Incremental printing of symbolic information – Ink jet – Controller
Reexamination Certificate
2000-04-14
2002-05-21
Hallacher, Craig A. (Department: 2853)
Incremental printing of symbolic information
Ink jet
Controller
C347S019000, C358S406000
Reexamination Certificate
active
06390583
ABSTRACT:
This application is based on Patent Application No. 11-111500 (1999) filed Apr. 19, 1999 in Japan, the content of which is incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an information processing apparatus, a printing apparatus, an information processing method and a printing method for correcting density variations of an image. The present invention can use a variety of types of print heads, each having a plurality of print elements, for printing an image. Particularly the present invention can suitably use an ink jet print head having an array of ink ejection nozzles and a heat transfer print head having an array of heat generating elements.
2. Description of the Prior Art
Currently known printing systems include a heat transfer printing system that transfers ink of an ink ribbon onto a printing medium such as paper by thermal energy, and an ink jet printing system that ejects ink droplets to adhere them to a printing medium such as paper.
Of these printing systems, the ink jet printing system has been widely used, as in printers and copying machines, because of low noise, low running cost, and an ease with which to reduce size and realize color printing. The printing apparatus using such an ink jet printing system generally employs print heads with a dense array of print elements to enhance the printing speed. The print elements include, for example, ink ejection nozzles or orifices.
A serial scan printing method of the ink jet printing apparatus that scans the print head in a main scan direction produces lines of varying densities extending along the main scan direction (also referred to as striped density variations or banding). This is considered one of the causes for image quality degradation. The striped density variations often appear periodically in the sub scan direction and show conspicuously. In a so-called multi-nozzle type print head having a plurality of ink ejection nozzles, when thermal energy of a heater (electrothermal transducer) provided in an ink passage communicating with each nozzle is used to eject an ink droplet, for example, the striped variations are caused by the following factors. One of the factors is variations among nozzles in the amount of ink ejected and in the ejection direction, which are caused by variations among nozzles in the size of heaters and nozzles produced during the manufacture. Another factor is discrepancies between a printing medium feed and a printing width that occur in the serial scan printing method. Still another factor is ink density variations and ink displacements on the printing medium caused by printing time variations.
A variety of methods for eliminating such density variations to enhance print quality have been proposed.
One such method is a dividing printing method (multipass printing method) that completes printing of one area on a printing medium by a plurality of scans of the print head. This dividing printing method is effective in eliminating the striped density variations. However, to produce a satisfactory effect this method needs to increase the number of print head scans for each printing area, i.e., increase the number of divisions. This reduces the area that is printed in each scan of the print head, lowering the throughput.
Another method of suppressing the striped density variations without using the dividing printing method, for example, a head shading method, is disclosed in U.S. Pat. No. 5,528,270. This method is implemented in a sequence of steps shown in
First, a preset test pattern for determining a correction value is printed on a printing medium by using a print head (step S
11
), and the density of the printed test pattern is read by a scanner (step S
12
). After the position of the read image is properly corrected, the density of the image is averaged in the direction of column (main scan direction) (step S
13
) and then allocated to a raster of the associated nozzle of the print head (step S
14
). Density variations are caused by variations among nozzles in the ejected ink amount and the ink ejection direction or by the spreading or wicking of ink on the printing medium. The next step S
15
determines a density correction value for each nozzle from the density data allocated to each raster at step S
14
.
Based on the correction values, the image data for each nozzle is corrected (step S
16
). In more specific terms, a &ggr; table for each nozzle is changed, or a drive table for each nozzle is changed, to adjust the density of the image to be printed. The image data is corrected based on the correction values so that a raster, which prints darker than normal when no correction is made, will print lighter and that a rater, which prints lighter than normal when no correction is made, will print darker, thereby reducing the density variations. A correction method that changes the density of the original image data by changing the output &ggr; table for each nozzle in particular is very effective for the correction of density variations. Further, U.S. Pat. No. 5,528,270 describes a method of printing an image without producing unwanted stripes or density variations in the whole gradation range by taking an input gradation into account and by not performing correction for low-density printing areas but performing correction for high-density printing areas.
However, when the correction of the original image data using the output &ggr; table is performed by focusing only on averaging the print density for each raster, the following problems arise.
In a binary printing system, such as an ink jet printing system, each pixel can only be represented by the presence or absence of dots, so that a halftone is represented by changing the percentage of dots with respect to a predetermined printing area in a so-called area gradation method. In the area gradation method, the number of dots in a predetermined printing area is changed according to the print density. In a quantization method that mainly uses an error diffusion technique, as the number of dots changes, the spatial frequency characteristic, such as granularity of a printed image, also changes. In a printed image, when areas with different granularities adjoin, the granularity. difference will mar the evenness of image quality. Hence, even if the optical reflection density of the printed image is uniform, the spatial frequency difference is recognized by the human eye with the result that the image looks as though there are density variations.
This is explained in more detail by referring to
FIGS. 11
,
12
and
13
.
FIG. 11
is a front view of an ink jet print head
100
, showing the front of the print head that faces the printing medium. For simplicity of explanation, the print head
100
is assumed to have six ink ejection nozzles, which are designated, from the first to sixth nozzle, as
101
a
,
101
b
,
101
c
,
101
d
,
101
e
and
101
f
. It is also assumed that the first to sixth nozzle have variations in the ink ejection amount but no variations in the ink ejection direction.
FIG. 12
is an explanatory diagram showing the dots formed on the printing medium by one scan operation of the print head
100
. Dots formed by ink droplets ejected from the nozzles
110
a
,
101
b
,
101
c
,
101
d
,
101
e
,
110
f
are denoted
102
a
,
102
b
,
102
c
,
102
d
,
102
e
,
102
f
. In this example, as can be seen from
FIG. 12
, the ink ejection amount varies from one nozzle to another, with the amounts of ink ejected from the nozzles
101
a
,
101
b
,
101
e
being “medium”, those from the nozzles
101
c
,
101
d
being “large”, and that from the nozzle
101
f
being “small.” Variations among the nozzles in the amount of ink ejected cause variations in raster density among the nozzles. That is, as shown in
FIG. 12
, the print densities of the rasters corresponding to the first, second and fifth nozzles
101
a
,
101
b
,
101
e
(referred to as “first raster”, “second raster”, and “fifth raster”) are “medium”, those of the rasters corresponding to the third and fourth
Kanematsu Daigoro
Kato Masao
Kato Minako
Ono Mitsuhiro
Yano Kentaro
Canon Kabushiki Kaisha
Hallacher Craig A.
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