Image forming apparatus

Abstract: An apparatus for forming an electrostatic latent image in a form of dot for each of pixels on a photoreceptor, wherein each of pixels has an imaging unit area and the dot is formed in the imaging unit area. There are provided plural types of reference wave signals to provide plural different modulating image signals for each of pixels so that the dot can be exposed at one of different positions in the imaging unit area in accordance with position informaiton obtained by processing image signals of pixels neighboring a target pixel.

Description: The present invention relates to image forming apparatus for forming two-dimensional images composed of dots, and particularly for reproducing a halftone image. The dots are recorded by modulated signals in which density data are modulated by reference wave signals. EP-A-212990 and US-A-4782398 describe such apparatus. In this field of image forming apparatus using the electrophotographic method, a digital halftone image is reproduced in the following manner: an original image is read by a scanner to provide an image signal; and image density data in which the image signal is gradation-corrected, A/D converted, and shading-corrected, is modulated by a reference wave signal and thus a digital image is obtained. When an original image is read by the scanner, an edge portion of the image is read in a halftone density due to the aperture of a solid state image pick-up element installed in the scanner. When a latent image is formed on a photoreceptor with a pulse width modulation signal in which the image density data obtained from the image signal is modulated by a reference wave signal having a constant period, a recording pixel corresponding to the edge portion of the latent image is recorded by the method of pulse width modulation in a constant pitch, in the case where the image density is intermediate, and thereby dot breakage is generated, and as a result, the sharpness of the recorded image is lowered. This problem can arise in the above-mentioned EP-A-212990 and US-A-4782398. Referring to Fig. 27, the aforementioned will be explained as follows. Fig. 27(b) shows a block diagram in which a pulse width modulated signal is generated by comparing the reference wave signal having a constant period with the input image data signal. The modulated signal is sent to a laser driving circuit, and light-modulates a semiconductor laser 431 in Fig. 1, which will be explained later. A laser beam emitted from the semiconductor laser 431 is raster-scanned by a rotating polygonal mirror, and projected on the photoreceptor through a lens system so that a latent image composed of dots is formed on the photoreceptor. Fig. 27(a) is a time chart showing the operation of each block in Fig. 27(b), in which (1) is image data, (2) is a standard clock, (3) is an analog output of image data latched at the leading edge of the standard clock and D/A converted, (4) is a reference wave, (5) is an output from a comparator, and (6) shows latent image dots formed on the photoreceptor. This image data corresponds to an edge portion of the image in which a line or the like is read. In the image data, it can be seen that intermediate density of the edge exists on "b" and "d" portions in Fig. 27(b). However, these portions are recorded separately from the "c" portion. This causes a white density-lack portion in the edge portion of the recorded image, and thereby it causes the apparent resolution of the image to be lowered. On the other hand, even when an interpolated character or figure is formed from computer graphic (C.G.) data or front data, a similar problem is caused. That is, when the edge portion is interpolated smoothly with the intermediate density using the interpolated data, a recording pixel corresponding to the edge portion is recorded in pixels as average density, and thereby the resolution is lowered in the same way as the aforementioned. Conventionally, it is widely known that MTF correction for shaping the image can be conducted by a differential filter, a Laplacian filter, or the like in order to maintain the sharpness of the image. However, this emphasizes only the edge portion of the image, so that uniformity of the halftone image is relatively lowered. For the reasons mentioned above, intermediate density processing, which effectively operates on the edge portion of the image, is required. US-A-4868684 describes apparatus in which a triangular wave whose phase is shifted is used as a reference wave signal. However, there is a problem in which the adjoining pixels are affected in the aforementioned apparatus. Further, when intermediate density processing is conducted on each color in a color image forming apparatus, there occurs the problem in which color tone is varied, or characters become not sharp. In view of the foregoing problems, an object of the present invention is to provide an image forming apparatus in which sharpness and resolution of the image, which is formed from scanner data, C.G. data, font data, or the like, is improved. The above-mentioned US-A-4868684 describes image forming apparatus as set out in the preamble of Claim 1. The present invention is characterised as set out in the characterising portion of Claim 1. EP-A-220568 describes an image forming method and apparatus also utilizing a form of sub-pixel analysis. Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a perspective view showing an outline structure of an embodiment of image forming apparatus according to the present invention. Fig. 2 is a characteristic curve of a high γ photoreceptor used in the present embodiment. Fig. 3 is a sectional view showing an example of a specific structure of the high γ photoreceptor used in the present embodiment. Fig. 4 is a block diagram of an image processing circuit of another embodiment of image forming apparatus of the present invention. Fig. 5 is a circuit diagram showing an example of a triangular wave generation circuit shown in Fig. 4. Fig. 6 is a time chart showing a signal wave of each portion in the triangular wave generation circuit shown in Fig. 5. Fig. 7 is a block diagram showing an example of the RE (Resolution Enhancing) processing circuit in Fig. 4. Fig. 8 is a block diagram showing an example of the modulation circuit in Fig. 4. Fig. 9 is a view for explaining RE processing used for determination of phases of reference waves. Fig. 10 is a view showing an example in the case where an objective pixel for RE processing is divided into 3 x 3, and P = 0.5. Fig. 11 is a view showing an example in the case where the objective pixel is divided into 2 x 2. Fig. 12 is a view showing another example in the case where the objective pixel for RE processinq is divided into 2 x 2. Fig. 13 is a view showing a shape of the reference wave in the case where the image signal exists in the character region. Fig. 14 is a timing chart showing each signal of a modulation signal generating circuit of the example in Fig. 4 in the case where the image signal exists in the character region. Fig. 15 is a view showing a shape of the reference wave in the case where the image signal exists in a halftone region. Fig. 16 is a time chart showing a signal of each portion of the modulation signal generating circuit of the example in Fig. 4 in the case where the image signal exists in the halftone region. Fig. 17 is a block diagram and its output diagram showing another example of the triangular wave generating circuit. Fig. 18 is a view showing a semiconductor laser array of the example in Fig. 7. Fig. 19 is a view showing a scanning locus of laser spots using the semiconductor laser array in Fig. 15. Fig. 20 is a graph showing the relation between a driving current and an output of emission of the semiconductor laser. Fig. 21 is a graph showing an example in the case where the relation between the center of gravity in the primary scanning direction and a recording position of a small scanning line, is changed. Fig. 22 is a graph showing the case where average density of the small scanning line in the subsidiary direction is changed. Fig. 23 is a block diagram showing an image processing circuit of another embodiment of the present invention. Fig. 24 is a block diagram showing an RE processing circuit in Fig. 23. Fig. 25 is a block diagram showing an image processing circuit of an embodiment of the present invention. Fig. 26 is a block diagram showing an image processing circuit of another embodiment of the present invention. Fig. 27(b) is a block diagram in which a reference wave with a constant period is compared with the image data and then a pulse width modulation signal is generated, and Fig. 27(a) is a timing chart showing the operation of each block in Fig. 27(b). The structure of an image forming apparatus which is an embodiment of the present invention, will be described as follows. Fig. 1 is a perspective view showing the outline of the structure of the image forming apparatus of the present embodiment. In image forming apparatus 400, the photoreceptor is uniformly charged; after that, a dot-like electrostatic latent image is formed by a spot light which is pulse width modulated according to a modulation signal obtained by differentially amplifying an analog image density signal, which is obtained by D/A converting digital image density data obtained from a computer or a scanner, and a reference wave signal; a dot-like toner image is formed by reversal development of the latent image; charging, exposing, and developing are repeatedly conducted so that a color toner image can be formed on the photoreceptor; the color toner image is transferred onto a recording sheet; the recording sheet are separated from the photoreceptor; then the image is fixed so that the color image can be obtained; and gradation of the image is expressed by changing areas of dots by pulse width modulation. In the image signal which is made in the computer or read from the scanner, when an edge portion having high image density touches a reading pixel, signal density in the corresponding pixel is the same as intermediate density in a uniform image. In conventional pulse width modulation, the recording dots are formed in the manner that they are isolated at the center of the pixels in the case of recording at an edge portion or in a halftone region, and therefore only an expression in which resolution is rough, can be obtained. In the image forming apparatus of the present invention, recording position modulation is conducted, in the manner to be described, in which a position of an electrostatic latent image of recording dots is deviated in the primary scanning direction, and the recording dots are enlarged so that an image having improved resolution and gradation can be obtained. The image forming apparatus 400 is composed of: a photoreceptor which is a drum-like image forming body rotated in the arrowed direction (hereinafter, it will be called simply a photoreceptor) 401; a scorotron charger 402 by which the surface of the photoreceptor 401 is uniformly charged; an optical scanning system 430; developing units 441 to 444 in which yellow, magenta, cyan, and black toner are loaded; a pre-transfer charger 461; a scorotron transfer unit 462; a fixing roller 464; a separator 463; a cleaning unit 470; a discharger 474; and the like. In the optical system 430, a laser light emitted from the semiconductor laser 431 is made nearly parallel beam by the collimeter lens 432. The image is exposed by this laser beam in the following manner: the laser beam passes through a cylindrical lens 433; the laser beam is reflected by a rotational polygonal mirror 434 which is rotated at a constant speed; a minute laser spot is focused on the peripheral surface of the uniformly charged photoreceptor 401 by an f lens 435 and a cylindrical lens 436; and then the laser beam becomes a laser spot that scanning the surface and thus the image exposure can be conducted. In the aforementioned optical system, the f lens 435 is a correction lens by which a constant speed optical scanning is conducted, and the cylindrical lenses 433 and 436 are lenses by which deviation of the position of the spot is corrected when the rotational polygonal mirror 434 is tilted. The photoreceptor 401 used in the present embodiment has a high γ characteristic and Fig. 3 shows an example of its specific structure. The photoreceptor 401 is formed by a conductive support 401A, an intermediate layer 401B, and a photosensitive layer 401C, as shown in Fig. 3. Thickness of the photosensitive layer 401C is about 5 to 100 µm, and preferably 10 to 50 µm. The photoreceptor drum 401 is structured in the following manner: a drum-like conductive support 401A, which is made of aluminum and has a diameter of 150 mm; the intermediate layer 401B, which is made of ethylene-acetic acid vinyl copolymer and whose thickness is 0.1 µm, is formed on the conductive support 401A; and the photosensitive layer 401C, whose thickness is 35 µm, is provided on the intermediate layer 401B. As the conductive support 401A, a drum, which is made of aluminum, steel, copper, or the like, and has a diameter of about 150 mm, is used, however, a belt-like body in which a metal layer is laminated or vapor-deposited on a paper or a plastic film, or a metallic belt such as a nickel belt, which is made by the method of electroforming, may be used as the conductive support. The intermediate layer 401B is preferably provided with the following properties: it can resist high charging voltage of ± 500 V to ± 2000 V; for example, in the case of positive charging, injection of electrons from the conductive support 401A can be prevented; and holl mobility can be provided so that superior light decay characteristics due to an avalanche phenomenon can be obtained. For the aforementioned reasons, positive charging type electric charge conveyance material , for example, disclosed in Japanese Patent Application Publication JP-A-63-44662 which has been proposed by the inventors of the present invention, is preferably added by not more than 10 weight % to the intermediate layer 401B. For the intermediate layer 401B binder resin, the following resins, for example, which are commonly used for a photosensitive layer of electrophotography, can be used. (1) vinyl polymer such as polyvinyl alcohol (Poval), polyvinyl methyl ether, and polyvinyl ethyl ether, (2) nitrogen vinyl polymer such as polyvinylamine, poly-N-vinyl imidazole, polyvinyl pyridine (quarternary salt), polyvinyl pyrrolidone, and vinyl pyrrolidone-vinyl acetate copolymer, (3) polyether polymer such as polyethylene oxide, polyethylene glycol, and polypropylene glycol, (4) acrylic acid polymer such as polyacrylic acid and its salt, polyacrylamide, poly-β-hydroxy ethylacrylate, (5) methacrylate polymer such as polymethacrylate and its salt, polymethacrylamide, and polyhydroxy propyl methacrylate, (6) ether cellulose polymer such as methyl cellulose, ethyl cellulose, carboxy methyl cellulose, hydroxy ethyl cellulose, and hydroxy propyl methyl cellulose, (7) polyethylene imine polymer such as polyethylene imine, (8) polyamino acid such as polyalanine, polyserine, poly-L-glutamine acid, poly-(hydroxy ethyl)-L-glutamine, poly-δ-carboxy methyl-L-cysteine, polyproline, lysine-tyrosine copolymer, glutamic acid-lysine-alanine copolymer, silk fibroin, and casein, (9) starch and its derivatives sucn as starch acetate, hydroxyl ethyl starch, starch acetate, hydroxy ethyl starch, amine starch, and phosphate starch, (10) polymer which is soluble in mixed solvent of water and alcohol, such as soluble nylon, and methoxy methyl nylon (8 type nylon) which are polyamide. The electric charge conveyance material is not used for the photosensitive layer 401C basically, and the photosensitive layer 401C is formed by the following manner: phthalocyanine fine particles, which are made of photoconductive pigment and whose diameter is 0.1 to 1 µm, antioxidant and binder resin are mixed and dispersed in a solvent for the binder resin so that a coating liquid is prepared; the coating liquid is coated on the intermediate layer; and it is dried and thermally processed. When the photoconductive material is used with the electric charge conveyance material, the photosensitive layer is structured in the following manner: the photoconductive material which is composed of the photoconductive pigment and a small amount of the electric charge conveyance material whose weight % is not more than 1/5, and preferably 1/1000 to 1/110 (weight ratio) to the photoconductive pigment, and the antioxidant are dispersed into the binder resin. When the high γ photoreceptor is used, a sharp latent image can be formed even when the diameter of the laser beam is spread, and thereby recording can be effectively conducted with high resolution. In the present example, since color toner images are superimposed on the photoreceptor 401 surface, the photoreceptor, which has the spectral sensitivity on the infrared side, and a laser diode, which emits an infrared ray, are used so that a laser beam emitted from the optical scanning system 430 is not shaded by the color toner images. Next, light decay characteristics of the high γ photoreceptor used in the present example, will be explained as follows. Fig. 2 is a graph showing characteristics of the high γ photoreceptor. In the drawing, V1 (Volts) is a charging potential, V 0 (Volts) is an initial potential before exposure, L 1 is an amount of exposure (µJ/cm 2 ) by a laser beam which is necessary for that the initial potential V 0 is decayed to 4/5, and L2 is an amount of exposure (µJ/cm 2 ) by a laser beam which is necessary for that the initial potential V 0 is decayed to 1/5. A preferable range of L2/L1 is 1.0 < L 2 /L 1 ≦ 1.5 In the example, V 1 = 1000 (Volts), V 0 = 950 (Volts), L 2 /L 1 =1.2, and the photoreceptor surface potential at the exposed section is 10 Volts. When the light sensitivity at the position corresponding to a middle period of exposure at which the initial potential (V 0 ) is decayed to 1/2 in the light decay curve is defined as E1/2, and that at the position corresponding to an initial period of the exposure at which the initial potential (V 0 ) is decayed to 9/10 is defined as E9/10, a photoconductive semiconductor which gives the following relations is selected. (E1/2)/(E9/10) ≧ 2 and preferably, (E1/2)/(E9/10) ≧ 5 In the aforementioned, the light sensitivity is defined as the absolute value of the potential lowering amount to a minute amount of exposure. In the light decay curve of the photoreceptor 401, the absolute value of the differential factor of the potential characteristics, which means the light sensitivity, is small, as shown in Fig. 2, at the time of a small amount of exposing light, and it sharply increases when the amount of exposing light reaches an increased point. That is, the light decay curve shows the following characteristics: it shows almost horizontal light decay characteristics because sensitivity characteristics are not good for a small period of time at the initial period L1 of exposure, as shown in Fig. 2; and however, it shows super high γ characteristics which is lowered almost vertically because it has super high sensitivity, ranging from the middle period of exposure to the latter period thereof. It is considered that the photoreceptor 401 has a high γ characteristic, due to the avalanche phenomenon under the high charging voltage of, specifically, +500 to +2000 V. That is, it is considered that carriers generated on the surface of photoconductive pigment at the initial period of exposure are effectively trapped by an interface layer of the pigment and coated resin so that the light decay is positively prevented, and thereby extremely sudden avalanche phenomenon is generated after the middle period of exposure. Next, a color image forming apparatus according to the present invention will be explained as follows. In the color image forming apparatus, an objective matrix pixel of the image density data is composed of sub-pixels m × n (width × length), and a distribution of the density data of adjoining pixels including the objective pixel, is replaced by the distribution of sub-pixels m × n in one pixel, and the image is formed by the following method: a position in which dots of n rows are written is displaced when a phase of a reference wave in each row of sub-pixels is displaced according to image density data of sub-pixels obtained by distributing data of the objective pixel multiplied by constant P corresponding to the distribution. Displacement of the position in which dots are written will be referred to as recording position modulation, hereinafter. Further, processing to convert the image density data of the objective pixel into the image density data of sub-pixels obtained by dividing the objective pixel into m × n, will be referred to as resolution enhancing processing (RE processing), hereinafter. Due to RE processing, high density recording can be conducted. In this case, a high γ photoreceptor is specifically effective in order to form a latent image corresponding exactly to the reference wave. In this invention, RE processing is conducted when 1 ○ the image density data of the objective pixel is not less than a first threshold value, that is, not less than the specific density in which a recorded optical reflection density is 0.1, namely the first threshold value. In many portions of an area corresponding to a highlight portion, RE processing is not conducted on a background portion of a document, and small pixels m × n are set to have a uniform density. In the case of CRT, this data display can be possible. However, in the case of laser recording which will be described later, it is difficult to display the data uniformly, and therefore, the reference wave whose density center exists in the center of the image density, is selected. Due to the aforementioned, uniformity in the highlight portion can be kept, and a noisy image can be prevented from occurring. 2 ○ In the case of high density and a steep density gradient, when a reference wave whose density recording position does not exist in the center is selected, dots are formed in the manner that they overlap with the adjoining pixel. In order to prevent a density change and recording dot blocking between pixels, when the image density data of the objective pixel is not less than a specific second threshold value, and also in a high density portion in which a recorded optical reflection density is 0.5, a reference wave whose density center exists in the center of the image density, is selected. When either one of the conditions of threshold values 1 ○ and 2 ○ is used, it is effective for image formation. Since a uniform display can be conducted in the case of CRT, the densities of sub-pixels m × n are processed as a uniform density. That is, RE processing is not conducted. In a color image forming apparatus in which high density image recording is conducted according to density distribution data in the objective pixel which is determined corresponding to density data of the pixels adjoining the objective pixel, a color image forming apparatus which is characterized as follows is preferable: when a specific density data of the obiective pixel is not less than the first threshold value in the low density portion, recording position modulation is conducted according to the determined density distribution; or when a specific density data of the pixel is not more than the second threshold value in the high density portion, recording position modulation is conducted according to the determined density distribution. Fig. 9(a) is a plan view in which the adjoining pixels including the objective pixel m5 are expressed as m1 to m9 when the objective pixel is defined as m5, and the objective pixel m5 is divided into 3 × 3 sub-pixels. Fig. 9(b) is an enlarged view in which each sub-pixel is expressed by s1 to s9 when the objective pixel is divided into sub-pixels of 3 × 3. m1 to m9 and s1 to s9 also express the density of each portion. RE processing will be explained in detail as follows. Taking the example of the case where the objective pixel m5 is divided into 3 × 3 sub-pixels, density of a sub-pixel si is determined by the following equation. si = (9 × m5 × P × mi/A) +(1-P) × m5 Where i = 1, 2, ····, 9, and P is a constant, which is called strength for RE processing, and in which the range of 0.1 to 0.9 is used, and A is the sum total of m1 to m9. In the above equation, a term of (9 × m5 × P × mi/A) expresses a density in which the density of the objective pixel m5 multiplied by P is distributed to each sub-pixel according to density ratio of the adjoining pixels, and the term (1-P) × m5 expresses a density in which the residual density of the objective pixel m5 is distributed equally to each sub-pixel, so that an element of unsharpness is taken into the equation. Fig. 10 is an illustration showing an example in which the objective pixel m5 is divided into 3 × 3 sub-pixels, and P = 0.5. Fig. 10(a) is an illustration showing an example of the density distribution of the adjoining pixels including the objective pixel m5. Fig. 10(b) is an illustration showing the density distribution in the objective pixel m5 which is calculated by P = 0.5. Fig. 11 and Fig. 12 show an example in which the objective pixel m5 is divided into 2 × 2 sub-pixels. Fig. 11(a) is an illustration showing an example in which the objective pixel m5 is divided into 2 × 2 sub-pixels. Fig. 11(b) is an illustration showing an example of the adjoining pixels relating to sub-pixels s1 to s4 in the objective pixel. Density of s1, s2, s3, and s4 is calculated according to Equation 1. (Equation 1) s1 = 4m1+2(m2+m4)+m5 A × m5 × P+(1-P) × m5 s2 = 4m3+2(m2+m6)+m5 A × m5 × P+(1-P) × m5 s3 = 4m7+2(m4+m8)+m5 A × m5 × P+(1-P) × m5 s4 = 4m9+2(m6+m8)+m5 A × m5 × P+(1-P) × m5 where A is the total sum of m1 to m9. Fig. 12(a) is an illustration showing another example in which the objective pixel m5 is divided into 2 × 2 sub-pixels. Fig. 12(b) is an illustration showing another example of the adjoining pixels relating to sub-pixels s1 to s4 in the objective pixel. Density calculation of s1, s2, s3, and s4 is conducted according to Equation 2. (Equation 2) s1 = m1+m2+m4+m5 A × 9 4 × m5 × P+(1-P) × m5 s2 = m2+m3+m5+m6 A × 9 4 × m5 × P+(1-P) × m5 s3 = m4+m5+m7+m8 A × 9 4 × m5 × P+(1-P) × m5 s4 = m5+m6+m8+m9 A × 9 4 × m5 × P+(1-P) × m5 where A is the total sum of ml to m9. Fig. 4 is a block diagram showing an example of an image processing circuit which is used in a color image forming apparatus of the present invention (an example in which the objective pixel is divided into 3 x 3). Fig. 5 is a circuit diagram showing an example of a triangular wave generator in Fig. 4. Fig. 7 is a block diagram showing an RE processing circuit in Fig. 4, and Fig. 8 is a block diagram showing a modulation circuit in Fig. 4. Image processing circuit 1000 of the present embodiment is a circuit which structures a driving circuit of an optical scanning system, and is composed of an image data processing circuit 100, a modulation signal generator circuit 200 and a raster scanning circuit 300. The image data processing circuit 100 is a circuit which interpolates an edge portion of font data and outputs it, and composed of an input circuit 110 including a computer, a font data generator circuit 120, a font data memory circuit 130, and an interpolation data generator circuit 140, and sends a character code signal from the input circuit 110, a size code signal, a position code signal and a color code signal to a font data generator circuit 120. The font data generator circuit 120 selects an address signal from four kinds of input signals and sends it to the font data memory circuit 130. The font data memory circuit 130 sends an outline font data corresponding to one character which corresponds to an address signal to the font data generator circuit 120. The font data generator circuit 120 sends the outline font data to the interpolation data generator circuit 140. The interpolation data generator circuit 140 interpolates notches or jumping of the image density data, which is generated at the edge portion of the outline font data in conventional binary development, and sends it to an image density data memory circuit 210 which is composed of a frame memory in 8 bit image data by using intermediate density. Concerning the generated color, corresponding colors are converted into density data of yellow (Y), magenta (M), cyan (C),and black (B) corresponding to a color code. Before the data is sent to the image density data memory circuit 210, each density data may be sent again after data has been processed by masking, and UCR. Due to the aforementioned, a font is bit-map-developed in each frame memory under the condition that each color has the same shape and the ratio of density is different. The modulation signal generator 200 is structured by the image density data memory circuit 210, a read-out circuit 220, a latch circuit 230, an image discrimination circuit 231, an MTF correction circuit 232, a γ correction circuit 233, an RE processing circuit 240, triangular wave generators 250A to 250C which are means to generate reference waves, modulation circuits 260A to 260C, a standard clock generator 280, a frequency multiplied clock generator 281, a selector circuit 282, and the like. The image density data memory circuit 210 is an ordinary page memory (hereinafter, it will be referred to as a page memory), and a RAM (random access memory) which stores data by every one page, and has a capacity which can store multi-value image density data corresponding to at least one page (1 image frame). When it is accepted for a color printer, the printer is provided with a page memory which can store an image density signal corresponding to each color component of a plurality of colors, for example, yellow, magenta, cyan, and black. The read-out circuit 220 reads out continuous image density data of every one scanning line, which is continued in synchronization with the standard clock DCK 0 , from the image density data memory circuit (page memory) 210, and sends it to the RE processing circuit, the image discrimination circuit 231, and MTF correction circuit 232, wherein an index signal which determines the front timing of the raster scanning is used as a trigger. The latch circuit 230 is a circuit which latches the image density data while processing in RE processing circuit (which will be explained later) and triangular wave generators 250A to 250C are conducted. The standard clock generator 280 is a pulse generating circuit, and generates the standard clock DCK 0 which is a pulse signal with a cycle period the same as the pixel clock, and sends it to the read-out circuit 220, the frequency multiplied clock generator 281, and the selector circuit 282. The frequency multiplied clock generator 281 generates a frequency multiplied clock DCK 1 having a frequency which is two times as high as that of the pixel clock and sends it to the selector circuit 282, according to the reference clock DCK 0 . The selector circuit 282 selects one of the reference clock DCK 0 and the frequency multiplied clock DCK 1 , inputted from the frequency multiplied circuit 281, by a selection signal from the image discrimination circuit 231 which will be described later, and outputs it to the triangular wave generators 250A to 250C, the modulation circuits 260A to 260C, and the like. The triangular wave generators 250A to 250C are structured by the same circuit as each other as shown in Fig. 5, and with a saw-tooth-wave from the saw-tooth-wave generator 251, and position data of gravity center of each primary scanning line sent from the RE processing circuit 240, the generators 250A to 250C generate the reference waves which are composed of triangular waves having different top positions corresponding to their position data of gravity center, and send them to the modulation circuits 260A to 260C. Fig. 6 is a timing chart showing signal wave forms at each position of the triangular wave generators 250A to 250C shown in Fig. 5. Operations of the triangular wave generator 250 will be explained as follows according to the timing chart. In Fig. 6, (a) shows a wave form of the clock signal at point a, and (b) shows a detection signal in which a rising portion of the clock signal is detected by a rising edge detection circuit at point b. Tr2 is turned on for a short period of time by the detected signal, C1 is discharged once, and after that, charging is started through Tr1, and a saw-tooth-wave shown in (c) is generated at point c. VR1 shown in the drawing is a variable resistor to adjust a charging current, and VB1 is a D.C. variable power source to adjust a D.C. level of the generated saw-tooth-wave. (d) shows a wave form in which 8 bit position data of gravity center obtained from the RE processing circuit 240 is converted into an analog signal by a D/A converter installed in a circuit 251, and (e) is an output signal in which the analog signal and the saw-tooth-wave which is inputted from a saw-tooth-wave generator installed in the circuit 251, are compared by a comparator 252. The output signal is inputted into bases of Tr4 and Tr5 through a capacitor and turns them on. The analog signal of the position data of gravity center is inputted into a central point among Tr5, R5, R6, and Tr6 which are connected in series between power source Vcc and -Vcc after the D.C level of the analog signal has been adjusted by VB3. Due to the aforementioned, signal wave forms or point f and point g are shown respectively by (f), and (g). Levels shown by a1 and a2 in (f) determine a magnitude of a charging current of a capacitor C2, and levels shown by b1 and b2 in (g) determine the magnitude of a discharging current of the capacitor C2. Therefore, a triangular wave in which a top position, shown in (h) is determined corresponding to the position data of gravity center, is obtained in point h in Fig. 5. A triangular wave, which displaces its top position corresponding to the position of gravity enter of density, can be generated also by a circuit shown in Fig. 17(a). That is, a CPU 255 selects a rising clock pulse for reference wave generation from a table of a ROM 258 according to 8 bit position data of gravity center inputted from the RE processing circuit 240. Due to this, when a counter 257 counts the number of pulses inputted from an oscillator 256 and the number of pulses reaches a predetermined number (when voltage of the reference wave becomes equal to a predetermined voltage Vp), the counter 257 is counted down. Next, the CPU selects a falling clock pulse of the reference wave which is determined by the position data of gravity center from a table of the ROM258, and the counter is counted down by the clock. As a result, a wave form shown in Fig. 17(b) is obtained, and converted into an analog signal by a D/A converter 259. Due to the aforementioned, a triangular wave whose cycle frequency is the same as that of the pixel clock, and in which the top position corresponding to the position data of gravity center is displaced, can be obtained. The modulation circuits 260A to 260C are structured by the same circuit as each other as shown in Fig. 8, and composed of a D/A converter 261, a comparator 262, and an input section T of the triangular wave corresponding to the position data of gravity center. The modulation circuit D/A converts the image density data inputted through the latch circuit 230, by the D/A converter 261 in synchronization with the clock DCK 0 or DCK 1 , and the triangular waves inputted from the triangular wave generators 250A to 250C are compared as reference waves so that the pulse width modulation signal can be obtained. The image discrimination circuit 231 discriminates that the inputted image density data exists in a character region or a halftone region, and outputs a selection signal, which selects the standard clock DCK 0 with respect to the character region, or the frequency multiplied clock DCK 1 with respect to the halftone region, to the selector circuit 282. Further the circuit 231 sends a signal which makes an MTF correction circuit 232 and a y correction circuit 233 operate or not operate, to these circuits, as will be described later. As shown in Fig. 7, the RE processing circuit 240 is composed of a one line delay circuit 242, a one clock delay circuit 243, and an arithmetic processing circuit 241. The one line delay circuit operates as follows: the image density data of the first one scanning line of three scanning lines of the image density data in which one scanning line is sent at a time, is delayed for two line scanning times by the one line delay circuit 242: the image density data of the second scanning line is delayed for one line scanning time by the circuit 242; and the image density data of the last one scanning line is not delayed. Further, each image density data is delayed for the time of two standard clocks or one standard clock by the one clock delay circuit 243, and all image density data of the pixels, which include the objective pixel and adjoin the pixel, are sent at the same time to the arithmetic processing circuit 241. In Fig. 7, a standard clock DCK 0 input is neglected. In the arithmetic processing circuit 241, the image density data of the sub-pixel is obtained by the RE processing. At first, the density distribution in one pixel is obtained by the following method. The image density data of the obtained sub-pixel is divided into a sub-scanning line including s1, s2, s3···, the sub-scanning line including s4, s5, s6···, and the sub-scanning line including s7, s8, s9 ···, and one scanning line of the original pixel corresponds to these three scanning lines of the sub-pixels. The arithmetic processing circuit 241 calculates average density of each sub-scanning line and the position of gravity center of the density data in each sub-scanning line, outputs an analog signal of the average density data to laser drivers 301A to 301C from output terminals 04, 05, and 06, and outputs the position data of gravity center to triangular wave generators 250A to 250C from the output terminals OA to OC. That is, the arithmetic processing circuit 241 outputs the following position data of gravity center corresponding to the positions of gravity center of density from the output terminal OA to triangular wave generator 250A: the maximum digital signal when the gravity center of density of s1, s2, and s3 of the pixel m5 (the first sub-scanning line) is positioned at a left end of s1; a digital signal of an intermediate value when the gravity center of density is positioned at the center of s2; and the minimum digital signal when the gravity center of density is positioned at a right end of s3. In the same manner, from the output terminal OB, the position data of gravity center of the second sub-scanning line which is determined by the position of gravity center of density of s4, s5, and s6 of the pixel m5 (in this case, a central sub-scanning line) is outputted to the triangular wave generator 250B, and from the output terminal OC, the position data of gravity center of the third sub-scanning line which is determined by the position of gravity center of density of s7, s8, and s9 of the pixel m5, is outputted to the triangular wave generator 250C. Fig. 13 is an illustration showing an example of the relation between the triangular wave whose top positions are different and the objective pixel. The arithmetic processing circuit 241 controls an emission output of laser drivers 301A to 301C corresponding to the average density in the pixel m5 of each sub-scanning line. For example, the circuit 241 controls a semiconductor laser 301A to emit the beam in proportion to the average density of s1, s2 and s3. Fig. 20 is a graph showing an example of the relation between a driving current of the semiconductor laser and the output of laser emission. Next, the case where a density distribution in two pixels, which is necessary when the image exists in a halftone region, is found will be explained as follows. The density data of the obtained sub-pixel is divided into a sub-scanning line including s1, s2, s3 ···, the sub-scanning line including s4, s5, s6, and the sub-scanning line including s7, s8, and s9, in m5 and m6 in Fig. 9, and three sub-scanning lines of the sub-pixels correspond to one scanning line in every two pixels of the original pixel. When a signal which exists in the halftone region is inputted from the image discrimination circuit 231 into the arithmetic processing circuit 241, the position of gravity center of density including two pixels of each sub-scanning line and the average density in the original one pixel of each sub-scanning line are calculated in the circuit 241, and the circuit 241 outputs an analog signal of the average density data to laser drivers 301A to 301C from output terminals O4, 05, and 06, and the position data of gravity center to triangular wave generators 250A to 250C from the output terminals OA to OC. Namely, the arithmetic processing circuit 241 outputs the following position data of gravity center to the triangular wave generator 250A from the output terminal OA: the maximum digital signal when the gravity center of density of s1, s2, and s3 of the pixel m6 adjoining the pixel m5 (the first sub-scanning line) is located at a left end of s1 of m5; the digital signal with an intermediate value when the gravity center is located on a boundary point between s3 of m5 and s1 of m6; and the digital signal having the minimum value when the gravity center is located at a right end of s3 of m6. In the same manner, from the output terminal OB, the position data of gravity center of the second sub-scanning line which is determined by the position of gravity center of density of s4, s5, and s6 of the pixels m5 and m6 is outputted to the triangular wave generator 250B, and from the output terminal OC, the position data of gravity center of the third sub-scanning line which is determined by the position of gravity center of density of s7, s8, and s9 of the pixels m5 and m6, is outputted to the triangular wave generator 250C. Fig. 15 is an illustration showing an example of the relation between the triangular waves whose top positions are different, and the objective pixel. As described above, the arithmetic processing circuit 241 outputs an analog signal corresponding to the average density in pixels m5 and m6 of each sub-scanning line to laser drivers 301A to 301C, and controls their emission outputs. For example, the circuit 241 controls the semiconductor laser 301A to emit the laser beam in proportion to the average density of s1, s2, and s3 between each pixel. Fig. 20 is a graph showing an example of the relation between a driving current of a semiconductor laser and an output of the laser emission. The image discrimination circuit 231 compares the image data of the objective pixel with a predetermined first low threshold value, and with a predetermined second high threshold value. After that, when the image data of the objective pixel is discriminated to be out of the region of the first and second threshold values, the circuit 231 sends out, to the RE processing circuit 240, a signal which let the position data of gravity center in which the top position is positioned at the center with respect to all color components, send to triangular wave generators 250A to 250C, and does not operate the MTF correction circuit 232. Due to the aforementioned, the image density data which is read out from the read-out circuit 220 is not corrected by the MTF correction circuit 232, and after it has been corrected by the γ correction circuit 233, it is sent to modulation circuits 260A to 260C through the latch circuit 230. Due to the aforementioned, in the highlight and high density regions, MTF correction and position modulation are not conducted, and thereby an image which has high uniformity and no noise can be formed. The image discrimination circuit 231 further discriminates generally, under the aforementioned conditions, which of the character region and halftone region the image belongs to. The discrimination is conducted by the density change in 16 x 16 pixels including the objective pixel. When the density change of the region is large, the discrimination circuit discriminates that the objective pixel belongs to the character region, and when the density change of the region is small, the circuit 231 discriminates that the objective pixel belongs to the halftone region. Further, when the result of the discriminated region is different only in a minute region, for example, when the halftone region exists, being isolated, in the character region, the halftone region is discriminated as the character region. In the case of the halftone region, the image is discriminated in the same manner. When the character and line are discriminated to belong to the character region, the discrimination circuit 231 outputs a selection signal which outputs standard clock DCK 0 to the selector circuit 282 so that a period of the reference wave can be the same as that of an image clock, and does not operate the MTF correction circuit 232 and the γ correction circuit 233, and sends out the image density data, while it is not processed, through the latch circuit 230 to modulation circuits 260A to 260C. Due to the aforementioned, a sharp character or edge portion whose color is not changed, can be reproduced. Further, when the circuit 231 discriminates that the image belongs to the halftone region, the circuit 231 outputs a selection signal which outputs the frequency multiplied clock DCK 1 to the selector circuit 282 so that a period of the reference wave can be twice that of the image clock. Due to the aforementioned processing, a high gradation image can be formed in the halftone region, and also an effect by which a character image is given sharpness and neatness. A density data in which a specific color, for example, is converted into the density data of R + 2G + B, (in this case, R is a density data of red, G is that of green, and B is that of blue), is used as the image density data used for determination of the top position of the reference wave. For convenience sake, the density data of (R + 2G + B) will be expressed by N, hereinafter. When the top position of the reference wave is used in common with respect to each recording color, gradation of an image can be assured, and color change can be prevented. For determination of the top position of the reference wave, a G component which visually coincides with the image density data, or achromatic data having the G component is preferably used. For the same reason, the data which is used for the image discrimination circuit 231 is data which is common with each color. The modulation circuits 260A to 260C operate in the following manner: the signal of the image density data inputted through the latch circuit 230 by the triangular wave in which the top position is changed corresponding to the position of gravity center of density, is modulated; after that, the modulation signal which is pulse width modulated by the circuit, is generated; and the circuit sends out the modulation signal to the raster scanning circuit 300 after three sub-scanning lines (one scanning line in the case of the original image density data) in which these modulated signals are continued in parallel, are made to be one unit. Next, operation of the modulation signal generator 200 will be explained as follows. Figs. 14(a) to (d) are time charts showing a signal in each position of the modulation signal generator in the case where the signal is recording-position-modulated in the region which is discriminated as the character region. A period of the reference wave is the same as that of the original pixel. In Fig. 14, (a) shows a portion of the signal in which the image density data read out according to the standard clock DCK 0 from the page memory 210 making the index signal a trigger, is converted into an analog value by the D/A converter 261. The higher the level is, the lower is the density shown, and the lower the level is, the darker is the density shown. Fig. 14(b) shows the triangular wave which is the reference wave outputted successively from the triangular wave generators 250A to 250C. Fig. 14(c) shows the triangular wave (continuous line), and the image density signal (one dotted chain line) which is converted into the analog value, and shows the modulation operations in the modulation circuits 260A to 260C. Fig. 14(d) shows the pulse width modulation signal which is generated after the signal has been compared by the comparator 262, and shows that the recording position modulation, in which the recording position is shifted corresponding to the position of gravity center of density, has been conducted. Due to the result of generation of the modulation signal, the recording position modulation is not conducted in the case of the pixel in the low density portion, or the high density portion, and in an region in which density change is large, the recording position modulation by which the position of small dots of n lines in the objective pixel is moved to the position along the original character and the line direction of the line image, is conducted according to the density data of the original adjoining pixel, so that the character and the line image can be reproduced sharply. Figs. 16(a) to (d) are time charts showing the signal of each portion in the modulation signal generator in the case where the signal is recording-position-modulated in the region which is discriminated as the halftone region. As shown in Fig. 15, the period of the reference wave is twice the period of the original pixel. In Fig. 16, (a) shows a portion of the signal in which the image density data read out from the page memory 210 according to the standard clock DCK 0 by making the index signal a trigger, is converted into an analog value by the D/A converter 261. The higher the level is, the lower is the density shown, and the lower the level is, the darker is the density shown. The image data has the image density in which data of two pixels are averaged in the primary scanning direction. Fig. 16(b) shows the triangular wave which is the reference wave outputted successively from the triangular wave generators 250A to 250C. Fig. 16(c) shows the triangular wave (continuous line), and the image density signal (one dotted chain line) which is converted into the analog value, and shows the modulation operations in the modulation circuits 260A to 260C. Fig. 16(d) shows the pulse width modulation signal which is generated after the signal has been compared by the comparator 262. Due to the result of generation of the modulation signal, the recording position modulation is not conducted in the case of the pixel positioned in the low density portion, or the high density portion, and in a region in which density change is large, the recording position modulation by which the position of small dots of n lines in the objective pixel is moved to the position along the direction in which the density change is large, is conducted according to the density data of the original adjoining pixel, so that even halftones in photography or the like can be reproduced sharply. Further, when a phase of the reference wave is shifted in the subsidiary scanning direction successively, a dot corresponding to a halftone dot having a screen angle can be structured. For example, when the screen angle is 45° for a yellow component, 26.6° for a magenta component, - 26,6° for a cyan component, and 0° for a black component, uniformity of the color reproduction can be improved, and generation of Moire pattern can be eliminated. In particular, it is a merit in this invention that when the screen angle for the black component is 0°, the recording position modulation means can be used without any alteration. The raster scanning circuit 300 is provided with a δ delay circuit 311, a 2δ delay circuit 312, laser drivers 301A to 301C, an index detection circuit which is not shown in the drawings,and a polygonal mirror driver, and the like. Laser drivers 301A to 301C oscillate a semiconductor laser array 431 having a plurality of (in this case, three) laser emission sections 431A to 431C by the modulation signal from the modulation circuits 260A to 260C, and a signal corresponding to an amount of light beams from the semiconductor laser array 431, is fed back, and the laser drivers 301A to 301C drive the semiconductor laser array 431 so that the light amount can be kept constant. The index detection circuit detects the surface position of the rotating polygonal mirror 434 which is rotated at a predetermined speed by an index signal outputted from an index sensor 439 shown in Fig. 1, and conducts optical scanning by the image density signal modulated by the raster scanning method according to the period of the primary scanning direction. The scanning frequency is 2204.72 Hz, the effective printing width is not less than 297 mm, and the effective exposing width is not less than 306 mm. The polygonal mirror driver rotates a D.C motor at a predetermined speed uniformly, and a rotary polygonal mirror 434 at 16535.4 rpm. As shown in Fig. 18, the semiconductor laser array 431 is used in the manner that three emitting portions 431A to 431C are positioned in the array at same intervals. Since it is normally difficult that a space d between emitting portions is not more than 20 µm, a shaft which passes through the center of emitting portions 431A to 431C is mounted to be parallel with the rotating shaft of the rotary polygonal mirror 434, and to be inclined at a predetermined angle with respect to the primary scanning direction. Due to the aforementioned, laser spots sa, sb, sc of the laser beam by the semiconductor laser array 431 on the photoreceptor 401 can scan upwardly and downwardly in the manner that they are close to each other, as shown in Fig. 19. However, due to the aforementioned, positions of laser spots sa, sb, and sc in the scanning direction are deviated respectively with respect to the primary scanning direction. In order to correct the deviation, when the δ delay circuit 311 is inserted between the modulation circuit 260B and the laser driver 301B, and the 2δ delay circuit 312 is inserted between the modulation circuit 260C and the laser driver 301C so that an appropriate amount of time is delayed and the timing is adjusted, laser spots sa, sb, and sc emitted from the semiconductor laser array 431 are replaced by sa, sb' and sc' which are vertically arranged with respect to the primary scanning direction so that sa, sb' and sc' can be recorded. When the RE processing is conducted in the manner that the objective pixel is divided into 2 × 2 sub-pixels, the semiconductor laser array having two emitting portions is used. In the aforementioned embodiments of the present invention, as for the density information in each scanning direction, the average density in the primary scanning direction is regarded as the laser emitting output, and the data outputted from the read-out circuit 220 is used for the image data. However, the following structure can be adopted in which: the average density of each sub-scanning line obtained by the RE processing circuit 240, as shown in Fig. 23 and Fig. 24, is used for the density information; and the density data is inputted into the modulation circuits 260A to 260C by each reference wave so that laser drivers 301A to 301C are modulated. Next, the image forming process of the image forming apparatus 400 shown in Fig. 1, will be explained as follows. At first, the photoreceptor 401 is charged uniformly by the scorotron charger 402. An electrostatic latent image corresponding to yellow is formed on the drum-like photoreceptor 401 by the following method that: the laser beam modulated by yellow data (8 bit digital density data) read out from the image density data memory circuit 210 is focused through the cylindrical lens 433 and the rotating polygonal mirror 434, the f lens 435, the cylindrical lens 436, and the reflection mirror 437. The electrostatic latent image corresponding to yellow is developed by the first developing unit 441, and the extremely sharp dot-like first toner image (yellow toner image) is formed on the photoreceptor 401 surface. The first toner image is not transferred onto a recording sheet, passes under the cleaning device 470 which is withdrawn, and the scorotron charger 402 charges again the surface of the photoreceptor 401. Next, the electrostatic latent image is formed as the laser beam modulated by magenta data (8 bit digital density data) is focussed and scanned on the photoreceptor 401. The latent image is developed by the second developing unit 442 so that the second toner image (magenta toner image) is formed. In the same way as the above described, the electrostatic latent image by the cyan data is developed by the third developing unit 443 so that the third toner image (cyan toner image) is formed, and three color toner images, in which three images of different colors are superimposed successively, is formed on the photoreceptor 401. Lastly, the fourth toner image (black toner image) is formed, and four color toner image, in which four images of different colors are superimposed successively, is formed on the photoreceptor 401. According to the image forming apparatus 400 of the present embodiment, the photoreceptor 401 has excellent high γ characteristics, and even when the toner image is formed in the manner that a plural number of times of charging and exposing processes are conducted repeatedly so that toner images are superimposed, the latent image can be formed stably due to the excellent high γ characteristics. That is, even when the laser beam is focused on toner images according to the digital signal, a dot-like electrostatic latent image which has high sharpness and no fringe, is formed, and thereby a toner image having high sharpness can be obtained. These four color toner images are transferred onto a recording sheet fed from a sheet feed device by the transfer unit 462. The recording sheet on which the transferred toner images are carried, is separated from the photoreceptor 401 by a separator 463, conveyed to a fixing roller 464 by a guide and a conveyance belt, thermally fixed and delivered to a sheet delivery tray. In the present embodiment, as the result of experimenting in which values of factor P for RE processing were changed variously, a fine image could be obtained in the range of 0.1 to 0.9 of P. However, since sharpness of characters is insufficient when P is small, and an edge portion of a line image or a dot image is too emphasized when P is large, it was found that the preferable range of P is 0.3 to 0.7. Due to this, when an original document was written by line images or dots, the edge portion could be formed clearly, so that, even in the case of small characters, detail could be reproduced. Further, bad effects did not occur in the low density portion or high density portion. This is due to the reason that this method stops the recording position modulation with respect to these pixels, and makes the factor P = 0. Although, in this method, P can be used as a constant, it is preferable that P is changed in synchronization with the recording period, that is, corresponding to the image (in a character region or halftone region). When the P value is defined as P 1 in the case where the recording period is short, that is, the image is in the character region, and the P value is defined as P 2 in the case where the recording period is long, that is, the image is in the halftone region, the relation P 1 > P 2 is preferable. That is, when the image exists in the character region, the P value is set large, preferably 0.9 to 0.4, and when the image exists in the halftone region, the P value is set small, preferably 0.6 to 0.1. P = 0 corresponds to the value in the case where recording position modulation is not conducted. Further, the ratio of the RE processing can be changed. Fig. 21 is a graph showing an example of the case where the relation between the top position of the triangular wave corresponding to the recording position in the primary scanning direction and the gravity center is converted. Fig. 22 is a graph showing an example of the case where the average density in the subsidiary scanning direction is converted. The recording position can be changed in the manner that the position data of gravity center is converted after being multiplied by the factor according to a conversion equation which is set beforehand, for example, as shown in Fig. 21 using a ROM 245 in which the result obtained from the image density data by arithmetic processing is contained. In the same way, the average density in subsidiary scanning direction can also be converted as shown in Fig. 22. "0" in Fig. 21 shows the central position of colors. It is preferable to change the ratio of conversion shown in Fig. 21, and Fig. 22 corresponding to the case where the period of the reference wave is changed. When the period of the reference wave is large, it is preferable that gradation is seriously considered, and the ratio of the recording position modulation is decreased, or γ of gradation is decreased. Further, in the present embodiment, though the pulse width is formed by comparing the selected reference wave with the density signal, the following structure can be adopted: a plurality of pulse widths are formed previously by comparing a plurality of reference waves with the density signal, and the pulse width corresponding to the specific reference wave may be selected from the recording position information. Another embodiment of the selection circuit will be described below with reference to Figs. 25 and 26. In the described embodiments, a plurality of reference waves which have the same period and amplitude and different top positions are used. The advantages of the aforementioned are the following points: the pulse width formed by the comparison of the reference wave with the density information is the same; and the formed pulse width does not differ from other pulse widths. Due to this, there is an advantage in that the image can be formed corresponding to the density information. In the aforementioned flow of the image data , an example of a laser printer which outputs the data having been stored once in the page memory 210 is explained. However, the present invention is not limited to the specific embodiments, but when the image data processing circuit 100 is replaced by the image data processing circuit 150 composed of the color scanner 151, the A/D converter 152, the density converter 153, the masking UCR circuit 154 and the like, and the image density data is inputted into the processing circuit 150 from the scanner and the image is processed in the circuit, the present invention can be adopted to other image forming apparatus such as a copying apparatus and the like. Further, although the period of the reference wave is changed at each pixel corresponding to the result of image discrimination, the entire image frame can be changed uniformly by an external command such as a character, photographic mode and the like. The apparatus can incorporate a device, such as a light emitting panel EL, whereby the pulse width can be modulated in the subsidiary scanning direction. Further embodiments of the selection circuit will now be described with reference to Figures 25 and 26. In the embodiment shown in Fig. 25, an image processing circuit (1000) is a circuit by which a driving circuit of the optical scanning system 430 is structured, and composed of an image data processing circuit 100, a modulation signal generator 200, a raster scanning circuit 300, and an amplitude data generator 510. The amplitude data generator 510 generates amplitude data of 4 bits, for example, by which the amplitude of a triangular wave, which is a reference wave and will be described later, is determined, and sends the data of large amplitude to modulation circuits 260A to 260C when a sharpness button 512 to improve the sharpness, and a gradation button 513 to improve the gradation, which are provided on an operational panel not shown in the drawing, are pressed. Numeral 511 is a standard button by which the operation, is returned from the condition in which the sharpness button 512 or the gradation button 513 is pressed to the standard condition, and it is automatically returned to the standard condition when the image forming process is completed. At the standard condition, as the amplitude data a digital signal of 8, for example, is sent to modulation circuits 260A to 260C, and when the sharpness button 512 is pressed, as the amplitude data, a digital signal of 12, for example, is sent to two triangular wave generators 262B and 262C provided in the modulation circuits 260B and 260C as the amplitude data. When the gradation button 513 is pressed, an amplitude data of 12, for example, is sent to the triangular wave generators 262A, 262B And 262C. The amplitude data generator 510 is not limited to output the fixed amplitude data, but it may be structured in the manner that the amplitude data generator 510 can output the data which is sequentially varied, by using a slide resistor or the like. Further, it may be structured in the manner that it can send different amplitude data to the triangular wave generators 262A, 262B, and 262C respectively. The image data processing circuit 100 is a circuit which interpolates an edge portion of font data and outputs it, and is composed of: an input circuit 110 composed of a computer; a font data generator 120; a font data memory circuit 130; and an interpolation data generator 140. The image data processing circuit 100 sends a character code signal, a size code siqnal, a position code signal, and a color code signal to a font data generator 120. The font data generator 120 selects an address signal from four kinds of input signals and sends it to the font data memory circuit 130. The font data memory circuit 130 sends font data corresponding to one character, which corresponds to an address signal, to the font data generator 120. The font data generator 120 sends the font data to the interpolation data generator 140. The interpolation data generator 140 interpolates notches of the image density data generated in the edge portion of the font data with intermediate density, and sends it to an image density data memory circuit 210. Concerning generated color, a corresponding color is converted into density data of yellow(Y), magenta(M), cyan(C), black(BK) respectively in accordance with the color code. Due to the aforementioned, font data is bit-map-developed in each frame memory under the condition that each color has the same configuration and a density ratio of each color is different. The modulation signal generator 200 functions as follows: it reads out the image density data obtained from one scanning line, out of the image density data memory circuit 210, the image density data to be recorded from the continuous image density data obtained from one scanning line and its surrounding image density data are calculated in an arithmetic circuit 240; and according to the result, the first image signal which is obtained from that an output from the modulation circuit 260A is delayed in a delay circuit B271 by the first triangular wave, the second image signal and the third image signal which are obtained from that an output from the modulation circuit 260B with the second triangular wave and an output from the modulation circuit 260C with the third triangular wave are passed through a replacing circuit 270 alternately every half period, are switched over in a selector circuit 250, and the continuous modulation signal in one scanning line unit is sent to the raster scanning circuit 300 which has a driving means for a semiconductor laser 431. In the embodiment shown in Fig. 25, a specific pulse signal is selected after a pulse width modulated signal corresponding to the density has been obtained with respect to each reference wave. However, the following structure may be adopted: after the specific reference wave has been selected previously as shown in Fig. 26, a pulse width modulated signal is obtained by comparing an image density signal with the selected reference wave. As explained above, the objective pixel is divided into sub-pixels with respect to the objective pixel included in the specific density corresponding to the density data of the objective pixel; concerning density of each sub-pixel, a means to generate the reference wave in which the top position of the triangular wave is deviated corresponding to the position of gravity center of density of the image data, on which the RE processing to distribute the density of the objective pixel corresponding to the distribution of the density data of the adjoining pixel including the objective pixel is conducted, is provided; and the recording position modulation signal, in which the density signal of the objective pixel is modulated by the reference wave generated by the means, is generated. Due to the aforementioned, since the reference wave does not overlap the other pixel, the other pixel is not affected, so that the recording image of high image quality can be obtained. Further, the area of the reference wave is not changed, so that the pulse width corresponding to the image density data can be formed without depending on the reference wave. Further, since color image recording is conducted by the following methods: the recording position modulation is not conducted with respect to the low density portion and high density portion; the image discrimination is conducted by the image discrimination circuit; when the image exists in the character region, the recording position modulation is conducted by the reference wave of a short period; and when the image exists in the halftone region, the recording position modulation is conducted by the reference wave of a long period, an excellent color image forming apparatus, in which sharpness can be increased without causing change in color tone of the color image which is generated from the scanner, CG, or font data, can be provided. The aforementioned apparatus shows the case where the number of recording beams for the pixel is three. Further, the following apparatus can be adopted: the pixel can be scanned by one or two recording beams; the recording position modulation can be conducted only in the primary scanning direction, or only in the subsidiary scanning direction; or the recording position modulation can be conducted on an EL head. When a high y photoreceptor is used for the color image forming apparatus, effects can be further improved.

Claims: Apparatus for forming, from an input image signal, a two-dimensional image composed of dots in respective pixels of plural image lines of pixels, said apparatus comprising: a photoreceptor (401); wave generating means (250A-250C) for generating a reference wave signal; comparing means (260A-261C,262) for comparing an input image signal representing a desired density level for each target pixel with the reference wave signal to generate a modulating image signal; and beam generating means (301A-301C) for modulating a light beam on the basis of the modulating image signal so as to form a latent image composed of dots each in an image unit area of the respective target pixel on the photoreceptor (401); characterised by: processing means (240) for processing the input image signals representing desired density levels of neighbouring pixels (m1-m4,m6-m9) surrounding the target pixel (m5) so that a density distribution of the neighbouring pixels (m1-m4,m6-m9) around the target pixel (m5) is obtained, wherein the neighbouring pixels (m1-m4,m6-m9) comprise pixels on three image lines consisting of a current image line including the target pixel, a preceding image line preceding the current image line and a succeeding image line succeeding the current image line, and wherein the imaging unit area of each pixel is divided into at least three dot forming positions being a centre portion (s2,s5,s8), a right-hand portion (s3,s6,s9) and a left-hand portion (s1,s4,s7); said processing means (240) determining a dot forming position of the target pixel (m5) on the basis of the density distribution of the neighbouring pixels (m1-m4,m6-m9), and outputting a position signal indicating the determined dot forming position; wherein the wave generating means (250A-250C) generates at least three types of reference wave signals for each pixel, the three types of reference wave signals differing in wave form in accordance with the three dot forming positions; the wave generating means (250A-250C) outputting one of the three types of said reference wave signals corresponding to the determined dot forming position of the target pixel (m5) in response to the position signal outputted from the processing means (240); and said comparing means (260A-260C,262) comparing the image signal representing the desired density level of the target pixel (m5) with said one of the three types of reference wave signals and generating the modulating image signal for the target pixel (m5), whereby the dot formed by the modulated light beam in the target pixel (m5) is formed at said determined dot forming position in the imaging unit area of that said target pixel on the basis of the density distribution of at least four neighbouring pixels (m1-m4,m6-m9) around the target pixel (m5). The apparatus of claim 1, further characterized by selection means (250) for selecting one of the three types of reference signals in response to the position signal and outputting the selected type of reference wave signals. The apparatus of claim 1, characterized in that the wave generating means (250A-250C) outputs the three types of reference wave signals and the comparing means (260A-260C, 262) compares the image signal of the target pixel (m5) with each of the three types of reference wave signals so that at least three types of modulating image signals are provided for each pixel, and wherein the apparatus further comprises selection means (250) for selecting one of the three types of the modulating image signals in response to the position signal and outputs the selected modulating image signals corresponding to the determined dot forming position. The apparatus of claim 1, characterized by clock means (280) to generate a reference clock having a basic cycle period wherein a first one of the reference wave signals has the same cycle period as the basic cycle period, a second one has a cycle period twice the basic cycle period, and a third one has the same cycle period as that of the second one and differs in phase by 180° from the second one. The apparatus of claim 1, characterized in that the neighbouring pixels (m1-m4, m6-m9) surround the target pixel (m5) in the form of a matrix and the target pixel (m5) is divided into plural sub-pixels (s1-s9) in the form of the matrix, and wherein a sub-density distribution of the sub-pixels (s1-s9) in the target pixel (m5) is obtained on the basis of the density distribution of the neighbouring pixels (m1-m4, m6-m9) and the dot forming position in the imaging unit area of the target pixel (m5) is determined on the basis of the sub-density distribution. The apparatus of claim 5, characterized in that the matrix is formed in n rows and m columns and the dot forming position is determined for each row of the sub-pixels (s1-s9) in accordance with the sub-density distribution of each row of the sub-pixels (s1-s9). The apparatus of claim 6, characterized in that the processing means (240) applies a reference triangular wave signal for each of the rows of the sub-pixels (s1-s9), and the wave generation means (250A-250C) changes the position of the apex of the reference triangular wave signal in accordance with the density distribution in each of the rows of the sub-pixels (s1-s9). The apparatus of claim 7, characterized in that the processing means (240) obtains a centre of gravity in the density distribution and the wave generation means (250A-250C) changes the position of the apex of the reference triangular wave signal in accordance with position of the centre of gravity.

Inventors: HANEDA SATOSHI, HASEBE TAKASHI, ITIHARA YOSHIYUKI, KOIZUMI NOBORU, NIITSUMA TETSUYA, HANEDA, SATOSHI, HASEBE, TAKASHI, ITIHARA, YOSHIYUKI, KOIZUMI, NOBORU, NIITSUMA, TETSUYA
Applicants: KONISHIROKU PHOTO IND, KONICA CORPORATION
Application year: 1992
Main classification: H04N1
IPC codes: H04N1