Method and apparatus for generating screened halftone images

Apparatus and method for generating screened halftone images, which includes means for assuming an area of halftone dots with desired periodicity and tone reproducibility, subdividing the area into minute cells and setting address values (X, Y) for each of the minute cells, computing out a threshold value of density for each of said cells as a function f (X, Y) of the relevant address values (X, Y), and using the computed value as threshold value of density for the cell, means for obtaining a density-related video signal of the portion of the original corresponding to each of said cells by scanning the original, and means for producing halftone dot signals by comparing the video signals and the threshold value of density with each other.

The present invention relates to a method of and apparatus for generating 
halftone images, and more particularly, to a novel apparatus and method 
for generating halftone images electronically without the use of contact 
screens. 
BACKGROUND OF THE INVENTION 
Up to the present time, when an original such as a photograph or a picture 
having a continuous tone image is to be reproduced by printing in large 
quantities, it has been necessary to convert the continuous tone image of 
the original to an image formed by dots (called screened halftone or 
halftone dots) having various sizes (size to area ratio) according to the 
image density of the original. This screened halftone image is, then, 
formed on a printing plate in order to reproduce the image of the 
photograph, picture, etc. in large quantities, using conventional printing 
plates and inks. 
When converting a continuous tone image into a screened halftone image 
consisting of various size halftone dots, contact screens having a regular 
density gradient are normally employed. In practice, when a screened 
halftone image is to be formed, a contact screen is placed directly on a 
recording material (such as a lith type film) for a screened halftone 
image, and the original, such as a transparency, having a continuous tone 
image is exposed by a process camera through the contact screen onto the 
recording material; a screened halftone image consisting of various size 
halftone dots is thereby formed on the recording material, the sizes of 
the dots being a function of the quantity of light corresponding to the 
density of the original and the density gradient of the contact screen. 
In recent years, other methods using a scanning instrument called a scanner 
and directly converting a continuous tone original to a screened halftone 
image, without the use of a process camera, have become the main stream 
method of recording screened halftone images. The scanner used in such a 
process is called a direct scanner. The methods for recording screened 
halftone images by use of such direct scanners can be divided into two 
types; one type is to use a contact screen similar to those used in the 
aforementioned process of forming a screened halftone image using a 
process camera so as to generate screened halftone images, and the other 
type is to electronically generate halftone images by means of an 
electronic halftone dot generator stored in a direct scanner, thereby 
eliminating the contact screen. 
In the first type of direct scanner mentioned above using a contact screen, 
an original with continuous tone image is scanned to obtain electronic 
video signals corresponding to the density of the image. A light beam of a 
light source which is modulated by the video signals thus obtained is used 
to scan and expose, through a contact screen, the recording material that 
is placed in direct contact with the screen, so as to record the screened 
halftone image. Since this method is similar to the conventional one that 
uses a process camera, it is a familiar method, but it poses a variety of 
problems due to its use of a contact screen. As contact screens themselves 
suffer scratches, stains, etc. in use, they cannot be used repeatedly, and 
since they are expensive, the cost of this process is high. To obtain a 
screened halftone image having good quality, it is essential to keep the 
contact screen and the recording material in sufficiently close contact, 
but insufficient contact often occurs and results in irregularities in the 
configuration and the size of the halftone dots. Dirt or dust may be 
present between the contact screen and the recording material and prevent 
close contact. Further, the use of a contact screen itself will often 
produce fringes around halftone dots, similar to the case of optically 
recorded screened halftone images using a conventional process camera. 
Accordingly, the size of the halftone dots will become unstable, and, in 
turn, after the completion of the screened halftone image recording, it 
will be often required to correct the size of the halftone dots to the 
desired one by some method such as dot etching. 
In addition, since the exposure is made through a contact screen on a 
recording material, the light source for the exposure is required to have 
a high power, requiring a higher cost, and the exposure speed (scanning 
speed) cannot be increased much. It also takes time to set up the contact 
screen. Thus, a variety of disadvantages are present in this method. 
In order to eliminate the aforementioned various disadvantages caused by 
the use of a contact screen, the second method, which requires no contact 
screen, has been used. 
This latter method electronically generates halftone dots, and a halftone 
dot-generator is included in direct scanners used for this purpose. Such a 
generator of the scanner is also called a dot-generator or halftone 
generator. The use of an electronic halftone dot-generator will solve all 
of the problems arising from the use of a contact screen, and hard 
halftone dots without any fringes will be obtained, and the scanning speed 
will be enhanced; this method is thus advantageous in terms of 
operability, quality, stability, material costs, etc., and has become the 
primary method for generating halftone images. A variety of methods for 
generating halftone dots by means of electronic halftone dot-generators 
have been devised and put to practical use up to the present time. 
In one such method, values of all halftone dots of various sizes (e.g. 
halftone dots of from 5% to 95% of maximum at 5% interval) are stored in a 
memory. Values of halftone dots corresponding to the density-related video 
signal levels obtained by scanning the original, are read out in sequence 
from the memory, to compare with and to control the exposure light beam 
from the recording light source and to record the halftone image. In this 
method wherein the tone reproduction of the halftone image is determined 
by the number of the values stored in the memory, it will be necessary to 
increase the number of the values of the halftone dots stored in the 
memory in order to obtain a smoother tone reproduction, but this, in turn, 
poses a problem of requiring an increased memory capacity. This method is 
disclosed in Behane et al. U.S. Pat. No. 3,604,846 and French Pat. No. 
1,585,163. 
Another method is to divide the unit area of a contact screen used in the 
conventional methods into minute cells, and allot a specific threshold 
value of density to the address of each minute cell, store said value in a 
memory, scan the original and generate a density-related video signal, 
electronically compare and control whether to expose or not the minute 
cell of the reproduction according to the density-related video signal, 
which is obtained by scanning the original, corresponding to the scanned 
minute cell, and thus record a screened halftone image in sequence. This 
method is disclosed in Schreiber U.S. patent application Ser. No. 576,851 
(Japanese Patent Provisional Pub. No. SHO. 51-138445). 
Accordingly, in this method, each halftone dot consists of a group of 
plural minute cells, the exposure of which is individually controlled by 
the electronic halftone dot generating signals. The size of each 
individual minute cell is always constant, and the number of minute cells 
will vary according to the optic density (highlight middle shadow) of the 
continuous tone image of the original. 
The inventors of the present invention have concentrated their energies on 
the study of electronic screened halftone image recording and developed a 
novel apparatus and method for computing out threshold values of density 
from some functions without storing them in a memory, thereby avoiding the 
need for a large memory. 
SUMMARY OF THE INVENTION 
According to the present invention, apparatus and method are provided for 
generating screened halftone images, which includes means for assuming an 
area of halftone dots with desired periodicity and tone reproducibility, 
subdividing the area into minute cells and setting address values (X, Y) 
for each of the minute cells, computing out a threshold value of density 
for each of said cells as a function f (X, Y) of the relevant address 
values (X, Y), and using the computed value as threshold value of density 
for the cell, means for obtaining a density-related video signal of the 
portion of the original corresponding to each of said cells by scanning 
the original, and means for producing halftone dot signals by comparing 
the video signals and the threshold value of density with each other. 
According to this method, the reading point of the original image, which 
shifts consecutively from one cell to the next as the scanning proceeds, 
is assumed to be the address values (X, Y) of a minute cell, which is 
obtained by subdividing a specific or assumed area of halftone dots, 
having the desired tone reproducibility and periodicity, and the threshold 
value of density is computed out from a function f (X, Y) using the 
address values (X, Y) to be used as the threshold value of density of the 
address; as the scanning proceeds, the address values (X, Y) will be 
updated, and the threshold value of density will be obtained by computing 
out the function f (X, Y) again. The density-related video signal of a 
cell, obtained by scanning the original, and the computed threshold value 
of density are compared with each other by a comparator, and a halftone 
dot signal having either ON or OFF values for exposure on a reproducing or 
recording material, will be generated depending on whether the video 
signal is greater or less than the threshold value. A light beam from a 
light source will be turned on or off by this halftone dot generating 
signal to expose or not expose the cell of the relevant address on the 
recording material, etc. A plurality of such cells will, all together, 
form the whole of a halftone dot of desired size.

DETAILED DESCRIPTION OF THE DRAWINGS 
In FIG. 1, an original image 1, such as a continuous-tone photograph, is 
placed on an original transfer belt 2. The light beams, which may be laser 
beams, from a light source 5 are polarized in the transverse axis or 
direction X by a movable galvanometer-mirror 4 or the like to transversely 
scan the original, and the electronic video signals corresponding to the 
image darkness or density of the original 1 are measured by an 
opticelectro transducer 7, which may be a photodiode. These electric video 
signals are shaped or processed including amplification in a unit 9, and 
the video analog signals are converted by an analog-to-digital converter 
10 to digital video signals. If necessary, corrections, such as tone 
correction and edge enhancement, may be made. At the same time, the 
original is scanned in the longitudinal direction Y in a similar manner by 
moving the transfer belt 2 by an electric motor 3. The threshold value of 
density generated by an arithmetic unit of threshold value of density 
generator 13, and the video signals obtained from the analog-to-digital 
converter 10 are compared with each other by a comparator 14, and the 
resulting signals (either ON or OFF) for the screened halftone image 
generation are fed to a photo-modulator 15 or the like. The light beams 
emitted from a light source 16 (which may emit laser beams or the like) 
are controlled or modulated by the photo-modulator 15, before the beam is 
passed to scan and expose a recording material 18. The beam is directed to 
the material 18 by means of a movable output galvanometer-mirror 17 to 
record the screened halftone image in the transverse X direction. The 
recording material 18 is simultaneously moved in the longitudinal 
direction Y by a motor 19. 
For timing the scanning operation, a circuit 11 for generating X-axis clock 
pulses or signals and a circuit 12 for generating the Y-axis clock pulses 
or signals are provided to transmit respective timing signals to an input 
galvanometer-mirror drive circuit 6, an output galvanometer-mirror drive 
circuit 20, an original transfer motor drive circuit 8, and a recording 
materials shift motor drive circuit 21, and, at the same time, to transmit 
both X-axis and Y-axis clock signals to the arithmetic unit of threshold 
value of density generator 13 so as to generate halftone dots. The 
generators 11 and 12 produce series or trains of timing signals which 
cause the various circuit components to operate in coordinated fashion, 
and these generators may be connected for coordinated action. Thus the 
beams from the sources 5 and 16 scan the image 1 and the material 18 in 
synchronism and the motors 3 and 19 move the image 1 and the material 18 
in synchronism. Further, the generator 13 is operated in synchronism with 
these movements. The X-axis scanning speed governed by the generator 11 is 
coordinated with the longitudinal movement produced by the generator 12 so 
that the entire area of the original 1 is scanned. 
FIG. 1 illustrates a flat-bed scanning system, but other types of scanning 
systems, including a revolving drum system, are also applicable. The 
devices for implementing the present invention should not be understood to 
be confined to those shown in FIG. 1. 
The threshold value of density arrangement using functions will now be 
explained. 
As an example, a halftone dot area which is equivalent to two customary 
halftone dot areas is assumed as shown in FIG. 2, as a halftone dot area 
having periodicity, and is divided into four quarter sections (I), (II), 
(III) and (IV), as shown. Each section has the dimension a on the X axis 
and the dimension b on the Y axis. Each section is further divided into a 
plurality of minute cells. When the threshold value of density for the 
cells having positions x, y and contained in the sections and having 
address values (X, Y), is to be expressed as functions of the address 
values (X, Y) on the X- and Y-axes, the following functions may be used: 
for cells in the section (I), or when 0.ltoreq.x&lt;a, and 0.ltoreq.y&lt;b, 
EQU f(I)(x,y)=(x-1/2a).sup.2 +(y-1/2b).sup.2 ; 
for the section (II), or when a.ltoreq.x&lt;2a, and b.ltoreq.y&lt;2b, 
EQU f(II)(x,y)=(x-3/2a).sup.2 +(y-3/2b).sup.2 ; 
for the section (III), or when 0.ltoreq.x&lt;a, and b.ltoreq.y&lt;2b, 
EQU f(III)(x,y)=a.times.b-[(x-1/2a).sup.2 +(y-3/2b).sup.2 ] 
and for the section (IV), or when a.ltoreq.x&lt;2a, and 0.ltoreq.y&lt;b, 
EQU f(IV)(x,y)=b.times.a-[(x-3/2a).sup.2 +(y-1/2b).sup.2 ] 
(where a and b are positive even numbers). 
With the threshold value of density expressed as these functions, in the 
sections (I) and (II), the threshold value of density increases from the 
center towards the periphery of each section; in the sections (III) and 
(IV), the threshold value of density increases from a value greater than 
the maximum value of the sections (I) and (II) further towards the center. 
Now, in order to form halftone dots with a regular period, the maximum 
value of the X-axis address, which is (2a-1), and the maximum value of the 
Y-axis address, which is (2b-1), are set as the repeat periods. When, as a 
specific example, the maximum value of the X-axis address and the maximum 
value of the Y-axis address are both 127, as shown in FIG. 3, both 
coordinates of the X- and Y-axes can be any values of from 0 up to 127. 
When the screen ruling for halftone dots is L lines per inch, the minimum 
pitches .DELTA.x and .DELTA.y of the respective X- and Y-axes are as 
follows: 
halftone dot pitch S=25,400/L (um); 
X-axis minimum pitch .DELTA.X=S/128 cos 45.degree.; and 
Y-axis minimum pitch .DELTA.Y=S/128 cos 45.degree.. 
The functions f (X, Y) in this case are expressed by the following 
equations: 
when a=64 and b=64, 
(I) when 0.ltoreq.x&lt;64, and 0.ltoreq.y&lt;64, 
EQU f(x,y)=(x-32).sup.2 +(y-32).sup.2 (I); 
(II) when 64.ltoreq.x&lt;128, and 64.ltoreq.y&lt;128, 
EQU f(x,y)=(x-96).sup.2 +(y-96).sup.2 (II); 
(III) when 0.ltoreq.x&lt;64, and 64.ltoreq.y&lt;128, 
EQU f(x,y)=4096-[(x-32).sup.2 +(y-96).sup.2 ] (III); 
and (IV) when 64.ltoreq.x&lt;128, and 0.ltoreq.y&lt;64, 
EQU f(x,y)=4096-[(x-96).sup.2 +(y-32).sup.2 ] (IV). 
The construction and operation of halftone dot generation in the generator 
13 is further described in connection with FIG. 4. For each output pulse 
on the X-axis clock line 33 from the generator 11, or from the Y-axis 
clock line 34 from the generator 12, the scanning of the X-axis or the 
Y-axis is advanced by the distance .DELTA.X or .DELTA.Y, respectively (see 
FIG. 3). A counter 35 of the X-axis address 31 and a counter 36 of the 
Y-axis address 32 are both cleared to zero or nil by the coincidence 
circuits 54 and 55 when their associated counters 35 and 36 proceed to 128 
.DELTA.X or 128 .DELTA.Y, respectively. Accordingly, the same functions 
are repeatedly computed for every distance or step through the values 128 
.DELTA.X and 128 .DELTA.Y. As the equations differ in sections (I), (II), 
(III), and (IV), respectively, the section is identified by a section or 
area computing circuit 37. 
Now, the case when X-axis and Y-axis address values are 0.ltoreq.x&lt;64 and 
0.ltoreq.y&lt;64, which is in the section (I), will be explained. The output 
of a multiplexer XA 39 which receives an input from a subtractor 38 
(preset to 32) will be (x-32), and similarly, the output of a multiplexer 
YA 41 which receives an input from a subtractor 40 (preset to 32) will be 
(y-32). These outputs are inputted to squaring circuits 42 and 43 to 
compute the squares of the respective values, and the outputs of a 
multiplexer XB 44 will be (x-32).sup.2, and the output of a multiplexer YB 
45 will be (y-32).sup.2. These outputs are inputted to an adder 46 to 
obtain the value from Equation (I), of f(x,y)=(x-32).sup.2 +(y-32).sup.2. 
This output f (x, y) is the result of the computation of the halftone dot 
function, and is to be compared with a video signal on line 47 (such as 
from the converter 10) by a comparator 49 (corresponds to comparator 14). 
If the value of the output f (x, y) is larger than the video signal on 
line 47, the halftone dot signal on the output 48 will represent ON, and 
if smaller, it will represent OFF. 
Next, for the section (II), to produce Equation (II), subtractors 50 and 51 
(preset to 96) are used. For the section (III), to produce Equation (III), 
subtractors 38, 51, 52 and 53 are used. Subtractors 52 and 53 are preset 
to 2048. Further, for the section (IV), to produce Equation (IV), 
subtractors 40, 50, 52 and 53 are used together and a calculation is made 
in the same manner as for section (I). 
Examples of halftone dot patterns obtained by the present invention are 
shown in FIGS. 5A and 5B. When the video signal density value ranges 
between 0 to 255, the whole area is white or blank when the value is 0, 
and it is entirely black when the value is 255. The pattern for a video 
signal having a value of 68 is shown in FIG. 5A where 68 of the 255 cells 
are black, and the pattern for a video signal having a value of 240 is 
shown in FIG. 5B where 240 of the cells are black. In these examples, the 
X-axis address is incremented from 0 to 127 by .DELTA.X=1, and the Y-axis 
address is incremented from 0 to 127 by .DELTA.Y=8. 
As described so far, according to the method of the present invention, the 
patterns of halftone dots can be represented by functions without dividing 
one unit area of a contact screen and storing the threshold values of 
density in a memory, etc., and threshold value of density can be easily 
generated by computing out their values without relying on a storage such 
as memories. Furthermore, according to the method of the present 
invention, it is possible to generate symmetric patterns other than a 
square pattern by varying the values of a, b, and the method is 
advantageous in that halftone dot recording can be effected by a simple 
circuit configuration. 
In the aforementioned embodiment, the present invention has been described 
for the case in which the screen angles are set at 45.degree., or halftone 
dots are generated from black and white continuous tone original. The 
method, however, is also applicable to the generation of halftone dots 
from color originals. For generating halftone dots from color originals, 
as the objective is to prepare printing plates for multicolor printing, 
color separated screened halftone image plates must be prepared for the 
required colors. In this case, it is sufficient to make color separation 
of the color original and to scan one of the color separated originals, 
and compare the video signal of each cell of said original with the 
threshold value of density calculated from the function f (X, Y) of the 
address values (X, Y) according to the method of the present invention. In 
doing so, what is most important is to vary the screen angle for each 
color, and according to the method of the present invention, the threshold 
values of density required for generating halftone dots for the respective 
screen angles can be generated by converting the X-Y coordinate system to 
any X'-Y' coordinate system set at an angle relative to the former one and 
computing the address values (X', Y') for the corresponding address values 
(X, Y). 
Furthermore, the signals for generating screened halftone images obtained 
by the present invention can directly control the output unit (such as the 
laser beams for exposing a recording material) or the halftone dot signals 
can be temporarily stored in an external storage.