Colour image processing apparatus and method

The present invention relates to an image processing apparatus capable of estimating and correcting color data which vary according to the illuminating light, and a method and a recording medium therefor.

Utilizing the spectral reflectance R(λ) of a reflective object, the spectral distribution P(λ) of the illuminating light and the isochromatic functions: <maths id="math0001" num="A"><math display="block"><mrow><mover accent="true"><mi mathvariant="normal">x</mi><mo>‾</mo></mover><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mo>,</mo><mover accent="true"><mi mathvariant="normal">y</mi><mo>‾</mo></mover><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mo>,</mo><mover accent="true"><mi mathvariant="normal">z</mi><mo>‾</mo></mover><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow></mrow></math><img file="EP0891077B1_D0001.tif" /></maths> the colorimetric values (X, Y, Z; three stimulation values) of the reflective object under certain illuminating light can be represented by: <maths id="math0002" num="B"><math display="block"><mtable columnalign="left"><mtr><mtd><mi>X</mi><mo>=</mo><mi>k</mi><mstyle displaystyle="true"><mrow><msub><mo>∫</mo><mrow><mi mathvariant="normal">vis</mi></mrow></msub><mrow><mi>R</mi><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mo>⋅</mo><mi>P</mi><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mo>⋅</mo><mover accent="true"><mi>x</mi><mo>‾</mo></mover><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mi>d</mi><mi mathvariant="normal">λ</mi></mrow></mrow></mstyle></mtd></mtr><mtr><mtd><mi>Y</mi><mo>=</mo><mi>k</mi><mstyle displaystyle="true"><mrow><msub><mo>∫</mo><mrow><mi mathvariant="normal">vis</mi></mrow></msub><mrow><mi>R</mi><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mo>⋅</mo><mi>P</mi><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mo>⋅</mo><mover accent="true"><mi>y</mi><mo>‾</mo></mover><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mi>d</mi><mi mathvariant="normal">λ</mi></mrow></mrow></mstyle></mtd></mtr><mtr><mtd><mi>Z</mi><mo>=</mo><mi>k</mi><mstyle displaystyle="true"><mrow><msub><mo>∫</mo><mrow><mi mathvariant="normal">vis</mi></mrow></msub><mrow><mi>R</mi><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mo>⋅</mo><mi>P</mi><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mo>⋅</mo><mover accent="true"><mi>z</mi><mo>‾</mo></mover><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mi>d</mi><mi mathvariant="normal">λ</mi></mrow></mrow></mstyle></mtd></mtr></mtable></math><img file="EP0891077B1_D0002.tif" /></maths> wherein the constant k is represented by: <maths id="math0003" num="C"><math display="block"><mrow><mi>k</mi><mo>=</mo><mrow><mrow><mn>100</mn></mrow><mo>/</mo><mrow><mstyle displaystyle="true"><mrow><msub><mo>∫</mo><mrow><mi mathvariant="normal">vis</mi></mrow></msub><mrow><mi>P</mi><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mo>⋅</mo><mover accent="true"><mi>y</mi><mo>‾</mo></mover><mrow><mo>(</mo><mi mathvariant="normal">λ</mi><mo>)</mo></mrow><mi>d</mi><mi mathvariant="normal">λ</mi></mrow></mrow></mstyle></mrow></mrow></mrow></math><img file="EP0891077B1_D0003.tif" /></maths> and the integration: <maths id="math0004" num="D"><math display="block"><mrow><mrow><mo>(</mo><mrow><mstyle displaystyle="true"><mrow><msub><mo>∫</mo><mrow><mi mathvariant="normal">vis</mi></mrow></msub>&lt;mrow /&gt;</mrow></mstyle></mrow><mo>)</mo></mrow></mrow></math><img file="EP0891077B1_D0004.tif" /></maths> is executed within the visible wavelength range.

Consequently the colorimetric values of the reflective object vary according to the variation of the illuminating light. Also according to the definitions described above, the spectral distribution P(λ) of the illuminating light and the spectral reflectance distribution R(λ) are required in order to obtain the exact colorimetric values in such state.

For this reason, the colorimetric values of the reflective object under arbitrary illuminating light have conventionally been determined from the spectral reflectance R(λ) of the reflective object and the spectral distribution P(λ) of the illuminating light.

The above-mentioned method can be easily executed and can determine the exact colorimetric values in case the reflective object consists of areas of several colors (spectral reflectance R(λ)). On the other hand, in case the reflective object is for example an image, the object in general has color information in each of finely divided many areas (pixels). Consequently there is required a large memory capacity for storing spectral reflectance R(λ) for each pixel, and the information ordinarily held for each pixel is the chromaticity values (X, Y, Z) under a specified condition (specifying illuminating light or colorimetric method) or equivalent RGB chromaticity values. In order to determine the colorimetric values of the reflective object for the arbitrary illuminating light in the above-described method, there is required the spectral reflectance R(λ) for each pixel, so that the spectral reflectance R(λ) is determined again from the information corresponding to the aforementioned colorimetric values (X, Y, Z) for each pixel or the spectral reflectance R(λ) is measured again for each pixel.

In case the information obtained for each pixel is the values corresponding to the colorimetric values (such as the aforementioned XYZ values of RGB values) under a specified condition, the colorimetric values of the reflective object under arbitrary illuminating light can be determined, in addition to the above-described method according to the foregoing definitions based on the spectral reflectance R(λ) of the object and the spectral distribution P(λ) of the illuminating light, by a method of directly converting the information for each pixel, corresponding to the colorimetric values under the specified condition, into the colorimetric values under the arbitrary illuminating light utilizing a matrix, a three-dimensional look-up table or a neural network. The conversion function (above-mentioned matrix, three-dimensional look-up table or neural network) is determined for each of the required plural illuminating lights.

As explained in the foregoing, the colorimetric values of the reflective object vary according the change in the illuminating light. In an image reproducing process or the like, there are often required the colorimetric values of the reflective object under arbitrary illuminating light.

In case the information obtained for each pixel is the values corresponding to the colorimetric values (such as the aforementioned XYZ values of RGB values) under a specified condition, the colorimetric values of the reflective object under arbitrary illuminating light can be determined by a method of directly converting the information for each pixel, corresponding to the colorimetric values under the specified condition, into the colorimetric values under the arbitrary illuminating light utilizing a matrix, a three-dimensional look-up table or a neural network. The conversion function (above-mentioned matrix, three-dimensional look-up table or neural network) is determined for each of the required plural illuminating lights.

The above-described method is acceptable in case the number of the required illuminating lights is limited. However, for example in the ordinary office environment, the condition of lighting changes in various manner according to the kind of the illuminating light source, the time-dependent change thereof, and the change in the state of the incoming external light such as the solar light, and it is difficult to prepare or store in advance the conversion functions required corresponding to such changes.

As explained in the foregoing, in order to determine the colorimetric values under arbitrary illuminating light for an image or the like that requires information for each of a large number of pixels, the conventional methods have been associated with drawbacks of requiring a large amount of information such as preparing the spectral reflectance for each pixel or preparing a large number of conversion functions corresponding to various illuminating light conditions.

On the other hand, owing to the recent commercialization of various color image processing equipment, the color images can be handily processed not only in the special fields such as designing based on the computer graphics but also in the ordinary offices. It has however been difficult to consider the color of the printout on the monitor, because the color of the image prepared on the monitor does not in general match that of the printout obtained from the printer. In order to solve such drawback, there has been considered and proposed the color management system.

The color management system is to cancel the difference in color between the different devices by employing a common color space. This system is based on a principle that a color described by same coordinate values in a same color space should always look same and is to match the apparent color by representing all the colors in a common color space and matching the coordinate values. One of the currently employed methods for canceling the difference between the devices employs the CIE-XYZ color space and utilizes the XYZ stimulation values which are the internal description coordinates therein.

However such method may still be insufficient in case the media used for reproduction are different, for example the image on a monitor and the printed image. Fig. 15 shows the environment for observing an image on a monitor and a printout. In the following it is assumed that an image 1202 same as that on a print 1201 is displayed on a monitor 1203.

The printed image or the image displayed on the monitor is not observed under constant ambient light, but the ambient light 1204 shown in Fig. 15 changes by the opening or closing of a window or the replacement of the illuminating light source, and the image appears differently by such change. Consequently, even when isochromaticity can be obtained under certain ambient light, it cannot be preserved even under the same conditions in case the ambient lighting condition is altered.

Though the foregoing consideration has been explained in the comparison of the printed image and the image displayed on the monitor, it is generally applicable to the comparison of a color presentation by reflecting illuminating light and a light-emitting color presentation. More specifically, such phenomenon occurs also in case of taking an object such as a person or a sculpture and displaying it on a monitor or presenting as a transmissive display.

As explained in the foregoing, the appearance of the image varies according the change in the image observing environment. In the conventional art employing different media, the images providing isochromaticity under a certain situation no longer provide isochromatic feeling by the change in the image observing environment.

WO 92/17982 discloses an image processing method for converting data for illumination by a first illuminating light into corresponding data for illumination by a second illuminating light. Conversion data for each of a plurality of illuminating lights having different characteristics is stored in a computer.

US-A-4992963 discloses an apparatus for characterising the light produced by a light source by determining the surface spectral reflectance for the light reflected by a surface. The reflected light is divided into constituent parts of the spectral power distribution.

One aspect of the present invention obtains data under desired illumination in a simple manner with a limited memory capacity.

According to a first aspect of the present invention, there is provided an image processing method for converting data dependent on a first illuminating light into data dependent on a second illuminating light, comprising the steps of: converting data dependent on said first illuminating light into data dependent on said second illuminating light, the method being characterised by the steps of: storing conversion data for a light source having high colour rendering properties and conversion data for a light source having low colour rendering properties; generating data indicating the proportion of synthesis of the stored conversion data corresponding to said second illuminating light; generating a first conversion condition from the stored conversion data according to the data indicating the proportion of synthesis; and generating a second conversion condition based on colour temperature information of the second illuminating light, wherein said converting step executes the conversion using the first conversion condition and the second conversion condition.

One embodiment of the present invention compensates the appearance of the color of the displayed image regardless of the observing environment.

Another embodiment of the present invention provides a user interface allowing easy setting of the correction for the ambient light.

According to a second aspect of the present invention, there is provided an image processing apparatus for converting data dependent on a first illuminating light into data dependent on a second illuminating light, comprising: conversion means for converting data dependent on said first illuminating light into data dependent on said second illuminating light, the apparatus being characterised by comprising: storage means for storing conversion data for a light source having high colour rendering properties and for a light source having low colour rendering properties; means for generating data indicating the proportion of synthesis of the stored conversion data corresponding to said second illuminating light; generating means for generating a first conversion condition from the stored conversion data according to the data indicating the proportion of synthesis; and means for generating a second conversion condition based on colour temperature information of the second illuminating light, wherein said means for converting executes the conversion using the first conversion condition and the second conversion condition.

Embodiments of the present invention will now be described with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a chart showing spectral characteristics of light sources of different spectral distributions;</li><li>Fig. 2 is a chart showing examples of colorimetric values (a*, b*) of a same reflective object illuminated by the light sources of different spectral distribution characteristics;</li><li>Figs. 3, 4 and 5 are charts showing examples of the result obtained by the algorithm of a comparative example to the present invention.</li><li>Fig. 6 is a view showing an example of the colorimetric value estimating system;</li><li>Fig. 7 is a view of a test chart consisting of 77 color patches, employed for determining the illuminating light characteristic matrix;</li><li>Figs. 8, 9, 10 and 11 are views showing examples of the colorimetric value estimating system;</li><li>Fig. 12 is a chart showing the spectral sensitivity of a sensor in the illuminating light sensing unit;</li><li>Fig. 13 is a view showing an example of the colorimetric value estimating system;</li><li>Fig. 14 is a view showing an example of the user interface for designating the illuminating light; and</li><li>Fig. 15 is a view showing an image observing environment.</li></ul>

[Comparative Example]

At first there will be explained the principle of estimating the colorimetric values of the reflective object, to be employed in the following embodiments. For converting three stimulation values under certain illuminating light into those under another illuminating light, there is known, for example, a conversion method corresponding to the change in the color temperature, such as the color adaptation conversion (such as the method of von Kries). However, as often felt in the image observation under the natural daylight and under the daylight type fluorescent lamp, certain colors may be felt differently even if the color temperature is same. Also certain colors may be felt different even if the isochromaticity is attained by the color adaptation conversion for the colors close to the achromatic color. Such phenomena are assumed to result from the spectral distribution characteristics of the illuminating light, and the evaluating method therefor is already known, for example the method of evaluating the color rendering of the light source according to JIS-Z-8726 (1990). Fig. 1 shows examples of the light sources substantially same in the color temperature but different in the spectral distribution. In Fig. 1, a curve 11 indicates the spectral distribution of a white, ordinary fluorescent lamp while a curve 12 indicates that of a white, high color rendering fluorescent lamp. Fig. 2 shows the colorimetric values of a same reflective object, illuminated by the two light sources shown in Fig. 1. In Fig. 2, square marks 21 indicate the colorimetric values corresponding to the illuminating light of the spectral characteristics 12 in Fig. 1, and the ends 22 of line segments corresponding to that of the characteristics 11 in Fig. 1.

The method for estimating the colorimetric values of the reflective object to be employed in the following embodiments serves to estimate and correct the difference of color, resulting from the difference in the spectral distribution characteristics of the illuminating light.

In this method, a lighting characteristic matrix CR, defined by the following equation: <maths id="math0005" num="E"><math display="block"><mrow><mi mathvariant="normal">C</mi><mi mathvariant="normal">R</mi><mo>=</mo><msub><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow></msub><mo>·</mo><mi mathvariant="normal">C</mi><msub><mi mathvariant="normal">R</mi><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">r</mi></mrow></msub><mo>+</mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>−</mo><msub><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow></msub></mrow><mo>)</mo></mrow><mo>·</mo><mi mathvariant="normal">C</mi><msub><mi mathvariant="normal">R</mi><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">l</mi></mrow></msub></mrow></math><img file="EP0891077B1_D0005.tif" /></maths> is utilized as means for estimating and correcting the difference of color, resulting from the difference in the spectral distribution characteristics of the illuminating light.

In the above-mentioned equation, CR_hr is a lighting characteristic matrix corresponding to the natural daylight, the incandescent lamp or the illuminating light, obtained from a standard light source defined in the Japan Industrial Standards or a light source of satisfactory color rendering such as a high color-rendering fluorescent lamp.

CR_hl is a lighting characteristic matrix corresponding to the illuminating light obtained from a light source of low color rendering, as defined in the Japan Industrial Standards, such as an ordinary fluorescent lamp.

Also IH_ks is a lighting characteristic coefficient, assuming a value between 0 and 1.

In case IH_ks = 1, CR coincides with CR_hr. The matrix obtained in this case corresponds to the illuminating light of satisfactory color rendering, such as the natural daylight, the incandescent lamp, the JIS defined standard light source of the JIS defined high color rendering fluorescent lamp.

In case IH_ks = 0, CR coincides with CR_hl. In this case, the matrix corresponds the illuminating light of low color rendering, such as the JIS defined ordinary fluorescent lamp.

In case 0 &lt; IH_ks &lt; 1, the lighting characteristic matrix CR corresponds to the illuminating light obtained by mixing CR_hr and CR_hl with a mixing ratio IH_ks.

In this manner it is rendered possible to generate the lighting characteristic matrix corresponding to the illuminating lights of various color rendering properties, based on the lighting characteristic matrixes corresponding to the illuminating lights of different color rendering properties.

Figs. 3, 4 and 5 show the results of estimation of the colorimetric values under arbitrary illuminating light, utilizing those under a certain specified condition. In the illustrated cases, the high color rendering illumination of daylight color is used as the specified condition. As the arbitrary illuminating light, the case shown in Fig. 3 employs an ordinary white fluorescent lamp (light source of low color rendering), while the case in Fig. 4 employs a high color rendering white fluorescent lamp (light source of high color rendering), and the case in Fig. 5 employs a daylight white fluorescent lamp of three-wavelength type (light source of medium color rendering). In these drawings, square marks indicate the colorimetric values of pixels under the above-mentioned specified condition, while crosses indicate the actually measured colorimetric values under the arbitrary illuminating light and triangles indicates the estimated colorimetric values.

Fig. 6 shows an example of the flow of the colorimetric value estimating process utilizing the estimating method described above. The estimating process can be realized through execution of different process units shown in Fig. 6, by a CPU according to a program stored in a memory medium and adapted to execute the different process units.

An image data conversion unit 61 converts the input image data XYZ into estimated image data X'Y'Z' under an arbitrary light source according to the following equation, utilizing the lighting characteristic matrix CR calculated in an illuminating light characteristic matrix calculation unit 62, wherein the input pixel data XYZ are normalized to the values under a high color rendering light source of a color temperature of 6500°: <maths id="math0006" num="F"><math display="block"><mrow><mrow><mo>[</mo><mrow><mtable><mtr><mtd><msup><mi>X</mi><mo>′</mo></msup></mtd></mtr><mtr><mtd><msup><mi>Y</mi><mo>′</mo></msup></mtd></mtr><mtr><mtd><msup><mi>Z</mi><mo>′</mo></msup></mtd></mtr></mtable></mrow><mo>]</mo></mrow><mo>=</mo><mi>C</mi><mi>R</mi><mrow><mo>[</mo><mrow><mtable><mtr><mtd><mi>X</mi></mtd></mtr><mtr><mtd><mi>Y</mi></mtd></mtr><mtr><mtd><mi>Z</mi></mtd></mtr></mtable></mrow><mo>]</mo></mrow></mrow></math><img file="EP0891077B1_D0006.tif" /></maths>

The lighting characteristic matrix CR is calculated, in the lighting characteristic matrix calculation unit 62, according to the following equation: <maths id="math0007" num="G"><math display="block"><mrow><mi mathvariant="normal">C</mi><mi mathvariant="normal">R</mi><mo>=</mo><msub><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow></msub><mo>·</mo><mi mathvariant="normal">C</mi><msub><mi mathvariant="normal">R</mi><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">r</mi></mrow></msub><mo>+</mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>−</mo><msub><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow></msub></mrow><mo>)</mo></mrow><mo>·</mo><mi mathvariant="normal">C</mi><msub><mi mathvariant="normal">R</mi><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">l</mi></mrow></msub></mrow></math><img file="EP0891077B1_D0007.tif" /></maths> based on the lighting characteristic matrix CR_hr stored in a standard lighting characteristic matrix storing unit 63 and corresponding to the illuminating light obtained from the light source of high color rendering such as the natural daylight, incandescent lamp, JIS-defined standard light source or JIS-defined high color rendering fluorescent lamp, the lighting characteristic matrix CR_hl corresponding to the illuminating light obtained from the light source low color rendering such as the JIS-defined ordinary fluorescent lamp and a lighting characteristic coefficient IH_ks given by a lighting characteristic coefficient instructing unit 64.

The lighting characteristic matrixes CR_hr, CR_hl can be obtained by determining the three stimulation values under the above-mentioned illuminating lights and those under the standard light source, for example with a test chart containing 77 color patches as shown in Fig. 7 and executing optimization for example by the attenuated minimum square method.

The lighting characteristic coefficient IH_ks indicates information on the mixing ratio of the high color-rendering light source and the low color-rendering light source as explained in the foregoing, and is set by the lighting characteristic coefficient instructing unit 64 according to the kind of the designated illuminating light.

A plurality of the lighting characteristic coefficients IH_ks are prestored in the lighting characteristic coefficient instructing unit 64 corresponding to the kinds of the illuminating lights.

Fig. 14 shows an example of the user interface (UI) relating to the setting of the lighting characteristic coefficient. The setting of the present embodiment includes, as shown in the user interface 140, a selection mode 141 and a setting mode 142.

The selection mode 141 displays the kinds of the illuminating lights stored in the lighting characteristic coefficient instructing unit 64 in the form of a pull-down menu, for selection by the user.

The setting mode 142 sets the value of the lighting characteristic coefficient according to the instruction of the user. According to the position of a black triangle mark on a bar, selected by the manual instruction of the user, the lighting characteristic coefficient instructing unit 64 calculates and sets the value of the lighting characteristic coefficient. The black triangle mark positioned at the left-hand end of the bar sets 0 as IH_ks and that at the right-hand end sets 1. Also for setting the lighting characteristic coefficient while confirming the effect of the actually set lighting characteristic coefficient, there are provided a mode 143 for displaying the color patches in a user interface 145 and a mode 144 for displaying the original image.

In case the patch 143 is selected by the user in the UI 140, the patch data representing the predetermined colors are processed according to the lighting characteristic coefficient set on the bar of the UI 140 and the obtained X'Y'Z' data are corrected according to the monitor profile and displayed on the monitor, whereby the user can confirm the effect of the set lighting characteristic coefficient.

Also in case the preview mode 144 is selected by the user in the UI 140, the original image read according to the instruction of the user is processed according to the lighting characteristic coefficient set on the bar and displayed as shown in the UI 146, whereby the user can confirm the effect.

The above-described example allows to easily estimate and correct the colorimetric values under arbitrary light source with a limited memory capacity. [Embodiment 1]

The foregoing example does not execute the correction of the color temperature. The present embodiment provides the process including the correction of the color temperature in the process of the example.

As shown in Fig. 8, a color temperature conversion unit 81 is added to the colorimetric value estimation process shown in Fig. 6, whereby the conversion of the color temperature can be easily achieved. The color temperature conversion unit 81 converts the estimated output image data X'Y'Z' into image data X"Y"Z" under the illuminating light of a desired color temperature according to the following equation: <maths id="math0008" num="H"><math display="block"><mrow><mrow><mo>[</mo><mrow><mtable><mtr><mtd><msup><mi>X</mi><mo>″</mo></msup></mtd></mtr><mtr><mtd><msup><mi>Y</mi><mo>″</mo></msup></mtd></mtr><mtr><mtd><msup><mi>Z</mi><mo>″</mo></msup></mtd></mtr></mtable></mrow><mo>]</mo></mrow><mo>=</mo><mi>C</mi><mi>T</mi><mrow><mo>[</mo><mrow><mtable><mtr><mtd><msup><mi>X</mi><mo>′</mo></msup></mtd></mtr><mtr><mtd><msup><mi>Y</mi><mo>′</mo></msup></mtd></mtr><mtr><mtd><msup><mi>Z</mi><mo>′</mo></msup></mtd></mtr></mtable></mrow><mo>]</mo></mrow></mrow></math><img file="EP0891077B1_D0008.tif" /></maths> for example by the method of Von. Kries utilizing a color temperature conversion matrix CT corresponding to the information (for example XYZ stimulation values) relating to the arbitrary color temperature and given from the lighting characteristic coefficient instructing unit 64.

Instead of the method of Von. Kries, there may be applied another method of color temperature conversion. The method of preparation of the color temperature conversion matrix is detailedly explained for example in "Color Engineering; published by Publishing Office of Tokyo Denki University; Chapter 6 Development of CIE color representation system".

In the process shown in Fig. 8, the image data conversion unit 61 and the color temperature conversion unit 81 are separately provided, but the process may be collectively executed in the image data conversion unit 61 as shown in Fig. 9.

The colorimetric values under desired illumination can be easily estimated and corrected, including the conversion of color temperature, by synthesizing the color temperature conversion matrix CT with the lighting characteristic matrix CR, calculated in the lighting characteristic matrix calculation unit 62, as indicated by the following formula to obtain a lighting characteristic matrix CR' including the color temperature conversion: <maths id="math0009" num="I"><math display="block"><mrow><mi mathvariant="normal">C</mi><msup><mi mathvariant="normal">R</mi><mo>′</mo></msup><mo>=</mo><mi mathvariant="normal">C</mi><mi mathvariant="normal">T</mi><mo>·</mo><mi mathvariant="normal">C</mi><mi mathvariant="normal">R</mi></mrow></math><img file="EP0891077B1_D0009.tif" /></maths> and converting the image data by such matrix CR'.

It is also possible, as shown in Fig. 10, to calculate the color temperature conversion matrix CT in the lighting characteristic matrix calculation unit 62 and to execute the matrix calculation based on the obtained result.

It is also possible to automatically set the lighting characteristic coefficient or the color temperature information without the instruction of the user through the user interface as shown in Fig. 14, by providing, as shown in Fig. 11, a light sensing unit 111 and a lighting characteristic coefficient calculation unit 112.

The light sensing unit 111 is provided with a sensor of sensitivity characteristics for example as shown in Fig. 12 for measuring the illuminating light, and the BGR output signals of the sensor are processed to obtain individually the above-mentioned set values (color temperature, lighting characteristic coefficient).

[Embodiment 2]

Though in the foregoing embodiment and example, the lighting characteristic matrix is employed for estimating and correcting the difference in color resulting from the difference in the spectral distribution of the illuminating lights, the present invention is not limited to a method utilizing such matrix and can also be attained for example with a three-dimensional look-up table.

In this case, for estimating and correcting the difference in color resulting from the difference in the spectral distribution of the illuminating lights, there is employed a lighting characteristic three-dimensional look-up table CR3LUT, which is defined by the following equation: <maths id="math0010" num="J"><math display="block"><mrow><mi mathvariant="normal">C</mi><mi mathvariant="normal">R</mi><mn>3</mn><mi mathvariant="normal">LUT</mi><mo>=</mo><msub><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow></msub><mo>·</mo><mi mathvariant="normal">C</mi><mi mathvariant="normal">R</mi><mn>3</mn><msub><mrow><mi mathvariant="normal">LUT</mi></mrow><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">r</mi></mrow></msub><mo>+</mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>−</mo><msub><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow></msub></mrow><mo>)</mo></mrow><mo>·</mo><mi mathvariant="normal">C</mi><mi mathvariant="normal">R</mi><mn>3</mn><msub><mrow><mi mathvariant="normal">LUT</mi></mrow><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">l</mi></mrow></msub></mrow></math><img file="EP0891077B1_D0010.tif" /></maths> wherein CR3LUT_hr is a lighting characteristic three-dimensional look-up table corresponding to the natural daylight, the incandescent lamp or the illuminating light, obtained from a standard light source defined in the Japan Industrial Standards or a light source of satisfactory color rendering such as a high color-rendering fluorescent lamp.

CR3LUT_hl is a lighting characteristic three-dimensional look-up table corresponding to the illuminating light obtained from a light source of low color rendering, as defined in the Japan Industrial Standards, such as an ordinary fluorescent lamp.

Also IH_ks is a lighting characteristic coefficient, equivalent to that in the foregoing embodiments.

Fig. 13 shows an example of the colorimetric value estimating system utilizing such colorimetric value estimating method, wherein an image data conversion unit 131 converts the input image data XYZ into estimated image data X'Y'Z' under a designated light source according to the following equation: <maths id="math0011" num="K"><math display="block"><mrow><mrow><mo>(</mo><mrow><msup><mi mathvariant="normal">X</mi><mo>′</mo></msup><mo>,</mo><msup><mi mathvariant="normal">Y</mi><mo>′</mo></msup><mo>,</mo><msup><mi mathvariant="normal">Z</mi><mo>′</mo></msup></mrow><mo>)</mo></mrow><mo>=</mo><mi mathvariant="normal">C</mi><mi mathvariant="normal">R</mi><mn>3</mn><mi mathvariant="normal">LUT</mi><mrow><mo>(</mo><mrow><mi mathvariant="normal">X</mi><mo>,</mo><mi mathvariant="normal">Y</mi><mo>,</mo><mi mathvariant="normal">Z</mi></mrow><mo>)</mo></mrow></mrow></math><img file="EP0891077B1_D0011.tif" /></maths> utilizing the lighting characteristic three-dimensional look-up table CR3LUT calculated in the lighting characteristic three-dimensional look-up table calculation unit 132.

It is also possible to estimate and correct the difference in color resulting from the difference in the spectral distribution of the illuminating lights, employing a function as more general means, and, in such case, the arbitrary illuminating light characteristics are defined by a function (for example CRF). The function CRF satisfies the following relationship: <maths id="math0012" num="L"><math display="block"><mrow><mi mathvariant="normal">CRF</mi><mo>=</mo><mi mathvariant="normal">CRF</mi><mrow><mo>(</mo><mrow><msub><mrow><mi mathvariant="normal">CRF</mi></mrow><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">r</mi></mrow></msub><mo>,</mo><msub><mrow><mi mathvariant="normal">CRF</mi></mrow><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">l</mi></mrow></msub><mo>,</mo><msub><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow></msub></mrow><mo>)</mo></mrow></mrow></math><img file="EP0891077B1_D0012.tif" /></maths> which may assume a specific form as in the foregoing embodiments: <maths id="math0013" num="M"><math display="block"><mrow><mi mathvariant="normal">CRF</mi><mo>=</mo><msub><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow></msub><mo>·</mo><msub><mrow><mi mathvariant="normal">CRF</mi></mrow><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">r</mi></mrow></msub><mo>+</mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>−</mo><msub><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow></msub></mrow><mo>)</mo></mrow><mo>·</mo><msub><mrow><mi mathvariant="normal">CRF</mi></mrow><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">l</mi></mrow></msub></mrow></math><img file="EP0891077B1_D0013.tif" /></maths> or <maths id="math0014" num="N"><math display="block"><mrow><mi mathvariant="normal">CRF</mi><mo>=</mo><msubsup><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow><mn>2</mn></msubsup><mo>·</mo><msub><mrow><mi mathvariant="normal">CRF</mi></mrow><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">r</mi></mrow></msub><mo>+</mo><msup><mrow><mrow><mo>(</mo><mrow><mn>1</mn><mo>−</mo><msub><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow></msub></mrow><mo>)</mo></mrow></mrow><mn>2</mn></msup><mo>·</mo><msub><mrow><mi mathvariant="normal">CRF</mi></mrow><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">l</mi></mrow></msub></mrow></math><img file="EP0891077B1_D0014.tif" /></maths> or <maths id="math0015" num="O"><math display="block"><mrow><mi mathvariant="normal">CRF</mi><mo>=</mo><msubsup><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow><mn>2</mn></msubsup><mo>·</mo><msub><mrow><mi mathvariant="normal">CRF</mi></mrow><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">r</mi></mrow></msub><mo>+</mo><mrow><mo>(</mo><mrow><mn>1</mn><mo>−</mo><msubsup><mrow><mi mathvariant="normal">IH</mi></mrow><mrow><mi mathvariant="normal">k</mi><mi mathvariant="normal">s</mi></mrow><mn>2</mn></msubsup></mrow><mo>)</mo></mrow><mo>·</mo><msub><mrow><mi mathvariant="normal">CRF</mi></mrow><mrow><mi mathvariant="normal">h</mi><mi mathvariant="normal">l</mi></mrow></msub></mrow></math><img file="EP0891077B1_D0015.tif" /></maths> wherein CRF_hr is a lighting characteristic function corresponding to the natural daylight, the incandescent lamp or the illuminating light, obtained from a standard light source defined in JIS or a light source of satisfactory color rendering such as a high color-rendering fluorescent lamp; and

CRF_hl is a lighting characteristic function corresponding to the illuminating light obtained from a light source of low color rendering, as defined in JIS, such as an ordinary fluorescent lamp.

Based on the above-described function, the colorimetric values XYZ are converted into the values X'Y'Z' as indicated by the following equation: <maths id="math0016" num="P"><math display="block"><mrow><mrow><mo>(</mo><mrow><msup><mi mathvariant="normal">X</mi><mo>′</mo></msup><mo>,</mo><msup><mi mathvariant="normal">Y</mi><mo>′</mo></msup><mo>,</mo><msup><mi mathvariant="normal">Z</mi><mo>′</mo></msup></mrow><mo>)</mo></mrow><mo>=</mo><mi mathvariant="normal">CRF</mi><mrow><mo>(</mo><mrow><mi mathvariant="normal">X</mi><mo>,</mo><mi mathvariant="normal">Y</mi><mo>,</mo><mi mathvariant="normal">Z</mi></mrow><mo>)</mo></mrow></mrow></math><img file="EP0891077B1_D0016.tif" /></maths> wherein IH_ks is the lighting characteristic coefficient equivalent to that in the foregoing embodiment.

As explained in the foregoing embodiments, in order to match the apparent colors in the display on the monitor and in the print, it is necessary to execute the conversion of the color signals in full consideration of the characteristics (spectral characteristics, color rendering property etc.) of the ambient light. The foregoing embodiments are different in the method of setting the data required for such conversion, and the intention of the present invention is, in an image display method, an image display apparatus and an image display system, to provide means for setting the input device, display device and ambient illuminating light, and color signal conversion means capable, based on the characteristics of thus set input device, display device and ambient illuminating light, of executing a color signal conversion based on the characteristics of the input device, a color signal conversion based on the characteristics of the ambient illuminating light, a color signal conversion including the color adaptation based on the characteristics of the ambient illuminating light and the displayed white color of the display device, and a color signal conversion based on the characteristics of the display device.

More specifically, based on the information (chromaticity, color temperature or spectral intensity (luminosity)) on the illuminating light (ambient light), there are obtained information (chromaticity, XYZ stimulation values etc.) on the white color sensed under such illuminating light and information (for example two-dimensional matrix) for converting other colors, and the color signal conversion is executed utilizing these information.

According to the foregoing embodiments, there is achieved precise conversion of the color signals corresponding to various ambient light sources, thereby attaining sufficiently precise isochromaticity for the display on the monitor and for the printed image.

Also the present invention is applicable to various hardware configurations and sequence processes corresponding thereto. Such sequence process may be realized into a logic circuit or a software or an algorithm within the aforementioned scope of the present invention, and the hardware or the apparatus can be realized according to such algorithm.

[Other embodiments]

The present invention is applicable not only to a system consisting of plural equipment (for example host computer, interface equipment, reader, printer etc.) but also to an apparatus consisting of a single equipment (for example a copying apparatus or a facsimile apparatus).

The present invention also includes an embodiment of supplying a computer in an apparatus or a system, so connected with various devices as to realize the functions of the foregoing embodiments, with program codes of a software for realizing the functions of the foregoing embodiments whereby the effects of the present invention are achieved by the functions of such various devices according to the program stored in the computer of such system or apparatus.

In such case, the program codes themselves of the above-mentioned software realize the functions of the foregoing embodiments, and the program codes themselves and means for supplying the computer with such program codes, for example a memory medium storing such program codes, constitute the present invention.

The memory medium for storing the program codes can be, for example, a floppy disk, a hard disk, an optical disk, a magnetooptical disk, a CD-ROM, a magnetic tape, a non-volatile memory card or a ROM.

Also the present invention naturally includes not only a case in which the functions of the foregoing embodiments are realized by the execution of the supplied program codes by the computer but also a case in which the functions are realized by the cooperation of the program codes with the operating system of the computer or another application software.

Furthermore the present invention includes a case in which the supplied program codes are stored in an expansion board of the computer or a memory provided in an expansion unit connected to the computer and a CPU or the like provided in such expansion board or expansion unit executes all or a part of the actual process according to the instruction of such program codes thereby realizing the functions of the foregoing embodiments.

The present invention is not limited to the foregoing embodiments but is subject to any and all modifications and variations within the scope of the appended claims.