Apparatus for binarizing image signals

An apparatus for converting image signals into binary signals is disclosed. The apparatus comprises: a binarizing section for binarizing image signals; a connectivity distribution section for obtaining connectivities between pixels from binary signals and for producing a connectivity distribution; and a judging section 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.

BACKGROUND OF THE INVENTION 
1. Field of the invention: 
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. 
2. Description of the prior art: 
FIG. 7 shows a conventional apparatus of such a kind. The apparatus of FIG. 
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 object 53 (hereinafter, such a 
threshold level is referred to as "a binary level"), from the density 
distribution of the image signals as described below. 
FIG. 8 shows one example of the density distribution of the image signals 
stored in the memory 56. As shown in FIG. 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 FIG. 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 obtained binary image become unstable. 
SUMMARY OF THE INVENTION 
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 signals; a connectivity distribution means for obtaining 
connectivities between pixels from binary signals and for producing a 
connectivity distribution; 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. 
The apparatus for binarizing image signals of this invention further 
comprises: a binarizing means for binarizing image signals; a smoothing 
means for smoothing binary image signals; a connectivity distribution 
means for obtaining connectivities between pixels from smoothed binary 
signals and for producing a connectivity distribution; 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. 
The apparatus for binarizing image signals of this invention further 
comprises: a filtering means for filtering image signals; a binarizing 
means for binarizing filtered image signals; a smoothing means for 
smoothing binary image signals; a connectivity distribution means for 
obtaining connectivities between pixels from smoothed binary signals and 
for producing a connectivity distribution; 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. 
The apparatus for binarizing image signals of this invention further 
comprises: a plurality of binarizing means each for temporarily binarizing 
the same set of image signals with a different binary level; a plurality 
of connectivity distribution means 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 a 
judging means 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. 
The apparatus for binarizing image signals of this invention further 
comprises: a plurality of binarizing means each for temporarily binarizing 
the same set of image signals with a different binary level; a plurality 
of smoothing means provided correspondingly for each of said binarizing 
means and each for smoothing said binary image signals obtained by the 
corresponding binarizing means; a plurality of connectivity distribution 
means provided correspondingly for each of said smoothing means and each 
for obtaining connectivities between pixels from binary image signals 
supplied from the corresponding smoothing means and for obtaining a 
frequency distribution of connectivities; and a judging means 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. 
The apparatus for binarizing image signals of this invention further 
comprises: a filtering means for filtering a set of image signals; a 
plurality of binarizing means each for temporarily binarizing the set of 
said filtered image signals with a different binary level; a plurality of 
smoothing means provided correspondingly for each of said binarizing means 
and each for smoothing said binary image signals obtained by the 
corresponding binarizing means; a plurality of connectivity distribution 
means provided correspondingly for each of said smoothing means and each 
for obtaining connectivities between pixels from binary image signals 
supplied from the corresponding smoothing means and for obtaining a 
frequency distribution of connectivities; and a judging means for judging 
one of said different binary levels as an optimum binary level for said 
set of filtered 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 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.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an apparatus according to the invention. The apparatus shown 
in FIG. 1 converts image signals of an object 1 and background 1a obtained 
by a television camera 2 into binary image signals, and comprises 
binarization units B.sub.1 -B.sub.N, and a control unit 7. The 
binarization units B.sub.1 -B.sub.N include binarizing sections 3.sub.1 
-3.sub.N, binary signal memory sections 4.sub.1 -4.sub.N, connectivity 
distribution sections 5.sub.1 -5.sub.N, and connectivity memory sections 
6.sub.1 -6.sub.N, respectively. The binarization units B.sub.1 -B.sub.N 
are controlled by the control unit 7. Actually, the binarization units 
B.sub.1 -B.sub.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 3.sub.1 -3.sub.N. In each of the binarizing sections 3.sub.1 
-3.sub.N, the image signals are temporarily converted into binary signals 
with a predetermined binary level. The binary levels of the binarizing 
sections 3.sub.1 -3.sub.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 3.sub.1 -3.sub.N are stored 
respectively in the binary signal memory sections 4.sub.1 -4.sub.N. FIG. 
3A shows one example of images of binary signals. In FIG. 3A, the hatched 
squares indicate pixels of "1", and the blank squares indicate pixels of 
"0". The connectivity distribution sections 5.sub.1 -5.sub.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: 
EQU n=S(x,y)+S(x-1,y)+S(x+1,y)+S(x,y-1)+S(x,y+1) (1) 
EQU h(n)=h(n)+1 (2) 
over the entire area of the object 1 and background 1a to be recognized. 
The relationship of expression (1) is illustrated in FIG. 2. When all of 
an object pixel and its 4-neighbors are "0", the connectivity of the pixel 
is "0" ((a) of FIG. 2). When only one of an object pixel and its 
4-neighbors is "0", the connectivity is "1" ((b) of FIG. 2). Similarly, 
connectivities "2" to "5" are obtained ((c) to (f) of FIG. 2). The 
connectivities of the image signals of FIG. 3A which are calculated by the 
expression (1) are shown in FIG. 3B. 
The thus-calculated connectivities are arranged in the form of histograms 
as shown in FIG. 4 which indicate the distribution of connectivities, and 
stored in the connectivity memory sections 6.sub.1 -6.sub.N, respectively. 
The control unit 7 reads the connectivity distributions from the 
connectivity memory sections 6.sub.1 -6.sub.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 FIG. 4. If the temporary binary level is too high, in contrast, the 
frequencies concentrate around connectivity "0" as shown in (c) of FIG. 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 4.sub.1 -4.sub.N which belongs to the 
binarization unit producing the optimum binary level. 
The connectivity distribution sections 5.sub.1 -5.sub.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 FIG. 1, the operation of temporarily 
converting the image signals into binary signals is concurrently conducted 
in all of the binarization units B.sub.1 -B.sub.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. 
FIG. 5 shows another apparatus according to the invention. In the apparatus 
of FIG. 5, smoothing sections 8.sub.1 -8.sub.N are disposed respectively 
between the binarizing sections 3.sub.1 -3.sub.N and the binary signal 
memory sections 4.sub.1 -4.sub.N. Other components may be the same as 
those of the apparatus of FIG. 1. The smoothing sections 8.sub.1 -8.sub.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 3.sub.1 -3.sub.N. The smoothed binary signals are stored in the 
binary signal memory sections 4.sub.1 -4.sub.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. 
FIG. 6 shows a further apparatus according to the invention. The apparatus 
of FIG. 6 comprises the same configuration as that of the apparatus of 
FIG. 5, except that a filtering unit 9 having a plurality of filtering 
characteristics is disposed between the television camera 2 and the 
binarization units B.sub.1 -B.sub.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 3.sub.1 -3.sub.N of the 
binarization units B.sub.1 -B.sub.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. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description as 
set forth herein, but rather that the claims be construed as encompassing 
all the features of patentable novelty that reside in the present 
invention, including all features that would be treated as equivalents 
thereof by those skilled in the art to which this invention pertains.