JP4726211B2 - Image forming apparatus, image forming method, and signal generating apparatus - Google Patents

Description

  The present invention relates to a pulse width modulation signal generating apparatus and an image forming apparatus using the same.

  Conventionally, in an image forming apparatus, high resolution printing is realized by supplying a pulse width modulation signal for turning on / off a laser at high speed to a laser driver (Patent Document 1).

By the way, when the pulse width modulation signal is generated, the pulse width can be changed at an interval shorter than one pixel by comparing the reference wave and each image data for each pixel. When the reference wave is a triangular wave, first, three triangular waves having vertexes on the left side, right side, and center of one pixel are generated. Next, one of the three triangular waves is selected. Then, the image formation position can be adjusted to any one of the left side, the right side, and the center in one pixel based on the comparison result between the selected triangular wave and the image data.
JP 2004-050673 A

  However, if a reference wave is generated at a high frequency in order to generate a high-speed pulse width modulation signal, it is likely to be affected by minute fluctuations such as a power supply voltage. In this case, the reference wave is distorted with respect to the ideal waveform. Due to this distortion, even for the same image data, the width of the pulse width modulation signal varies depending on the difference in image forming position in one pixel. This is not preferable because a pseudo contour is generated in the image.

  Therefore, in order to realize a light quantity and a waveform that can output a stable image, a stable high-frequency pulse width modulation signal can be provided for any of a plurality of image forming positions in one pixel. desirable.

  Therefore, an object of the present invention is to solve at least one of such problems and other problems. Other issues can be understood throughout the specification.

According to the present invention, for example, it is possible to provide an image forming apparatus that is more suitable than the prior art and forms an image by driving a light source with a pulse width modulation signal. The image forming apparatus includes a generating unit that generates position data representing an image forming position of less than one pixel according to input image data. Further, the image forming apparatus includes a changing unit that changes the amount of laser light output from the light source or the width of the pulse width modulation signal to be supplied to the light source according to the image data and the position data. Yes. In particular, the change means uses a storage means for storing a plurality of tables for determining the light amount, a selection means for selecting any one of the tables according to the position data, and the selected table. Determining means for determining the light quantity.

  According to the present invention, in particular, the amount of laser light output from the light source is changed or the width of the pulse width modulation signal to be supplied to the light source is changed according to the image data and the position data. As a result, an image with a stable quality can be supplied regardless of the image forming position in the main scanning direction in one pixel.

  An embodiment of the present invention is shown below. Of course, the individual embodiments described below will be helpful in understanding various concepts such as the superordinate concept, intermediate concept and subordinate concept of the present invention. Further, the technical scope of the present invention is determined by the scope of the claims, and is not limited by the following individual embodiments.

[First Embodiment]
FIG. 1 is a block diagram illustrating an example of a main part of an image forming apparatus including a pulse width modulation signal generation unit according to the first embodiment.

  In FIG. 1, image data 1 is 8-bit data sent from the controller 100. Therefore, the image data 1 can represent 16 levels of density. When the image data 1 is received, the image data converter 2 converts the image data 1 into an image data conversion signal 4 using the internal table 3. The image data conversion signal 4 is a signal for determining the final pulse width. The image data conversion signal 4 is input to the D / A converter 5. The D / A converter 5 outputs an analog signal 6 corresponding to the image data conversion signal 4 to the comparator 12.

  The image position data 7 is 2-bit data output from the controller 100. The image position data 7 is data representing an image forming position of less than one pixel (eg, left, center, right, etc.) in the controller 100 according to the image data 1. In the present embodiment, since three image forming positions are designated, the image position data 7 is 2 bits. Note that the image position data 7 for the target pixel is determined by the controller 100 in accordance with the contents of the adjacent pixel with respect to the target pixel. For example, when the target pixel and the adjacent pixel correspond to the edge of the image, the image position data of the target pixel is determined so that the edge is smoothly expressed.

  The image position data 7 is input to the reference wave generator 8. The reference wave generator 8 generates a plurality of reference waves (reference wave <center> 9a, reference wave <left> 9b, reference wave <right> 9c) by reference wave generating circuits 8a to 8c and the like. The selector 10 selects any one reference wave according to the input image position data 7 and outputs the selected reference wave to the comparator 12. That is, the output reference wave 11 is a signal corresponding to the image position data 7.

  The comparator 12 compares the analog signal 6 with the reference wave 11 and outputs a pulse width modulation signal 13 when the reference wave 11 is larger than the value of the analog signal 6.

  FIG. 2 is a diagram illustrating waveforms of the analog signal 6, the reference wave 11, and the pulse width modulation signal 13 output from the comparator 12 according to the embodiment. In particular, FIG. 2 shows an example of the waveform of each signal when a pulse width modulation signal having a width of 50% of the width corresponding to one pixel is output at three positions. Since each reference wave is generated at a high frequency (eg, 50 MHz or more), even if the power supply voltage fluctuates minutely, it is affected. Therefore, the reference wave is distorted. Note that the distortion method varies depending on the reference wave.

  Therefore, the actual pulse width modulation signal 13 output according to the distorted reference wave has an error with respect to the ideal pulse width modulation signal. Further, the pulse width of the pulse width modulation signal 13 varies depending on the reference wave (image position data). Therefore, in this embodiment, this problem is solved by introducing the laser light quantity determination unit 14 and the like.

  The laser light quantity determination unit 14 receives the image data conversion signal 4 and the image position data 7. The laser light quantity determination unit 14 includes a plurality of tables (for example, a first table 15a, a second table 15b, and a third table 15c) for determining the light quantity according to the image data conversion signal 4. Each table is stored in a storage device such as a memory, for example. The image data conversion signal 4 is converted into laser light amount determination signals 17a to 17c by the tables 15a to 15c, respectively. For example, when the image data conversion signal 4 is a signal having a pulse width corresponding to 70% of one pixel, a signal for setting the light amount to 60% is output by the table 15a. In this case, for example, a signal for setting the light amount to 50% is output by the table 15b. Further, a signal for setting the light amount to 40% is output by the table 15c. These numerical values are only examples. Thus, the relationship between the input signal and the output signal is registered in each table.

  The selector 16 selects one of the laser light amount determination signals 17 a to 17 c according to the input image position data 7 and outputs it as the laser light amount determination signal 17. In other words, it can be said that the laser light quantity determination signal 17 is determined according to the table 15 selected by the image position data 7 and the image data conversion signal 4. The laser light quantity determination signal 17 is output to the D / A converter 18.

  The laser light quantity determination signal 17 is an 8-bit signal, for example. The D / A converter 18 outputs light amount data 19 corresponding to the laser light amount determination signal 17 to the laser driver 20 with reference to the light amount reference signal 21 output from the laser driver 20.

  FIG. 3 is an exemplary circuit diagram of the D / A converter 18 and the laser driver 20 according to the embodiment. The LD 30 in the figure is a laser diode element including a light quantity monitoring photodiode PD and a laser diode LD. The cathode terminal of the laser diode LD and the anode terminal of the photodiode PD are each grounded to GND. The cathode terminal of the photodiode PD is connected to the negative input terminals of the operational amplifiers IC30 and IC31, and further connected to the Vcc power supply via the variable resistor R35.

  Reference power supplies E30 and E31 are connected to the positive input terminals of the operational amplifiers IC30 and IC31, respectively. The output terminal of the operational amplifier IC30 is connected to the positive input terminal of the operational amplifier IC32 and the capacitor C30 via the analog switch SW30. The other end of the capacitor C30 is grounded. The control signal CL0 from the controller 100 is input to the control terminal of the analog switch SW30.

  The output terminal of the operational amplifier IC32 is connected to the base terminal of the PNP transistor Tr30. The emitter terminal of the PNP transistor Tr30 is connected to the negative input terminal of the operational amplifier IC32, and further connected to the Vcc power supply via the R30 resistor. The collector terminal of the PNP transistor Tr30 is connected to the anode terminal of the laser diode LD of the laser diode element LD30. The output terminal of the operational amplifier IC31 is connected to the positive input terminal of the operational amplifier IC33 and the capacitor C31 via the analog switch SW31. The other end of the capacitor C31 is grounded. The control signal CL1 from the controller 100 is input to the control terminal of the analog switch SW31.

  The output terminal of the operational amplifier IC33 is connected to the base terminal of the NPN transistor Tr31. The emitter terminal of the NPN transistor Tr31 is connected to the negative input terminal of the operational amplifier IC33, and is further grounded to GND through the R31 resistor. The collector terminal of the NPN transistor Tr31 is connected to the anode terminal of the laser diode LD via the analog switch SW32. The pulse width modulation signal from the comparator 12 of FIG. 1 is input to the control terminal of the analog switch SW32 via the inverter IC37.

  The emitter terminal of the PNP transistor Tr30 is connected to the plus input terminal of the operational amplifier IC35. The output of the operational amplifier IC35 is connected to the negative input terminal, and further connected to the negative input terminal of the operational amplifier IC35 via the resistor R33. A reference power supply E32 is connected to the positive input terminal of the operational amplifier IC35. The output terminal of the operational amplifier IC35 is connected to the negative input terminal via the resistor R34, and further connected to the Vref_H terminal of the D / A converter 18.

  The Vref_L terminal of the D / A converter 18 is grounded. An 8-bit laser light amount determination signal 17 is input from the laser light amount determination unit 14 to the DATA_IN terminal of the D / A converter 18. The OUTPUT output terminal (light quantity data 19) of the D / A converter 18 is connected to the plus input terminal of the operational amplifier IC36.

  The output terminal of the operational amplifier IC 36 is connected to the base terminal of the NPN transistor Tr32. The emitter terminal of the NPN transistor Tr32 is connected to the negative input terminal of the operational amplifier IC36, and is further grounded to GND through the resistor R32. The collector terminal of the NPN transistor Tr32 is connected to the anode terminal of the laser diode LD.

  The analog switches SW30, SW31, and SW32 are turned on when Hi is input to the control terminal, and are turned off when Lo is input to the control terminal.

  Next, in FIG. 4, the operation will be described for each operation mode. First, when the control signal CL0 is Hi and the pulse width modulation signal is Hi, the auto power control mode (APC mode) is set. At this time, if the laser diode LD is turned off, no current flows through the photodiode PD, so that the voltages input to the negative input terminals of the operational amplifiers IC30 and IC31 rise.

  When the voltage input to the negative input terminal of the operational amplifier IC30 is larger than the reference voltage E30 input to the positive input terminal of the operational amplifier IC30, the output voltage of the operational amplifier IC30 decreases. In the APC mode, the light amount of the laser diode LD can be adjusted by changing the variable resistor R35.

  The analog switch SW30 is turned on because Hi is input to the control terminal, and the output voltage of the operational amplifier IC30 is charged in the capacitor C30. Further, the output voltage of the operational amplifier IC30 is connected to the positive input terminal of the operational amplifier IC32.

The operational amplifier IC32 and the transistors Tr30 and R30 resistors constitute a constant current source. When the resistance value of the resistor R30 is “R30” and the voltage applied to the positive input terminal of the operational amplifier IC32 is “E32”, the lighting current Ion output from the collector of the transistor Tr30 is determined by the following equation:
Ion = (Vcc−E32) / R30.

  Therefore, when the voltage applied to the positive input terminal of the operational amplifier IC32 decreases, the output current from the collector of the transistor Tr30 increases. When the pulse width modulation signal is Hi, the signal inverted by the inverter IC 37 is input to the control terminal of the switch SW32, so that SW32 is turned OFF.

  Also, assuming that the transistor Tr32 is turned off in accordance with the output of the D / A converter 18, the current output from the collector of the transistor Tr30 is input to the anode terminal of the laser diode LD. Thereby, the laser diode LD is turned on.

  When the light quantity of the laser diode LD increases, a current flows through the photodiode PD. As a result, the voltage input to the negative input terminal of the operational amplifier IC30 decreases. When the voltage input to the negative input terminal of the operational amplifier IC30 is smaller than the reference voltage E30 input to the positive input terminal of the operational amplifier IC30, the output voltage of the operational amplifier IC30 increases. When the output voltage of the operational amplifier IC30 increases, the output current of the transistor Tr30 decreases. When the output current of the transistor Tr30 decreases, the light amount of the laser diode LD decreases.

  As described above, the light amount of the laser diode LD is controlled by the light amount determined by the reference voltage E30.

  FIG. 4 is a diagram illustrating exemplary current-light output characteristics of a laser diode. It is assumed that there is almost no light quantity at the threshold current Ith. In the figure, in the APC mode, the input current is controlled to Ion so that the optical output becomes the APC output.

  Next, the bias auto power control mode (bias APC mode) will be described. When the control signal CL0 is Lo, CL1 is Hi, and the pulse width modulation signal is Lo, the image forming apparatus is in the bias APC mode.

  At this time, the analog switch SW30 is turned OFF because Lo is input to the control terminal. The capacitor C30 holds the charged voltage in the APC mode. Since the held voltage of the capacitor C30 is connected to the positive input terminal of the operational amplifier IC32, the same current Ion as in the APC mode is output from the collector of the transistor Tr30.

  When the current output from the collector of the transistor Tr30 is applied to the laser diode LD, the laser diode LD is turned on. Since the current flows through the photodiode PD by turning on the light, the voltage input to each negative input terminal of the operational amplifiers IC30 and IC31 decreases. When the voltage input to the negative input terminal of the operational amplifier IC31 is smaller than the reference voltage E31 input to the positive input terminal of the operational amplifier IC31, the output voltage of the operational amplifier IC31 increases.

The analog switch SW31 is turned on because Hi is input to the control terminal, and the output voltage of the operational amplifier IC31 is charged in the capacitor C31. The output voltage is connected to the positive input terminal of the operational amplifier IC33. The operational amplifier IC33, the transistor Tr31, and the resistor R31 constitute a constant current source. In addition, the resistance value of the resistor R31 is “R31”, and the voltage applied to the positive input terminal of the operational amplifier IC33 is “E33”. In this case, a current Ioff determined by the following equation is input to the collector of the transistor Tr31:
Ioff = E33 / R31.
Further, since the pulse width modulation signal is Lo and the signal inverted by the inverter IC 37 is input to the control terminal of the switch SW32, the switch SW32 is turned on. Therefore, the current supplied from the transistor Tr30 flows to the transistor Tr31 via the switch SW32. For this reason, since the current applied to the laser diode LD is reduced, the amount of light is reduced.

  When the amount of light from the laser diode LD decreases, no current flows through the photodiode PD. Therefore, the voltage input to the negative input terminal of the operational amplifier IC31 is lowered. When the voltage input to the negative input terminal of the operational amplifier IC31 is higher than the reference voltage E31 input to the positive input terminal of the operational amplifier IC31, the output voltage of the operational amplifier IC31 decreases. When the output voltage of the operational amplifier IC31 decreases, the input current of the transistor Tr31 decreases. When the input current of the transistor Tr31 decreases, the current applied to the laser diode LD increases. Therefore, the light quantity of the laser diode LD increases.

  As described above, the light amount of the laser diode LD is controlled by the light amount determined by the reference voltage E31. The light quantity at this time is controlled to be the same as or lower than the light quantity in the APC mode.

  According to FIG. 4, in the bias APC mode, the input current is controlled so that the optical output becomes the bias APC output. That is, the input current is set so as to be lower than the current value of Ion by the amount of current Ioff.

  Next, when the control signal CL0 becomes Lo and CL1 becomes Lo, the image forming apparatus operates in the print mode. At this time, since Lo is input to the control terminal, the analog switches SW30 and SW31 are turned OFF. Capacitor C30 holds the charged voltage in the APC mode. Therefore, the same current Ion as in the APC mode is output from the collector of the transistor Tr30.

  Further, the capacitor C31 holds the charged voltage in the bias APC mode. Therefore, the same current Ioff as in the bias APC mode is input to the collector of the transistor Tr31. When the pulse width modulation signal is Hi, the switch SW32 is turned off. Therefore, the same current Ion as that in the APC mode is applied to the laser diode LD. Therefore, the laser diode LD is lit with the same light amount as in the APC mode.

  When the pulse width modulation signal is Lo, the switch SW32 is turned on. Therefore, the same current as that in the bias APC mode is applied to the laser diode LD. Therefore, the laser diode LD is lit with the same amount of light as in the bias APC mode.

  In FIG. 4, in the printing mode, the optical output becomes an APC output when the pulse width modulation signal becomes Hi, and the optical output becomes a bias APC output when the pulse width modulation signal becomes Lo. The above description relates to the operation of the laser when the light amount data 19 output from the D / A converter 18 is OFF.

  On the other hand, the timing at which the D / A converter 18 operates is only in the image area where the pulse width modulation signal 13 is output. Therefore, the light amount during image formation can be changed by subtracting the current flowing through the laser diode LD in accordance with the light amount data 19 output from the D / A converter 18.

  FIG. 5 is a diagram showing each waveform of the reference wave 11, the pulse width modulation signal 13, the laser light quantity determination signal 17, and the laser beam output from the laser driver 20 when the image data 1 that is half of one pixel is output.

  In the image forming area, the value of the light quantity reference signal 21 input to the D / A converter 18 is 80h. When the laser light quantity determination signal 17 is also 80h, the current value is Ion-Ioff / 2 shown in FIG. Therefore, the amount of laser light output from the laser diode LD is ½ of the APC output in FIG.

  The tables 15a to 15c shown in FIG. 1 are set so as to reduce the influence of distortion of the corresponding reference waves 9a to 9c. Therefore, in any reference wave 9a to 9c, when the same image data 1 is input, the laser light quantity becomes the same integrated light quantity.

  According to FIG. 5, an actual pulse width modulation signal is generated according to the reference wave for each position data. Each reference wave distortion causes an error in the corresponding pulse width.

  In FIG. 5, when the image position is <right>, the actual pulse width is narrowed by 17% with respect to the ideal pulse width. When the image position is <center>, the actual pulse width is narrowed by 30% with respect to the ideal pulse width. Further, when the image position is <left>, the actual pulse width is narrowed by 10% with respect to the ideal pulse width.

On the other hand, the laser light quantity determination signal 17 is reduced by the amount that is reduced from 80h. For example, if the image position is <right>
80h × 0.83 = 6Ah
Only when the laser light quantity determination signal 17 is reduced, and the image position is <center>,
80h × 0.7 = 59h
Only the laser light quantity determination signal 17 is reduced.

Furthermore, if the image position is <left>
80h × 0.9 = 73h
Only the laser light quantity determination signal 17 is reduced.

  As a result, the current subtracted by the transistor Tr32 from the current flowing through the laser diode LD is reduced, and thus the amount of laser light output from the laser diode LD is increased. In addition, since the current flowing through the laser diode LD increases, the image data for each reference wave is corrected. Therefore, according to FIG. 5, the waveform of the laser beam with the light amount corrected is wider than the waveform when the light amount is not corrected. Also, the integrated light quantity becomes equal.

As described above, according to the present embodiment, the quality of the image formed is changed by changing the amount of laser light output from the laser diode LD according to the image data 1 and the image position data 7. Can be stabilized. In particular, even if a triangular wave serving as a reference wave is generated at a high frequency, the influence of fluctuations in the power supply can be reduced.
The high frequency means a frequency that is affected by fluctuations in the power source. For example, although it says 50 MHz or more, this invention is not necessarily limited to this. Even if the frequency is less than 50 MHz, if the frequency is affected by fluctuations in the power supply, the frequency also belongs to “high frequency”.

  For example, by selecting a plurality of tables 15a to 15c for determining the amount of light according to the image position data 7, the amount of light is determined so that the distortion of the waveform of the output laser light is reduced.

[Second Embodiment]
FIG. 6 is a block diagram illustrating an example of a main part of an image forming apparatus including a pulse width modulation signal generation unit according to the second embodiment. In addition, about the part which is common in 1st Embodiment, description is abbreviate | omitted by attaching | subjecting the same referential mark.

  Compared with the configuration of FIG. 1, it can be understood that in FIG. 6, first, the laser light quantity determination unit 14 and the D / A converter 18 are omitted. It can also be understood that the internal configuration of the image data conversion unit has been changed. Accordingly, the laser driver will be changed accordingly.

  The image data converter 2 'includes a plurality of tables 3a, 3b and 3c corresponding to a plurality of image positions. For example, the table <center> 3a corresponds to the center image position. The table <left> 3b corresponds to the left image position. The table <right> 3c corresponds to the right image position. The image data 1 input to the image data conversion unit 2 'is converted into image data conversion signals 4a, 4b and 4c by each table.

  The selector 22 selects one of the image data conversion signals 4 a according to the image position data 7 and outputs the image data conversion signal 4. For example, if the image position data is “right”, the image data signal 4c is selected.

  FIG. 7 is a diagram illustrating an example of an image data conversion table according to the embodiment. For example, when the image data 1 is “C0h”, the image data conversion signal 4a that is “F5h” is output from the table <center> 3a. The table <left> 3b outputs an image data conversion signal 4b of “BCh”. Further, the image data conversion signal 4c which becomes “E2h” is output from the table <right> 3c.

  The operations of the D / A converter 5, the reference wave generator 8, and the comparator 12 are as described in the first embodiment. However, the laser driver 20 ′ is different from the first embodiment in that it is driven by the pulse width modulation signal 13.

  FIG. 8 is an exemplary circuit diagram of the laser driver according to the embodiment. As compared with FIG. 3, it can be understood that the circuit related to the D / A converter 18 is omitted.

  FIG. 9 is a diagram illustrating an example of each waveform of the analog signal 6, the reference wave 11, and the pulse width modulation signal 13 output from the comparator 12 according to the second embodiment. Note that the waveform of each signal when the table is not switched (when the data conversion unit 2 in FIG. 1 is used) is as shown in FIG.

  Both FIG. 9 and FIG. 2 are based on the premise that a pulse width modulation signal having a width of 50% is output for each pixel width. Since each reference wave 9a thru | or 9c is produced | generated by high frequency as mentioned above, even if the fluctuation | variation of a power supply voltage is minute, it will receive the influence. Therefore, the waveform is distorted. Further, the distortion method is different for each reference wave.

  As described in the first embodiment, when the table is not switched according to the image position data, the actual pulse width modulation signal has an error with respect to the ideal pulse width modulation signal (FIG. 2). . Further, for each reference wave (image position data), a pulse width modulation signal 13 having a different width is output to the laser driver 20 '.

  On the other hand, when the tables 3a to 3c are switched according to the image position data 7 as in the second embodiment, the signal level of the analog signal 6 is also changed according to the image position.

  The signal level of the analog signal 6 is, for example, 80h × 1.17 = 95h when the image position is <right>. When the image position is <center>, the signal level of the analog signal 6 is 80h × 1.3 = A6h. Further, when the image position is <left>, the signal level of the analog signal 6 is 80h × 1.1 = 8Ch. That is, the tables 3a to 3c are set so that such values are output.

  As a result, the pulse width modulation signal 13 becomes an ideal signal as shown in FIG. That is, for the same image data 1, even if the image forming position data 7 is different, the pulse width modulation signal 13 having the same width is output.

  As described above, according to this embodiment, it is formed by suitably changing the width of the pulse width modulation signal supplied to the laser diode LD according to the image data 1 and the image position data 7. Image quality can be stabilized. In particular, even if a triangular wave serving as a reference wave is generated at a high frequency, the influence of fluctuations in the power supply can be reduced.

  For example, the width of the pulse width modulation signal is suitably determined by selecting a plurality of tables 3a to 3c for determining the width of the pulse width modulation signal in accordance with the position data.

[Other Embodiments]
The image forming apparatus according to the above-described embodiment can be realized as a printer, a copier, a facsimile machine, a multifunction machine, or the like. In addition, the part of the image forming apparatus that generates the pulse width modulation signal can be used in an apparatus other than the image forming apparatus.

1 is a block diagram illustrating an example of a main part of an image forming apparatus including a pulse width modulation signal generation unit according to a first embodiment. It is a figure which shows each waveform of the pulse width modulation signal 13 output from the analog signal 6, the reference wave 11, and the comparator 12 as a comparative example. 1 is an exemplary circuit diagram of a D / A converter 18 and a laser driver 20 according to a first embodiment. It is the figure which showed the example electric current-light output characteristic of a laser diode. It is a figure which shows each waveform of the laser beam output from the reference wave 11, the pulse width modulation signal 13, the laser light quantity determination signal 17, and the laser driver 20 which concern on 1st Embodiment. It is a block diagram which shows an example of the principal part of the image forming apparatus containing the production | generation part of the pulse width modulation signal which concerns on 2nd Embodiment. It is a figure which shows an example of the image data conversion table which concerns on 2nd Embodiment. It is an exemplary circuit diagram of the laser driver concerning a 2nd embodiment. It is a figure which shows an example of each waveform of the pulse width modulation signal 13 output from the analog signal 6, the reference wave 11, and the comparator 12 which concern on 2nd Embodiment.

Claims (3)

An image forming apparatus for forming an image by driving a light source with a pulse width modulation signal,
Generating means for generating position data representing an image forming position of less than one pixel according to the input image data;
And the image data, in response to said position data, look-containing and changing means for changing the width of the pulse width modulation signal to be supplied to the light quantity or the light source of the laser light output from said light source,
The changing means is
Storage means for storing a plurality of tables for determining the light amount;
Selecting means for selecting any of the tables according to the position data;
Determining means for determining the amount of light using the selected table;
An image forming apparatus comprising containing Mukoto a.   A triangular wave generating means for generating a plurality of triangular waves each having a different shape;
  Selecting means for selecting a triangular wave corresponding to the position data among the plurality of triangular waves;
Further comprising
  The image forming apparatus according to claim 1, wherein the changing unit generates the pulse width modulation signal using a triangular wave selected by the selecting unit. An image forming method for forming an image by driving a light source with a pulse width modulation signal,
Generating position data representing an image formation position of less than one pixel according to the input image data;
And the image data, in response to said position data, look-containing and changing the width of the pulse width modulation signal to be supplied to the light quantity or the light source of the laser light output from said light source,
The changing step includes
Selecting any one of the plurality of tables for determining the amount of light according to the position data;
An image forming method comprising the step of determining the amount of light using the selected table .

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