EP1125751A1

EP1125751A1 - Method and apparatus for rotary printing, and method of image conversion

Info

Publication number: EP1125751A1
Application number: EP99940462A
Authority: EP
Inventors: Kenichi Nagai; Yasunori Tsukuda; Norihiro Sawamoto
Original assignee: Star Micronics Co Ltd
Current assignee: Star Micronics Co Ltd
Priority date: 1998-10-30
Filing date: 1999-08-25
Publication date: 2001-08-22
Also published as: WO2000026036A1; EP1125751A4; CN1325345A; JP2000135853A; KR20010110295A

Abstract

A rotary printing apparatus comprising: a thermal head 11 for printing in a main scanning direction along a radial direction of a disk print medium M; a stepping motor 15 for rotating the disk print medium in a sub-scanning direction along a circumferential direction of the disk print medium M; and a CPU 61 for converting rectangular coordinate-type image data, supplied from an external host device, to rotational coordinate-type image data consisting of a plurality of pixels arranged in the main scanning and sub-scanning directions of the disk print medium. With this construction, an image expressed in a rectangular coordinate system can be printed on the disk print medium without distorting the image.

Description

Technical Field

The present invention relates to a rotary printing method and apparatus for printing on a disk print medium such as a CD-R (Compact Disk-Recordable). The invention also relates to a method of image conversion for converting rectangular coordinate-type image data into rotational coordinate-type image data.

Background Art

CD-Rs (compact disk-recordables) are write-once optical disks onto which data can be recorded only once unlike disks such as read-only CDs and CD-ROMs (read only memories). Data recorded on CD-Rs can be reproduced by a reproducing apparatus for CD-Rs as well as CDs and CD-ROMs. Because of their inexpensiveness, easiness to handle and large recording capacity, CD-Rs are frequently used for software publication on a small scale of approximately 100 copies.
One surface of a CD or CD-ROM is used as a record surface for data-recording and data-reading, and the other surface is generally used as a print surface for printing a title or the like. Since CDs or CD-ROMs having the same design are generally published in large quantity, printing is performed by use of a large-size printer. Since CD-Rs are used for small-scale publication and private uses, a small-size and inexpensive printer is desired which is capable of easily producing and changing print contents for a small number of disks or for each disk.
Examples of such printers are described in Japanese Unexamined Patent Publications JP-A 5-238005 (1993) and JP-A 6-31906 (1994). In these information recorders, information is recorded onto a record surface of an optical disk through a pickup, while ink-jet printing is performed on the print surface. The printing the optical disk is performed while moving an ink-jet nozzle in the radial direction of the optical disk and rotating the optical disk.
Such information recorders are large-sized and complicated because the information recorders have an ink cartridge and a mechanism for supplying ink to an ink-jet head for an ink-jet printing method and as well a pickup for information recording. Moreover, since the ink-jet head and the pickup are provided in the same apparatus, information recording may be hindered for a reason such that ink scattering from the ink-jet head adheres to the pickup. Further, maintenance such as replacement of the ink cartridge and cleaning of parts around the ink-jet head takes time.
Furthermore, since the main scanning direction is set along the radial direction of the optical disk, and the sub-scanning direction along the circumferential direction of the optical disk, if rectangular coordinate-type image data of M rows × N columns, as handled in a conventional serial printer or line printer, is used directly as the print data, the printed image will be distorted with the image in the inner radius portion being compressed in the circumferential direction and the image in the outer radius portion elongated in the circumferential direction, because the printing area is curved in a fan-shaped form along the circumferential direction of the optical disk.

Disclosure of Invention

An object of the invention is to provide a rotary printing method and apparatus, capable of forming an image expressed in rectangular coordinates on a disk print medium without causing distortions.
Another object of the invention is to provide a method of image conversion, capable of faithfully converting image data in rectangular coordinates into image data in rotational coordinates.
The invention provides a rotary printing apparatus comprising:
a print head for printing in a main scanning direction along a radial direction of a disk print medium;
medium rotating means for rotating the disk print medium in a sub-scanning direction along a circumferential direction of the disk print medium; and
image converting means for converting rectangular coordinate-type image data, supplied from an external host device, to rotational coordinate-type image data consisting of a plurality of pixels arranged in the main scanning and sub-scanning directions of the disk print medium.
According to the invention, by providing the image converting means for converting the rectangular coordinate-type image data into rotational coordinate-type image data, an image identical in appearance to the image expressed in rectangular coordinates can be printed on the disk print medium even if the arrangement pattern of print pixels is changed. Accordingly high quality image printing can be achieved by solving the problems of image curving, compression and elongation of printed images in conventional apparatus.
In the invention it is preferable that the print head is a line print head having a plurality of recording elements arranged in a line at predetermined pitches, and the main scanning direction crosses at a predetermined angle with the radial direction of the disk print medium.
According to the invention, since the print line in the main scanning direction is made to cross at a predetermined angle with the radial direction of the disk print medium, the gap between lines printed in the sub-scanning direction by the recording elements can be reduced by an amount equivalent to the crossing angle. Furthermore, by suitably setting the crossing angle and the length in the sub-scanning direction of the pixel printed by each recording element, the gap between lines printed in the sub-scanning direction can be reduced to zero.
Accordingly, printing with high density can be accomplished with reduced non-print area, and besides, a pixel density higher than the resolution of the print head can be achieved. Moreover, since the contact length between the line print head and the disk print medium increases, medium warping and deformation due to the pressing force of the head can be suppressed.
The invention also provides a rotary printing method comprising the steps of converting rectangular coordinate-type image data, supplied from an external host device, to rotational coordinate-type image data consisting of a plurality of pixels arranged in a main scanning direction along a radial direction of a disk print medium and a sub-scanning direction along a circumferential direction thereof; and performing printing of the disk print medium in the main scanning direction of the disk print medium based on the rotational coordinate-type image data while rotating the disk print medium, to form an image on the disk print medium.
According to the invention, by converting the rectangular coordinate-type image data into rotational coordinate-type image data, an image identical in appearance to the image expressed in rectangular coordinates can be printed on the disk print medium even if the arrangement pattern of print pixels is changed. Accordingly high quality image printing can be achieved by solving the problems of image curving, compression and elongation of printed images in conventional apparatus.
In the invention it is preferable that the printing in the main scanning direction is performed using a line print head having a plurality of recording elements arranged in a line at predetermined pitches, and the main scanning direction crosses at a predetermined angle with the radial direction of the disk print medium.
According to the invention, since the print line in the main scanning direction is made to cross at a predetermined angle with the radial direction of the disk print medium, the gap between lines printed in the sub-scanning direction by the recording elements can be reduced by an amount equivalent to the crossing angle. Furthermore, by suitably setting the crossing angle and the length in the sub-scanning direction of the pixel printed by each recording element, the gap between lines printed in the sub-scanning direction can be reduced to zero.
Accordingly, printing with high density can be accomplished because of reduced non-print area, and besides, a pixel density higher than the resolution of the print head can be achieved. Moreover, since the contact length between the line print head and the disk print medium increases, medium warping and deformation due to the pressing force of the head can be suppressed.
The invention also provides a method of image conversion for converting rectangular coordinate-type image data into rotational coordinate-type image data, comprising the steps of placing an image area for rotational coordinate-type image data within an image area for rectangular coordinate-type image data; specifying a rotational coordinate point (r, c) using a radial position coordinate c defining a pixel position in a radial direction from a rotational coordinate center and an angular position coordinate r defining a pixel position in a circumferential direction from a rectangular coordinate axis; obtaining a rectangular coordinate of a pixel containing the rotational coordinate point, out of pixels in the rectangular coordinate system; and the extracting image data in the rectangular coordinate as image data in rotational coordinate.
According to the invention, a circular image area expressed in a rotational coordinate system is placed within a rectangular image area expressed in a rectangular coordinate system, the rectangular coordinates of a pixel containing the desired rotational coordinate point (r, c) are calculated, image data in rectangular coordinates is extracted as image data in rotational coordinates, and by performing this process over an entire range of the radial position coordinate c and angular position coordinate r or over part of that range, the image data can be converted into the rotational coordinate-type image data.
By rotary-printing the rotational coordinate-type image data, obtained by the data conversion, onto the disk print medium, an image identical in appearance to the image expressed in rectangular coordinates can be printed on the disk print medium.

Brief Description of Drawings

Fig. 1 is a perspective view explaining a thermal recording method according to the invention.
Fig. 2A is a cross-sectional view showing the structure of a heat-sensitive print sheet 21 used as a disk print medium M of Fig. 1 and Fig. 2B is a cross-sectional view showing the structure of a medium 19 in which the heat-sensitive print sheet 21 is pasted to an optical disk 20.
Fig. 3 is a block diagram showing the electrical structure of a rotary printing apparatus 10.
Fig. 4 is a timing chart showing the operation of the rotary printing apparatus 10.
Figs. 5A to 5F are views showing printing steps of the disk print medium M.
Fig. 6 is an explanatory diagram illustrating a first embodiment of the invention.
Fig. 7 is an explanatory diagram illustrating the method of image conversion for converting rectangular coordinate-type image data into rotational coordinate-type image data.
Fig. 8A is a flow chart illustrating the method of image conversion, and Fig. 8B is a flow chart illustrating the entire process of rotary printing.
Figs. 9A to 9C are explanatory diagrams illustrating a second embodiment of the invention.
Fig. 10 is an explanatory diagram illustrating a second embodiment of the invention.
Fig. 11 is an explanatory diagram illustrating the method of image conversion for converting rectangular coordinate-type image data into rotational coordinate-type image data when the thermal head 11 is oriented obliquely.
Fig. 12 is an explanatory diagram of parameters used in mathematical equations.

Best Mode for Carrying out the Invention

Fig. 1 is a perspective view explaining a thermal recording method according to the invention. A rotary printing apparatus 10 comprises a thermal head 11, a backup roller 12 and cathode tubes 13 and 14, for printing on a disk print medium M. The disk print medium M is a disk-shaped print medium having heat-sensitive coloring layers which are colored when heat is applied. The thermal head 11 is a line thermal head extending in the radial direction of the disk print medium M. A stepping motor 15 rotates the disk print medium M about its axis. The backup roller 12, whose surface is covered with rubber, supports the disk print medium M from its rear surface against the pressure to its top surface by the thermal head 11, and rotates as the disk print medium M rotates. The cathode tubes 13 and 14 emit ultraviolet rays of wavelengths at which the coloring layers of the disk print medium M are fixed.
Printing of the rotary printing apparatus 10, in which the radial direction of the disk print medium M is the main scanning direction and the circumferential direction of the disk print medium M is the sub scanning direction, is carried out by selectively supplying heat to pixel areas arranged in the radial and circumferential directions of the disk print medium M to color them.
The thermal head 11 as shown in Fig. 1 may be a serial head capable of scanning in the radial direction of the disk print medium M. Moreover, a turntable 31 may be used in place of the backup roller 12 as shown in Fig. 1.
Fig. 2A is a cross-sectional view showing the structure of a heat-sensitive print sheet 21 used as the disk print medium M as shown in Fig. 1. Fig. 2B is a cross-sectional view showing the structure of a medium 19 in which the heat-sensitive print sheet 21 is pasted on an optical disk 20.
The heat-sensitive print sheet 21 as shown in Fig. 2A has a cross-sectional structure similar to those of multicolor heat-sensitive record sheets described in Japanese Unexamined Patent Publications JP-A 3-43293 (1991) and JP-A 5-69566 (1993). The printing method of coloring a multicolor heat-sensitive record sheet by applying heat to the sheet is called a TA (thermo-auto chrome) method. The rotary printing apparatus 10 of this embodiment is also a TA printer.
In the heat-sensitive print sheet 21, a heat-sensitive coloring layer 23 is formed on the surface of a base material 22 such as paper, and the plane shape of the sheet 21 is a disk shape. The heat-sensitive coloring layer 23 is composed of a yellow coloring layer 23a, a magenta coloring layer 23b and a cyan coloring layer 23c.
The yellow coloring layer 23a contains a yellow pigment material encapsulated in microcapsules-and a coupler. By applying thermal energy of 20 mJ/mm² or more, the pigment material passes through the microcapsules to react with the coupler, so that the layer 23a is colored. Moreover, by applying ultraviolet rays of a wavelength of 420 nm to the yellow coloring layer 23a, the unreacted yellow pigment material is decomposed, so that coloring does not continue any more.
The magenta coloring layer 23b contains a magenta pigment material encapsulated in microcapsules and a coupler. By applying thermal energy of not less than 40 mJ/mm², the pigment material passes through the microcapsules to react with the coupler, so that the layer 23b is colored. Moreover, by applying ultraviolet rays of a wavelength of 365 nm to the magenta coloring layer 23b, the unreactedmagenta pigment material is decomposed, so that coloring does not progress any more.
The cyan coloring layer 23c contains a dye encapsulated in microcapsules, and is colored by applying thermal energy of approximately 80 mJ/mm² or more.
The thermal head 11 as shown in Fig. 1 is capable of supplying any of thermal energy of 20 mJ/mm² to 40 mJ/mm², thermal energy of 40 mJ/mm² to 80 mJ/mm² and thermal energy of 80 mJ/mm² to 120 mJ/mm². The cathode tube 13 emits ultraviolet rays of a wavelength of 420 nm to fix the yellow coloring layer 23a. The cathode tube 14 emits ultraviolet rays of a wavelength of 365 nm to fix the magenta coloring layer 23b.
In the medium 19 as shown in Fig. 2B, the heat-sensitive print sheet 21 is pasted on the print surface 20a of the optical disk 20 such as a CD-R with an adhesive layer 24 in between.
In the optical disk 20, an organic pigment layer 25, a reflective layer 26 made of metal and a protective layer 27 are laminated in this order on a polycarbonate substrate 30. For the optical disk 20, data recording is performed by phase-changing the organic pigment layer 25 by applying a laser beam from the record surface 20b. The optical disk 20 usable for the rotary printing apparatus 10 is not limited to a disk having such a structure, but may be, for example, a CD, CD-ROM or CD-RW (rewritable). Moreover, the optical disk 20 may be a DVD (digital video disk) -ROM, DVD-RAM (random access memory) , DVD-R, DVD-RW or the like.
As the disk print medium M, the heat-sensitive print sheet 21 as shown in Fig. 2A may be used as it is, or the medium 19 as shown in Fig. 2B may be used. When the heat-sensitive print sheet 21 is used as it is, the heat-sensitive print sheet 21 can be pasted on the optical disk 20 after printing. Moreover, the heat-sensitive coloring layer 23 may be directly formed on the optical disk 20 by vapor deposition or the like.
Fig. 3 is a block diagram showing the electrical structure of the rotary printing apparatus 10. An interface (I/F) 64 performs data transmission with an external host such as a personal computer through communication such as parallel communication or serial communication, for example, receives image data to be printed from the external host and transmits status data representative of the operation condition of the printing apparatus 10. A CPU (central processing unit) 61 operates in accordance with a predetermined program stored in a ROM (read only memory) 62 or the like, and controls general operations such as processing of signals for the thermal head 11 and operations of the stepping motor 15 and the cathode tubes 13 and 14. The ROM 62 is a nonvolatile memory in which the program for the CPU 61 and various data are stored.
A RAM 63 is a volatile memory in which printing data and various data are stored, and functions also as a buffer memory for continuously expanding image data thereinto. The function of the buffer memory for data development may be assigned to a memory on the side of the external host to thereby save the memory capacity on the side of the printing apparatus 10.
Fig. 4 is a timing chart showing the operation of the rotaryprinting apparatus 10. Figs. 5A to 5F are views stepwisely showing the print condition of the disk print medium M. When printing is started, the data of a print image produced by the external host is transmitted to the thermal head 11 through the I/F 64, the CPU 61 and the like. At the same time, energization of the stepping motor 15 is started, so that the stepping motor 15 rotates the disk print medium M at predetermined rotation speed.
In Fig. 5A, energization of the thermal head 11 is started, and thermal energy of the lowest heat-sensitive coloring level is applied to the disk print medium M. Consequently, the yellow coloring layer 23a is colored. Then, as shown in Fig. 5B, when a forefront line 73 of a colored area 75 of the yellow coloring layer 23a reaches a rearmost part 71b of a light irradiated area 71, energization of the cathode tube 13 is started. Consequently, light from the cathode tube 13 is applied to the disk print medium M. The light irradiated area 71 is an area irradiated with light from the cathode tube 13. Then, when the forefront line 73 has reached the thermal head 11 again, energization of the thermal head 11 is stopped to end coloring of the yellow coloring layer 23a.
Then, as shown in Fig. 5C, when a forefront line 74 of a fixed area 77 where fixing of the yellow coloring layer 23a is completed reaches the thermal head 11, energization of the thermal head 11 is started, and thermal energy of the second lowest heat-sensitive coloring level is applied to the disk print medium M. Consequently, coloring of the magenta coloring layer 23b is started from the forefront line 74. Then, when the forefront line 74 of the fixing-completed area of the yellow coloring layer again reaches the rearmost part 71b of the light irradiated area 71, energization of the cathode tube 13 is stopped to end fixing of the yellow coloring layer 23a.
Then, as shown in Fig. 5D, when the forefront line 74 of a colored area 79 where the magenta coloring layer 23b is colored reaches a rearmost part 72b of a light irradiated area 72, energization of the cathode tube 14 is started. Consequently, light from the cathode tube 14 is applied to the disk print medium M, so that the magenta coloring layer 23b is fixed. The light irradiated area 72 is an area irradiated with light from the cathode tube 14. Then, when the forefront line 74 of the colored area of the magenta coloring layer reaches the thermal head 11, energization of the thermal head 11 is stopped to end coloring of the magenta coloring layer 23b.
Then, as shown in Fig. 5E, when a forefront line 78 of a fixed area 81 where fixing of the magenta coloring layer 23b is completed reaches the thermal head 11, energization of the thermal head 11 is started, and thermal energy of the highest heat-sensitive coloring level is applied to the disk print medium M. Consequently, coloring of the cyan coloring layer 23c is started from the forefront line 78. Then, when the forefront line 78 of the fixing-completed area of the magenta coloring layer again reaches the rearmost part 72b of the light irradiated area 72, energization of the cathode tube 14 is stopped to end fixing of the magenta coloring layer 23b.
Then, as shown in Fig. 5F, when the forefront line 78 of the colored area of the cyan coloring layer again reaches the thermal head 11, energization of the thermal head 11 is stopped to end coloring of the cyan coloring layer 23c. At this time, the disk print medium M is all-covered with a colored area 82 where the cyan coloring layer 23c is colored, and printing is completed.
By successively performing coloring and fixing of the yellow coloring layer 23a, coloring and fixing of the magenta coloring layer 23b and coloring of the cyan coloring layer 23c as described above, full-color printing can be performed.
Fig. 6 is an explanatory diagram illustrating a first embodiment of the invention. The thermal head 11 has a plurality of heating elements arranged in a line along the radial direction of the disk print medium M, and prints on the disk print medium M while the medium M is being rotated in a clockwise direction about the center of rotation O.
When printing is started, the thermal head 11 prints a total number, HDN, of pixels from the outermost pixel P (1, 1) to the innermost pixel P (HDN, 1) along a first main scanning line (r=1). Next, the disk print medium M is rotated by an amount equal to the pixel sub-scan pitch, and the thermal head 11 prints pixels from the outermost pixel P (1, 2) to the innermost pixel P (HDN, 2) along a second main scanning line (r=2). This main scan printing and step rotation process is repeated, and printing for one revolution is complete when pixels from the outermost pixel P (1, SLN) to the innermost pixel P (HDN, SLN) along the final main scanning line (r=SLN) have been printed. When performing the full-color printing described earlier, printing is performed for three revolutions, one for each of the three primary colors.
Pixel P (c, r) designates the pixel specified by the rotational coordinates of the r-th main scanning line and the c-th sub-scanning line, HDN is a numeric value expressing the effective print width in the ring-shaped printable area in terms of the number of pixels, DIN is a numeric value expressing the ineffective print width inward of the innermost circumference in terms of the number of pixels, and SLN represents the number of sub-scanning lines in one revolution of the disk print medium M. Accordingly, a print image consisting of a number, HDN × SLN, of pixels is formed on one side of the disk print medium M. For example, if the print image pitch in the main scanning direction is 200 dpi, then HDN = 320 pixels and SLN = 2940 lines.
Fig. 7 is an explanatory diagram illustrating the method of image conversion for converting rectangular coordinate-type image data into rotational coordinate-type image data. A rectangular image area Q showing the arrangement of image data in rectangular coordinates is described in a rectangular coordinate system having the x axis along the rightward direction, the y axis along the downward direction, and the origin (0, 0) at the upper left corner of the image. Such image data expressed in rectangular coordinates is supplied from an external host device.
A circular image area U is defined in order to extract from the image area Q the rotational coordinate-type image data necessary for printing on the disk print medium M. The image area U has a radius equivalent to the number of pixels (= HDN + DIN) from the center of rotation O (Xo, Yo) to the outermost first sub-scanning line. The square area circumscribed about the image area U is defined by rectangular coordinate points P1 (X1, Y1) and P2 (X2, Y2). Here, the innermost area corresponding to the ineffective print width may be excluded since this area is not needed for printing on the disk print medium M.
Next, a method for extracting image data PixelData(c, r) of the pixel P (c, r) will be described. The image data PixelData(c, r) can be expressed as shown below by using the rectangular coordinates X, Y.

[MATHEMATICAL 1]

PixelData(c, r) = PixelData(X(c, r), Y(c, r)) X(c, r) = INT[Xo + L*cos()*{0.5*(X2 - X1 + 1)/(DIN + HDN)}] Y(c, r) = INT[Yo - L*sin()*{0.5*(Y2 - Y1 + 1)/(DIN + HDN)}] L = DIN + (HDN - c) + 1  =2 * π * r / SLN
Here, L is a numeric value expressing the radius from the center of rotation O to the pixel P in terms of the number of pixels,  is an angle measured anti-clockwise from the X axis direction to the pixel P, and INT is a function for rounding down the calculation result to the nearest integer.
When obtaining the image data of the pixel P (c, r), first the radial position coordinate c is substituted into the equation (4) to calculate L, and the angular position coordinate r is substituted into the equation (5) to calculate . Next, L and 6 are substituted into the equations (1) and (2) to calculate the rectangular coordinates (X, Y) of the pixel containing the rotational coordinate point (r, c). Then, the image data of the pixel specified by the rectangular coordinates (X, Y) is extracted as the image data PixelData(c, r). By performing this operation over an entire range of the radial position coordinate c and angular position coordinate r or over part of that range, the image data can be converted into the rotational coordinate-type image data.
Fig. 8A is a flow chart illustrating the method of image conversion, and Fig. 8B is a flow chart illustrating the entire process of rotary printing. First, in step al in Fig. 8A, the top address TOP_ADRS of a memory free space is set as a buffer address Buf_ADRS in order to secure a buffer area for storing rotational coordinate-type image data. Next, in steps a2 and a3, the angular position coordinate r as a sub-scan counter is set to 1, and the radial position coordinate c as a main scan counter is set to 1. Then, in step a4, the rotational coordinates (r, c) are converted into rectangular coordinates (X, Y) by using the equations (2) to (5), and in step a5, the image data of the pixel specified by the rectangular coordinates (X, Y) is extracted as image data PixelData(1, 1) and stored in the memory area designated by the buffer address Buf_ADRS. Next, in step a6, the buffer address Buf_ADRS is incremented by one, and in step a7, the radial position coordinate c is incremented by one. In step a8, it is determined whether the radial position coordinate c has exceeded the effective print width HDN, that is, whether the conversion for one main scanning line has been completed. Since the present value is c=2, the process returns to step a4, and image data PixelData(2, 1) is extracted and stored at the buffer address Buf_ADRS. In this way, the image data for the first main scanning line from PixelData(1, 1) to PixelData (HDN, 1) are sequentially extracted and stored in the buffer.
Next, in step a9, the angular position coordinate r is incremented by one, and in step a10, it is determined whether the angular position coordinate r has exceeded the number of sub-scanning lines SLN for one revolution, that is, whether the conversion for all data has been completed. Since the present value is r=2, the process returns to step a3, and the image data for the second main scanning line from PixelData(1, 2) to PixelData(HDN, 2) are sequentially extracted and stored in the buffer.
When the image data for all the main scanning lines from PixelData(1, 1) to PixelData(HDN, SLN) have been stored in the buffer, the image conversion for one revolution is complete.
The image conversion described above corresponds to the process of step bl in Fig. 8B; in step b2, gamma correction is applied to the image data so that the gray scale level of the image data converted into rotational coordinates will match the gray scale characteristics of a printing process. In step b3, density unevenness is corrected that arises from the difference in surface velocity between the inner and outer radii of the disk print medium M. This correction is applied to eliminate the problem of density unevenness in which the print density at the inner radius appears higher and that at the outer radius appears lower because the surface speed of the disk print medium M is lowest at the inner radius and gradually increases toward the outer radius. In step b4, density unevenness is corrected that arises from the thermal history prior to the start of the print operation of the thermal head 11. This correction is applied to eliminate the problem of density unevenness in which if printing is started when the temperature of the thermal head 11 is high, more than necessary heat is applied to the recording medium, and conversely if printing is started when the thermal head 11 is cool, the heat applied to the recording medium is insufficient.
As described above, the rectangular coordinate-type image data, supplied from the external host device, is converted into the rotational coordinate-type image data, and the various corrections are applied to the converted image data; in this way, an image identical in appearance to the image expressed in rectangular coordinates can be formed on the disk print medium M by rotary printing using the thermal head 11.
Figs. 9A to 9C and Fig. 10 are explanatory diagrams illustrating a second embodiment of the invention. The thermal head 11 has a plurality of heating elements arranged in a line oriented obliquely so as to cross at a predetermined angle with the radial direction of the disk print medium M, and prints on the disk print medium M while the medium M is being rotated in a clockwise direction about the center of rotation O.
When the heating elements 11a of the thermal head 11 are arranged, for example, at 200 dpi, as shown in Fig. 9A, the pitch between each heating element 11a is 0.125 mm ( = 25.4 mm / 200). If each thermal element 11a is rectangular in shape whose dimension along the main scanning direction is 0.105 mm and whose dimension along the sub-scanning direction is 0.175 mm, then the gap between each heating element 11a is 0.02 mm. Therefore, if the thermal head 11 is arranged parallel to the radial direction of the disk print medium M, a gap of about 0.02 mm will be left between lines printed along the sub-scanning direction.
Depending on the kind of image, such a gap is not so distracting, but in applications where-a higher image quality is required, the gap must be eliminated. To achieve this, the thermal head 11 is oriented at an angle so that the dimension along the sub-scanning direction of each heating element 11a becomes equal to 0.125 mm. It can be seen that when a diagonal (0.204 mm long) of the heating element 11a is tilted from 59 degrees to 52.4 degrees, as shown in Fig. 9C, the projection of the diagonal becomes equal in dimension to 0.125 mm (= 0.204 mm × cos 52.4°).
As shown in Fig. 10, when printing is started, the thermal head 11 prints a total number, HDN, of pixels from the outermost pixel P (1, 1) to the innermost pixel P (HDN, 1) along the first main scanning line (r=1) that crosses at an angle with the radial direction of the disk print medium M. Next, the disk print medium M is rotated by an amount equal to the pixel sub-scan pitch, and the thermal head 11 prints pixels from the outermost pixel P (1, 2) to the innermost pixel P (HDN, 2) along the second main scanning line (r=2). This main scan printing and step rotation process is repeated, and printing for one revolution is complete when pixels from the outermost pixel P (1, SLN) to the innermost pixel P (HDN, SLN) along the final main scanning line (r=SLN) have been printed. When performing the full-color printing described earlier, printing is performed for three revolutions, one for each of the three primary colors.
Pixel P (c, r) designates the pixel specified by the rotational coordinates of the r-th main scanning line and the c-th sub-scanning line, HDN indicates the total number of heating elements 11a of the thermal head 11, DN is a numeric value expressing the effective print width in the ring-shaped printable area in terms of the number of pixels, DIN is a numeric value expressing the ineffective print width inward of the innermost circumference in terms of the number of pixels, and SLN represents the number of sub-scanning lines in one revolution of the disk print medium M. Accordingly, a print image consisting of a number, HDN × SLN, of pixels is formed on one side of the disk print medium M.
Fig. 11 is an explanatory diagram illustrating the method of image conversion for converting rectangular coordinate-type image data into rotational coordinate-type image data when the thermal head 11 is oriented obliquely. Fig. 12 is an explanatory diagram of parameters used in mathematical equations. A rectangular image area Q showing the arrangement of image data in rectangular coordinates is described in a rectangular coordinate system having the x axis along the rightward direction, the y axis along the downward direction, and the origin (0, 0) at the upper left corner of the image. Such image data expressed in rectangular coordinates is supplied from an external host device.
A circular image area U is defined in order to extract from the image area Q the rotational coordinate-type image data necessary for printing on the disk print medium M. The image area U has a radius equivalent to the number of pixels (= HDN + DIN) from the center of rotation O (Xo, Yo) to the outermost first sub-scanning line. The square area circumscribed about the image area U is defined by rectangular coordinate points P1 (X1, Y1) and P2 (X2, Y2). Here, the innermost area corresponding to the ineffective print width may be excluded since this area is not needed for printing on the disk print medium M.
Next, a method for extracting image data PixelData(c, r) of the pixel P (c, r) will be described. The image data PixelData(c, r) can be expressed as shown below by using the rectangular coordinates X, Y.

[MATHEMATICAL 2]

PixelData(c, r) = PixelData(X(c, r), Y(c, r)) X(c, r) = INT[Xo+L1(c) * cos {  (c, r)} * {0.5*(X2 - X1 + 1)/(DIN + DN)}] Y(c, r) = INT[Yo - L1(c) *sin{((c, r)} *{0.5 * (Y2 - Y1 + 1)/(DIN + DN)}] (c, r) = 2 * π * r / SLN + arctan{L3(c)/L2(c)} L1(c) = (L2(c)^2 + L3(c)^2)^0.5 L2(c) = L4 - (c - 1) * (L4 - DIN)/(HDN - 1) L3(c) = (HDN - c) * L4 * tanα/(HDN - 1) L4 = cos α * (DIN + DN)
Here, L1 to L4 are the parameters shown in Fig. 12,  is an angle measured anticlockwise from the X axis direction to the pixel P, α is the angle that the outermost pixel P and the innermost pixel P make when viewed from the center of rotation O, and INT is a function for rounding down the calculation result to the nearest integer.
When obtaining the image data of the pixel P (c, r), first the radial position coordinate c is substituted into the equations (15) to (18) to calculate L1 to L4, and the angular position coordinate r is substituted into the equation (14) to calculate . Next, L1 to L4 and  are substituted into the equations (12) and (13) to calculate the rectangular coordinates (X, Y) of the pixel containing the rotational coordinate point (r, c). Then, the image data of the pixel specified by the rectangular coordinates (X, Y) is extracted as the image data PixelData (c, r). By performing this operation over an entire range of the radial position coordinate c and angular position coordinate r or over part of that range, the image data can be converted into rotational coordinate-type image data.
The image conversion method described above may be carried out using the CPU 61 of the rotary printer, but in an alternative configuration, the external host device such as a personal computer equipped with a high speed CPU and large capacity memory may be utilized to carry out the image conversion and to transfer the obtained rotational coordinate-type image data to the rotary printer.

Effect of the Invention

As described in detail above, according to the invention, even if the arrangement pattern of print pixels is changed from a rectangular coordinate system to a rotational coordinate system, an image identical in appearance to the image expressed in the rectangular coordinate system can be printed on the disk print medium. In this way, high quality image printing can be achieved by solving the problem of image curving, compression and elongation of printed images in conventional apparatus.
Furthermore, since the gap between lines printed in the sub-scanning direction can be reduced to zero by making the printing line in the main scanning direction cross at a predetermined angle with the radial direction of the disk print medium, printing with high density can be accomplished because of reduced non-print area, and besides, a pixel density higher than the resolution of the print head can be achieved.

Claims

A rotary printing apparatus comprising:

a print head for printing in a main scanning direction along a radial direction of a disk print medium;

medium rotating means for rotating the disk print medium in a sub-scanning direction along a circumferential direction of the disk print medium; and

image converting means for converting rectangular coordinate-type image data, supplied from an external host device, to rotational coordinate-type image data consisting of a plurality of pixels arranged in the main scanning and sub-scanning directions of the disk print medium.
The rotary printing apparatus of claim 1, wherein the print head is a line print head having a plurality of recording elements arranged in a line at predetermined pitches, and the main scanning direction crosses at a predetermined angle with the radial direction of the disk print-medium.
A rotary printing method comprising the steps of:

converting rectangular coordinate-type image data, supplied from an external host device, to rotational coordinate-type image data consisting of a plurality of pixels arranged in a main scanning direction along a radial direction of a disk print medium and a sub-scanning direction along a circumferential direction thereof; and

performing printing of the disk print medium in the main scanning direction of the disk print medium based on the rotational coordinate-type image data while rotating the disk print medium, to form an image on the disk print medium.
The rotary printing method of claim 3, wherein the printing in the main scanning direction is performed using a line print head having a plurality of recording elements arranged in a line at predetermined pitches, and the main scanning direction crosses at a predetermined angle with the radial direction of the disk print medium.
A method of image conversion for converting rectangular coordinate-type image data into rotational coordinate-type image data, comprising the steps of:

placing an image area for rotational coordinate-type image data within an image area for rectangular-coordinate-type image data;

specifying a rotational coordinate point (r, c) using a radial position coordinate c defining a pixel position in a radial direction from a rotational coordinate center and an angular position coordinate r defining a pixel position in a circumferential direction from a rectangular coordinate axis;

obtaining a rectangular coordinate of a pixel containing the rotational coordinate point, out of pixels in the rectangular coordinate system; and

extracting image data in the rectangular coordinate as image data in rotational coordinate.