1. Field of the Invention
This invention relates to an image display device and a driving method thereof for improving picture quality.
2. Related Art
With the advancement of various image processing techniques, image display systems capable of selectively displaying 2D images and 3D images are developed.
Methods of generating 3D images are classified into stereoscopic technique and autostereoscopic technique.
The stereoscopic technique uses disparity images of left and right eyes which have high 3D effect, and includes a stereoscopic method and an autostereoscopic method which are practically used. The autostereoscopic method provides an optical plate such as a parallax barrier for separating optical axes of left and right disparity images from each other in front of or behind a display screen. The stereoscopic method displays left and right disparity images having different polarization directions on a liquid crystal display panel and generates 3D images by using polarizing glasses or liquid crystal shutter glasses.
The stereoscopic method is further classified into a first polarizing filter method which uses a pattern retarder film and polarizing glasses, a second polarizing filter method which uses a switching liquid crystal layer and polarizing glasses, and a liquid crystal shutter glasses method. In the first and second polarizing filter methods, 3D images have low transmissivity due to the pattern retarder film or the switching liquid crystal layer which is arranged on a liquid crystal display panel to function as a polarizing filter.
The stereoscopic method which uses a liquid crystal shutter glasses alternately displays left-eye and right-eye images on a display frame by frame and then opens and closes left-eye and right-eye shutters of liquid crystal shutter glasses in synchronization with the display timing to generate a 3D image. The liquid crystal shutter glasses open only the left-eye shutter for an nth frame period in which a left-eye image is displayed and open only the right-eye shutter for an (n+1)th frame period in which a right-eye image is displayed to generate binocular disparity in a time division manner.
In the above image display systems, a liquid crystal display (LCD) is widely used as an image display device. The LCD, a hold-type display device, holds data charged in a previous frame right before new data is written because of the maintenance characteristic of liquid crystal. The response of liquid crystal is delayed when data is written. The response delay of liquid crystal causes motion blurring when a left-eye image is changed to a right-eye image or when a right-eye image is changed to a left-eye image while the LCD generates a 3D image, resulting in 3D crosstalk in the form of a ghost.
Various methods for improving the response characteristic of liquid crystal for 2D images are known. For example, Over Driving Control (ODC) compares previous frame data with current frame data, detects a data variation according to the comparison result, reads a compensation value corresponding to the data variation from a memory and modulates input data with the read compensation value. Referring to FIG. 1, the ODC modulates the current frame data into “223” larger than “191” when the previous frame data is “127” and the current frame data is “191” and modulates the current frame data into “31” smaller than “63” when the previous frame data is “191” and the current frame data is “63”, thereby adjusting data voltages applied to liquid crystal so as to improve the response characteristic of liquid crystal. Besides, Black Data Insertion (BDI) inserts a black frame between neighboring frames to improve motion blurring and thereby enhance the response characteristic of liquid crystal.
To improve the 3D crosstalk occurred due to the overlapping of left eye images and right eye images caused by the brightness difference, it is considered to apply the above-described methods for improving the response characteristic of liquid crystal to image display devices.
However, there are some problems relating to a luminance deviation using the existing ODC logic and compensation values as shown in FIG. 2. In FIG. 2, an (n−2)th frame Fn−2 represents a left-eye data frame displaying a left-eye image, an nth frame Fn represents a right-eye data frame displaying a right-eye image, and an (n−1)th frame Fn−1 represents a black data frame displaying a black image. A variation in the luminance of the nth frame Fn to which the ODC is applied is generated between a case (A) where the target gray-scale values of each frame corresponds to “180”, “0” and “150” respectively and a case (B) where the target gray-scale values of each frame corresponds to “255”, “0” and “150”, respectively. This is because liquid crystal rises due to the applied voltage such that the initial luminance Li of the nth frame Fn in the case (A) is different from the initial luminance Li of the nth frame Fn in the case (B) due to a response delay of liquid crystal even when the same compensation value is applied with reference to the target gray-scale value “0” of the (n−1)th frame Fn−1 in order to achieve the target gray-scale value “150” of the nth frame Fn. The response of liquid crystal is proportional to a gray-scale difference between the (n−2)th frame Fn−2 and the (n−1)th frame Fn−1, and thus the initial luminance Li in the case (B) is higher than the initial luminance Li in the case (A). Similar case will happen when a black data frame displaying a black image is inserted between an (n−2)th frame Fn−2 represents a left-eye data frame displaying a left-eye image and an nth frame Fn represents a right-eye data frame displaying a right-eye image.