Image analysis – Color image processing – Color correction
Reexamination Certificate
2000-09-12
2004-03-30
Mehta, Bhavesh M. (Department: 2625)
Image analysis
Color image processing
Color correction
C382S169000, C382S274000, C358S001900, C358S461000
Reexamination Certificate
active
06714673
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image processing method and to an image processing apparatus for stabilizing output characteristics of an output apparatus, and to a recording medium therefor.
2. Description of the Related Art
In recent years, various types of peripheral apparatuses, such as personal computers and printers, have become popular, and anyone can easily produce hardcopy output of word-processor documents and graphic images produced by a computer. As a representative construction thereof, an image processing system such as that shown in
FIG. 10
is known.
FIG. 10
shows an outline of a construction of a system which produces a page layout document such as DTP, and word-processor and graphic documents by using a host computer
101
and which produces hardcopy output by a laser printer, an ink jet printer, etc.
Reference number
102
denotes an application which operates in a host computer. Representative well known examples are word processor software such as MS Word (trademark) of Microsoft Corporation, and page layout software such as PageMaker (trademark) of Adobe Systems Incorporated.
A digital document produced by this software is passed to a printer driver
103
via an operating system (OS) of a computer (not shown).
The digital document is usually expressed as a set of command data representing figures and characters which constitute one page, and these commands are sent to the printer driver
103
. The series of commands forming the screen are expressed as a language system called “PDL (Page Description Language)”. Representative well known examples of PDL are GDI (Graphics Device Interface) (trademark) and PS (PostScript) (trademark).
The printer driver
103
transfers the received PDL command to a rasterizer
105
inside a raster image processor
104
. The rasterizer
105
forms the characters, figures, etc., expressed by PDL commands, into a two-dimensional bit-mapped image, performs a gradation correction process on each pixel by using a look-up table, and performs a quantization process, such as a dithering process. Since the bit-mapped image is an image in which a two-dimensional plane is filled with a repetition of a one-dimensional raster (line), the rasterizer
105
is called a “rasterizer”. The expanded bit-mapped image is temporarily stored in an image memory
106
.
The above operation is schematically shown in
FIG. 11. A
document image
111
displayed on the host computer is sent, as a PDL command sequence
112
, to the rasterizer via the printer driver. The rasterizer expands the two-dimensional bit-mapped image onto the image memory, as indicated by reference numeral
113
.
The expanded image data is sent to a color printer
107
. In the color printer
107
, an image forming unit
108
employing a well-known electrophotographic method or ink-jet recording method is used, and by using this, a visible image is formed on paper and a printout is produced. It is a matter of course that the image data in the image memory is transferred in synchronization with a synchronization signal (not shown), a clock signal (not shown), or a transfer request (not shown) of a specific color-component signal, which is required to operate the image forming unit.
In the conventional technology such as that described above, when an image forming unit used for output is considered, it is clear that various problems will occur.
The problems arise from instability of image output characteristics of the image forming unit and from variations among apparatuses, and in output images of the same original document, hue changes each time an output is made or differs when the output is made by a different printer.
This occurs due to the following reasons. For example, when an electrophotographic method is used in the image forming unit, steps, such as laser exposure, latent-image formation on a photosensitive body, toner development, toner transfer onto a paper medium, and fixing by heat, in the electrophotographic process are affected by environmental temperature and humidity or by factors such as aging of components, and the amount of toner which is finally fixed onto the paper changes for each situation.
Such instability is not characteristic of the electrophotographic method, and it is known that such instability occurs in a similar manner even in an ink-jet recording method, a heat-sensitive transfer method, and various other methods.
In order to overcome such problems, conventionally, a system shown in
FIG. 12
is conceived. This is designed to output a test pattern image, such as that indicated by reference numeral
121
, from the printer
107
, to measure the density of the output pattern, and to correct the characteristics of the image forming unit. In this system, a look-up table which is used in a gradation correction process performed by the rasterizer
105
is created.
The operations at this time are described below step by step.
Initially, the host computer
101
sends a command for outputting a predetermined gradation pattern to the raster image processor
104
. The raster image processor
104
generates a bit-mapped pattern for printer output on the basis of the sent command and transfers it to the printer section
107
. The printer section
107
outputs the supplied bit-mapped pattern on a paper medium. Here, for the output pattern, it is assumed that a pattern is output such that a toner adherence area ratio changes in eight steps from 0% to 100% with respect to cyan (C), magenta (M), yellow (Y), and black (K) corresponding to a four-color toner of a printer as indicated by the test pattern image
121
. In
FIG. 12
, each of the eight steps is given a number from 0 to 7, and the gradation pattern of each color is such that a horizontal row of reference numeral
122
is for C, a horizontal row of reference numeral
123
is for M, a horizontal row of reference numeral
124
is for Y, and a horizontal row of reference numeral
125
is for K.
In the output pattern, there are a total of 32 rectangular printing areas (patch areas) in 4 colors×8 steps, and each of the areas is measured by using a reflection densitometer
126
. The measured value (the density value of each patch) is sent to the host computer.
The host computer compares the measured value with a prestored reference value, creates a correction table for each of the colors C, M, Y, and K, and registers this table in a table conversion section of the raster image processor. The table conversion section corrects a value which is written as bit-mapped data when the raster image processor creates a bit-mapped image.
For example, in a case where the density of the third patch of cyan in the test pattern
121
is measured lower than the reference value, in the correction table, by correcting the bit-mapped data corresponding to the third patch of cyan to a high value, the density characteristics of the printer can be brought closer to the reference value.
This state is shown in
FIGS. 13A and 13B
.
FIG. 13A
shows that the density value obtained for a cyan patch is plotted with respect to the gradation number of the patch.
FIG. 13B
shows a correction table which is created based on this measured value.
In
FIG. 13A
, the horizontal axis indicates a gradation number, the vertical axis indicates a measured value, ∘ marks
131
each indicate a measured value, and a curve
132
connects the measured values by straight lines.
The gradation number along the horizontal axis is considered here. This is equivalent to that signal values to be output to the printer which is an image forming unit are sampled at predetermined intervals and are assigned numbers. A conventional printer unit is capable of image output at the number of gradations formed of eight bits for each of C, M, Y, and K, and forms and outputs an image having continuous gradations on paper by using a binarization process using a well-known dithering process according to the level of each signal value.
In the embodiment described here, since patches of 0 to 7 are output by signa
Bayat Ali
Mehta Bhavesh M.
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