Image processing apparatus and image processing method

1. An image processing apparatus comprising:

an image display unit that includes pixels arranged in a matrix, each of which is formed of a first sub-pixel, a second sub-pixel, a third sub-pixel, and a fourth sub-pixel, and that performs image display; and

a luminance control unit that adjusts a ratio between a generation amount of first luminance generated by the first sub-pixel, the second sub-pixel, and the third sub-pixel and a generation amount of second luminance generated by the fourth sub-pixel,

wherein, over all input tones, the luminance control unit makes the generation amount of the second luminance lower than the generation amount of the first luminance and generates the second luminance so that a function representing a luminance value of the second luminance is continuous,

wherein the first sub-pixel is a red sub-pixel, the second sub-pixel is a green sub-pixel, the third sub-pixel is a blue sub-pixel, and the fourth sub-pixel is a white sub-pixel, and

wherein the luminance control unit

adjusts a ratio between a generation amount of first white luminance generated by the red sub-pixel, the green sub-pixel, and the blue sub-pixel and a generation amount of second white luminance generated by the white sub-pixel, and

over all input tones, makes the generation amount of the second white luminance lower than the generation amount of the first white luminance and generates the second white luminance so that a function representing a luminance value of the second white luminance is continuous,

wherein, when an ability to represent an image with an n-bit tone is given, the luminance control unit calculates the function representing a luminance value of the second white luminance by performing a spline interpolation using formulas defined by

X =(1− t ) 2×Ax +2 t (1− t )× Bx+t 2×Cx ,

Y =(1− t ) 2×Ay +2 t (1− t )× By+t 2×Cy , and

t =λ/(2 n −1),

where X is a X coordinate value, Y is a Y coordinate value, t is an input tone, (Ax, Ay), (Bx, By), and (Cx, Cy) are control points, and λ is a value of an input tone.

2. The image processing apparatus according to claim 1 ,

wherein the luminance control unit determines, as the function representing a luminance value of the second white luminance, a spline curve obtained by performing the spline interpolation on three points of (0, 0), (b, 0), and (255, Yc), where Yc is a value less than or equal to a maximum value of the second white luminance generated by the white sub-pixel and b is a value of an input tone.

3. The image processing apparatus according to claim 1 ,

wherein, when an ability to represent an image with an n-bit tone is given, the luminance control unit determines, as the function representing a luminance value of the second white luminance, an exponential function defined by

Y=Yc ×( X /(2 n −1)) 4 ,

where Yc is a value less than or equal to a maximum value of the second white luminance generated by the white sub-pixel, X is an input tone, and Y is a value of the second white luminance.

4. An image processing method comprising:

performing image display with pixels arranged in a matrix, each of which is formed of a first sub-pixel, a second sub-pixel, a third sub-pixel, and a fourth sub-pixel;

adjusting a ratio between a generation amount of first luminance generated by the first sub-pixel, the second sub-pixel, and the third sub-pixel and a generation amount of second luminance generated by the fourth sub-pixel; and

over all input tones, making the generation amount of the second luminance lower than the generation amount of the first luminance and generating the second luminance so that a function representing a luminance value of the second luminance is continuous,

wherein the first sub-pixel is a red sub-pixel, the second sub-pixel is a green sub-pixel, the third sub-pixel is a blue sub-pixel, and the fourth sub-pixel is a white sub-pixel,

wherein a ratio between a generation amount of first white luminance generated by the red sub-pixel, the green sub-pixel, and the blue sub-pixel and a generation amount of second white luminance generated by the white sub-pixel is adjusted, and

wherein, over all input tones, the generation amount of the second white luminance is made lower than the generation amount of the first white luminance and the second white luminance is generated so that a function representing a luminance value of the second white luminance is continuous,

wherein, when an ability to represent an image with an n-bit tone is given, the function representing a luminance value of the second white luminance is calculated by performing a spline interpolation using formulas defined by

X =(1− t ) 2×Ax +2 t (1− t )× Bx+t 2×Cx ,

Y =(1− t ) 2×Ay +2 t (1− t )× By+t 2×Cy , and

t= λ/(2 n −1),

where X is a X coordinate value, Y is a Y coordinate value, t is an input tone, (Ax, Ay), (Bx, By), and (Cx, Cy) are control points, and λ is a value of an input tone.

5. The image processing method according to claim 4 ,

wherein a spline curve obtained by performing the spline interpolation on three points of (0, 0), (b, 0), and (255, Yc), where Yc is a value less than or equal to a maximum value of the second white luminance generated by the white sub-pixel and b is a value of an input tone, is determined as the function representing a luminance value of the second white luminance.

6. The image processing method according to claim 4 ,

wherein, when an ability to represent an image with an n-bit tone is given, an exponential function defined by

Y=Yc ×( X /(2 n −1)) 4 ,

where Yc is a value less than or equal to a maximum value of the second white luminance generated by the white sub-pixel, X is an input tone, and Y is a value of the second white luminance, is determined as the function representing a luminance value of the second white luminance.