Abstract: When a user changes a density adjusting value (developing contrast), and even variations and usage state of the process characteristics (photosensitive body and developing) of the cartridge change, the amount of recording agent consumed can be suppressed low, and a sharp line image can be recorded. An image forming apparatus of this invention includes an edge detecting portion for detecting an edge of image data, an image data correcting portion for correcting image data inside the edge so as to decrease its density, a laser driving circuit for forming an electrostatic latent image on a photosensitive body, and a correction value determining table for acquiring an instruction value for giving an instruction to correct a density contrast, wherein a correction condition for image data changes in accordance with an instruction value.

Abstract: An image forming and apparatus is provided in which a halftone density of an image is stabilized irrespective of a timing at which writing on scanning lines is started, even when using multiple laser beams. First, it is determined whether or not a portion exposed by adjacent laser beams (i.e., exposed by both of two beams simultaneously) is present. If the result of the determination is affirmative, it is then determined whether or not a portion adjacent to the exposed portion and not exposed by a laser beam is present. If the result of this determination is affirmative, exposure by fine dots is added to the portion not exposed by a laser beam.

Abstract: An image forming apparatus which includes an image bearing body and a light beam scanner for exposing the image bearing body, which is charged, based on an image signal and forming an electrostatic latent image on the image bearing body. A rise time or a fall time of a light beam emitted from the light beam scanner is controlled when a pixel density of the electrostatic latent image is switched.

Abstract: A copier or a printer 10 includes a controller 16/30 with a rendering algorithm that groups sets of adjacent pixels into sets of adjacent cells where each cell corresponds to a halftone dot of an image. The algorithm operates in conjunction with a gray scale printhead 22. In sending data to the printhead, the controller 16 parses a scanned image and set the exposure for each pixel in accordance with a growth and density program. The algorithm selectively grows halftone dots from zero size to a desired size equal to or less than a maximum size. It repeats this step for the rest of the pixels until the cell is at its desired size and at an initial density. If a higher density is desired, the algorithm selectively adjusts the fully grown cells by sequentially increasing the level of exposure of each pixel in the cell. The image is developed with developer including a hard magnetic carrier and toner.

Abstract: A digital photolithography system is provided that is capable of making smooth diagonal components. The system includes a computer for providing a first digital pattern to a digital pixel panel, such as a deformable mirror device (DMD). The DMD is capable of providing a first plurality of pixel elements for exposure onto a plurality of wafer sites. After exposure, the wafer can be scanned a distance less than the site length. The DMD then receives a second digital pattern for exposing a second plurality of pixel elements onto the plurality of sites of the subject. The exposed second plurality of pixel elements overlaps the exposed first plurality of pixel elements. This overlapping allows incremental changes to be made in the image being exposed, thereby accommodating the creation of diagonal components.

Abstract: A method for setting the amount of light in an array of light-emitting thyristors, each thyristor having I-L characteristic in which a luminous efficiency is decreased in a lower current field, is provided. According to the method, the amount of light emitted from a light-emitting thyristor is set so that a predetermined exposure energy may be obtained without decreasing a luminous efficiency of a light-emitting thyristor. The density D of a current to be supplied to the light-emitting thyristor to obtain a predetermined exposure energy is selected so as to satisfy the range of 3×Dth<D<100 MA/m2, wherein Dth is a threshold current density for light emission which is defined as a current density corresponding to the value of a current at a point where a tangent drawn at the value of a current corresponding to a current density of 50 MA/m2 with respect to the curve of the I-L characteristic intersects a current axis.

Abstract: When a user changes a density adjusting value (developing contrast), and even when variations in the usage state of process characteristics (of the photosensitive body and the developing device) of the cartridge change, the amount of recording agent consumed can be suppressed to be low, and a sharp line image can be recorded. An image forming apparatus of this invention includes an edge detecting portion for detecting an edge of image data, an image data correcting portion for correcting image data inside the edge so as to decrease its density, a laser driving circuit for forming an electrostatic latent image on a photosensitive body, and a correction value determining table for acquiring an instruction value for giving an instruction to correct a density contrast, wherein a correction condition for image data changes in accordance with an instruction value.

Abstract: An image forming apparatus of the present invention includes a developing device including two developing rollers arranged in parallel in a direction in which an image carrier moves each for forming a magnet brush thereon. The magnetic poles of the developing rollers for effecting hand-over of a developer between the rollers are provided with the same polarity. An optical writing device optically writes an image on the image carrier in accordance with tonality data of four levels or less for a single dot. The developing device prevents the developer from moving through a gap between the developing rollers and bringing about irregular image density and background contamination. In addition, the optical writing device successfully obviates ghosts, which are likely to appear when a uniform halftone image including solid portions or bold characters is reproduced.

Abstract: A method and arrangement for embedding a watermark in an image are disclosed. The watermark consists of a pseudo-random, dense subset of image pixels, e.g. a pattern of lines (20). A number of salient image pixels (21-26), for example, local extremes, corners or edges, is identified and it is determined whether they lie on (i.e. within a vicinity &dgr; of) the line pattern (21-23) or not (24-26). In an unwatermarked image (FIG. 2A), the number of most salient pixels (21) lying on the watermark is substantially the same as the number of most salient pixels (25,26) not lying on the watermark. The image is watermarked (FIG. 2B) by modifying the saliency of the salient pixels in such a way that a significant majority (21,23) of the most salient pixels (21,23,25) is eventually located within the vicinity of the line pattern.

Abstract: An image forming apparatus which writes an electrostatic image onto a photoconductor by laser light. An image writing resolution is set and a controller controls writing laser power according to the set image writing resolution. The controller also controls the writing laser power for a high resolution mode to be greater than the writing laser power for a low resolution mode.

Abstract: An image forming apparatus for outputting an image through an electrophotographic process in accordance with a digital image signal includes an electrophotographic photosensitive member; an optical system for applying light information to the photosensitive member; a developing device for forming a toner image on the photosensitive member; an input path for a plurality of digital image signals containing a bi-level digital image signal and a multi-level digital image signal; a controller for changing developing condition of the developing device when the image is outputted in accordance with the bi-level digital image signal, and changing an image forming condition which is different from the developing condition when an image is outputted in accordance with a multi-level digital image signal.

Abstract: In a digital electrophotographic imaging process in which a latent electrostatic image is written to a charged photoreceptor using an exposure beam diameter Ds larger than pixel pitch Dp and the image is visualized by development, an unsaturated region of photo-induced discharge characteristics of the photoreceptor is used for an image portion of isolated 1 line, and a saturated region of the photo-induced discharge characteristics of the photoreceptor or gamma characteristics of development is used for an image portion for n lines (n≧2).

Abstract: The method reproduces on an output device an electronic image having color pixels, each color pixel being represented by K spatially corresponding component pixels, K>1, each component pixel having an input pixel value, the output device being capable of rendering Nj intensity levels for a component pixel j, 1≦j≦K, thereby defining N1*N2* . . .

Abstract: The CPU counts the number of printings made with use of a photosensitive member so as to monitor the usage of the photosensitive member. When executing printing, the CPU determines a modulating rate, which is based on the reducing rate and some correction of the rate in accordance with the usage of the photosensitive member, with reference to a table. The CPU sets the determined modulating rate to the pulse modulating section. The pulse modulating section executes pulse modulation at the set modulating rate, and thus generates video data used for the printing operation of the print processing section.

Abstract: Image data is read in through a scanner unit, and, when a toner saving mode is set, the image data having a pixel value larger than a predetermined threshold is converted into a predetermined PWM pattern, while, for the image data having a pixel value not larger than the predetermined threshold, no output is made.

Abstract: An optical scanning device for optically scanning a surface of a photoconductor to form an electrostatic latent image thereupon. The scanning device includes a light source to emit a light flux and pulse modulate the light flux. A deflector deflects the light flux from the light source. A scanning image formation element condenses the light flux deflected by the deflector to form a scanning beam spot on the scanned surface. The scanning beam spot scans the scanned surface to form the electrostatic latent image on the scanned surface. A stationary beam spot, formed by stationary light flux on the scanned surface, has a stationary beam spot diameter &ohgr;m in a main scanning direction that is smaller than a stationary beam spot diameter &ohgr;s in a sub-scanning direction.

Abstract: An electrophotographic machine has at least two printing modes, with each printing mode having a respective printing density. A method of calibrating the electrophotographic machine includes depositing at least one toner patch on an image-bearing surface. The depositing is performed in a first of the printing modes. Light is emitted onto the at least one toner patch. An amount of light that is reflected off of the at least one toner patch is measured. At least one first electrophotographic condition for the first printing mode is adjusted dependent upon the measuring step. At least one second electrophotographic condition is adjusted for a second of the printing modes. The adjusting of the at least one second electrophotographic condition is dependent upon the measuring step.

Abstract: The present invention relates to an image forming apparatus in which an electrostatic latent image is formed on an electrophotographic photosensitive member with digital light, and electrostatically developed with a developer to form a developed image. The apparatus has an exposure device for normally increasing the reflection density only in a specified region on the photosensitive member in a white image portion where no image is basically formed, in the horizontal scanning direction of the digital light. For example, a small amount of developer is supplied to either end of the photosensitive member having the tendency that the amount of the developer supplied is small, thereby preventing the occurrence of image flow at either end.

Abstract: Image forming apparatus and method for identifying the type of transfer material and controlling irradiation of a laser beam for latent image formation based on the identified type of transfer material. A latent image is formed by modulating a pulse number, pulsewidth, or amplitude of an outputted laser beam to achieve scan-line density of 250 Lines/inch or more. By this, it is possible to suppress occurrence of an optical interference caused by the optical system and prevent image quality deterioration due to the optical interference.

Abstract: An exposure apparatus for image formation uses power control means provided to a laser driver for controlling exposure energy and accordingly forming an image. Specifically, exposure energy is varied depending on an isolated-dot pattern and an isolated-line pattern by changing the pulse height (drive power) of a drive pulse for each dot. Both of the isolated-line and isolated-dot patterns of a high resolution can thus be reproduced with a high image quality without increase of cost due to requirements for accuracy in processing and positioning of a lens and without decrease of life of a photoreceptor due to a decreased thickness of its photosensitive layer.

Abstract: When a user changes a density adjusting value (developing contrast), and even variations and usage state of the process characteristics (photosensitive body and developing) of the cartridge change, the amount of recording agent consumed can be suppressed low, and a sharp line image can be recorded. An image forming apparatus of this invention includes an edge detecting portion for detecting an edge of image data, an image data correcting portion for correcting image data inside the edge so as to decrease its density, a laser driving circuit for forming an electrostatic latent image on a photosensitive body, and a correction value determining table for acquiring an instruction value for giving an instruction to correct a density contrast, wherein a correction condition for image data changes in accordance with an instruction value.

Abstract: A control unit of an image forming apparatus for controlling exposure energy includes a reference image storage unit storing a plurality of reference images, an image recognition unit comparing image information consisting of a plurality of pieces of pixel information with data stored in the reference image storage unit, an exposure energy density storage unit storing an exposure energy density for each reference image stored in the reference image storage unit, an exposure energy density determination unit based on data from the image recognition unit and data in the exposure energy density storage unit an exposure energy density to be applied to a dot position corresponding to each piece of the pixel information, and a laser drive driving a laser of an exposure unit based on data from the exposure energy density determination uni. Accordingly, the image forming apparatus can be provided which exhibits an excellent image reproducibility for images with various dot densities.

Abstract: In a digital electrophotographic imaging process in which a latent electrostatic image is written to a charged photoreceptor using an exposure beam diameter Ds larger than pixel pitch Dp and the image is visualized by development, an unsaturated region of photo-induced discharge characteristics of the photoreceptor is used for an image portion of isolated 1 line, and a saturated region of the photo-induced discharge characteristics of the photoreceptor or gamma characteristics of development is used for an image portion for n lines (n≧2).

Abstract: A dot-matrix image-forming device such as an electrophotographic printer alters the size of a dot according to the surrounding dot pattern, thereby compensating for the tendency of isolated dots or small groups of dots to be undersized due to characteristics of the image formation process. Alternatively, the device may add dots to or delete dots from a contiguous group of dots to achieve substantially the same effect. Preferably, the device both adds or deletes dots and alters the dot size. This compensation scheme enables the total area occupied by dots in a given image region to be proportional to the number of dots in the region, regardless of the dot density and arrangement in the region.

Abstract: A method for suppressing phantom images in a digital copying or printing process wherein toner material in the form of pixels of image information (D (x,y), in which D is the image density for point x,y, is transferred through a sequence of process steps from a photoconductive surface to an image-recording medium, where at least one process step is influenced in a predetermined manner by at least one preceding process step for the same surface point x,y, which comprises connecting the image information for each pixel of image information to be printed on the basis of image information D(x,y-&Dgr;y1) for at least the pixel associated with the same surface point x,y, and said preceding process step, thereby compensating for the influence of said preceding process step.

Abstract: A device for spotwise imaging printing surfaces includes a laser light source producing at least one laser beam movable relative to a printing surface, the laser beam defining an image spot on the printing surface, the laser beam having a laser power. A laser control varies the laser power or an exposure time as a function of a distance of the laser light source from the image spot.

Abstract: An object of the present invention is to provide an image forming apparatus that has an image bearing body, exposing device for exposing the image bearing body, which is charged, based on an image signal and forming an electrostatic latent image on the image bearing body, wherein a pixel density of the electrostatic latent image can be switched, and control device for controlling a responsibility to the image signal of the exposing device when the pixel density is switched.

Abstract: An image forming apparatus is provided for producing an array of dots in the primary scanning direction based on image data prepared for one line. The apparatus includes enlarging means for enlarging the image data n times in the primary scanning direction to form images with a resolution equal to (1/n) times a maximum resolution, where the n is a positive integer. The image forming apparatus also includes dot forming means for forming dots for n lines in the secondary scanning direction based on the enlarged image data.

Abstract: Image data is read in through a scanner unit, and, when a toner saving mode is set, the image data having a pixel value larger than a predetermined threshold is converted into a predetermined PWM pattern, while, for the image data having a pixel value not larger than the predetermined threshold, no output is made.

Abstract: The CPU counts the number of printings made with use of a photosensitive member so as to monitor the usage of the photosensitive member. When executing printing, the CPU determines a modulating rate, which is based on the reducing rate and some correction of the rate in accordance with the usage of the photosensitive member, with reference to a table. The CPU sets the determined modulating rate to the pulse modulating section. The pulse modulating section executes pulse modulation at the set modulating rate, and thus generates video data used for the printing operation of the print processing section.

Abstract: An image forming apparatus includes an image bearing member, and an image processing device whic processes image data of an image including pixels forming the image such that a gradation of the image is represented by a density of toner adhering pixels of the image forming pixels per unit area of the image. The image forming apparatus further includes a latent image forming device which forms an electrostatic latent image of the image including the image forming pixels on the image bearing member in accordance with the image data processed by the image processing device, a developing device which visualizes the electrostatic latent image of the image on the image bearing member to a toner image, and a transfer device which transfers the toner image on the image bearing member to a transfer member.

Abstract: A multi-beam image recording apparatus changes a diameter of a laser beam using a beam diameter changing board, an order in which the line data is to be read from the memory unit, a rotational speed of a polygon mirror, and a clock frequency employed when the image data is read in units of pixels, in accordance with the specified resolution. Accordingly, the multi-beam image recording apparatus can switch between the resolutions without changing a distance between the centers of two laser beams emitted from an optical system. The resolution switching can be achieved with high accuracy at low costs.

Abstract: An electrophotographic apparatus includes a photosensitive body and electrostatic image forming device for forming an electrostatic image on the photosensitive body. The electrostatic image forming device includes an exposing device for exposing the photosensitive body by a digital light in accordance with image information. The exposing device exposes a portion that is a background for an image. A developing device develops the electrostatic image using a developer, wherein a rate throughout the developer of the contained particles have a diameter equal to or smaller than 1 &mgr;m is 5 to 40 number %, and wherein, when A denotes a one-pixel width and Wv denotes a width at half value of a peak in a potential distribution of the electrostatic image formed by exposing the photosensitive drum using the digital light of one pixel, 0.6≦Wv/A≦1.0 is satisfied.

Abstract: An electrophotography apparatus is provided with one or a plurality of recording units which optically form a latent image on a body. Each of the recording units includes a plurality of light emitting elements having a shape which satisfies a relationship t/y<T/Y<1, where t indicates a width of a pixel of the latent image in a sub scanning direction, y indicates a length of the pixel in a main scanning direction, T indicates a width of a light emitting element in the sub scanning direction, and Y indicates a length of the light emitting element in the main scanning direction.

Abstract: An image forming apparatus has the following characteristics: the diameter of an exposure beam for exposing a latent image is not more than twice a minimum dot recording period p in peak intensity 1/e2, and the value of a spatial frequency characteristic function of a latent image defined by the thickness of a photoreceptor layer, the dielectric constant of the photoreceptor layer, the thickness of a developer layer, the dielectric constant of the developer layer and a spatial frequency &ohgr; defined as &ohgr;=&pgr;/p is at least 0.3. The image forming apparatus having such characteristics can reproduce an image of high resolution exceeding 600 to 1200 DPI with high fidelity.

Abstract: An exposure apparatus for image formation uses power control means provided to a laser driver for controlling exposure energy and accordingly forming an image. Specifically, exposure energy is varied depending on an isolated-dot pattern and an isolated-line pattern by changing the pulse height (drive power) of a drive pulse for each dot. Both of the isolated-line and isolated-dot patterns of a high resolution can thus be reproduced with a high image quality without increase of cost due to requirements for accuracy in processing and positioning of a lens and without decrease of life of a photoreceptor due to a decreased thickness of its photosensitive layer.

Abstract: Electrostatic developing units are arranged in the record paper conveyance direction, form dot latent images in conformity with image dot data through the issue of scanning light on photosensitive drums in rotation, and after developing by toner components, transfer dots onto record paper for development. For each lighting noticeable dot amount image dot data, a quantity-of-light control unit provides control and output of optical energy of the noticeable dot on the basis of peripheral dots which may have influence on the size of the noticeable dot on the record paper, for example, in response to the distance from the peripheral dots.

Abstract: An image forming apparatus includes a photoreceptor, an exposure device for exposing the photoreceptor with a laser beam dot by dot so as to form a dotted latent image on the photoreceptor, and a developing device for developing the dotted latent image with developer. The apparatus also includes a dot diameter correcting device for correcting a diameter of the dot formed on the photoreceptor. The dot diameter correcting device includes an image patch producing device for forming an image patch having a specific dot pattern with a laser beam having a predetermined power strength by the exposure device, an image patch detector for detecting a density level of the image patch formed on the photoreceptor, and a memory for storing data of laser power strengths with correlation to the detected density levels. With this structure, a controller controls laser power strength of the exposure device based on the correlated data stored in the memory and the density level detected by the image patch detector.

Abstract: An image forming apparatus is provided for producing an array of dots in the primary scanning direction based on image data prepared for one line. The apparatus includes enlarging means for enlarging the image data n times in the primary scanning direction to form images with a resolution equal to (1/n) times a maximum resolution, where the n is a positive integer. The image forming apparatus also includes dot forming means for forming dots for n lines in the secondary scanning direction based on the enlarged image data.

Abstract: Bit data of a received video signal is partially converted into a first signal representing a data sequence corresponding to the resolution of an LED head. The first signal is transmitted to the LED head in the form of a real printing data signal that is to be printed on a basic raster line in synchronism with a line timing signal. The remaining bit data of the received video signal, which are not converted into the first signal, are converted into a second signal representing another data sequence, and then stored in a line buffer. The second signal stored in the line buffer is transmitted to the LED head in the form of a real printing data signal that is to be printed on an additional raster line in synchronism with an additional line timing signal. The LED head drive energy with which the basic raster lines are printed and another LED head drive energy with which the additional raster lines are printed are set independently of each other. The LED head may be provided with such a resolution function.

Abstract: The present invention can provide an electrophotographic apparatus and electrophotographic method which are less affected by dot gain, can realize smooth gray scale expression, are high in gray scale reproducibility, and are hard to generate moire. In an embodiment of the present invention, a continuous tone image is binarized by using the frequency modulation screening, to form a latent image free from the generation of moire and high in relative resolution. The latent image thus formed is developed using a liquid developer, to form a thin toner layer, for forming a toner image less in the generation of mechanical and optical dot gain.

Abstract: An image forming apparatus of the present invention includes a developing device including two developing rollers arranged in parallel in a direction in which an image carrier moves each for forming a magnet brush thereon. The magnetic poles of the developing rollers for effecting hand-over of a developer between the rollers are provided with the same polarity. An optical writing device optically writes an image on the image carrier in accordance with tonality data of four levels or less for a single dot. The developing device prevents the developer from moving through a gap between the developing rollers and bringing about irregular image density and background contamination. In addition, the optical writing device successfully obviates ghosts, which are likely to appear when a uniform halftone image including solid portions or bold characters is reproduced.

Abstract: An exposure energy control unit of an image forming apparatus controls exposure energy density Edot (&mgr;J/cm2) to be applied to an isolated one-dot pattern of image information to satisfy a relation −0.11P+6.4≦Edot/Eline≦−0.2P+10.7 where Eline (&mgr;J/cm2) represents exposure energy density applied to a line pattern of the image information having lines on every second lines and P (&mgr;m) represents dot pitch. Accordingly, image reproducibility of both of a periodic line pattern and an isolated dot pattern can be enhanced without decreasing the film thickness of a photoreceptor even if the resolution is high.

Abstract: This specification discloses an electrophotographic apparatus for forming an image by digital processing improved in a gradation property, provided with image data processing means for processing input image data to the apparatus body into data for forming an electrostatic latent image, electrostatic latent image forming means for effecting image exposure by digital optical means conforming to the electrostatic latent image data to thereby form the electrostatic latent image on the electrophotographic photosensitive member, and developing means for developing the electrostatic latent image with a toner to thereby form a developed image, wherein the image data processing means effects an image calculation taking into account the mutual action around a pixel constituting the electrostatic latent image in conformity with the size of the pixel when it processes the input image data into the electrostatic latent image data.

Abstract: An image forming apparatus provided with an electrostatic latent image bearing member, a charger for charging the surface of the electrostatic latent image bearing member, an exposure device for exposing the surface of the electrostatic latent image bearing member to light on the basis of image information, a developer device for visualizing the electrostatic latent image by a developer, a pixel density changing device for changing over the pixel density of the electrostatic latent image to a different set value, and an exposure intensity changing device for changing over the exposure intensity of the exposure means in conformity with the setting of the pixel density. The electrostatic latent image bearing member is 2000 (V·cm2/&mgr;J) or greater in sensitivity.

Abstract: A modulator drive function, for a hyperacuity laser printer that produces text and line art and halftones from numerically sampled input, incorporates device dependent characteristics, such as xerographic development and laser spot size, to position these edges precisely. The modulation drive function converts r,&thgr; channel coordinates to drive values for each scan within the span of its own channel pitch. A spline-based edge reproduction curve (ERC) compensates for slowscan phase errors and a temporal bias compensates for fastscan spot size. An electronic digital-to-analog converter, pulse width modulator or shift register can be used to control exposure.

Abstract: An object of the present invention is to offer an image stabilizer device capable of effectively improving quality of images produced by an image forming apparatus of a type that changes the size of a basic pixel by controlling an irradiation time of write-in light, and to offer a pattern image reader device used for the purpose. A pattern image reader device 100 is constituted by a reader unit 2 that at least contains a semiconductor laser 3, converging lens 6, and photosensor 5 for reading the amount of reflected light, and the driving section 7 that contains a drive coil 7c for moving the reader unit 2 in the principal scanning direction, and can measure on the order of microns the size of a toner image formed on the surface of a transfer and transport belt 26 in the principal scanning direction by monitoring the output of the photosensor 5 while moving in the principal scanning direction under the control by the CPU 48.

Abstract: A writing devices for an image forming apparatus of the present invention determines whether or not an image at and around an interval between nearby light emitter chips of a light emitter array is of halftone. If the image is of halftone, the writing device corrects image data corresponding to the above interval in order to increase the quantity of light. This successfully reduces or fully obviates the local omission of an image ascribable to intervals between nearby light emitter chips and insures a smooth output image.

Abstract: Provided are an image processing method and apparatus in which there is little deterioration in picture quality when specific information specifying a copying machine or the user thereof is added to image data. Pixel data in an 8.times.4 array at a predetermined position of the image data is modulated by subtracting a predetermined value .alpha. from 2.times.4 pixel data 2302 and 2.times.4 pixel data 2303 and adding the predetermined value a to 4.times.4 pixel data 2301 in such a manner that the number of pixels from which the predetermined value is subtracted and the number of pixels to which the predetermined value is added will become equal. The modulated data serves as a unit of added information. A plurality of units of the added information are arranged on the image data and the added information is represented by the mutual positional relationship of the plurality of information units.

Abstract: An image forming apparatus with toner saving mode is disclosed. This image forming apparatus comprising a normal ditheringing matrix and a toner saving mode ditheringing matrix having threshold data higher than that of the normal ditheringing matrix. The normal ditehring matrix is for image data processing under non-toner saving mode and the toner saving mode matrix is for image data processing under toner saving mode. The selection of the dithering matrix is executed by a central processing unit in response with the existance of order for toner saving. The processed image data processed by one of the dithreing matrix maintains the expression of linear tonality.