CA1325267C - Image processing apparatus - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An apparatus for generating a high quality image from a digital video signal includes a system for gamma correcting the digital video signal with a digital look up table and for converting the resultant digital signal to an analog video signal. Another circuit generates a triangular wave reference pattern signal and a comparator compares the analog video signal with the triangular wave reference pattern signal to form a pulse-width-modulated signal. A raster scanning print engine producing, for example, a laser beam, scans over a recording medium in accordance with the pulse-width-modulated signal, thereby forming an image of high quality on the recording medium of a print engine. This apparatus can also be used with an analog video signal by first converting the analog video signal to a digital video signal with an analog to digital converter.

Description

This application is a division of Application Serial No. 515,897, filed August 13, 1986.

BACRGROU~D OF THE I~ENTION

Field of the Invention The present inventiorl relates to an apparatus for generating an image from a digital video input signal.
The apparatus is improved so as to reproduce an image with high quality.

Description of the Prior Art In the past, methods generally referred to as the dither j method and the density pattern method have been proposed : for reproducing images of half tones. These known methods, however, cannot provide satisfactory gradation of dot size when the size o~f the threshold dot matrix is small and, therefore, require the use of a threshold matrix having a larger size. This is turn reduces the resolution and undesirably allows tne texture of the image to appear too distinctive due to the periodic structure of the matrix. Therefore, deterioration of the quality o~
; 20 the output image results.
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In order to mitigate the above described problems, it has been proposed to modify the dither method so as to allow finer control of the dot size by the use of a plurality of dither matrices. This metnod, however, requires a complicated circuit arrangement for obtaining syncnronism of operation ~etween the dither matrices so that the system as a whole is large in size, complicated in construction, and slow. Thus, there is a practical limit in the incremental increase of dot size and the resultant increment of density available by the use of a plurality of dither matrices. In U.S. Patent No. 3,916,096, a method of improving the conventional screening process is described. As set fortn in this U.S. Patent No.
3,916,096, at column 8, lines 19 through 31:

The conventional screening process when applied to a scanned image can be regarded as a form of pulse-widt~)-modulation whereDy a line of length X is laid down and repeated at intervals of Y. The percentage transmission (or reflection) of the reproduced image is then Y - X/Y [sic.
should read (Y-X)/Y]. To be a linear process (Y - X) must be directly proportional to the amplitude of the scanned video signal where the signal amplitude represents the percentaye optical transmission of the recorded original image. A way of achieving this is by comparing the amplitude of the video signal with a sawtooth wave form and laying a line forming a portion of a dot whenever the sawtooth is larger than the video signal.

i See also U.S. Patent No. 4,040,094, which relates to ~, similar suoject matter.

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However, even if the method described in this patent is used in an apparatus for reproduction of an image, the precision of gradation reproduction deteriorates due to tne delay of response of the apparatus.

The conventional method described in U.S. Patent No.
3,91~ 6, produces a linear mapping from the analog video signal to the pulse-width-modulated signal. As is known in the art of printing, this linear mapping doés not produce acceptable results because of the non-linear distortions introduced in the nalf-tone printing process, in particular when used with a laser beam print engine.
Therefore, to obtain high quality half-tone printing, a method of non-linear mapping must be found. And, the method di~closed in the noted U.S. Patent, as quoted above, uses a complex arrangement to allow the use of different sawtooth waveforms on successive scans.

SUMMARY OF THE INVEN~ION

Accordingly, an object of the pre~ent invention is to provide an image processing apparatu~ for generating an image from a digital video signal, that can overcome the problems of the prlor art described above.

Anotner object of the present invention is to provide an image processing apparatus, for generating an image from a digital video signal, that permits reproduction of images with high ~uality.

Still another object of the present invention is to provide an image processing apparatus, for generating an image from a digital video signal, that can provide, with a very simple arrangement, a superior quality half-tone image.

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Another object of the present invention is to provide an imaye processing apparatus, for geslerating an image from a digital video signal, that permits reproduction of images with high ~uality at high speed, A furtber object of the present invention is to provide an image processing apparatus, for generating an image from a digital video signal, that can reproduce tone information with a high gradation and without impairing resolution.

Still another object of tne present invention is to provide an image processing apparatus that can correct the tonal properties of the video image by providing a non-linear mapping of the video signal onto a pulse-width-modulated signal with a very flexible arrangement, In accordance with a preferred emDodiment, the image processing apparatus of the present invention processes a digltal image input signal and includes a raster scanning print engine for generating a series of successive ~canning lines. ~ pulse-widtn-moaulated signal generator generates a pulse-width-modulated signal from a digital image input signal input to the apparatus. A circuit then applies the pulse-width-modulated signal to the print engine to cause it to generate each line as a succession of line segments. Tne lengths of the line segments are controlled to produce a variaDle density line screen from the line segments with the line screen comprising a plurality of columns of the line segments.

In accordance with another aspect of a preferred em~odiment of the present invention, the image processing apparatus includes a pattern signal generator for generating a pattern signal of predetermined period. A
pulse- widtn-modulated signal generator then generates a pulse-width-modulated signal in accordarlce with the video .' .
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signal and the pattern signal that can be utilized by a raster scanning print ensine or image forming device to form an image.

More specifically, the print engine scans lines on a recording medium with a beam in accordance with the pulse-width-modUlated signal, and a synchronizing signal generator generates a synchronizing signal for each line scanned on the recording medium. The pattern signal generator generates the pattern signal of predetermined period in accordance with the synchronizing signal.

In accordance with still another aspect of the invention, the digital input signal has a characteri~tic, and a characteristic converting device converts the cnaracteristic in ~rder to produce a converted digital video signal. This signal is converted to an analog video signal Dy a digital to analog converter. A
pulse-width-modulated signal is thereafter generated from tni--- analog video signal and the pattern signal.

Other aspects, features, and advantages of the present invention will Decome apparent from the following detailed description of the preferred embodiments taken in conjunction witn the accompanying drawing, as well as from the concluding claims.

BRIEF DESCRIPTION OF THE DRA~JING

Fig. l is a simplified schematic illustration of a preferred embodiment of the apparatus for generating an image from a digital video signal in accordance with the present invention;

Fig. 2 shows waveform-~ of ~ignal obtained at different portions of the apparatus for generating an image from a , .
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digital video signal shown in Fig. l.

Fig. 3 shows how Figs. 3A and 3B are assembled together to illustrate details of the embodiment of the apparatus for generating an image from a digital video signal shown in Fig. l;

Fig. 4 is a scheMatic illustration of an optical scanning system in a laser Deam printer to which the invention is applicable;

Fig. 5 shows waveforms of signals o~tained at different portions of the circuit shown in Figs. 3A and 3B;

Fig. 6 is an illustration of triangular wave signals formed in the circuit shown in Figs. 3A and 3B;

Figs. 7(a) to 7(c) are illustrations of how triangular wave signals may be adjusted in the embodiment of the invention;

Fig. B is an illustration of a look-up table of a gamma converting ROI~ 12;

Fig. 9 is a diagram showing the relationship between input video signals and converted video signals;

Figs. lO(a) and lO(b) illustrate the relationship between the scanning lines and the conversion table as used;

Fig. ll is a circuit diagram of a circuit for causing phase shift of triangular wave signals between lines;

Fig. 12 is an illustration of trianyular wave signals appearing in respective lines at different phases; and .. . .
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Fig. 13 is an illustration of another embodiment of the invention.

DESCRIPTIOl~ OF THE PREF~RED EMBODIMENTS

A preferred emDodlment of the invent ~ will De described in detail herein with reference to the accompanying drawing.

Referring first to Fig. 1 schematically showing an embodiment of the invention, a digital data output device 1 is adapted to receive an analog video data from a CCD
sensor or a video camera (neither of whicn is shown) and to perform an A/D (analog-to-digital) conversion of the analog video signal so as to convert that signal into a digital video signal, where each picture element (pixel) is represented by a predetermined number of bits carrying tone information. The digital video signal may be temporarily stored in a memory or, alternatively, may be supplied from an external device b~, for example, telecommunication. The signal from the digital data output device 1 is used as the address for a digital look-up table for gamma correction 9. The resultant output, which in tne preferred embodiment is an eight (8) bit digital number ranging from OOH to FFH reprèsenting 256 possiDle tonal gradation levels as described further below, is converted back into an analog signal by means of a DtA (digital to analog) converter 2 so as to form an analog signal which is updated for each picture element.
The analog video signal representing the picture elements is fed to one of the input terminals of a comparator circuit 4. simultaneouSly, analoy reference pattern signals having a triangular waveform are produced by a pattern signal generator 3 at a period corresponding to the desired pitch of the half-tone screen. The pattern .
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signals (a triangle wave) are fed to the other input terminal of the comparator circuit 4. Meanwhile a horizontal synchronizing signal generating circuit 5 generates horizontal synchronizing signals for respective lines, while an oscillator lreference clock generating circuit) 6 generates reference clocks. In synchronism with the horizontal synchronizing signal, a timing signal generating circuit 7 counts down the reference clocks, to, for example, l/4 period. The signal derived fro~ the timing signal generating circuit 7 is used as the clock for the transfer of the digital video signal and also as the latcn timing signal for the D/A converter 2.

In the embodiment described, since the apparatus is intended for use in a laser beam printer, the horizontal synchronizing signal corresponds to a beam detect (BD) signal which is known per se. The comparator circuit 4 compares the level ol the analog video signal with the level of the pattern signal of triangular waveform and produces a pulse-width-modulated signal. The pulse-width-modulatea signal is supplied to, for example, the laser modulator circuit of a raster scanning print engine 8 for modulating the laser beam. As a result, the laser beam is turned on and off in accordance with the pulse width thereoy forming a half-tone image on the recording medium of the raster scanning print engine 8.

Fig. 2 shows the waveforms of signals obtained in certain components of the apparatus shown in Fig. l. More specifically, the portion (a) of Fig. 2 diagrammatically shows the reference clocks generated by the oscillator 6, while the portion tb) shows the horizontal synchronizing signal mentioned above. The portion (c) shows the pixel clocKs which are produced by counting down the reference clocks with the timing signal generating circuit 7. More specifically, the pixel clock shown in the portion (c) of ,~ ~ ' .

., , g Fig. 2 is the signal which is obtained by counting down the reference clocks into 1/4 period by the operation of the timing signal generating circuit 7 in synchronism with the norizontal synchrollizing signal. The pixel clock thus obtained is delivered to the D/A converter 2 to be used as the digital video signal transfer clock. The portion (d) of Fig. 2 shows the pattern signal synchronizing clock (screen clock) which is obtained by counting down the reference clock into 1/12 period by operation of the timing signal generating circuit 7 in synchronism with the horizontal syncnronizing signal. In the illustrated case, one pattern signal synchroni2ing clock is generated for every -three pixel clocks. ~he pattern signal synchr-onizing or screen clock thus obtained is delivered to the pattern signal generator 3 to be used as the synchroni~ing signal in the generation of the pattern signal. The portion (e) of Fig. 2 shows the digital video signal which is output from the digital data output device 1. And the portion (f) shows the analog video signal after the D/A conversion conducted by the D/A converter 2. It will be seen from the portions of Fig. 2 that picture element data of analog level are produced in synchronism with the pixel clocks. It will also be seen that the density of image Decomes higher, i.e., approaches blac~, as the level of the analog video signal rises.

As shown by a solid line curve in the portion (g) of Fig.
2, the output from the pattern generator 3 is obtained in synchronism with the clocks shown in the portion (d) and is input to the comparator circuit 4. The broken line curve in the portion (9) of Fig. 2 shows the analog video signal shown in the portion (f). This video signal is comparea by the comparator circuit 4 with the pattern signal of triangular waveform derived from the pattern signa~ generator 3 so that the analog video signal is converted into a pulse-width-modulated signal as shown in the portion ~h) of Fig. 2.

'' l 325261 ~ 10 --The described embodiment of the invention permits a substantially continuous or linear pulse modulation and, hence, ensures a high gradation of the image output by virtue of the fact that the digital video signal is converted into an analog video signal which is then compared with the triangular wave signal of a predetermined period.

It is to be noted also that, in the described embodiment of the invention, the pattern signal synchronizing clock (screeil clock) for generation of the pattern signal, e.g., the triangular wave signal, is generated in synchronism witn the ho~izolltal synchronizing signal by making use of reference clocks having a frequency much higner than tnat of the pattern signal synchronizing signal. Therefore, the ~itter of the pattern signal derived from the pattern signal generator ~, e.g., the offset of the pattern signal from one scan line to the next, is reduced to less than l/12 of the period of the pattern signal. This precision is required to insure a high quality half-tone reproduction in which the line screen is formed uniformly and smoothly from one scan line to the next. Therefore, the density information can be accurately pulse-width-modulated by making use of this pattern signal which has a small fluctuation, so that the image can be reproduced with high quality.

Fig. 4 is a schematic perspective view of the optical scanning system incorporated in the laser beam printer (a raster scanning print engine) to which the present invention is applied. The scanning system has a semiconductor laser adapted to emit a laser beam moàula~ed in accordance with the pulse-width-modulated signal mentioned a~ove.

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~ 3252b1 The optical laser beam modulated by the semiconductor laser 21 is collimated by a collimator lens 20 asld is optically deflected by a polygonal mirror ~applying means) 22 having a plurality of reflecting surfaces. The deflected ~eam is focused to form an image on a photosensitive drum 12 by an image forming lens 23 referred to an fe lens, so as to scan the drum 12. During tne scanning by the beam, and when the beam reaches the end of each scanning line, it is reflected by a mirror 24 and is directed to a Deam detector 25. ~he beam detection (BD~ signal produced by tne beam detector 25 is used as the norizontal syncnronizing signal as ~s ~nown. ~rhus, in tne descriDed emDodiment, the horizontal synchronizing slgnal is consti~u~ed Dy tne BD signal.

It will be seen that the BD signal is detected for each of the lines of -~canning by the laser beam and is used as the timing signal for the transmission of tne pulse-width-modUlated signal to the semiconductor laser.

As used in the subject specification in description of the preferred embodinlents and as used in the concluding claims, the term ~line-segment~ means a dot which is formed on a recording medium~ the length (size) of which is variable in accordance with the width of the pulse widtn in the supplied pulse-width-modulated signal.

The apparatus for generating an image from a digital video signal ~f the invention will be descrioed more fully with specific reference to Figs. 3A and 3B which show details of the apparatus shown in Fig. 1.

As stated before, the preferred emDodiment described herein makes use of the BD signal as the horizontal synchronizing signal. The BD signal, however, is ~asically asynchronou~ witn the pixel clock and, therefore, would normally cause jitter in the horizontal direction. In the described embodiment, therefore, jitter is reduced to less than 1/4 of the width of a pixel, by making use of an oscillator 100 tnat can produce reference clocks ~72M-CLK) (72 megahertz clock) of a frequency which is 4 times higner than that of the pixel clocks. A BD
synchronizing circuit 200 is used for this purpose. The reference clock (72M-CL~) from the oscillator 100 is supplied to D latches 201, 202, and 203 through a buffer 101, while the BD signal is input to the data terminal D
of the D latch 201 through a terminal 200a so as to be syncnronized with tne reference clocks. In addition, the BD signal is delayed by the D latches 202 and 203 by an amount corresponaing to 2 (two) reference clock pulses.
The BD signal thus delayed is delivered to one of the input terminals of a ~OR gate 103, while tne other input terminal of the NGR gate 103 receives the inverted output of the D latch 201. The output from the NOR gate 103 is input to one of the input terminals of a NOR gate 104, while the other input terminal of the NOR gate 104 receives the output of a flip-flop circuit 102.

~ith this arran~ement, the flip-flop circuit 102 produces clocks (36M-CLK) (36 megahertz clockl which are obtained by dividing the frequency of the reference clock by 2 (two). Thus, the output (36M-CLK) from the flip-flo?
circuit 102 is synchronous witn the BD signal to within one period of the clock 72M-CLK.

The output of the D latch 203 is delayed by the D latches 204, 205, and 206 by an amount corresponding to 3 (three) pulses of the output (36M-CLK) of the flip-flop circuit 102.

The inverted output from the D latch 201 and the output from the D latch 206 are delivered to a NO~ gate 207, so ' :
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that an internal horizontal synchronizing signal (BD-Pulse) is formed in synchronism (within one period) with the reference clock.

Fig. 5 shows the timing of the signals obtained at various portions of tne BD synchronizing circ~:it 200. More specifically, A-l ShOws the BD signal, A-2 shows the reference clock ~72M-CLK) produced by the oscillator 100, and A-3 snows the inverted out~ut from the D latch 201, obtained by synchronizing the BD signal with the reference clock (72M-CLK). A-4 shows the output from the D latch 203, obtained by delaying the signal A-3 by an amount corresponding to 2 ~ two) reference clock pulses. A-5 shows the clock (36M-CLK) output from the flip-flop circuit 102, A-6 shows the output from the D latch 206, obtained by delaying the signal A-4 by an amount corresponding to 3 (three) pulses of the clocks (36M-CLK), and A-7 shows the internal horizontal synchroniæing (BD-pulse). It will be seen that the internal horizontal synchronizing signal (BD-Pulse) rises in synchronism with the rise of the first reference clock (72M-CLK) after the rise of the BD signal, and is held at level ~1~ for a period corresponding to 8 (eight) pulses of the reference clock. This internal horizontal synchronizing signal (BD-pulse) constitutes the reference for the horizontal driving of the circuit of this em~odiment.

An explanation of the video signals will now be made again with reference to Figs. 3A and 3B. The pixel clocks ~PIXEL-CLK) are formed by dividlng the frequency of the signal (36M-CLK) by 2 (two) by means of the J-K flip-flop circuit 105. A 6-bit digital video signal is latched in the D latch 10 by the pixel clock (PIXEL-CLK), and the output is delivered to a ROM 12 for gamma conversion. The 8-bit video signal produced through the conversion by the ROM 12 is further converted into an analog signal by the ., , , :

D/A converter 13 and is delivered to one of the input terminals of the comparator 15 in order to be compared with the triangular wave signal explained below. The pulse~width-modulated signal obtained as a result of the comparison is delivered to the laser driver of a raster scanning print engine.

Still referring to Figs. 3A and 3B, reference numeral 300 designates a screen clock generating circuit which generates the screen clock, i.e., the analog reference pattern signal synchronizing clock, which is used as the reference for the generation of the triangular wave signal. A counter 301 is used as a frequency divider for dividing the fre~uency of the signal (36M-CLK) output from tAe flip-flop circuit 102. The counter 301 has input terminals D, C, B, and A which are preset with predetermined data by means of a switch 303. The ratio of the frequency division i~ determined by the values set at these input terminals D, C, B, and A. For instance, when tne values ~ , and ~1~ are set in the terminal~
D, C, B, and A, respectively, tne frequency of the signal ~36M-CLK) is divided into 1/~.

Meanwhile, horizontal synchronization is attained by the NOR gate 302 and tne tBD-pulse) signal. The frequency of the divided signal is further divided into 1/2 by a J-K
flip-flop circuit 304, so that a screen clock having a duty ratio of 50~ is formed. A triangular wave generating circuit 500 generates triangular waves by using this screen clock as the reference, Fig. 6 shows waveforms or signals appearing at various components of the screen clock generating circuit 300.
~It is noted, nowever, that the scales of Figs. 5 and 6 are different). More specifically, L-l shows the internal synchronizing signal (BD-PULS~ -2 shows the signal , . .

' (36M-CLK) and B-3 shows the screen clock (SCREEN CLK) as obtained wnen values ~ 0~ are set in the terminals D, C, B, A of the counter 301, respectively.
B-4 represents tne triangular wave signal as obtained when the screen clock B-3 is used as the reference. On the other hand, B-5 shows the screen clock (SCREEN CL~) as obtained when values ~ 0~ are set in the input terminals ~, C, B, A of the counter 301. B-6 shows the triangular wave signal as obtained when the screen clock (SCREEN CLK) shown in B-S is used as the reference obtained. It will be seen that the period of the triangular wave signal shown by ~-4 corresponds to 2 (two) picture elements, while the period of the triangular wave signal snown by B-6 corresponds to 4 (four) picture elements. l~hus, the period of the triangular wave signal can De varied as desired by appropriately setting the switch 303. In the embodiment described, the period of tne triangular wave is cnan~eable between a duration corresponding to 1 (one) picture element and a duration corresponding to 16 (sixteen) picture elements.

The triangular wave signal generating circuit 500 will now be descriDed, again with reference to Figs. 3A and 3~.
The screen clock (SCREEN CLK) is received by the buffer 5Ul, and the triangular wave is generated by an integrator comprising by a variable resistor 502 and a capacitor 503. The triangular wave signal is then delivered to one of the input terminals of the comparator 15 through a capacitor 504, a protective resistor 506, and a buffer amplifier 507. The triangular wave signal generating circuit 500 nas two variable resistors, namely, variable resistor 502 for adjusting the amplitude of the triangular wave signal, and a variable resistor 505 ror adjusting the bias or ofrset of the triangular wave signal. The a~justment of the amylitude and the offset of the triangular wave signal by the variable resistors 502 and .. 7 S05 is conducted in a manner which will be explained witn reference to Figs. 7(a) to 7(c). In Fiy. 7(a), a triangular wave signal Tri-l before adjustment is shown by a solid line curve. By adjusting the variable resistor 502, the signal Tri-l is changed into an amplified triangular wave signal Tri-2 shown by a broken line curve. Then, the variable resistor S05 can be adjusted to snift or adjust the offset of the wave so as to form a triangular wave signal Tri-3 shown by a one-dot-an~-one-dash line curve. It is thus possible to obtain a triangular wave signal having the desired amplitude and ofr~et.

As statea Defore, the triangular wave signal tAus formed is compared by the comparator 15 with the output of the D/A converter 1~, i.e., witn the analog vlaeo signal. The relationship between the triangular wave signal and the analog vlaeo signal is preferaDly sucn tnat the maximum level of the triangular wave equals the level of the output of the D~A cQnverter 13 as obtained wnen the input to the converter 13 has the maximum level (FF~, where H
indicates a hexidecimal numDer), wnile the minimum value of the triangular wave signal equals the level of the output of the ~/A converter 13 as obtained when the input to this converter has the minimum level (OOH). Since the amplitude and the orfset of the triangular wave can be controlled as desired, it is possible to obtain this preferred condition without difficulty.

More particularly, according to the invention, the amplituae and the offset of the trianyular wave signal are adjusted in the following manner. In general, a laser driver for emitting a laser ~eam has a certain delay time in its operation. The delay time until the laser beam is actually emitted is further increased due to the beam emitting characteristics of the laser. Therefore, the . . . .

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1 3252b7 laser does not start emitting the laser beam until the width of the pulse input t~ the driver exceeds a predetermined value. This means that, in the case where tne input signal is a series of periodic pulses as in the case of the described embodiment, the laser does not emit a Deam unless the input signal pulse has a duty ratio greater than a predetermined value. Conversely, when the duty ratio o the pulse is increased beyond a certain level, i.e., when the period of low level of the pulse is shortened, tne laser tends to stay on, that is, the beam is continuously emitted. For these reasons, if the adjustment of the triangular wave signal is conducted in the manner shown in Fig. 7(b), the gradation levels around the minimum level (00~) and near the maximum level (FFH) are omitted from the 256 gradation levels of the input data which ~ay ~e input to tne D/A converter 13, so that the gradation deteriorates undesirably. In the embodiment described, therefore, the variable resistors 502 and 505 are adjusted so that the pulse width just below that which will cause the laser to begin emission is obtained at the OOH level of the data input to the D~A converter 13, and 80 that the pulse width which will render the laser continuously on is obtained at tne FFH level of the data input to the D/A converter 13. This manner of adjustment of the variable resistors 502 and 505 is shown in Fig.
7~c).

As will be understood from Fig. 7(c), tnis preferred embodiment is designed so that the comparator lS produces an output pulse of a certain pulse widtll (a pulse width just below that which will cause the laser to begin emission) when the minimu~ input data OOH is supplied to the D/A converter 13. '~he preferred embodiment is also designed so tnat, when the maximum input data FFH is supplied to the D/A converter 13, the comparator produces output pulses the duty ratio of which is not 100% but : . - :. . -: - ,.~., : - :

which is large enough to allow the laser to emit the beam continuously. This arrangement permits the emission time of the laser to vary nearly over the entire range of the 256 graaation levels of the input data, thus ensuring high gradation of the reproduced image.

It snoulà ve under-~toaà that the method descriDed above is not limited to a laser printer but may also be utilized in to an inK jet printer, a thermal printer or other raster scanning devices.

The ROM 12 for gamma conversion will now De explained in detail with reference to Fig. 8. The ROM 12 is provided to allow a ni~h gradation of density in the reproduced image. Although the described embodiment employs a ROl'l having a capacity of 256 bytes as ROM 12, a capacity of 64 bytes is basically enough because the input digital video signal is a 6-bit signal. Fig. 8 shows the memory map of the ROM 12 for gamma conversion. Since this ROM has a capacity of 25~ bytes, it can contain 4 (four) separate correction tables, namely TABLE-l including addresses UOi~
20 to 3FH, TABLE-2 including aadresses 4~ to 7FH, TABLE-3 including addresses 80H to BFH, and TABLE-4 including the acaresses CO~ to FFH.

Fig. 9 shows a practical example of the input-output characteristics of each of the conversion tables, i.e., the relationship between the input video signals and the converted output video signal. As will be seen from tnis Figure, the 64 (sixty-four) levels of the input video signal are converted into levels O to 255 (OOH to FFH) in accordance witn the respective conversion tables. The change-over between the conversion tables can be made by varying the signal applied to upper terminals A6 and A7 oE
the ROM 12 as shown in Fi9s. 3A and 3B. The descriDed embodiment is designed to allow this switching for each ... .
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In operation, the internal norizontal synchronizing signal ~BD-Pulse) is input to a counter 4~1 the output of which i~ delivere~ througn terminals Q~ and ~ to the terminals A6 and A7 of the ROM 12. The counter 401, in cooperation with an RCO inverter 402 and a switch 403 constitutes a ring counter, so tnat the period of ~ ~-ching of the conversion taDle can be varied in accordance witn the state of the switch 403. For instance, when the switch 403 has the state ~1~ (at terminal B), ~1~ (at terminal A), TABLE-4 is always selected, whereas, when the state of tne switch 403 is ~1~ (at terminal B), ~0~ (at terminal A), TABLE-4 and TABLE-3 are selected alternately. When the swltch 403 has the state of ~U~ (at terminal B), ~0 (at terminal A), TABLE-l, TABLE-2, TABLE-3, and TABLc-4 are successively selected for successive lines, as shown in Fig. 10a. Moreover, it is possible to improve the gradation by cnangin5 tne conversion table for successive lines.

In general, ln tile electropnotograpnic reproduction of an image, the gradation is more difficult to obtain in the light portion of the image than in the daek portion of the image. Therefore, as in the example shown in Fig. 9, the conversion taDles are substanti~lly duplicated in the dark portions of the image and differ in the light po~tion so as to provide optimal gradation.

In the preferred embodiment, the switching of the table can also be made in the direction of the main scan by the laser beam.

More specifically as shown in Figs. 3A and 3B, a signal can be formed by dividing the frequency of the screen clock (SCREEN-CL~) by 2 (two) by means of a J-K flip-flop circuit 404, inputting the resulting signal to one input .:
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::'-' ': : -: '' .` . : .' i :-' terminal of an exclusive OR circuit 406, the other input of whicn is connected to terminal QB of the counter 401 and the output terminal of which is then connected to ROM
12 tnrough a latch ll. With this arrangement, it is possible to change the conversion table in a staggered manner as shown in Fig. 10(b), thus attaining a further improvement in the gradation. A reference numeral 405 denotes a switcil for selecting either switching of the table in the staggered manner described above or not so switching. The staggered switching of the table is selected when this switch has the ~l~ level and is not selected when the switch has the ~0~ level. The numerals appearing in frames of the table snown in Fig. l0(b) represent the numDers of the selected conversion ta~les l to 4. Thus, the period of the screen clock in the em~odiment corresponds to ~he period of 3 ltnree) pixel clocks.

It will De understood from the description provided above that the scanning lines produced by the laser in accordance with data from the conversion tables of the ROM
12 are each generated as a succession of line segments.
The line segments of successive scanning lines collectively form a plurality of columns that aefine a line screen.

More particularly, when the video signal processed by the circuit shown in Figs. 3A and 3B is directly delivered to a reproducing means such as a laser beam printer, the reproduced image has a structure with vertical columns (in the described embodiment, the line screen is composed of vertical columns of line segments of successive scanning lines which form in the reproduced image) due to the fact that the phase of the triangular wave signal is the same as that of tne internal horizontal synci~ronizing signal (BD-Pulse) for each line. l'he circuit in the present . :
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embodiment is one in which the triangular wave is formed after the reference clocks are counted by 12 (twelve) from the rise of the BD-Pulse signal. The timing for the generation of triangular waves is the same for each line, and so each phase of the triangular waves on each line is the same. The image data is output from the digital data output device 1 as stated above. The digital data output device 1 outputs image data with a predetermined timing in synchronism with a signal equivalent to the BD-Pulse signal. More particularly, the data output device 1 is adapted to receive the BD signal. This device 1 starts to count tne reference cloc~ after receiving the BV signal, and begins transmission of the image data after counting tne rei~rence clocks up to a predetermined nur~ber. As a conseyuence, the timing of transfer of the image data necessary ror image reproduction is the same on each line, and a high quality reproduced image with no image jitter can be pro~uced. As the timlng of the generation of the triangular waves and the timing of transfer of the image data necessary for image reproduction have the same relation on all of the lines, the reproduced image has its vertical column structure with no image jitter, which is effective, for example, in reducing a particular Moire pattern. Again this vertical column structure comprises a line screen having a vertical columnar axis extending at an angle, that is perpendicular to the raster scanning lines.

It is also possible to obtain a reproduced image having a structure comprising oblique line screen columns, if the phase of the triangular wave signals is made to be offset slightly for successive lines. This is effective in reduciny the l~oire pattern wnich appears undesirably when an original dot image is read and processed. The angle of inclination of tne obli~ue columns can be determined as desired by suitably selecting the amount of shift of the . .
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phase of the screen clocks for successive lines. For instance, a reproduced image comprisiny scanning lines having oblique columns inclined at 45 degrees can be obtained by shifting tne triangular wave signal by an amount corresponding to one picture element, i.e., by phase snifting tne triangular wave signal 120 degrees for each of the successive columns. Fig. 11 shows a circuit for reproducillg an image comprising oblique columns. More specifically, a reproduced image comprising oblique columns can ~e oDtained by using tnis circuit in place or tne screen clock generating circuit 300 in the circuit sAown in Fig. -~.

Referring again to Fiy. 11, the internal horizontal synchronizing signal (BD-Pulse) is latcned by the pixel clocks (PIXEL-CLK) by means of D latches 356 and 3;7, so that three internal horizontal synchronizing signals (BD-Pulse) ilaving dirferent phases are produced. Then, one of these three internal horizontal synchronizing signais ~BD-~ulse) is selected for eacll line Dy operation of a counter 35a, inverters 359 and 360, and gate circuits 361 to 367. The selected signal is input as a LOAD signal to a counter 351, thereby changing the phase of the screen clocks for successive lines- The counter 351 is adapted to divide the frequency of the signal (36M-CLK) into 1/3, while tne J-K ~lip-flop circuit 354 furtner divide~ the frequency of the output from tne counter 351 into 1/2.
With tnis arrangement, it is possiDle to generate one screen clock for every three picture elements.

Fig. 12 shows timing of the screen clock generated by the circuit of Fig. 11 and the triangular wave signal for successive lines. These three triangular wave signals are generated in sequence of eacn set of each 3 lines.

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~Ihen the reference pattern signal is generated in syncilronism with a group of picture elements as in the case of the embodiment described, it is possible to shift the synchronizing signal used in tne generation of the pattern signal by an amount corresponding to one half of the reference pattern signal period for each successive set of scan lines equal to the widtn of the pattern signal. ~ucn a method allows the position of the center of growth of the pulse width to be shifted in each of successive lines, so that the output image can have an appearance resemDling that produced by half-tone dots arranged along oblique lines.

In the circuit shown in Figs. 3A and 3B, the RO.~I 12 is u~ed for the purpose of gamma conversion. This, nowever, is not tne only element suitable for this purpose and the R~M 12 may De replacea Dy an S-~All connectea to tAe DATA
BUS line of a computer. ~1ith such an arrangement, it is possi~le to rewrite the ga~na conversion ta~le as desired in accordance w-th, for example, a change in the kind of the original, tnus increasing the adaptaDility of the apparatus of tne invent-on.

Fig. 13 snow~ an exam~le of a circuit which is usable in place of the ROM 12 in the circuit shown in Figs. 3A anà
3B. This circuit llas, as will be seen from this Figure, an S-RAI~ 12a for gamma conversion, a decoder 30, a microcomputer 31 ror re~riting the gamma conversion tables, tri-state buffers 32 and 33, and a bi-directional tri-state buffer 34.

The mode changing switcnes 304, 403 and 405 in the circuit shown in Figs. 3A and 3B may be controlled by the microcomputer 31 so as to increase the flexiDility of the system as a whole.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Image processing apparatus comprising:
digital video signal generating means for generating a digital video signal having a characteristic;
characteristic converting means for converting the characteristic of said digital video signal generated by said digital video signal generating means and for producing a converted digital video signal therefrom, said characteristic converting means including changing means for periodically changing a plurality of factors for converting the characteristic of said digital video signal at a predetermined interval thereby to convert the characteristic of said digital video signal generated by said digital video signal generating means and to produce said converted digital video signal by using said plurality of factors; and image forming means for forming an image in accordance with said converted digital video signal, said image forming means forming said image by scanning lines on a recording medium in accordance with said converted digital video signal;
wherein said characteristic converting means uses the same ones of said plurality of factors for at least one scanning line scanned by said image forming means, and said changing means changes said plurality of factors after scanning of said at least one scanning line scanned by said scanning means.

2. Image processing apparatus according to claim 1, wherein said characteristic converting means includes a table for entering as an address the digital video signal generated from said video signal generating means, the converted digital video signal being produced from the table.

3. Image processing apparatus according to claim 2, wherever a plurality of tables are provided for entering as an address the digital video signal generated from the video signal generating means, and wherein said changing means changes the table for each line scanned by said image forming means.

4. Image processing apparatus according to claim 1, wherein said characteristic converting means comprises storage means containing digital information for providing at least one non-linear transformation of said digital video signal.

5. Image processing apparatus according to claim 4, wherein said storage means comprises a read only memory for storing a digital look-up table for gamma correction.

6. Image processing apparatus according to claim 3, wherein said image forming means includes digital-to-analog converting means for converting the converted digital video signal generated by said characteristic converting means into an analog video signal, pattern signal generating means for generating a pattern signal of predetermined period, pulse-width-modulated signal generating means for generating a pulse-width-modulated signal in accordance with said analog video signal and said pattern signal, and a raster scanning print engine for generating a series of successive scan lines with a beam in accordance with said pulse-width-modulated signal.

7. Image processing apparatus according to claim 6, wherein said image forming means includes means for generating a synchronizing signal for each line scanned on the recording medium, and said changing means changes the table in accordance with the synchronizing signal.

8. Image processing apparatus according to claim 7, wherein said synchronizing signal generating means includes detecting means for detecting a scanning position of the beam and generates the synchronizing signal on the basis of a detection output from said detecting means.

9. Image processing apparatus according to claim 7, wherein said pattern signal generating means generates the pattern signal of predetermined period in accordance with said synchronizing signal.

10. Image processing apparatus according to claim 6, wherein said digital video signal ranges between maximum and minimum values, and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having predetermined pulse width when said digital video signal has the minimum value.

11. Image processing apparatus according to claim 6, wherein said digital video signal ranges between maximum and minimum values and wherein said pulse-width-modulated signal generating means generates a pulse-width-modulated signal having predetermined pulse width when said digital video signal has the maximum value.

12. Image processing apparatus according to claim 7, wherein one period of said pattern corresponds to a plurality of pixels of the digital video signal.

13. Image processing apparatus according to claim 6, wherein said pattern signal generating means includes an integrator for entering a clock having a predetermined period and for generating a triangular wave signal as the pattern signal on the basis of the clock having a predetermined period.

14. Image processing apparatus according to claim 1, wherein said image forming means includes pulse-width-modulated signal generating means for generating a pulse-width-modulated signal in accordance with the converted video signal.