Image processing apparatus with reduced image deterioration in highlight portions

An image processing apparatus binarizes input multi-level pixel data by changing the same either to the minimum density data corresponding to white or to a minimum reproducible density data corresponding to the minimum density level reproducible by an image forming apparatus such as a printer, when the density defined by the input multi-level pixel data is of such a low level that it cannot be reproduced by the image forming apparatus. Error incurred by the binarization is reflected in the succeeding binarizing process so that the density of the whole image is preserved, whereby an image with a highlight portion can be recorded with reduced degradation of the image quality at the highlight portion.

BACKGROUND OF THE INVENTION 
The present invention relates to an image processing apparatus, and more 
particularly, to an image processing apparatus which can produce a half 
tone image from multi-level image data input thereto. 
In recent years, a laser beam printer making use of electrophotographic 
process is attracting attention as one type of printers which can operate 
at a high speed with reduced noise. Such a laser beam printer is typically 
used for the purpose of recording data such as characters and line 
patterns. The images of the characters and line patterns are so-called 
binary images which can be expressed by two states: namely, black and 
white. Since reproduction of half-tone image is unnecessary, the 
construction of the printer can be simplified. 
Methods are known which reproduce quasi-half tone image with a binary 
recording apparatus, such as the dither method and density pattern method. 
As well known to those skilled in the are however, the dither method and 
density pattern method are disadvantageous in that they cannot produce 
images with high resolution. Under this circumstance, a printer has been 
recently developed in which a semiconductor laser is driven by a 
pulse-width-modulated (PWM) image signal so that half-tone image can be 
formed even by binary recording methods. According to this PWM method, it 
is possible to obtain a print output with high degrees of resolution and 
gradation. In particular, this printing technique has become indispensable 
in color image printing apparatus. 
Laser beam printers relying upon PWM methods, however, encounter with 
various problems peculiar to this type of printer. One of these problems 
pertains to lack of stability of the density in the printing of image. 
This problem is inherent in electrophotography. The other problem is 
encountered when a semiconductor laser is driven through pulse width 
modulation. 
These problems will be discussed in more detail. 
FIG. 11 shows the general construction of a printer portion used in an 
electrophotographic system. The printer portion has a photosensitive drum 
301 adapted to be rotated in the direction of the arrow about the axis of 
a shaft 306, components arranged around the photosensitive drum 301 such 
as a charger 302, a developing unit 303, a transfer charger 304 and a 
cleaning device 305, and an optical system arranged at the upper side of 
the photosensitive drum 301 as viewed in the drawings. 
The optical system includes a semiconductor laser unit 306, a polygonal 
mirror which rotates at a constant high speed, an f-.theta. lens 308, a 
light-shielding plate, and so forth. Time-serial digital pixel signals 
computed and output from an image reader or an electronic computer (not 
shown) are PWM modulated and delivered to the semiconductor laser unit 
306. The semiconductor laser unit 306 turns on and off the generation of a 
laser beam in accordance with the levels of the PWM-modulated pixel 
signals and directs the beam towards the polygonal mirror 307. Since the 
polygon mirror 307 is rotating at a high constant speed, the laser beam 
applied to one side of the polygonal mirror 307 is reflected in an 
oscillatory manner so as to scan and expose the portion of the 
photosensitive drum between the charger 302 and the developing unit 303, 
from the proximal end to the distal end as viewed in the drawings. 
In general, the photosensitive drum 301 exhibits changes in the exposure 
sensitivity and residual potential, due to a change in the environmental 
condition and elapse of time. In addition, the developing material such as 
a toner used in the developing unit 303 exhibits a large fluctuation in 
the developing density according to a change in the amount of charges. 
This problem, i.e., lack of stability of the image density, is a problem 
inherently possessed by electrophotographic technique itself, but 
significantly affects formation of low-density image by a PWM type laser 
printer. 
FIG. 9 is a circuit diagram of a PWM circuit proposed by the assignors, 
while FIG. 10 is a circuit diagram showing a laser driver circuit. FIG. 12 
is a timing chart illustrative of the operation of the PWM circuit. 
Referring to FIG. 9, the PWM circuit includes a TTL latch circuit 401 for 
latching 8-bit pixel signals, a level converter 402 for converting a TTL 
logical level to a high-speed ECL logical level, an ECL D/A converter 403, 
an ECL comparator 404 for generating a PWM signal, a level converter 405 
for converting an ECL logical level to a TTL logical level, a clock 
generator 406 for generating a clock signal 2f of a frequency which is 
twice as high as the pixel clock signal f, a triangular wave generator 407 
for generating substantially ideal triangular wave signals, and a 1/2 
frequency dividing circuit for conducting a 1/2 frequency division of the 
clock signal 2f. In order to enable the circuit to operate at a high 
speed, ECL logical circuits are arranged everywhere in the circuit. 
The operation of this circuit will be explained with reference to FIG. 12. 
In these Figures, signals (a) and (b) represent, respectively, the clock 
signal 2f and the pixel clock signal f having a period which is twice as 
large that of the clock signal 2f. In the triangular wave generator 40 
also, a triangular wave signal (c) is generated after a 1/2 
frequency-division of the clock signal 2f, in order to maintain the duty 
ratio of the triangular wave signal at 50%. Furthermore, the triangular 
wave signal (c) is converted to an ECL level (0 to -1 V) so as to form a 
triangular wave signal (d). 
Meanwhile, the pixel signal latched by the latch circuit 401 is variable 
over 256 gradation levels between 00H (white) to FFH (black). The symbol 
"H" represents a hexadecimal notation code. The pixel signal (e) 
represents ECL voltage levels as obtained through a D/A conversion of a 
plurality of pixel signal values by a D/A converter 403. For instance, the 
pixel signal for the first pixel has a voltage of black pixel level FFH, 
the pixel signal for the second pixel has a voltage of a half tone level 
of 80 H, the pixel signal for the third pixel has a voltage of a half tone 
level of 40 H and the pixel signal for the fourth pixel has a voltage of a 
half tone level 20 H. The comparator 404 is adapted to produce, through a 
comparison between the triangular wave signal (d) and the pixel signal 
(e), PWM signals such as pulse widths T, t.sub.2, t.sub.3 and t.sub.4. The 
PWM signal is then converted to a TTL level of 0 V or 5 V so as to become 
a PWM signal (f) which i delivered to a laser driver circuit 500. 
FIG. 10 shows the laser driver circuit which is of a constant current type, 
and the semiconductor laser device 501. This semiconductor laser device 
501 emits a laser beam when the switching transistor 502 is on and 
terminates the emission when the switching transistor is turned off. The 
switching transistor 502 cooperated with a transistor 504 in forming a 
transistor pair which in turn forms a current switching circuit capable of 
controlling on/off (conversion) of the constant current which is to be 
supplied to the semiconductor laser device 501, in accordance with the PWM 
signal input thereto. This constant current is supplied from a constant 
current source transistor 505 and can be varied. The input laser power 
value input thereto is converted into an analog voltage by a D/A converter 
503 and is compared with a reference voltage. The level of the constant 
current is determined in accordance with the result of the comparison. 
However, the following problem is still encountered even when the 
above-described control is conducted, due to response characteristics of 
the semiconductor laser device 501. Referring to FIG. 12, representing the 
maximum emission time per pixel by T (sec), a change in the pulse width 
between 0 and T (sec) theoretically should cause the semiconductor laser 
device 501 to emit the beam over a time which corresponds to the pulse 
width. Actually, however, a signal waveform (g) for driving the laser 
device is different from the PWM signal (f) due to the fact that the PWM 
signal (f) is transmitted through the semiconductor laser device 501 and 
the driving circuit 501, with the result that a delay is caused in the 
turning on and off of the laser beam. This delay does not cause any 
problem when the pulse width is T or t.sub.2. However, when the pulse 
width is t.sub.3, the signal for driving the semiconductor laser device 
cannot be completely switched to ON state. When the pulse width is 
t.sub.4, the semiconductor laser device 501 fails even to operate 
materially. A beam effect (h) two-dimensionally illustrates the state of 
emission of the laser beam. The first pixel is completely black, so that 
the laser beam is kept on whole through one pixel period. However, when 
the pulse width of the PWM signal is extremely short as, for example, 
t.sub.3 =10 ns, the state of generation of the laser beam is too unstable 
to form an image by electrophotographic process, not to mention a problem 
as to whether the laser beam is actually generated. In such a case, stable 
formation of density can no more be expected. Thus, in the gradation 
expression according to the PWM method, there is a practical limit in the 
minimum pulse width which can form an appreciable density. If this limit 
is t3=10 ns for example, gradation is always white whenever the pulse 
width is below this lower limit of 10 ns, i.e., in the highlight portion. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide an image 
processing apparatus which can overcome the above-described problems of 
the prior art. 
Another object of the present invention is to provide an image processing 
apparatus which enables the known image forming apparatus having the 
described shortcomings to form an image which includes reduced degradation 
in the highlight portion. 
To these ends, according to one aspect of the present invention, there is 
provided an image processing apparatus which receives multi-level pixel 
data and for delivering the received data to an image forming apparatus 
capable of forming a gradation image, the apparatus comprising: 
determining means for determining whether the density level of the input 
multi-level objective pixel data is below a predetermined density level; 
and binarizing means for binarizing the multi-level objective pixel data 
by changing the data either to a minimum density level or to the 
predetermined density level, when the determining means has determined 
that the density level of the multi-level objective pixel data is below 
the predetermined density level. 
In a preferred form of the present invention, the binarizing means 
includes: a memory for storing data which represents whether the 
binarization has been done to the minimum density level at the positions 
of a plurality of binarized pixels in the vicinity of the multi-level 
objective pixel; calculating means for calculating the average density at 
the position of the multi-level objective pixel data in the memory; 
comparing means for comparing the calculated average density with the 
density of the multi-level objective pixel data; output means for 
outputting the predetermined density level as the multi-level objective 
pixel data when the comparing means has determined that the multi-level 
objective pixel data is not lower than the average density and for 
outputting the minimum density level as the multi-level objective pixel 
data when the comparing means has determined that the multi-level 
objective data is not higher than the average density; and storage means 
for storing, as the result of the binarization of the multi-level 
objective pixel data, the result of the comparison in the memory at the 
position of the objective pixel. 
A further object of the present invention is to provide an image processing 
apparatus which can stabilize the density in the highlight portion so as 
to form am output image of a high quality. 
To this end, the present invention provides an image processing apparatus 
which receives multi-level pixel data and for forming a gradation image on 
the basis of the received multi-level pixel data, the apparatus 
comprising: determining means for determining whether the density level of 
the input multi-level objective pixel data is below a predetermined 
density level; and binarizing means for binarizing the multi-level 
objective pixel data by changing the data either to a minimum density 
level or to the predetermined density level, when the determining means 
has determined that the density level of the multi-level objective pixel 
data is below the predetermined density level. 
Other features and advantages of the present invention will be apparent 
from the following description taken in conjunction with the accompanying 
drawings, in which like reference characters designate the same or similar 
parts throughout the figures thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the present invention will be described in 
detail with reference to the accompanying drawings. 
FIG. 1 is a block diagram which schematically shows the construction of a 
laser beam printer embodying the present invention. 
In this Figure, a reference numeral 200 denotes a low-density portion 
binarizing circuit which binarizes pixel data of densities below a 
predetermined half-tone density. The detail of this circuit 200 will be 
described later. Numeral 400 denotes a PWM circuit which receives the 
pixel signal output from the low-density portion binarizing circuit 200 
and conducts a pulse width modulation of the received signal, 500 denotes 
a driver circuit, and 501 denotes a semiconductor laser device. An image 
forming portion including a photosensitive drum 301 and other components 
is denoted by 300. Although not exclusive, the PWM circuit 400, laser 
driver circuit 500 and the image forming portion 300 have the 
constructions which are the same as those described before in connection 
with FIGS. 9, 10 and 11. 
In this arrangement, an 8-bit input pixel signal A having 256 gradation 
levels is converted by the low-density portion binarizing circuit 200 into 
an 8-bit output image signal B suitable for a laser beam printer which 
operates in accordance with the PWM method. The image signal B is further 
input to the PWM circuit 400 and is used in forming a high-gradation image 
by an electrophotographic process performed by the image forming portion 
300, through the operation of the laser driver circuit 500 and the 
semiconductor laser device 501. 
FIG. 2 is a block diagram showing internal blocks of the low-density 
portion binarizing circuit used in the embodiment. 
As will be seen from this Figure, the low-density portion binarizing 
circuit used in this embodiment has a low-density portion detecting 
circuit 201, a binarizing circuit 202, an average density calculating 
circuit 203, a binary memory 204 having a memory capacity corresponding to 
data of three lines, and a selector 205. 
Briefly, the operation of this low-density portion binarizing circuit 200 
is as follows. When the density value of the input image data A is not 
greater than the minimum half tone density "M" which can be reproducible 
by the image forming portion 300, the low-density portion binarizing 
circuit 200 changes the density value of this data either to "0" or "M" so 
as to form an output pixel which is delivered to the PWM circuit 400. 
However, when the density of the input pixel data A is greater than the 
above-mentioned half tone density level "M", the low-density portion 
binarizing circuit 200 directly passes this pixel data A to the PWM 
circuit 400 without effecting any change. 
The detail of the operation of the low-density portion binarizing circuit 
200 shown in FIG. 2 will be described in sequence. 
FIG. 4(D) shows an image data A (see FIG. 1) which is output from an 
external host computer or an image reader. For information, this image 
data A includes also data after a correction, as in the case of ordinary 
image processing system. FIG. 4(C) shows an example L(i, j) of conversion 
of the image data after the detection of only the low-density portion of 
the input image at a G(i, j). The conversion is conducted by using a 
threshold or a reference level which is a predetermined half tone density 
"M" in consideration of the gradation reproducibility in the highlight 
portion of image to be printed by a PWM type laser beam printer. In order 
to simplify the explanation, it is assumed here that the minimum pulse 
width t.sub.3 which enables the laser beam printer to stably form a 
density is 10 ns (t3=10 ns) and that the half tone density value M 
corresponding to this minimum pulse width is 30 (M=30). The value 30 is a 
value of decimal notation. In this embodiment, the value 30 means the 
30th gradation level from among the 256 gradation levels, i.e., from 0 to 
255, which can be reproduced in this embodiment. 
In FIG. 4(D) , values appearing in the respective frameworks represent the 
density values of these pixels. It is assumed here that the framework 
demarcated by thick lines corresponds to the objective pixel G (=G(i, j) . 
The value of the objective pixel G is compared by the value M, i.e., 
threshold level 30, which is beforehand stored in the low-density portion 
detecting circuit 201, and the result of the comparison is delivered 
through a signal line 206. At the same time, the low-density portion 
detecting circuit 201 delivers to the binarizing processing circuit 202 
the following value as the converted value L (i, j) . 
EQU On condition of G&lt;M, L(i, j).rarw.G(i, j) (1) 
EQU On condition of G&gt;M, L(i, j).rarw.M (2) 
Thus, any density level of the objective pixel is delivered without being 
changed when the density level is below the threshold value "M", whereas, 
when the density level of the objective pixel is equal to or greater than 
the threshold level "M", the threshold level M is delivered as the 
converted value L(i, j) . 
The binarizing processing circuit 202 compares the converted value L(i, j) 
received from the low-density portion detecting circuit 201 with a average 
density value S which is delivered by the average density calculating 
circuit 203, thereby binarizing the converted value L(i, j) using the 
average density value S as the threshold, i.e., either into "0" (when the 
level of L(i, j) is equal to or below the average density level S) or "1" 
(when the level of L(i, j) is equal to or greater than the man density 
level "S") . The thus binarized value is stored in the binary memory 204. 
The calculation of the average density level S by the average density 
calculating circuit 203 is conducted in a manner which will be described 
hereinunder. A group of data which have already been binarized are read 
from the binary memory 204 and is processed by a previously set weight 
mask. As shown in FIG. 4(A), the values in the weight mask form a 
substantially constant gradient in accordance with the distances from the 
objective pixel and the sum of the matrix elements is equal to the 
aforementioned predetermined half tone density value M which equals to 30. 
The average density value S is calculated by superposing this weight mask 
to the binary memory 204. In the case of the binary data shown in FIG. 
4(B) , the average density value S with respect to the objective pixel is 
calculated as follows: 
EQU S=6.times.0+10.times.0+5.times.1+9.times.1=14 
As will be seen from FIG. 5, the average density value S is variable within 
a range between 0 and M, i.e., between 0 and 30. 
In the process shown in FIGS. 4(D) to 4(A) , the initial value of the input 
objective pixel data is "20", so that the value "20" is output as the 
converted value L(i, j) from the low-density portion detecting circuit 201 
to the binarizing processing circuit 202. At the same time, the average 
density calculating circuit 203 delivers "14" as the mean density value S 
to the binarizing processing circuit 202. In consequence, the condition 
L(i, j)&gt;S is confirmed as a result of the determination by the binarizing 
processing circuit 202, so that the value "1" is written in the address of 
the objective pixel in the binary memory 204 (see FIG. 5). 
On the basis of this result of determination, the binarizing processing 
circuit 202 outputs a correction data G' in accordance with the following 
rule. 
______________________________________ 
On condition of B = 1 
G' = 30 
On condition of B = 0 
G' = 0 
______________________________________ 
Therefore, in the case of the process explained in connection with FIGS. 
4(D) to 4(A) in which the condition of B=1 is met, the correction data 
G'=30 is input from the binarizing processing circuit 202 to one of the 
input terminals of the selector 205. Meanwhile, the other input terminal 
of the selector 205 receives the initial value G (=20) of the objective 
pixel. 
The selector 205 selects the correction data G' when it has judged that the 
initial value of the data of the objective pixel is below the minimum 
density M which is reproducible by the image forming portion 30, otherwise 
it selects the initial value G. The level of the signal delivered from the 
low-density detecting circuit 201 to the signal line 206 is used as the 
criterion for the selection performed by the selector. In the case of the 
process explained in connection with FIGS. 4(D) to 4(A), the density G of 
the objective pixel is "20" so that the selector 205 selects the data G' 
(=30) which is output from the binarizing processing circuit 202 and 
delivers the same to a PWM circuit 400 shown in FIG. 1. 
An example of the process which is conducted when the density G of the 
objective pixel meets the condition of G.gtoreq.M will be described with 
reference to FIG. 6. 
The density G of the input objective pixel is "45" so that "M(=30)" is 
output as the data L(i, j) from the low-density portion detecting circuit 
201 and delivered to the binarizing processing circuit 202 (see FIGS. 6(C) 
and 6(D)). Therefore, the binarized signal B of the objective pixel is "1" 
regardless of the average density value S which is determined by the 
weight mask (see FIG. 6(A)). As a consequence, "1" and "30" are 
respectively delivered to the binary memory 204 and the selector 205. In 
this state, a signal indicating that the density value G of the objective 
pixel meets the condition of G.gtoreq.M is delivered by the low-density 
portion detecting circuit 201 to the selector 205 through the signal line 
206. Therefore, the selector 205 selects and outputs the initial data 
(data representing the density "45"). 
Thus, the input data is directly output without change when the level of 
the data ranges from M to 255 so that the density of the input data is 
maintained. 
The condition B=1 is stored as the binary data in the binary memory 204 
whenever the condition of G.gtoreq.M is confirmed. 
The content of the process performed by the low-density portion binarizing 
processing circuit 200 in the described embodiment is as shown in the flow 
chart of FIG. 3. 
In Step S1, the objective pixel G is input. Then, in Step S2, the value of 
the density of the objective pixel is compared with the minimum density 
value M which is reproducible by the image forming portion 300. If the 
condition of G.gtoreq.M is determined as a result of the comparison, the 
process proceeds to Step S3 in which the density value G of the objective 
pixel is delivered to the PWM circuit 400 without any change. 
Conversely, if the density G is determined as being G&lt;M as a result of the 
comparison, the process proceeds to Step S4 in which the average density S 
is calculated on the basis of the binarized data around the objective 
pixel. Then, the process proceeds to Step S5 which compares the calculated 
average density value S with the density G of the objective pixel. If a 
condition G&gt;S is confirmed as a result of the comparison, the value "M" is 
delivered to the PWM circuit 400 (Step S6), otherwise, i.e., when 
G.ltoreq.S is confirmed, "0" is delivered to the PWM circuit 400 (Step 
S7). 
By the process described hereinbefore, the input data having continuous 256 
levels of gradation, i.e., from 0 to 255, is output as pixel data of "0" 
level or one of "M to 255" levels, i.e., data which do not employ levels 
of "1" to "M-1". When the input data has a level which is not employed, 
i.e., when the level of this input data ranges from "1" to "M-1", this 
input data is changed either to "0" or "M", but the overall density i 
preserved so that image of a good quality is obtained with a high degree 
of stability. 
In the described embodiment, the threshold level M of the density is 
determined as the minimum density which is reproducible by the image 
forming portion 300. This, however, is only illustrative and the threshold 
level M may be set at any desired level so as to enable reproduction of 
the highlight portion of the image without using unstable image forming 
condition inherent in the electrophotographic process, whereby a stable 
reproduction of gradation is attained over the entire range of the 
density. 
In the embodiment described hereinbefore, the minimum PWM pulse width 
t.sub.3 which can produce a visible image is assumed to be 10 ns and the 
minimum density level M corresponding to this minimum pulse width is 30. 
These values, however, are introduced only for the purpose of illustration 
and are not intended for restricting the scope of the invention. Rather, 
it is expected that the factors such as the minimum pulse width t.sub.3 
and minimum density M fluctuate according to the characteristics of the 
image forming portion 300. It is therefore preferred that a switch for 
varying the set values of such factors is provided to enable a more 
delicate adjustment of gradation reproduction. 
Another advantage offered by the described embodiment resides in the 
process for binarizing the data of the low-density portion, i.e., portions 
of low densities of 0 to M. 
In general, when binarizing the data by changing it either to "0" or "M", 
it is a common measure to use a value M/2 as the threshold for the 
binarization. However, the use of such a value a the threshold poses a 
problem in that, even when a continuous smooth change of density level 
exists in the low-density portion, such a gradation is lost since the data 
is binarized by the fixed threshold, resulting in a large binarizing error 
and a serious degradation in the image quality. 
In contrast, in the present invention, the threshold is varied to follow up 
a change in the image density, as will be understood from the foregoing 
description of the embodiment. It is therefore possible to reduce any 
binarizing error to an acceptable level and to reproduce any gentle change 
of density in a highlight portion with a high degree of exactness, 
regardless of the image density. 
The distribution of the weight coefficient used in the average density 
calculating circuit 203 of the low-density portion detecting circuit 200 
is related to edge stressing effect, such that the greater the range of 
distribution, the stronger the edge stressing effect. It is therefore 
possible to enhance the edge stressing effect by enlarging the mask 
pattern, e.g., from the weight matrix of FIG. 7 to the weight matrix shown 
in FIG. 8. It is advisable to prepare a plurality of weight matrices for 
free selection by the operator. This can be done by storing a plurality of 
mask patterns in the ROM and arranging such that the contents of the ROM 
is selectively delivered to the average density calculating circuit 203 
through a selector switch. Alternatively, a RAM is used in lace of the ROM 
so as to enable the user to freely change the values in the matrix. In 
this case, restriction concerning the type of the matrix are substantially 
eliminated. 
Although a laser beam printer is used in the described embodiment, this is 
not exclusive and other types of printers which reproduce the gradation 
through the control of the intensity of beam applied to a photosensitive 
member such as a drum. 
As has been described, according to the present invention, it is possible 
to obtain an output image with a high degree of stability of density in 
highlight portion. 
In particular, when the density levels of the input multi-level pixel data 
is equal to or below the minimum reproducible level, such levels are 
binarized by being changed either to the minimum density or the minimum 
reproducible density, whereby the width of the gradation level which can 
safely be preserved is maximized. 
Furthermore, since the binarizing processing is executed such that the 
densities are preserved within the range of binarization, whereby the 
whole density levels of the output image are preserved with respect to the 
input image. 
In the described embodiment, the low-density portion binarizing circuit 200 
is provided in the laser beam printer as a component of the latter. This, 
however, is only illustrative and the arrangement may be such that the 
low-density portion binarizing circuit 200 as an independent device is 
connected between a host computer and an image forming device such as a 
laser beam printer. In such a case, the low-density portion binarizing 
device, constructed in accordance with the present invention, makes it 
possible to form an image with reduced deterioration of the image quality 
in the highlight portion even with conventional printers. 
As many apparently widely different embodiments of the present invention 
can be made without departing from the spirit and scope thereof, it is to 
be understood that the invention is not limited to the specific 
embodiments thereof except as defined in the appended claims.