1. Field of the Invention
The present invention relates to a display apparatus, a display panel driver and a display panel driving method, and more particularly relates to a driving technique of a display panel to execute a color reducing process and an enlarging process to image data at a same time.
2. Description of Related Art
One of requests to an LCD panel (liquid crystal display panel) installed in a portable terminal is in increase in the number of colors to be displayed. In order to satisfy this request, the LCD driver for driving the LCD panel is needed to deal with a multiple gradation display. One problem lies in increase in a chip size, when the number of the displayable gradations of the LCD driver is increased. In order to increase the number of the displayable gradations, a D/A converter used to drive signal lines is needed to deal with a large number of gradations, and this causes the increase in the chip size.
One scheme for suppressing the increase in the chip size that results from the increase in the number of the gradations lies in that the LCD driver includes a color reducing circuit, and a pseudo gradation display is performed to substantially attain the multiple gradation display. For example, Japanese Patent No. 3,735,529 and Japanese Patent Application Publication (JP-A-Heisei, 9-90902) disclose a technique that a color reducing process is executed through error diffusion and further attains the pseudo gradation display by using FRC (frame rate control).
Another request to the LCD panel installed in the portable terminal lies in increase in the number of pixels. In recent years, the LCD panel is used that has the number of pixels more than the number of pixels defined by VGA (video graphic array). However, the increase in the number of pixels increases a data transfer amount to the LCD driver from an image processing unit such as CPU or DSP (digital signal processor), and consequently increases consumption of electric power and EMI (electromagnetic interference) of the LCD driver.
The inventor considers one scheme to solve the above problems of the increases in the electric power consumption and EMI that result from the increase in the number of pixels, in which the size of an image is selected on the basis of a kind of the image to be displayed (for example, VGA, QVCA (quarter VGA) and the like) and also an enlarging process, namely, a function of enlarging the image is given to the LCD driver. For example, it is assumed that the LCD panel has the number of pixels corresponding to VGA. In displaying the image for which a high quality display is requested such as a photograph, the image data of VGA is sent to the LCD driver, and the image is displayed at a same magnification. On the other hand, in displaying the image in which a relatively low resolution is allowable such as the display screen of a game or mail, the enlarging process is executed such that the image data of QVGA is sent to the LCD driver and then the image is enlarged to twice in both of a horizontal direction and a vertical direction by the LCD driver. The enlargement of the image in the horizontal direction is attained by driving the two pixels arrayed in the horizontal direction in accordance with the same image data, as the easiest manner. The enlargement of the image in the vertical direction is attained by driving the adjacent two scan lines sequentially (or at the same time), in the state that the signal line is driven to a desirable drive voltage. Since such a scheme is used to perform the image display, it is possible to decrease the data transfer amount to the LCD driver and decrease consumption of electric power and EMI.
In order to attain the correspondence to the multiple gradation display and the reduction in the consumption of electric power and the EMI at the same time, the color reducing process and the enlarging process are desired to be used at the same time. However, according to the consideration of the inventor, when the color reducing process and the enlarging process are simply combined, there is a possibility of deterioration of the image such as generation of flicker. For example, FIGS. 1A and 1B are diagrams showing an example of operation of the LCD driver, in which although the image of VGA is kept in its original state, the enlarging process to double in a column and a row directions is executed on the image of QVGA.
At first, it is assumed that the image data of VGA is supplied in which the gradation values of all of pixels of the image data are 18. In this case, as shown in FIG. 1A, the color reduction image data in which the pixel whose gradation value is 16 and the pixel whose gradation value is 20 are alternately repeated is generated through the color reducing process. Then, the LCD panel is driven in accordance with this color reduction image data.
On the other hand, it is assumed that the image data of QVGA is supplied in which the gradation values of all the pixels of the image data are 18. After the color reducing process is executed on the image data of QVGA, when the enlarging process to double in the column and row directions is executed, a matrix of 2×2 pixels in which the gradation value is 20 and a matrix of 2×2 pixels in which the gradation value is 20 are arranged on the LCD panel in a checker-wise pattern, as shown in FIG. 1B. In this way, when the color reducing process and the enlarging process are executed simply at the same time, a spatial frequency of a brightness change falls, thereby generating flicker.