Apparatus and method for driving a display device including four color pixels

1. An apparatus for driving a display device including a plurality of four color pixels, the apparatus comprising:

an input unit receiving input three-color image signals;

an image signal modifier converting the three-color image signals into output four-color image signals such that a maximum gray among an input R signal, an input G signal, and an input B signal of the input three-color image signals is equal to a maximum gray among an output R signal, an output G signal, and an output B signal of the output four-color image signals, and another gray of the output R signal, G signal, and B signal is derived from the maximum gray and at least one other gray of the input R signal, G signal, and B signal; and

an output unit outputting the output four-color image signals,

wherein the image signal modifier compares grays of the input three-color image signals, determines a maximum input gray, a middle input gray, and a minimum input gray, and assigns order indices based thereon,

gamma and reverse gamma converts the maximum input gray, the middle input gray, and the minimum input gray to obtain a maximum output gray (Max′), a middle output gray (Mid′), a minimum output gray (Min′), and an output white gray (W) of the output four-color image signals, and

generates the output four-color image signals based on the order indices, wherein the maximum input gray (Max), the middle input gray (Mid), and the minimum input gray (Min) have relations with the maximum output gray (Max′), the middle output gray (Mid′), the minimum output gray (Min′), and the output white gray (W) as follows:

when Γ(Max)>[ s 1 / s 1 −1]Γ(Min),

Max′=Max;

Mid′=Max Γ −1 [Γ(Mid)−Γ(Min)]/[Γ(Max)−Γ(Min)];

Min′=0; and

W =Max Min/Γ −1 [Γ(Max)−Γ(Min)],

where Γ is a gamma conversion function, Γ −1 is a reverse gamma conversion function, and s 1 is a scaling factor, and

wherein the maximum input gray (Max), the middle input gray (Mid), and the minimum input gray (Min) have relations with the maximum output gray (Max′), the middle output gray (Mid′), the minimum output gray (Min′), and the output white gray (W) as follows:

when Γ(Max)≦[ s 1 / s 1 −1]Γ(Min),

Max′=Max;

Mid′=Γ −1 {s 1 [Γ(Mid)−Γ(Max)]+Γ(Max)]; and

W=Γ −1 [( s 1 −1)Γ(Max)],

where Γ is a gamma conversion function, Γ −1 is a reverse gamma conversion function, and s 1 is a scaling factor.

2. The apparatus of claim 1 , wherein the gamma function satisfies:

Γ( xy ) =Γ( x ) Γ( y ); and

Γ −1 ( pq )=Γ −1 ( p )Γ −1 ( q ).

3. The apparatus of claim 2 , wherein the gamma function is an exponential function.

4. The apparatus of claim 3 , wherein the power of the gamma function is equal to 2.4.

5. The apparatus of claim 1 , wherein the scaling factor is equal to two.

6. The apparatus of claim 1 , further comprising:

a gray voltage generator generating a plurality of gray voltages; and

a data driver that selects gray voltages among the plurality of gray voltages corresponding to the output four-color image signals and outputs the selected gray voltages to the pixels as data voltages.

7. The apparatus of claim 1 , wherein an output white signal and output three-color image signals of the output four-color image signals are used together for substantially all grays of achromatic color.

8. An apparatus for driving a display device including a plurality of four color pixels, the apparatus comprising:

an input unit receiving input three-color image signals;

an image signal modifier converting the three-color image signals into output four-color image signals such that a gamma curve for achromatic color of the display device has no inflection point below about a 94 percent transmittance of the gamma curve, wherein the gamma curve is a relationship between a gray of one of the input three-color image signals and the transmittance of a-corresponding one of the four-color image signals; and

an output unit outputting the output four-color image signals,

wherein the image signal modifier

compares grays of the input three-color image signals, determines a maximum input gray (Max), a middle input gray (Mid), and a minimum input gray (Min), and assigns order indices based thereon,

gamma and reverse gamma converts the maximum input gray, the middle input gray, and the minimum input gray to obtain a maximum output gray (Max′), a middle output gray (Mid′), a minimum output gray (Min′), and an output white gray (W) of the output four-color image signals, and

generates the output four-color image signals based on the order indices, wherein the maximum input gray (Max), the middle input gray (Mid), and the minimum input gray (Min) have relations with the maximum output gray (Max′), the middle output gray (Mid′), the minimum output gray (Min′), and the output white gray (W) as follows:

when Γ(Max)>[ s 1 /( s 1 −1)]Γ(Min),

Max′=Max;

Mid′=Max Γ −1 [Γ(Mid)−Γ(Min)]/[Γ(Max)−Γ(Min)];

Min′=0; and

W =Max Min/Γ −1 [Γ(Max)−Γ(Min)],

where Γ is a gamma conversion function, Γ −1 is a reverse gamma conversion function, and s 1 is a scaling factor, and

wherein the maximum input gray (Max), the middle input gray (Mid), and the minimum input gray (Min) have relations with the maximum output gray (Max′), the middle output gray (Mid′), the minimum output gray (Min′), and the output white gray (W) as follows:

when Γ(Max)≦[ s 1 / s 1 −1]Γ(Min),

Max′=Max;

Mid′=Γ −1 {s 1 [Γ(Mid)−Γ(Max)]+Γ(Max)];

Min′=Γ −1 {s 1 [Γ(Min)−Γ(Max)]+Γ(Max)]; and

W=Γ −1 [( s 1 −1)Γ(Max)],

where Γ is a gamma conversion function, Γ −1 is a reverse gamma conversion function and s 1 is a scaling factor.

9. The apparatus of claim 8 , wherein the gamma function is an exponential function.

10. The apparatus of claim 9 , wherein the scaling factor is equal to two.

11. The apparatus of claim 8 , wherein a gray among an input R signal, an input G signal, and an input B signal of the input three-color image signals is equal to a maximum gray among an output R signal, an output G signal, and an output B signal of the output four-color image signals.

12. The apparatus of claim 8 , wherein an output white signal and output three-color image signals of the output four-color image signals are used together for substantially all grays of achromatic color.

13. A method for driving a display device including a plurality of four color pixels, the method comprising:

assigning order indices after comparing grays of the input three-color image signals and determining a maximum input gray (Max), a middle input gray (Mid), and a minimum input gray (Min);

gamma converting (Γ) and reverse gamma converting (Γ −1 ) the maximum input gray, the middle input gray, and the minimum input gray;

obtaining a maximum output gray (Max′), a middle output gray (Mid′), a minimum output gray (Mid′), and an output white gray (W),

from relations

Max′=Max;

Mid′=Max Γ −1 [Γ(Mid)−Γ(Min)]/Γ(Max)−Γ(Min)],

Min′=0, and

W =Max MinΓ −1 [Γ(Max)−Γ(Min)],

when Γ (Max)>[s 1 / s 1 −1)]Γ (Min) (where s 1 is a scaling factor); and

from relations

Max′=Max;

Mid′=Γ −1 {s 1 [Γ(Mid)−Γ(Max)]+Γ(Max)],

Min′=Γ −1 {s 1 [Γ(Min)−Γ(Max)]+Γ(Max)], and

W=Γ −1 [( s 1 −1)Γ(Max)],

when Γ (Max)≦[s 1 / s 1 −1]Γ (Min); and

generating four-color image signals having the maximum output gray, the middle output gray, the minimum output gray, and the output white gray according to the order given by the order indices.

14. The method of claim 13 , wherein the gamma conversion and the reverse gamma conversion are performed by using a look-up table.

15. The method of claim 13 , further comprising:

generating a plurality of gray voltages;

selecting data voltages among the plurality of gray voltages corresponding to the four-color image signals; and

applying the data voltages to the pixels.