Image processing apparatus

An image processing apparatus for converting input color image signals to image recording signals to be sent to an image forming apparatus. The image processing apparatus comprises: a first color converting unit for converting the input color image signals to three-variable color signals independent of devices; a second color converting unit for converting the three-variable color signals to the image recording signals; and a parameter determining unit for determining parameters of the first color converting unit. The parameter determining unit determines the parameters so as to make characteristic colors in the input color image signals coincide with predetermined colors and to maintain linearities of color reproduction characteristics of an output image from the image forming apparatus.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image processing apparatus for 
converting input color image signals to image recording signals that are 
sent to an image forming apparatus such as a color printer, the input 
color image signals including image data that is obtained through scanning 
of a photographic film or acquired from a digital still camera. 
2. Description of the Related Art 
Recent years have seen advances in computer technology, improvements in 
communication networks and the introduction of mass storage media, 
accompanied by widespread use of scanners and digital still cameras. The 
trend has given impetus to a growing need for processing photographic 
image data, i.e., the need for photo image data to be printed out with 
high quality. 
In the area of photographic films, images are output less often in analog 
fashion today. Instead, image processing and editing are carried out 
increasingly on digitized images to comply with a need for 
enhanced-quality, multi-functional image print-out. 
The photographic image data to be printed out typically comprises so-called 
memory colors of characteristic image portions such as the color of the 
human skin, the blue of the skies and the green of the greenery. A need 
has been recognized to reproduce such characteristic portions in visually 
preferred or desired colors. 
The need for such memory color reproduction is addressed illustratively by 
Japanese Published Unexamined Patent Application No. Hei 6-121159. The 
publication discloses a method for extracting memory colors from an image 
and converting the extracted portions into visually desirable colors that 
were previously determined by sensor evaluation. The extracted portions 
with their colors thus determined are then printed out. 
Image forming apparatuses such as printers and image display units have 
different ranges of color reproduction from one device to another. In 
particular, printers differ significantly from display units in terms of 
color reproduction ranges. This can lead to cases where input image data 
acquired from an image input source such as a scanner is not necessarily 
printed exactly as displayed, part of the input image data being left out 
upon print-out. 
FIG. 17 is a graphic representation showing differences in color 
reproduction ranges between devices. In FIG. 17, a region op indicates a 
typical color reproduction range of a color printer (Kodak XL7700), and a 
region Gm stands for a representative color reproduction range of an 
ordinary ROB color video monitor. The figure sketches the color 
reproduction ranges on an a*b* plane at L*=50.0 in a CIE.multidot.L*a*b* 
color space. 
In FIG. 17, the image data at point Pi cannot be printed out as such 
because it is outside the color reproduction range Gp of the printer. To 
have the image data at point Pi output by the printer requires additional 
procedures of operation or data conversion. 
Several such procedures have been proposed so far. For example, Japanese 
Published Unexamined Patent Application No. Hei 7-298073 discloses ways to 
"clip" image data outside a given color reproduction range in a chroma 
direction such that the data will be located on a boundary of the target 
range. More specifically, the image data at point Pi in FIG. 17 is 
converted to image data at point Po1 where a line 1 connecting point Pi 
with the origin of the a*b* coordinates intersects the boundary of the 
color reproduction range Gp, the data conversion being performed in such a 
manner that the lightness and hue of the data at point Po1 match those of 
the data at point Pi. 
Japanese Published Unexamined Patent Application No. Hei 5-115000 discloses 
another technique. The disclosure involves establishing a region in which 
to reproduce desired data inside the color reproduction range of the 
printer without chroma contraction; image data outside the range is 
partially contracted in chroma. 
Japanese Published Unexamined Patent Application Nos. Hei 6-253138 and Hei 
6-253139 disclose a method whereby specific points in color space are 
explicitly mapped, the remaining portions being mapped by interpolation or 
like manner. According to this method, the image data at point Pi in FIG. 
17 is converted to image data at point Po2 on a boundary of the color 
reproduction range Gp. 
Japanese Published Unexamined Patent Application No. Hei 6-189121 discloses 
another method. The disclosed method utilizes evaluation functions 
supplemented with deviations weighted in terms of lightness, chroma and 
hue between monitor-displayed coloration and print colors in response to 
input signals. These functions are used to optimize color converting 
parameters, whereby the target color reproduction range is contracted for 
preferred sensory perception. 
The method disclosed by the above-cited Japanese Published Unexamined 
Patent Application No. Hei 6-121159 appears to be effective in reproducing 
memory colors in visually preferable coloration upon print-out of 
photographic image data. 
However, the disclosed method has a disadvantage stemming from a color 
correcting process performed on the extracted memory color regions, the 
process being different from the one on the remaining regions. The 
dissimilar process can produce a discontinuity between the memory colors 
and the remaining colors as shown in FIG. 18A, generating a false contour 
or other flaws in color reproduction. 
Japanese Published Unexamined Patent Application No. Hei 7-298073 cited 
above resorts to the clipping of image data in addressing a narrower color 
reproduction range of the printer than that of the display unit. However, 
the method of clipping image data outside the color reproduction range in 
the direction of chroma to relocate the data onto a boundary of the range 
can lead to a chroma fusion in highly saturated portions, as indicated by 
a linear segment 3 in FIG. 18B. The result can be a significantly degraded 
quality of output images. 
The technique disclosed in the above-cited Japanese Published Unexamined 
Patent Application No. Hei 5-115000 aims to establish a region in which to 
reproduce image data inside the color reproduction range of the printer 
without chroma contraction, while image data outside the range is 
partially contracted in chroma. This method, while not as conspicuously as 
the above-mentioned clipping method, can still cause a chroma fusion in 
highly saturated portions as shown by a linear segment 4 in FIG. 18B, 
resulting in a degraded quality of output images. 
The method disclosed in Japanese Published Unexamined Patent Application 
Nos. Hei 6-253138 and Hei 6-253139 eases any chroma fusion in highly 
saturated portions. Still, this method has a disadvantage, as evident from 
a curved segment 9 in FIG. 18C, of causing a warped hue and of disturbing 
linearity of the chroma gradation characteristic. In addition, the method 
has difficulty in guaranteeing the continuity of colors in cases where 
there are many points to be matched in color. 
The technique of the above-cited Japanese Published Unexamined Patent 
Application No. Hei 6-189121 has a disadvantage similar to that of the 
clipping method regarding output chroma characteristics as opposed to 
input chroma. That is, the technique also tends to cause a chroma fusion 
in highly saturated portions, although not as severely as the clipping 
method, as indicated by a broken line curve 5 in FIG. 18B. The disclosed 
technique has another disadvantage of requiring a great deal of effort in 
determining the necessary evaluation functions. Specifically, color 
converting parameters need to be determined by establishing a large number 
of points as evaluation points in color space. For each evaluation point, 
a weighting factor needs to be determined through sensor evaluation. 
As the digital photo system is coming into general use, there will be a 
growing number of cases in which users bring their photo films or CD-ROMs 
into local print shops and later receive prints there derived from the 
submitted materials. In such cases, there will conceivably be more users 
wishing to get better color reproduction in their prints than those who 
prefer having their prints coincide with monitor-displayed colors. 
Aged deterioration of photo films or their inadequate exposure, as well as 
diverse image input devices to be dealt with, can contribute to color 
balance variations from one print image to another. It follows that 
faithfully printing out input data according to its color balance can lead 
to undesirable color reproduction of prints if the color balance of the 
input data has been subsequently changed. 
As outlined above, there have yet to be developed color image processing 
techniques for converting digital photo data into colors exactly as 
desired by users. The method for contracting the color reproduction range, 
extensively used so far, rests on the precondition that print colors 
coincide visually with monitor-displayed colors. The requirement involves 
having the color reproduction range contracted in the chroma direction 
while keeping the level of overall saturation as high as possible. This 
means theoretically that highly saturated portions tend to produce a 
fusion of chroma gradation. 
It may happen that a color imbalance on the monitor results in flawed color 
reproduction thereby. This will make it impossible to acquire the user's 
preferred colors even if the print colors match the monitor-displayed 
colors. 
The hitherto-proposed technique of extracting memory colors of the humans 
and subjecting the extracted colors to a different color correcting 
process apparently improves reproduction of the memory colors. But the 
technique is deficient in overcoming the discontinuity between the memory 
colors and other colors or in eliminating the generation of a false 
contour. 
The point is not that print colors should match some extraneous criteria 
but that the colors are to be reproduced as preferred by users. This 
simply requires making colors attracting users' attention such as memory 
colors coincide with what the users keep in their memory. 
It happens frequently that the need for reproducing memory colors and the 
need for preventing chroma fusion in highly saturated portions are 
mutually exclusive. A trade-off needs to be made between these two 
requirements when color converting characteristics are determined. 
There is room for variations in the colors that are kept in the human 
memory. This means that, unlike under conventional schemes, memory colors 
need not necessarily match predetermined values. What is more important is 
for hue and chroma settings to be brought to better levels than before. 
In order to implement gradation without chroma fusion in highly saturated 
portions, it is imperative to utilize the widest possible range of color 
reproduction on the part of the image forming apparatus. To make memory 
colors coincide with what users keep in their memory requires that a 
memory color range be extracted from input image data with the 
characteristics of the input data taken into consideration. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an image 
processing apparatus which reproduces any input image in prints always 
having visually preferred colors; which permits satisfactory reproduction 
of lightness, chroma and hue of images portions such as memory colors 
attracting attention of the humans; and which offers good color 
reproduction characteristics guaranteeing the gradation and hue linearity 
of the whole image with no chroma fusion in highly saturated portions. 
It is another object of the present invention to provide an image 
processing apparatus offering good color reproduction characteristics 
using a small number of parameters, whereby the parameters involved are 
optimized in a reliable manner. 
It is a further object of the invention to provide an image processing 
apparatus which utilizes a limited number of evaluation points to 
reproduce simply and reliably memory colors or the like exactly as desired 
by the user. 
In carrying out the invention and according to one aspect thereof, there is 
provided an image processing apparatus for converting input color image 
signals to image recording signals to be sent to an image forming 
apparatus, the image processing apparatus comprising: first color 
converting means for converting the input color image signals to 
three-variable color signals independent of devices; second color 
converting means for converting the three-variable color signals to the 
image recording signals; and parameter determining means for determining 
parameters of the first color converting means; wherein the parameter 
determining means determines the parameters so as to make characteristic 
colors in the input color image signals coincide with predetermined colors 
and to maintain linearity of color reproduction characteristics of an 
output image from the image forming apparatus. 
In the inventive image processing apparatus of the above constitution, the 
input color image signals are converted by the first color converting 
means into the three-variable color signals that are not dependent on any 
specific devices. The three-variable color signals are converted by the 
second color converting means into the image recording signals. 
The first color converting means converts the input color image signals to 
the three-variable color signals using the color converting parameters 
determined by the parameter determining means. The parameter determining 
means determines the color converting parameters of the first color 
converting means for two purposes: to make the characteristic colors such 
as memory colors in the input color image signals coincide with 
predetermined colors; and to maintain the linearity of the color 
reproduction characteristics of the output image from the image forming 
apparatus. 
The color converting parameters thus determined to address the above two 
purposes are used by the first color converting means to convert the input 
color image signals into the three-variable color signals. The setup above 
permits adequate reproduction of lightness, chroma and hue in the 
characteristic color portions of the image, guarantees gradation 
characteristics and hue linearity of the whole image, and suppresses 
nonlinear image quality degradation such as chroma fusion in highly 
saturated portions. 
These and other objects, features and advantages of the invention will 
become more apparent upon a reading of the following description and 
appended drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Preferred embodiments of the present invention will now be described with 
reference to the accompanying drawings. 
(Color Image Output System Embodying the Invention) 
FIG. 1 is a block diagram of a typical color image output system using an 
image processing apparatus of the invention. The color image output system 
as a whole is made up of an image input apparatus 100, an image processing 
apparatus 200 and an image forming apparatus 300. 
The image input apparatus 100 receives color images in various formats from 
the outside. In turn, the image input apparatus 100 of this example 
outputs color image signals constituted by 24-bit RGB data made of red 
(R), green (G) and blue (B), each color being composed of eight bits and 
furnished in 256 grades. 
Specifically, the image input apparatus 100 supplies the image processing 
apparatus 200 with RGB data acquired in one of several processes. That is, 
the apparatus 100 may use a CCD sensor to read RGB data from 35-mm color 
negative or positive films or APS films representative of the silver salt 
photographic film; the apparatus 100 may read image data from CD-ROMs in 
the KODAK Photo CD format and convert the read data into RGB data; the 
apparatus 100 may receive picture data from a digital still camera such as 
the Canon DCS1c and convert the received data into RGB data; the apparatus 
100 may read color image data edited by the user operating a computer and 
stored in a storage medium such as MO (magneto-optical disk) or ZIP disk 
and may convert the read data into RGE data; or the apparatus 100 may 
convert to RGB data the image data sent from a device on a network. 
The image processing apparatus 200 as a whole is made up of first color 
converting means 210, second color converting means 220 and parameter 
determining means 230. The RGB data received from the image input 
apparatus 100 is converted by the first color converting means 210 into 
data in a CIE.multidot.L*a*b* color space, which Is one of uniform color 
spaces. 
The color converting characteristic of the first color converting means 210 
is defined by color converting parameters. The parameters are determined 
by the parameter determining means 230 based on the input RGB data and on 
the color reproduction characteristic such as the color reproduction range 
of the image forming apparatus 300, the color reproduction characteristic 
being transferred from the image forming apparatus 300. The color 
converting parameters thus determined are sent to the first color 
converting means 210. 
The L*a*b* data from the first color converting means 210 is converted by 
the second color converting means 220 into image recording signals for the 
color space of the image forming apparatus 300. In this example, the image 
recording signals comprise four-color data, i.e., Y (yellow), M (magenta), 
C (cyan) and K (black). The image recording signals thus prepared are 
transferred to the image forming apparatus 300. Using the YMCK data, the 
image forming apparatus 300 draws an image on a suitable sheet of paper. 
The most common input color signals from the image input apparatus 100 are 
RGB data. The examples that follow will use RGB data. However, other types 
of data may also be used such as YMC color space data or YCC color space 
data for use on Photo CDs. 
The first color converting means 210 is typically one which converts the 
input color signals into L*a*b* color space data. The examples that follow 
will utilize the type of means 210 for converting the input color signals 
to L*a*b* color space data. However, the first color converting means 210 
may alternatively be one which converts the input color signals into other 
color space data such as XYZ color space data or Luv color space data as 
long as the color space in question is not dependent on any specific 
devices. It is preferable that the first color converting means 210 be one 
which converts the input color signals into uniform color space data. 
In the examples that follow, the color space for the image forming 
apparatus 300 will refer to the YMCK color space. However, this is not 
limitative of the invention and other color spaces such as the YMC color 
space or RGB color space may alternatively be used by the image forming 
apparatus 300. The image drawing medium for use with the image forming 
apparatus 300 is not limited to sheets of paper as will be the case in the 
description that follows. Other suitable media may also be used instead. 
FIG. 2 is a schematic view of a typical image forming apparatus 300. The 
apparatus of this example is a single engine type electrophotographic 
color printer. Four-color (YMCK) data from the image processing apparatus 
200 is converted for each color by a screen generator 390 into a binary 
signal with its pulse width modulated in accordance with the data value. 
The binary signals constitute what is called a screen signal. 
The screen signal drives a laser diode 381 when introduced into a laser 
beam scanner 380. The laser beam scanner 380 has the laser diode 381 
output a laser beam L that is emitted onto a photosensitive drum 310. 
The photosensitive drum 310 is electrically charged by a charging device 
320 for electrostatic latent image formation. The laser beam L emitted by 
the laser beam scanner 380 forms electrostatic latent images over the 
surface of the drum 310. 
The photosensitive drum 310 with electrostatic latent images formed thereon 
comes into contact with image developing units 331, 332, 333 and 334, 
respectively for the four colors (KYMC), in a rotary developing device 
330. The contact operation causes the electrostatic latent images in the 
respective colors on the photosensitive drum 310 to be developed into 
toner images. 
A sheet of paper in a paper tray 301 is collected by a sheet feeder 302, 
fed to a transfer drum 340 and wrapped around the latter. At the same 
time, a transfer charging device 341 applies corona discharge from behind 
the sheet. This causes the developed toner images on the photosensitive 
drum 310 to be transferred onto the sheet of paper. Where a multi-color 
image is to be obtained, the sheet is brought into contact with the 
photosensitive drum 310 two to four times in a row. The repeated contact 
of the sheet with the drum 310 causes the multiple images in up to four 
colors (KYMC) to be all transferred to the sheet of paper. 
After image transfer, the sheet is sent to a fusing device 370 whereby the 
toner images are heated and fused onto the sheet. With the toner images 
transferred to the sheet, the photosensitive drum 310 is cleaned by a 
cleaner 350. After cleaning, the photosensitive drum 310 is prepared by a 
preliminary exposing device 360 for another use. 
Although the example in FIG. 2 is a single engine type, this is not 
limitative of the invention. The image forming apparatus 300 may also be 
any one of other electrophotographic printers including the tandem engine 
type and the image-on-image type whereby color images formed on a 
photosensitive drum are transferred collectively onto a suitable medium. 
As will be described later, this invention also applies to other types of 
image forming apparatus than the electrophotography type, such as the 
silver salt photography type, thermal transfer type and ink jet type. Such 
alternative applications still yield the same effects. 
(First Embodiment in the Form of an Image Processing Apparatus) 
FIG. 3 is a block diagram of a typical image processing apparatus 200 shown 
in FIG. 1. The apparatus of this example comprises the first and second 
color converting means 210 and 220. The first color converting means 210 
has a specialized three-dimensional interpolation color converting circuit 
211 and a color adjusting circuit 213, to be described later. The second 
color converting means 220 includes a DLUT interpolation computing circuit 
221 of a known type and a gradation correcting circuit 222, to be 
described later. 
The parameter determining means 230 in this embodiment comprises 
characteristic color region extracting means 231, characteristic 
describing means 232, output image predicting means 233, evaluation data 
setting means 234, lightness deviation computing means 241, chroma 
deviation computing means 242, hue deviation computing means 243, 
lightness gradation characteristic computing means 244, chroma gradation 
characteristic computing means 245, hue linearity computing means 246, 
evaluation value integrating means 235, and parameter changing means 236. 
Input RGB data from the image input apparatus 100 is fed to the 
characteristic color region extracting means 231. In turn, the 
characteristic color region extracting means 231 extracts from the input 
RCB data the characteristic color regions attracting the user's attention 
most in the input image. These characteristic color regions are typically 
constituted by memory colors such as the skin color i.e., colors of the 
human being, the green of greenery and the blue of the skies. The RGB data 
representing the extracted characteristic color regions is sent to the 
output image predicting means 233. 
As will be described later, the characteristic describing means 232 
communicates with the image forming apparatus 300 and receives data 
therefrom, the received data indicating the color reproduction range of 
the image forming apparatus 300. The data from the characteristic 
describing means 232 is sent to the output image predicting means 233, 
which is also supplied with the RGB data on the characteristic color 
regions from the characteristic color region extracting means 231. Using 
the received data, the output image predicting means 233 predicts as 
L*a*b* color data the output colors following color conversion by the 
first color converting means 210 that has the three-dimensional 
interpolation color converting circuit 211 and color adjusting circuit 
213. The predicted L*a*b* color data is transferred from the output image 
predicting means 233 to the lightness deviation computing means 241, 
chroma deviation computing means 242 and hue deviation computing means 
243. 
The lightness deviation computing means 241 computes deviations in 
lightness between the predicted data about the characteristic colors on 
the one hand and lightness settings defined beforehand by use of L*a*b* 
data on the other hand. The computed result is transferred to the 
evaluation value integrating means 235. The chroma deviation computing 
means 242 computes deviations in chroma between the predicted data about 
the characteristic colors on the one hand and predetermined chroma 
settings on the other hand. The computed result is likewise transferred to 
the evaluation value integrating means 235. The hue deviation computing 
means 243 computes deviations in hue between the predicted data about the 
characteristic colors on the one hand and predetermined hue settings on 
the other hand. The computed result is also transferred to the evaluation 
value integrating means 235. 
Meanwhile, the evaluation data setting means 234 sets up evaluation data in 
advance using RGB data. The predetermined evaluation data comprises 
illustratively gradation data about the colors that are likely to attract 
users' attention for a particular gradation, for a fused gradation in 
highly saturated portions or for a nonlinear hue. The color gradation data 
thus established is sent to the output image predicting means 233. 
The output image predicting means 233 receives from the characteristic 
describing means 232 the data written therein to designate the color 
reproduction range of the image forming apparatus 300. The evaluation data 
setting means 234 feeds the output image predicting means 233 with RGB 
data as evaluation data. On the basis of both the color reproduction range 
data and the evaluation data, the output image predicting means 233 
predicts as L*a*b* color data the output colors following color conversion 
by the first color converting means 210 made of the three-dimensional 
interpolation color converting circuit 211 and color adjusting circuit 
213. The L*a*b* data is transferred as the predicted data to the lightness 
gradation characteristic computing means 244, chroma gradation 
characteristic computing means 245 and hue linearity computing means 246. 
The lightness gradation characteristic computing means 244 computes the 
linearity of lightness gradation in the predicted data reflecting the 
evaluation data. The computed result is sent to the evaluation value 
integrating means 235. The chroma gradation characteristic computing means 
245 computes the linearity of chroma gradation in the predicted data 
reflecting the evaluation data. The computed result is also transferred to 
the evaluation value integrating means 235. The hue linearity computing 
means 246 computes hue linearity in the predicted data reflecting the 
evaluation data. The computed result is likewise sent to the evaluation 
value integrating means 235. 
The evaluation value integrating means 235 is thus supplied with image 
quality evaluation values from the lightness deviation computing means 
241, chroma deviation computing means 242, hue deviation computing means 
243, lightness gradation characteristic computing means 244, chroma 
gradation characteristic computing means 245 and hue linearity computing 
means 246. The respective image quality evaluation values are weighted, 
added up and integrated, whereby an overall image quality evaluation value 
is obtained. The acquired overall evaluation value is sent to the 
parameter changing means 236. 
The parameter changing means 236 changes parameters of the 
three-dimensional interpolation color converting circuit 211 and color 
adjusting circuit 213 if the overall evaluation value from the evaluation 
value integrating means 235 fails to meet predetermined converging 
conditions. The changed parameters are transferred to the output image 
predicting means 233 so that the predicting means 233 and other related 
components will repeat the image quality evaluation process until the 
overall evaluation value meets the converging conditions. When the 
converging conditions are met by the overall evaluation value, the 
parameters in effect at that point are regarded as optimized parameters 
which are then transferred to the three-dimensional interpolation color 
converting circuit 211 and color adjusting circuit 213. 
After the optimized parameters have been determined, the image input 
apparatus 100 supplies input RGB data to the three-dimensional 
interpolation color converting circuit 211 in the first color converting 
means 210. In turn, the three-dimensional interpolation color converting 
circuit 211 converts the input RGB data into L*a*b* data, as will be 
described later. The L*a*b* data after conversion is color-adjusted by the 
color adjusting circuit 213 in the L*a*b* color space, i.e., a uniform 
color space, as will also be described later. After color adjustment, the 
L*a*b* data is output from the color adjusting circuit 213 in the first 
color converting means 210. 
The color-adjusted L*a*b* data is then converted to YMCK data reflecting 
the L*a*b* data. The conversion process is carried out by that DLUT 
interpolation computing circuit 221 in the second color converting means 
220 which is a known circuit to be described later and which comprises 
DLUTs (direct look-up tables) and an interpolation computing circuit. The 
YMCK data after conversion is corrected in gradation by the gradation 
correcting circuit 222. After gradation correction, the YMCK data is 
output as image recording signals from the gradation correcting circuit 
222 in the second color converting means 220. 
The image recording signals composed of the gradation-corrected YMCK data 
are transferred to the image forming apparatus 300. In turn, the image 
forming apparatus 300 draws images on a sheet of paper in the manner 
described earlier with reference to FIG. 2. 
As described, the input RGB data is fed to the character color region 
extracting means 231 in the parameter determining means 230. With the 
optimized parameters determined, the input RGB data is supplied to the 
three-dimensional interpolation color converting means 211 in the first 
color converting means 210. Thus the image input apparatus 100 includes 
illustratively an image buffer memory through which the input RGB data is 
read out repeatedly. Alternatively, the silver salt photographic film 
mentioned above may be read out repeatedly in order to output the input 
RGB data as many times as needed. 
The DLUT interpolation computing circuit 221 comprises DLUTs and an 
interpolation computing circuit as described above. Of the L*a*b* data 
from the first color converting means 210 (eight bits long per point), the 
high-order four bits are used to generate the address of a lattice point 
near each point determined by the L*a*b* data. The addresses of such 
nearby lattice points are used as the basis for reading nearby lattice 
point data from the DLUTs. The data on each lattice point thus read out is 
interpolated by use of the low-order four bits of the L*a*b* data, whereby 
output YMCK data is obtained. 
Illustratively, a publication "Display and Imaging" (SCI, Volume 2, Number 
1, 1993, pp. 17-25) describes ways to perform cubic interpolation with 
reference to eight nearby lattice points, prism interpolation with 
reference to six nearby lattice points, and tetrahedron interpolation with 
reference to four nearby lattice points. The first embodiment may utilize 
some of these methods, prism interpolation for instance. In such a case, 
the address of any nearby lattice point need not be limited to the 
high-order four bits of L*a*b* data. 
The gradation correcting circuit 222 is intended to correct nonlinear 
gradation characteristics of the image forming apparatus 300. The circuit 
222 is composed of as many eight-bit LUTs (look-up tables) as the 
component colors making up the YMCK data. Alternatively, the contents of 
the LUTs constituting the gradation correcting circuit 222 may be updated 
periodically to correct secular changes of the image forming apparatus 
300. 
The first embodiment uses the DLUT interpolation computing circuit 221 in 
the second color converting means 220 in order to convert L*a*b* data to 
YMCK data with high precision. Alternatively, the second color converting 
means 220 may be replaced by other types of color converting circuit such 
as a matrix type color converting circuit and a neural network type color 
converting circuit used extensively as color converting means. 
A feature characteristic of the first embodiment is a color converting 
process carried out by the three-dimensional interpolation color 
converting circuit 211 in the first color converting means 210. This color 
converting process will now be described with reference to FIGS. 4A 
through 4C as well as 5A and 5B. 
According to the color converting process of the three-dimensional 
interpolation color converting circuit 211, the color reproduction range 
of the image forming apparatus 300 in the L*a*b* color space is expressed 
by a dodecahedron having a total of eight vertexes in L*a*b* coordinates, 
as shown in FIGS. 4A and 4B. The vertexes are made up of maximum density 
points of red (R), green (G), blue (B), yellow (Y), magenta (M), cyan (C) 
and gray (S) as well as white (W) of the sheet of paper used by the image 
forming apparatus 300. 
The dodecahedron is divided into six tetrahedrons each including a maximum 
gray density point S and a white point W representing the sheet of paper, 
as shown by regions 1 through 6 in FIG. 4C. Which tetrahedron contains the 
input RGB data is judged using inequalities shown in FIG. 4A. For example, 
the input RGB data in which R=100, B=128 and G=255 is judged to be the 
data contained in the tetrahedron of region 5 because B.gtoreq.R and 
G.gtoreq.B. 
The input RGB data within a given tetrahedron is converted to the 
corresponding L*a*b* data by use of the interpolation process described 
below. 
Suppose that, as shown in FIG. 5A, vertexes (xi, yi, zi), (xj, yj, zi), 
(xp, yp, zp) and (xq, yq, zq) of a tetrahedron are represented 
respectively by given data items Di, Dj, Dp and Dq constituting the L*a*b* 
data. In that case, an interpolation value D corresponding to a point (x, 
y, z) inside the tetrahedron is defined as 
EQU D=Di .phi. i(x, y, z)+Dj .phi. j(x, y, z)+Dp .phi. p(x, y, z)+Dq .phi. q(x, 
y, z) (1) 
as shown in FIG. 5A. 
Coordinates .phi. i(x, y, z), .phi. j(x, y, z), .phi. p(x, y, z) and .phi. 
q(x, y, z) are defined respectively by expressions (2), (3), (4) and (5) 
shown in FIG. 5B. 
As described, the color reproduction range of the image forming apparatus 
300 is described using a dodecahedron. The dodecahedron is divided into 
six tetrahedrons. Inside each of the tetrahedrons, linear interpolation 
computations are carried out to convert the input RGB data into L*a*b* 
data. 
Compared with conventional color converting schemes such as the matrix 
type, neural network type and DLUT type employed extensively, the color 
converting method based on the above interpolation process offers the 
following major advantages: 
First, the color reproduction range of the image forming apparatus 300 is 
maximized for use. As will be described later, the linearity of lightness 
direction and chroma direction is guaranteed, which in turn ensures the 
linearity of hue direction. 
Second, the color converting parameters involved are as few as eight 
vertexes R, G, B, Y, M, C, S and W of a dodecahedron. Because the 
dodecahedron is divided into tetrahedrons reflecting the hues involved, it 
is easy to color-adjust and optimize each of the hues, as will be 
described later. 
Third, the gray axis is represented by a side of each of the tetrahedrons 
making up the dodecahedron. This guarantees the continuity of the gray 
axis. 
Furthermore, the inventive color converting process may utilize the 
hardware of the DLUT color converting means furnished for the conventional 
tetrahedron interpolation method. In such a case, the addresses to be 
stored in the DLUT arrangement involve the coordinates of only eight 
vertexes. This offers an implementation of the process at very low costs. 
FIG. 6 is a block diagram of a typical color adjusting circuit 213 in the 
first color converting means 210. In this example, of the L*a*b* data from 
the three-dimensional interpolation color converting circuit 211, the a*b* 
data is fed to the chroma hue converting circuit 214 for conversion into 
chroma data C* and hue data H, as defined by the expressions (6) and (7) 
below: 
EQU C*={(a*).sup.2 +(b*).sup.2 }.sup.1/2 (6) 
EQU H=tan.sup.-1 (b*/a*) (7) 
The L* data from the three-dimensional interpolation color converting 
circuit 211 is input to an L*LUT 215. The chroma data C* from the chroma 
hue converting circuit 214 is input to a C*LUT 216. The hue data H is sent 
to an H.multidot.LUT 217. 
The L*LUT 215, C*LUT 216 and H.multidot.LUT 217 are a one-dimensional 
look-up table each. The L*LUT 215, C*LUT 216 and H.multidot.LUT 217 
provide adjustments in the direction of lightness, chroma and hue 
respectively. 
FIGS. 7A, 7B and 7C are graphic representations showing typical settings of 
the L*LUT 215, C*LUT 216 and H.multidot.LUT 217. In the L*LUT 215 and 
C*LUT 216, the maximum input data is standardized at 255. 
The chroma data C* and hue data H following adjustment by the C*LUT 216 and 
H.multidot.LUT 217 are input to an a*b* converting circuit 218 for 
conversion into a*b* data, as defined by the expressions (8) and (9) 
below: 
EQU a*=C*cos(H) (8) 
EQU b*=C*sin(H) (9) 
The a*b* data from the conversion above and the L* data after adjustment by 
the L*LUT 215 are supplied to the DLUT interpolation computing circuit 221 
in the second color converting means 220. 
The chroma hue converting circuit 214 and a*b* converting circuit 218 are 
not limited to executing definition expressions as described above. 
Alternatively, the circuits 214 and 218 may be constituted by a 
two-dimensional look-up table each. 
When the L*a*b* data is adjusted by LUTs in the L*C*H color space as 
described above, the three-dimensional interpolation color converting 
circuit 211 may adjust linearly arranged data in the L*a*b* color space 
separately for lightness, chroma and hue. The adjustments may be made so 
as to attain the respective characteristics desired. 
The color adjusting process above is not limitative of the invention. 
Alternatively, the L*a*b* data may be converted not to the L*C*H color 
space but to any other suitable color space for color adjustment. 
FIG. 8 is a flowchart of steps constituting a characteristic color region 
extracting routine 10 carried out by the characteristic color region 
extracting means 231 in the -parameter determining means 230 of FIG. 3. In 
this example, the skin color region, i.e., one of memory colors, is 
extracted. 
Data ranges and a data item ratio for the skin color region are set in 
advance for the characteristic color region extracting means 231. In this 
example, the RGB data is regarded as skin color region data if the data 
item R comes between 100 and 240, G between 60 and 200, and B between 60 
and 220, and if the data item G falls between 0.7 and 0.9, and B between 
0.75 and 0.95 with respect to R being set for 1. 
In step 11 of the characteristic color region extracting routine 10, the 
characteristic color region extracting means 231 reads input RGB data. In 
step 12, the RGB data items that fall within the ranges of 100&lt;R&lt;240, 
60&lt;G&lt;200 and 60&lt;B&lt;220 are extracted. 
In step 13, the ratio of the RGB data items extracted in step 12 is 
computed and the RGB data items having the ratio of 
EQU R:G:B=1:0.7 to 0.9:0.75 to 0.95 
are extracted as the skin color region. 
In step 14, histograms for the RGB data items extracted in step 13 are 
prepared as shown in FIGS. 9A, 9B and 9C. In step 15, a representative 
value of each of the histograms (specifically the mode, central value or 
mean value) is output as the RGB data designating the skin color in the 
input image. This is an example in which modes 191, 118 and 106 are output 
as shown in FIGS. 9A, 9B and 9C. 
In like manner, data ranges and a data item ratio are preset for each of 
the characteristic colors other than the skin color, whereby the region 
for each characteristic color is extracted. Where a data item ratio is to 
be established as described, the data ranges need not be set for all RGB 
data items; a data range need only be set for at least one data item. 
Alternatively, the characteristic color region extracting means 231 may be 
structured so that an operator can extract characteristic color regions 
while watching a screen displayed illustratively on a CRT display unit. 
For example, the operator may extract characteristic color regions 
interactively using an application program having functions equivalent to 
those of the automatic selection tool furnished in PhotoShop 3.0J from 
Adobe Systems. 
Where the color reproduction range of the image forming apparatus 300 is 
expressed by a dodecahedron in the L*a*b* color space as shown in FIGS. 4A 
and 4B, the characteristic describing means 232 communicates with the 
image forming apparatus 300. The communicating process sets the 
characteristic describing means 232 with L*a*b* coordinates of eight 
vertexes R, G, B, Y, M, C, S and W of the dodecahedron. The coordinates 
are sent from the characteristic describing means 232 to the output image 
predicting means 233 as data designating the color reproduction range of 
the image forming apparatus 300. 
Upon power-up or during a user-initiated operation, the image forming 
apparatus 300 may control image quality as follows: a reference patch is 
first output to the photosensitive drum 310 in FIG. 2 or onto a sheet of 
paper. The reference patch is then measured by use of a calorimeter. 
Operation quantities necessary for image formation are adjusted so that 
the colorimeter readings will match target values. In such cases, every 
time the operation quantities are controlled, the characteristic 
describing means 232 should preferably be set with the L*a*b* coordinates 
of the eight vertexes R, G, B, Y, M, C, S and W specifying the color 
reproduction range of the image forming apparatus 300. 
The output image predicting means 233 receives from the characteristic 
describing means 232 the L*a*b* coordinates of the eight vertexes R, G, B, 
Y, M, C, S and W which define the color reproduction range of the image 
forming apparatus 300. In response, the output image predicting means 233 
simulates color conversion characteristics for the three-dimensional 
interpolation color converting circuit 211 and color adjusting circuit 
213. 
In the example above, the lattice points of the three-dimensional 
interpolation color converting means 211 are supplied as initial values 
with the L*a*b* coordinates of the eight vertexes R, G, B, Y, M, C, S and 
W designating the color reproduction range of the image forming apparatus 
300. The L*LUT 215, C*LUT 216 and H.multidot.LUT 217 of the color 
adjusting circuit 213 in FIG. 6 are fed with straight lines tilted at 45 
degrees each, i.e., output values equal to the input values. 
Then the output image predicting means 233 acquires as prediction values 
the L*C*H coordinates through color conversion. That color conversion is 
carried out by the three-dimensional interpolation color converting 
circuit 211 and by the chroma hue converting circuit 214, L*LUT 215, C*LUT 
216 and H.multidot.LUT 217 in the color adjusting circuit 213 on the RGB 
data about the characteristic color regions extracted by the 
characteristic color region extracting means 231. 
The lightness deviation computing means 241, chroma deviation computing 
means 242 and hue deviation computing means 243 compute deviations 
.DELTA.L, .DELTA.C and .DELTA.H of prediction values L*, C* and H with 
respect to predetermined settings (desired values) L*o, C*o and Ho through 
the use of the expressions (10), (11) and (12) below: 
EQU .DELTA.L=.vertline.L*-L*o.vertline. (10) 
EQU .DELTA.C=.vertline.C*-C*o.vertline. (11) 
EQU .DELTA.H=.vertline.H-Ho.vertline. (12) 
In addition, the output image predicting means 233 obtains as prediction 
values the L*C*H coordinates through a color converting process that is 
effected by the three-dimensional interpolation color converting circuit 
211 and by the chroma hue converting circuit 214, L*LUT 215, C*LUT 216 and 
H.multidot.LUT 217 in the color adjusting circuit 213 on the RGB data set 
as color gradation data by the evaluation data setting means 234. 
Furthermore, the lightness gradation characteristic computing means 244, 
chroma gradation characteristic computing means 245 and hue linearity 
computing means 246 compute variances .sigma.L, .sigma.C and .sigma.H of 
prediction values L*i, C*i and Hi with respect to predetermined settings 
(ideal values) L*si, C*si and Hsi by use of the expressions (13), (14) and 
(15) below: 
EQU .sigma.L={(L*0-L*s0).sup.2 +(L*1-L*s1).sup.2 + . . . +(L*n-L*sn).sup.2 
}.sup.1/2 /n (13) 
EQU .sigma.C={(C*0-C*s0).sup.2 +(C*1-C*s0).sup.2 + . . . +(C*n-C*sn).sup.2 
}.sup.1/2 /n (14) 
EQU .sigma.H={(H0-Hs0).sup.2 +(H1-Hs1).sup.2 + . . . +(Hn-Hsn).sup.2 }.sup.1/2 
/n (15) 
where "n" represents the number of grades in color gradation data, and "i" 
denotes any integer between 0 and n. In the above example, n=255. 
The process in which the variances .sigma.L, .sigma.C and .sigma.H are 
computed is illustrated in FIGS. 10A, 10B and 10C. 
The evaluation value integrating means 235 computes an overall evaluation 
value .OMEGA. of image quality using the following expression: 
EQU .OMEGA.-(j=1-m).SIGMA.(w1j.multidot..DELTA.Lj+w2j.multidot..DELTA.Cj+w3j.mu 
ltidot..DELTA.Hj)+(q=1-k).SIGMA.(w4q.multidot..sigma.Lq+w5q.multidot..sigma 
.Cq+w6q.multidot..sigma.(Hq) (16) 
where, .DELTA.Lj, .DELTA.Cj and .DELTA.Hj are deviations for the respective 
characteristic colors in which j=1-m, and .sigma.Lq, .sigma.Cq and a Hq 
are variances regarding the color gradation data about the respective 
colors in which q=1-k. The deviations and variances are thus weighted, 
added up and integrated. 
The term (j=1-m).SIGMA. denotes the total sum ranging from j=1 to j=m. The 
term (q=1-k).SIGMA. represents the total sum from q=1 to q=k. Values w1j, 
w2j, w3j, w4q, w5q and w6q are weighting coefficients. If color gradation 
data is set to comprise red, green, blue, yellow, magenta, cyan and gray 
as in the example above, then k=7. 
In that case, appropriately establish ing the weighting coefficients w1j, 
w2j, w3j, w4q, w5q and w6q makes it possible to realize color conversion 
characteristics that will meet the user's preferences. 
For example, the skin color, one of the memory colors, is known to have a 
low tolerance towards hue shift and a relatively high tolerance regarding 
chroma. In such a case, the weighting coefficient w3j with respect to hue 
is set to be greater than the weighting coefficient w2j regarding chroma. 
This provides color conversion characteristics that will minimize hue 
shift. 
The blue of the skies, another memory color, is generally preferred to have 
a high chroma level. A distinct sky blue is acquired by setting a large 
weighting coefficient w2j regarding chroma. 
It may be desired to have a specific color from among the characteristic 
colors (e.g., skin color) coincide with a predetermined color. In that 
case, the need for emphasis on the color in question is met flexibly by 
setting a large weighting coefficient concerning that color. 
Where it is desired to emphasize gradation characteristics, a large 
weighting coefficient w4q regarding the lightness gradation characteristic 
need only be set. To meet the need for preventing fused gradation in 
highly saturated portions, a large weighting coefficient w5q relative to 
the chroma gradation characteristic may be established. 
The parameter changing means 236 receives from the evaluation value 
integrating means 235 the overall evaluation value .OMEGA. defined by the 
expression (16). In turn, the parameter changing means 236 changes 
parameters for the three-dimensional interpolation color converting 
circuit 211 and for the color adjusting circuit 213 until the converging 
conditions set with the overall evaluation value 0 have been met, as 
described. In changing the parameters, the parameter changing means 236 
causes the output image predicting means 233 and other related components 
to perform the above-decribed image quality evaluation. 
Optimized color converting parameters are obtained by minimizing the 
overall evaluation value .OMEGA.. It follows that the changing of color 
converting parameters by the parameter changing means 236 may be regarded 
as a question of nonlinear optimization through the minimizing of the 
overall evaluation value .OMEGA. taken as an objective function. 
Of the numerous nonlinear optimization techniques including the method of 
least squares and direct search method, the simplex method (a variation of 
the direct search method) is used by the first embodiment. The simplex 
method is described illustratively in "Nonlinear Optimization" (by J. 
Kowalik et al., translated into Japanese by Yamamoto et al., Baifu-kan). 
Suitable techniques other than the simplex method may also be used for 
nonlinear optimization and the effects are still the same. 
With the first embodiment, the color converting parameters are made up of 
the L*a*b* coordinates constituting the eight vertexes R, G, B, Y, M, C, S 
and W for the three-dimensional interpolation color converting circuit 211 
as well as of the values for the L*LUT 215, C*LUT 216 and H.multidot.LUT 
217 in the color adjusting circuit 213. To optimize all these parameters, 
however, would take an inordinately long time because they are numerous. 
Instead, the first embodiment optimizes independently the parameters for 
the three-dimensional interpolation color converting circuit 211 and the 
parameters for the color adjusting circuit 213 as will be described below. 
The three-dimensional interpolation color converting circuit 211 makes 
adjustments in the hue direction regarding characteristic colors, while 
the L*LUT 215 and C*LUT 216 in the color adjusting circuit 213 adjust 
light and chroma in terms of gradation. 
Specifically, the parameter changing means 236 carries out a parameter 
changing routine 20 shown in FIG. 11. In step 21, as mentioned earlier, 
the lattice points of the three-dimensional interpolation color converting 
circuit 211 are supplied as initial values with the L*a*b* coordinates 
representing the eight vertexes R, G, B, Y, M, C, S and which define the 
color reproduction range of the image forming apparatus 300. The L*LUT 
215, C*LUT 216 and H.multidot.LUT 217 in the color adjusting circuit 213 
are fed with straight lines tilted at 45 degrees each. 
In step 22, a first optimization process is carried out using the hues of 
the lattice points R (red), G (green), B (blue), Y (yellow), M (magenta) 
and C (cyan) as parameters for the three-dimensional interpolation color 
converting circuit 211. 
In the first optimization, the lattice points R, g, B, Y, M and C for the 
three-dimensional interpolation color converting circuit 211 are changed 
in terms of hue, whereby the characteristic colors are adjusted in the hue 
direction. There are six parameters used in the first optimization. 
In step 23, a second optimization process is performed using a parameter 
.gamma. for the L*LUT 215 and parameters made of the coordinates of 
setpoints P1 through P3 for the C*LUT 216 in the color adjusting circuit 
213. 
In the second optimization, the parameter .gamma. for the L*LUT 215 is 
adjusted so as to regulate lightness gradation. The coordinates of the 
setpoints P1 through P3 for the C*LUT 216 are adjusted to control chroma 
gradation. There are seven parameters used in the second optimization. 
FIG. 12A shows the parameter .gamma. for the L*LUT 215, and FIG. 122 
illustrates the three setpoints P1 through P3 for the C*LUT 216. The 
parameter .gamma. is involved in the following expression: 
EQU y=255(x/255).sup..gamma. (17) 
where "x" represents input L* data and "y" denotes output L* data for the 
L*LUT 215. A broken line passing through the three setpoints P1 through P3 
is described by the C*LUT 216. 
In step 24 following optimization, the optimized parameters are transferred 
to the three-dimensional interpolation color converting circuit 211 and to 
the color adjusting circuit 213. This terminates execution of the 
parameter changing routine. 
Instead of describing the setpoints P1 through P3 using a broken line 
passing therethrough, the C*LUT 216 may interpolate the points P1 through 
P3 by use of a polynomial or a spline function. The L*LUT 215, as with the 
C*LUT 216, may have its setpoints furnished in the form of parameters. 
When the optimization process above is performed as described, a small 
number of color converting parameters are optimized and determined at high 
speed. This allows the image forming apparatus 300 to produce prints in 
preferred colors at all times. 
In the example above, the RGB data is linearly mapped in the L*a*b* color 
space by the third-dimensional interpolation color converting means 211, 
and the mapped state is considered to represent initial values for 
parameter optimization. This enables the optimization process to be 
carried out unfailingly using only a limited number of characteristic 
colors to be given to the evaluation function. 
The objective function is computed by the evaluation value integrating 
means 235 weighting and adding up the individual factors for image quality 
evaluation. Thus suitably adjusting the weighting coefficients makes it 
possible to reflect the user's preferences in the output image. 
The characteristic color region extracting means 231 automatically extracts 
characteristic color regions. This feature makes it possible always to 
output, in preferred colors matching the memory colors, those images whose 
color balances may have been disturbed because they come illustratively 
from different input devices or from age-deteriorated photographic films. 
The three-dimensional interpolation color converting means 211 performs the 
color converting process of modeling the color reproduction range of the 
image forming apparatus 300. This feature always permits color 
reproduction that makes the most of the color reproduction characteristics 
of the image forming apparatus 300. 
Within the color reproduction range of the image forming apparatus 300, 
linear interpolation is carried out through tetrahedron interpolation. 
This makes it possible to guarantee the linearity of lightness and chroma 
gradation as well as the linearity of hue. Because the parameter 
determining means 230 determines parameters by evaluating gradation 
characteristics and fused gradation in highly saturated portions, the 
colors thus reproduced are more striking and more advantageous in 
gradation than conventionally obtained colors. 
(Second Embodiment in the Form of an Image Processing Apparatus) 
FIG. 13 is a block diagram of another example of the image processing 
apparatus 200 shown in FIG. 1 and practiced as the second embodiment of 
the invention. The second embodiment is characterized in that the first 
color converting means 210 utilizes a commonly employed color converting 
circuit 212 in place of the specialized three-dimensional interpolation 
color converting circuit 211 described with reference to FIGS. 3 and 6. 
The circuit is designed to convert input RGB data to L*a*b* data. 
The color converting circuit 212 of the second embodiment may be any known 
color converting circuit as long it converts RGB data into L*a*b* data. 
Thus the circuit may be any one of a matrix type color converting circuit, 
a neural network type color converting circuit, a DLUT type color 
converting circuit and a converting circuit based on defining equations. 
The second embodiment adopts a neural network type color converting circuit 
212. This type of color converting circuit is disclosed illustratively in 
Japanese Published Unexamined Patent Application No. Hei 6-95723 and 
Japanese Published Unexamined Patent Application No. Hei 7-87347. 
With the second embodiment, the parameter changing means 236 in the 
parameter determining means 230 is used to bchange parameters only for the 
color adjusting circuit 213. As shown in FIG. 14, the color adjusting 
circuit 213 is allowed to have the same structure as the circuit shown in 
FIG. 6. 
As described with reference to FIG. 1, the input color signals are not 
limited to RGB data alone. The signals may be constituted by data 
applicable to any appropriate color space. Illustratively, input data in 
the CMYK color space, which is the standard color space for printing, may 
be converted by the color converting circuit 212 into L*a*b* data. In 
another example, input data in the YCC color space used by the Photo CD 
may be converted by the color converting circuit 212 into L*a*b* data. 
The output image predicting means 233 receives from the characteristic 
describing means 232 the L*a*b* coordinates of the eight points R, G. B, 
Y, M, C, S and W designating the color reproduction range of the image 
forming apparatus 300, the coordinates having been set to the 
characteristic describing means 232. On the basis of the L*a*b* 
coordinates thus received, the output image predicting means 233 simulates 
the color converting characteristics of the color converting circuit 212 
and color adjusting circuit 213. 
More specifically, the color reproduction range of the image forming 
apparatus 300 is considered to be represented by a dodecahedron having 
eight vertexes R, G, B, Y, M, C, S and W. When any value converted by the 
color converting circuit 212 and color adjusting circuit 213 occurs 
outside the dodecahedron, a straight line connecting the point represented 
by that value to the origin of the L*a*b* coordinates is allowed to 
intersect the surface of the dodecahedron. The point of that intersection 
is taken as a new value brought about by the color conversion. 
As their initial values, the L*LUT 215, C*LUT 216 and H.multidot.LUT 217 in 
the color converting circuit 213 shown in FIG. 14 are supplied with 
straight lines tilted at 45 degrees each, i.e., output values equal to the 
input values. 
The parameter changing means 236 receives from the evaluation value 
integrating means 235 the overall evaluation value .OMEGA. defined by the 
expression (16) above. Until the converging conditions set with the 
overall evaluation value .OMEGA. have been met, the parameter changing 
means 236 changes the parameters for the color adjusting circuit 213 so as 
to let the output image predicting means 233 and other related components 
perform image quality evaluation. 
In this example, the color parameters belong to the L*LUT 215, C*LUT 216 
and H.multidot.LUT 217 in the color adjusting circuit 213 shown in FIG. 
14. These parameters, however, are too many to be all optimized in a short 
time. 
Instead, this example causes the parameters of the color adjusting circuit 
213 to be optimized independently in the hue direction and in the 
lightness and chroma direction as will be described below. That is, the 
H.multidot.LUT 217 in the color adjusting circuit 213 makes adjustments in 
the hue direction regarding characteristic colors, while the L*LUT 215 and 
C*LUT 216 in the color adjusting circuit 213 adjust the gradation 
characteristics of lightness and chroma. 
FIG. 15 is a flowchart of steps constituting a parameter changing routine 
30 carried out by the parameter changing means 236. In step 31 of the 
routine 30, as indicated in FIG. 15 and as described above, the L*LUT 215, 
C*LUT 216 and H.multidot.LUT 217 in the color adjusting circuit 213 are 
fed with 45-degree-tilted straight lines as their initial values. 
In step 32, a first optimization process is carried out using as parameters 
the coordinates of the setpoints P1 through P6 corresponding in hue to the 
six points (red, green, blue, yellow, magenta, cyan) of the H.multidot.LUT 
217 in the color adjusting circuit 213. 
In the first optimization process, the characteristic colors are adjusted 
in the hue direction by the H.multidot.LUT 217. There are 12 parameters 
involved in the process. FIG. 16 graphically represents the six setpoints 
P1 through P6 of the H.multidot.LUT 217. 
In step 33, as in step 23 of the parameter changing routine 20 in FIG. 11 
for the example in FIG. 3, a second optimization process is performed 
using as parameters the parameter .gamma. of the L*LUT 215 in the color 
adjusting circuit 213 as well as three setpoints P1 through P3 of the 
C*LUT 216 in the same color adjusting circuit 213. 
In the second optimization process, the gradation characteristic of 
lightness is adjusted by suitably changing the parameter .gamma. of the 
L*LUT 215; the gradation characteristic of chroma is adjusted by 
appropriately changing the coordinates of the setpoints P1 through P3 of 
the C*LUT 216. There are seven parameters involved in the process. 
In step 34 following the optimization, the optimized parameters are 
transferred to the color adjusting circuit 213. This terminates execution 
of the parameter changing routine. 
Instead of describing the setpoints P1 through P6 using a broken line 
passing therethrough, the H.multidot.LUT 217 may interpolate the points P1 
through P6 by use of a polynomial or a spline function. The L*LUT 215, as 
with the C*LUT 216, may have its setpoints furnished in the form of 
parameters. 
When the optimization is performed as described above, a small number of 
color converting parameters are optimized and determined at high speed. 
With the second embodiment, the L*a*b* data converted by the commonly used 
color converting circuit 212 is color-adjusted by the color converting 
circuit 213 in accordance with the image quality evaluation value, whereby 
preferred colors are reproduced. This permits reproduction of images in 
preferred colors in diverse kinds of input color space. 
Furthermore, the second embodiment sets the color converting circuit 212 in 
such a way that causes print colors based on L*a*b* data to coincide with 
displayed colors derived from RGB data through the use of the conventional 
Gamut compression techniques. In that state, the color adjusting circuit 
213 is adjusted to attain preferred colors. This reproduces visually 
desirable colors that are fairly close to the colors shown on the display 
unit. 
The major benefits of the present invention are summarized as follows: 
according to the invention, the color reproduction characteristics of the 
output image are evaluated from the characteristics of input image data 
and from the color reproduction characteristics of the image forming 
apparatus. The color reproduction characteristics thus evaluated are used 
as the basis for automatically determining optimum color converting 
parameters. This allows the image forming apparatus always to produce 
prints in preferred colors representing any input image. 
The inventive apparatus evaluates concurrently two factors that are often 
mutually exclusive: coincidence of characteristic colors with preferred 
colors, and tendency toward chroma fusion in highly saturated portions. 
Where the evaluation process yields a suitable trade-off, there occur few 
instances in which characteristic colors are allowed to match preferred 
colors only at the expense of chroma fusion in highly saturated portions 
or in which characteristic colors are left to deteriorate in chroma so as 
to avoid chroma fusion in highly saturated portions. Whereas such cases 
have plagued conventional image processing apparatuses, the inventive 
apparatus provides the kind of image processing which satisfies the two 
requirements at the same time. 
The inventive apparatus automatically extracts characteristic color regions 
such as memory colors. In so doing, the apparatus always outputs, in 
preferred colors matching the memory colors, those images whose color 
balances may have been disturbed because they come illustratively from 
different input devices or from age-deteriorated photographic films. 
According to the invention, the parameters of the color converting means 
having continuous converting characteristics are optimized. The parameters 
thus optimized are finalized after gradation characteristics and chroma 
fusion in highly saturated portions have been evaluated. This feature, 
unlike conventional setups, eliminates any discontinuity between the 
memory color regions on the one hand and the remaining regions on the 
other hand, and suppresses any false contour in gradation. 
Because the inventive apparatus weighs and adds up individual factors of 
image quality evaluation in computing the objective function, it is 
possible to reflect the user's preferences in the output image by 
adjusting appropriate weighting coefficients. 
The inventive apparatus makes it possible to describe the color converting 
process in a limited number of parameters. This means that the color 
converting parameters for reproducing preferred colors are optimized 
unfailingly at high speed. 
The specialized three-dimensional interpolation color converting circuit of 
the invention performs the color converting process of modeling the color 
reproduction range of the image forming apparatus. This feature always 
permits color reproduction that makes the most of the color reproduction 
characteristics of the image forming apparatus. Within the color 
reproduction range of the image forming apparatus, linear interpolation is 
carried out through tetrahedron interpolation. This guarantees the 
linearity of lightness and chroma gradation as well as the linearity of 
hue. 
According to the invention, the color converting parameters are determined 
by evaluating gradation characteristics and fused gradation in highly 
saturated portions. The colors thus reproduced are more striking and more 
advantageous in gradation than conventionally obtained colors. 
The specialized three-dimensional interpolation color converting circuit of 
the invention performs the optimization process using as its initial 
values RGB data being linearly mapped in the L*a*b* color space. This 
makes it possible to carry out the optimization unfailingly with only a 
small number of characteristic colors given to the evaluation function. 
After such optimization, the color converting characteristics are free of 
defects. 
As many apparently different embodiments of this invention may be made 
without departing from the spirit and scope thereof, it is to be 
understood that the invention is not limited to the specific embodiments 
thereof except as defined in the appended claims.