Abstract:
An optical disk image forming apparatus that forms an image on an optical disk, comprises: an optical disk drive that includes a rotating unit for rotating an optical disk and a laser light irradiating unit being movable in a radial direction of the optical disk; and a control unit that sets a plurality of pixel areas on the optical disk, allocates predetermined writing densities to the plurality of pixel areas, respectively, and controls the optical disk drive to irradiate the laser light onto each pixel area according to the allocated writing densities. When the allocated writing density is a halftone writing density, the control unit corrects the halftone writing density so as to be lighter than the allocated writing density.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to an optical disk recording apparatus that forms images, such as characters or pictures, on a label surface or a recording surface of an optical disk and an optical disk image forming method.  
         [0002]     There is disclosed a technology for an optical disk recording apparatus that forms images, such as characters or pictures, on a data recording surface or a label surface of a recordable optical disk, such as CD-R or CD-RW, on which data can be recorded (for example, see Patent Document 1).  
         [0000]     Patent Document 1: JP-A-2002-367173  
         [0003]     An optical disk recording apparatus disclosed in Patent Document 1 forms one pixel by plural tracks, in which multiple-values of the pixel are expressed by the number and arrangement of the tracks so as to obtain a multi gray-scale levels with binary recording system. When the image is formed on the optical disk with the above method of the optical recording apparatus, however, there is a problem that a viewer recognizes that density (gray scale level) of the original image seems is different from that of the image formed on the optical disk.  
       SUMMARY OF THE INVENTION  
       [0004]     The invention has been made in order to solve the above-described problems, and an object of the invention is to provide an optical disk image forming apparatus and an optical disk image forming method that can form on an optical disk an image in which the visibility of the image is the same as that of an original image.  
         [0005]     In order to solve the above-described problems, the invention has the following configuration. 
    (1) An optical disk image forming apparatus that forms an image constituted by a plurality of pixels on an optical disk by irradiating laser light onto the optical disk including a color changing layer, the color changing layer being capable of changing a characteristic of a color by the laser light, the optical disk image forming apparatus comprising:    
 
         [0007]     an optical disk drive that includes a rotating unit for rotating the optical disk and a laser light irradiating unit being movable in a radial direction of the optical disk; and  
         [0008]     a control unit that virtually sets a plurality of pixel areas on the optical disk, allocates predetermined writing densities to the plurality of pixel areas based on image data representing the image to be formed, respectively, and controls the optical disk drive to irradiate the laser light onto each pixel area according to the allocated writing densities,  
         [0009]     wherein, when the allocated writing density is a halftone writing density, the control unit corrects the halftone writing density so as to be lighter than the allocated writing density. 
    (2) The optical disk image forming apparatus according to (1), wherein the control unit controls the optical disk drive to irradiate the laser light onto each pixel area by a number of times or for an amount of time corresponding to the corrected writing density.     (3) The optical disk image forming apparatus according to (1), wherein a coefficient for correcting the writing density by the control unit is set according to a number of times or an amount of time the laser light passes the pixel area.     (4) The optical disk image forming apparatus according to (1), wherein the control unit defines the pixel area by a central angle and a pixel length which is a length in a radial direction so that the plurality of pixel areas overlap one another at an interval shorter than the pixel length in the radial direction.     (5) The optical disk image forming apparatus according to (1), wherein the control unit controls the laser light irradiating unit to irradiate and vibrate the laser light in the radial direction within the pixel length.     (6) A method of forming an image constituted by a plurality of pixels on an optical disk by irradiating laser light onto the optical disk including a color changing layer, the color changing layer being capable of changing a characteristic of a color by the laser light, the method comprising:    
 
         [0015]     virtually setting a plurality of pixel areas on the optical disk;  
         [0016]     allocating predetermined writing densities to the plurality of pixel areas based on image data representing the image to be formed, respectively;  
         [0017]     correcting, when the allocated writing density is a halftone writing density, the halftone writing density so as to be lighter than the allocated writing density;  
         [0018]     irradiating the laser light onto the optical disk with moving a laser light irradiating unit in a radial direction of the optical disk while a rotating unit rotates the optical disk based on the corrected writing density. 
    (7) The method according to (6) further comprising setting control condition so as to irradiate the laser light onto each pixel area by a number of times or for an amount of time corresponding to the corrected writing density,    
 
         [0020]     wherein the laser light is irradiated on the optical disk based on the control condition.  
         [0021]     With this arrangement, when forming an image on a color changing layer of an optical disk, the optical disk image forming apparatus irradiates laser light onto each pixel area defined as a predetermined central angle and a pixel length as a predetermined length in the radial direction by the number of revolutions or for an amount of time corresponding the writing density so as to form an image of a predetermined density (a plurality of pixels). In this case, when the original image is formed on the optical disk, a viewer recognizes the difference in density (gray scale level) between the original image and the image formed on the optical disk. Accordingly, in order to compensate a variation in density, the optical disk image forming apparatus corrects the halftone writing density so as to be lighter than the allocated writing density. Therefore, it is possible to make visibility of the image formed on the optical disk same as the original image.  
         [0022]     With this configuration, the coefficient for correcting the writing density of the halftone to be lighter than the allocated writing density is set to a value according to the number of times or an amount of time the laser light passes the pixel area. Accordingly, the image density can be suitably corrected and the visibility of the image formed on the optical disk can be made same as the original image.  
         [0023]     With this configuration, each pixel area of the image formed on the optical disk is set so that the plurality of pixel areas overlap one another at an interval shorter than the pixel length in the radial direction. Accordingly, since pixel areas overlap one another, a gap is not generated between pixel areas and it is possible to prevent a radial stripe from being generated in the image formed on the optical disk due to light interference.  
         [0024]     According to the present invention, the writing density of the halftone of the image to be formed on the optical disk is corrected to as to be lighter than the allocated writing density. Therefore, it is possible to make visibility of the image formed on the optical disk same as the original image. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]      FIG. 1  is a block diagram showing the schematic construction of an optical disk image forming apparatus according to an embodiment of the invention.  
         [0026]      FIG. 2  is a diagram showing an example of the irradiation locus of laser light.  
         [0027]      FIGS. 3A and 3B  are diagrams showing a state where pixels of each row overlap pixels of adjacent rows.  
         [0028]      FIG. 4  is a graph showing the relationship of density conversion at each visual characteristic value α.  
         [0029]      FIG. 5  is a graph showing the relationship between the number of revolutions and the visual characteristic value α at a certain thread position.  
         [0030]      FIG. 6  is a table showing a density before correction and a density after correction when an image having 16 gray-scale levels is formed.  
         [0031]      FIG. 7  is a table showing the conversion contents of a data converter.  
         [0032]      FIG. 8  is a diagram showing the state of one pixel whose color is changed by laser light irradiated on the basis of gray-scale data.  
         [0033]      FIG. 9  is a diagram showing a gray-scale display pattern based on a duty ratio. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0034]     An optical disk image forming apparatus according to an embodiment of the invention has an image forming function of forming an image on an optical disk, in addition to an information recording function on an optical disk and an information reading function of the optical disk of an optical disk recording/reproducing apparatus. The optical disk image forming apparatus according to the embodiment of the invention has a function of correcting the gray-scale level of the image formed on the optical disk so as to make the density of the image formed on the optical disk same as the original image, thereby making visibility of the image formed on the optical disk same as the original image. In order to prevent a gap from being generated between rows of an image formed on an optical disk, the optical disk image forming apparatus according to the embodiment of the invention forms an image such that some of pixels of each row overlap pixels of a plurality of adjacent rows.  
         [0035]     First, the configuration of the optical disk image forming apparatus will be described. Since a function of recording information on a recording surface of an optical disk and a function of reading out information recorded on a recording surface of an optical disk are known, the descriptions thereof will be omitted. In the following description, a case where an image is formed on an optical disk having a color changing layer provided at a label surface of, for example, a DVD-R will be described, but the invention is not limited thereto. It is possible to form an image on other kinds of recordable optical disks.  
       Configuration of Optical Disk Image Forming Apparatus  
       [0036]      FIG. 1  is a block diagram showing the schematic configuration of an optical disk image forming apparatus according to an embodiment of the invention. As shown in  FIG. 1 , an optical disk image forming apparatus  10  includes an optical pick-up  100 , a spindle motor  130 , a rotation detector  132 , a RF (Radio Frequency) amplifier  134 , a decoder  136 , a servo circuit  138 , a stepping motor  140 , a motor driver  142 , a PLL (Phase Locked Loop) circuit  144 , a frequency dividing circuit  146 , an interface  150 , a buffer memory  152 , an encoder  154 , a strategy circuit  156 , a frame memory  158 , a data converter  160 , a LPC (Laser Power Control) circuit  162 , a laser driver  164 , and a control unit  170 . The optical disk image forming apparatus  10  is connected to a host computer  300  through the interface  150 .  
         [0037]     In the following description, it is assumed that the optical disk image forming apparatus  10  records information or forms an image on an optical disk  200  in a CAV (Constant Angular Velocity) mode. Other modes, such as a CLV (Constant Linear Velocity) mode or a ZCLV (Zone Constant Linear Velocity) mode, can be applied to the invention.  
         [0038]     The spindle motor  130  rotates the optical disk  200  held by a holding mechanism (not shown).  
         [0039]     The rotation detector  132  outputs a signal FG having a frequency according to a spindle rotation velocity using a counter electromotive current of the spindle motor  130 .  
         [0040]     The optical pick-up  100  has a laser diode (not shown), a plurality of lenses, such as objective lenses, a tracking servo mechanism, a focus servo mechanism, and the like. The optical pick-up  100  irradiates laser light focused by the lenses onto the revolving optical disk  200 .  
         [0041]     The stepping motor  140  rotates to move the optical pick-up  100  in a radial direction of the optical disk  200 .  
         [0042]     The motor driver  142  outputs a driving signal for moving the optical pick-up  100  in a direction and by a distance according to an instruction from the control unit  170  to the stepping motor  140  so as to perform a thread control.  
         [0043]     The RF amplifier  134  amplifies a light receiving signal Rv output from the optical pick-up  100  and outputs the amplified signal to the decoder  136  and the servo circuit  138 .  
         [0044]     When the recording surface of the optical disk  200  is reproduced and information recorded on the optical disk  200  is read out, the light receiving signal Rv output from the optical pick-up  100  is subjected to 8-to-16 modulation. Accordingly, the decoder  136  demodulates the 8-to-16 modulated signal and outputs the demodulated signal to the control unit  170 .  
         [0045]     The servo circuit  138  performs a feedback control (rotation control) such that the rotation velocity of the spindle motor  130  detected by the signal FG becomes an angular velocity according to an instruction from the control unit  170 . Further, the servo circuit  138  performs a tracking control and a focus control for the optical pick-up  100 , in addition to the rotation control.  
         [0046]     The PLL circuit  144  generates a clock signal Dck that is synchronous with the signal FG and has a frequency corresponding to a multiple of the frequency of the signal FG, and supplies the generated clock signal Dck to the control unit  170 .  
         [0047]     The frequency dividing circuit  146  generates a reference signal SFG by frequency-dividing the signal FG by a predetermined number and supplies the reference signal SFG to the control unit  170 .  
         [0048]     Although the detailed configuration of the control unit  170  is not shown, the control unit  170  includes a CPU, a ROM, a RAM, and the like. The control unit  170  operates individual units according to a program stored in the ROM so as to record information on the recording surface of the optical disk  200  or to form an image on the recording surface or the label surface of the optical disk  200 . Further, when an image is formed to the optical disk  200 , the control unit  170  corrects the density (gray-scale level) of each pixel, which will be described below.  
         [0049]     The interface (I/F)  150  is an interface through which the optical disk image forming apparatus  10  receives information such as image (hereinafter referred to original image) supplied from a host computer  300 .  
         [0050]     When information to be recorded on the recording surface of the optical disk  200  (hereinafter, referred to as record data) is supplied from the host computer  300  through the interface  150 , the buffer memory  152  stores the record data in an FIFO (First In, First Out) method.  
         [0051]     The encoder  154  performs the 8-to-16 modulation on the record data read out from the buffer memory  152  and outputs the modulated signal to the strategy circuit  156 .  
         [0052]     The strategy circuit  156  performs a time axis correction processing on the 8-to-16 modulated signal supplied from the encoder  154  and outputs the corrected signal to the laser driver  164 .  
         [0053]     When information of an image to be formed on the optical disk  200  (hereinafter, referred to as image data) is supplied from the host computer  300  through the interface  150 , the frame memory  158  accumulates the image data. The image data is a set of gray-scale data defining the densities of the individual pixels P of the image to be rendered on the disc-shaped optical disk  200 .  
         [0054]     In order to form the image on the optical disk  200 , the data converter  160  converts the intensity of laser light into a signal indicating a write level or a servo level according to the gray-scale data read out from the frame memory  158  and the number of revolutions on the basis of an instruction from the control unit  170 , and outputs the converted data to the laser driver  164 . Here, the write level means intensity enough to change the color of a color changing layer of the optical disk when laser light is irradiated, and the servo level means intensity almost not causing a change in color of the color changing layer of the optical disk even though the laser light is irradiated.  
         [0055]     The laser power control circuit  162  controls the intensity of laser light irradiated from a laser diode (not shown). Specifically, the laser power control circuit  162  controls a current value of a driving signal Li such that the amount of emergent light of the laser diode (not shown) detected by a front monitor diode (not shown) is consistent with a target value of optimum laser power supplied by the control unit  170 .  
         [0056]     Upon information recording, the laser driver  164  generates the driving signal Li, in which the control content of the laser power control circuit  162  reflects, according to modulated data supplied from the strategy circuit  156  and supplies the generated driving signal Li to the laser diode of the optical pick-up  100 . Further, upon image formation, the laser driver  164  generates the driving signal Li, in which the control content of the laser power control circuit  162  reflects, according to the data converted by the data converter  160  and supplies the generated driving signal Li to the laser diode of the optical pick-up  100 . Accordingly, the intensity of the laser beam from the laser diode is subject to the feedback control so as to be consistent with the target value to be supplied from the control unit  170 .  
       Irradiation Locus of Laser Light  
       [0057]      FIG. 2  is a diagram showing an example of the irradiation locus of laser light. In  FIG. 2 , rows and columns are obtained by developing rows and columns virtually arranged on the optical disk in a planar manner. The radial direction of the optical disk refers to rows and a circumferential direction thereof refers to columns. The rows are obtained by dividing the radius and are disposed to have a first width. The columns are disposed to have a width obtained by dividing the circumference by a predetermined number. The column width is expressed by an angle on the optical disk and is expressed by a time during the recording operation. When images, such as characters or pictures, are disposed on the optical disk, gray-scale levels are extracted from the images by associating the images with individual pixel ranges and then are set as image formation data. An image is formed by irradiating a laser beam at high or low intensity according to each gray-scale level of the image formation data. In  FIG. 2 , the vibration center Q of the laser beam is located at the center of the rows.  
         [0058]     When an image having 16 gray-scale levels is formed on the optical disk  20 , the optical disk image forming apparatus  10  forms individual pixels for one row of the image by rotating the optical disk  200  fifteen times such that laser light passes through each pixel fifteen times. Specifically, when a timing at which laser light passes through a reference line is set to zero of a time axis, the control unit  170  instructs the servo circuit  138  to generate, as a tracking signal Tr, a triangular wave signal in which the phase is zero in a first revolution and is subsequently delayed by 2π/15 in a second revolution and later. When the tracking signal Tr is supplied to the optical pick-up  100 , as shown in  FIG. 8 , the irradiation locus of laser light onto the optical disk  200  has an amplitude of approximately 100 μm, and varies from the locus ( 1 ) of the first revolution to the locus ( 15 ) of the fifteenth revolution.  
         [0059]     Here, the minimum movement resolution of the optical pick-up  100  by the stepping motor  140  is approximately 10 μm, and the resolution of a switchable distance of laser power in the circumferential direction is approximately 10 μm. Then, the size of one pixel is set to 100 μm×10 μm. The spot diameter of laser light that is irradiated from the optical pick-up  100  onto the color changing layer of the optical disk  200  is approximately 1 μm.  
         [0060]     In the optical disk image forming apparatus  10 , when an image having 16 gray-scale levels is formed, it is assumed that gray-scale data is 4 bits and gray-scale data of (0000) defines the darkest (deepest) density, and then the formation of a dot of a brighter (lighter) density is instructed in an order of (0001), (0010), (0011), (0100), (0101), (0110), (0111), (1000), (1001), (1010), (1011), (1100), (1101), (1110), and (1111).  
         [0061]      FIGS. 3A and 3B  are diagrams showing a state where the pixels of each row overlap the pixels of adjacent rows.  FIG. 3A  shows one column of the image and  FIG. 3B  shows a state where the pixels of the i-th column are horizontally developed so as not to be overlapped. In  FIG. 3A , for convenience, the irradiation locus of laser light is not shown. As described above with reference to  FIG. 2 , in order to form an image having 16 gray-scale levels on the optical disk  200  along the irradiation locus of laser light, when the pixels corresponding to one row of the image are formed by rotating the optical disk  200  fifteen times, the optical disk image forming apparatus  10  moves the optical pick-up  100  by 10 μm that is the minimum movement resolution of the optical pick-up  100  by the stepping motor  140 . By repeating this operation, the optical disk image forming apparatus  10  forms the image, in which a plurality of adjacent rows partially overlap one another as shown in  FIG. 3A , on the recording surface or the label surface of the optical disk  200 . In the example shown in  FIG. 3A , the pixels of each row overlap the pixels of adjacent rows by 90 μm in the radial direction of the optical disk  200 .  
         [0062]     As such, when an image in which each row partially overlaps a plurality of adjacent rows is formed, it is possible to prevent a gap from being generated between rows of the image formed on the optical disk  200 . As a result, it is possible to prevent a radial stripe from being generated in the image formed on the optical disk  200  due to light interference.  
       Correction of Visual Characteristic  
       [0063]     As described above, when an image is formed by switching the intensity of laser light every revolution, it is possible to form an image with a shade. However, when the image is formed on the optical disk  200  at the density (gray-scale level) of the original image, the density of the image formed on the optical disk  200  looks different from the density of the original image. That is, a halftone portion of the image becomes darker.  
         [0064]     The inventors compare visibility of the original image and visibility of the image formed on the optical disk  200  and confirm (experiment) how to correct the density in order to make both images look the same. Based on the result, the inventors determine values for density correction of the image. This values is hereinafter referred to visual characteristic value, and expressed by  
         [0065]     In the following description, data when an image is formed on the optical disk (DVD-R)  200  is illustrated. However, data for density correction varies according to the kind of a material (dye) used in a recording layer or a color changing layer formed in the optical disk  200 . Therefore, a visual characteristic value varies according to the kind of an optical disk.  
         [0066]      FIG. 4  is a graph showing the relationship of density conversion at each visual characteristic value α.  FIG. 5  is a graph showing the relationship between the number of revolutions at a certain thread position and the visual characteristic value α.  
         [0067]     As the experiment result, the relationship shown in  FIG. 4  is obtained.
 
(Density After Correction)=(Density Before Correction)α(where α is visual characteristic value)  Equation 1
 
         [0068]     From Equation 1, it can be that, when the writing density of a halftone of the image is corrected on the basis of the relationship shown in  FIG. 4  so as to be lighter than the pre-corrected image density, the image formed on the optical disk looks same as the original image. Further, it can be seen found that, when the image is formed on the optical disk, the relationship shown in  FIG. 5  is established between the visual characteristic value α and the number of revolutions when laser light is irradiated onto the same area.  
         [0069]      FIG. 6  is a table showing the density before correction and the density after correction when an image having 16 gray-scale levels is formed. For example, as described above, when 15 revolutions are required to form pixels for one row, that is, when an image having 16 gray-scale levels is formed, it is preferable to set the visual characteristic value α to 1.55, as shown in  FIG. 5 . For example, when gray-scale data is (1010), since the decimal value of the gray-scale data is 10, the density of the original image is ( 10/15)=(0.667), as shown in  FIG. 6 . However, when the image is formed on the optical disk  200  at this density, the formed image looks deeper than the original image. For this reason, the density of the image to be formed is corrected to be lighter than the pre-corrected density at a halftone writing density. On the basis of Equation 1, the density of the image to be formed on the optical disk  200  is preferably corrected to ( 10/15) 1.55 =0.533, as shown in  FIG. 6 . When gray-scale data is (1000), the corrected density is ( 8/15)=0.533. Therefore, when the gray-scale data of the original image is (1010), the gray-scale data is corrected to (1000), thereby making visibility of the image formed on the optical disk  200  same as visibility of the original image.  
         [0070]     As described above, when an image having 16 gray-scale levels is formed on the optical disk  200 , it is preferable to correct other gray-scale data of the original image, as shown in  FIG. 6 . That is, gray-scale data (0000), (0001), (0010), (0011), (0100), (0101), (0110), (0111), (1000), (1001), (1010), (1011), (1100), (1101), and (1111) are corrected to gray-scale data (0000), (0000), (0001), (0001), (0010), (0011), (0100), (0101), (0110), (0111), (1001), (1001), (1011), (1100), (1101), (1110) and (1111), respectively. Therefore, the density (shade) of the image formed on the optical disk  200  looks same as that of the original image.  
         [0071]     When the number of revolutions is changed to form an image having no 16 gray-scale levels, as shown in  FIG. 5 , the visual characteristic value α is corrected to another value according to the number of revolutions, whereby the shade of the image formed on the optical disk  200  looks substantially same as the original image.  
         [0072]     For example, when the number of revolutions when laser light is irradiated onto the same area is 31, that is, when an image 32 gray-scale levels is formed, it is preferable to correct gray-scale data using the visual characteristic value α of 1.72.  
         [0073]     As described above, since the characteristic varies depending on the thickness or the kind of dye used in the color changing layer of the optical disk on which an image is formed, it is required to prepare density correction data for every disk to be used.  
         [0074]     When the movement amount of the optical pick-up  100  by the stepping motor  140  is set to another value, for example, 30 μm whenever the optical disk image forming apparatus  10  forms pixels for one row on the optical disk  200 , visibility of the image formed on the optical disk varies. Therefore, in this case, it is also required to prepare the density correction data.  
       Gray-scale Display  
       [0075]     In the optical disk image forming apparatus  10 , when a mode for forming an image on the optical disk  200  is set, the control unit  170  converts image data read in the frame memory  158  into image data (of a polar coordinate system) recordable on the optical disk  200 . A main control unit  170  of the optical disk image forming apparatus  10  corrects density information of each pixel on the basis of an image formation mode set by a user and the setting of the number of gray-scale levels of the image to be formed on the optical disk.  
         [0076]     When the preparation of the image formation data and the density correction are completed, the control unit  170  outputs the image formation data to the data converter  160 .  
         [0077]     The preparation of the image formation data and the density correction may be performed by the host computer  300 .  
         [0078]     As described above, in order to form the pixels for one row, a control operation may be performed such that the irradiation locus of laser light varies every revolution. In this case, a control operation may be performed on a specified pixel such that the color of the color changing layer of the optical disk  200  is changed by the irradiation of laser light onto the optical disk  200  at a certain revolution time, whereas the color of the color changing layer may not be changed at another revolution time. With this configuration, an area ratio of a discolored portion to a non-discolored portion in the specified pixel is changed, thereby displaying the shade of the image.  
         [0079]     Specifically, in this embodiment, the optical disk  200  is irradiated with laser light by the number of revolutions corresponding to a decimal value of the gray-scale data of the 15 revolutions required in order to forming pixels for one row, so as to change the color of the color changing layer of the optical disk. For example, when the gray-scale data having the corrected density is (0101), the optical disk  200  is irradiated with laser light having intensity capable of changing the color of the color changing layer of the optical disk for only 5 revolutions of the 15 revolutions and thus the color of the portion on the locus of laser light is changed. When the gray-scale data having the corrected density is (0011), the optical disk  200  is irradiated with laser light having intensity capable of changing the color of the color changing layer of the optical disk for only 3 revolutions of the 15 revolutions and thus the color of the portion on the locus of laser light is changed.  
         [0080]      FIG. 7  is a table showing the conversion contents of the data converter. The data converter  160  defines the intensity of laser light for every revolution over the 15 revolutions required for forming the pixels for one row. That is, the data converter  160  converts the gray-scale data read out from the frame memory  158  into any one of a write level (ON data) and a servo level (OFF data) according to the number of revolutions based on the instruction from the main control unit  170 , as shown in  FIG. 7 . For example, when the gray-scale data read out from the frame memory  158  is (0010), the data converter  160  converts the read gray-scale data into the ON data in the first revolution and the second revolution and outputs the converted ON data. Then, the data converter  160  converts the read gray-scale data into the OFF data in the third to fifteenth revolutions and outputs the converted OFF data.  
         [0081]     The write level means intensity enough to change the color of the color changing layer of the optical disk when laser light is irradiated, and the servo level means intensity not causing a change in color of the color changing layer of the optical disk even though laser light is irradiated. The reason why laser light having the intensity of the servo level is output when the color of the color changing layer of the optical disk is not changed is to perform the focus control.  
         [0082]      FIG. 8  is a diagram showing the state of one pixel whose color is changed by laser light irradiated on the basis of the gray-scale data. In each pixel shown in  FIG. 8 , a bold line indicates a path through which laser light of the write level is irradiated, and the change in color actually occurs. Further, a fine line (a dotted line) indicates a path through which laser light of the servo level is irradiated, and the change in color does not actually occurs. When laser light of the write level or the servo level is irradiated every revolution on the basis of the gray-scale data shown in  FIG. 7 , the color of one pixel is changed as shown in  FIG. 8 . In the optical disk image forming apparatus  10 , since the gray-scale data is set for every single pixel, the image having 16 gray-scale levels is formed on the label surface of the optical disk.  
       Gray-scale Display Based on Duty Ratio  
       [0083]     According to the embodiment of the invention, in addition to a function of switching the intensity of laser light irradiated onto each pixel for every revolution as described above so as to give the shape to the image, a function of changing a duty ratio of laser light irradiated onto each pixel is provided. In the optical disk image forming apparatus  10 , for example, 25 duty ratios can be set for one pixel.  
         [0084]      FIG. 9  is a diagram showing a gray-scale display pattern based on the duty ratio. As shown in  FIG. 9 , the optical disk image forming apparatus  10  can equally divide one pixel into 24 parts in order to irradiate light onto the corresponding pixel area for an amount of time corresponding to the writing density, and change the intensity of laser light to the write level or the servo level for each part.  
         [0085]     According to this embodiment of the invention, as shown in  FIG. 9 , as for any gray-scale level, the irradiation pattern of laser light is set such that laser light of the write level (ON data) is irradiated in the vicinity of the center of the pixel, while laser light of the servo level (OFF data) is irradiated in the vicinity of the end of the pixel. Then, it is possible to make the density balance of each pixel good.  
         [0086]     Since approximately 90% of the pixel (the irradiation locus of laser light) overlap a plurality of adjacent rows, as microscopically viewed, it is difficult to distinguish the gray-scale display pattern based on the duty ratio. However, since the size of one pixel is 100 μm×10 μm, as macroscopically viewed, points of different gray-scale levels overlap each other to be slightly shifted from each other. Therefore, in the image actually formed on the optical disk, it is possible to distinguish the gray-scale display pattern without any difficulty.  
         [0087]     As described above, the optical disk image forming apparatus  10  can display the shade of 25 gray-scale levels by changing the intensity ratio (duty ratio) of laser light irradiated onto one pixel.  
         [0088]     As described above, when the intensity level of laser light irradiated onto each pixel is corrected every revolution and the duty ratio of laser light irradiated onto one pixel is changed, it is possible to further increase the number of gray-scale levels of the image according to the number of revolutions. For example, as described above, when one row of an image is formed by rotating the optical disk  200  fifteen times, it is possible to form an image having 400 (=25×15) gray-scale levels. Therefore, in the optical disk image forming apparatus  10  according to the embodiment of the invention, it is possible to form images having, for example, 25 to 400 gray-scale levels on the recording surface or the label surface of the optical disk  200 . Accordingly, it is possible to adjust the density more finely such that the density of the image formed on the optical disk becomes same as that of the original image.  
         [0089]     As described above, when the duty ratio for one pixel is changed to change the gray-scale display of the image, if density correction is similarly performed, it is possible to make visibility of the image formed on the optical disk  200  same as visibility of the original image.