Patent Publication Number: US-6222570-B1

Title: Thermal printing method and thermal printer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thermal printing method and thermal printer. More particularly, the present invention relates to a thermal printing method and thermal printer in which irregularities in a recorded density are prevented from occurrence. 
     2. Description Related to the Prior Art 
     A thermal printer includes a thermal head, which applies heat to recording material to print an image. There are two examples of thermal printers, including a direct thermal recording type in which the thermal head heats thermosensitive recording paper to color it directly, and a thermal transfer type in which a back surface of ink ribbon is heated by the thermal head to transfer ink to paper. 
     To record a color image according to the direct thermal recording, a thermosensitive recording paper called “Thermo-Autochrome paper” is used. The recording paper includes a support and at least three thermosensitive coloring layers. The coloring layers are cyan, magenta and yellow coloring layers. The recording paper and the thermal head are conveyed relative to one another in a sub scan direction. The thermal head presses and heats the recording paper to record a full-color image. To color the coloring layers selectively, the recording paper has heat sensitivity different between the coloring layers. The cyan coloring layer is positioned the most deeply, and has the highest heat sensitivity. The yellow coloring layer is positioned the least deeply, and has the lowest heat sensitivity. Before recording to the next one of the coloring layers, a previously colored one of the coloring layers is fixed by application of ultraviolet rays, and prevented from being colored for higher coloring density. 
     The thermal head includes an array of numerous heating elements arranged in a main scan direction, for recording one line after another of each of the colors. For this operation, the heating elements apply bias heat energy to the recording paper. The bias heat energy is such an amount that it heats the recording paper to set the recording paper in a state short of developing color and prepares it for further application of heat energy. Then the heating elements apply image heat energy to the recording paper. The image heat energy is an amount for coloring at a desired density. Pixels arranged on the recording paper virtually are colored to record dots. The bias heat energy is constant and depends on each of the coloring layers. The image heat energy changes and depends on input image data representing a gradation data. When the image heat energy is finished, the heating elements are left to stand in a cooling period. After the heating elements are cooled, one other line is recorded. 
     A coefficient μ of friction between the thermal head and the recording paper changes and depends upon a surface temperature of the recording paper. If the temperature of the vicinity of the heating elements in the thermal head is low, the coefficient μ of friction is great. Load to the conveyance is high. If the temperature of the vicinity of the heating elements is high, the coefficient of friction is small. The load to the conveyance is low. 
     When the temperature of the thermal head is changed to increase or decrease the load to the conveyance, there is an increase or decrease in distortion of the platen roller or platen drum, distortion of a feed roller or conveyor roller set, extension or shrinkage of belts for transmission of rotation to the conveyor roller set, and distortion of a roller shaft. A paper conveyor system for the recording paper is associated with a stepping motor as a power source. A rotor of the motor may be stopped in a position different from an accurate stop position, but where the magnetic force and the load to the conveyance are balanced. Note that such recoverable changes in the state are herein referred to as distortions of the paper conveyor system. At each time that the load to the conveyance changes, the distortions of the paper conveyor system are either increased or decreased. Then a conveying speed of the recording paper changes in a temporary manner. 
     If the conveying speed is higher than a predetermined, value an interval between recorded lines is increased. Heat energy (mJ/mm 2 ) applied by the heating elements to the recording paper per unit area is small. Coloring density of a line being recorded becomes low, to create a low-density stripe which looks blank or white. As the conveying speed is lower than a predetermined, value the interval between adjacent lines is lowered. The coloring density of the line being recorded rises, to create a high-density stripe which looks dark or black. Those high- and low-density stripes constitute unevenness in the coloring density. 
     It is conceivable that a synthesized image is printed by combining an input image, a template image preset to be printed about the input image, and a frame to be disposed about the input image and inside the template image. In other words, the frame constitutes a periphery of an insertion region into which the input image is inserted inside the template image. The frame may have portions extending in parallel with the main scan direction. When the heating elements are positioned at portions of the frame, the heating elements have a relatively low temperature. If an image surrounded by the frame has a high density, portions for such an image are positioned at the heating elements so that a greater part of the heating elements abruptly comes to generate high heat energy. The temperature of the thermal head rises abruptly. The surface temperature of the recording paper in contact with the heating elements rises. The friction coefficient decreases. The conveying speed of the recording paper becomes higher temporarily than a predetermined value. There occurs a low-density stripe where the coloring density of a line is considerably low. 
     If blank portions of the frame are printed immediately after printing a high-density portion, the conveying speed of the recording paper becomes lower temporarily than a predetermined value. A high-density stripe occurs, in which the coloring density of a line being recorded is remarkably high. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, an object of the present invention is to provide a thermal printing method and thermal printer in which unevenness in a recorded density is prevented from occurrence. 
     In order to achieve the above and other objects and advantages of this invention, a thermal printing method is provided, in which recording heat energy is applied by a thermal head to an effective recording region on a recording material, the thermal head includes an array of heating elements arranged in a main scan direction, the thermal head and the recording material are conveyed relative to one another in a sub scan direction substantially perpendicular to the main scan direction, for recording at least one input image to the recording material. In the thermal printing method, the effective recording region is separated into an insertion region, a template region and a blank frame space, the frame space extending in a linear shape with a small width, a first borderline being defined between the insertion region and the frame space, a second borderline being defined between the template region and the frame space, the first borderline including at least one first borderline segment being straight or curved, extending crosswise to the sub scan direction and being inclined with reference to the main scan direction. The input image is recorded in the insertion region. At least one template image is recorded in the template region, so as to constitute a synthesized image in combination with the input image. 
     In a preferred embodiment, the second borderline includes at least one second borderline segment being straight or curved, extending crosswise to the sub scan direction and being inclined with reference to the main scan direction. 
     The heating element array, while positioned at the first or second borderline segment, changes progressively from one of first and second states to the other, and when in the first state, a small number of heating elements included in the heating element array are driven, and when in the second state, a great number of heating elements included in the heating element array are driven. 
     By this construction, unevenness in a recorded density is prevented from occurrence, because there occurs no such event that the number of driven heating elements in the array would abruptly decrease. 
     In a preferred embodiment, the insertion region has a substantially quadrilateral shape, and the template region is disposed around the insertion region. 
     The template image is predetermined. 
     The input image has a rectangular quadrilateral shape, and has first and second side lines extending in the main scan direction, the first side line is associated with the first borderline segment, but offset therefrom with a difference. Furthermore, the input image data is corrected in consideration of the difference, for adapting a portion of the input image along the first side line to the first borderline segment. 
     According to another aspect of the invention, the effective recording region is separated into an insertion region and a template region by use of a borderline, the borderline including at least one borderline segment being straight or curved, extending crosswise to the sub scan direction and being inclined with reference to the main scan direction. The input image is recorded in the insertion region. At least one template image is recorded in the template region, so as to constitute a synthesized image in combination with the input image. 
     According to still another aspect of the invention, the effective recording region is separated into an insertion region and a template region by use of a frame image, the frame image extending in a linear shape with a small width, including at least one frame image segment extending in the main scan direction, and having a predetermined density. The input image is recorded in the insertion region. At least one template image is recorded in the template region. The frame image is recorded in the effective recording region between the insertion region and the template region, so as to constitute a synthesized image in combination with the input image and the template image. 
     The insertion region has a substantially quadrilateral shape, and the template region is disposed around the insertion region. 
     The template image and the frame image are predetermined. 
     An entirety of a frame image has the predetermined density. 
     The frame image is gray. 
     The recording heat energy is a combination of bias heat energy and image heat energy, the bias heat energy is so predetermined as to set the recording material in a heated state directly short of starting coloring the recording material, and the image heat energy is determined according to the density to be recorded to the recording material. The predetermined density is recordable by applying at least 120% as high heat energy to the recording material as the bias heat energy. 
     According to a further aspect of the invention, a thermal printer has a first memory for storing the input image data. A second memory stores template data and information of first and second borderlines, the template data representing at least one template image, the first and second borderlines being disposed inside the effective recording region, the effective recording region being separated into an insertion region, a template region and a blank frame space, the frame space extending in a linear shape with a small width, the first borderline being defined between the insertion region and the frame space, the second borderline being defined between the template region and the frame space, the first borderline including at least one first borderline segment being straight or curved, extending crosswise to the sub scan direction and being inclined with reference to the main scan direction. An image synthesis circuit produces synthesized image data in accordance with the input image data, the template data and the information of the first and second borderlines, the synthesized image data representing a synthesized image in which the input image is disposed in the insertion region and the template image is disposed in the template region. A third memory stores the synthesized image data, the thermal head recording the synthesized image according thereto. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent from the following detailed description when read in connection with the accompanying drawings, in which: 
     FIG. 1 is an explanatory view illustrating a thermal printer of the present invention; 
     FIG. 2 is an explanatory view in plan, illustrating a relationship between a thermal head and a recording sheet with pixels; 
     FIG. 3 is an explanatory view in plan, illustrating a template image, a frame region and an insertion region; 
     FIG. 4 is an explanatory view in plan, illustrating a synthesized image which includes an input image and the template image; 
     FIG. 5 is an explanatory view in plan, illustrating a thermosensitive recording sheet with the synthesized image of FIG. 4; 
     FIG. 6 is an explanatory view in plan, illustrating another preferred template image, a frame region and an insertion region; 
     FIG. 7 is an explanatory view in plan, illustrating still another preferred template image which is associated with a gray frame image; 
     FIG. 8 is an explanatory view in plan, illustrating another preferred template image at which there is no frame region; and 
     FIG. 9 is an explanatory view in plan, illustrating a recording sheet with a synthesized image printed according to the prior art. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE PRESENT INVENTION 
     In FIG. 1, a color thermal printer is illustrated. A color thermosensitive recording sheet  10  is fed from a sheet supply cassette (not shown), and conveyed to a platen roller  11 . A thermal head  12  is positioned opposite to the platen roller  11 . A bottom of the thermal head  12  has an array  12   a  of heating elements  13  arranged in a main scan direction or width direction of the recording sheet  10 . See FIG.  2 . The thermal head  12  is movable pivotally at a shaft  12   b  and between a press position and a retracted position. The thermal head  12 , when in the press position, is pressed against the recording sheet  10  on the platen roller  11  for image recording, and when in the retracted position, is away from the recording sheet  10 . 
     The recording sheet  10  includes a support, thermosensitive coloring layers, and a transparent protective layer. The coloring layers are colorable in cyan, magenta and yellow. The magenta coloring layer is optically fixable in response to ultraviolet rays peaking at a wavelength of 356 nm. The yellow coloring layer is optically fixable in response to near ultraviolet rays or peaking at a wavelength of 420 nm. Recording to the coloring layers are in a sequence according to a sequence of the overlaid manner of the coloring layers. If a thermal printer should be used with the recording sheet  10  in which the yellow and magenta coloring layers would be positioned in reverse to those in the present invention, then the recording is effected in the sequence of magenta, yellow and cyan. 
     In relation to the coloring layers, the magnitude in recording heat energy to be applied thereto is according to the depth in the layer position. In the recording sheet  10 , the yellow coloring layer requires the lowest energy of recording heat for developing its color. The cyan coloring layer requires the highest energy of recording heat for developing its color. The recording heat energy is a combination of bias heat energy and image heat energy. The bias heat energy is such an amount that it heats the recording sheet  10  to set the recording sheet  10  in a heated state short of developing color and prepares it for further application of heat energy. The image heat energy is applied after application of the bias heat energy. The bias heat energy is a predetermined constant for each of the coloring layers. The image heat energy is determined for each pixel and according to the intended density to be recorded. 
     A thermal head driver  14  drives the thermal head  12 , which applies heat of bias heat energy and image heat energy to the recording sheet  10  being conveyed forwards, namely to the left in the drawing. An image of each of the three colors is recorded line after line, so that a full-color image is recorded in three-color frame-sequential recording in which the recording sheet  10  is conveyed back and forth for three times. 
     A feed roller or conveyor roller set  15  is disposed downstream from the platen roller  11 , and includes a capstan roller  15   a  and a pinch roller  15   b . The capstan roller  15   a  is driven by a stepping motor  16 . The pinch roller  15   b  is a driven roller rotated during conveyance of the recording sheet  10 . The pinch roller  15   b  is movable between a nip position and a released position, and when in the nip position, nips the recording sheet  10  between it and the capstan roller  15   a , and when in the released position, is away from the recording sheet  10 . When the capstan roller  15   a  is rotated by the stepping motor  16 , the feed roller set  15  rotates in forward and backward directions, to convey the recording sheet  10  back and forth. For the image recording, the stepping motor  16  is supplied with drive pulses at a constant frequency, and is rotated continuously. 
     A roller shaft  17  of metal is included in the capstan roller  15   a , and directly connected with an output shaft of the stepping motor  16 . Of course, it is possible to transmit rotation of the stepping motor  16  to the roller shaft  17  by use of pulleys and belts, or a train of gears. There are a yellow fixer  20  and a magenta fixer  21  arranged downstream from the feed roller set  15 . The yellow fixer  20  is constituted by an ultraviolet lamp  20   a  and a reflector  20   b , and emits ultraviolet rays, of which a peak is at the wavelength of 420 nm to fix the yellow color. The magenta fixer  21  is constituted by an ultraviolet lamp  21   a  and a reflector  21   b , and emits ultraviolet rays, of which a peak is at the wavelength of 365 nm to fix the magenta color. 
     For an image to be recorded, a video camera, a scanner or the like is used for photographing an object in a photographic field or for reading an image on an original sheet material. Or an image may be initially stored in recording media such as magnetic recording media, and a memory card, from which the image is read for operation of the printer. An input image memory  25  stores the yellow, magenta and cyan image data. In the color thermal printer, an input image is inserted in an insertion region inside a template image, to record a synthesized image. The input image memory  25  as a first memory is used, to which input image data of the input image is written. An image synthesis circuit  26  reads the input image from the input image memory  25 . The input image data of each of the colors is, for example, 8-bit data, and represents a gradation value with highness according to density of a pixel to be recorded. The density is high according to the highness of the value of the input image data. 
     A template ROM  27  as a second memory is connected with the image synthesis circuit  26 . The template ROM  27  stores template data which represent plural template images. For recording the synthesized image, an operation panel  28  is operated for selection of template images as desired, and revisions of an input image to be inserted, such as trimming, enlargement, reduction, and rotational changes of orientation of the input image. Note that, instead of the template ROM  27 , a magnetic recording medium or memory card may be used for storing the template image, and inserted into the body of the thermal printer. 
     The image synthesis circuit  26  reads the template data of the template image from the template ROM  27  according to the selected one of the template image at the operation panel  28 . The template data is written to a work memory  29  as a third memory. The work memory  29  is a work area which is used for producing synthesized images, and to which the synthesized image data of a synthesized image is written. The image synthesis circuit  26  produces the synthesized image by writing the input image data from the input image memory  25  at an address in the template data in the work memory  29  for the predetermined insertion region. 
     For the image recording, synthesized image data of each of the colors is read from the work memory  29  sequentially line by line, and sent to the thermal head driver  14 . In the process of the bias heating, the thermal head driver  14  drives the heating elements  13  in the heating element array  12   a  at the same time. In the process of the image heating, the thermal head driver  14  selectively drives the heating elements  13  according to the synthesized image data. 
     In FIG. 2, a recording state of the recording sheet  10  is illustrated. The array  12   a  of the heating elements  13  in the thermal head  12  records one line after another for each of the colors. Each line extends in the main scan direction, and includes a plurality of pixels PS. The pixels PS are recorded by the heating elements  13 . The thermal head  12  records each one line by operation of bias heating, image heating, cooling of the heating elements  13  while the feed roller set  15  conveys the recording sheet  10  by a range of each one line in the sub scan direction. Upon the finish of the conveyance by the one-line range, the next line starts being recorded. 
     In FIG. 3, one example among the template images stored in the template ROM  27  is illustrated. In FIG. 3, a template image  31  is constituted by a background image  32 . A frame region or frame space  33  is located about an insertion region  34 , into which an input image as principal image is inserted and recorded as a part of a synthesized image. It is to be noted that plural input images may be inserted into the template image  31 . The background image  32 , although preset in the thermal printer, may be an externally entered background image, and also may be selectable from a plurality of preset background images. 
     The insertion region  34 , surrounded by the frame region  33 , has a rectangular quadrilateral shape. The frame region  33  has a predetermined small width, and has a white color without coloring of any of the yellow, magenta and yellow. It is possible for the frame region  33  to be colored lightly. In other words, the frame region  33  may have a frame image where the small-width portion has a certain color at a small density. Frame region segments  33   a  of the frame region  33  are extended almost in the main scan direction, but with an inclination, and are non-parallel to the main scan direction. In the drawing, the broken lines indicate the parallelism to the main scan direction, with reference to which the frame region segments  33   a  are inclined. The inclination is for the purpose of avoiding irregularities in a recorded density due to changes in the load in the conveyance. It is to be noted that the inclination in FIG. 3 is depicted with exaggeration, and is considerably smaller than illustrated, in such a manner that users or viewers of the recording sheet  10  as a hard copy apparently recognizes the exactly horizontal orientation for the frame region segments  33   a  as if the frame region segments  33   a  were not inclined. Note that it is possible to provide the frame region segments  33   a  with a relatively great inclination for the purpose of appearance. 
     Thus unevenness in the density in the printing is suppressed by use of the template image  31  with the frame region  33  non-parallel with the main scan direction. The portions of the frame region  33  extending in the sub scan direction are not correlated with changes in the load in the conveyance, and may be parallel to the sub scan direction or inclined. 
     The operation of the present embodiment is described now. To print a synthesized image, at first the operation panel  28  is operated by a user to enter a signal for instructing synthesis of an image. A desired one of the preset template images  31  is selected. According to the selected template image, the image synthesis circuit  26  reads three-color template data of the yellow, magenta and cyan from the template ROM  27 , and writes them to the work memory  29 . Then the user operates the operation panel  28  and causes a main component of the thermal printer to obtain an input image or principal image. The input image is subjected to photometry in the manner of three-color separation by means of a scanner or the like, so that three-color image data of the yellow, magenta and cyan are written to the input image memory  25 . 
     Upon entry of a command signal for starting printing by operating the operation panel  28 , the image synthesis circuit  26  reads three-color image data of the principal image from the input image memory  25 , and writes the same to the work memory  29  at an address associated with the background image  32 . In FIG. 4, synthesized image data is written to the work memory  29 , and represents a synthesized image  42 , which is a combination of the template image  31  and an input image  41  or principal image. In the synthesized image  42 , the periphery of the input image  41  is surrounded by the frame region  33 . Borderlines between the input image  41  and the frame region segments  33   a  and between the background image  32  of the template image  31  and the frame region segments  33   a  are extended nearly in the main scan direction, but are exactly inclined with reference to the main scan direction. 
     When the image synthesis is finished, the recording sheet  10  is supplied from the supply cassette, moved between the platen roller  11  and the thermal head  12  at retracted position, and sent toward the feed roller set  15 . When a front edge of the recording sheet  10  comes to the position of the feed roller set  15 , the pinch roller  15   b  is shifted from the released position to the nip position, and nips the front edge of the recording sheet  10 . A photo sensor (not shown) is disposed in the vicinity of the feed roller set  15 , and detects whether or not the front edge of the recording sheet  10  has come to the position of the feed roller set  15 . 
     When the feed roller set  15  nips the recording sheet  10 , the thermal head  12  is moved to a press position. The ultraviolet lamp  20   a  is turned on. Then the stepping motor  16  rotates forwards upon supply of drive pulses at the constant frequency. The stepping motor  16  rotates the capstan roller  15   a  forwards, to convey the recording sheet  10  forwards at a constant speed. 
     A front edge of an effective recording region of the recording sheet  10  comes to the heating element array  12   a  of the thermal head  12 . Then a first line of the synthesized image data of yellow is read from the work memory  29 , and sent to the thermal head driver  14 . The thermal head driver  14  drives the heating elements  13  of the thermal head  12  simultaneously at first, for application of bias heat energy for yellow to the recording sheet  10 . 
     Then the thermal head driver  14  drives the heating elements  13  according to first yellow line data in the synthesized image data, for image heating. The heating elements  13  generate the image heat energy according to the yellow synthesized image data, and apply it to the recording sheet  10 . If a pixel has the yellow synthesized image data of zero (0), then corresponding ones of the heating elements  13  are not driven, and generate no heat. 
     The heating elements  13  are colored at a density according to the synthesized image data of yellow on the condition of the coloring characteristic of the yellow coloring layer. Yellow dots are formed in the pixels PS to constitute the first yellow line. After the application of the image heat energy, the heating elements  13  are left to stand for the purpose of cooling. 
     During the cooling period, second yellow line data in the synthesized image data is read from the work memory  29 , and sent to the thermal head driver  14 . When a position in the recording sheet  10  for a second line reaches the heating element array  12   a , the cooling period finishes. The second line starts being recorded. In a manner similar to the first line, the heating elements  13  are driven simultaneously for the bias heating. At the end of this, the heating elements  13  are selectively driven according to the synthesized image data for the second line of yellow, so that the image heat energy is applied in order to record the second line. Then a third line and succeeding lines are recorded for the synthesized image of yellow. 
     Portions of the recording sheet  10  with the yellow synthesized image recorded are moved to the position of the yellow fixer  20 . Yellow fixing ultraviolet rays are emanated by the ultraviolet lamp  20   a  and fix the yellow coloring layer. After the recording of the final line of the yellow synthesized image, the recording sheet  10  are conveyed farther until the rear edge of the effective recording region is moved past the yellow fixer  20 . 
     When a rear edge of the effective recording region is conveyed past the yellow fixer  20 , then the ultraviolet lamp  20   a  is turned off. The stepping motor  16  is stopped provisionally. The thermal head  12  is swung to the retracted position. Then the stepping motor  16  is rotated backwards. The feed roller set  15  conveys the recording sheet  10  to an upstream position along the conveying path. During the conveyance, the front edge of the effective recording region reaches the position of the thermal head  12 . Rotation of the feed roller set  15  is stopped. The thermal head  12  is swung to the press position. Furthermore, the ultraviolet lamp  21   a  is turned on. 
     After the thermal head  12  is set in the press position, the stepping motor  16  is rotated again in the forward direction, for the feed roller set  15  to convey the recording sheet  10  forwards along the conveying path. In the course of the conveyance, the heating element array  12   a  applies magenta bias heat energy and magenta image heat energy to the recording sheet  10 , and records a magenta synthesized image one line after another. In the magenta image heating, the heating elements  13  are selectively driven according to the synthesized image data of magenta read from the work memory  29  one line after another. 
     The portion of the recording sheet  10  with the magenta image recorded is subjected to magenta fixing ultraviolet rays from the ultraviolet lamp  21   a . The magenta coloring layer is fixed optically. 
     The rear edge of the effective recording region is conveyed past the magenta fixer  21 . The feed roller set  15  conveys the recording sheet  10  in the upstream direction in the manner the same as above. Then the recording sheet  10  is conveyed again forwards in the downstream direction. The heating element array  12   a  records the cyan synthesized image one line after another. The recording sheet  10  after recording the final cyan line is further conveyed, and ejected through the exit slot. 
     In FIG. 9, a synthesized image  44  recorded according to the prior art is illustrated. The template image  31  is provided with a frame region or frame space  45 , which is defined about the insertion region  34  in the manner of the template image  31  in FIG.  3 . Frame region segments  45   a  in the frame region  45  are parallel with the main scan direction. The synthesized image  44  is printed in a combination of the input image  41  and the background image  32  in the template image  31  having a considerably high density. 
     In the recording of the area A 1  with the thermal head  12 , nearly all the heating elements  13  are driven for the bias heating and image heating. The average temperature of the heating elements  13  or the temperature of the heating element array  12   a  is high. The temperature of the surface of the recording sheet  10  is high in contact with the heating element array  12   a . Thus the coefficient of friction between the heating element array  12   a  and the recording sheet  10  is kept small. The load to the conveyance is relatively small. There occurs no great distortion of the platen roller  11  or the feed roller set  15 , no great distortion of the roller shaft  17  as a transmission, no great error in the stop position of the rotor or the stepping motor  16 , and no great amount of distortion in the conveyor system. 
     After the area A 1  is recorded, the heating element array  12   a  relatively comes to an area A 2  having the frame region segment  45   a  parallel with the main scan direction. The first line of the area A 2  starts being recorded. All the heating elements  13  operate for the bias heating. But some of the heating elements  13  associated with the frame region segment  45   a  in the heating element array  12   a  are stopped and do not generate the image heat energy. The temperature of the heating element array  12   a  abruptly becomes low. A friction coefficient between the recording sheet  10  and the heating element array  12   a  becomes high to increase the load in conveyance. Distortion in the conveyor system increases to lower the conveying speed of the recording sheet  10 . A conveying amount of the recording sheet  10  is decreased. 
     Thus the heat energy per unit area (mJ/mm 2 ) applied to the first line of the template image  31  in the area A 2  becomes high. A high-density stripe  50  being deeply colored in yellow is recorded in the background image  32 . 
     For the second line and its succeeding lines of the area A 2 , the heating elements  13  generate only a small amount of heat, because only the bias heating is effected. The temperature of  12   a  becomes still lower. However this change in the temperature is not abrupt. The load in the conveyance comes to balance with the distortion in the conveyor system. The recording sheet  10  being conveyed comes again to have the conveying speed. The background image  32  is recorded at the yellow density being expected originally. 
     Then the heating element array  12   a  is relatively moved to the area A 3 . In the area A 3 , portions of the template image  31  and the input image  41  are recorded. Nearly all the heating elements  13  start recording the input image  41  in accordance with the frame region segment  45   a  parallel to the main scan direction. The temperature of the heating element array  12   a  abruptly rises. The coefficient of friction between the recording sheet  10  and the heating element array  12   a  drops, to decrease the load to the conveyance. In response to the decrease in the load, an amount of distortion of the conveyor system is also decreased. The speed of conveying the recording sheet  10  becomes high abruptly to increase the conveying amount of the recording sheet  10 . Thus the heat energy per unit area (mJ/mm 2 ) becomes low. A low-density stripe  51  being colored in yellow only lightly is recorded in the background image  32 . 
     In the recording to the area A 4  after the area A 3 , the high-density stripe  50  appears in the image in the same manner as the recording to the area A 2  after the area A 1 . In the recording to the area A 5  after the area A 4 , the low-density stripe  51  appears. Also, high- and low-density stripes occur in the magenta recording and the cyan recording. The high-density stripes of the three colors are overlapped and become black stripes finally. 
     Note that a drop in the density occurs also upon the start of recording the area A 1 . However, starting points of the area A 1 , even if colored lightly, are indiscernible with a blank margin of the recording sheet  10 , and do not cause any problem. Furthermore, no problem occurs at the starting points of the area A 3  between two portions of the low-density stripe  51 , or at the ending points of the area A 4  between two portions of the low-density stripe  51 . 
     In contrast to the recording of the synthesized image  44 , the frame region segments  33   a  with the inclination according to the present invention reliably prevents unevenness in the recorded density. In the sequence from the recording of the background image  32  to the recording of the frame region segment  33   a , the number of the heating elements being turned off for the image heating is increased gradually. The number of the heating elements being turned on for the image heating is decreased gradually. This operation is also effective in the sequence from the recording of the input image  41  to the recording of the frame region segment  33   a . There does not occur an event in which all the heating elements  13  stop in the image heating. The heating element array  12   a  is kept from abruptly having a low temperature. The conveying speed does not abruptly become low. No black stripe occurs. 
     In the sequence from the recording of the frame region segment  33   a  to the recording of the background image  32  in the template image  31 , the number of the heating elements being turned on for the image heating is increased gradually. The number of the heating elements being turned off for the image heating is decreased gradually. This operation is also effective in the sequence from the recording of the frame region segment  33   a  to the recording of the input image  41 . The temperature of the heating element array  12   a  does not rise abruptly. The conveying speed does not rise abruptly. No white stripe occurs. 
     Consequently, the synthesized image  42  in FIG. 5 without dark or blank stripes can be recorded on the recording sheet  10  with the small inclination of the frame region segments  33   a  with reference to the main scan direction. Of course, the inclination of the frame region segments  33   a  is sufficiently small. The image quality of the print of the recording sheet  10  is not influenced. 
     In FIG. 6, another preferred embodiment is illustrated, in which a template image  55  is provided with a frame region or frame space  56 , of which frame region segments  56   a  are horizontal, but curved, and not parallel with the main scan direction. Thus no stripes of low or high density are created. Of course, it is possible to enhance or reduce the curvature of the frame region segments  56   a  for the purpose of providing the frame region segments  56   a  either with an agreeably conspicuous appearance or with an indiscernible appearance. 
     In FIG. 7, a template image  57  is provided with a frame image  58  of a light gray color for the purpose of avoiding the occurrence of dark or blank stripes. Frame image segments  58   a  are parallel with the main scan direction. In the use of the template image  57 , the temperature of the heating element array  12   a  does not change so abruptly as when the frame is blank as a space. Dark or blank stripes are effectively prevented from occurring. 
     The frame image segments  58   a  herein described are colored in gray. It is possible to leave blank a pair of vertical frame image segments in the frame image  58  without coloring in gray, and only to color the frame image segments  58   a  in gray. Also the frame image  58  may be so colored that its density is gradually changed from the color of the background image  32  to that of the insertion region  34  in a continuous manner, or is gradually changed from the color of the background image  32  to gray and from gray to the color of the insertion region  34 . Any suitable color may be used for coloring the frame image  58  at a medium density. For this suitable color, all of the yellow, magenta and cyan coloring layers should be colored over the minimum density in the density range. For any of the three colors, it is preferable that the predetermined medium density of the color of the frame image  58  is preferably as high as to be recordable upon application of at least 120% as much heat energy to the recording sheet  10  as the bias heat energy. 
     According to the above embodiment, the frame image segments  58   a  are a simple gray area without an object image. However, it is possible in a grayish manner to color a narrow part inside the frame image segments  58   a  on the outermost side of the input image  41 , and also a narrow part inside the frame image segments  58   a  on the innermost side of the background image  32 . 
     In the above embodiments, the frame region or frame space exists around the insertion region inside the template image. Also, a template image  60 , illustrated in FIG. 8, may have an insertion region  61  without a frame region or frame image. A borderline  62  between the template image  60  and the insertion region  61  is set determined non-parallel with the main scan direction, so as to prevent occurrence of unevenness in density. This is effective even if a difference in the density between the background image  32  of the template image  60  and an inserted input image is considerable. In FIG. 8, two portions of the borderline  62  extending in the main scan direction are straight with an inclination. However those portions may be curved. 
     In the above embodiment, the thermal printer is a one-head three-pass type in which the single thermal head is used, and a full-color image is recorded by three-color frame-sequential recording. But a thermal printer in the present invention may be a three-head one-pass type in which three thermal heads are used and the recording sheet  10  is conveyed in one direction for one time. Also a platen drum of a great diameter may be used for supporting the color thermosensitive recording sheet on the periphery thereof. 
     In the above embodiments, the thermal recording of the type of the color direct thermal recording is used. However, images can be recorded by a printer of a thermal transfer type, examples of which are a sublimation type and a wax-transfer type. The embodiment of FIG. 7 can be used for printers of the sublimation type or the wax-transfer type in which ink ribbon is heated to transfer ink to recording paper by sublimating or melting the ink. The predetermined density of the gray color of the frame image  58  is preferably as high as to be recordable upon application of at least 120% as much heat energy to the ink ribbon as the bias heat energy. 
     In general, the input image  41  has a rectangular quadrilateral shape, and has first and second side lines extending in the main scan direction. In view of the inclination or curvature, the first side line is associated with an inner borderline of the frame region segments  33   a ,  56   a , but offset therefrom with a difference. Consequently, the image synthesis circuit  26  corrects the input image data in consideration of the difference, for adapting a portion of the input image  41  along the first side line to the inner borderline of the frame region segments  33   a ,  56   a.    
     For effecting this correction, the image synthesis circuit  26  produces magnification-changed image data. The magnification-changed data is obtained by processing the input image data to enlarge the input image  41 , and by deleting partial data from the processed data in the image synthesis circuit  26  regarding portions overlapping with the frame region segments  33   a ,  56   a . Accordingly it is possible to eliminate the above-mentioned difference, because the first side line of the input image  41  is caused to lie on the inner borderlines of the frame region segments  33   a ,  56   a.    
     In the above embodiments, the thermal printer is a full-color printer. However the thermal printer may be monochromatic. 
     In the above embodiments, the insertion region is located at the center of the template image. However an insertion region may be located at any off-centered position inside the template image, for example along one of the four edges, or at one of the four corners. The frame regions  33  and  56 , the frame image  58  and the borderline  62  may have one straight line shape, an L-shape, or a channel shape. 
     Furthermore, the insertion region may have a rectangular quadrilateral shape, and may be disposed with an inclination inside the template region. 
     In the above embodiments, forms of the input image and the template image are foreground and background scenes, of which examples are a golf player and a view of a golf course. However, the input image and the template image may have any relationship in their forms, or may be combined in any manner desired by a user. The template image may be a decorative pattern, or letters and words to constitute a phase or passage. 
     In the above embodiments, the frame region  33  and  56  and the frame image  58  are a continuous line without gap. However, the frame region  33  and  56  and the frame image  58  can have a form of a broken line. For the frame region  33  and  56 , each of their inner and outer borderlines may be comb-shaped, sawtooth-shaped, or shaped in any intermittent manner. The frame image  58  may be constituted by a train of dots, between which small blank sections are disposed. 
     In the above embodiment, each of the frame region segments  33   a  and the horizontal portions of the borderlines  62  has a shape of a single inclined linear portion. Furthermore, each of the frame region segments  33   a  and the horizontal portions of the borderlines  62  can have a zigzag shape in combination of two or more inclined linear portions, or a patterned shape of inclined linear portions of plural kinds. In the above embodiment, each of the frame region segments  56   a  consists of a combination of two long portions curved in opposite directions. Furthermore, each of the frame region segments  56   a  can have a corrugated shape in combination of two or more curved long portions, or a patterned shape of curved long portions of plural kinds. 
     In the above embodiment, the borderlines of the frame region segments  33   a  and  56   a  and the borderlines  62  do not include any portion parallel with the main scan direction. However, those borderlines may have a combined shape including a small portion parallel with the main scan direction. Of course, it is desirable in the present invention that such a small portion should be as small as possible. A major horizontal part of the borderlines of the frame region segments  33   a ,  56   a  and the borderline  62  should be inclined. 
     Although the present invention has been fully described by way of the preferred embodiments thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field. Therefore, unless otherwise these changes and modifications depart from the scope of the present invention, they should be construed as included therein.