Method and printer for printing an image formed of 2-dimensionally arranged pixels

Abstract: A printer has a shift register (4), an emission-time setting section (5), a laser diode (62), and a drive section (67). The register (4) supplies the emission-time setting section (5) with bits representing consecutive pixels including at least one black pixel. The emission-time setting section (5) determines the period of time during which the drive section (67) is to drive the laser diode (62), from the pixel patterns represented by the bits supplied from the register (4), and outputs an emission-time signal which represents said period of time. In response to the emission-time signal, the drive section (67) drives the laser diode (62) for the period of time, whereby a black pixel of a desired size is formed on a printing medium.

Description: The present invention relates to a method of printing an image formed of 2-dimensionally arranged pixels, and also to a printer such as an electronic photography apparatus or a thermal printer, which is designed to print an image formed of 2-dimensionally arranged pixels. A laser printer comprises a photosensitive member and a laser diode. The laser diode is driven in accordance with input image, thus emitting a laser beam to the photosensitive member. The surface of the photosensitive member is thereby scanned with the laser beam. An item of the image data, which represents a pixel, is a signal S1 having the waveform shown in Fig. 1. As is evident from Fig. 1, the signal S1 is at either a high (H) level or a low (L) level for a period T 1 during which the photosensitive member is scanned to form the pixel on its surface. The laser diode is driven by a drive signal S2, which has the same wave form as the image data signal S1 as is illustrated in Fig. 1. Hence, the laser diode emits a laser beam for the period T 1 , to form a pixel on the photosensitive member. The electric resistance at the surface of the photosensitive member changes with the amount of light applied to the member. The electric charge of the surface of the member attenuates in accordance with the time the member is exposed to light. Hence, when the laser diode is driven by the signal S2 (Fig. 1) and emits a laser beam to the photosensitive member for the period T 1 , a circular pixel will be formed on the member, which circumscribes a square one-pixel region as is shown at (a) in fig. 2. When driven by two or more consecutive drive signals S2, the laser diode continuously emits a laser beam to the photosensitive member, thereby forming pixels which overlap one another as is shown at (b) in Fig. 2. As can be understood from Fig. 2, these overlapping pixels form a true black image since there are left no blank spaces among them. Therefore, the drive signals S2 are good for printing true black images. The drive signals S2 shown in Fig. 1 are far form desirable for printing gray scale images by means of a Bayer-type dither. Nor are they desirable for printing characters, particularly complex ones (e.g., Chines characters), each consisting of many strokes. When driven by a signal S2, the laser diode emits a beam for the period T 1 , whereby a pixel will be formed which circumscribes the one-pixel region. Hence, even if some of consecutive drive signals S2 are at a low level as in the case of printing a gray image or characters, the resultant pixels overlap in most cases, as is illustrated at (c) in Fig. 2. To print clear-cut gray images or characters, the laser diode can be driven by a signal S3 which is at the high level for a period T 2 shorter than the period T 1 as is evident from Fig. 1. When driven by the signal S3, the laser diode emits a laser beam to the photosensitive member for the period T 2 , forming a pixel on the member. This pixel is small, circumscribed by the square defining the one-pixel region as is shown at (a) in Fig. 3. Hence, if some of consecutive drive signals S3 are set at a low level, the resultant pixels are discrete, forming a clear-cut gray image or distinct characters as is illustrated at (b) in Fig. 3. Obviously, the drive signals S3 are good for printing gray images and complex characters. The signals S3, which remain at the high level for the period T 2 (T 2 < T 1 ) are not good for printing a true black image, however. When driven by two or more consecutive drive signals S3, the laser diode intermittently emits laser beams, each for the period T 2 , thereby forming discrete pixels, with blank spaces among them, as is illustrated at (c) in Fig. 3. The discrete pixels, thus formed, fail to define a true black image. The above-described problem with the conventional laser printer is also inherent in other types of electrophotographic printers, such as a light-emitting diode (LED) printer, a liquid-crystal printer. Further, the problem is found in thermal printers such as a heat-transfer printer, ink-jet printers, and electrostatic printers. In the case of the thermal printer disclosed in US-A-4567488 the problem of compensating for cumulative heating is met by reducing thermal exposure as a function of the number of surrounding black pixels. The present invention provides a printer as claimed in Claim 1, and a printing method as claimed in Claim 9. The invention provides a printer which can print not only a clear quasi-gray image or clear characters, but also true black images, either on different sheets of printing paper or on the same sheet of printing paper. The invention also provides a method of printing not only a clear quasi-gray image or clear characters, but also true black images, either on different sheets of printing paper or on the same sheet of printing paper. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a timing chart showing the image-data signals, laser-driving signals of one type, and laser-driving signals of another type -- all used in a conventional laser printer; Fig. 2 shows various pixel-patterns which the conventional laser printer forms when laser-driving signals of the first type are used; Fig. 3 shows various pixel patterns which the conventional laser printer forms when laser-driving signals of the second type are used; Fig. 4 is a block diagram illustrating a laser printer which is a first embodiment of the present invention; Fig. 5 shows 3-pixel patterns which can be detected by the emission-time setting section incorporated in the laser printer of Fig. 4; Fig. 6 is a timing chart showing the pixel signals, emission-time setting signals, and laser-driving signals -- all used in the printer shown in Fig. 4; Fig. 7 represents the pixel pattern formed by the printer of Fig. 4 and defining a true black image; Fig. 8 is a block diagram showing another laser printer which is a second embodiment of the present invention; Fig. 9 schematically represents the data table used in the emission-time setting section of the printer shown in Fig. 8; Fig. 10 shows 3-pixel patterns which can be detected by the emission-time setting section of the laser printer illustrated in Fig. 8; Fig. 11 is a timing chart showing the pixel signals, emission-time setting signals, and laser-driving signals -- all used in the printer shown in Fig. 8; Fig. 12 shows various pixel patterns which the printer of Fig. 8 forms; Fig. 13 represents a pixel pattern formed by the printer of Fig. 8 and defining a true black image; Fig. 14 is a block diagram showing still another laser printer which is a third embodiment of this invention; Fig. 15 schematically shows the data table used in the emission-time setting section of the printer shown in Fig. 14; Fig. 16 represents a variety of pixel-patterns which the emission-time setting section of the printer shown in Fig. 14 can detect; Fig. 17 is a timing chart showing the pixel signals, emission-time setting signals, and laser-driving signals -- all used in the printer of Fig. 14; Fig. 18 is a diagram showing various pixel patterns which the printer of Fig. 14 can form; Fig. 19 represents a pixel pattern formed by the printer of Fig. 14 and defining a true black image; Fig. 20 is a block diagram showing a laser printer according to a fourth embodiment of the invention; Fig. 21 is a schematic representation of the data table used in the emission-time section of the laser beam printer shown in Fig. 20; Fig. 22 is a timing chart showing the pixel signals, emission-time setting signals, and laser-driving signals -- all used in the printer of Fig. 20 to print images; Fig. 23 illustrates pixels of various sizes which the printer of Fig. 20 can form on a printing medium; Fig. 24 represents a pixel pattern formed by the printer of Fig. 20 and defining a true black image; Fig. 25 is a block diagram illustrating another laser printer which is the fifth embodiment of the present invention; Fig. 26 schematically shows a pixel pattern which the printer of Fig. 25 can detects; Fig. 27 is a schematic representation of the data table used in the emission-time setting section of the laser printer illustrated in Fig. 25; and Fig. 28 shows laser-driving signals having different durations, which are used in the printer of Fig. 26 to print images. Embodiments of the present invention will now be described, with reference to the accompanying drawings. First Embodiment Fig. 4 illustrates a laser printer according to the first embodiment of the invention. As is shown in Fig. 4, the laser printer comprises an interface section 1, a control section 2, an image memory 3, a shift register 4, an emission-time setting section 5, and a laser scanner unit (LSU) 6, and a photosensitive drum 7. The interface section 1 transfers image data between the control section 2 and an external device (not shown). (When the laser printer is incorporated in a facsimile apparatus, the section 1 transfers the image data between the section 2 and the data receiving/transmitting section of the facsimile apparatus.) The control section 2 supplies the image data to the image memory 3. The memory 3 has storage capacity great enough to store at least one page of data. Every time the memory 3 stores one page of data, the one-page data is read from the memory 3 and supplied to the shift register 4 under the control of the control section 2. The shift register 4 is a 3-bit one, which shifts the input data, bit by bit, in response to the pulses forming the image data, which are used as transfer clock pulses. The shift register 4 outputs three bits Q 0 , Q 1 and Q 2 simultaneously to the emission-time setting section 5. In other words, the register samples every three bits of the image data, which represents three consecutive pixels to be formed on the photosensitive drum 7 and be aligned in the horizontal direction, i.e., the direction of the main scanning of the drum 7. The bit Q 1 is supplied to the LSU 6 as pixel signal representing the pixel corresponding to the bit Q 1 . The emission-time setting section 5 has a ROM (not shown) storing data items showing reference 3-bit pat terns. The section 5 detects a 3-pixel patterns from the values of the three bits Q 0 , Q 1 and Q 2 supplied from the shift register 4, and outputs an emission-time signal SA to the LSU 6. The signal SA represents the time for which the LSU 6 is driven to print the pixel corresponding to the bit Q 1 . As is evident from Fig. 4, the LSU 6 comprises a laser diode 62, a collimator lens 63, an electric motor 64, a polygon mirror 65, an f- lens 66, a drive section 67. Upon receipt of the pixel signal SB from the shift register 4, the drive section 67 outputs a drive signal SC which is a pulse having the width corresponding the emission time represented by the signal SA output by the time-emission setting section 5. The drive signal SC is supplied to the laser diode 62. Driven by the signal SC, the diode 62 emits a laser beam 61 for the time set by the section 5. The beam 61 is applied to the collimator lens 63, which converges the beam 61 into a laser beam having a predetermined diameter. The beam from the collimator 63 is applied to the polygon mirror 65 being rotated by the motor 64 at a constant speed. The mirror 65 reflects the beam and applies it to the f- lens 66. The lens 66 applies the beam to the photosensitive drum 7, achieving a uniform-speed scanning of the drum 7 and forming a latent pixel image on the drum 7. Various devices (not shown), such as a drum-charging device, an image-developing device, an image-transferring device, an image-fixing device, a drum-cleaning device -- all being of the known types --are located around the photosensitive drum 7. Equipped with these devices, the laser printer shown in Fig. 4 is an electrophotographic printer of so-called "reverse developing type." The operation of the laser printer will now be explained, with reference to Figs. 5, 6 and 7. First, the image data stored in the image memory 3 is input to the shift register 4, bit by bit, under the control of the control section 2. Every time the image data is shifted by one bit in the register 4, three consecutive bits Q 0 , Q 1 and Q 2 are supplied to the emission-time setting section 5, and the bit Q 1 is supplied as a pixel signal SB to the drive section 67. Whenever the bit Q 1 is "1," thus representing a black pixel, the emission-time setting section 5 detects the 3-pixel pattern defined by the bits Q 0 , Q 1 and Q 2 , from the values of these bits Q 0 , Q 1 and Q 2 which represents three consecutive pixels arranged in the horizontal direction (i.e., the main scanning direction). When the bit Q 1 is "1," the three bits Q 0 , Q 1 and Q 2 can define the following four 3-pixel patterns: Pattern 1: Only the middle pixel is black, as is shown at (a) in Fig. 5. Pattern 2: The middle and left pixels are black, as is shown at (b) in Fig. 5. Pattern 3: The middle and right pixels are black, as is shown at (c) in Fig. 5. Pattern 4: All three pixels are black, as is shown at (d) in Fig. 5. When the section 5 detects the 3-pixel pattern 1, it outputs a signal SA representing a beam-emission period T A . When it detects the 3-pixel pattern 2 or 3, it generates a signal SA representing a beam-emission period T B . When it detects the 3-pixel pattern 4, it outputs a signal SA representing a beam-emission period T C . As is shown in Fig. 6, the drive section 67 outputs, in response to the signal SA representing the period T A , a drive signal SC which is at a high level for the period T A ; it outputs, in response to the signal SA representing the period T B , a drive signal SC which is at a high level for the period T B ; it outputs, in response to the signal SA showing the period T C , a drive signal SC which is at a high level for the period T C . Here, T A < T B < T C ≦ T 1 . The period T C is equal to a one-pixel scanning period T 1 in the present embodiments. When driven for the shortest period T A , the laser diode 62 emits a laser beam for this period T A , whereby a pixel is formed on the drum 7, which is small and located well within the one-pixel region. When driven for the longer period T B , the laser diode 62 emits a laser beam for the period T B , whereby a pixel is formed on the drum 7, which is circumscribed by the one-pixel region, as is shown in at (a) in Fig. 3. When driven for the longest period T C , the laser diode 62 emits a laser beam for this period T C , whereby a pixel is formed on the drum 7, which is large, circumscribing the one-pixel region, as is shown at (a) in Fig. 2. To print a quasi-gray image or complex characters, which are defined by discrete black pixels, the laser diode 62 is repeatedly driven, each time for the period T A or T B and emitting a laser beam for this period, whereby discrete black pixels, each located well within or circumscribed by the one-pixel region, are formed ultimately on a printing medium. The black pixels, thus formed, define a clear-cut quasi-gray image or distinct characters. On the other hand, to print a true black image, which is defined by overlapping black pixels, the laser diode 62 is repeatedly driven, each time for the period T C (i.e., one-pixel scanning period T 1 ) and emitting for the period T C , whereby black pixels, each circumscribing the one-pixel region and overlapping another, are formed ultimately on a printing medium. The black pixels, thus formed, define a true black image. A pattern of three consecutive pixels is recognized in the present embodiment. Then when either one of right and left side pixels in the pattern having more than three consecutive pixels is detected, the condition of the end pixels is discriminated as two consecutive black pixels. As a result, a black pixel of which size is smaller than that obtained in three consecutive pixels is formed, thereby obtaining a true black image of Fig. 7 as apparent from Fig. 7, the size of the black pixels at right and left side is smaller than that of the other black pixels. In such an image, the boundary between the true black image and white portion surrounding the two black image can be clearly recognized when it is observed by eyes. The printed condition can, therefore, be improved. Second Embodiment Fig. 8 illustrates a laser printer which is a second embodiment of the present invention and which is similar to the laser printer shown in Fig. 4. Some of the components of this printer, which are identical to those shown in Fig. 4 are designated at the same numerals in Fig. 8 and will not be described in detail. As is evident from Fig. 8, the laser printer comprises an interface section 1, a control section 2, an image memory 3, a laser scanner unit (LSU) 6, a photosensitive drum 7, three shift registers 104a, 10b and 104c, and an emission-time setting section 105. The shift registers 104a, 104b and 104c are each an n-bit shift register, where n is the number of pixels which form a one-line image. They shift the input data, bit by bit, in response to the transfer clock pulses, i.e., the pulses forming the image data supplied to the interface section 1. These registers 104a, 104b and 104c are connected in series. Hence, the output of the register 104a is input to the register 104b, and the output of the register 104b is input to the register 104c. Every time the one-line image data items are shifted by one bit in the registers 104a, 104b and 104c, these registers output three bits Q A , Q B and Q C , respectively. These three bits represent three consecutive pixels to be printed in the vertical direction, i.e., the sub-scanning direction. In other words, the shift registers 104a, 104b and 104c cooperate to sample every three bits of the image data, which represent three consecutive pixels to be formed on the photosensitive drum 7 and be aligned in the vertical direction, or the direction of the sub-scanning of the drum 7. The three bits Q A , Q B and Q C output from the registers 104a, 104b and 104c, respectively, are supplied to the emission-time setting section 105. Of these three bits, the bit Q B , i.e., the output of the register 104b, is supplied to the drive section 67 of the LSU 6 since the output of the register 104b is connected to the drive section 67. The emission-time setting section 105 has a ROM (not shown) storing the data table shown in Fig. 9. The section 5 detects one of the four 3-pixel patterns shown in Fig. 10, from the values of the three bits Q A , Q B and Q C supplied from the shift registers 104a, 104b and 104c, and outputs an emission-time signal SA to the LSU 6. The signal SA represents the time for which the LSU 6 is driven to print the pixel corresponding to the bit Q B . As in the laser beam shown in Fig. 4, various devices (not shown), such as a drum-charging device, an image-developing device, an image-transferring device, an image-fixing device, a drum-cleaning device -- all being of the known types -- are located around the photosensitive drum 7. The operation of the laser printer shown in Fig. 8 will now be explained, with reference to Fig. 9 through Fig. 13. At first, the shift registers 104a, 104b and 104c are empty. Then, the image data is supplied to the shift register 104a, hence to the shift register 104b, and further to the shift register 104c. Thus, the image data is shifted, bit by bit, through the registers 104a, 104b and 104c under the control of the control section 2. As has been described, the shift registers 104a, 104b and 104c are each an n-bit register, capable of storing a one-line data, and are connected in series. Hence, when the shift register 104a starts receiving the third one-line data, the shift register 104b starts outputting the first one-line data to the drive section 67, bit by bit, each being being a pixel SB. The drive section 67 drives the laser diode 62 incorporated in the LSU 6 in accordance with the data output from the shift register 104b. Meanwhile, the emission-time setting section 105 receives the three bits Q A , Q B and Q C from the registers 104a, 104b and 104c, detects the 3-pixel pattern from these bits Q A , Q B and Q C , and and output an emission-time signal SA which corresponds to one of the data items stored in the ROM. The signal SA is supplied to the drive section 67. More specifically, the emission-time setting section 105 detects the 3-pixel pattern defined by the bits Q A , Q B and Q C , from the values of these bits which represents three consecutive pixels arranged in the vertical direction (i.e., the sub-scanning direction). When the bit Q B is "1," the three bits Q A , Q B and Q C can define the following four 3-pixel patterns: Pattern 1: Only the middle pixel is black, as is shown at (a) in Fig. 10. Pattern 2: The middle and upper pixels are black, as is shown at (b) in Fig. 10. Pattern 3: The middle and lower pixels are black, as is shown at (c) in Fig. 10. Pattern 4: All three pixels are black, as is shown at (d) in Fig. 10. When the section 105 detects the 3-pixel pattern 1, it data item A (Fig. 9 ) and outputs a signal SA representing a beam-emission period T A . When it detects the 3-pixel pattern 2 or 3, it selects data item B and generates a signal SA representing a beam-emission period T B . When it detects the 3-pixel pattern 4, it selects data item C and outputs a signal SA representing a beam-emission period T C . As is shown in Fig. 11, the drive section 67 outputs, in response to the signal SA representing the period T A , a drive signal SC which is at a high level for the period T A ; it outputs, in response to the signal SA representing the period T B , a drive signal SC which is at a high level for the period T B ; it outputs, in response to the signal SA showing the period T C , a drive signal SC which is at a high level for the period T C . Here, T A < T B < T C ≦ T 1 . The period T C is equal to a one-pixel scanning period T 1 in the present embodiments. When driven for the shortest period T A , the laser diode 62 emits a laser beam for this period T A , whereby a pixel is formed on the drum 7, which is small and located well within the one-pixel region as is shown at (a) in Fig. 12. When driven for the longer period T B , the laser diode 62 emits a laser beam for the period T B , whereby a pixel is formed on the drum 7, which is circumscribed by the one-pixel region, as is shown in at (b) in Fig. 12. When driven for the longest period T C , the laser diode 62 emits a laser beam for this period T C , whereby a pixel is formed on the drum 7, which is large, circumscribing the one-pixel region, as is shown at (c) in Fig. 12. To print a quasi-gray image or complex characters, which are defined by discrete black pixels, the laser diode 62 is repeatedly driven, each time for the period T A or T B and emitting a laser beam for this period, whereby discrete black pixels, each located well within or circumscribed by the one-pixel region, are formed ultimately on a printing medium. The black pixels, thus formed, define a clear-cut quasi-gray image or distinct characters. On the other hand, to print a true black image, which is defined by overlapping black pixels, the laser diode 62 is repeatedly driven, each time for the period T C (i.e., one-pixel scanning period T 1 ) and emitting for the period T C , whereby black pixels, each circumscribing the one-pixel region and overlapping another, are formed ultimately on a printing medium. The black pixels, thus formed, define a true black image. A pattern of three consecutive pixels is recognized in the present embodiment. Then when either one of upper and lower side pixels in the pattern having more than three consecutive pixels is detected, the condition of the end pixels is discriminated as two consecutive black pixels. As a result, a black pixel of which size is smaller than that obtained in three consecutive pixels is formed, thereby obtaining a true black image of Fig. 13 as apparent from Fig. 13, the size of the black pixels at upper and lower side is smaller than that of the other black pixels. In such as image, the boundary between the true black image and white portion surrounding the two black image can be clearly recognized when it is observed by eyes. The printed condition can, therefore, be improved. Third Embodiment Fig. 14 illustrates a laser printer which is a third embodiment of the invention and which is similar to the laser printer shown in Fig. 4. Some of the components of this printer, which are identical to those shown in Fig. 4 are designated at the same numerals in Fig. 14 and will not be described in detail. As is evident from Fig. 14, the laser printer comprises an interface section 1, a control section 2, an image memory 3, a laser scanner unit (LSU) 6, a photosensitive drum 7, three shift registers 204a, 204b and 204c, and an emission-time setting section 205. As in the laser beam shown in Fig. 4, various devices (not shown), such as a drum-charging device, an image-developing device, an image-transferring device, an image-fixing device, a drum-cleaning device -- all being of the known types -- are located around the photosensitive drum 7. The shift registers 204a, 204b and 204c are each an n-bit shift register, where m is the number of pixels which form a one-line image. They shift the input data, bit by bit, in response to the transfer clock pulses, i.e., the pulses forming the image data supplied from the control section 2. These registers 204a, 204b and 204c are connected in series. Hence, the output of the register 204a is input to the register 204b, and the output of the register 204b is input to the register 204c. Every time the one-line image data items are shifted by one bit in the registers 204a, 204b and 204c, these registers output three bits Q A , Q B and Q C , respectively, and the registers 204b and 204c outputs two bits Q D and Q E which immediately follows and precedes the bit Q B , respectively. The bits Q A , Q B and Q C represent three consecutive pixels to be printed in the vertical direction, i.e., the sub-scanning direction. On the other hand, the bits Q B , Q D and Q E represents three consecutive pixels to be printed in the horizontal direction, i.e., the main scanning direction. The five pixels represented by the five bits Q A , Q B , Q C , Q D and Q E form a cross, the center of which is defined by the bits Q B . In other words, the shift registers 204a, 204b and 204c cooperate to sample every five bits of the image data, which represent a cross to be printed on a printing medium. These five bits Q A , Q B , Q C , Q D and Q E are supplied from the shift registers 294a, 204b and 204c to the emission-time setting section 205. Further, of these five bits, the bit Q B is supplied to the drive section 67 incorporated in the LSU 6, as a pixel signal representing a pixel to be printed. The emission-time setting section 205 has a ROM (not shown) which stores the data table shown in Fig. 15. It detects one of the sixteen 5-pixel patterns shown in Fig. 15, from the values of the bits Q A , Q B , Q C , Q D and Q E output from the shift registers 204a, 204b and 204c, and outputs an emission-time signal SA to the LSU 6. The signal SA represents the time for which the LSU 6 is driven to print the pixel corresponding to the bit Q B . As in the laser beam shown in Fig. 4, various devices (not shown), such as a drum-charging device, an image-developing device, an image-transferring device, an image-fixing device, a drum-cleaning device -- all being of the known types -- are located around the photosensitive drum 7. The operation of the laser printer shown in Fig. 14 will now be explained, with reference to Fig. 15 through Fig. 19. At first, the shift registers 204a, 204b and 204c are empty. Then, the image data is supplied to the shift register 204a, hence to the shift register 204b, and further to the shift register 204c. Thus, the image data is shifted, bit by bit, through the registers 204a, 204b and 204c under the control of the control section 2. As has been described, the shift registers 204a, 204b and 204c are each an n-bit register, capable of storing a one-line data, and are connected in series. Hence, when the shift register 204a starts receiving the third one-line data, the shift register 204b outputs the first bit Q B of the first one-line data to the drive section 67, as pixel signal SB. The drive section 67 drives the laser diode 62 incorporated in the LSU 6 in response to the pixel signal SB. Meanwhile, the emission-time setting section 205 receives the five bits Q A , Q B , Q C , Q D and Q E from the registers 204a, 204b and 204c, detects the 5-pixel pattern from these five bits and outputs an emission-time signal SA which corresponds to one of the data items stored in the ROM. The signal SA is supplied to the drive section 67. More specifically, the emission-time setting section 205 detects the 5-pixel pattern defined by the bits Q A , Q B , Q C , Q D and Q E from the values of these bits which represents five pixels arranged in the form of a cross, the center of which is the pixel represented by the bit Q B . When the bit Q B is "1," the three bits Q A , Q B and Q C can define the sixteen 5-pixel patterns (a) to (p) which are illustrated in Fig. 16 which are stored in the ROM. The 5-pixel patterns are classified into the following three types: Type A: Only the center pixel is black, or the center pixel and only another pixel is black. Type B: The center pixel and two or three other pixels are black. Type C: All five pixels are black. When the section 105 detects a 5-pixel pattern of type A, it outputs an emission-time signal SA representing a beam-emission period T A . When it detects a 5-pixel pattern of type B, it generates an emission-time signal SA representing a beam-emission period T B . When it detects the 5-pixel pattern C, it outputs an emission-time signal SA representing a beam-emission period T C . As is shown in the timing chart of Fig. 17, the drive section 67 outputs a drive signal SC which is at a high level for the period T A , in response to the signal SA representing the period T A ; it outputs, a drive signal SC which is at a high level for the period T B , in response to the signal SA representing the period T B ; it outputs, a drive signal SC which is at a high level for the period T C , in response to the signal SA showing the period T C . Here, T A < T B < T C . The period T C is equal to a one-pixel scanning period T 1 . The drive signal SC is supplied to the laser diode 62 of the LSU 6, thus driving the laser diode 62. When driven for the shortest period T A , the laser diode 62 emits a laser beam for this period T A , whereby a pixel represented by the signal SB is formed on the drum 7, which is small and circumscribed by the one-pixel region as is shown at DA to DL in Fig. 18. When driven for the longer period T B , the laser diode 62 emits a laser beam for the period T B , whereby a pixel represented by the signal SB is formed on the drum 7, which has a diameter longer than the side of the one-pixel region but not long enough to circumscribe the region, as is shown at DM and DN in Fig. 18. When driven for the longest period T C , the laser diode 62 emits a laser beam for this period T C , whereby a pixel is formed on the drum 7, which is large, circumscribing the one-pixel region, as is shown at DO in Fig. 12. To print a quasi-gray image or complex characters, which are defined by discrete black pixels, the laser diode 62 is repeatedly driven, each time for the period T A or T B and emitting a laser beam for this period, whereby discrete black pixels, each circumscribed by the one-pixel region or having a diameter longer than the side of the one-pixel region but not long enough to circumscribe the region, are formed ultimately on a printing medium. The black pixels, thus formed, define a clear-cut quasi-gray image or distinct characters. On the other hand, to print a true black image, which is defined by overlapping black pixels, the laser diode 62 is repeatedly driven, each time for the period T C (i.e., one-pixel scanning period T 1 ) and emitting for the period T C , whereby black pixels, each circumscribing the one-pixel region and overlapping another, are formed ultimately on a recording medium. The black pixels, thus formed, define a true black image. As has been described, the laser diode 62 is driven for the period T C and forms a large pixel when the five bits Q A , Q B , Q C , Q D and Q E supplied from the shift registers 204a, 204b and 204c are all "1," each representing a black pixel, and is driven for the shorter period T A or T B and forms a smaller pixel when any one of the five bits, except the bits Q B is "0," representing a white pixel. Hence, there can be printed a clearly outlined, true black image, as can be understood from Fig. 19. Fourth Embodiment Fig. 20 illustrates a laser printer which is a fourth embodiment of the invention and which is similar to the laser printer shown in Fig. 4. Some of the components of this printer, which are identical to those shown in Fig. 4 are designated at the same numerals in Fig. 20 and will not be described in detail. As is evident from Fig. 20, the laser printer comprises an interface section 1, a control section 2, an image memory 3, a laser scanner unit (LSU) 6, a photosensitive drum 7, three shift registers 304a, 304b and 304c, and an emission-time setting section 305. As in the laser beam shown in Fig. 4, various devices (not shown), such as a drum-charging device, an image-developing device, an image-transferring device, an image-fixing device, a drum-cleaning device -- all being of the known types -- are located around the photosensitive drum 7. The shift registers 304a, 304b and 304c are each an n-bit shift register, where n is the number of pixels which form a one-line image. They shift the input data, bit by bit, in response to the transfer clock pulses, i.e., the pulses forming the image data supplied from the control section 2. These registers 304a, 304b and 304c are connected in series. Hence, the output of the register 304a is input to the register 304b, and the output of the register 304b is input to the register 304c. Every time the registers 304a, 304b and 304c shift the one-line image data items by one bit, they output three bits Q A , Q B and Q C , respectively. At the same time, the register 304a also outputs two bits Q E and Q D which follows the bit Q A , the register 304b also outputs two bits Q G and Q F which follows the bit Q B , and the register 304c also outputs two bits Q I and Q H which follows the bit Q C , as can be understood from Fig. 20. The three bits Q A , Q E and Q D simultaneously output from the shift register 304a represent three consecutive ones of the pixels forming a first horizontal line, i.e., the pixels arranged in the main scanning line. The three bits Q B , Q G and Q F output from the shift register 304b represent three consecutive ones of the pixels forming a second horizontal line, i.e., the pixels arranged in the main scanning line. The three bits Q C , Q I and Q H output from the shift register 304c represent three consecutive ones of the pixels forming a third horizontal line, i.e., the pixels arranged in the main scanning line. Since the shift registers 304a, 304b and 304c, each holding a one-line data item, are connected in series, the bits Q A , Q B and Q C output from the registers 304a, 304b and 304c, represent three consecutive pixels arranged in the vertical direction, i.e., the sub-scanning direction; the bits Q E , Q G and Q I output from the registers 304a, 304b and 304c, represent three consecutive pixels arranged in the vertical direction; and the bits Q D , Q F and Q H output from the registers 304a, 304b and 304c, represent three consecutive pixels arranged in the vertical direction. Hence, the nine bits Q A to Q I represent a 3 × 3 pixel matrix of the type illustrated in Fig. 21, the bit Q G representing the center pixel of the 3 × 3 matrix. In other words, the shift registers 304a, 304b, and 304c cooperate to sample every nine bits of the image data, which represent a 3 × 3 pixel matrix. These nine bits Q A to Q I are supplied from the shift registers 304a, 304b and 304c to the emission-time setting section 205. Further, of these five bits, the bit Q G is supplied to the drive section 67 incorporated in the LSU 6, as a pixel signal representing a pixel to be printed. The emission-time setting section 205 has a ROM (not shown) which stores a data table showing various 9-bit patterns and three periods T A , T B and T C . It detects one of the various patterns which the bits Q A to Q I define when the bit Q G represents a black pixel, in accordance with the values of the bits Q A to Q I output from the shift registers 304a, 304b and 304c. Then, it outputs an emission-time signal SA to the LSU 6. The signal SA represents the time for which the LSU 6 is driven to print the pixel corresponding to the bit Q G . As in the laser beam shown in Fig. 4, various devices (not shown), such as a drum-charging device, an image-developing device, an image-transferring device, an image-fixing device, a drum-cleaning device -- all being of the known types -- are located around the photosensitive drum 7. The operation of the laser printer shown in Fig.20 will now be explained, with reference to Figs. 22, 23 and 24. At first, the shift registers 304a, 304b and 304c are empty. Then, the image data is supplied to the shift register 304a, hence to the shift register 304b, and further to the shift register 304c. Thus, the image data is shifted, bit by bit, through the registers 304a, 304b and 304c under the control of the control section 2. As has been described, the shift registers 304a, 304b and 304c are each an n-bit register, capable of storing a one-line data, and are connected in series. Hence, when the shift register 304a receives the last bit of the second one-line data, the shift register 304b outputs the bit Q G , which is supplied to the drive section 67, as pixel signal SB. The drive section 67 drives the laser diode 62 incorporated in the LSU 6 in response to the pixel signal SB. Meanwhile, the emission-time setting section 305 receives the five nine Q A to Q I from the registers 304a, 304b and 304c, detects the 9-pixel pattern from these nine bits, and outputs an emission-time signal SA which represents of the periods T A , T B and T C stored in the ROM. The signal SA is supplied to the drive section 67. More specifically, the emission-time setting section 305 detects the 9-pixel pattern defined by the bits Q A to Q I , from the values of these bits which represents nine pixels forming a 3x 3 matrix, the center of which is the pixel represented by the bit Q G . When the bit Q G is "1," the nine bits Q A to Q I can define 256-pixel patterns (= 2 8 patterns) which are stored in the ROM. The 9-pixel patterns are classified into the following three types: Type A: Only the center pixel is black, or the center pixel and only another pixel is black. Type B: The center pixel and three to five other pixels are black. Type C: The center pixel and six to eight other pixels are black. When the section 205 detects a 9-pixel pattern of type A, it outputs an emission-time signal SA representing a beam-emission period T A . When it detects a 9-pixel pattern of type B, it generates an emission-time signal SA representing a beam-emission period T B . When it detects the 9-pixel pattern of pattern C, it outputs an emission-time signal SA representing a beam-emission period T C . As is shown in the timing chart of Fig. 22, the drive section 67 outputs a drive signal SC which is at a high level for the period T A , in response to the signal SA representing the period T A ; it outputs, a drive signal SC which is at a high level for the period T B , in response to the signal SA representing the period T B ; it outputs, a drive signal SC which is at a high level for the period T C , in response to the signal SA showing the period T C . Here, T A < T B < T C ≦ T 1 . The period T C is equal to a one-pixel scanning period T 1 in the present embodiments. The drive signal SC is supplied to the laser diode 62 of the LSU 6, thus driving the laser diode 62. When driven for the shortest period T A , the laser diode 62 emits a laser beam for this period T A , whereby a pixel represented by the signal SB is formed on the drum 7, which is small and circumscribed by the one-pixel region as is shown at (a), (b) or (c) in Fig. 23. When driven for the longer period T B , the laser diode 62 emits a laser beam for the period T B , whereby a pixel represented by the signal SB is formed on the drum 7, which has a diameter longer than the side of the one-pixel region but not long enough to circumscribe the region, as is shown at (d), (e) and (f) in Fig. 23. When driven for the longest period T C , the laser diode 62 emits a laser beam for this period T C , whereby a pixel is formed on the drum 7, which is large, circumscribing the one-pixel region, as is shown at (g), (h) or (i) in Fig. 23. In Fig. 23, each square indicates a one-pixel region, and any shaded square indicates a one pixel region in which a black pixel is printed if any one of the three possible sizes. To print a quasi-gray image or complex characters, which are defined by discrete black pixels, the laser diode 62 is repeatedly driven, each time for the period T A or T B and emitting a laser beam for this period, whereby discrete black pixels, each circumscribed by the one-pixel region or having a diameter longer than the side of the one-pixel region but not long enough to circumscribe the region, are formed ultimately on a printing medium. The black pixels, thus formed, define a clear-cut quasi-gray image or distinct characters. On the other hand, to print a true black image, which is defined by overlapping black pixels, the laser diode 62 is repeatedly driven, each time for the period T C (i.e., one-pixel scanning period T 1 ) and emitting for the period T C , whereby black pixels, each circumscribing the one-pixel region and overlapping another, are formed ultimately on a printing medium. The black pixels, thus formed, define a true black image. As has been described, the laser diode 62 is driven for the period T C and forms a large pixel when six or more adjacent pixels are black ones, and is driven for the shorter period T A or T B and forms a smaller pixel when five or less adjacent pixels are black ones. Hence, there can be printed a clearly outlined, true black image, as can be understood from Fig. 24. Fifth Embodiment Fig. 25 illustrates a laser printer which is a fifth embodiment of the invention and which is similar to the laser printer shown in Fig. 4. Some of the components of this printer, which are identical to those shown in Fig. 4 are designated at the same numerals in Fig. 25 and will not be described in detail. As is evident from Fig. 25, the laser printer comprises an interface section 1, a control section 2, an image memory 3, a laser scanner unit (LSU) 6, a photosensitive drum 7, two shift registers 81a and 81b, an emission-time setting section 81, a 2-input AND gate 84, and 3-input AND gate 84. As in the laser beam shown in Fig. 4, various devices (not shown), such as a drum-charging device, an image-developing device, an image-transferring device, an image-fixing device, a drum-cleaning device -- all being of the known types -- are located around the photosensitive drum 7. The shift registers 81a and 81b are each an n-bit shift register, where n is the number of pixels which form a one-line image. They shift the input data, bit by bit, in response to the transfer-clock pulses, i.e., the pulses forming the image data supplied from the control section 2. These registers 81a and 81b are connected in series. Hence, the output of the register 81a is input to the register 81b. Every time the register 81a shifts the one-line image data by one bit, it outputs the first seven bits it holds, i.e., bits Q A to Q G . Of these seven bits, the bits Q A , Q B , Q C , Q E , Q F and Q G are supplied to the emission-time setting section 82, while the bit Q D is supplied as a pixel signal SB to the first input terminal of the AND gate 83. On the other hand, every time the register 81b shifts the one-image data by one bit, it outputs three consecutive bits Q H , Q I and Q J which are at the same places as the bits Q C , Q D and Q E , respectively. These three bits Q H , Q I and Q J are supplied to the three input terminals of the AND gate 84, respectively. The bit Q X output by the AND gate 84 is supplied to the emission-time setting section 82. The seven bits Q A to Q G output by the shift register 81a, and the three bits Q H , Q I and Q J output by the shift register 81b represent ten pixels A to J have the positional relation specified in Fig. 26. The emission-time setting section 305 has a ROM (not shown) which stores a data table showing various 10-bit patterns and seven periods T A to T G . It detects one of the various patterns which the 10 bits Q A to Q J define when the bit Q D represents a black pixel, in accordance with the values of the bits Q A , Q B , Q C , Q E , Q F and Q G supplied from the register 81a and the bit Q X supplied from the AND gate 84. Then, it outputs an emission-time signal SD representing the period of time which is determined from the values of these input seven bits and for which the LSU 6 is driven to print the pixel represented by the bit Q D . The signal SD is supplied to the second input terminal of the AND gate 83. The AND gate 83 obtains the logic product of the pixel signal SB (i.e., the bit Q D ) output from the shift register 81a and the signal SD output from the emission-time setting section 82, and output a signal representing this logic product. The signal output by the AND gate 83 is supplied to the drive section 67 incorporated in the LSU 6. As in the laser beam shown in Fig. 4, various devices (not shown), such as a drum-charging device, an image-developing device, an image-transferring device, an image-fixing device, a drum-cleaning device -- all being of the known types -- are located around the photosensitive drum 7. The operation of the laser printer shown in Fig. 25 will now be explained, with reference to Figs. 27 and 28. At first, the shift registers 81a and 82b are empty. Then, the image data is supplied to the shift register 81a, and hence to the shift register 81b. Thus, the image data is shifted, bit by bit, through either register under the control of the control section 2. As has been described, the shift registers 81a and 18b are each an n-bit register, capable of storing a one-line data, and are connected in series. Hence, when the shift register 81a receives the first bit of the second one-line data, the shift register 81b starts receiving the first one-line data. The drive section 67 incorporated in the LSU 6 drives the laser diode 62 incorporated in the LSU 6 in response to the pixel signal SB, i.e., the bit Q D , supplied from the shift register 81a through the AND gate 83. Meanwhile, the emission-time setting section 82 receives six bits Q A to Q C and Q E to Q G output by the shift register 81a and the Q X output by the shift register 81b, detects the 10-pixel pattern from the values of the seven input bits, and outputs an emission-time signal SD which represents of one of the seven periods T A to T G stored in the ROM. The signal SD is supplied to the drive section 67. More specifically, the ROM of the emiss ion-time setting section 82 stores the data table shown in Fig. 27. This table shows the relation between the number of consecutive black pixels, always including the black pixel represented by the bit Q D , on the one hand, and the period of time for which the drive section 67 will drive the laser diode 62 to print the black pixel represented by the bit Q D . As can be understood form Fig. 27, when only one pixel (i.e., the one represented by the bit Q D ) is black, the bits Q A to Q C and Q E to Q G output by the register 81a can form only one 6-bit pattern "000000." When two consecutive pixels are black, the bits Q A to Q C and Q E to Q G can form two 6-bit patterns "000100" and "001000." When three consecutive pixels are black, the bits Q A to Q C and Q E to Q G can form three 6-bit patterns "000110," "001100," and "011000." When four consecutive pixels are black, the bits Q A to Q C and Q E to Q G can form four 6-bit patterns "bits are "000111," "001110," "011100," and "111000." When five consecutive pixels are black, the bits Q A to Q C and Q E to Q G can form three 6-bit patterns "001111," "011110," and "111100." When six consecutive pixels are black, the bits Q A to Q C and Q E to Q G can form two 6-bit patterns "011111" and "111110." When seven consecutive pixels are black, the bits Q A to Q C and Q E to Q G can form only one 6-bit pattern "111111." When the section 82 detects the 6-bit pattern showing there is only one black pixel, it outputs an emission-time signal SD representing a beam-emission period T A , whether the bit Q X is "1" or "0." When the section 82 detects a 6-bit pattern showing there are two consecutive black pixels, it generates an emission-time signal SD which represents a beam-emission period T B , whether the bit Q X is "1" or "0." When the section 82 detects a 6-bit pattern showing there are three consecutive black pixels, it generates an emission-time signal SD which represents a beam-emission period T C , whether the bit Q X is "1" or "0." When the section 82 detects a 6-bit pattern showing there are four consecutive black pixels, it generates an emission-time signal SD which represents a beam-emission period T D , whether the bit Q X is "1" or "0." When the section 82 detects a 6-bit pattern showing there are five consecutive black pixels, it generates an emission-time signal SD which represents a beam-emission period T E if the bit Q X is "1" or a beam-emission period T D if the bit Q X is "0." When the section 82 detects a 6-bit pattern showing there are six consecutive black pixels, it generates an emission-time signal SD which represents a beam-emission period T F if the bit Q X is "1" or a beam-emission period T D if the bit Q X is "0." When the section 82 detects a 6-bit pattern showing there are seven consecutive black pixels, it generates an emission-time signal SD which represents a beam-emission period T G if the bit Q X is "1" or a beam-emission period T D if the bit Q X is "0." As is shown in the diagram shown in Fig. 28, T A < T B < T C < T D < T E < T F < T G , and T G = T1. The emission-time signal SD output by the section 82 is supplied to the second input terminal Of the AND gate 83, to which the pixel signal SB (i.e., the bit Q D ) is also supplied. Hence, the AND gate 83 is turned on for the period of time represented by the emission-time signal SD and outputs a signal for this period of time. The output signal of the AND gate 83 is supplied to the drive section 67. The drive section 67 outputs a drive signal SC which is at a high level for the period represented by the output signal of the AND gate 83, whereby the laser diode 62 keeps emitting a laser beam for this period of time. As can be understood from Figs. 26, 27 and 28, the period represented by the emission-time signal SD is proportional to the number of consecutive black pixels arranged in the main-scanning direction, provided that there are at most four consecutive black pixels including the pixel represented by the bit Q D . When there are five to seven consecutive black pixels arranged in the main-scanning direction, the signal SD represents a period which is proportional to the number of consecutive black pixels if the three pixels represented by the bits Q H , Q I and Q J and located above the three pixels represented by the bits Q C , Q D and Q E , respectively, are all black, and represents the same period, i.e., the period T D , if the three pixels represented by the bits Q H , Q I and Q J are not all black. Obviously, the longer the period the drive section 67 drives the laser diode 62, the larger the black pixel printed, which is represented by the bit Q D . To print a quasi-gray image or complex characters, which are defined by discrete black pixels, the laser diode 62 is repeatedly driven, each time for the period T A or T B and emitting a laser beam for this period, whereby discrete black pixels, each circumscribed by the one-pixel region or having a diameter longer than the side of the one-pixel region but not long enough to circumscribe the region, are formed ultimately on a printing medium. The black pixels, thus formed, define clear-cut quasi-gray image or distinct characters. On the other hand, to print a true black image, which is defined by overlapping black pixels, the laser diode 62 is repeatedly driven, each time for the period T C (i.e., one-pixel scanning period T 1 ) and emitting for the period T C , whereby black pixels, each circumscribing the one-pixel region and overlapping another, are formed ultimately on a printing medium. The black pixels, thus formed, define a true black image. The present invention is not limited to the embodiments described above. The number and arrangement of pixels, the pattern of which is detected, are not limited to those adopted in the embodiments described above. Further, these pixels need not be consecutive; they can be spaced away from one another. Also, the embodiments can easily be modified into laser printers of the positive developing type, merely by inverting logic with respect to the values of bits representing pixels. Other various changes and modifications can be made, without departing the scope of the present invention.

Claims: A printer having pixel-forming means (6) for printing pixels in accordance with image data consisting of an arrangement of pixel data items representing black pixels and white pixels, thereby to print on a printing medium an image represented by the image data, and pixel-size setting means (5) for setting the energy for the printing of each black pixel by the pixel-forming means at a value which depends on the values of the pixels in at least one line of pixels including the said black pixel, the line or lines being parallel, perpendicular and/or oblique to the scanning direction of printing; in which the printer has a photosensitive body (7) and the pixel-forming means has exposure means (6) for applying light thereon electrophotographically to print the image, the printer has pixel formation control means (67) for causing said exposure means (6) to apply light at the energy set by said pixel-size setting means (5) onto said photosensitive body (7), and the pixel-size setting means (5) determines the said energy as a predetermined function whose value increases with the number of consecutive black pixels in said line or lines. A printer according to Claim 1, in which said function is proportionate to the number of consecutive black pixels, including the black pixel, in the or each line. A printer according to Claim 1, wherein said pixel formation control means (67) sets the period during which said exposure means (6) applies light, thereby to set the energy for printing the black pixel. A printer according to Claim 1, wherein said pixel formation control means (67) sets the amount of light which said exposure means (6) applies, thereby to set the energy for printing the black pixel. A printer according to Claim 1, in which the pixel-size setting means sets the value in accordance with the values of pixels in only one line, the line being parallel to the scanning direction. A printer according to Claim 1, in which the pixel-size setting means sets the value in accordance with the values of pixels in only one line, the line being perpendicular to the scanning direction. A printer according to Claim 1, in which the pixel-size setting means sets the value in accordance with the values of pixels in only two lines, the lines being parallel and perpendicular to the scanning direction. A printer according to Claim 1, in which the pixel-size setting means sets the value in accordance with the values of pixels in only four lines, the lines being parallel and perpendicular and oblique to the scanning direction. A method of printing on a medium an image from image pixel data representing black and white pixels, including setting the energy for the printing of each black pixel at a value which depends on the values of the pixels in at least one line of pixels including the said black pixel, the line or lines being parallel, perpendicular and/or oblique to the scanning direction of printing; in which the image is formed electrophotographically by applying light onto a photosensitive body (7), and the energy value is set as a predetermined function whose value increases with the number of consecutive black pixels in said line or in each of said lines. A method according to Claim 9, in which the value of the function is proportionate to the number of consecutive black pixels, including the said black pixel, in the or each line. A method according to Claim 9, in which the energy value is set in accordance with the values of pixels in only one line, the line being parallel to the scanning direction. A method according to Claim 9, in which the energy value is set in accordance with the values of pixels in only one line, the line being perpendicular to the scanning direction. A method according to Claim 9, in which the energy value is set in accordance with the values of pixels in only two lines, the lines being parallel and perpendicular to the scanning direction. A method according to Claim 9, in which the energy value is set in accordance with the values of pixels in only four lines, the lines being parallel, perpendicular and oblique to the scanning direction.

Inventors: AIHARA MASAYOSHI, FUKUYAMA HIROTAKA, KINOSHITA HARUKI, MURANO KATSUMI, NAKAMURA TETSUYA, NANNICHI TOSHIHIKO, TERASAKI MASANORI, AIHARA, MASAYOSHI,, FUKUYAMA, HIROTAKA,, KINOSHITA, HARUKI,, MURANO, KATSUMI,, NAKAMURA, TETSUYA,, NANNICHI, TOSHIHIKO,, TERASAKI, MASANORI,, AIHARA, MASAYOSHI, C/O INTELLECTUAL PROPERTY DIV., Fukuyama, Hirotaka, c/o Intellectual Property Div., Kinoshita, Haruki, c/o Intellectual Property Div., Murano, Katsumi, c/o Intellectual Property Div., NAKAMURA, TETSUYA, C/O INTELLECTUAL PROPERTY DIV., Nannichi, Toshihiko, c/o Intellectual Property Div, Terasaki, Masanori, c/o Intellectual Property Div.
Applicants: TOKYO ELECTRIC CO LTD, TOSHIBA KK, KABUSHIKI KAISHA TEC, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: B41J2
IPC codes: G06K15