabilized image of the object 1 is obtained can be uniquely determined. Figure 6 shows a further apparatus according to the invention. The apparatus of Figure 6 comprises the same configuration as that of the apparatus of Figure 5, except that a filtering unit 9 having a plurality of filtering characteristics is disposed between the television camera 2 and the binarization units B₁ - B N . The filtering unit 9 has, and performs the filtering operation in accordance with one of the filtering characteristics. The image signals output from the television camera 2 are supplied to the filtering unit 9 in which the image signals are filtered according to the filtering characteristics selected by a command from the control unit 7 so as to match the image characteristics of the object 1 and background 1a. The command for selecting one of the filtering characteristics may be produced in accordance with the manual selection by an operator. Alternatively, the control unit 7 may select automatically one of the filtering characteristics by performing a feedback control, so that resulting binary images can be optimized. Then, the filtered image signals are supplied to binarizing sections 3₁ - 3 N of the binarization units B₁ - B N , and subjected to the same processes as described above. By filtering the image signals, the edges of the image are emphasized, and thus the contour shape can be adequately extracted. In the above-described configuration, after being filtered, the image signals are temporarily converted into binary signals, the binary signals are then smoothed, and the optimum binary level is determined on the basis of the smoothed binary signals. Therefore, the optimum binary level at which a stable contour shape of the object is obtained can be determined even when the density difference between the object 1 and the background 1a is small.

Claims: An apparatus for binarizing image signals comprising: a binarizing means (B₁-B N ) for binarizing image signals; a connectivity distribution means (5₁-5 N ) for obtaining connectivities between pixels from binary signals and for producing a connectivity distribution where the temporary binary level, at which frequencies of the minimum connectivity and the maximum connectivity are high and the sum of the frequencies of intermediate connectivities are minimal in the connectivity distribution, is judged as the optimum binary level; and a judging means (7) for, from connectivity distributions obtained with different binary levels, judging the level at which the frequencies of maximum and minimum connectivities are highest and the sum of the frequencies of intermediate connectivities is minimal. An apparatus according to claim 1, comprising: a smoothing means (8₁-8 N ) for smoothing binary image signals before arriving at said connectivity distribution means. An apparatus according to claim 2, comprising: a filtering means (9) for filtering image signals before arriving at said binarizing means. An apparatus according to one of claims 1 to 3, comprising: a plurality of said binarizing means (B₁-B N ) each for temporarily binarizing the same set of image signals with a different binary level; a plurality of said connectivity distribution means (5₁-5 N ) provided correspondingly for each of said binarizing means and each for obtaining connectivities between pixels from binary image signals obtained by the corresponding binarizing means and for obtaining a frequency distribution of connectivities; and said judging means (7) is for judging one of said different binary levels as an optimum binary level for said set of image signals, the frequency distribution obtained by using said one of said different binary levels showing that the frequencies of maximum and minimum connectivities are highest and the sum of the frequencies of the intermediate connectivities is minimal. An apparatus according to claim 4, comprising: a plurality of said smoothing means (8₁-8 N ) provided correspondingly for each of said binarizing means and each for smoothing said binary image signals obtained by the corresponding binarizing means for each of said plurality of connectivity distribution means (t₁-5 N ). An apparatus according to one of claims 4 or 5, comprising: a filtering means (9) for filtering a set of image signals to be directed to said plurality of binarizing means. An apparatus according to one of claims 1 to 6, wherein said connectivity distribution means (5₁-5 N ) obtains connectivities of 4 neighbors. An apparatus according to one of claims 1 to 7, wherein said connectivity distribution means (5₁-5 N ) obtains connectivities of 8 neighbors. An apparatus according to one of claims 1 to 8, wherein said connectivity distribution means (5₁-5 N ) obtains connectivities between pixels separated from each other by one or more pixels.

Inventors: HADA JUNICHI, KOBAYASHI AKIRA, MIZUOKA SEIJI, OKUMURA KAZUMASA, HADA, JUNICHI, KOBAYASHI, AKIRA, MIZUOKA, SEIJI, OKUMURA, KAZUMASA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: H04N1
IPC codes: H04N1, G06T5, G06K9