Image forming apparatus

An image forming apparatus that forms an image according to light emitted from a light source is provided. The image forming apparatus includes: a first image processor that performs image processing on image data having a first resolution and outputs the resulting image data; a resolution converter that acquires the image data having the first resolution output from the first image processor and converts the image data to image data having a second resolution that is higher than the first resolution; a modulation signal generator that modulates the image data having the second resolution according to a clock signal to thereby generate a modulation signal; a light source driver that drives the light source according to the modulation signal; and a second image processor that performs image processing on the image data having the second resolution to be modulated to the modulation signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2013-054368 filed in Japan on Mar. 15, 2013.

BACKGROUND OF THE INVENTION

1. Field of the Invention

2. Description of the Related Art

Digital printers employing an electrophotographic process have lately become widely used in the production printing field. The digital printers employing the electrophotographic process are thus required to offer higher image quality and greater reliability. The digital printers employing the electrophotographic process are particularly required to offer, for example, improved fine line reproducibility, improved character reproducibility (e.g., improved reproducibility of characters of minute sizes corresponding to 2 to 3 points), inhibition of characters from becoming broader due to the electrophotographic process, and improved color shift correction accuracy.

In order to achieve the higher image quality, the digital printer employing the electrophotographic process includes an image processor that corrects image data through image processing. The image processor performs image processing for, for example, multi-bit data having a high resolution of 1200 dots per inch (dpi) or 2400 dpi.

The digital printer employing the electrophotographic process further includes, for example, a photosensitive drum, a light source, a polygon mirror, and a scanning optical system. Specifically, the photosensitive drum has a surface that functions as a scanned surface having photosensitivity. The light source emits a laser beam. The polygon mirror deflects the laser beam from the light source. The scanning optical system guides the laser beam deflected by the polygon mirror onto the surface (scanned surface) of the photosensitive drum. The digital printer employing the electrophotographic process modulates the light beam emitted from the light source according to the image data to thereby irradiate the scanned surface with the light beam from the light source. And by scanning the scanned surface with the light beam, the digital printer employing the electrophotographic process forms an electrostatic latent image on the photosensitive drum according to the image data.

The digital printer employing the electrophotographic process having the configuration as described above includes as the light source a laser diode array (LDA), a vertical-cavity surface-emitting laser (VCSEL), or other element having a plurality of light emitting points. This enables the digital printer employing the electrophotographic process to form an electrostatic latent image having a resolution higher than image data of 1200 dpi, specifically, a 2400-dpi or 4800-dpi electrostatic latent image.

Japanese Patent Nos. 4968902 and 4640257 each disclose a technique in which, through processing performed by an image processor, outlined portions in the image are detected and outlines are extended or pixels around white-on-black inverted characters are corrected. Thereby, inverted characters are prevented from being collapsed and improved character reproducibility is achieved. Japanese Patent No. 4912071 discloses an arrangement in which a light source drive circuit includes a light source modulation signal generating circuit that corrects bend and skew in a scanning line (a locus of a light beam deflected by a polygon mirror).

Processing of a high-density image involves a problem in data transfer from the image processor to the light source drive circuit downstream thereof. If the image processor processes multi-bit data images with a resolution, for example, of 2400 dpi or 4800 dpi, the degree of freedom in image processing is enhanced and reproducibility of 1200-dpi characters and lines of minute sizes can be improved. In high-density image processing, however, an enormous amount of data needs to be transferred from the image processor to the downstream light source drive circuit, which is a bottleneck in productivity.

If the correction is made with the light source modulation signal generating circuit of the downstream light source drive circuit as in Japanese Patent No. 4912071, the amount of data transferred from the image processor to the light source drive circuit does not increase. The data transferred to the light source drive circuit is, however, converted to light source ON/OFF information, which makes it difficult to perform complicated corrections.

In view of the foregoing situation, there is a need to provide an image forming apparatus capable of performing image processing at high resolutions without increasing an image data transfer amount.

SUMMARY OF THE INVENTION

According to the present invention, there is provided an image forming apparatus that forms an image according to light emitted from a light source, the image forming apparatus comprising: a first image processor that performs image processing on image data having a first resolution and outputs the resulting image data; a resolution converter that acquires the image data having the first resolution output from the first image processor and converts the image data to image data having a second resolution that is higher than the first resolution; a modulation signal generator that modulates the image data having the second resolution according to a clock signal to thereby generate a modulation signal; a light source driver that drives the light source according to the modulation signal; and a second image processor that performs image processing on the image data having the second resolution to be modulated to the modulation signal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A color printer2000as an exemplary image forming apparatus according to an embodiment of the present invention will be described in detail with reference to the accompanying drawings.FIG. 1is a schematic diagram illustrating the configuration of the color printer2000according to the embodiment.

The color printer2000is a tandem type multi-color printer that forms a full-color image by superimposing four colors (black, cyan, magenta, and yellow), one on top of another.

The color printer2000includes an optical scanning device2010, four photosensitive drums2030a,2030b,2030c,2030d(to be generically referred to as a photosensitive drum2030), four cleaning units2031a,2031b,2031c,2031d(to be generically referred to as a cleaning unit2031), and four charging devices2032a,2032b,2032c,2032d(to be generically referred to as a charging device2032). The color printer2000further includes four developing rollers2033a,2033b,2033c,2033d(to be generically referred to as a developing roller2033) and four toner cartridges2034a,2034b,2034c,2034d(to be generically referred to as a toner cartridge2034). The color printer2000still further includes a transfer belt2040, a transfer roller2042, fixing rollers2050, a feed roller2054, a pair of registration rollers2056, discharging rollers2058, a paper feed tray2060, a discharge tray2070, a communication control device2080, a density detector2245, four home position sensors2246a,2246b,2246c,2246d(to be collectively referred to as a home position sensor2246), and a printer control device2090.

The communication control device2080controls bi-directional communications with a host device (e.g., a computer) via, for example, a network.

The printer control device2090generally controls different elements of the color printer2000. The printer control device2090includes, for example, a central processing unit (CPU), a ROM that stores therein a computer program described in codes to be executed by the CPU and various types of data used for executing the program, a RAM that serves as a working memory, and an AD converter circuit that converts analog data to the corresponding digital data. The printer control device2090, while controlling each of the different elements according to a request from the host device, transmits image data from the host device to the optical scanning device2010.

The photosensitive drum2030a, the charging device2032a, the developing roller2033a, the toner cartridge2034a, and the cleaning unit2031aare used as one unit. These elements constitute an image forming station to form a black image (may be referred to as a K station).

The photosensitive drum2030b, the charging device2032b, the developing roller2033b, the toner cartridge2034b, and the cleaning unit2031bare used as one unit. These elements constitute an image forming station to form a cyan image (may be referred to as a C station).

The photosensitive drum2030c, the charging device2032c, the developing roller2033c, the toner cartridge2034c, and the cleaning unit2031care used as one unit. These elements constitute an image forming station to form a magenta image (may be referred to as an M station).

The photosensitive drum2030d, the charging device2032d, the developing roller2033d, the toner cartridge2034d, and the cleaning unit2031dare used as one unit. These elements constitute an image forming station to form a yellow image (may be referred to as a Y station).

The photosensitive drum2030has a photosensitive layer formed on its surface. Specifically, the surface of the photosensitive drum2030assumes a scanned surface. The photosensitive drums2030a,2030b,2030c,2030deach have a rotational axis extending in parallel with each other and each rotate, for example, in an identical direction (e.g., in the direction indicated by the arrowed line in plane inFIG. 1).

The following description is based on a viewer's perspective in which, in an XYZ three-dimensional Cartesian coordinate system, a direction extending in parallel with the central axis of the photosensitive drum2030is the Y-axis direction and a direction in which the photosensitive drums2030are arrayed is the X-axis direction.

The charging device2032uniformly charges the surface of the photosensitive drum2030. The optical scanning device2010irradiates the charged surface of the photosensitive drum2030with a light beam modulated for each color based on the image data (black image data, cyan image data, magenta image data, yellow image data). As a result, on the surface of the photosensitive drum2030, an electric charge is erased only on portions irradiated with the light and a latent image corresponding to the image data is formed on the surface of the photosensitive drum2030. The latent image thus formed moves toward the developing roller2033as the photosensitive drum2030rotates. The configuration of the optical scanning device2010will be described in detail later.

In the photosensitive drum2030, an area in which image data is written may be called an “effective scanning area”, an “image forming area”, or an “effective pixel area”.

The toner cartridge2034astores therein black toner. The black toner is supplied to the developing roller2033a. The toner cartridge2034bstores therein cyan toner. The cyan toner is supplied to the developing roller2033b. The toner cartridge2034cstores therein magenta toner. The magenta toner is supplied to the developing roller2033c. The toner cartridge2034dstores therein yellow toner. The yellow toner is supplied to the developing roller2033d.

As the developing roller2033rotates, a light and uniform coat of toner from the corresponding toner cartridge2034is applied to the surface of the developing roller2033. The toner on the surface of the developing roller2033, upon its contact with the surface of the corresponding photosensitive drum2030, is transferred only to portions irradiated with the light of the surface and adheres thereto. Specifically, the developing roller2033causes the toner to adhere to the latent image formed on the surface of the corresponding photosensitive drum2030to thereby visualize the latent image.

The transfer belt2040is trained over a belt rotating mechanism, rotating in a predetermined direction. The transfer belt2040has an outer surface contacting the surface of each of the photosensitive drums2030a,2030b,2030c,2030dat a position opposite to the optical scanning device2010. In addition, the outer surface of the transfer belt2040contacts the transfer roller2042.

An image to which the toner adheres on the surface of the photosensitive drum2030(a toner image) is moved toward the transfer belt2040as the photosensitive drum2030rotates. The toner images of yellow, magenta, cyan, and black are then transferred, in sequence, onto the surface of the transfer belt2040at predetermined timing and are superimposed one on top of another to form a color image. The color image formed on the transfer belt2040is moved toward the transfer roller2042as the transfer belt2040rotates.

The paper feed tray2060stores therein recording sheets. The feed roller2054is disposed near the paper feed tray2060. The feed roller2054takes up the recording sheet, one at a time, from the paper feed tray2060and conveys the recording sheet to the pair of registration rollers2056.

The pair of registration rollers2056feeds the recording sheet toward a nip between the transfer belt2040and the transfer roller2042at predetermined timing. The color image on the transfer belt2040is transferred onto the recording sheet. The recording sheet onto which the color image has been transferred is fed to the fixing rollers2050.

The fixing rollers2050apply heat and pressure to the recording sheet. This enables to fixing rollers2050to fix the toner on the recording sheet. The recording sheet on which the toner has been fixed is fed onto the discharge tray2070by way of the discharging rollers2058and stacked in sequence on the discharge tray2070.

The cleaning unit2031removes toner remained (residual toner) on the surface of the photosensitive drum2030. The surface of the photosensitive drum2030from which the residual toner has been removed returns to a position facing the charging device2032again.

The density detector2245is disposed at a position on the negative X side of the transfer belt2040(upstream of the fixing rollers2050in the traveling direction of the transfer belt2040and downstream of the four photosensitive drums2030). Exemplarily, the density detector2245includes three optical sensors2245a,2245b,2245cas illustrated inFIG. 2.

The optical sensor2245ais disposed at a position facing a position near the end portion on the negative Y side within the effective pixel area in the transfer belt2040(on a first end side in the width direction of the transfer belt2040). The optical sensor2245cis disposed at a position facing a position near the end portion on the positive Y side within the effective pixel area in the transfer belt2040(on a second end side in the width direction of the transfer belt2040). The optical sensor2245bis disposed substantially at the center between the optical sensor2245aand the optical sensor2245cin the main-scanning direction (at a central position in the width direction of the transfer belt2040). In this specification, in the main-scanning direction, the central position of the optical sensor2245ais denoted Y1, the central position of the optical sensor2245bis denoted Y2, and the central position of the optical sensor2245cis denoted Y3.

The optical sensors2245a,2245b,2245ceach exemplarily include an LED11, a regularly reflected light receiving element12, and a diffusely reflected light receiving element13as illustrated inFIG. 3. Specifically, the LED11emits light (hereinafter referred to also as detection light) toward the transfer belt2040. The regularly reflected light receiving element12receives light reflected regularly from the transfer belt2040or a toner pad on the transfer belt2040. The diffusely reflected light receiving element13receives light reflected diffusely from the transfer belt2040or the toner pad on the transfer belt2040. Each of the regularly reflected light receiving element12and the diffusely reflected light receiving element13outputs a signal corresponding to the amount of received light (photoelectric conversion signal).

The home position sensor2246adetects a home position in rotation of the photosensitive drum2030a. The home position sensor2246bdetects a home position in rotation of the photosensitive drum2030b. The home position sensor2246cdetects a home position in rotation of the photosensitive drum2030c. The home position sensor2246ddetects a home position in rotation of the photosensitive drum2030d.

FIG. 4is a diagram illustrating the configuration of an optical system of the optical scanning device2010.FIG. 5is a diagram illustrating an exemplary optical path from a light source2200ato a polygon mirror2104and an exemplary optical path from a light source2200bto the polygon mirror2104.FIG. 6is a diagram illustrating an exemplary optical path from a light source2200cto the polygon mirror2104and an exemplary optical path from a light source2200dto the polygon mirror2104.FIG. 7is a diagram illustrating an exemplary optical path from the polygon mirror2104to the respective photosensitive drums2030.

The configuration of the optical system of the optical scanning device2010will be described below. The optical scanning device2010includes as its optical system the four light sources2200a,2200b,2200c,2200d, four coupling lenses2201a,2201b,2201c,2201d, four aperture plates2202a,2202b,2202c,2202d, and four cylindrical lenses2204a,2204b,2204c,2204d. The optical scanning device2010further includes as the optical system the polygon mirror2104, four scanning lenses2105a,2105b,2105c,2105d, and six folding mirrors2106a,2106b,2106c,2106d,2108b,2108c. These components are disposed at respective predetermined positions in an optical housing.

The optical scanning device2010still further includes an electric circuit which will be described with reference toFIG. 8and onward.

The light sources2200a,2200b,2200c,2200deach include a surface emitting laser array in which a plurality of light emitting elements are arrayed two-dimensionally. The light emitting elements of the surface emitting laser array are arrayed so as to be equidistant from each other when all light emitting elements are orthographically projected onto a virtual line extending in the direction corresponding to the sub-scanning direction. The light sources2200a,2200b,2200c,2200dare, for an example, each an exemplary vertical-cavity surface-emitting laser (VCSEL).

The coupling lens2201ais disposed on the light path of a light beam emitted from the light source2200a, changing the light beam passing therethrough to a substantially parallel light beam. The coupling lens2201bis disposed on the light path of a light beam emitted from the light source2200b, changing the light beam passing therethrough to a substantially parallel light beam. The coupling lens2201cis disposed on the light path of a light beam emitted from the light source2200c, changing the light beam passing therethrough to a substantially parallel light beam. The coupling lens2201dis disposed on the light path of a light beam emitted from the light source2200d, changing the light beam passing therethrough to a substantially parallel light beam.

The aperture plate2202ahas an aperture and shapes the light beam that has passed through the coupling lens2201a. The aperture plate2202bhas an aperture and shapes the light beam that has passed through the coupling lens2201b. The aperture plate2202chas an aperture and shapes the light beam that has passed through the coupling lens2201c. The aperture plate2202dhas an aperture and shapes the light beam that has passed through the coupling lens2201d.

The cylindrical lens2204afocuses the light beam that has passed through the aperture of the aperture plate2202aonto a position near a deflecting reflection surface of the polygon mirror2104along the Z-axis direction. The cylindrical lens2204bfocuses the light beam that has passed through the aperture of the aperture plate2202bonto a position near the deflecting reflection surface of the polygon mirror2104along the Z-axis direction. The cylindrical lens2204cfocuses the light beam that has passed through the aperture of the aperture plate2202conto a position near the deflecting reflection surface of the polygon mirror2104along the Z-axis direction. The cylindrical lens2204dfocuses the light beam that has passed through the aperture of the aperture plate2202donto a position near the deflecting reflection surface of the polygon mirror2104along the Z-axis direction.

An optical system comprising the coupling lens2201a, the aperture plate2202a, and the cylindrical lens2204ais a pre-deflector optical system for the K station. An optical system comprising the coupling lens2201b, the aperture plate2202b, and the cylindrical lens2204bis a pre-deflector optical system for the C station. An optical system comprising the coupling lens2201c, the aperture plate2202c, and the cylindrical lens2204cis a pre-deflector optical system for the M station. An optical system comprising the coupling lens2201d, the aperture plate2202d, and the cylindrical lens2204dis a pre-deflector optical system for the Y station.

The polygon mirror2104comprises a four-face mirror having a two-stage structure rotating about an axis extending in parallel with the Z-axis, each face of the polygon mirror2104assuming a deflecting reflection surface. The polygon mirror2104is disposed such that the four-face mirror of a first stage (lower stage) deflects the light beam from the cylindrical lens2204band the light beam from the cylindrical lens2204c, while the four-face mirror of a second stage (upper stage) deflects the light beam from the cylindrical lens2204aand the light beam from the cylindrical lens2204d.

In addition, the light beam from the cylindrical lens2204aand the light beam from the cylindrical lens2204bare deflected to the negative X side of the polygon mirror2104, while the light beam from the cylindrical lens2204cand the light beam from the cylindrical lens2204dare deflected to the positive X side of the polygon mirror2104.

The scanning lenses2105a,2105b,2105c,2105deach have an optical power that converges the light beam on a position near the photosensitive drum2030and an optical power that causes an optical spot to move on the surface of the photosensitive drum2030in the main-scanning direction at a constant speed as the polygon mirror2104rotates.

The scanning lens2105aand the scanning lens2105bare disposed on the negative X side of the polygon mirror2104. The scanning lens2105cand the scanning lens2105dare disposed on the positive X side of the polygon mirror2104.

The scanning lens2105aand the scanning lens2105bare stacked in the Z-axis direction. The scanning lens2105bfaces the four-face mirror of the first stage. The scanning lens2105afaces the four-face mirror of the second stage.

The scanning lens2105cand the scanning lens2105dare stacked in the Z-axis direction. The scanning lens2105cfaces the four-face mirror of the first stage. The scanning lens2105dfaces the four-face mirror of the second stage.

The photosensitive drum2030ais irradiated, via the scanning lens2105aand the folding mirror2106a, with the light beam from the cylindrical lens2204adeflected by the polygon mirror2104, which forms an optical spot. The optical spot moves in the longitudinal direction of the photosensitive drum2030aas the polygon mirror2104rotates. Specifically, the optical spot scans the surface of the photosensitive drum2030a. The direction in which the optical spot moves at this time is the “main-scanning direction” in the photosensitive drum2030aand the direction in which the photosensitive drum2030arotates is the “sub-scanning direction” in the photosensitive drum2030a.

The photosensitive drum2030bis irradiated, via the scanning lens2105b, the folding mirror2106b, and the folding mirror2108b, with the light beam from the cylindrical lens2204bdeflected by the polygon mirror2104, which forms an optical spot. The optical spot moves in the longitudinal direction of the photosensitive drum2030bas the polygon mirror2104rotates. Specifically, the optical spot scans the surface of the photosensitive drum2030b. The direction in which the optical spot moves at this time is the “main-scanning direction” in the photosensitive drum2030band the direction in which the photosensitive drum2030brotates is the “sub-scanning direction” in the photosensitive drum2030b.

The photosensitive drum2030cis irradiated, via the scanning lens2105c, the folding mirror2106c, and the folding mirror2108c, with the light beam from the cylindrical lens2204cdeflected by the polygon mirror2104, which forms an optical spot. The optical spot moves in the longitudinal direction of the photosensitive drum2030cas the polygon mirror2104rotates. Specifically, the optical spot scans the surface of the photosensitive drum2030c. The direction in which the optical spot moves at this time is the “main-scanning direction” in the photosensitive drum2030cand the direction in which the photosensitive drum2030crotates is the “sub-scanning direction” in the photosensitive drum2030c.

The photosensitive drum2030dis irradiated, via the scanning lens2105dand the folding mirror2106d, with the light beam from the cylindrical lens2204ddeflected by the polygon mirror2104, which forms an optical spot. The optical spot moves in the longitudinal direction of the photosensitive drum2030das the polygon mirror2104rotates. Specifically, the optical spot scans the surface of the photosensitive drum2030d. The direction in which the optical spot moves at this time is the “main-scanning direction” in the photosensitive drum2030dand the direction in which the photosensitive drum2030drotates is the “sub-scanning direction” in the photosensitive drum2030d.

The folding mirrors2106a,2106b,2106c,2106d,2108b,2108care disposed such that each has an optical path length between the polygon mirror2104and the corresponding photosensitive drum2030identical to each other and the position of incidence and the incident angle of the light beam at the corresponding photosensitive drum2030are identical to each other.

The optical system disposed along the optical path between the polygon mirror2104and the photosensitive drum2030is also referred to as a scanning optical system. In the embodiment, the scanning lens2105aand the folding mirror2106aconstitute a scanning optical system for the K station. Similarly, the scanning lens2105band the two folding mirrors2106b,2108bconstitute a scanning optical system for the C station. The scanning lens2105cand the two folding mirrors2106c,2108cconstitute a scanning optical system for the M station. The scanning lens2105dand the folding mirror2106dconstitute a scanning optical system for the Y station. In each of these scanning optical systems, the scanning lens2105a,2105b,2105c, or2105dmay comprise a plurality of lenses.

FIG. 8is a diagram illustrating the configuration of an electrical system of the optical scanning device2010. The optical scanning device2010includes as its electrical system an interface unit3101, an image processing unit3102, and a drive control unit3103.

The interface unit3101acquires, from the printer control device2090, image data transferred from the host device (e.g., a computer). The interface unit3101transfers the acquired image data to the image processing unit3102downstream thereof.

In the embodiment, the interface unit3101acquires image data in the RGB format having a resolution of 1200 dpi and consisting of eight bits and transfers the image data to the image processing unit3102.

The image processing unit3102, having acquired the image data from the interface unit3101, converts the image data to color image data corresponding to the applicable printing system. Exemplarily, the image processing unit3102converts the image data in the RGB format to image data of the tandem type (CMYK format). In addition to the data format conversion, the image processing unit3102performs image processing with the aim of, for example, improving image quality of the image data.

In this embodiment, the image processing unit3102outputs image data in the CMYK format having a resolution of 1200 dpi and consisting of two bits. The image data output from the image processing unit3102may have any resolution other than 1200 dpi. The resolution of the image data output from the image processing unit3102is referred to as a first resolution.

The drive control unit3103acquires, from the image processing unit3102, image data having the first resolution, and converts the image data to color image data having a second resolution corresponding to light source driving. The second resolution is higher than the first resolution. In this embodiment, the drive control unit3103converts the image data to one in the CMYK format having a resolution of 4800 dpi and consisting of one bit.

Additionally, the drive control unit3103modulates the image data to a clock signal that indicates pixel light-emitting timing, thus generating an independent modulation signal for each color. The drive control unit3103drives and causes each of the light sources2200a,2200b,2200c,2200dto emit light according to the modulation signal associated with the corresponding color.

The drive control unit3103is exemplarily a single-chip IC circuit disposed near the light sources2200a,2200b,2200c,2200d. The image processing unit3102and the interface unit3101are disposed farther away from the light sources2200a,2200b,2200c,2200drelative to the drive control unit3103. The image processing unit3102and the drive control unit3103are connected with a cable3104.

The optical scanning device2010having the arrangements as described above can form a latent image by causing the light sources2200a,2200b,2200c,2200dto emit light corresponding to the image data.

FIG. 9is a diagram illustrating the configuration of the interface unit3101. The interface unit3101exemplarily includes a flash memory3211, a RAM3212, an IF circuit3213, and a CPU3214. The flash memory3211, the RAM3212, the IF circuit3213, and the CPU3214are connected to each other by a bus.

The flash memory3211stores therein a computer program to be executed by the CPU3214and various types of data required by the CPU3214for executing the program. The RAM3212is a work storage area used by the CPU3214to execute the program. The IF circuit3213performs bi-directional communications with the printer control device2090.

The CPU3214operates according to the program stored in the flash memory3211, thus generally controlling the optical scanning device2010. The interface unit3101configured as described above transfers the image data (in the RGB format having a resolution of 1200 dpi and consisting of eight bits) transmitted from the printer control device2090to the image processing unit3102.

FIG. 10is a diagram illustrating the configuration of the image processing unit3102. The image processing unit3102includes a color converter3221, an ink generator3222, a gamma corrector3223, a quasi-halftone processor3224, and a first image processor3225.

The color converter3221converts the image data in the RGB format to image data in a CMY format. The ink generator3222generates a black component from the image data in the CMY format generated by the color converter3221to thereby generate image data in the CMYK format.

The gamma corrector3223uses, for example, a table to subject the level of each color to linear conversion. The quasi-halftone processor3224uses, for example, a dithering technique to process halftones, thereby reducing the number of gradations of the image data.

The first image processor3225performs image processing on the image data output from the quasi-halftone processor3224with the aim of, for example, improving image quality. The first image processor3225uses filtering, pattern matching, or the like to detect, within the image, a specific area for which image quality is to be improved and performs predetermined image processing on the detected image area.

Specific examples of the processing performed by the first image processor3225will further be described in detail with reference toFIGS. 12A,12B,13A,13B, and14. In addition, the first image processor3225performs the image processing on an area different from that processed in image processing performed by the drive control unit3103at a later stage and using parameters different from those used in the image processing performed by the drive control unit3103. The differences will be described in detail later.

The image processing unit3102as described above outputs the image data in the CMYK format having the first resolution (e.g., 1200 dpi) and consisting of two bits to the drive control unit3103. The image processing unit3102may be achieved by hardware partially or entirely or by a CPU executing a software program.

FIG. 11is a diagram illustrating the configuration of the drive control unit3103. The drive control unit3103includes a resolution converter3231, a clock generator3232, a modulation signal generator3233, a light source driver3234, and a second image processor3235.

The resolution converter3231acquires image data having the first resolution from the image processing unit3102and converts the image data to image having the second resolution that is higher than the first resolution. In the embodiment, the resolution converter3231converts the image data in the CMYK format having a resolution of 1200 dpi and consisting of two bits to image data in the CMYK format having a resolution of 4800 dpi and consisting of one bit.

Specifically, the resolution converter3231quadruples the resolution by converting one dot (two bits (four gradations)) of 1200-dpi image data to four dots (one bit) of 4800-dpi image data. It is noted that the resolution converter3231may convert image data to that of any gradations, as long as the conversion process converts image data with a resolution N (N being a natural number) to image data with a resolution of m×N (m being 2 or any other natural number more than 2).

The clock generator3232generates a clock signal that indicates the pixel light-emitting timing. The clock signal can be phase-modulated with a resolution of ⅛ clock, for example.

The modulation signal generator3233modulates image data of each color to a corresponding clock signal to thereby generate an independent modulation signal for the color. In the embodiment, the modulation signal generator3233generates a modulation signal for each color of C, M, Y, and K. Additionally, the modulation signal generator3233modulates, for each color, the image data to a clock signal in synchronism with write start timing based on the angular position of rotation of the photosensitive drum2030. The modulation signal generator3233then supplies the independent modulation signal for each color to the light source driver3234.

The light source driver3234drives a corresponding one of the light sources2200a,2200b,2200c,2200daccording to the independent modulation signal for each color output from the modulation signal generator3233. This enables the light source driver3234to make each of the light sources2200a,2200b,2200c,2200demit light with an intensity corresponding to the modulation signal.

The second image processor3235performs image processing for the image data having the second resolution (e.g., 4800 dpi) to be modulated to a modulation signal. The second image processor3235exemplarily includes a pattern matcher3241and a corrector3242.

The pattern matcher3241detects, of the image data, an image area to be subject to processing by the second image processor3235. Exemplarily, the pattern matcher3241detects from the image data having the second resolution an area with a space component close to that of a previously registered image pattern. Alternatively, the pattern matcher3241may perform filtering for the image data having the second resolution to thereby detect an area with a frequency component close to that of a previously registered image pattern.

The corrector3242corrects the detected image area through image processing. For example, the corrector3242may perform image processing for the image data before modulation. Alternatively, the corrector3242may even perform image processing for the image data by adjusting signal intensity by, for example, changing the pulse width of the modulation signal during the modulation.

As briefly noted earlier, the first image processor3225of the image processing unit3102and the second image processor3235of the drive control unit3103perform image processing using processing parameters different from each other or relative to areas subject to image data processing different from each other.

For example, the first image processor3225performs image processing rendering a coarseness level coarser than a predetermined coarseness level for the image data having the first resolution (e.g., 1200 dpi). At this time, the second image processor3235performs image processing rendering a fineness level finer than a predetermined fineness level for the image data having the second resolution (e.g., 4800 dpi). This allows the first image processor3225to perform a coarse adjustment and the second image processor3235to perform a fine adjustment relative to an identical image area.

Exemplarily, the first image processor3225performs image processing for objects (e.g., characters or graphics) equal in size to or larger in size than a predetermined size on the image data having the first resolution (e.g., 1200 dpi). In this case, the second image processor3235performs image processing for objects (e.g., characters or graphics) smaller in size than the predetermined size, the objects not being subject to the image processing by the first image processor3225. Exemplarily, the second image processor3235performs its image processing for at least part of a pattern of predetermined characters having a predetermined size or smaller. This allows the first image processor3225to perform its image processing for a coarse image area and the second image processor3235to perform the same image processing as that of the first image processor3225for a fine image area.

Thus, the color printer2000performs image processing for a minute pattern or fine image processing on high-resolution image data, which enables the color printer2000to form an image with high quality. Furthermore, the color printer2000performs image processing for a relatively large pattern or relatively coarse image processing on low-resolution image data. This reduces processing load on the drive control unit3103, while reducing the amount of data transferred from the image processing unit3102to the drive control unit3103.

In detecting the object, such as a character or a graphic figure, to be subjected to image processing, if it is difficult for the first image processor3225and the second image processor3235to detect an entire object from the image data, at least part of the character or graphic figure may be detected by, for example, pattern matching. For example, the first image processor3225and the second image processor3235register at least a characteristic shape pattern of a predetermined character or graphic figure in advance and detect the whole or part of the character by pattern matching. The first image processor3225and the second image processor3235then perform the image processing on the detected whole or part of the character.

FIG. 12Ais a diagram illustrating an exemplary 5-point white-on-black inverted character and exemplary enlarging steps in units of 1200 dpi.FIG. 12Bis a diagram illustrating an exemplary 3-point white-on-black inverted character and exemplary enlarging steps in units of 4800 dpi. The first image processor3225and the second image processor3235detect, for example, a white (blank) portion that is represented by blanking out the shape of an object (e.g., a character or a graphic figure) from a background color as illustrated inFIGS. 12A and 12Bthrough matching between the space component or the frequency component of the image data and a previously registered pattern. Then, the first image processor3225and the second image processor3235perform steps of enlarging a white (blank) part in the detected white (blank) portion.

This enables the first image processor3225and the second image processor3235to form a high-quality image by minimizing a disadvantage in electrophotographic printing of aggravated reproducibility due to collapsed fine lines.

In performing the steps of enlarging the white part, the first image processor3225enlarges, relative to the white-on-black inverted character of a predetermined first size or larger (e.g., 5 points or larger), the white part in units of the first resolution (e.g., in units of 1200 dpi), but not relative to the white-on-black inverted character smaller than the first size (e.g., smaller than 5 points), as illustrated inFIG. 12A.

Alternatively, as illustrated inFIG. 12B, the second image processor3235enlarges, relative to the white-on-black inverted character not subjected to the image processing performed by the first image processor3225, specifically, the white-on-black inverted character smaller than the first size (e.g., smaller than 5 points), the white part in units of the second resolution (e.g., in units of 4800 dpi), but not relative to the white-on-black inverted character equal to or larger than the first size (e.g., 5 points or larger).

This allows the first image processor3225to perform its image processing for a relatively coarse image area and the second image processor3235to perform the same image processing as that of the first image processor3225for a relatively fine image area. It is noted that, in this case, the second image processor3235may enlarge the white part by, for example, changing the pulse width of the modulation signal of parts surrounding the white part.

FIG. 13Ais a diagram illustrating exemplary thinning steps in units of 1200 dpi.FIG. 13Bis a diagram illustrating exemplary thinning steps in units of 4800 dpi. Exemplarily, the first image processor3225detects a line-shaped object as illustrated inFIG. 13Athrough matching between the space component or the frequency component of the image data and a previously registered pattern in units of the first resolution (e.g., in units of 1200 dpi). The first image processor3225then performs a step of changing the width of the line (e.g., thinning) in units of the first resolution (e.g., in units of 1200 dpi) relative to the detected line-shaped object. The second image processor3235detects a line-shaped object as illustrated inFIG. 13Bthrough matching between the space component or the frequency component of the image data and a previously registered pattern in units of the second resolution (e.g., in units of 4800 dpi). The second image processor3235then performs a step of changing the width of the line (e.g., thinning) in units of the second resolution (e.g., in units of 4800 dpi) relative to the detected line-shaped object.

This enables the first image processor3225and the second image processor3235to form a high-quality image by minimizing a disadvantage of character thickening resulting from electrophotographic printing.

In performing the step of changing the width of the line as described above, the first image processor3225changes the width of a line with a predetermined width or larger (e.g., 5 dots or more at 1200 dpi), but not for a line with a width smaller than the predetermined width. The second image processor3235changes, relative to the line with a width not subjected to the step performed by the first image processor3225, specifically, the width of the line with a width smaller than the predetermined width (e.g., a line of less than 20 dots at 4800 dpi), but not for a line with the predetermined width or larger.

This allows the first image processor3225to perform its image processing for a relatively coarse image area and the second image processor3235to perform the same image processing as that of the first image processor3225for a relatively fine image area. It is noted that, in this case, the second image processor3235may narrow edges of the line by, for example, changing the pulse width of the modulation signal of line edge portions.

FIG. 14is a diagram illustrating exemplary smoothing steps. As illustrated inFIG. 14, the first image processor3225and the second image processor3235exemplarily detect a line-shaped object drawn in an oblique direction relative to an array of dots of image data through matching between the space component or the frequency component of the image data and a previously registered pattern. The first image processor3225and the second image processor3235then performs a smoothing step that smoothes edges of the detected oblique line.

This enables the first image processor3225and the second image processor3235to improve line reproducibility, thereby forming a high-quality image.

In performing the smoothing step for the oblique line as described above, the first image processor3225smoothes the oblique line in units of predetermined pixels (e.g., one dot at 1200 dpi). The second image processor3235smoothes the oblique line in units of pixels (e.g., one dot at 4800 dpi) with which an oblique line that cannot be smoothed in units of pixels applicable to the first image processor3225can be smoothed.

Assume, for example, that the first image processor3225smoothes the oblique line in units of one dot at 1200 dpi as illustrated by figure (A) inFIG. 14. In this case, the second image processor3235first thickens the oblique line as illustrated by figure (B) inFIG. 14and then smoothes the thickened oblique line in units of one dot at 4800 dpi as illustrated by figure (C) inFIG. 14.

As such, the first image processor3225and the second image processor3235can further improve line reproducibility by performing the thickening and smoothing steps for the oblique line. This allows the first image processor3225to perform coarse adjustments and the second image processor3235to perform fine adjustments relative to the same image area.

FIG. 15is a diagram illustrating a modification of the drive control unit3103of the optical scanning device2010. The drive control unit3103may receive object information together with the image data from the image processing unit3102. The object information indicates, for each image area (e.g., for each pixel dot) of the image data, the type of an object of the image area.

If, for example, the corresponding dot is part of a character, the object information indicates an attribute that represents a “character”. Alternatively, if the corresponding dot is part of a graphic figure, the object information indicates an attribute that represents a “graphic figure”. If the corresponding dot is part of a photo, the object information indicates an attribute that represents a “photo”.

According to a specific detail of the received object information, the second image processor3235determines whether to perform image processing. If, for example, the received object information indicates the attribute that represents a “character”, and if, for example, the area in question is subject to processing for a white-on-black inverted character, the second image processor3235performs the image processing. If the received object information indicates an attribute representing one other than a character, the second image processor3235does not perform image processing for the area subject to processing for the white-on-black inverted character.

As described above, the second image processor3235can improve image quality with even higher accuracy by determining whether to perform image processing using the object information.

The present invention achieves an advantageous effect of performing image processing at high resolutions without increasing the amount of optical image data to be transferred.