A method of determining image input contrast to dynamically adjust screen amplitude. The screen amplitude is controlled on a dot by dot basis to selectively enhance the original image. There is a higher amplitude when the input contrast is low and a lower amplitude when the input contrast is high. Thus, the partial dots more closely follow image detail at high contrast, while at low contrasts, noise is not enhanced. In a specific embodiment, depending upon the input contrast, a predetermined screen amplitude is used.

This invention relates generally to an electronic halftoning system for 
reproducing images and more specifically to a system for the automatic 
enhancement of selected portions of a halftone image. 
Halftone images are commonly used in printed material, and now additionally 
in computer displays, to represent continuous-tone images in systems where 
only two levels (usually black and white) can be represented at any point. 
The term halftone, or equivalently binary image, means an image formed by 
black dots of various sizes so as to give the effect of continuous tone 
when viewed at normal reading distance. This definition includes the 
digital case where each black or white spot is a fixed size in a fixed 
array, such that the variable size dots are generated by turning several 
adjacent spots black. 
A common method of producing a halftone image is the use of a screen 
consisting of a pattern which has the same fundamental frequency in two 
orthogonal directions. The halftone screen is combined with the pictorial 
information. In a photographic method this involves imaging the picture 
through the screen, thus multiplying the transmittances. In most 
electronic systems, the screen and pictorial information are added. It 
should also be noted that in a digital system both the screen information 
and the pictorial information are sampled functions. 
When the screen is periodic, the fundamental period of the screen is 
substantially larger than the sample interval, giving many samples per 
period. For both photographic and digital methods the combined pictorial 
and screen information is next subjected to a threshold. This is 
accomplished by recording on a high-contrast film for photographic 
halftoning or by a numerical comparison for digital halftoning. In either 
case, the result of the threshold operation is to produce a binary image. 
In the usual method, the threshold is a fixed value and adjustment of the 
screen pattern (including so-called bump and flash exposures in the 
photographic case) is used to adjust the effective grey scale of the 
halftone as desired. This process results in dots of varying size, shape, 
and location within their repetitive pattern. In the digital case, each 
sample of the halftone screen and corresponding sample of the pictorial 
information are combined and result in one bit, which is then printed 
either black or white at a given location. Since there are a number of 
samples within each cycle of the halftone screen, several adjacent bits 
normally combine to give the effect of a single halftone dot with size, 
shape, and location depending on the pattern of bits. 
Most halftone methods do a good job of giving the proper illusion of grey 
scale for low-spatial-frequency information on the continuous-tone image 
and partial dots in the halftone allow representation of higher frequency 
detail when detail contrast is sufficient. When the fine detail is 
periodic, however, spurious low-frequency patterns occur. 
One prior art method for converting a continuous-tone image to a halftone 
incorporates the capability for both suppression of spurious (aliasing) 
signals and edge enhancement. The basis of the method for suppression of 
spurious signals and edge enhancement is to adjust the threshold for each 
halftone cycle in a manner which guarantees that the resultant halftone 
image matches the average reflectance of the original image. U.S. Pat. No. 
4,051,536 is an example. This approach preserves the characteristics of 
the original halftone process such as partial dots. In areas of uniform 
grey in the original, the adjustable threshold will remain constant. 
In general, the pictorial information over the area corresponding to one 
halftone cycle in two dimensions is averaged or else a low-pass filtered 
value is used, giving only low spatial frequency information in either 
case. If grey scale information is to be preserved, this average 
determines precisely what percentage of the area must be covered by the 
halftone dot. In a digital system, this is equivalent to the number of 
bits which must be black of the total number of bits. Starting with the 
complete area of the binary image over one halftone cycle either all black 
or all white, the threshold is set at the extreme value to generate this 
case. The threshold is next adjusted monotonically. Either the total 
number of bits or dot size is examined during the adjustment process. As 
soon as the correct dot size is reached, the threshold value is fixed and 
the binary image is generated for that cycle of the halftone. 
Another prior art method is shown in U.S. Pat. No. 4,246,614. In 
particular, there is disclosed a method by which additional information of 
location and contrast of input picture detail is used to control phase of 
the halftone screen to achieve improved binary representation of the 
image, further reducing spurious patterns and possibly improving detail. 
It also is known in the prior art to be able to control the degree of 
enhancement of an image by adjusting the amplitude of the screen. See 
Journal of the Optical Society of America, Volume 66, No. 10, October 
1976. A relatively small amplitude screen will provide a sharper detailed 
image. On the other hand, this can also provide a noisy appearance. For 
some images it would be desirable to be able to selectively and 
automatically provide various screen amplitudes for various portions of 
the image. In particular, it is desirable to provide more enhancement in 
highly detailed portions of an image and to provide less enhancement in 
uniform portions of an image. This is not shown in the prior art. 
It is, therefore, an object of the invention to provide a new and improved 
method of enhancing a digital halftone image. It is another object of the 
invention to provide for the automatic selective switching of screen 
amplitudes for various portions of the image. 
Further advantages of the present invention will become apparent as the 
following description proceeds, and the features characterizing the 
invention will be pointed out with particularity in the claims annexed to 
and forming a part of this specification. 
Briefly, the present invention is a method of determining image input 
contrast to dynamically adjust screen amplitude. The screen amplitude is 
controlled on a dot by dot basis to selectively enhance the original 
image. There is a higher amplitude when the input contrast is low and a 
lower amplitude when the input contrast is high. Thus, the partial dots 
more closely follow image detail at high contrast, but without enhancing 
noise in uniform areas. In a specific embodiment, depending upon the input 
contrast, a predetermined screen amplitude is used.

With respect to FIG. 1, there is shown a generalized block diagram showing 
a prior art image enhancement. The halftone screen block is a non-image 
related pattern which could be random, but is usually a periodic pattern. 
Generally, the screen consists of a pattern which has the same fundamental 
frequency in two orthogonal directions. As shown in FIG. 1, the halftone 
screen is combined with the image or pictorial information. In most 
electronic systems, this is done by adding the signals together as 
illustrated in the add block. 
The signal from the add block is then conveyed to an adjustable threshold 
circuit. Usually, the threshold is a fixed value and adjustment of the 
screen pattern is used to adjust the effective grey scale of the halftone 
as desired. In the digital case, each sample of the halftone screen and 
corresponding sample of the pictorial information are combined to provide 
one bit. This bit is printed either black or white at a given location. 
Since there are a number of samples within each cycle of the halftone 
screen, several adjacent bits normally combine to give the effect of a 
single halftone dot with size, shape and location depending on the pattern 
of bits. 
The threshold for each halftone cycle is adjusted in a manner to guarantee 
that the resultant halftone image matches the average reflectance of the 
original signal. This approach preserves the characteristics of the 
original halftone process such as partial dots. In areas of uniform grey 
in the original, the adjustable threshold will remain constant. 
To adjust the threshold, the pictorial information over the area 
corresponding to one halftone cycle in two dimensions is averaged or a low 
pass filtered value is used, giving only low spatial frequency 
information. This average determines what percentage of the area must be 
covered by the halftone dot. The threshold is set at the extreme value 
which would provide either all black or all white over one halftone cycle. 
The threshold is next adjusted monotonically. Either the total number of 
bits or dot size is examined during the adjustment process. 
As soon as the correct dot size is reached, the threshold value is fixed 
and the binary image is generated for that cycle of the halftone. It 
should be pointed out that this is equivalent to a number of other 
computational procedures such as, for example, calculating the histogram 
of the combined pictorial and screen values and thereby finding the 
appropriate threshold for which the proper percentage of the values exceed 
threshold. 
Once the threshold value is established for one cycle of the halftone, it 
will cause the appropriate number of bits to be turned white, and the rest 
remaining black. Which bits in the pattern are white is chosen by locating 
the N largest values of the sum of picture and screen. As in normal 
halftones, if a black/white edge passes through the area, the largest sum 
values will exactly match the white regions (screen plus black never 
exceeds white plus any value), thus correctly representing full contrast 
details. At lower edge contrast, the screen values added to the picture 
values create some asymetry such that more of the resulting white area is 
on the lighter side of the edge. 
With reference to FIG. 2, there is shown an enlarged view of a matrix of 
pixel or image dots. In accordance with the present invention, the local 
image contrast is computed by finding the maximum and minimum pixel grey 
values over the area of the halftone cell being worked on. In FIG. 2, the 
area being worked on is represented by the crosshatched block 5. The 
maximum grey value minus the minimum grey value is contrast. 
EQU CONTRAST=MAX.sub.G.sbsb.1.sub.V.sbsb.1 -MIN.sub.G.sbsb.1.sub.V.sbsb.1 
This difference within one cell or over one screen period of pixels, is 
used to select a particular screen value related to the difference. 
Any arbitrary number of screen values can be used for a range of contrasts 
or maximum value minus minimum value ranges. Thus, by adjusting the 
amplitude of the screen (by selecting a particular screen pattern) the 
degree of enhancement of the picture can be controlled. The selective 
degree of enhancement of the image can be done automatically simply by 
determining the contrast and branching or switching to a particular screen 
pattern. Appendix A is a listing of a preferred embodiment of the present 
invention. 
For low contrast, the screen is used at normal values. For medium contrast, 
in a preferred embodiment, the screen is divided by two, giving half the 
screen values. For the highest contrast, the screen is not used (i.e. zero 
screen values). FIG. 3 illustrates typical screens to be used for small or 
large contrasts. That is, curve A would be used if the difference between 
the maximum and minimum grey levels is small and curve B would be used if 
the difference is large. 
FIG. 4 is a general flow chart and FIG. 5 is a schematic block diagram of 
an embodiment of the halftone image system according to the present 
invention. A scanner 10 sequentially illuminates an original image which 
is attached to the periphery of drum 12 which rotates about shaft 13. Of 
course, the original image may be a transparency as well as an opaque 
document. Scanner 10 operates at speeds of a millisecond or less per 
halftone period covered, though the actual speeds will depend upon the 
characteristics of the particular optical and electronic components used 
in any embodiment of the invention. 
Each halftone dot period of the original image illuminated by scanner 10 is 
sequentialy imaged by imaging lens 14 onto a light sensitive element 16 
such as, for example, a photodiode array. The light sensitive element 
includes a plurality of light sensitive detectors, one for each discrete 
element in the halftone period area. Typically, the halftone period area 
is broken down into at least 5.times.5 discrete elements. The various 
intensities of light striking the light sensitive element 16 are dependent 
upon the densities of the toner in the original image. 
The light is transduced by photodiode array 16 into analog electronic 
signals. The analog electronic signal from each light sensitive detector 
in photodiode array 16 is fed into summing circuit 18, to be added to give 
a signal representative of the total amount of light reflected by the 
halftone dot period. 
Alternatively, a beam splitter could be interposed between imaging lens 14 
and photodiode array 16 and a portion of the light in the beam directed to 
a separate photodiode having only a single light sensitive detector. In 
this case, the light would be transduced into a single electronic signal 
representative of the total amount of light reflected by the halftone dot 
period and the electronic signal would replace the output of circuit 18. 
Although the embodiment shown in FIG. 5 is illustrated with a moving 
scanner 10, rotating drum 12, lens 14 and photodiode array 16 moving with 
scanner 10, any suitable arrangement of these elements may be employed. 
For example, the scanner may be stationary and the shaft 13 on which the 
drum is mounted may be a lead screw so that the drum moves transversely as 
it is rotated. 
The electronic signal from each light sensitive detector in photodiode 
array 16 is then fed into separate summing circuits 20, 22 to be combined 
with a reference voltage representative of the halftone screen function. 
For purposes of illustration, the signals from two of the light sensitive 
detectors in photodiode array 16 are shown; however, it is understood that 
the electronic signal from each detector is treated in a similar fashion. 
A different reference voltage, illustrated by v.sub.i and v.sub.j is added 
to the output signal from the respective detectors in photodiode array 16 
by summing circuits 20 and 22, respectively. It is again noted that the 
values of the respective reference voltages can be varied to control 
detail contrast. 
The signals from the summing circuits are then brought into sample and hold 
circuits 24 and 26, respectively. These sample and hold circuits are not 
required if the original image is scanned slowly enough; however, they are 
preferred since they allow the image to be scanned electronically, that 
is, at speeds of a millisecond or less per halftone period covered. Each 
electronic signal at this point represents the sum function of the subcell 
pictorial information and the halftone screen function. The sum function 
for each subcell is next added to a dynamically adjusted amplitude varying 
voltage signal common to all subcells supplied by a single ramp generator 
28. Each electronic signal is then directed through fixed level threshold 
circuits 30 and 32, respectively, to a light emitting diode array 38 which 
includes as many light emitting diodes as the number of light sensitive 
detectors in photodiode 16. The light emitting diodes are arranged in a 
pattern similar to that of the light sensitive detectors in photodiode 
array 16 which control them. 
In accordance with the present invention, a maximum detector 60 and a 
minimum detector 62 are electrically connected to the photodiode array 16 
to detect the maximum and minimum pixel grey levels. A difference circuit 
64 subtracts the minimum level from the maximum level to provide local 
image contrast. This image contrast or the signal from circuit 64 controls 
the gain of a set of amplifiers illustrated at 66. Alternatively, a set of 
values loaded into look-up tables could be controlled in a digital 
implementation. 
The control of the amplifiers 66 yields a small amplifier gain for a large 
local image contrast and a large amplifier gain for a small local image 
contrast. Suitable reference voltages are sent through variable gain 
amplifier 66 to yield adjusted reference voltages. These adjusted 
reference voltages v.sub.i and v.sub.j are conveyed to summing circuits 20 
and 22. 
Not shown, but comprised of well known elements, is a timing mechanism 
which starts a sequence by triggering the sample and hold circuits 24, 26 
to sample the signals from summing circuits 20 and 22, respectively. The 
timing mechanism starts the ramp generator 28 voltage increasing, thus 
effectively producing the dynamically varying threshold. The outputs of 
the threshold circuits 30 and 32 are fed into circuit 34 where they are 
summed. The signal representing the sum is directed into circuit 36 which 
compares the electronic signal from circuit 34 with the electronic signal 
from circuit 18. When the two voltage signals match, circuit 36 emits a 
pulse which stops any further increase of the voltage signal from ramp 
generator 28. 
In operation, the electronic signals representing the sum functions having 
the largest values will cause the appropriate light emitting diodes in 
array 38 to become operative until the total light output matches the 
total amount of light reflected by the original image in the area of the 
halftone period being examined. The remaining light emitting diodes will 
not be energized. A short time after these actions the timing mechanism 
opens shutter 40 momentarily allowing the energized light emitting diodes 
to expose a photoreceptor 42 through lens 44. The photoreceptor 42 may be 
any suitable light sensitive recording medium such as a photographic film 
or a charged xerographic member. In one embodiment, as illustrated, the 
photoreceptor comprises a charged xerographic drum which rotates about 
shaft 13 and is matched to the movement of drum 12 which carries the 
original image. Alternatively, photoreceptor 42 could be moved by another 
scanner, similar to 10 and matched to it. After the photoreceptor is 
exposed the timer resets the circuits in the system and starts the next 
cycle. This sequence is repeated for each halftone period of the original 
image. 
While there has been illustrated and described what is at present 
considered to be a preferred embodiment of the present invention, it will 
be appreciated that numerous changes and modifications are likely to occur 
to those skilled in the art, and it is intended in the appended claims to 
cover all those changes and modifications which fall within the true 
spirit and scope of the present invention. 
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