Abstract:
A method is provided to increase the gray level resolution of a thermal print head. The thermal print head includes a plurality of heaters which are used to heat color dye so as to deposit the color dye on a paper to form a line image. The line image contains a plurality of pixels, each formed by one heater. When generating a pixel, a pulse sequence is fed to one heater to control the heater″s heating period. The pulse sequence contains first and second portions. The first portion contains a plurality of consecutive pulses which is used to generate a first gray level. And the second portion contains at least one empty pulse period followed by at least one pulse which is used to generate a second gray level. The combination of the first and second gray levels will determine the gray level of each pixel of the line image.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing method, and more particularly, to a printing method for interpolating gray levels. 
     2. Description of the Prior Art 
     Photo printers are different from general printers. The major difference is that the photo printer can print out an image such as a photo picture on paper with high picture quality. Please refer to FIG.  1  and FIG.  2 . FIG. 1 is a diagram of a prior art photo printer  10 . FIG. 2 is a simplified exploded view of the photo printer  10  shown in FIG.  1 . As shown in FIG. 1, the photo printer  10  has a ribbon  14 , a thermal print head  12 , a ribbon driver  18 , and a roller set  20 . The ribbon  14  has a plurality of sectors, and each sector is used for storing one kind of different color dyes. The thermal print head  12  is fixed inside the photo printer  10  for heating the color dyes so that the color dyes are transferred onto a photo paper  16 . The ribbon driver  18  is used for moving the ribbon  14  back and forth so that the thermal print head  12  can transfer a specific color dye stored on the ribbon  14  onto the corresponding photo paper  16 . The roller set  20  is used for holding the photo paper  16  and moving the photo paper  16  along a predetermined direction. Therefore, the fixed thermal print head  12  is capable of printing a color image on the photo paper  16 . 
     As shown in FIG. 2, the thermal print head  12  has a plurality of heaters  22  that are arranged linearly and spaced equally for heating the ribbon  14 . The color dye stored on the ribbon  14  is heated, and is transferred onto the photo paper  16 . When the thermal print head  12  starts printing images, each heater  22  positioned on the thermal print head  12  will heat the ribbon  14  so that a plurality of corresponding pixels X 1  will form a line image Y 1 . Then, the photo paper  16  driven by the roller set  20  is moved along the predetermined direction according to a predetermined speed. Therefore, another line image Y 2  is printed on the same photo paper  16  next to the line image Y 1 . Accordingly, a plurality of line images are successfully printed on the same photo paper  16  to complete the printing operation. 
     As mentioned above, the total number of heaters  22  positioned on the thermal print head  12  determines the corresponding number of the pixels X 1  of each line image printed on the photo paper  16 . Moreover, the color concentration, that is, the gray level of each pixel X 1  printed on the photo paper  16  is determined by the corresponding heater  22  with a specific duration of each heating operation and a total number of heating cycles. 
     Please refer to FIG.  3 A and FIG.  3 B. FIG. 3A is a diagram of gray levels and a corresponding driving signal  30  according to the photo printer  10  shown in FIG.  1 . FIG. 3B is a diagram of a binary data sequence of the driving signal  30  shown in FIG. 3 a . As shown in FIG.  3 A and FIG. 3B, before the thermal print head  12  of the photo printer  10  starts printing images onto the photo paper  16 , all of the heaters  22  positioned on the thermal print head  12  are activated during a predetermined period Tp so that each heater  22  will first approach a predetermined printing temperature. The above-mentioned procedure is called a preheating operation. In addition, the driving signal having a pulse with a binary value “1” will activate the corresponding heater  22 , and the driving signal corresponding to a binary value “0” will not activate the heater  22 . Next, the photo printer  10  will continuously activate the same heater  22  according to the corresponding gray level of the pixel X 1 . In other words, each heater  22  positioned on the thermal print head  12  is activated repeatedly according to the desired gray level of the corresponding pixel. The overall heating operation of the heater  22  is represented by a driving signal  30  and its corresponding binary values. Each duration Tu of a pulse  32  is a heating time unit for activating the heater  22 . In addition, the energy generated by the heater  22  onto the corresponding pixel X 1  during the duration Tu of each pulse  32  is nearly identical. That is, the quantity of color dyes transferred onto the photo paper  16  during the fixed duration Tu is almost identical. The reason why the quantity of color dyes is almost identical is because of a thermal accumulation effect. It is well known that the thermal accumulation effect is adjusted according to a prior art control method so that the quantity of color dyes is controlled with acceptable inaccuracy. A lengthy description of the prior art control method is skipped for brevity. 
     The heater  22  of the photo printer  10  can produce 256 (0˜255) gray levels to print the corresponding pixel X 1  with an appropriate gray level. A gray level corresponding to a lightest color concentration is equal to 0, and a gray level corresponding to a darkest color concentration is equal to 255. In other words, when the pixel X 1  acquires a corresponding gray level equaling N, which is an integer between 0 and 255, the corresponding heater  22  has to be successively activated N times. Therefore, N pulses  32  of the driving signal  30  are generated repeatedly. That is, N binary “1” values are inputted to the heater  22  continuously. Please note that the photo paper  16  is printed one line at a time. Because each pixel X 1  positioned on the same line may have different gray levels, each heater  22  has to wait for 255 durations Tu so that the thermal print head  12  can then print the next line image. That is, one heater  22  finishes printing a corresponding pixel X 1  with a smaller gray level within a short time. But, another heater  22  printing a corresponding pixel X 1  with a greater gray level may take a long time. When the total number of different gray levels is doubled, each heater  22  has to wait for 511 durations Tu. Therefore, if the number of different gray levels is increased, each heater  22  has to operate for a longer period to complete printing one line image. That is, if the color resolution is improved, the execution time is longer. The printing efficiency, therefore, is greatly deteriorated. 
     SUMMARY OF INVENTION 
     It is therefore a primary objective of the claimed invention to provide a printing method for interpolating gray levels of a thermal print head to solve the above mentioned problem. 
     Briefly, the claimed invention provides a printing method using a thermal print head having a plurality of heaters linearly arranged and equally spaced for heating a dye and transferring the dye onto an object, thereby forming a plurality of pixels corresponding to the heaters on the object. A color of each pixel is determined by a gray level. Each gray level comprises a first portion and a second portion. When controlling the heater to generate a pixel of a predetermined gray level, the printing method comprises activating a heater for a number of cycles corresponding to the first portion of the predetermined gray level, thereby transferring the dye onto the object in a position corresponding to the heater.The first portion is larger than or equal to zero. Each cycle lasts a substantially equal amount of time. Each activation of the heater within a cycle lasts a substantially equal amount of time, and quantity of the dye transferred onto the object is substantially equal for each activation of the heater. The printing method further comprises deactivating the heater for a first predetermined number of cycles corresponding to the second portion of the predetermined gray level, then activating the heater a second predetermined number of cycles corresponding to the second portion of the predetermined gray level. Both the first predetermined number and the second predetermined number are integers larger than or equal to 1. A total quantity of the dye transferred onto the object in printing the second portion of the predetermined gray level is less than the quantity of dye transferred onto the object during each cycle of printing in first portion of the predetermined gray level. 
     It is an advantage of the claimed invention that the claimed printing method can improve the output picture quality and the printing efficiency of the photo printer by interpolating gray levels based on the thermal accumulation effect. 
     These and other objectives of the claimed invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment which is illustrated in the various figures and drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a diagram of a prior art photo printer. 
     FIG. 2 is a simplified exploded view of the photo printer shown in FIG.  1 . 
     FIG. 3A is a diagram of gray levels and a corresponding driving signal according to the photo printer shown in FIG.  1 . 
     FIG. 3B is a diagram of a binary data sequence of the driving signal shown in FIG. 3 a.    
     FIG. 4 is a diagram of a first printing method according to the present invention. 
     FIG. 5 is a diagram of a second printing method according to the present invention. 
     FIG. 6 is a diagram of a third printing method according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     The structures of the photo printer  10  and the thermal print head  12  according to the present invention are identical to the structures of the prior art photo printer  10  and the prior art thermal print head  12  shown in FIG.  1  and FIG.  2 . Therefore, they are not described again for simplicity. As shown in the prior art description, before the thermal print head  12  of the photo printer  10  starts printing images onto the photo paper  16 , all of the heaters  22  positioned on the thermal print head  12  are activated during a predetermined period Tp so that each heater  22  will first approach a predetermined printing temperature. For example, the driving signal  30  having a pulse  32  with a binary value “1” will activate the corresponding heater  22 , and the driving signal  30  having a binary value “0” will not activate the corresponding heater  22 . The above-mentioned procedure is called a preheating operation. Next, the photo printer  10  will continuously activate the same heater  22  according to the corresponding gray level of the pixel X 1 . In other words, each heater  22  positioned on the thermal print head  12  is activated according to the desired gray level of the corresponding pixel X 1 . 
     Please refer to FIG. 4, FIG. 5, and FIG.  6 . FIG. 4 is a diagram of a first printing method according to the present invention. FIG. 5 is a diagram of a second printing method according to the present invention. FIG. 6 is a diagram of a third printing method according to the present invention. The overall heating operation with respect to the heater  22  is represented by a driving signal  70  and its corresponding binary values. Each duration Tu of a pulse  72  is a heating time unit for activating the heater  22 . In addition, the energy generated by the heater  22  onto the corresponding pixel X 2  within the duration Tu of each pulse  72  is nearly identical. That is, the quantity of color dyes transferred onto the photo paper  16  within the fixed duration Tu is almost identical. The color concentration of each pixel X 2  is controlled by a corresponding predetermined gray level. When a plurality of heaters  22  are activated for heating corresponding pixels, the gray level generated by the heater  22  is basically affected by the total number of heating operations imposed on the corresponding pixel X 2 . In the preferred embodiment, a combination of a first portion W and a second portion T is used for expressing a gray level. The first portion W represents a number with regard to successive activations of the same heater  22 . Therefore, the color dye is continuously transferred onto a corresponding pixel X 2  of a photo paper  16 . The first portion W is greater than or equal to 0. The duration Tu of each pulse  72  is almost identical with acceptable inaccuracy. That is, the quantity of color dyes transferred onto the photo paper  16  is almost the same according to each pulse  72 . The second portion T of the gray level corresponds to an interruption of the overall heating process. The heater  22  is deactivated within a duration Tu, and then the heater  22  is activated once or is activated for a number of cycles with the same duration Tu. Within one fixed duration Tu, the quantity of dyes transferred onto the photo paper  16  associated with the second portion T is less than the quantity of dyes transferred onto the photo paper  16  associated with the first portion W. That is, the heating process without any interruptions will output a great deal of energy within one fixed duration Tu for transferring more color dyes onto the corresponding photo paper  16 . The result is caused by the thermal accumulation effect as mentioned before. If the total number of times of successively activating the heater  22  is increased, much energy accumulates at the heater  22 . Furthermore, the thermal accumulation effect with successive heating operations is greater than the thermal accumulation effect with interruptions induced during original consecutive heating operations. When the heater  22  is deactivated for one duration Tu or a plurality of durations Tu so as to break the consecutive heating operations, the energy generated from the heater  22  to heat the color dyes within the duration Tu is reduced when the heater  22  is activated again. Similarly, when the heater  22  is activated again for heating the color dyes successively, each pulse  72  will have different energy output within the same duration Tu, and the thermal accumulation effect also makes the later pulse  72  with a higher energy output within the same duration Tu as usual. 
     As shown in FIG. 4, the printing method according to the present invention applies the above-mentioned principle to interpolating gray levels between a gray level with a value N and a gray level with a value “N+1”. In the preferred embodiment, heating operations with proper interruptions are combined together for increasing total number of gray levels. The steps are described as follows. 
     Step  100 : 
     Activate one heater  22  for a predetermined period Tp so that the heater  22  approaches a predetermined temperature required for performing a printing operation properly; 
     Step  102 : 
     Activate the heater  22  continuously for N times, that is, input successive N pulses to activate the heater  22 ; 
     Step  104 : Deactivate the heater  22  for one duration Tu; 
     Step  106 : Activate the heater  22  again for one duration Tu or a plurality of durations Tu, that is, input at least one pulse to activate the heater  22  again; and 
     Step  108 : Generate one gray level with a value “N+½” to achieve the objective of increasing total number of gray levels. 
     As mentioned above, the objective of step  100  is to preheat the heater  22  so that the heater  22  can approach a required printing temperature. If the heater  22  is deactivated for one duration Tu after a long heating process, the accumulated energy at the heater  22  starts radiating within the duration Tu, and the corresponding temperature of the heater  22  is lowered. Therefore, the energy outputted from the heater  22  within one duration Tu when the heater  22  is activated again is less than the energy outputted from the heater  22  within one duration Tu before the related interruption. In addition, the energy outputted from the heater  22  is measured as a new energy unit to heat the color dyes when the heater  22  is activated again. Similarly, each pulse, after the interruption, will have different energy outputs within the same duration Tu, and the thermal accumulation effect also gives the later pulse  72  a higher energy output within the same duration Tu. The objective of step  102  to step  106  is to use the new energy unit to heat the color dyes so that a new gray level is generated after step  108 . For example, when the heater  22  is activated continuously for N times, the energy outputted from the heater  22  within one duration Tu is equal to E. If the same heater  22  is deactivated for one duration Tu, the accumulated energy will radiate to make the heater  22  have a lower temperature than before. When the heater  22  is activated again, the energy outputted from the heater  22  within one duration Tu will be equal to 0.5*E. That is, a new gray level with a value “N+½” is interpolated between the gray level with a value “N” and the gray level with a value “N+1”. Similarly, if the heater  22  is deactivated again for one duration Tu, the accumulated energy will radiate again to make the heater  22  have a much lower temperature than before. Then, the heater  22  is activated again, and the energy outputted from the heater  22  within one duration Tu will be equal to 0.25*E now. Finally, a new gray level with value “N+½+¼” is interpolated between the gray level with a value “N” and the gray level with a value “N+1”. Please note that if the heater  22  is then activated for one duration Tu, the accumulated energy will be increased to make the heater  22  have a higher temperature than before. Then, the energy outputted from the heater  22  within one duration Tu will be equal to 0.5*E again. 
     As shown in FIG. 5, the color concentration of each pixel X 3  is controlled by a corresponding predetermined gray level. When a plurality of heaters  22  are activated for heating corresponding pixels X 3 , the gray level is determined according to the total number of times the heater  22  is activated for heating the pixel X 3 . In addition, the combination of the first portion W and the second portion T is used for expressing the gray level. The first portion W represents a number with regard to successive activations of the same heater  22 . Therefore, the color dye is continuously transferred onto a corresponding pixel X 3  of the photo paper  16 . The first portion W is greater than or equal to 0. The duration Tu of each heating operation is almost identical with acceptable inaccuracy. That is, the quantity of dyes transferred onto the photo paper  16  is almost the same within each pulse  72 . The second portion T of the gray level corresponds to an interruption of the overall heating process. The heater  22  is deactivated within a plurality of durations Tu, then the heater  22  is activated once or is activated for a number of cycles with the same duration Tu. Within the fixed duration Tu, the quantity of color dyes transferred onto the photo paper  16  associated with the second portion T is less than the quantity of color dyes transferred onto the photo paper  16  associated with the first portion W. That is, the heating operations without any interruptions will generate a great deal of energy within one duration Tu for transferring color dyes onto the corresponding photo paper  16 . The result is caused by the thermal accumulation effect as mentioned before. If the total number of times of successively activating the heater  22  is increased, much energy accumulates at the heater  22 . Furthermore, the thermal accumulation effect with successive heating operations is greater than the thermal accumulation effect with interruptions induced during original consecutive heating operations. When the heater  22  is deactivated for one duration Tu or a plurality of durations Tu so as to break the consecutive heating operations, the energy generated from the heater  22  to heat the dyes within the duration Tu is reduced when the heater  22  is activated again. Similarly, when the heater  22  is activated again for heating the color dyes successively, each pulse  72  will have different energy output within the same duration Tu, and the thermal accumulation effect also makes the later pulse  72  with a higher energy output within the same duration Tu. As shown in FIG. 5, the printing method according to the present invention applies the above-mentioned principle to generate gray levels between the gray level with a value N and the gray level with a value “N+1”. In the preferred embodiment, heating operations with proper interruptions are combined together for increasing the number of gray levels. The steps are described as follows. 
     Step  120 : 
     Activate one heater  22  for a predetermined duration Tp so that the heater  22  approaches a predetermined temperature required for performing a printing operation properly; 
     Step  122 : 
     Activate the heater  22  continuously for N times, that is, input N successive pulses having a binary value “1” to activate the heater  22 ; 
     Step  124 : 
     Deactivate the heater  22  for two durations Tu, that is, input  2  pulses having a binary value “0” continuously to deactivate the heater  22 ; 
     Step  126 : 
     Activate the heater  22  again for one duration Tu or a plurality of durations TU, that is, input at least one pulse to activate the heater  22  again; and 
     Step  128 : 
     Generate one gray level with a value “N+¼” to achieve the objective of increasing the number of gray levels. 
     As mentioned above, the objective of step  120  is to preheat the heater  22  so that the heater  22  can approach a required printing temperature. If the heater  22  is deactivated for two durations Tu after a long heating process, the accumulated energy at the heater  22  starts radiating within these two durations Tu, and the corresponding temperature of the heater  22  is lowered. Therefore, the energy outputted from the heater  22  within one duration Tu when the heater  22  is activated again is much less than the energy outputted from the heater  22  within one duration Tu before the related interruption. In addition, the energy outputted from the heater  22  is measured as a new energy unit to heat the color dyes after the heater  22  is activated again. Similarly, each pulse  72 , after the interruption, will have different energy output within the same duration Tu, and the thermal accumulation effect also makes the later pulse  72  have a higher energy output within the same duration Tu. The objective of step  122  to step  126  is to use the new energy unit to heat the dyes so that a new gray level is generated after step  108 . For example, when the heater  22  is activated continuously for N times, the energy outputted from the heater  22  within one duration Tu is equal to E. If the same heater  22  is deactivated for two durations Tu, the accumulated energy will radiate to make the heater  22  have a lower temperature than before. When the heater  22  is activated again, the energy outputted from the heater  22  within one duration Tu will be equal to 0.25*E. That is, a new gray level with a value “N+¼” is interpolated between the gray level with a value “N” and the gray level with a value “N+1”. If the heater  22  is activated for one duration Tu again, the accumulated energy will be increased to make the heater  22  have a higher temperature than before. Then, the energy outputted from the heater  22  within one duration Tu will be equal to 0.5*E again. Finally, a new gray level with a value “N+¼+½” is interpolated between the gray level with a value “N” and the gray level with a value “N+1”. 
     As shown in FIG. 6, the third printing method according to the present invention applies the above-mentioned principle to interpolate gray levels between gray level with a value “N” and gray level with a value “N+1”. In the preferred embodiment, heating operations and proper interruptions are combined together to increase the number of gray levels. The steps are described as follows. 
     Step  130   
     Activate one heater  22  for a predetermined duration Tp so that the heater  22  approaches a predetermined temperature required for performing a printing operation properly; 
     Step  132 : Activate the heater  22  continuously for N times, that is, input N pulses having a binary value “1” continuously to activate the heater  22 ; 
     Step  134 : Deactivate the heater  22  for one duration Tu; 
     Step  136 : Activate the heater  22  again for one duration Tu, that is, input one pulse having a binary value “1” to activate the heater  22  again; 
     Step  138 : Deactivate the heater  22  for one duration Tu; 
     Step  140 : Activate the heater  22  again for one duration Tu, that is, input one pulse having a binary value “1” to activate the heater  22  again; and 
     Step  142 : Generate one gray level with a value “N+¾” to achieve the objective of increasing the number of gray levels. 
     As mentioned above, the objective of step  130  is to preheat the heater  22  so that the heater  22  can approach a required printing temperature. If the heater  22  is deactivated for one duration Tu after a long heating process, the accumulated energy at the heater  22  starts radiating within the duration Tu, and the corresponding temperature of the heater  22  is lowered. Therefore, the energy outputted from the heater  22  within one duration Tu when the heater  22  is activated again is less than the energy outputted from the heater  22  within one duration Tu before the related interruption. In addition, the energy outputted from the heater  22  is measured as a new energy unit to heat the color dyes after the heater  22  is activated again. Similarly, each pulse  72 , after the interruption, will have different energy outputs within the same duration Tu, and the thermal accumulation effect also makes the later pulse  72  have a higher energy output within the same duration Tu. The objective of step  132  to step  140  is to use the new energy unit to heat the dyes so that a new gray level is generated after step  142 . For example, when the heater  22  is activated continuously for N times, the energy outputted from the heater  22  within one duration Tu is equal to E. If the same heater  22  is deactivated for one duration Tu, the accumulated energy will radiate to make the heater  22  have a lower temperature than before. When the heater  22  is activated again, the energy outputted from the heater  22  within one duration Tu will be equal to 0.5*E. Similarly, if the heater  22  is deactivated again for one duration Tu, the accumulated energy will radiate again to make the heater  22  have a much lower temperature than before. Then, the heater  22  is activated again, and the energy outputted from the heater  22  within one duration Tu will be equal to 0.25*E. Finally, a new gray level with a value “N+½+¼” is interpolated between the gray level with a value “N” and the gray level with a value “N+1”. 
     From the disclosure mentioned in the first, second, and third printing methods according to the present invention, each heater  22  can acquire other gray levels between any two successive gray levels such as the gray level with a value “N” and the gray level with a value “N+1”. Therefore, all the heaters  22  can output a plurality of gray levels within original 255 durations Tu with a proper control to activate or deactivate the heaters  22 . The number of different gray levels is greatly increased without additional operation time. On the other hand, if the number of different gray levels such as 255 is fixed, the heater  22  can generate the required gray level within a smaller number of durations Tu by controlling a proper sequence of activating or deactivating the heater  22 . Please refer to the following table. 
     
       
         
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Gray level 
                 Driving signal 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 1 
                 0 
                 0 
                 0 
                 0 
               
               
                   
                 1 
                 1 
                 0 
                 1 
                 0 
                 0 
               
               
                   
                 2 
                 1 
                 1 
                 0 
                 0 
                 0 
               
               
                   
                 3 
                 1 
                 1 
                 1 
                 0 
                 0 
               
               
                   
                 4 
                 1 
                 1 
                 1 
                 0 
                 1 
               
               
                   
                 5 
                 1 
                 1 
                 1 
                 1 
                 0 
               
               
                   
                 6 
                 1 
                 1 
                 1 
                 1 
                 1 
               
               
                   
                   
               
             
          
         
       
     
     The driving signal with a binary value “0” is used for deactivating the heater  22 , and the driving signal with a binary value “1” is used for activating the heater  22 . Each gray level corresponds to different binary data sequences. For example, the heater  22  used for heating the dyes is driven by the driving signal. As mentioned before, the heater  22  must be preheated first to reach the predetermined printing temperature. If the driving signal has a binary data sequence “1”, “0”, “0”, “0”, “0”, the heater  22  is first activated for the duration Tp, then is deactivated for successive four durations Tu. Please note that the binary value “1” in the beginning of the driving signal represents the preheating operation. Therefore, a corresponding gray level with a value “0” is generated. If the driving signal has a binary data sequence “1”, “0”, “1”, “0”, “0”, the heater  22  is activated once after the preheating operation. Therefore, the heater  22  related to the gray level with a value “1” will transfer more dyes onto the photo paper  16  than the heater  22  related to the gray level with a value “0”. If the driving signal has a binary data sequence “1”, “1”, “0”, “0”, “0”, the heater  22  is activated once after the preheating operation, too. Therefore, the heater  22  related to the gray level with a value “2” is activated continuously after the preheating operation so that the heater  22  related to the gray level with a value “2” will transfer more dyes onto the photo paper  16  than the heater  22  related to the gray level with a value “1”. Similarly, the heater  22  related to the gray level with a value “3” is activated twice after the preheating operation so that the heater  22  related to the gray level with a value “3” will transfer more dyes onto the photo paper  16  than the heater  22  related to the gray level with a value “2”. As shown in the table, one heater  22  only requires an operation time equaling Tp+4*Tu to transfer dyes onto the photo paper  16  according to any of the six different gray levels. However, the prior art printing method needs the operation time equaling TP+6*Tu to transfer dyes onto the photo paper  16  according to any of the six different gray levels. Please note that only six gray levels are shown in the table for simplicity, and the claimed printing method is not limited to only six gray levels. That is, the claimed printing method can be used for generating a fixed number of different gray levels with a shorter operation time or generating a greater number of different gray levels with a fixed operation time when compared with the prior art printing method. 
     Furthermore, the photo printer  10  mentioned above further comprises a fixture (not shown) to hold and move the photo paper  16 . The thermal print head  12  is fixed inside the photo printer  10  to transfer the color dyes onto the photo paper  16 . In addition, the thermal print head  12  can be movably positioned inside the photo printer  10 , and the fixture is used for fixing the photo paper  16 . Then, the thermal print head  12  is gradually moved to transfer the color dyes onto the photo paper  16  line by line. 
     In contrast to the prior art printing method, the claimed printing method makes use of the thermal accumulation effect to interpolate new gray levels between original successive gray levels. On one hand, each pixel on the photo paper will have a better color resolution because of the increased gray levels when the required printing time for one line is fixed. On the other hand, the printing speed is improved because the printing time for one line is reduced when the required number of gray levels is fixed. In conclusion, the claimed printing method can improve output quality and printing efficiency. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teaching of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.