Patent Publication Number: US-9842548-B2

Title: Device for controlling display device, method of controlling display device, display device, and electronic apparatus

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to devices for controlling a display device, methods for controlling a display device, display devices, and electronic apparatuses. 
     2. Related Art 
     An electro-optic device described in JP-A-2004-085606 uses colored electrophoretic particles. When an electric field is applied to the electrophoretic particles in the dispersion medium, the colored electrophoretic particles are moved thereby displaying an image. In an electro-optic device that uses dispersion medium and electrophoretic particles, the viscosity of the dispersion medium changes depending on the temperature. Therefore, such an electro-optic device has a characteristic that desired gray levels might not be obtained depending on the temperature even when the same electric filed is applied to act on the electrophoretic particles. Therefore, even when displaying the same gray level, the electro-optic device may change the electric field to be applied depending on the temperature in order to control the amount of the electrophoretic particles to be moved, so that the desired gray level can be displayed even when the temperature changes. 
     Though the electro-optic device described in JP-A-2004-085606 controls the drive voltage depending on the temperature to control the gray level, electrophoretic particles may move greatly with even a small change in the drive voltage when the temperature is high and thus the viscosity of the dispersion medium is low. Accordingly, in order to control the gray level in a high temperature state, a circuit capable of continuously and highly accurately controlling the drive voltage is necessary. Also, a drive voltage suitable for displaying a certain gray level changes depending on the temperature, and a greater voltage needs to be applied as the temperature lowers. Therefore, it is necessary to have a circuit that is capable of controlling the drive voltage continuously from a low voltage to a high voltage depending on the temperature. 
     SUMMARY 
     In accordance with some aspects of the invention, kinds of voltages to be applied to pixels are reduced, and gray levels corresponding to the characteristic of the device can be displayed. 
     An embodiment of the invention pertains to a control device for a display device that includes a plurality of first electrodes respectively provided for pixels, a second electrode disposed opposite the plurality of first electrodes, and an electro-optical material disposed between the plurality of first electrodes and the second electrode, the control device including an image acquisition part that designates a gray level value of each of the pixels, and acquires first image data with the number of gray levels being a-gray levels; a parameter acquisition part that acquires a parameter that decides the number of gray levels displayed by the pixel; a color reduction part that decides a number of gray levels to be displayed by the pixel according to the parameter acquired by the parameter acquisition part, and generates second image data in which the first image data acquired in the image acquisition part is color-reduced to the number of gray levels decided; and a write part that changes the gray level of the pixel to a gray level of the gray level value designated by the second image data generated in the color reduction part. In an aspect, the write part performs a first write operation of applying a first voltage to the first electrode of the pixel a plurality of times, when changing the gray level of the pixel in a direction from the second gray level to the first gray level, and performs a second write operation of applying a second voltage having a polarity different from the first voltage to the first electrode of the pixel a plurality of times, when changing the gray level of the pixel in a direction from the first gray level to the second gray level. 
     According to this configuration, the number of gray levels to be displayed at the pixel is decided according to the acquired parameter, which corresponds to the characteristic of the device, and the first image data that designates the gray level of the pixel is color-reduced to the number of gray levels decided, whereby the second image data is generated. Also, the first voltage or the second voltage is applied to the pixel multiple times such that the gray level of the pixel is changed to a gray level value designated by the second image data. As an image that has been color-reduced according to the characteristic of the device is displayed through application of the first voltage or the second voltage, the gray level corresponding to the characteristic of the device can be displayed with fewer kinds of voltages (e.g., two kinds of voltages in this example). 
     In the control device, the parameter acquisition part may acquire data indicative of the temperature as the parameter, and the color reduction part may decide the number of gray levels according to the data acquired by the parameter acquisition part, and may reduce the number of gray levels with an increase in the temperature. According to this configuration, when the gray level display characteristic of the pixel of the display device changes according to the temperature, the number of gray levels displayed by the pixel is decided in proportion to the temperature indicated by the parameter acquired, such that the gray level display corresponding to the characteristic of the device can be performed. 
     In the control device, the parameter acquisition part may acquire data indicative of the number of gray levels to be displayed by the pixel as the parameter, and the color reduction part may decide the number of gray levels to be displayed by the pixel to be the number of gray levels indicated by the data acquired by the parameter acquisition part. According to this configuration, the number of gray levels to be displayed by the pixel is decided to be the number of gray levels designated by the parameter, such that the gray level can be display according to the characteristic of the device. 
     In the control device, the color reduction part may have a dither matrix of arrayed threshold values. When color-reduction to two gray levels is performed, the color reduction part may generate the second image data with two gray levels according to a comparison result between the gray level values designated for each of the pixels by the first image data and the threshold values in the dither matrix respectively corresponding to the pixels. When color-reduction to n-gray levels of three or more gray levels is performed, the color reduction part may generate (n−1) threshold values for color-reduction to n-gray levels from the threshold values in the dither matrix corresponding to the respective pixels, and may generate the second image data with n-gray levels according to a comparison result between the (n−1) threshold values and the gray level values designated by the first image data. Because plural sets of threshold values can be generated from one dither matrix, the number of gray levels after color reduction can be changed according to the characteristic of the device. 
     Moreover, another embodiment of the invention pertains to a display device that includes a plurality of first electrodes respectively provided for pixels, a second electrode disposed opposite the plurality of first electrodes, and an electro-optical material disposed between the plurality of first electrodes and the second electrode. The display device includes an image acquisition part that designates a gray level value of each of the pixels, and acquires first image data with the number of gray levels being a-gray levels; a parameter acquisition part that acquires a parameter that decides the number of gray levels displayed by the pixel; a color reduction part that decides a number of gray levels to be displayed by the pixel according to the parameter acquired by the parameter acquisition part, and generates second image data in which the first image data acquired in the image acquisition part is color-reduced to the number of gray levels decided; and a write part that changes the gray level of the pixel to a gray level of the gray level value designated by the second image data generated in the color reduction part. In an aspect, the write part performs a first write operation of applying a first voltage to the first electrode of the pixel a plurality of times, when changing the gray level of the pixel in a direction from the second gray level to the first gray level, and performs a second write operation of applying a second voltage having a polarity different from the first voltage to the first electrode of the pixel a plurality of times, when changing the gray level of the pixel in a direction from the first gray level to the second gray level. 
     According to this configuration, the number of gray levels to be displayed by the pixel is decided according to the acquired parameter, which corresponds to the characteristic of the device, and the first image data that designates the gray level of the pixel is color-reduced to the number of gray levels decided, whereby the second image data is generated. Also, the first voltage or the second voltage is applied to the pixel multiple times such that the gray level of the pixel is changed to a gray level value designated by the second image data. As an image that has been color-reduced according to the characteristic of the device is displayed through application of the first voltage or the second voltage, the gray level corresponding to the characteristic of the device can be displayed with fewer kinds of voltages (e.g., two kinds of voltages in this example). 
     Note that the invention can be realized not only as the control device for a display device, and the display device, but also realized as a method for controlling the display device and an electronic apparatus having the display device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a hardware configuration of a display device  1000  and an electro-optic device  1  in accordance with a first embodiment of the invention. 
         FIG. 2  is a view showing a cross section of a display area  100 . 
         FIG. 3  is a view showing an equivalent circuit of a pixel  110 . 
         FIG. 4  is a block diagram showing a configuration of functions realized by a controller  5 . 
         FIG. 5  is an illustration showing numbers of times of voltage application required for changing gray levels. 
         FIG. 6  is a flow chart showing a flow of processings executed by the controller  5 . 
         FIG. 7  is an illustration showing an example of a first dither matrix. 
         FIG. 8  is an appearance of an electronic book reader  2000 . 
     
    
    
     PREFERRED EMBODIMENTS 
     Configuration of Embodiment 
       FIG. 1  is a block diagram showing a hardware configuration of a display device  1000  in accordance with an embodiment of the invention. The display device  1000  is a device that displays an image, and is equipped with an electrophoretic electro-optic device  1 , a control part  2 , a VRAM (Video Random Access Memory)  3  and a RAM  4  that is an example of a storage part. Also, the electro-optic device  1  is equipped with a display part  10  and a controller  5 . 
     The control part  2  is a microcomputer that is equipped with a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM, etc., and controls each of the parts of the display device  1000 . Also, the control part  2  accesses the VRAM  3 , and writes image data indicative of an image to be displayed in the display area  100  in the VRAM  3 . The controller  5  supplies various signals, to a scanning line drive circuit  130  and a data line drive circuit  140  of the display part  10 , for displaying the image in the display area  100  of the display part  10 . The controller  5  corresponds to a control device of the electro-optic device  1 . Note that the control part  2  and the controller  5  combined can be defined as a control device of the electro-optic device  1 . Alternatively, the control part  2 , the controller  5 , the VRAM  3  and the RAM  4  as a whole can be defined as a control device of the electro-optic device  1 . 
     The VRAM  3  is a memory that stores image data written by the control part  2 . The VRAM  3  has a memory area (buffer) for each of the pixels  110  arranged by m rows×n columns to be described later. The image data contains data indicative of a gray level of each of the pixels  110 , and data indicative of a gray level of one pixel  110  is stored in one memory area corresponding to the pixel  110  in the VRAM  3 . The data written in the VRAM  3  is read out by the controller  5 . In the embodiment, the image data takes one of the integers from 0 to 255, where 0 expresses black and 255 expresses white, and the gray level changes from black to white as the value becomes greater. 
     A temperature sensor  6  is provided for detecting the temperature. The temperature sensor  6  outputs a signal that expresses the detected temperature. Note that the temperature sensor  6  is disposed near the display area  100 . The RAM  4  stores various data to be used for displaying an image in the display area  100 . The RAM  4  has an image memory area A, a processed image memory area B, and a prior image memory area C. Each of the memory areas has a storage area corresponding to each of the pixels  110  of m rows×n columns, respectively. The image memory area A is an area where the image data read from the VRAM  3  is stored. The processed image memory area B is an area that stores processed image data of which the image data stored in the image memory area A has been processed. The prior image memory area C is an area that stores image data stored in the processed image storage area B, when the content of the VRAM  3  is detected as having been rewritten. 
     In the display area  100 , a plurality of scanning lines  112  are provided along the row (X) direction in  FIG. 1 , and a plurality of data lines  114  are provided along the column (Y) direction in a manner to be electrically insulated from the scanning lines  112 . Further, the pixel  110  is provided corresponding to the intersection between each of the scanning lines  112  and each of the data lines  114 . Assuming for the sake of convenience that the number of rows of the scanning lines  112  is “m” and the number of columns of the data lines  114  is “n”, the pixels  110  are arrayed in a matrix (m rows×n columns) to form the display area  100 . 
       FIG. 2  is a view showing the cross section of the display area  100 . As shown in  FIG. 2 , the display area  100  is formed generally from a first substrate  101 , an electrophoretic layer  102 , and a second substrate  103 . The first substrate  101  is a substrate in which a circuit layer is formed on an insulating and flexible substrate  101   a . In the present embodiment, the substrate  101   a  is formed from polycarbonate. Note that a resin material which is light and flexible, and has elasticity and insulation property may also be used for the substrate  101   a  without any particular limitation to polycarbonate. Also, the substrate  101   a  may be formed from glass which does not have flexibility. An adhesive layer  101   b  is provided on the surface of the substrate  101   a , and a circuit layer  101   c  is laminated on the surface of the adhesive layer  101   b . The circuit layer  101   c  has a plurality of scanning lines  112  arrayed in the row direction and a plurality of data lines  114  arrayed in the column direction. Also, the circuit layer  101   c  has a pixel electrode  101   d  (a first electrode) corresponding to each of the intersections between the scanning lines  112  and the data lines  114 . 
     The electrophoretic layer  102 , that is an example of an electro-optic material, is formed from a binder  102   b  and a plurality of microcapsules  102   a  fixed by the binder  102   b , and is formed on the pixel electrodes  101   d . It is noted that an adhesive layer formed from an adhesive may be provided between the microcapsules  102   a  and the pixel electrodes  101   d.    
     The binder  102   b  may be made of any material that has good affinity with the microcapsules  102   a , excellent adhesion to the electrodes, and dielectric property, without any particular limitation. Each of the microcapsules  102   a  contains a dispersion medium and electrophoretic particles. The microcapsules  121  may preferably be made of a material having flexibility, such as, composites of gum arabic and gelatin, urethane compounds, and the like. 
     As the dispersion medium, it is possible to use any one of materials including water; alcohol solvents (such as, methanol, ethanol, isopropanol, butanol, octanol, and methyl cellosolve); esters (such as, ethyl acetate and butyl acetate); ketones (such as, acetone, methyl ethyl ketone, and methyl isobutyl ketone); aliphatic hydrocarbons (such as, pentane, hexane, and octane); alicyclic hydrocarbons (such as, cyclohexane and methylcyclohexane); aromatic hydrocarbons (such as, benzene, toluene, long-chain alkyl group-containing benzenes (such as, xylenes, hexylbenzene, heptylbenzene, octylbenzene, nonylbenzene, decylbenzene, undecylbenzene, dodecylbenzene, tridecylbenzene, and tetradecylbenzene)); halogenated hydrocarbons (such as, methylene chloride, chloroform, carbon tetrachloride, and 1,2-dichloroethane); and carboxylates. Also, the dispersion medium may be made of any one of other various oils. As the dispersion medium, one or any of the materials described above in combination may be used. Further in another example, the dispersion medium may be further mixed with a surfactant. 
     The electrophoretic particles are particles (polymer or colloid) having a property in which the particles move in the dispersion medium by electric fields. In the present embodiment, white electrophoretic particles and black electrophoretic particles are contained in each of the microcapsules  102   a . The black electrophoretic particles are particles formed of black pigments, such as, aniline black, carbon black and the like, and are positively charged in the present embodiment. The white electrophoretic particles are particles formed of white pigment, such as, titanium dioxide, aluminum oxide and the like, and are negatively charged in the present embodiment. 
     The second substrate  103  includes a film  103   a  and a transparent common electrode layer  103   b  (second electrode) formed on the bottom surface of the film  103   a . The film  103   a  serves to seal and protect the electrophoretic layer  102 , and is a polyethylene terephthalate film, for example. The film  103   a  is transparent and has an insulation property. The common electrode layer  103   b  is formed from a transparent conductive film, such as, an indium tin oxide film (ITO film). 
       FIG. 3  is a view showing an equivalent circuit of the pixel  110 . In the present embodiment, in order to distinguish the scanning lines  112 , the scanning lines  112  shown in  FIG. 1  may be called scanning lines on the first, the second, the third, . . . , the (m−1) -th , and the m -th  rows in this order from the top. Similarly, in order to distinguish the data lines  114 , the data lines  114  shown in  FIG. 1  may be called data lines on the first, the second, the third, . . . , the (n−1) -th , and the n -th  columns in this order from the left. 
       FIG. 3  shows an equivalent circuit of the pixel  110  corresponding to the intersection between the scanning line  112  on the i -th  row and the data line  114  on the j -th  column. Since the pixels  110  corresponding to the intersections between other data lines  114  and other scanning lines  112  also have the same configuration shown in the drawing, an equivalent circuit of the pixel  110  corresponding to the intersection between the data line  114  on the i -th  row and the scanning line  112  on the j -th  column will be described as representative, and explanation regarding equivalent circuits of the other pixels  110  will be omitted. 
     As shown in  FIG. 3 , each of the pixels  110  has an n channel type thin film transistor (hereinafter, abbreviated as a “TFT”)  110   a , a display element  110   b , and an auxiliary capacitor  110   c . In the pixel  110 , the TFT  110   a  has a gate electrode connected to the scanning line  112  on the i -th  row, a source electrode connected to the data line  114  on the j -th  column, and a drain electrode connected to a pixel electrode  101   d , which is an end of the display element  110   b , and an end of the auxiliary capacitor  110   c . The auxiliary capacitor  110   c  has a configuration in which a dielectric layer is interposed between a pair of electrodes formed in the circuit layer  101   c . The other electrode of the auxiliary capacitor  110   c  is given a voltage common to all pixels. The pixel electrode  101   d  faces the common electrode layer  103   b , and the electrophoretic layer  102  is interposed between the pixel electrode  101   d  and the common electrode layer  103   b . Therefore, when seen in the equivalent circuit, the display element  110   b  defines a capacitor in which the electrophoretic layer  102  is interposed between the pixel electrode  101   d  and the common electrode layer  103   b . In addition, the display element  110   b  retains (stores) a voltage between these electrodes and performs display according to the direction of an electric field caused by the retained voltage. Moreover, in the present embodiment, a common voltage Vcom is applied to the other end of the auxiliary capacitor  110   c  of each pixel  110  and the common electrode layer  103   b  by an external circuit (not shown). 
     Referring back to  FIG. 1 , the scanning line driving circuit  130  is connected to each of the scanning lines  112  of the display area  100 . Under the control of the controller  5 , the scanning line drive circuit  130  sequentially selects the scanning lines  112  at the first, the second, . . . , and the m -th  rows in this order, and supplies a high-level (High) signal to the selected scanning line  112  and supplies a low-level (Low) signal to the other scanning lines  112  which are not selected. The data line drive circuit  140  is connected to each of the data lines  114  in the display area, and supplies a data signal to the data line  114  on each of the columns according to the display content of the pixels  110  for one row which are connected to the selected scanning line  112 . 
     During a period starting from the selection of the scanning line  112  in the 1 st  row by the scanning line drive circuit  130  until the completion of the selection of the scanning line in the m -th  row (hereafter referred to as a “frame period” or, simply a “frame”), each of the scanning lines  112  is selected once, and a data signal is supplied to each of the pixels  14  once in each frame. 
     When the scanning line  112  assumes the high level, the TFT  110   a  whose gate is connected to the scanning line  112  is set to an ON state, whereby the pixel electrode  101   d  is connected to the data line  114 . If a data signal is supplied to the data line  114  when the scanning line  112  is at a high level and, the data signal is applied to the pixel electrode  101   d  through the TFT  110   a  which is in the ON state. When the scanning line  112  assumes the low level, the TFT  110   a  is set to an OFF state. However, the voltage applied to the pixel electrode  101   d  by the data signal is accumulated in the auxiliary capacitor  110   c , and electrophoretic particles move according to the potential difference (voltage) between the electric potential of the pixel electrode  101   d  and the electric potential of the common electrode layer  103   b.    
     For example, when the voltage on the pixel electrode  101   d  is +15V (second voltage) with respect to the voltage Vcom on the common electrode layer  103   b , white electrophoretic particles negatively charged move toward the pixel electrode  101   d  and black electrophoretic particles positively charged move toward the common electrode layer  103   b . As a result, the pixel  110  is displayed in black. On the other hand, when the voltage on the pixel electrode  101   d  is −15V (first voltage) with respect to the voltage Vcom on the common electrode layer  103   b , black electrophoretic particles positively charged move toward the pixel electrode  101   d  and white electrophoretic particles negatively charged move toward the common electrode layer  103   b . As a result, the pixel  110  is displayed in white. Note that the voltage on the pixel electrode  101   d  is not limited to the above-described voltage, and may be a voltage other than +15V or −15V as long as it is a positive or negative voltage with respect to the voltage Vcom of the common electrode layer  103   b.    
     In the present embodiment, when changing the display state of each pixel  110  from white (low gray level) as the first gray level to black (high gray level) as the second gray level or from black to white, the display state is changed by a write operation in which data signals are supplied to the pixel  110  over a plurality of frames, instead of changing the display state by supplying a data signal to the pixel  110  in only one frame. This is because, when changing the display state from white to black, black electrophoretic particles do not move to the display side completely even if the potential difference is given to the electrophoretic particles in only one frame, and therefore the display state does not become a fully black display state. This is the same for white electrophoretic particles when changing the display state from black to white. Therefore, for example, when changing the display state of the pixel  110  from white to black, data signals for displaying the black on the pixel  110  are supplied to the pixel  110  over a plurality of frames, and when changing the display state of the pixel  110  from black to white, data signals for displaying the white on the pixel  110  are supplied to the pixel  110  over a plurality of frame. Note that, in the present embodiment, the “write operation” refers to a data signal supply sequence of supplying data signals to pixels for changing the display state of the pixels to a display state of a desired gray level, or a sequence of applying voltages between the common electrode layer  103   b  and the pixel electrode  101   d , which is performed based on the data signal supply sequence. 
     Moreover, in the present embodiment, the pixel electrode  101   d  of a certain pixel  110  in one frame may be set as a positive electrode with a higher electric potential than the common electrode layer  103   b , and the pixel electrode  101   d  of another pixel  110  in the same frame maybe set as a negative electrode with a lower electric potential than the common electrode layer  103   b . On other words, a driving operation that is capable of selecting both electrodes of positive and negative electrodes with respect to the common electrode layer  103   b  in one frame (hereinafter, referred to as bipolar driving) is performed. More specifically, in one frame, the pixel electrode  101   d  of the pixel  110  whose gray level is changed to the high gray level side (second gray level side) is set as a positive electrode, and the pixel electrode  101   d  of the pixel  110  whose gray level is changed to the low gray level side (first gray level side) is set as a negative electrode. Note that, when black electrophoretic particles are negatively charged and white electrophoretic particles are positively charged, the pixel electrode  101   d  of the pixel  110  whose gray level is changed to the high gray level side (second gray level side) may be set as a negative electrode, and the pixel electrode  101   d  of the pixel  110  whose gray level is changed to the low gray level side (first gray level side) may be set as a positive electrode. 
     Next, the configuration of the controller  5  will be described.  FIG. 4  is a block diagram showing functions realized in the controller  5  in accordance with the present embodiment. In the controller  5 , an image acquisition part  501 , a parameter acquisition part  502 , a color reduction part  503 , and a write part  502  are realized. Note that the blocks may be realized by hardware, or may be realized by a program to be executed by a CPU provided in the controller  5 . 
     The image acquisition part  501  is a block that acquires image data (first image data) stored in the VRAM  3 , and stores the acquired image data in the image memory area A of the RAM  4 . Moreover, after storing the image data memorized in the VRAM  3  in the image memory area A of the RAM  4 , the image acquisition part  501  stores the image data stored in the processed image memory area B in the prior image memory area C. 
     The parameter acquisition part  502  is a block that acquires a parameter for deciding the number of gray levels to be displayed by the pixel  110 . In the embodiment, the signal output from the temperature sensor  6  is acquired as a parameter. 
     The color reduction part  503  is a block that renders a color reduction processing on the image data stored in the image memory area A. The color reduction part  503  specifies the temperature that is detected by the temperature sensor  6  based on the signal that is acquired by the parameter acquisition part  502 . Also, the color reduction part  503  decides a number of gray levels that is to be displayed by the pixel  110  according to the specified temperature, and renders the color reduction processing on the image data such that it assumes the decided number of gray levels. The image data rendered with the color reduction processing is stored in the processed image memory area B. 
     In the embodiment, when the temperature specified by the parameter acquisition part  502  is 30° C. or higher, the color reduction part  503  sets the gray levels that can be expressed by the image data to be two gray levels (black and white). When the temperature specified by the parameter acquisition part  502  is lower than 30° C., the color reduction part  503  sets the gray levels that can be expressed by the image data to be four gray levels (black, dark gray, light gray and white). In the image data after the color reduction processing has been rendered, the value that expresses the gray level of the pixel assumes to be one of 0 through 3, and the smaller the gray level value, the higher density the gray level becomes, where 0 expresses black, 1 expresses the dark gray, 2 expresses the light gray, and 3 expresses white. 
     The write part  504  controls the scanning line drive circuit  130  and the data line drive circuit  140 , and applies the first voltage or the second voltage to the pixel electrode  101   d  of each of the pixels  110  based on the image data stored in the processed image memory area B and the prior image memory area C. The controller  5  stores a table shown in  FIG. 5 . The table shown in  FIG. 5  stores the number of times of voltage application to the pixel  110  when the gray level of the pixel  110  is to be changed as a result of the content of the VRAM  3  having been rewritten. 
     The controller  5  acquires the gray level value before change from the prior image memory area C, and acquires the gray level value after change from the processed image memory area B. According to  FIG. 5 , when the gray level value before change, for example, is 0 (black), and the gray level value after change is 3 (white), when the voltage of −15V with respect to the voltage Vcom of the common electrode layer  10  is applied to the pixel electrode  101   d  six times, the pixel changes from the black state to the white state. When the gray level value before change is 0 (black), and the gray level value after change is 2 (light gray), when the voltage of −15V with respect to the voltage Vcom is applied to the pixel electrode  101   d  four times, the gray level value becomes to be 2. When the gray level value before change is 0 (black), and the gray level value after change is 1 (dark gray), when the voltage of −15V with respect to the voltage Vcom is applied to the pixel electrode  101   d  two times, the gray level value becomes to be 1. Also, when the gray level value before change is 3 (white), and the gray level value after change is 0 (black), when the voltage of +15V with respect to the voltage Vcom is applied to the pixel electrode  101   d  six times, the pixel changes from the white state to the black state. 
     By applying the voltage to the pixel electrode  101   d  according to the table shown in  FIG. 5 , the gray level of the pixel can be changed. However, at higher temperatures, the viscosity of the dispersion medium becomes lower, and the electrophoretic particles would move more easily. Accordingly, even if the voltage is applied the same number of times, the dark gray and the light gray could vary in gray level. Therefore, in accordance with the embodiment, in the state where the temperature is high and fine control of the gray level is therefore difficult, the number of gray levels to be displayed is reduced. Meanwhile, in the state where the temperature is low and fine control of the gray level is thus relatively easy, the number of gray levels to be displayed is increased, thereby suppressing variations in the displayed gray level. The operation will be described in greater detail. 
       FIG. 6  is a flow chart showing the flow of processings executed by the controller  5 . The controller  5  observes writing of image data to the VRAM  3 . The controller  5  (e.g., the image acquisition part  501 ) acquires image data stored in the VRAM  3  when there is a change in the content of the VRAM  3  (step SA 1  (image acquisition step)), and has the acquired image data stored in the image memory area A (step SA 2 ). Here, the gray level values of the pixels are stored in a matrix corresponding respectively to the pixels  110  of m rows×n columns. Moreover, the controller  5  has the image data that is stored in the processed image memory area B stored in the prior image memory area C (step SA 3 ). By step SA 3 , the image data of the image displayed at this point will be stored in the prior image memory area C. 
     Next, the controller  5  (e.g., the parameter acquisition part  502 ) acquires a signal output from the temperature sensor  6  (parameter acquisition step). Then, the controller  5  (e.g., the color reduction part  503 ) specifies the temperature detected with the temperature sensor  6  based on the acquired signal (step SA 4 ). The controller  5  (e.g., the color reduction part  503 ), that specified the temperature, renders the color reduction processing corresponding to the specified temperature on the image data stored in the image memory area A (step SA 5  (color reduction step)). 
     An example of the color reduction processing in accordance with the present embodiment will be described below. The controller  5  stores a first dither matrix of four rows by four columns (see  FIG. 7 ), and the first dither matrix stores threshold values to binarize the image data stored in the image memory area A. The threshold value is any one of values from 0 to 255. 
     When the temperature specified in step SA 4  is 30° C. or higher, the controller  5  superposes the first dither matrix over the image data arrayed in a matrix that is stored in the image memory area A. The controller  5  adds the gray level value of each of the pixels stored in the image memory area A respectively to each of the values of the first dither matrix superposed on these gray level values. When the addition result becomes less than 256, the controller  5  changes the gray level value of those of the pixels whose addition result is less than 256 to 0 (black), and changes the gray level value of those of the pixels whose addition result becomes 256 or more to 3 (white). The controller  5  has the gray level values after the change stored in the processed image memory area B. As a result, the image data stored in the processed image memory area B becomes an image in two gray levels of black and white. 
     On the other hand, when the temperature specified in step SA 3  is less than 30° C., the controller  5  creates a second dither matrix in which each of the values in the first dither matrix is divided by three. Next, the controller  5  superposes the second dither matrix over the image data arrayed in a matrix in the image memory area A. The controller  5  adds the gray level value of each of the pixels stored in the image memory area A and each of the values of the second dither matrix superposed respectively on these gray level values plus 170. When the addition result becomes less than 256, the controller  5  changes the gray level value of those of the pixels whose addition result is less than 256 to 0 (black), and has the gray level values after the change stored in the processed image memory area B. 
     On the other hand, when the addition result becomes 256 or more, the controller  5  adds the gray level value of each of such pixels stored in the image memory area A and each of the values of the second dither matrix superposed respectively on these gray level values plus 65. When the addition result becomes less than 256, the controller  5  changes the gray level value of those of pixels to 1 (dark gray), and has the gray level value after the change stored in the processed image memory area B. 
     On the other hand, when the addition result of the gray level value of the pixel and the value of the second dither matrix plus 65 is 256 or more, the controller  5  adds the gray level value of each of such pixels stored in the image memory area A to each of the values of the second dither matrix superposed respectively on the gray level values. When the addition result becomes less than 256, the controller  5  changes the gray level value of each of those of the pixels to 2 (light gray). When the addition result becomes 256 or more, the controller  5  changes the gray level value of each of those of the pixels to 3 (white). Then the gray level values after the change are stored in the processed image memory area B. As a result, the image data stored in the processed image memory area B becomes an image in four gray levels of black, dark gray, light gray and white. 
     When the color reduction processing is finished, the controller  5  (the writing part  504 ) executes a writing operation (step SA 6  (writing step)), using the image data stored in the processed image memory area B, and the image data stored in the prior image storage area C. More specifically, the controller  5  acquires, for each of the pixels, the gray level value stored in the processed image memory area B and the gray level value stored in the prior image memory area C. The controller  5  assumes the gray level value acquired from the processed image memory area B as a gray level value after change, and the gray level value acquired from the prior image memory area C as a gray level value before change, and decides the number of times of voltage application for each of the pixels by referring to the table of  FIG. 5 . Having decided the number of times of voltage application, the controller  5  specifies, based on the gray level value stored in the processed image memory area B and the gray level value stored in the prior image memory area C, as to whether the gray level of the pixel should be changed toward white or changed toward black. When the gray level is to be changed toward white, the controller  5  applies the voltage of −15V with respect to the voltage Vcom to the pixel electrode  101   d  by the decided number of times of voltage application. When the gray level is to be changed toward black, the controller  5  applies the voltage of +15V with respect to the voltage Vcom to the pixel electrode  101   d  by the decided number of times of voltage application. 
     According to the embodiment, the voltage to be applied to the pixel electrode is +15V, −15V or 0V with respect to the voltage Vcom. Even if the voltage to be applied to the pixel is not continuously controlled, the gray level of the pixel can be changed by controlling the number of times of application of these voltages. Moreover, in the state where the temperature is high and the electrophoretic particles would easily move, the number of gray levels to be displayed is suppressed. Accordingly, the pixels to be displayed have few variations in the gray level even when the temperature condition changes, whereby the gray level display corresponding to the situation can be performed. 
     Electronic Apparatus 
     Next, an example of an electronic apparatus to which the display device  1000  according to the embodiment is applied will be described.  FIG. 8  is a view showing the appearance of an electronic book reader using the display device  1000  in accordance with the embodiment described above. An electronic book reader  2000  includes a plate shaped frame  2001 , buttons  9 A to  9 F, the electro-optical device  1  according to the embodiment described above, the control unit  2 , the VRAM  3 , and the RAM  4 . In the electronic book reader  2000 , the display area  100  is exposed. In the electronic book reader  2000 , the content of electronic book is displayed in the display area  100 , and the pages of the electronic book are turned over by operating the buttons  9 A to  9 F. In addition, examples of an electronic apparatus to which the electro-optical device  1  according to the embodiment described above can be applied may include a timepiece, electronic paper, an electronic diary, a calculator, a mobile phone, and the like. 
     While the embodiments of the invention have been described above, the invention is not limited to the embodiments described above and various modifications may also be made. For example, the invention may also be implemented by modifying the above-described embodiments as follows. In addition, the above-described embodiments and the following modification examples may be combined. 
     In the embodiment described above, the electro-optical device including the electrophoretic layer  102  has been described as an example. However, the invention is not at all limited to such example. The invention is applicable to any electro-optical devices as long as the writing for changing the display state of a pixel from the first display state to the second display state is performed by a writing operation in which voltage is applied multiple times. For example, it is also possible to use an electro-optical device using electronic liquid powder. 
     In the embodiment described above, the number of gray levels after the color reduction processing is two gray levels or four gray levels. However, the invention is not limited to these numbers of gray levels. For example, the number of gray levels may be 16 gray levels when the temperature specified by the temperature sensor  6  is less than 20° C., 8 gray levels at 20° C. and above but less than 25° C., 4 gray levels at 25° C. and above but less than 30° C., 3 gray levels at 30° C. and above but less than 35° C., and 2 gray levels at 35° C. and above. 
     In the embodiment described above, the number of gray levels to be displayed by pixels is changed based on the temperature. However, the number of gray levels to be displayed by pixels may be changed according to parameters other than the temperature. For example, the user may operate the buttons  9 A- 9 F on the electronic book reader  2000  to designate the number of gray levels to be displayed by the pixels, and the controller  5  (the parameter acquisition part  502 ) may acquire data (a parameter) that expresses the number of gray levels specified by the user, and may perform the color reduction processing such that the number of gray levels is expressed according to the acquired data. 
     Moreover, by adjusting the dispersion medium and the electrophoretic particles that compose the micro capsules  102   a , the number of gray levels suitable for the display can be adjusted, whereby an electro-optic device  1  having pixels suitable for the display in two gray levels and an electro-optic device  1  having pixels suitable for the display in three or more gray levels can be composed. In this case, the controller  5  (the parameter acquisition part  502 ) may acquire, for example, from the control part  2 , data (a parameter) indicative of whether the pixels are suitable for displaying one of the numbers of gray levels or the other, and may perform the color reduction processing to the image data stored in the image memory area A such that the number of gray levels according to the acquired data is displayed. 
     In the embodiment described above, the first dither matrix is stored, and threshold values to be used for performing the color reduction processing are generated from the first dither matrix. However, threshold values from 0 to 255 may be set for each of the gray levels after the color reduction and a dither matrix may be provided for each of the gray levels. Further, although a dither matrix is used for the color reduction processing in the embodiment described above, the color reduction processing may be performed by any one of other known color reduction processings (such as, for example, an error diffusion method) besides the method of the embodiment. 
     The embodiment described above is composed such that an image is displayed by using a dispersion medium and electrophoretic particles. However, an electro-optic device that uses a liquid crystal as the electro-optic material may have a composition in which the number of gray levels to be displayed is changed according to the temperature. When the liquid crystal is used, the transmissivity decreases at low temperatures more than at high temperatures. Therefore, the color reduction processing may be performed such that the number of gray levels to be displayed is reduced at low temperatures, and the number of gray levels to be displayed is increased at high temperatures. 
     The entire disclosure of Japanese Patent Application No. 2012-067171, filed Mar. 23, 2012 is expressly incorporated by reference herein.