Printing method for interpolating gray levels

Details Printing method for interpolating gray levels Printing method for interpolating gray levels

2002-05-21

2004-09-28

Feggins, K. (Department: 2861)

Incremental printing of symbolic information

Thermal marking apparatus or processes

Gradational recording

Reexamination Certificate

active

06798432

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printing method, and more particularly, to a printing method for interpolating gray levels.
2. Description of the Prior Art
Photo printers are different from general printers. The major difference is that the photo printer can print out an image such as a photo picture on paper with high picture quality. Please refer to FIG.
1
and FIG.
2
.
FIG. 1
is a diagram of a prior art photo printer
10
.
FIG. 2
is a simplified exploded view of the photo printer
10
shown in FIG.
1
. As shown in
FIG. 1
, the photo printer
10
has a ribbon
14
, a thermal print head
12
, a ribbon driver
18
, and a roller set
20
. The ribbon
14
has a plurality of sectors, and each sector is used for storing one kind of different color dyes. The thermal print head
12
is fixed inside the photo printer
10
for heating the color dyes so that the color dyes are transferred onto a photo paper
16
. The ribbon driver
18
is used for moving the ribbon
14
back and forth so that the thermal print head
12
can transfer a specific color dye stored on the ribbon
14
onto the corresponding photo paper
16
. The roller set
20
is used for holding the photo paper
16
and moving the photo paper
16
along a predetermined direction. Therefore, the fixed thermal print head
12
is capable of printing a color image on the photo paper
16
.
As shown in
FIG. 2
, the thermal print head
12
has a plurality of heaters
22
that are arranged linearly and spaced equally for heating the ribbon
14
. The color dye stored on the ribbon
14
is heated, and is transferred onto the photo paper
16
. When the thermal print head
12
starts printing images, each heater
22
positioned on the thermal print head
12
will heat the ribbon
14
so that a plurality of corresponding pixels X
1
will form a line image Y
1
. Then, the photo paper
16
driven by the roller set
20
is moved along the predetermined direction according to a predetermined speed. Therefore, another line image Y
2
is printed on the same photo paper
16
next to the line image Y
1
. Accordingly, a plurality of line images are successfully printed on the same photo paper
16
to complete the printing operation.
As mentioned above, the total number of heaters
22
positioned on the thermal print head
12
determines the corresponding number of the pixels X
1
of each line image printed on the photo paper
16
. Moreover, the color concentration, that is, the gray level of each pixel X
1
printed on the photo paper
16
is determined by the corresponding heater
22
with a specific duration of each heating operation and a total number of heating cycles.
Please refer to FIG.
3
A and FIG.
3
B.
FIG. 3A
is a diagram of gray levels and a corresponding driving signal
30
according to the photo printer
10
shown in FIG.
1
.
FIG. 3B
is a diagram of a binary data sequence of the driving signal
30
shown in
FIG. 3
a
. As shown in FIG.
3
A and
FIG. 3B
, before the thermal print head
12
of the photo printer
10
starts printing images onto the photo paper
16
, all of the heaters
22
positioned on the thermal print head
12
are activated during a predetermined period Tp so that each heater
22
will first approach a predetermined printing temperature. The above-mentioned procedure is called a preheating operation. In addition, the driving signal having a pulse with a binary value “1” will activate the corresponding heater
22
, and the driving signal corresponding to a binary value “0” will not activate the heater
22
. Next, the photo printer
10
will continuously activate the same heater
22
according to the corresponding gray level of the pixel X
1
. In other words, each heater
22
positioned on the thermal print head
12
is activated repeatedly according to the desired gray level of the corresponding pixel. The overall heating operation of the heater
22
is represented by a driving signal
30
and its corresponding binary values. Each duration Tu of a pulse
32
is a heating time unit for activating the heater
22
. In addition, the energy generated by the heater
22
onto the corresponding pixel X
1
during the duration Tu of each pulse
32
is nearly identical. That is, the quantity of color dyes transferred onto the photo paper
16
during the fixed duration Tu is almost identical. The reason why the quantity of color dyes is almost identical is because of a thermal accumulation effect. It is well known that the thermal accumulation effect is adjusted according to a prior art control method so that the quantity of color dyes is controlled with acceptable inaccuracy. A lengthy description of the prior art control method is skipped for brevity.
The heater
22
of the photo printer
10
can produce 256 (0~255) gray levels to print the corresponding pixel X
1
with an appropriate gray level. A gray level corresponding to a lightest color concentration is equal to 0, and a gray level corresponding to a darkest color concentration is equal to 255. In other words, when the pixel X
1
acquires a corresponding gray level equaling N, which is an integer between 0 and 255, the corresponding heater
22
has to be successively activated N times. Therefore, N pulses
32
of the driving signal
30
are generated repeatedly. That is, N binary “1” values are inputted to the heater
22
continuously. Please note that the photo paper
16
is printed one line at a time. Because each pixel X
1
positioned on the same line may have different gray levels, each heater
22
has to wait for 255 durations Tu so that the thermal print head
12
can then print the next line image. That is, one heater
22
finishes printing a corresponding pixel X
1
with a smaller gray level within a short time. But, another heater
22
printing a corresponding pixel X
1
with a greater gray level may take a long time. When the total number of different gray levels is doubled, each heater
22
has to wait for 511 durations Tu. Therefore, if the number of different gray levels is increased, each heater
22
has to operate for a longer period to complete printing one line image. That is, if the color resolution is improved, the execution time is longer. The printing efficiency, therefore, is greatly deteriorated.
SUMMARY OF INVENTION
It is therefore a primary objective of the claimed invention to provide a printing method for interpolating gray levels of a thermal print head to solve the above mentioned problem.
Briefly, the claimed invention provides a printing method using a thermal print head having a plurality of heaters linearly arranged and equally spaced for heating a dye and transferring the dye onto an object, thereby forming a plurality of pixels corresponding to the heaters on the object. A color of each pixel is determined by a gray level. Each gray level comprises a first portion and a second portion. When controlling the heater to generate a pixel of a predetermined gray level, the printing method comprises activating a heater for a number of cycles corresponding to the first portion of the predetermined gray level, thereby transferring the dye onto the object in a position corresponding to the heater.The first portion is larger than or equal to zero. Each cycle lasts a substantially equal amount of time. Each activation of the heater within a cycle lasts a substantially equal amount of time, and quantity of the dye transferred onto the object is substantially equal for each activation of the heater. The printing method further comprises deactivating the heater for a first predetermined number of cycles corresponding to the second portion of the predetermined gray level, then activating the heater a second predetermined number of cycles corresponding to the second portion of the predetermined gray level. Both the first predetermined number and the second predetermined number are integers larger than or equal to 1. A total quantity of the dye transferred onto the object in printing the second portion of the predetermined gray level is less than the quantity of dye transferred o

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