IMAGE DISPLAY APPARATUS AND METHOD THEREFOR

There is provided an image display apparatus including: a setting unit, a correcting unit and a displaying unit. The setting unit sets an amount of correction value for at least one of lightness and chroma for each hues, wherein the amount of correction values are determined so that a relative relationship of evaluation values among the hues is kept from varying before and after correction, the evaluation values being values defined depending on lightness and chroma for the hues. The correcting unit obtains a corrected image by correcting at least one of the lightness and the chroma of the input image according to the amount of correction. The displaying unit displays the corrected image.

DETAILED DESCRIPTION

According to some embodiments, there is provided an image display apparatus including: a setting unit, a correcting unit and a displaying unit.

The setting unit sets an amount of correction value for at least one of lightness and chroma for each hues, wherein the amount of correction values are determined so that a relative relationship of evaluation values among the hues is kept from varying before and after correction, the evaluation values being values defined depending on lightness and chroma for the hues.

The correcting unit obtains a corrected image by correcting at least one of the lightness and the chroma of the input image according to the amount of correction.

The displaying unit displays the corrected image.

Embodiments will now be described with reference to the drawings.

First Embodiment

FIG. 1shows an image display apparatus100according to a first embodiment of the invention.

The image display apparatus100of the first embodiment includes a converting unit102, a setting unit104, an image quality adjusting unit105, a correcting unit108, and a displaying unit110.

The displaying unit110is a device for displaying images, and assumed to be a liquid crystal display by way of example in present embodiment. The displaying unit110may instead be a plasma or CRT display, or a projective device such as a projector.

The converting unit102applies color conversion to an input image signal101to convert the signal into a converted signal103representing lightness, chroma, and hue, and sends the converted signal103to the setting unit104. The image quality adjusting unit105displays an image quality adjustment screen on a display screen of the displaying unit110, and sends an amount of image quality adjustment106input through user operation to the setting unit104. The setting unit104calculates an amount of image quality correction107for the input image signal101from the converted signal103and amount of image quality adjustment106, and sends it to the image quality adjusting unit105and the correcting unit108. The image quality adjusting unit105automatically changes the amount of image quality adjustment106for displayed on the image quality adjustment screen in accordance with the amount of image quality correction107and displays the changed same. The correcting unit108corrects the converted signal103in accordance with the amount of image quality correction107, and sends a corrected image signal109to the displaying unit110. The displaying unit110displays the corrected image signal109.

The detailed operation of the image display apparatus100in the first embodiment will be now described.

FIG. 2is a flowchart illustrating the operation of the image display apparatus100in the first embodiment.

First, the converting unit102converts an input image signal101into a converted signal103representing lightness, chroma, and hue (S201). To be specific, the converting unit102first applies the gamma transform shown in Equation 1 to the gradation value of each of R, G, and B subpixels of each pixel included in the input image signal101inputted in RGB format.

In the equation, Rin′, Gin′, Bin′ are the gradation values of R, G, and B subpixels in the input video signal, where the gradation value is represented in 8 bits (0 to 255). Rin, Gin, Binare gradation values after gamma transform on Rin′, Gin′, Bin′, represented as a relative value between 0 and 1. The letter “γ” represents gamma coefficient.

Although in the first embodiment gamma transform is achieved according to Equation 1, gamma transform operation may be performed by preparing and referencing a lookup table in which input gradation values are associated with gamma-transformed gradation values in advance.

The aforementioned conversion is applied to R, G, and B subpixel values of all the pixels in the input video signal.

In the equation, “M” represents a 3×3 color conversion matrix and “M” is determined by the color reproduction range of the image to be displayed. The first embodiment employs a transformation matrix that converts the input video signal in accordance with the maximum color reproduction range that can be reproduced by the displaying unit.

The tristimulus values Xin, Yin, Zinmay be calculated from Rin, Gin, Binon a per-pixel basis by a preparing color conversion matrix as shown in Equation 2. Alternatively, Xin, Yin, Zinmay be determined by storing correspondence between Xin, Yin, Zin, and Rin′, Gin′, Bin′ in terms of color conversion in a lookup table and making reference to the lookup table based on Rin′, Gin′, Bin′ for each pixel. When the input image signal is input in a format other than RGB, such as YUV, the XYZ tristimulus values may be determined by reference to a LUT that directly converts input signal values in YUV form or the like into XYZ tristimulus values.

Here, f(Yin/Yw) is determined as shown in Equation 4, and f(Xin/Xw) and f(Zin/Zw) are determined in a similar manner.

The lightness L*in, chroma C*in, and hue hindetermined are sent to the setting unit104as the converted signal103.

Then, the image quality adjusting unit105displays a screen for image quality adjustment on the display screen and the user uses the screen to adjust image quality (S202).FIG. 3depicts exemplary display of a display screen301and an image quality adjustment screen302on the displaying unit110. InFIG. 3, the image quality adjustment screen302is shown in a lower right portion of the screen, allowing the user to perform image quality adjustment.

FIG. 4is an enlarged view of the image quality adjustment screen302.

InFIG. 4, three radar charts which respectively allow adjustment of lightness, chroma, and hue per hue are displayed.FIG. 4includes a chart401allowing adjustment of lightness per hue, a chart402allowing adjustment of chroma per hue, and a chart403allowing adjustment of hue. Although the present embodiment takes three radar charts of lightness, chroma, and hue as the factors of image quality adjustment, this is not limitation.

In the status shown inFIG. 4, lightness, chroma, and hue are all in reference state and in unadjusted state (the amount of adjustment=0). The user can adjust the lightness, chroma, and hue of each hue by moving a slide bar404displayed per hue in the image quality adjustment screen302shown inFIG. 4.

In the charts402and403for adjusting lightness and chroma respectively, moving the slide bar outward in the radar chart sets a positive amount of adjustment, and moving the slide bar inward in the radar chart sets a negative amount of adjustment. On the chart403for hue adjustment, outward movement of the slide bar sets an amount of adjustment such that the hue changes clockwise on the chart403and inward movement sets an amount of adjustment such that the hue changes counterclockwise.

AlthoughFIG. 4illustrates a case where there are six hues for which lightness, chroma, and hue can be adjusted, the number of hues for adjustment is not limited to six; there may be twelve hues as shown inFIG. 5, or more. Also, although the adjustment slide bars inFIGS. 4 and 5are disposed in a radar chart form, slide bars for the respective hues may be provided as shown inFIG. 6. The user can use the image quality adjustment screen302to adjust the lightness, chroma, or hue of a desired hue.

By way of example,FIG. 7shows the image quality adjustment screen in which the user has made an adjustment for increasing the chroma of the red hue. It can be seen that in the image quality adjustment screen ofFIG. 7, a slide bar702for the red hue in a radar chart701for controlling chroma has moved outward from the reference position.

The image quality adjusting unit105determines the hue chosen by the user and the amount of change in lightness, chroma, or hue in that hue from the amount of adjustment of image quality made by the user using the image quality adjustment screen302, and sends them to the setting unit104as the amount of adjustment106.

Then, the setting unit104sets the amounts of correction107for lightness and chroma for the input image on a per-hue basis from the converted signal103and the amount of adjustment106(S203). Specifically, it first calculates a corrected converted signal in the case that the lightness, chroma, or hue in the converted signal is corrected according to the amount of adjustment106, for the hue indicated by the amount of adjustment106.

Hereinbelow, the present embodiment assumes a case where the chroma of the red hue is varied by ΔC*inin the adjustment performed by the user. When a converted signal with the lightness of L*mand chroma of C*inin the red hue is represented as (L*in, C*in, hin(R)), a corrected converted signal corrected with the amount of adjustment106(L*in′, C*in′, hin(R)′) is calculated as in Equation 6:

The setting unit104then calculates perceived brightness for each of the converted signal (L*in, C*in, hin(R)) and the corrected converted signal (L*in′, C*in′, hin(R)′). The perceived brightness is described here. As known as the Helmholtz-Kohlrausch effect, when the lightness is the same, a chromatic color generally appears brighter than an achromatic to human eyes and it is perceived to be brighter as its vividness increases. Based on the Helmholtz-Kohlrausch effect, the embodiment assumes that the brightness B* perceived by human eyes is dependent on the lightness L*, chroma C*, and hue h of a target object, and defined by Equation 7:

In the equation, “F” is a function that outputs different values depending on hue, and “k” is a constant. In accordance with Equation 7, perceived brightness B*inbefore adjustment for the hue adjusted by the user and perceived brightness B*in′ after adjustment are calculated as in Equation 8:

Further, the ratio P of perceived brightnesses before and after the adjustment for the hue adjusted by the user is calculated for each of lightness and chroma values. The ratio of perceived brightness with the lightness of L*inand chroma of C*inafter the user adjustment, P(L*in, C*in, hin(R)), is calculated as in Equation 9:

Further, as shown in Equation 10, the ratio of perceived brightnesses P(L*in, C*in, hin(R)) before and after the adjustment for the hue hin(R) by the user is applied in common to all hues that have equal lightness and chroma. Thus, this ratio is defined as ratio Pmax(L*in, C*in)

In the hue hin(R) for which the user adjusted chroma, the perceived brightness has increased by the ratio Pmax(L*in, C*1n) as the result of the user adjustment. Accordingly, by multiplying the perceived brightness of other hues for which the user did not adjust image quality by the ratio Pmax(L*in, C*in), the ratio of perceived brightness among all hues that have equal lightness and chroma can be kept the same as the ratio before the user adjustment. In other words, variation in the relative relationship of an evaluation value (perceived brightness herein) among hues before and after the adjustment can be kept from varying. The brightness B*out(L*in, C*in, hin) perceived when the perceived brightness B*in(L*in, C*in, hin) with the lightness L*inand chroma C*inin hue hinis multiplied by ratio Pmax(L*in, C*in) is determined according to Equation 11:

The difference of perceived brightness ΔB*out(L*in, C*in, hin) may also be approximated by setting the difference in perceived brightness in hue hin(R) before and after the user adjustment determined by Equation 8, as shown in Equation 13. Setting with Equation 13 has an advantage of low computational complexity.

Next, from the difference of perceived brightness ΔB*out(L*in, C*in, hin), the amounts of correction for lightness and chroma are established. For example, when the difference of perceived brightness is corrected entirely with lightness, the amount of lightness correction ΔL*nutand the amount of chroma correction ΔC*nutare determined as shown in Equation 14:

In contrast, when the difference of perceived brightness is corrected entirely with chroma, the amount of lightness correction ΔL*outand the amount of chroma correction ΔC*outare determined as in Equation 15:

It is also possible to divide the amount of correction for perceived brightness between lightness and chroma at a certain ratio as in Equation 16, where “α” is a variable between 0 and 1. The variable “α” may be user-configurable.

The amounts of correction107for lightness and chroma are sent to the image quality adjusting unit105and the correcting unit108.

In response to the amounts of image quality correction107sent from the setting unit104, the image quality adjusting unit105moves the adjustment slide bars for lightness and chroma of the image quality adjusting unit105(S204). More specifically, it converts the amounts of image quality correction107into the amounts of slide bar movement, and moves the slide bars by the amounts. Thereby, the amounts of correction107for lightness and chroma determined by the setting unit104can be visualized.

In response to the chroma adjustment for the red hue performed by the user as shown inFIG. 7, the amounts of correction for lightness and chroma107for each hue are calculated by the setting unit104, and the slide bars of the image quality adjusting unit105automatically move into a state in which the lightness of each hue has increased by the amounts of correction107as shown inFIG. 8. Although the example inFIG. 8shows correction of only lightness of all hues in response to chroma adjustment for the red hue, only the chroma of all hues may be corrected, or both lightness and chroma may be corrected.

The correcting unit108calculates the corrected image signal109from the amount of correction107and the converted signal103and sends it to the displaying unit110(S205).

First, from the amount of lightness correction ΔL*out, the amount of chroma correction ΔC*out, and converted signal (L*in, C*in, hin), the corrected converted signal (L*out, C*out, hout) is calculated according to Equation 17:

Xoutand Zoutare calculated in a similar manner to Yout. Here, f(X/Xw), f(Y/Yw), f(Z/Zw) are determined as in Equation 20:

Further, Xout, Yout, Zoutare converted into output signals Rout, Gout, Boutin RGB format as shown in Equation 21 according to the color reproduction range of the displaying unit, and the corrected image signal109is sent to the displaying unit110.

where “N” is a 3×3 matrix determined by the color reproduction range of the displaying unit.

Finally, the displaying unit110displays the output image signal110on the display screen (S206).

As described, in the present embodiment, when the user adjusts lightness, chroma, or hue, lightness and/or chroma of each hue is corrected so that the relative relationship of perceived brightnesses among the hues of the original image is maintained or so that the relative relationship is kept from varying. This enables natural image display that maintains perceived brightnesses among hues while achieving image quality adjustment desired by the user.

Second Embodiment

The User Adjusts Lightness, Chroma, or Hue of all Hues at One Time

A second embodiment will be described. The second embodiment is similar to the first embodiment in overall configuration and different in the adjusting unit. Thus, the adjusting unit will be described in detail.

FIG. 9depicts the display screen and the image quality adjustment screen shown on the displaying unit according to the second embodiment. The image quality adjustment screen in the second embodiment has an image quality adjustment screen901in which the lightness, chroma, or hue of all hues can be adjusted at one time, in addition to the image quality adjustment screen302of the first embodiment.

The second embodiment illustrates a case where the user increases the chroma of all hues at one time using the image quality adjustment screen901. When the user moves a chroma slide bar902for controlling all hues together in increasing direction, the image quality adjusting unit105sends an amount of adjustment106indicating that the chroma of all hues has increased by the amount specified by the user, to the setting unit104.

In response to it, the setting unit104determines a corrected converted signal in the case that the lightness, chroma, or hue in the converted signal103is corrected with the amount of adjustment106for all hues. Denoting the amount of chroma adjustment made by the user for all hues as ΔC*in, the corrected converted signal (L*in′, C*in′, hin′) is determined from the converted signal (L*in, C*in, hin) and the amount of adjustment ΔC*inas in Equation 22:

The perceived brightness B*in(L*in, C*in, hin) before correction with the amount of adjustment106in hue hinand the perceived brightness after correction B*in′(L*in, C*in, hin) are determined as in Equation 23:

Further, the ratio P of perceived brightnesses before and after correction with the amount of adjustment106in each hue is calculated for each lightness and chroma value. The ratio of perceived brightness P(L*in, C*in, hin) after the user adjustment with the lightness of L*inand chroma of C*inin hue hinare calculated as in Equation 24:

The Helmholtz-Kohlrausch effect acts to varying degrees depending on hue; for example, colors like red and magenta have a greater effect of appearing bright, while yellow and green have a smaller effect of appearing bright. Therefore, the ratio of perceived brightness P(L*in, C*in, hin) assumes different values depending on hue hin. Thus, as shown in Equation 25, the ratio of the hue that has the largest ratio P(L*in, C*in, hin) of perceived brightnesses before and after correction with the amount of adjustment106among all hues is defined as the maximum value Pmax(L*in,C*in) of the ratio of perceived brightness for all hues having equal lightness and chroma. Although the maximum value is employed in the illustrated case, the minimum value, or a representative value between the minimum and maximum values, such as a mean value or median, may be adopted.

The brightness B*out(L*in, C*in, hin) perceived when the brightness B*in(L*in, C*in, hin) perceived with the lightness L*in, chroma C*in, and hue hinis multiplied by the ratio Pmax(L*in, C*in) is calculated as shown in Equation 26, such that the relative relationship of perceived brightness among hues remains the same before and after the correction with the amount of adjustment106.

The difference ΔB*out(L*in, C*in, hin) between the perceived brightnesses B*in(L*in, C*in, hm) and B*out(L*in, C*in, hin) is calculated as in Equation 27:

Alternatively, ΔB*out(L*in, C*in, hin) may be determined by calculating the difference of perceived brightness ΔB*in(L*in, C*in, hin) before and after the correction with the amount of adjustment106according to Equation 28 and defining the maximum difference of perceived brightness ΔB*in(L*in, C*in, hin) among all hues as ΔB*out(L*in, C*in, hin) as shown by Equation 29. Although the maximum value is employed in the illustrated case, the minimum value, or a representative value between the minimum and maximum values, such as a mean value or median, may be adopted.

Further, as in the first embodiment, the amount of lightness correction ΔL*outand the amount of chroma correction ΔC*outare set as the amount of correction107based on the difference of perceived brightness ΔB*out(L*in, C*in, hin), and the amount of correction107is sent to the adjusting unit105and the correcting unit108. The adjusting unit105automatically changes the indication of amount of adjustment on the adjustment screen302in accordance with the amount of correction107. The correcting unit108corrects the converted signal103in accordance with the amount of correction107to obtain the corrected image signal109, and sends it to the displaying unit110. The displaying unit110displays the corrected image signal109on the display screen.

As described, in the present embodiment, when the user adjusts lightness, chroma, or hue, lightness and/or chroma of each hue is corrected so that the relative relationship of perceived brightness among hues of the original image is maintained or so that the relative relationship is kept from varying. This enables image display that maintains perceived brightnesses among hues while achieving image quality adjustment desired by the user.

Third Embodiment

The User Adjusts Image Quality Specifying a Given Point on Screen

A third embodiment will be now described. The third embodiment is similar to the second embodiment in overall configuration and different in the adjusting unit. Thus, the adjusting unit will be described in detail.

FIG. 10depicts an image and an image quality adjustment screen shown on the displaying unit according to the third embodiment. The image quality adjustment screen in the third embodiment includes a pointer1001with which the user can specify a pixel on the screen, in addition to the image quality adjustment screens302and901. The third embodiment allows the user to specify a pixel in which the user wants to make an image quality adjustment using the pointer1001. Subsequently, the lightness, chroma, or hue of the pixel specified by the user is adjusted using the image quality adjustment screen901.

The third embodiment illustrates a case where the user increases chroma by ΔC*inin the screen901for a pixel specified with the pointer1001. When the user moves the chroma slide bar902in increasing direction, the image quality adjusting unit105sends an amount of adjustment106indicating that the chroma of the hue exhibited by the pixel indicated by the pointer1001has increased by the amount specified by the user, to the setting unit104.

When the hue of the pixel specified by the user with the pointer1001is hp, the setting unit104converts a converted signal with the lightness L*in, chroma C*in, and hue hpinto a corrected converted signal (L*in′, C*in′, hp′) according to the amount of adjustment106as in Equation 30:

The setting unit104further calculates perceived brightness for each of the converted signal (L*in, C*in, hp) and the corrected converted signal (L*in′, hp′). The perceived brightness B*in(L*in, C*in, hp) before adjustment of the hue hpexhibited by the pixel specified by the user with the pointer1001and the perceived brightness after adjustment B*in′(L*in, C*in, hp) are calculated as in Equation 31:

Further, the ratio P of perceived brightnesses before and after the adjustment of the hue exhibited by the pixel specified by the user with the pointer1001is calculated for each of lightness and chroma values. The ratio of perceived brightness P(L*in, C*in, hp) after the user adjustment with the lightness L*inand chroma C*inis calculated as in Equation 32:

Further, as shown in Equation 33, the ratio of perceived brightness P(L*in, C*in, hp) before and after the adjustment of the hue hpexhibited by the pixel specified by the user with the pointer1001is defined as the ratio to be applied in common to all hues that have equal lightness and chroma. Thus, this ratio is defined as Pmax(L*in, C*in)

Since the perceived brightness of the pixel specified by the user with the pointer1001has increased by the ratio Pmax(L*in, C*in) as a result of the user adjustment, by multiplying the perceived brightness of the other colors by the ratio Pmax(L*in, C*in), the ratio of perceived brightness among all hues that have equal lightness and chroma can be kept the same as before the user adjustment.

The brightness B*out(L*in, C*in, hin) perceived when the perceived brightness B*in(L*in, C*in, hin) with the lightness of L*inand chroma of C*inin hue hinis multiplied by ratio Pmax(L*in, C*in) is calculated as in Equation 34:

Alternatively, ΔB*out(L*in, C*in, hin) may be set as the difference in perceived brightness in the hue hpof the pixel specified by the user with the pointer1001, as shown in Equation 36:

Further, as in the first embodiment, the amount of lightness correction ΔL*outand the amount of chroma correction ΔC*outare set as the amount of correction107from the difference of perceived brightness ΔB*out(L*in, C*in, hin), and the amount of correction107is sent to the adjusting unit105and the correcting unit108. The adjusting unit105automatically changes the indication of amount of adjustment on the adjustment screen302in accordance with the amount of correction107. The correcting unit108corrects the converted signal103in accordance with the amount of correction107to obtain the corrected image signal109, and sends it to the displaying unit110. The displaying unit110displays the corrected image signal109on the display screen.

As described, in the present embodiment, when the user adjusts lightness, chroma, or hue, lightness and/or chroma of each hue is corrected so that the relative relationship of perceived brightnesses among the hues of the original image is maintained or so that the relative relationship is kept from varying. This enables image display that maintains perceived brightnesses among hues while achieving image quality adjustment desired by the user.

Fourth Embodiment

The User Adjusts the Perceived Brightness Per Hue

A fourth embodiment will be described. The fourth embodiment is similar to the first embodiment in overall configuration and different in the adjusting unit. Thus, the adjusting unit will be described in detail.

FIG. 11depicts an image and the image quality adjustment screen shown on the displaying unit according to the fourth embodiment. The image quality adjustment screen in the fourth embodiment includes an image quality adjustment screen1101in which the perceived brightness of each hue can be adjusted, in addition to the image quality adjustment screen302of the first embodiment. In the adjustment screen1101shown inFIG. 11, the user can adjust the perceived brightness in a desired hue.

The fourth embodiment illustrates a case where the user increases the perceived brightness in the red hue by ΔB*inin the image quality adjustment screen1101. When the user moves a slide bar1102for the perceived brightness of the red hue in increasing direction, the image quality adjusting unit105sends an amount of adjustment106indicating that the perceived brightness of the red hue has increased by ΔB*into the setting unit104. The perceived brightnesses before and after the user's adjustment in terms of lightness L*inand chroma C*inin the red hue are respectively represented as in Equation 37:

Further, the ratio P of perceived brightnesses before and after the adjustment of the hue hin(R) by the user is calculated for each of lightness and chroma values. The ratio of perceived brightness P(L*in, C*in, hin(R)) after the user adjustment with the lightness of L*inand chroma of C*inin hue hin(R) are calculated as in Equation 38:

Further, as shown in Equation 39, the ratio of perceived brightness P(L*in, C*in, hin(R)) before and after the adjustment of the hue hin(R) adjusted by the user is defined as the ratio for application in common to all hues that have equal lightness and chroma. Thus, the ratio is defined as Pmax(L*in, C*in):

The perceived brightness of the hue hin(R) has increased by the ratio Pmax(L*in, C*in) as a result of the user adjustment. Thus, by multiplying the perceived brightness of the other hues for which the user does not adjust image quality by the ratio Pmax(L*in, C*in) as shown in Equation 40, the ratio of perceived brightness among all hues having equal lightness and chroma can be kept the same as the ratio before the user adjustment.

The brightness B*out(L*in, C*in, hin) perceived when the perceived brightness B*in(L*in, C*in, hin) with the lightness L*inand chroma C*inin hue hinis multiplied by ratio Pmax(L*in, C*in) is calculated as in Equation 40:

Alternatively, the difference in perceived brightness ΔB*out(L*in, C*in, hin) may be set as the difference in perceived brightness in the hue hin(R) adjusted by the user, as shown in Equation 42:

Further, as in the first embodiment, the amount of lightness correction ΔL*outand the amount of chroma correction ΔC*outare set as the amount of correction107from the difference of perceived brightness ΔB*out(L*in, C*in, hin), and the amount of correction107is sent to the adjusting unit105and the correcting unit108. The adjusting unit105automatically changes the indication of amount of adjustment on the adjustment screen302in accordance with the amount of correction107. The correcting unit108corrects the converted signal103in accordance with the amount of correction107to obtain the corrected image signal109, and sends it to the displaying unit110. The displaying unit110displays the corrected image signal109on the display screen.

As described, in the present embodiment, when the user adjusts the perceived brightness, lightness and/or chroma of each hue is corrected so that the relative relationship of perceived brightnesses between different hues of the original image is maintained or so that the relative relationship is kept from varying. This enables image display that maintains perceived brightnesses among hues while achieving image quality adjustment desired by the user.

Fifth Embodiment

The Perceived Brightness is Adjusted for all Hues at One Time

A fifth embodiment will be described. The fifth embodiment is similar to the fourth embodiment in overall configuration and different in the adjusting unit. Thus, the adjusting unit will be described in detail.

FIG. 12depicts an image and an image quality adjustment screen shown on the displaying unit according to the fifth embodiment. The image quality adjustment screen in the fifth embodiment includes an image quality adjustment screen1201in which the perceived brightness of all hues can be adjusted at one time, in addition to the image quality adjustment screens302and1101of the fourth embodiment.

The fifth embodiment illustrates a case where the user increase the perceived brightness by ΔB*inof all hues at one time in the image quality adjustment screen1201. When the user moves a slide bar1102for controlling the perceived brightness of all hues together in increasing direction, the image quality adjusting unit105sends an amount of adjustment106indicating that the perceived brightness of all hues has increased by ΔB*into the setting unit104.

The perceived brightnesses before and after the user adjustment in terms of lightness L*inand chroma C*inin hue hinare respectively represented as in Equation 43:

Further, the ratio P of perceived brightnesses of all hues before and after the adjustment is calculated for each of lightness and chroma values. The ratio of perceived brightness P(L*in, C*in, hin) after the user adjustment with the lightness of L*inand chroma of C*inin hue hinis calculated as in Equation 44:

Further, as shown in Equation 45, the ratio of the hue that has the largest ratio P(L*in, C*in, hin) of perceived brightnesses before and after correction of all hues among all hues is defined as the maximum value Pmax(L*in,C*in) of the ratio of perceived brightness for all hues having equal lightness and chroma. Although the maximum value is employed in the illustrated case, the minimum value, or a representative value between the minimum and maximum values, such as a mean value or median, may be adopted.

The Helmholtz-Kohlrausch effect acts to varying degrees depending on hue; for example, colors like red and magenta have a greater effect of appearing bright, while yellow and green have a smaller effect of appearing bright. Therefore, the ratio of perceived brightness P(L*in, C*in, hin) assumes different values depending on hue hin, and the maximum ratio among all hues is defined as Pmax(L*in, C*in) The brightness B*out(L*in, C*in, hin) perceived when the brightness B*in(L*in, C*in, hin) perceived with the lightness L*in, chroma C*inin hue hinis multiplied by the ratio Pmax(L*in, C*in) is calculated as shown in Equation 46:

Alternatively, the difference of perceived brightness ΔB*out(L*in, C*in, hin) may be set to the amount of adjustment of perceived brightness ΔB*1n.

Further, as in the first embodiment, the amount of lightness correction ΔL*outand the amount of chroma correction ΔC*nutare set as the amount of correction107from the difference of perceived brightness ΔB*out(L*in, C*in, hin), and the amount of correction107is sent to the adjusting unit105and the correcting unit108. The adjusting unit105automatically changes the indication of amount of adjustment on the adjustment screen302in accordance with the amount of correction107. The correcting unit108corrects the converted signal103in accordance with the amount of correction107to obtain the corrected image signal109, and sends it to the displaying unit110. The displaying unit110displays the corrected image signal109on the display screen.

As described, in the present embodiment, when the user adjusts the perceived brightness, lightness and/or chroma of each hue is corrected so that the relative relationship of perceived brightnesses among the hues of the original image is maintained or so that the relative relationship is kept from varying. This enables image display that maintains perceived brightnesses among hues while achieving image quality adjustment desired by the user.

Sixth Embodiment

The User Specifies a Pixel and Perceived Brightness

A sixth embodiment will be described. The sixth embodiment is similar to the fifth embodiment in overall configuration and different in the adjusting unit. Thus, the adjusting unit will be described in detail.

FIG. 13depicts an image and the image quality adjustment screen shown on the displaying unit according to the sixth embodiment. The image quality adjustment screen in the sixth embodiment includes a pointer1301with which the user can specify a pixel on the screen, in addition to the image quality adjustment screens302,1101, and1201. The sixth embodiment allows the user to specify a pixel in which the user wants to make an image quality adjustment using the pointer1301. The user then adjusts the perceived brightness of the pixel the user specified using the image quality adjustment screen1201.

The sixth embodiment illustrates a case where the user increases the perceived brightness of the pixel specified with the pointer1301by ΔB*inin the screen1201. When the user moves the brightness slide bar in increasing direction, the image quality adjusting unit105sends an amount of adjustment106indicating that the brightness of the hue exhibited by the pixel indicated by the pointer1301has increased by ΔB*into the setting unit104.

When the hue of the pixel specified by the user with the pointer1301is hp, the setting unit104calculates the perceived brightness B*inbefore adjustment and the perceived brightness after adjustment B*in′ of the converted signal with lightness L*in, chroma C*in, and hue hp, as in Equation 48:

Further, the ratio P of perceived brightnesses before and after the adjustment of hue hprepresented by the pixel specified by the user with the pointer1301is calculated for each of lightness and chroma values. The ratio of perceived brightness P(L*in, C*in, hp) after the user adjustment with the lightness of L*inand chroma of C*inis calculated as in Equation 49:

Further, as shown in Equation 50, the ratio of perceived brightness P(L*in, C*in, hp) before and after the adjustment of the hue hpof the pixel specified by the user with the pointer1301is defined as the ratio for application in common to all hues that have equal lightness and chroma. Thus, this ratio is defined as Pmax(L*in, C*in).

Since the perceived brightness of the pixel specified by the user with the pointer1301has increased by the ratio Pmax(L*in, C*1n) as a result of the user adjustment, by multiplying the perceived brightness of the other hues by the ratio Pmax(L*in, C*in), the ratio of perceived brightness among all hues having equal lightness and chroma can be kept the same as the ratio before the user performed adjustment.

The brightness B*out(L*in, C*in, hin) perceived when the perceived brightness B*in(L*in, C*in, hin) with the lightness of L*inand chroma of C*inin hue hinis multiplied by ratio Pmax(L*in, C*in) is calculated as in Equation 51:

Alternatively, the difference of perceived brightness ΔB*out(L*in, C*in, hin) may be set as the difference in perceived brightness ΔB*inof the pixel specified by the user with the pointer1301, as shown in Equation 53:

Further, as in the first embodiment, the amount of lightness correction ΔL*outand the amount of chroma correction ΔC*outare set as the amount of correction107from the difference of perceived brightness ΔB*out(L*in, C*in, hin), and the amount of correction107is sent to the adjusting unit105and the correcting unit108. The adjusting unit105automatically changes the indication of amount of adjustment on the adjustment screen302in accordance with the amount of correction107. The correcting unit108corrects the converted signal103in accordance with the amount of correction107to obtain the corrected image signal109, and sends it to the displaying unit110. The displaying unit110displays the corrected image signal109on the display screen.

As described, in the present embodiment, when the user adjusts perceived brightness, lightness and/or chroma of each hue is corrected so that the relative relationship of perceived brightnesses among the hues of the original image is maintained or so that the relative relationship is kept from varying. This enables image display with perceived brightnesses among hues maintained while achieving image quality adjustment desired by the user.

Seventh Embodiment

When the Reference Color Gamut is Different from the Panel Color Gamut

A seventh embodiment will be described. As the seventh embodiment is similar to the first embodiment in overall configuration, differences from the first embodiment will be described in detail.

FIG. 14shows the configuration of the image display apparatus in the seventh embodiment. The seventh embodiment includes a color-gamut information holder1401in addition to the components of the first embodiment. In the seventh embodiment, a standardized color reproduction range such as ITU-R BT.709 is used as the reference color reproduction range, and the chroma or lightness of each hue is corrected so that the relative relationship of perceived brightness among hues in the reference color reproduction range is maintained when displayed on the image displaying unit. The seventh embodiment illustrates a case where the color reproduction range defined by ITU-R BT.709 is used as the reference color reproduction range.

The converting unit102receives color gamut information of BT.709, which is the reference color reproduction range, from the color-gamut information holder, and determines tristimulus values (X709, Y709, Z709) converted according to the color reproduction range defined by BT.709 and tristimulus values (Xd, Yd, Zd) converted according to the color reproduction range of the image displaying unit from the input image signal101, according to Equation 54:

In the equation, “M” represents a 3×3 color conversion matrix depending on the color reproduction range of the image displaying unit, and “L” represents a 3×3 color conversion matrix depending on the color reproduction range defined by ITU-R BT.709. “L” is maintained in the color-gamut information holder1401. “M” is maintained in the converting unit102or a storage accessible to the converting unit102.

The converting unit further calculates the lightnesses, chromas, and hues in CIEL*a*b* space (L*709, C*709, h709) and (L*d, C*d, hd) respectively from (X709, Y709, Z709) and (Xd, Yd, Zd). As conversion from XYZ tristimulus values to the lightness, chroma, and hue in CIEL*a*b* space is similar to the first embodiment, description of how to calculate them is omitted.

The image quality adjusting unit105can adjust the amount of adjustment of lightness, chroma, or hue on a per-hue basis as in the first embodiment, and sends the amount of adjustment106made by the user to the setting unit104. The seventh embodiment illustrates a case where the user adjusts the chroma of the red hue by ΔC*in.

First, the setting unit104calculates the perceived brightness of the red hue B*d′ (L*d, C*d, hd(R)) in an image as displayed on the image displaying unit reflecting amount of adjustment ΔC*in, and the perceived brightness in the BT.709 color reproduction range, i.e., the reference color reproduction range, B*709(L*709, C*709, h709(R)), respectively as in Equation 55:

The ratio P(L*709, C*709, h709(R)) between the perceived brightness in the reference color gamut BT.709 in the red hue and the perceived brightness on the display screen with the amount of adjustment ΔC*inreflected is represented as Equation 56:

For a given hue h709other than red whose chroma was adjusted by the user, the ratio P(L*709, C*709, h709) between the perceived brightness in the reference color gamut BT.709 and the perceived brightness on the display screen is represented as in Equation 57:

Then, as shown in Equation 58, the largest ratio P (L*709, C*709, h709) between the perceived brightness in the reference color gamut BT.709 and the perceived brightness on the display screen with the amount of adjustment ΔC*inreflected among all hues including red, for which the user adjusted chroma, is defined as Pmax(L*709, C*709). Although the maximum value is employed in the illustrated case, the minimum value, or a representative value between the minimum and maximum values, such as a mean value or median, may be adopted.

The brightness B*out(L*709, C*709, h709) perceived when the perceived brightness B*709(L*709, C*709, h709) with lightness L*709and chroma C*709in hue h709by the ratio Pmax(L*709, C*709) is calculated as in Equation 59 such that the relative relationship of perceived brightness among hues is the same between the BT.709 color reproduction range (i.e., the reference color gamut) and the color reproduction range of the display screen reflecting the amount of adjustment ΔC*in:

Alternatively, the difference in perceived brightness ΔB*out(L*709, C*709, h709) may be set as the difference in perceived brightness in the hue h709(R) adjusted by the user, as shown in Equation 61:

Further, as in the first embodiment, the amount of lightness correction ΔL*outand the amount of chroma correction ΔC*outare set as the amount of correction107from the difference of perceived brightness ΔB*out(L*709, C*709, h709), and the amount of correction107is sent to the adjusting unit105and the correcting unit108. The adjusting unit105automatically changes the indication of amount of adjustment on the adjustment screen302in accordance with the amount of correction107. The correcting unit108corrects (L*709, C*709, h709), representing the converted signal103, in accordance with the amount of correction107to obtain the corrected image signal109, and sends it to the displaying unit110. The displaying unit110displays the corrected image signal109on the display screen.

As described, in the present embodiment, when the user adjusts lightness, chroma, or hue, lightness and/or chroma of each hue is corrected so that the relative relationship of perceived brightnesses among the hues of the original image is maintained or so that the relative relationship is kept from varying. This enables image display that maintains perceived brightnesses among hues while achieving the image quality adjustment desired by the user.

Eighth Embodiment

When the Reference Color Gamut is Different from Panel Color Gamut

An eighth embodiment will be described.FIG. 15shows the configuration of the image display apparatus in the eighth embodiment. The image display apparatus100according to the eighth embodiment is different from the seventh embodiment in that it does not include the image quality adjusting unit105. In the eighth embodiment, the image quality adjusting unit105is not included, adjustment of image quality by the user is not performed, and the chroma or lightness of each hue is corrected so that the relative relationship of perceived brightness among hues in a standardized reference color reproduction range is maintained when displayed on the image displaying unit. The eighth embodiment illustrates a case where the color reproduction range defined by ITU-R BT.709 is used as the reference color reproduction range.

The converting unit102receives color gamut information1402of BT.709, which is the reference color reproduction range, from the color-gamut information holder1401. The converting unit102calculates (L*709, C*709, h709) and (L*d, C*d, hd), which are the lightness, chroma, and hue converted in accordance with the BT.709-defined color reproduction range and the color reproduction range of the image displaying unit respectively, from the input image signal101. The converting unit102sends (L*709, C*709, h709) and (L*d, C*d, hd) to the setting unit104as the converted signal103. As conversion to lightness, chroma, and hue is similar to the first embodiment, description of how to calculate them is omitted.

The setting unit104first calculates the perceived brightness B*d(L*d, C*d, hd) perceived when the input image signal value (Rin, Gin, Bin) is displayed by the image displaying unit and the perceived brightness B*709(L*709, C*709, h709) in the BT.709 color reproduction range, i.e., the reference color reproduction range, as in Equation 62:

The ratio P(L*709, C*709, h709) between the perceived brightness in the reference color gamut BT.709 and the perceived brightness on the display screen is represented as in Equation 63:

Then, as shown in Equation 64, the largest ratio P (L*709, C*709, h709) between the perceived brightness in the reference color gamut BT.709 and the perceived brightness on the display screen among all hues is defined as Pmax(L*709, C*709). Although the maximum value is employed in the illustrated case, the minimum value, or a representative value between the minimum and maximum values, such as a mean value or median, may be adopted.

The brightness B*out(L*709, C*709, h709) perceived when the brightness B*709(L*709, C*709, h709) perceived with the lightness L*709, chroma C*709, and hue h709is multiplied by the ratio Pmax(L*709,C*709) is calculated as shown in Equation 65, such that the relative relationship of perceived brightness among hues is the same in the reference BT.709 color reproduction range and the color reproduction range of the display screen:

Alternatively, ΔB*out(L*709, C*709, h709) may be determined by calculating the difference ΔB*709(L*709, C*709, h709) between B*d(L*d, C*d, hd) and B*709(L*709, C*709, h709), which were determined by Equation 62, as shown in Equation 67, and defining the largest difference ΔB*709(L*709,C*709,h709) among all hues as ΔB*out(L*7o9,C*709,h709), as shown by Equation 68. Although the maximum value is employed in the illustrated case, the minimum value, or a representative value between the minimum and maximum values, such as a mean value or median, may be adopted.

Further, as in the first embodiment, the amount of lightness correction ΔL*outand the amount of chroma correction ΔC*outare set as the amount of correction107based on the difference of perceived brightness ΔB*out(L*709, C*709, h709), and the amount of correction107is sent to the correcting unit108. The correcting unit108corrects (L*709, C*709, h709), representing the converted signal103, in accordance with the amount of correction107to obtain the corrected image signal109, and sends it to the displaying unit110. The displaying unit110displays the corrected image signal109on the display screen.

As described above, according to the eighth embodiment, by correcting the lightness or chroma of each hue so that the relative relationship of perceived brightness among hues in the color reproduction range of the display screen is the same as the relative relationship of perceived brightness among hues in the reference color gamut, it is possible to achieve image display that maintains the relative relationship of perceived brightness among hues of the reference color reproduction range even on a display device having a different color reproduction range.

Ninth Embodiment

When the Reference Color Gamut is Different from Panel Color Gamut

A ninth embodiment will be described.FIG. 16shows the configuration of the image display apparatus in the ninth embodiment. The image display apparatus100of the ninth embodiment includes an LUT holder1601, a correcting unit108, and a displaying unit110.

In the eighth embodiment, corrected image signal values obtained by applying correction to input image signal values (Rin, Gin, Bin) with the amount of correction107ΔB*out(L*709, C*709, h709) for perceived brightness is represented as (Rout, Gout, Bout). In the eighth embodiment, since image quality adjustment by the user is not performed, if reference color gamut information (specifically, transformation matrix M) and color gamut information for the display screen (specifically, transformation matrix L) are known, the amount of brightness correction107ΔB*out(L*709, C*709, h709) for the input image signal values (Rin, Gin, Bin) is uniquely determined and further the corrected image signal values (Rout, Gout, Bout) are uniquely determined.

Thus, from the reference color gamut information and color gamut information of the display screen, the corrected image signal values (Rout, Gout, Bout) for the input image signal values (Rin, Gin, Bin) is calculated in advance, and the relationship between (Rin, Gin, Bin) and (Rout, Gout, Bout) is kept in an LUT. By referencing the LUT, the corrected image signal values (Rout, Gout, Bout) are directly calculated from the input image signal value (Rin, Gin, Bin).

Here, corrected image signal values may be calculated for all possible input image signal values (for example, 256×256×256 signal values when RGB subpixels is each represented in 8 bits) and an LUT storing their correspondence may be used to determine the corrected image signal values. Alternatively, an LUT that holds the correspondence between representative values of input image signal values and corresponding corrected image signal values may be prepared, and the final corrected image signal value may be determined by interpolating plural corrected image signal values obtained by reference to the LUT.

As the processing for determining the corrected image signal values from the input image signal values is similar to the eighth embodiment, detailed description of how to calculate corrected image signal values is omitted. The difference of the ninth embodiment from the eighth embodiment is that the corrected image signal values are determined by referencing a prepared LUT contrary to the eighth embodiment which determines the corrected image signal values by calculation.

The correcting unit108calculates the corrected image signal109from the input image signal101with reference to the LUT1602, and sends it to the displaying unit110. The displaying unit110displays the corrected image signal109on the display screen.

As described, according to the ninth embodiment, by maintaining corrected image information with perceived brightness corrected in an LUT in advance in association with an input image, image display is enabled that maintains the relative relationship of perceived brightness among hues of the reference color reproduction range even on a display device having a different color reproduction range without involving complicated processing.