Printing method of color video printer
BACKGROUND OF THE INVENTION 
     The present invention relates to a printing method in a video printer and, in particular, a high-speed printing method for efficiently reducing a printing time needed to print one document line in a color video printing apparatus having a line memory, thereby enabling a high-speed printing of a document. 
     In general, it has been known that three elementary colors of Cyan, Magenta and Yellow for printing a given document image in a color video printer may be obtained from a given color conversion process of the so-called, three primary colors Red, Green and Blue of a light ray. The former three colors are mixed to each other to produce a desired color image in the color video printer. That is, the latter three primary colors Red, Green and Blue are respectively, or compositively, converted to the former three elementary colors Yellow, Magenta and Cyan to obtain a desired number of color gradation, for example, 256 gradations of representing a given color image. 
     In a conventional printing method, for the color video printer utilizing 256 gradations to print one document line from a line memory, a total of 256 times of thermal print head operations are required to complete printing of one line, which operations have no particular reference to the gradations and are carried out either in a thermal or in a non-thermal state. In this case, the total 256 times of thermal print head operations should be always carried out irrespectively of a gradation characteristic in the corresponding line, which inevitably may lead to an unnecessarily long printing time on a color video printer. 
     For example, when making a print for a given video image provided from a television screen to a color video printer, all of the three elementary colors Yellow, Magenta and Cyan may not be used in their respective maximum degree of thickness. Hence, there is would be no need to have a thermal print head of the color video printer operating 256 times for each color representation. 
     It is therefore an object of the present invention to provide a high-speed printing method capable of considerably reducing a printing time consumption in a color video printer by having at least one line memory. 
     According to one aspect of the invention, the method may be achieved by limiting a frequency of thermal operation in a thermal printing device of a printer below the maximum gradation number of a print line. 
     According to another aspect of the invention, the method preferably includes a method of controlling printing in a color video printer having at least one line memory provided with a plurality of gradation data corresponding to a respective color component, the line memory for receiving at least one line data indicative of a given color image from a video memory, the method further comprising the steps of: 
     searching the largest value of one line data from the line memory; 
     designating the largest value as a maximum value of heating gradation relative to the one line data; and 
     terminating one cycle of printing operation for the one line data when reaching the maximum value of the heating gradation, irrespectively of a total heating gradation number. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily enjoyed as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like members indicate the same or similar components, wherein: 
     FIG. 1 shows a schematic block diagram of a part of a color video printer that the present invention is applied to; 
     FIG. 2 shows a flow chart of a printing method according to the invention; 
     FIG. 3 shows a schematic diagram indicative of a preferred embodiment of a line memory used in the invention; and 
     FIG. 4a to 4e show some operation stages according to FIG. 1 block diagram. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a color video printer wherein a high-speed printing method is applied comprises a microcomputer 20 for controlling all the operations in the color video printer; a line memory 18 for storing gradation data to be read or written under control of the microcomputer 20; a gradation counter 19 responsive to the microcomputer 20 for counting a gradation number in the video printer. Three primary colors Red, Green and Blue provided by a video RAM (not shown) are converted into three elementary color components Cyan, Magenta and Yellow to be used later, which components are put into a memory 12 together with various control data to control a thermal print head in the printer. A comparator 14 compares the gradation data read out of the line memory 18 with the counted value of the gradation counter 19. A data switching circuit 13, coupled to the memory 12 and the line memory 18, provides a part of the data retrieved from the memory 12 to the line memory or enables transmission of data from the line memory to the comparator. A data driver 15 enables transmission of the output of the comparator 14 to either one of a first buffer 16 or a second buffer 17 in dependence upon an address provided from the line memory. Both of the first and second buffers are to enable transmission of the output of the data driver is in synchronism with a clock, to a latch 21 respectively. Upon reception of a latch signal (LTH), the latch 21 receives gradation data from the first and second buffers and latches the received data to a driver for driving a thermal head assembly upon reception of a strobe signal (STB). 
     FIG. 3 is an exemplary diagram showing that a given number of gradation is stored into a respective address location corresponding to each line of the line memory; and FIGS. 4a to 4e are timing diagrams illustrating various operative stages of FIG. 1. FIG. 4a represents a data waveform to be stored into the first buffer 16. FIG. 4b represents a data waveform to be stored into the second buffer 17. FIG. 4c a clock; FIG. 4d a latch signal (LTH), and 4e represents a strobe signal (STB), having a printing duration of T1 for each print line of the printer. 
     Referring back again to FIG. 1, data of one print line retrieved in sequence from line memory 18 is compared with a series of gradation count numbers from the gradation counter 19, in either one set of odd numbers, viz., 1,3,5,7 . . . 255 and even numbers, viz., 2,4,6,8 . . . 256by the comparator in order to produce a logic high or low signal in dependence upon the status of the data. A logic high signal is produced if the data is larger than the gradation counter numbers, while a logic low signal is produced if the data is less than the gradation counter number. The data driver 15 receives the logic high or low signal from the comparator 14 and drives the received data to either one of the first buffer 16 and the second buffer 17, in dependence upon an address of the retrieved data in the line memory 18, wherein the lower addresses of 1 to 256 are delivered to the second buffer and the higher addresses 512 are delivered to the first buffer. 
     The data stored in the first and second buffers, as shown in the waveforms of FIGS. 4a and 4b, is latched by the latch circuit 21 in dependence upon a reception of a latch signal (LTH) having a low logic level of FIG. 4d, generated subsequent to an occurrence of 256 clocks of FIG. 4c. Thereafter, when a strobe signal (STB) goes low, as shown in FIG. 4e prior to latching the next line memory data, the driver 22 receives the latched data from the latch 21 for driving the thermal head assembly 23. A driving time in the thermal head assembly needed to perform a color printing for one line in the driver 22 is limited to a maximum gradation number for the corresponding line. 
     The microcomputer 20 carries out a detection of the maximum gradation number to in order control the driver 22. For example, assuming that a series of addresses (1 through 512) in one line of the line memory 18 have a plurality of gradation data as shown in FIG. 3 and that its maximum gradation data may be `10` in the 511th address location, the microcomputer 20 initializes an address variable (n) designating a given address in the line memory, that is, n=1, and then sets gradation data (=3) of a first address location into a first variable value (k), in step 2a of FIG. 2. Thereafter, in step 2b, the data in the first address location is stored into a register (J). The first variable value (k) is then checked in step 2c whether it is equal to or smaller than the data of the first (n=1) address location. Here, because the data is 3, the first variable value (k) is also 3. If n=1, for example, then step 2d is carried out to maintain the first variable value (k) as it is. In a decision block of step 2e, the address variable (n) is checked whether it reaches a number of 512. If the address variable (n) does not reach the number of 512, is added in step 2h and returned to step 2b again. That is, the steps 2b through 2e are repeated until the address variable (n) finally reaches the number of 512 in the step 2e. As a result of a close loop operation of the steps 2b through 2e, the final first variable value (k) after 512 times of comparing operation will be 10&#39; in reference to the exemplary one line of the line memory of FIG. 3. Therefore, the value 10&#39; is designated as a maximum gradation data in step 2f, and the heating element 23 is driven at only 10 times prior to execution of a next line printing. 
     Therefore, for example, assuming that a Yellow color element is to be printed with a half degree (50%-thick Yellow), not a full degree (100%-thick Yellow), it would be enough to drive the heating element only 128 times, and not 256 times as required by the conventional video printer. A total time consumption necessary to one print line from the line memory having a 50%-thick is reduced by at least one half. For instance, a 128-gradation of a given color component, for example, of Yellow, will consume a maximum printing time of 8.192 msec (=64 μsec×128), which is considerably faster than 16.384 msec (=64 μsec ×256) for a 100%-thick, 256-gradation color component. Thus, a printing time is decreased for one print line up to a half of the time required for the case of a conventional 256 gradation printing. 
     Moreover, since a usual color video printer employs three kinds of colors such as Yellow, Magenta and Cyan, a total time of about 24 msec (8 msec×3=24 msec) would be saved and more time could be decreased in case where less thick color is applied to the video printer, for example, for 30% thick Yellow color component in one print line from the line memory. 
     As is understood from the aforementioned description of a preferred embodiment of the present invention, a maximum gradation data of one memory line in a line memory of a color video printer is determined, by which maximum data a heating element is driven to printing. Thus, the invention eliminates waste of printing time, thereby video printer.