Patent Publication Number: US-10768560-B2

Title: Image forming apparatus and image forming method

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus using an electrophotographic system such as printers including a laser printer and an LED printer, digital copiers, and the like, an image forming method, and a program. 
     Description of the Related Art 
     In conventional image forming apparatuses using an electrophotographic system, there is a technique for controlling a set temperature of a heating apparatus that heats and melts toner on a recording material in accordance with an amount of image data to be printed. Japanese Patent Application Laid-open No. 2016-4231 discloses a method of dividing image data into areas constituted by 32 dots×32 dots or the like and determining a set temperature on the basis of an image data amount of an area with a largest image data amount out of all areas and a print percentage of an entire image. A fixing process is performed by raising the set temperature when a maximum image data amount is large but by lowering the set temperature when the maximum image data amount is small. Accordingly, fixing at an unnecessarily high set temperature with respect to a toner image is avoided in order to reduce power consumption of the heating apparatus. 
     SUMMARY OF THE INVENTION 
     When printing is performed by overlapping toners of a plurality of colors on a recording material as in the case of a color image forming apparatus, even when a sum value of image density of image data is the same, an amount of unfixed toner that is actually laid onto the recording material may differ. Therefore, when a set temperature of a heating apparatus is determined in accordance with a sum value of image density in image data, an excessive amount of heat may be supplied to a recording material and the heating apparatus may end up consuming an excessive amount of power. 
     An object of the present invention is to reduce power consumption by more suitably controlling a set temperature of a heating apparatus in accordance with the number of colors of toners. 
     In order to achieve the object described above, an image forming apparatus including: 
     a fixing portion configured to fix a toner image formed in accordance with image data to a recording material; 
     a determining portion configured to acquire a density value indicating image density represented by the image data for each of colors of toners constituting the toner image, obtain a sum value of the density values with respect to the colors of the toners, obtain a numerical value indicating the number of the density values being values larger than 0 out of the density values corresponding to the colors of the toners, and determine a target temperature for maintaining a temperature of the fixing portion on the basis of the sum value and the numerical value; and 
     a control portion configured to control power to be supplied to the fixing portion so that the temperature of the fixing portion is maintained at the target temperature, wherein 
     in a case the sum value is a first value and the numerical value is a first number, the determining portion determines a first temperature as the target temperature, in a case the sum value is the first value and the numerical value is a second number that is larger than the first number, the determining portion determines a second temperature that is lower than the first temperature as the target temperature. 
     In order to achieve the object described above, an image forming apparatus including: 
     a fixing portion configured to fix a toner image formed in accordance with image data to a recording material; 
     a determining portion configured to acquire a density value indicating image density represented by the image data for each of colors of toners constituting the toner image, calculate a toner bearing amount for the each of the colors of the toners from the density values with respect to the respective colors of the toners, obtain a sum amount of the toner bearing amounts with respect to the respective colors of the toners, and determine a target temperature for maintaining a temperature of the fixing portion on the basis of the sum amount; and 
     a control portion configured to control power to be supplied to the fixing portion so that the temperature of the fixing portion is maintained at the target temperature. 
     In order to achieve the object described above, an image forming method, causing a computer included in an image forming apparatus to perform: 
     a fixing step of fixing a toner image formed in accordance with image data to a recording material using a fixing portion; 
     a determining step of acquiring a density value indicating image density represented by the image data for respective colors of toners constituting the toner image, obtaining a sum value of the density values with respect to the respective colors of the toners, obtaining a numerical value indicating the number of the density values being values larger than 0 out of the density values corresponding to the respective colors of the toners, and determining a target temperature for maintaining a temperature of the fixing portion on the basis of the sum value and the numerical value; and 
     a controlling step of controlling power to be supplied to the fixing portion so that the temperature of the fixing portion is maintained at the target temperature, wherein 
     the determining step includes, in a case the sum value is a first value and the numerical value is a first number, determining a first temperature as the target temperature, in a case the sum value is the first value and the numerical value is a second number that is larger than the first number, determining a second temperature that is lower than the first temperature as the target temperature. 
     In order to achieve the object described above, an image forming method, causing a computer included in an image forming apparatus to perform: 
     a fixing step of fixing a toner image formed in accordance with image data to a recording material using a fixing portion; 
     a determining step of acquiring a density value indicating image density represented by the image data for respective colors of toners constituting the toner image, calculating a toner bearing amount for the respective colors of the toners from the density value with respect to the respective colors of the toners, obtaining a sum amount of the toner bearing amounts with respect to the respective colors of the toners, and determining a target temperature for maintaining a temperature of the fixing portion on the basis of the sum amount; and 
     a controlling step of controlling power to be supplied to the fixing portion so that the temperature of the fixing portion is maintained at the target temperature. 
     According to the present invention, power consumption can be reduced by more suitably controlling a set temperature of a heating apparatus in accordance with the number of colors of toners. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a sectional view of an image forming apparatus according to a first embodiment; 
         FIG. 1B  is a hardware configuration diagram of the image forming apparatus according to the first embodiment; 
         FIG. 1C  is a functional block diagram of a control portion according to the first embodiment; 
         FIGS. 2A and 2B  are sectional views of a heating apparatus according to the first embodiment; 
         FIGS. 3A and 3B  are schematic views showing a configuration of a heater according to the first embodiment; 
         FIG. 4  is a flow chart showing a temperature control method of the heating apparatus according to the first embodiment; 
         FIGS. 5A to 5C  are schematic views for illustrating image data of a recording material; 
         FIGS. 6A to 6E  are diagrams showing a relationship between gradations and image density; 
         FIGS. 7A and 7B  are graphs showing a relationship of a sum toner bearing amount with respect to an image density value; 
         FIG. 8  is a table showing an example of temperature control parameters according to the first embodiment; 
         FIGS. 9A to 9D  are graphs showing a relationship between an image density value and a set temperature T according to the first embodiment; 
         FIG. 10  is a diagram showing an image pattern when performing a comparative experiment; 
         FIGS. 11A to 11C  are tables showing a result of a comparative experiment according to the first embodiment; 
         FIGS. 12A and 12B  are tables illustrating a first modification; 
         FIG. 13  is a table showing an example of temperature control parameters according to the first modification; 
         FIGS. 14A and 14B  are graphs showing a relationship between an image density value and a set temperature T according to a second embodiment; 
         FIG. 15  is a flow chart showing a temperature control method of a heating apparatus according to the second embodiment; 
         FIG. 16  is a graph showing a relationship between an image density value and a toner bearing amount according to the second embodiment; 
         FIG. 17  is a table showing an example of temperature control parameters according to the second embodiment; 
         FIGS. 18A and 18B  are graphs showing a relationship between a maximum toner bearing amount and a set temperature T according to the second embodiment; and 
         FIG. 19  is a table showing a result of a comparative experiment according to the second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, it is to be understood that dimensions, materials, shapes, relative arrangements, and the like of components described in the embodiments are intended to be changed as deemed appropriate in accordance with configurations and various conditions of apparatuses to which the present invention is to be applied and are not intended to limit the scope of the present invention to the embodiments described below. 
     First Embodiment 
     Description of Image Forming Apparatus 
     A configuration of a color image forming apparatus (hereinafter, expressed as an image forming apparatus)  1  according to a first embodiment will be described with reference to  FIG. 1A .  FIG. 1A  is a sectional view of the image forming apparatus  1  according to the first embodiment. The image forming apparatus  1  includes a paper feeding tray  12 , a paper feeding roller  13 , a resist roller pair  14 , and a registration sensor  15 . The image forming apparatus  1  includes an image forming portion constituted by image forming stations  10 Y,  10 M,  10 C, and  10 K for forming toner images of each of the colors yellow (Y), magenta (M), cyan (C), and black (K) on a recording material (a recording medium)  11 . In the first embodiment, the image forming stations  10 Y,  10 M,  10 C, and  10 K are arranged in a single row in a direction intersecting a vertical direction. Each of the image forming stations  10 Y,  10 M,  10 C, and  10 K has a photosensitive drum  22 Y,  22 M,  22 C, or  22 K, an injection charger  23 Y,  23 M,  23 C, or  23 K as primary charging portions, and a scanner portion  24 Y,  24 M,  24 C, or  24 K as exposing portions. In addition, each of the image forming stations  10 Y,  10 M,  10 C, and  10 K has a toner cartridge  25 Y,  25 M,  25 C, or  25 K, developing portions  26 Y,  26 M,  26 C, or  26 K, and a primary transfer roller  27 Y,  27 M,  27 C, or  27 K. The image forming apparatus  1  includes an intermediate transfer belt  28 , a secondary transfer roller  29 , a heating apparatus (a fixing apparatus)  40 , a paper discharge roller pair  61 , a control portion  108 , and a video controller  109 . The video controller  109  receives image data (image information) and print instruction signals transmitted from an external apparatus such as a personal computer. The control portion  108  is connected to the video controller  109  and controls respective portions constituting the image forming apparatus  1  in accordance with instructions from the video controller  109 . 
     The image forming portion forms an electrostatic latent image by exposure light having been lighted on the basis of an exposure time calculated by the control portion  108  as an image processing portion, and develops the electrostatic latent image to form a monochrome toner image. In addition, the image forming portion superimposes monochrome toner images to form a multicolor toner image, and transfers the multicolor toner image onto the recording material  11 . The multicolor toner image on the recording material  11  is fixed to the recording material  11  by the heating apparatus  40 . 
     The photosensitive drums  22 Y,  22 M,  22 C, and  22 K are constructed by applying an organic photoconductive layer on an outer circumference of an aluminum cylinder, and rotate as a driving force of a drive motor (not illustrated) is transmitted thereto. The drive motor rotates the photosensitive drums  22 Y,  22 M,  22 C, and  22 K in a clockwise direction in accordance with an image forming operation. The injection chargers  23 Y,  23 M,  23 C, and  23 K are provided with sleeves  23 YS,  23 MS,  23 CS, and  23 KS respectively corresponding thereto. The injection chargers  23 Y,  23 M,  23 C, and  23 K charge the photosensitive drums  22 Y,  22 M,  22 C, and  22 K. Exposure light is irradiated to the photosensitive drums  22 Y,  22 M,  22 C, and  22 K from the scanner portions  24 Y,  24 M,  24 C, and  24 K to selectively expose surfaces of the photosensitive drums  22 Y,  22 M,  22 C, and  22 K. Accordingly, an electrostatic latent image is formed on the photosensitive drums  22 Y,  22 M,  22 C, and  22 K. 
     The developing portions  26 Y,  26 M,  26 C, and  26 K develop yellow (Y), magenta (M), cyan (C), and black (K) in order to visualize the electrostatic latent images formed on the photosensitive drums  22 Y,  22 M,  22 C, and  22 K. The developing portions  26 Y,  26 M,  26 C, and  26 K are provided with sleeves  26 YS,  26 MS,  26 CS, and  26 KS respectively corresponding thereto. In addition, a power supply (not illustrated) applies a developing bias between the sleeves  26 YS,  26 MS,  26 CS, and  26 KS and the photosensitive drums  22 Y,  22 M,  22 C, and  22 K respectively corresponding thereto. During image formation, the photosensitive drums  22 Y,  22 M,  22 C, and  22 K rotate clockwise, and the developing portions  26 Y,  26 M,  26 C, and  26 K supply toner to the electrostatic latent images formed on the photosensitive drums  22 Y,  22 M,  22 C, and  22 K. Accordingly, a toner image of each color (hereinafter, also referred to as a multicolor toner image) is formed on the photosensitive drums  22 Y,  22 M,  22 C, and  22 K in accordance with image data transmitted from an external apparatus. 
     The intermediate transfer belt  28  is in contact with the photosensitive drums  22 Y,  22 M,  22 C, and  22 K due to a pressing force of the primary transfer rollers  27 Y,  27 M,  27 C, and  27 K. In addition, a power supply (not illustrated) applies a primary transfer bias between the primary transfer rollers  27 Y,  27 M,  27 C, and  27 K and the photosensitive drums  22 Y,  22 M,  22 C, and  22 K respectively corresponding thereto. During image formation, the intermediate transfer belt  28  and the primary transfer rollers  27 Y,  27 M,  27 C, and  27 K rotate so as to follow the photosensitive drums  22 Y,  22 M,  22 C, and  22 K and primarily transfer the toner images on the photosensitive drums  22 Y,  22 M,  22 C, and  22 K onto the intermediate transfer belt  28 . 
     The recording material  11  housed in the paper feeding tray  12  is transported by the paper feeding roller  13  and reaches the resist roller pair  14 . The registration sensor  15  detects a leading end or a trailing end of the recording material  11 . During image formation, the recording material  11  is transported so as coincide with a timing of detection by the registration sensor  15  to a timing where the multicolor toner image on the intermediate transfer belt  28  arrives at the secondary transfer roller  29 . In this manner, the recording material  11  arrives at the secondary transfer roller  29  from the resist roller pair  14  at an appropriate timing. 
     The intermediate transfer belt  28  is sandwiched by a pair of the secondary transfer rollers  29 . Accordingly, a secondary transfer nip portion N 2  as a secondary transfer portion is formed between the intermediate transfer belt  28  and the secondary transfer rollers  29 . In the secondary transfer nip portion N 2 , the secondary transfer rollers  29  come into contact with the intermediate transfer belt  28 , sandwiches and transports the recording material  11 , and transfers the multicolor toner image on the intermediate transfer belt  28  to the recording material  11 . A power supply (not illustrated) applies a secondary transfer bias between the secondary transfer rollers  29  and the intermediate transfer belt  28 . The transport guide  30  is a guiding member for transporting the recording material  11  from the secondary transfer nip portion N 2  to the heating apparatus  40 . 
     The heating apparatus  40  is a fixing portion which sandwiches and transports the recording material  11 , heats and melts a toner image on the recording material  11 , and fixes the toner image to the recording material  11 . The recording material  11  subjected to a fixing process by the heating apparatus  40  is transported to the outside of the image forming apparatus  1  by the paper discharge roller pair  61  and discharged to a paper discharge tray  62 . An image forming operation ends as the recording material  11  is discharged to the paper discharge tray  62 . 
     Hardware Configuration of Image Forming Apparatus 
       FIG. 1B  is a hardware configuration diagram of the image forming apparatus  1  according to the first embodiment. The image forming apparatus  1  includes a CPU  501 , a ROM  502 , a RAM  503 , a bus  504 , an I/O port  505 , a fixing motor drive circuit  506 , a fixing motor  507 , and the heating apparatus  40 . The heating apparatus  40  has a fixing film  41 , a pressure roller  45 , a heater  42 , a thermistor Th, a heater circuit  508 , and a thermistor circuit  509 . In order to drive the pressure roller  45 , the CPU  501  outputs a signal to the fixing motor drive circuit  506  via the bus  504  and the I/O port  505  to drive the fixing motor  507 . The fixing film  41  rotates so as to follow a rotation of the pressure roller  45 . The CPU  501  acquires a temperature detected by the thermistor Th via the bus  504 , the I/O port  505 , and the thermistor circuit  509 . The CPU  501  causes the heater  42  to generate heat via the bus  504 , the I/O port  505 , and the heater circuit  508  in order to perform temperature control. 
     Functional Configuration of Control Portion 
     Next, a functional configuration of the control portion  108  will be described.  FIG. 1C  is a functional block diagram of the control portion  108  according to the first embodiment. As shown in  FIG. 1C , the control portion  108  has a target temperature determining portion  601  and a power control portion  602 . The target temperature determining portion  601  and the power control portion  602  are realized as the CPU  501  shown in  FIG. 1B  executes a program stored in the ROM  502 . The target temperature determining portion  601  determines a target temperature (a set temperature of the heating apparatus  40 ) for maintaining the temperature of the heating apparatus  40 . The power control portion  602  controls power supplied to the heating apparatus  40  so that the temperature of the heating apparatus  40  is maintained at the target temperature. 
     Description of Configuration of Heating Apparatus 
     Next, the heating apparatus  40  will be described with reference to  FIG. 2A . The heating apparatus  40  includes the fixing film  41  as a fixing member, the heater  42  as a heating member that comes into contact with an inner surface of the fixing film  41 , and the pressure roller  45  as a pressing member. The heater  42  is held by a holding member  43  which also has a guiding function for guiding rotation of the fixing film  41 . A stay  44  is a member for applying pressure of a pressure spring (not illustrated) to the holding member  43  toward a side of the pressure roller  45  to form a fixing nip portion N for heating and fixing a toner image on the recording material  11 . For example, the stay  44  is formed by a metal with high rigidity. In this case, total pressure of the pressure spring is 250 N, and a width of the fixing nip portion N in a transport direction of the recording material  11  (hereinafter, expressed as a recording material transport direction) is set to 9.0 mm. The pressure roller  45  receives power from a motor (not illustrated) and rotates clockwise. Due to the rotation of the pressure roller  45 , the fixing film  41  rotates counterclockwise so as to follow the rotation of the pressure roller  45 . The recording material  11  bearing a toner image is heated while being sandwiched and transported in a direction R 1  at the fixing nip portion N to perform a fixing process of the toner image on the recording material  11 . 
     The fixing film  41  has, for example, an outer diameter of 24 mm and has a base layer made of polyimide resin with a thickness of 60 μm, an elastic layer made of a thermally-conductive rubber layer with a thickness of 200 μm on an outer side of the base layer, and a releasing layer made of a PFA tube with a thickness of 20 μm as an outermost layer. In addition, the pressure roller  45  has, for example, an outer diameter of 25 mm and has a steel core with an outer diameter of 19 mm, an elastic layer made of silicone rubber with a thickness of 3 mm, and a releasing layer made of a PFA tube with a thickness of 40 μm as an outermost layer. The thermistor Th as a temperature detecting portion of the heater  42  is installed on a rear surface side of the heater  42 , and the thermistor Th is connected to the control portion  108 . During normal use, a driven rotation of the fixing film  41  starts as a rotation of the pressure roller  45  starts, and an inner surface temperature of the fixing film  41  rises as a temperature of the heater  42  rises. The heater  42  is controlled by the control portion  108  as a temperature control portion and a power control portion, and the set temperature (target temperature) of the heating apparatus  40  is determined and input power to the heater  42  is controlled so that a surface temperature of the fixing film  41  reaches a prescribed temperature. In other words, on the basis of a detected temperature of the thermistor Th, the control portion  108  performs power control of the heater  42  so that the temperature of the heating apparatus  40  (the surface temperature of the fixing film  41 ) is maintained at the set temperature. For example, the heater  42  may be controlled by the control portion  108  by controlling power supplied to the heater  42  in accordance with a signal of the thermistor Th. Due to the heater  42  being controlled in this manner, temperature control of the heating apparatus  40  is performed by holding the temperature inside the fixing nip portion N (a fixing temperature, a heating temperature) during a heating-fixing operation at a desired temperature (a target temperature). In other words, the heater  42  is controlled so that the temperature detected by the thermistor Th is maintained at the set temperature of the heating apparatus  40 . Alternatively, the heater  42  may be controlled so that the temperature detected by the thermistor Th is kept within an allowable range (a prescribed temperature range) of the set temperature of the heating apparatus  40 . 
     The thermistor Th is arranged so as to come into contact with a center position of the heater  42  in a longitudinal direction of the heater  42  and a center position of the heater  42  in a transverse direction of the heater  42 . The longitudinal direction of the heater  42  is a direction perpendicular to the recording material transport direction. The transverse direction of the heater  42  is a direction perpendicular to the longitudinal direction of the heater  42  and coincides with the recording material transport direction. In the first embodiment, as shown in  FIG. 2A , temperature control of the heating apparatus  40  is performed by bringing the thermistor Th as a temperature detecting portion into contact with a rear surface of the heater  42  as a heating portion and controlling the heater  42 . In addition, as shown in  FIG. 2B , by using the thermistor Th as a contactless temperature detecting portion that detects infrared rays or visible light rays, the thermistor Th may be arranged in a state where the heater  42  and the thermistor Th are separated from each other. 
     A configuration of the heater  42  will be described with reference to the schematic views of  FIGS. 3A and 3B .  FIG. 3A  is a sectional view of the heater  42 . An aluminum nitride base material  401  of the heater  42  is constituted by an aluminum nitride substrate that is a ceramic substrate with a thickness of 0.6 mm. For example, a longitudinal width of the aluminum nitride base material  401  is 260 mm and a transverse width (a paper-passing direction) thereof is 9 mm A sliding glass layer  404  with a thickness of 15 μm is provided on a front surface side of the heater  42  which comes into contact with the fixing film  41 . The sliding glass layer  404  comes into contact with the fixing film  41  via a fluorine grease (not illustrated) and exhibits favorable slidability. In addition, a resistance heating layer  402  with a thickness of 10 μm and protective glass  403  with a thickness of 50 μm are provided on a rear surface side of the heater  42 . The resistance heating layer  402  is formed by applying a conductive paste containing a silver-palladium (Ag/Pd) alloy on the aluminum nitride base material  401  by screen printing.  FIG. 3B  is a schematic view of the heater  42  when viewed from the rear surface side of the heater  42 . The resistance heating layer  402  is formed in a band shape along the longitudinal direction of the heater  42 . A dotted line in  FIG. 3B  denotes the protective glass  403 . Due to the protective glass  403  covering the resistance heating layer  402  and a conductive portion  406 , insulation properties of the resistance heating layer  402  and the conductive portion  406  are secured. In addition, in the heater  42 , the resistance heating layer  402  generates heat when electrode portions  405 A and  405 B are energized by an external power supply. In this case, in the longitudinal direction of the heater  42 , a heated region A that is heated by the resistance heating layer  402  is, for example, 220 mm In the first embodiment, power-supply voltage of the external power supply is 120 V and resistance of the heater  42  is set to 10Ω. In order to measure power (to be described later), the external power supply is connected to cables (not illustrated) for feeding power to the electrode portions  405 A and  405 B via a power meter WT310 manufactured by Yokogawa Test &amp; Measurement Corporation. 
     &lt;Description of Temperature Control of Heating Apparatus&gt; 
     Temperature control of the heating apparatus  40  on the basis of an image density value and the number of colors of toners which is a feature of the first embodiment will now be described in detail with reference to the flow chart in  FIG. 4 . In the first embodiment, a method will be described of extracting a maximum sum image density value Dsum_max and a toner coefficient E indicating the number of colors of toners constituting a toner image from image data received by the video controller  109  and reflecting the maximum sum image density value Dsum_max and the toner coefficient E on a set temperature T of the heating apparatus  40 . The maximum sum image density value Dsum_max will be described later. In  FIG. 4 , printing is started as the image forming apparatus  1  receives a print job (S 501 ). The video controller  109  as an image data detecting portion receives image data (S 502 ). The control portion  108  calculates the maximum sum image density value Dsum_max of the recording material  11  to pass through the heating apparatus  40  next from the image data and extracts the toner coefficient E (S 503 ). The toner coefficient is an example of the numerical value. 
     The maximum sum image density value Dsum_max will now be described.  FIG. 5A  is a schematic view for illustrating an image density value of each recording material  11 . The longitudinal direction which is a print surface side of each recording material  11  and which is perpendicular to the recording material transport direction is adopted as X coordinates, the recording material transport direction is adopted as Y coordinates, and a left end of the X coordinates and a distal end of the Y coordinates are adopted as a coordinate origin (0, 0), whereby each pixel on X-Y coordinates at an image resolution of 600 dpi has image density. In this case, image density of 16 gradations can be expressed per pixel. With four pixels in all directions (a total of 16 pixels) on the X-Y coordinates as one pixel block, image density of 256 gradations (gradation data: 0 to 255) can be expressed within one pixel block and the image density is defined as an image density value of 0 to 100%. In other words, an image density value is a density value indicating image density expressed by image data and is a value indicating image density of image data (image information) as a percentage. 
       FIG. 6A  shows image data in a case where image density value: 0% and gradation data: 0, and shows a state where toner is unused.  FIG. 6E  shows image data in a case where image density value: 100% and gradation data: 255, whereby image density value: 100% represents an upper limit value of the image density value of each color and a maximum image density (O.D.) in this case is approximately 1.4 (O.D.) for each color.  FIGS. 6B to 6D  indicate image data in cases where image density value: 25%, 50%, and 75% and gradation data: 63, 127, and 191. The halftones in  FIGS. 6B to 6D  are indicated by numerical values having been linearly interpolated with respect to image density. Image density (O.D.) is a measurement value obtained by measuring an output image from the image forming apparatus  1  according to the first embodiment using X-rite  504  as a spectral densitometer. In the first embodiment, while an image density value has a linear relationship with respect to image density as shown in FIG.  5 B, this is not restrictive and, for example, an image density value may have a linear relationship with respect to color difference (ΔE). In addition, high white paper GF-0081, A4 size, manufactured by Canon Inc. was used as the recording material  11  and a 100% image pattern (30 mm×30 mm) of each color such as that shown in  FIG. 5C  was created at center of the A4-size recording material  11 . Image creation was performed using YMCK color mode of Photoshop CS4 manufactured by Adobe Inc. In addition, a toner bearing amount (toner laid-on level) per unit area on the recording material  11  is approximately 0.45 (mg/cm 2 ) at an image density value of 100% for all colors. This numerical value is a measurement value obtained by performing a weight measurement of unfixed toner when toner is present in an unfixed state on the recording material  11  in a section from the secondary transfer nip portion N 2  to the heating apparatus  40 . 
     The control portion  108  acquires a sum density value Dsum of each point on the X-Y coordinates. The sum density value Dsum is an image density value of each point on the X-Y coordinates. The sum density value Dsum is a sum value of image density values of the four YMCK colors in each pixel block in one page of the recording material  11  and is calculated using expression (1) below.
 
 D sum( x,y )= DY ( x,y )+ DM ( x,y )+ DC ( x,y )+ DK ( x,y )  (1)
 
     In expression (1), DY(x, y), DM(x, y), DC(x, y), and DK(x, y) denote image density values of the respective YMCK colors at each point on the X-Y coordinates. In this manner, the control portion  108  acquires an image density value for each color of toners constituting a toner image and calculates a sum value of image density values (a sum density value Dsum) for each color of toners constituting the toner image. 
     The maximum sum image density value Dsum_max represents a maximum value (a maximum amount) of sum density values Dsum(x, y) of the respective pixel blocks in one page of the recording material  11 . The toner coefficient E represents the number of colors constituting a pixel block indicating a maximum value out of the sum density values Dsum(x, y) of the respective pixel blocks in one page of the recording material  11 . The toner coefficient E is a numerical value indicating the number of image density values that are image density values larger than 0% out of the image density values corresponding to each color of toners constituting a toner image. In addition, in the first embodiment, the video controller  109  adjusts the maximum sum image density value Dsum_max to be within a range of 0% to 300%. 
     Image data includes a plurality of regions (pixel blocks). The control portion  108  determines a prescribed region of which the sum density value Dsum(x, y) is a maximum value out of the plurality of regions of image data. On the basis of the sum density value Dsum(x, y) or, in other words, the maximum sum image density value Dsum_max and the toner coefficient E in the determined prescribed region, the control portion  108  determines the set temperature T using expression (2) below (S 504 ).
 
 T= 200 +D sum_max×0.4 /√E   (2)
 
     In this case, expression (2) is a controlling expression indicating a relationship among the maximum sum image density value Dsum_max, the toner coefficient E indicating the number of colors of toners constituting a toner image, and the set temperature T. Expression (2) is based on a relationship of a toner bearing amount on the recording material  11  with respect to the image density value of each color shown in  FIGS. 7A and 7B . When the image density value of each color of toners constituting a toner image is lower than a reference value (for example, 10%), the control portion  108  does not include the number of colors of toners with respect to the image density values lower than the reference value in the toner coefficient E. In other words, when the image density value of each color of toners constituting a toner image is lower than a reference value (for example, 10%), the control portion  108  obtains the toner coefficient E by excluding the image density values lower than the reference value. Alternatively, when the image density value of each color of toners constituting a toner image is lower than a reference value (for example, 10%), the control portion  108  may include the number of colors of toners with respect to the image density values lower than the reference value in the toner coefficient E. 
       FIG. 7A  is a graph showing a relationship of a toner bearing amount (a bearing amount of unfixed toner) with respect to an image density value DY in the image forming apparatus  1  according to the first embodiment. As shown in  FIG. 7A , the relationship between the image density value DY and the unfixed toner amount on the recording material  11  per unit area is non-linear. Although the image density value is generally linear with respect to optical density (O.D.) or color difference (ΔE) relative to chromaticity of a reference color, the toner bearing amount on the recording material  11  may not be linear with respect to optical density and color difference and may have a non-linear relationship. As shown in  FIG. 7A , in a region where the image density value is small (around 0 to 30%), an increment in the toner bearing amount with respect to the image density value is small. On the other hand, in a region where the image density value is large (around 70 to 100%), an increment in the toner bearing amount with respect to the image density value is large. Tendencies of the image density values DM, DC, and DK with respect to the toner bearing amount are similar to a tendency of the image density value DY shown in  FIG. 7A .  FIG. 7B  is a graph showing a relationship between the maximum sum image density value Dsum_max and a sum toner bearing amount.  FIG. 7B  shows cases where a ratio of the respective colors of toners is (D 1 ) Y:M=1:1, (D 2 ) Y:M:C=1:1:1, and (D 3 ) Y:M:C:K=1:1:1:1. As shown in  FIG. 7B , when the maximum sum image density values Dsum_max of (D 1 ) to (D 3 ) are the same, the larger the number of colors of toners constituting a toner image, the smaller the sum toner bearing amount. 
       FIG. 8  is a table showing an example of temperature control parameters according to the first embodiment.  FIG. 8  shows the maximum sum image density value Dsum_max, an image density value of each YMCK color, a sum toner bearing amount, the toner coefficient E, and set temperatures T and T 0 .  FIG. 8  shows image density values of the respective YMCK colors in cases where the maximum sum image density value Dsum_max is 50%, 100%, 150%, 200%, 250%, and 300%. The set temperature T is the target temperature (control temperature) of the heating apparatus  40  calculated using expression (2) above. The set temperature T 0  will be described later. 
       FIG. 9A  is a graph showing a relationship between the maximum sum image density value Dsum_max and the set temperature T extracted from  FIG. 8 . As shown in  FIG. 9A , while the set temperature T rises as the maximum sum image density value Dsum_max increases, when Dsum_max is the same value, the larger the number of colors of toners constituting a toner image, the lower the set temperature T. A case where the maximum sum image density value Dsum_max as a sum of image density values is 200% (A- 1  to A- 3  inside a bold frame A in  FIG. 8 ) will now be described. In the case of (A- 1 ) in  FIG. 8 , the toner coefficient E as the number of colors of toners constituting a toner image is “2” and the set temperature T is “257° C.”. In the case of (A- 2 ) in  FIG. 8 , the toner coefficient E is “3” and the set temperature T is “246° C.”. In the case of (A- 3 ) in  FIG. 8 , the toner coefficient E is “4” and the set temperature T is “240° C.”. As shown in (A- 1 ) to (A- 3 ) in  FIG. 8 , the larger the toner coefficient E, the lower the set temperature T. When the maximum sum image density value Dsum_max is a prescribed value (for example, “200%”) and the toner coefficient E is a first number (for example, “2”), the control portion  108  determines a first temperature (for example, “257° C.”) as the set temperature T. When the maximum sum image density value Dsum_max is the prescribed value and the toner coefficient E is a second number (for example, “3” or “4”) that is larger than the first number, the control portion  108  determines a second temperature (for example, “246° C.” or “240° C.”) that is lower than the first temperature as the set temperature T. 
       FIG. 9B  is a graph showing a relationship between the sum toner bearing amount and the set temperature T extracted from  FIG. 8 .  FIG. 9B  shows that, even when the number of colors of toners (the toner coefficient E) and the maximum sum image density value Dsum_max differ, a set temperature T in accordance with the sum toner bearing amount can be adjusted. The set temperature T can be adjusted in this manner because using expression (2) above for determining the set temperature T enables the effect of both the maximum sum image density value Dsum_max and the number of colors of toners (the toner coefficient E) with respect to the set temperature T can be sufficiently taken into consideration. 
     Let us now return to the flow chart in  FIG. 4  to continue the description of temperature control of the heating apparatus  40 . The control portion  108  controls power supplied to the heating apparatus  40  so that the temperature of the heating apparatus  40  is maintained at the set temperature T. By passing the recording material  11  through the heating apparatus  40 , unfixed toner is fixed to the recording material  11  (S 505 ). The control portion  108  determines whether or not the recording material  11  is a last recording material  11  in the print job (S 506 ). When the recording material  11  is a last recording material  11 , the print operation is ended (S 507 ). When the recording material  11  is not a last recording material  11 , the job is continued, the process returns to S 502 , and processes of S 502  to S 506  are repeated until the control portion  108  determines that the recording material  11  is the last recording material  11 . In the first embodiment, the temperature control of the heating apparatus  40  is performed according to the flow shown in  FIG. 4 . 
     The following comparative experiment was performed in order to confirm an effect of performing temperature control of the heating apparatus  40  on the basis of the image density value and the number of colors of toners according to the first embodiment. Conditions of the comparative experiment included recording material transportation speed: 300 mm/sec, print speed (throughput): 60 ppm, recording material  11 : OCE Red Label paper (basis weight 80 g/m 2 ), A4 size, manufactured by Canon Inc., and the number of passed sheets: 110 sheets.  FIG. 10  is a diagram showing an image pattern used when performing the comparative experiment. As shown in  FIG. 10 , a high-printing rate image as a pattern B is printed in addition to a low-printing rate halftone image (Bk: 5%) as a pattern A with respect to the recording material  11  used in the comparative experiment. Image creation is performed using YMCK color mode of Photoshop CS4 manufactured by Adobe Inc. The pattern B printed on the recording material  11  varies for each experimental condition. Confirmation of the effect of the comparative experiment is performed by comparing power consumption and fixability of the heating apparatus  40  with respect to 101st to 110th printed sheets. Although the comparative experiment focuses on the 101st to 110th printed sheets after the heating apparatus  40  has been sufficiently warmed up, the effect of the first embodiment is not limited to the 101st to 110th printed sheets. 
       FIGS. 11A to 11C  are tables showing a result of the comparative experiment, and  FIG. 11A  shows an experimental result in a case where temperature control of the heating apparatus  40  was performed on the basis of the image density value and the number of colors of toners according to the first embodiment. In this case, a film surface temperature is a surface temperature of the fixing film  41  which comes into contact with the recording material  11  when the thermistor Th is controlled on the basis of each set temperature T in the 101st to 110th printed sheets. A thermocouple (ST-13E-010-GW1-W) manufactured by Anritsu Meter Co., Ltd. is used to measure the surface temperature of the fixing film  41 . In conditions A to C in  FIGS. 11A to 11C , although the maximum sum image density value Dsum_max is the same, the sum toner bearing amount differs. In the first embodiment, with respect to the conditions A to C, the set temperature T is controlled in accordance with the sum toner bearing amount and the film surface temperature also varies in accordance with the set temperature T. As a result, fixability is favorable (Good) under the conditions A to C and, at the same time, a reduction in power consumption can be achieved under the conditions B and C having a low sum toner bearing amount. 
     Next, a case where the temperature control according to the comparative example is performed will be described. The set temperature T 0  in the temperature control according to the comparative example is obtained by expression (3) below.
 
 T 0=230.5 +D sum_max/8  (3)
 
     In other words, the set temperature T 0  is determined solely based on the maximum sum image density value Dsum_max.  FIG. 9C  is a graph showing a relationship between the maximum sum image density value Dsum_max and the set temperature T 0  extracted from  FIG. 8 .  FIG. 9C  shows that the set temperature T rises in accordance with the maximum sum image density value Dsum_max regardless of the number of colors of toners constituting a toner image. In addition,  FIG. 9D  is a graph showing a relationship between the sum toner bearing amount and the set temperature T 0  extracted from  FIG. 8 .  FIG. 9D  shows that the set temperature T 0  is not appropriately determined when a difference in the sum toner bearing amount is created due to a difference in the number of colors of toners. 
       FIG. 11B  is a table showing an experimental result when performing the temperature control according to a first comparative example. In the first comparative example, temperature control is performed according to the condition A corresponding to a case where the sum toner bearing amount is high and the set temperature T 0  is set to 256° C. In the first comparative example, since temperature control is performed according to the condition A corresponding to a case where the sum toner bearing amount is high, fixability is favorable (Good) under any of the conditions A to C and power consumption is more or less the same under the conditions A to C. Since temperature control is performed at the same set temperature T 0  under the conditions B and C which correspond to a case where the sum toner bearing amount is low, although fixability is secured, excess power is being supplied to the heating apparatus  40 . 
       FIG. 11C  is a table showing an experimental result when performing the temperature control according to a second comparative example. In the second comparative example, temperature control is performed according to the condition C corresponding to a case where the sum toner bearing amount is low and the set temperature T 0  is set to 240° C. Therefore, the set temperature T 0  according to the second comparative example is lower than the set temperature T 0  according to the first comparative example by 16° C. In the second comparative example, since temperature control is performed according to the condition C corresponding to a case where the sum toner bearing amount is low, although a reduction in power consumption is achieved under the conditions A to C, fixability under the conditions A and B has not been secured. 
     In the first embodiment, the set temperature T is determined by extracting the maximum sum image density value Dsum_max and the toner coefficient E from image data. When the maximum sum image density value Dsum_max is a same prescribed value, the larger the toner coefficient (the number of colors), the lower the set temperature T. Accordingly, the set temperature T can be appropriately determined in accordance with an actual toner bearing amount on the recording material  11 . As a result, since excess heat can be prevented from being imparted to the recording material  11 , power consumption can be suppressed and, at the same time, stable fixability can be secured. 
     In addition, in the first embodiment, when an image density value related to a prescribed color is lower than a reference value (for example, 10%), since the toner bearing amount is a minute amount, the prescribed color is not included in the toner coefficient E used to calculate the set temperature T. However, when the toner bearing amount is high despite the image density value being low, the prescribed color may be included in the toner coefficient E, and when the toner bearing amount is low despite the image density value being high, the prescribed color may not be included in the toner coefficient E. For example, the reference value may be changed as deemed appropriate in accordance with properties of the image forming apparatus  1 . 
     In addition, while one image forming station each is arranged in the image forming apparatus  1  with respect to each toner color of four colors (YMCK) in the first embodiment, a plurality of image forming stations may be arranged in the image forming apparatus  1  for one toner color. In other words, at least two of a plurality of image forming stations may form a toner image with toners of a same color. For example, two of four image forming stations may be image forming stations of the K toner color and two of four image forming stations may be image forming stations of the M toner color. When the four image density values are all equal to or higher than the reference value, the toner coefficient E is 4. In other words, when different image forming stations having toner of a same color are arranged in the image forming apparatus  1 , each of the different image forming stations having the toner of a same color is an object of calculation of the toner coefficient. The control portion  108  increases the number of the toner coefficient E in accordance with the number of the plurality of image forming stations that form the toner image with toner of a same color and, on the basis of the maximum sum image density value Dsum_max and the toner coefficient E, determines the set temperature T using expression (2) above. 
     First Modification 
     As a first modification of the first embodiment, a method of changing the set temperature T in stages according to the maximum sum image density value Dsum_max and the toner coefficient E will be described.  FIG. 12A  shows, in stages, a reference temperature T 1  in accordance with the maximum sum image density value Dsum_max and shows that the maximum sum image density value Dsum_max is divided in a prescribed range. In addition,  FIG. 12B  shows, in stages, an adjusted temperature T 2  in accordance with the toner coefficient E and shows that the adjusted temperature T 2  rises as the toner coefficient E increases. The set temperature T according to the first modification is determined by subtracting the adjusted temperature T 2  from the reference temperature T 1  (T=T 1 −T 2 ).  FIG. 13  is a table showing an example of temperature control parameters according to the first modification.  FIG. 13  shows the maximum sum image density value Dsum_max, an image density value of each YMCK color, a sum toner bearing amount, the toner coefficient E, the reference temperature T 1 , the adjusted temperature T 2 , and the set temperature T. 
       FIG. 14A  is a graph showing a relationship between the maximum sum image density value Dsum_max and the set temperature T extracted from  FIG. 13 . As shown in  FIG. 14A , the set temperature T rises in stages in accordance with the maximum sum image density value Dsum_max and the set temperature T drops in stages as the number of colors of toners constituting a toner image increases.  FIG. 14B  is a graph showing a relationship between the sum toner bearing amount and the set temperature T extracted from  FIG. 13 .  FIG. 14B  shows that, even when the number of colors of toners (the toner coefficient E) and the maximum sum image density value Dsum_max differ, a set temperature T in accordance with the sum toner bearing amount can be adjusted. By determining the set temperature T in stages in accordance with the maximum sum image density value Dsum_max or the toner coefficient E as in the first modification, calculation processes can be simplified. A configuration of the first embodiment or the first modification may be selected in accordance with performance of the control portion  108 . 
     Second Embodiment 
     In a second embodiment, a method of deriving the set temperature T which differs from the first embodiment will be described. Otherwise, the configuration of the image forming apparatus  1  and the configuration of the heating apparatus  40  are the same and descriptions thereof will be omitted. 
     Description of Temperature Control of Heating Apparatus 
     Temperature control of the heating apparatus  40  on the basis of toner amount information according to the second embodiment will now be described with reference to the flow chart in  FIG. 15 . In the second embodiment, a method will be described of calculating a maximum sum toner bearing amount Wsum_max representing a largest sum toner amount of the recording material  11  from image data received by the video controller  109  and determining the set temperature T of the heating apparatus  40 . Printing is started as the image forming apparatus  1  receives a print job (S 601 ). The video controller  109  receives image data (S 602 ). The control portion  108  calculates the maximum sum toner bearing amount Wsum_max of the recording material  11  to pass through the heating apparatus  40  next from the image data (S 603 ). The maximum sum toner bearing amount Wsum_max will now be described. Image data of each recording material  11  is similar to contents described with reference to  FIG. 5A  in the first embodiment, and each pixel on X-Y coordinates at an image resolution of 600 dpi has image density. In addition, DY(x, y), DM(x, y), DC(x, y), and DK(x, y) described below are similar to the first embodiment. 
       FIG. 16  is a graph showing a relationship of a toner bearing amount WY on the recording material  11  relative to an image density value DY for the Y color acquired in advance according to the second embodiment. Based on the relationship shown in  FIG. 16 , the toner bearing amount WY can be calculated from the image density value DY using expression (4).
 
 WY= 0.45×(0.958×( DY ) 2 +0.0422 ×DY )  (4)
 
     Next, the control portion  108  acquires a sum toner bearing amount Wsum (a sum amount of toner bearing amounts) of the recording material  11  at each point on the X-Y coordinates. The sum toner bearing amount Wsum is a sum amount of toner bearing amounts of the four YMCK colors in each pixel block in one page of the recording material  11  and is calculated using expression (5) below.
 
 W sum( x,y )= WY ( x,y )+ WM ( x,y )+ WC ( x,y )+ WK ( x,y )  (5)
 
     In expression (5), WY(x, y), WM(x, y), WC(x, y), and WK(x, y) denote toner bearing amounts of the respective YMCK colors on the recording material  11  at each point on the X-Y coordinates. Each of WY(x, y), WM(x, y), WC(x, y), and WK(x, y) is calculated from each of DY(x, y), DM(x, y), DC(x, y), and DK(x, y) using expression (4). In a similar manner to the first embodiment, the control portion  108  acquires an image density value for each color of toners constituting a toner image. The control portion  108  calculates a toner bearing amount of each color of toners constituting a toner image from the image density value for each color of toners constituting the toner image. In this case, since a relationship of the toner bearing amounts WM, WC, and WK on the recording material  11  with respect to image density values DM, DC, and DK in the MCK colors is similar to the relationship of the toner bearing amount WY on the recording material  11  with respect to the image density value DY, the toner bearing amounts WM, WC, and WK can be calculated using expression (4) in a similar manner to the Y color. 
     The maximum sum toner bearing amount Wsum_max represents a maximum value (a maximum amount) of sum toner bearing amounts Wsum(x, y) of the respective pixel blocks in one page of the recording material  11 . A toner image includes a plurality of regions (pixel blocks). The control portion  108  determines a prescribed region of which the sum toner bearing amount Wsum(x, y) is a maximum value out of the plurality of regions of the toner image. On the basis of the sum toner bearing amount Wsum(x, y) or, in other words, the maximum sum toner bearing amount Wsum_max in the determined prescribed region, the control portion  108  determines the set temperature T using expression (6) below (S 604 ).
 
 T= 212.9−(17.994×( W sum_max) 2 −64.066 ×W sum_max)  (6)
 
     When the toner bearing amount of each color of toners constituting a toner image is lower than a reference value, the control portion  108  does not include the toner bearing amount that is lower than the reference value in the maximum sum toner bearing amount Wsum_max. Alternatively, when the toner bearing amount of each color of toners constituting a toner image is lower than a reference value, the control portion  108  may include the toner bearing amount that is lower than the reference value in the maximum sum toner bearing amount Wsum_max. 
       FIG. 17  is a table showing an example of temperature control parameters according to the second embodiment.  FIG. 17  shows the maximum sum image density value Dsum_max, an image density value of each YMCK color, the maximum sum toner bearing amount Wsum_max, the toner coefficient E, and the set temperature T.  FIG. 18A  is a graph showing a relationship between the maximum sum toner bearing amount Wsum_max and the set temperature T extracted from  FIG. 17 .  FIG. 18A  shows that, even when the number of colors of toners (the toner coefficient E) and the image density value of each YMCK color differ, since the set temperature T is determined on the basis of the maximum sum toner bearing amount Wsum_max, the set temperature T in accordance with an unfixed toner amount on the recording material  11  can be adjusted. 
     In addition,  FIG. 18B  is a graph showing, as a reference, a relationship among the set temperature T obtained in the second embodiment, the maximum sum image density value Dsum_max, and the toner coefficient E (the number of colors). As shown in  FIG. 18B , while the set temperature T rises as the maximum sum image density value Dsum_max increases, when Dsum_max is the same value, the larger the number of colors (the toner coefficient E) of toners constituting a toner image, the lower the set temperature T. A case where the maximum sum image density value Dsum_max as a sum of image density values is 200% (B- 1  to B- 3  inside a bold frame B in  FIG. 17 ) will now be described. In the case of (B- 1 ) in  FIG. 17 , the toner coefficient E as the number of colors of toners constituting a toner image is “2” and the set temperature T is “256° C.”. In the case of (B- 2 ) in  FIG. 17 , the toner coefficient E is “3” and the set temperature T is “245° C.”. In the case of (B- 3 ) in  FIG. 17 , the toner coefficient E is “4” and the set temperature T is “239° C.”. As shown in (B- 1 ) to (B- 3 ) in  FIG. 17 , the larger the toner coefficient E, the lower the set temperature T. When the maximum sum image density value Dsum_max is a prescribed value (for example, “200%”) and the toner coefficient E is a first number (for example, “2”), a first temperature (for example, “256° C.”) is determined as the set temperature T. When the maximum sum image density value Dsum_max is the prescribed value and the toner coefficient E is a second number (for example, “3” or “4”) that is larger than the first number, a second temperature (for example, “245° C.” or “239° C.”) that is lower than the first temperature is determined as the set temperature T. 
     Let us now return to the flow chart in  FIG. 15  to continue the description of temperature control of the heating apparatus  40 . The control portion  108  controls power supplied to the heating apparatus  40  so that the temperature of the heating apparatus  40  is maintained at the set temperature T. By passing the recording material  11  through the heating apparatus  40 , unfixed toner is fixed to the recording material  11  (S 605 ). The control portion  108  determines whether or not the recording material  11  is a last recording material  11  in the print job (S 606 ). When the recording material  11  is a last recording material  11 , the print operation is ended (S 607 ). When the recording material  11  is not a last recording material  11 , the job is continued, the process returns to S 602 , and processes of S 602  to S 606  are repeated until the last recording material  11  is processed. In the second embodiment, the temperature control of the heating apparatus  40  is performed according to the flow shown in  FIG. 15 . 
     In addition, while one image forming station each is arranged in the image forming apparatus  1  with respect to each toner color of four colors (YMCK) in the second embodiment, a plurality of image forming stations may be arranged in the image forming apparatus  1  for one toner color. In other words, at least two of a plurality of image forming stations may form a toner image of a same color. When calculating the sum toner bearing amount Wsum using expression (5) above, the control portion  108  calculates the sum toner bearing amount Wsum by multiplying a toner bearing amount of a same color by the number of the plurality of image forming stations that form a toner image using toner of the same color. 
       FIG. 19  is a table showing a result of a comparative experiment performed by a similar method to the first embodiment. Results of comparative examples 1 and 2 as comparison objects are similar to  FIGS. 11B and 11C . In conditions A to C in  FIG. 19 , although the maximum sum image density value Dsum_max is the same, the sum toner bearing amount differs. In the second embodiment, the set temperature T is determined in accordance with the sum toner bearing amount and the film surface temperature also varies in accordance with the set temperature T. As a result, fixability is favorable (Good) and, at the same time, a reduction in power consumption can be achieved under the conditions B and C having a low sum toner bearing amount. 
     In the second embodiment, a toner bearing amount in each pixel block of image data on the recording material  11  is calculated and the set temperature T is determined in accordance with a maximum sum toner bearing amount thereof. On the other hand, in the first embodiment, the set temperature T is determined with respect to each pixel block of image data from a relationship between a maximum sum image density value and a toner coefficient (the number of colors). The first embodiment has an advantage in that the absence of a calculation process of a toner bearing amount enables processing by the CPU to be simplified while the second embodiment enables the set temperature T to be determined in accordance with a toner bearing amount. Therefore, the second embodiment has an advantage in that the set temperature T can be adjusted more accurately in accordance with a pixel block with a high toner bearing amount. Whichever is suitable between the first and second embodiments may be selected in consideration of a calculation load on the CPU and fixing performance that is required of the heating apparatus  40 . 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. This application claims the benefit of Japanese Patent Application No. 2018-242510, filed on Dec. 26, 2018, which is hereby incorporated by reference herein in its entirety.