Image display device and image display method

1. An image display device for displaying an image on the basis of image data comprising:

an image feature quantity calculating portion which calculates a plurality of image feature quantities based on a luminance histogram of the image data;

an expansion coefficient determining portion which determines an expansion coefficient based on the plurality of image feature quantities by referring to a predetermined expansion coefficient lookup table; and

a luminance range expansion processing portion which performs a luminance range expansion process on the image data using the expansion coefficient, the luminance range expansion process being a process to extend a range of luminances of the image data, wherein:

the image data is moving picture data,

the expansion coefficient determining portion determines the expansion coefficient for each frame of the moving picture data by referring to the predetermined expansion coefficient lookup table, and

the image display device further comprises:

an expansion correcting portion which:

determines an expansion modification volume of which an absolute value is smaller than an absolute value of an ideal expansion modification volume, the ideal expansion modification volume being a differential of a current frame ideal expansion coefficient from a previous frame expansion coefficient, the current frame ideal expansion coefficient being an expansion coefficient determined by the expansion coefficient determining portion based on the plurality of image feature quantities of a current frame referring to the predetermined expansion coefficient lookup table, the previous frame expansion coefficient being an expansion coefficient used in the luminance range expansion process of a previous frame; and

generates a current frame expansion coefficient by correcting the current frame ideal expansion coefficient using the expansion modification volume, and

the luminance range expansion processing portion performs the luminance range expansion process on the image data based on the current frame expansion coefficient as the expansion coefficient.

2. The image display device according to claim 1 wherein

the luminance histogram is a frequency distribution of mean luminance values of pixels in a plurality of small regions into which an area of the image has been divided.

3. The image display device according to claim 1 wherein the plurality of image feature quantities include:

a white peak value which represents a maximum luminance in the luminance histogram; and

at least one of a mean value of the luminance histogram and a minimum value of the luminance histogram.

4. The image display device according to claim 1 wherein

in case where an absolute value of a previous expansion modification volume is smaller than a predetermined threshold, the expansion correcting portion determines a first value as the expansion modification volume based on the ideal expansion modification volume, the previous expansion modification volume being a differential of the previous frame expansion coefficient from a previous frame ideal expansion coefficient, the previous frame ideal expansion coefficient being an expansion coefficient determined by the expansion coefficient determining portion based on the plurality of image feature quantities of the previous frame referring to the predetermined expansion coefficient lookup table, and

in case where the absolute value of the previous expansion modification volume is equal to or greater than the predetermined threshold, the expansion correcting portion determines a second value as the expansion modification volume based on the ideal expansion modification volume, wherein an absolute value of the second value is greater than an absolute value of the first value in case where the ideal expansion modification volumes are same.

5. The image display device according to claim 4 wherein

in case where the absolute value of the previous expansion modification volume is equal to or greater than the predetermined threshold and the ideal expansion modification volume is a positive value, the expansion correcting portion determines a third value as the second value, and

in case where the absolute value of the previous expansion modification volume is equal to or greater than the predetermined threshold and the ideal expansion modification volume is a negative value, the expansion correcting portion determines a fourth value as the second value, wherein an absolute value of the fourth value is greater than an absolute value of the third value in case where the ideal expansion modification volumes are same.

6. The image display device according to claim 1 wherein

the image data is moving picture data,

the expansion coefficient determining portion determines the expansion coefficient for each frame of the moving picture data by referring to the predetermined expansion, coefficient lookup table,

the image display device further comprising

an expansion substituting portion which, in case where a current frame ideal expansion coefficient equals a second previous frame ideal expansion coefficient, but does not equal a first previous frame ideal expansion coefficient, substitutes the current frame ideal expansion coefficient with a first previous frame expansion coefficient to generate a current frame expansion coefficient, the current frame ideal expansion coefficient being an expansion coefficient determined by the expansion coefficient determining portion based on the plurality of image feature quantities of a current frame referring to the predetermined expansion coefficient lookup table, the first previous frame ideal expansion coefficient being an expansion coefficient determined by the expansion coefficient determining portion based on the plurality of image feature quantities of a frame previous by one the current frame referring to the predetermined expansion coefficient lookup table, the second previous frame ideal expansion coefficient being an expansion coefficient determined by the expansion coefficient determining portion based on the plurality of image feature quantities of a frame previous by two the current frame referring to the predetermined expansion coefficient lookup table, the first previous frame expansion coefficient being an expansion coefficient used in the luminance range expansion process of the frame previous by one the current frame, and

the luminance range expansion processing portion performs the luminance range expansion process on the image data using the current frame expansion coefficient as the expansion coefficient.

7. The image display device according to claim 1 further comprising:

a lighting device;

a modulation coefficient determining portion which determines a modulation coefficient based on the plurality of image feature quantities by referring to a predetermined modulation coefficient lookup table, the modulation coefficient representing a brightness of light of the lighting device; and

a light modulating portion which modulates the light of the lighting device based on the modulation coefficient.

8. The image display device according to claim 7 wherein

the expansion coefficient lookup table and the modulation coefficient lookup table are set up such that maximum luminance of the image is unchanged prior and subsequent to execution of both the luminance range expansion process and modulation.

9. An image display method for displaying an image based on image data, comprising:

calculating a plurality of image feature quantities based on a luminance histogram of the image data;

determining an expansion coefficient based on the plurality of image feature quantities by referring to a predetermined expansion coefficient lookup table; and

performing a luminance range expansion process on the image data using the expansion coefficient, the luminance range expansion process being a process to extend a range of luminances of the image data, wherein:

the image data is moving picture data,

determining the expansion coefficient includes determining the expansion coefficient for each frame of the moving picture data by referring to the predetermined expansion coefficient lookup table,

the image display method further comprises:

determining an expansion modification volume of which an absolute value is smaller than an absolute value of an ideal expansion modification volume, the ideal expansion modification volume being a differential of a current frame ideal expansion coefficient from a previous frame expansion coefficient, the current frame ideal expansion coefficient being an expansion coefficient determined based on the plurality of image feature quantities of a current frame referring to the predetermined expansion coefficient lookup table, the previous frame expansion coefficient being an expansion coefficient used in the luminance range expansion process of a previous frame; and

generating a current frame expansion coefficient by correcting the current frame ideal expansion coefficient using the expansion modification volume, and

the luminance range expansion process is performed on the image data based on the current frame expansion coefficient as the expansion coefficient.