As compared to conventional cathode-ray tubes (CRTs) primarily used for realizing moving images, LCDs (Liquid Crystal Displays) have a drawback, so-called motion blur, which is the blurring of outline of a movement portion perceived by a viewer when displaying an image with movement. It is suggested that this motion blur arises from the LCD display mode itself (see, e.g., patent document 1 and non-patent document 1).
Since fluorescent material is scanned by an electron beam to cause emission of light for display in CRTs, the light emission of pixels is basically impulse-like although slight afterglow of the fluorescent material exists. This is called an impulse-type display mode. On the other hand, in the case of LCDs, an electric charge is accumulated by applying an electric field to liquid crystal and is retained at a relatively high rate until the next time the electric field is applied. Especially, in the case of the TFT mode, since a TFT switch is provided for each dot configuring a pixel and each pixel normally has an auxiliary capacity, the ability to retain the accumulated electric charge is extremely high. Therefore, the light emission is continued until the pixels are rewritten by the application of the electric field based on image information of the next frame or field (hereinafter, represented by the frame). This is called a hold-type display mode.
Since the impulse response of the image displaying light has a temporal spread in the above hold-type display mode, temporal frequency characteristics are deteriorated along with spatial frequency characteristics, resulting in the motion blur. That is, since the human eye can smoothly follow a moving object, if the light emission time is long as in the case of the hold type, movement of image seems jerky and unnatural due to a time integration effect.
To improve the motion blur in the above hold-type display mode, a frame rate (number of frames) is converted by interpolating an image between frames in a known technology. This technology is called FRC (Frame Rate Converter) and is put to practical use in liquid crystal display devices, etc.
Conventionally known methods of converting the frame rate include various techniques such as simply repeating read-out of the same frame for a plurality of times and frame interpolation using linear interpolation between frames (see, e.g., non-patent document 2). However, in the case of the frame interpolation process using the linear interpolation between frames, unnaturalness of motion (jerkiness, judder) is generated due to the frame rate conversion, and the motion blur disturbance due to the above hold-type display mode cannot sufficiently be improved, resulting in inadequate image quality.
To eliminate effects of the jerkiness, etc., and improve quality of moving images, a motion-compensated frame interpolation (motion compensation) process using motion vectors has been proposed. In this motion compensation process, since a moving image itself is captured and compensated, highly natural moving images can be acquired without deteriorating the resolution and generating the jerkiness. Since interpolation image signals are generated with motion compensation, the motion blur disturbance due to the above hold-type display mode can sufficiently be improved.
Above patent document 1 discloses a technology of motion-adaptively generating interpolation frames to increase a frame frequency of a display image for improving deterioration of spatial frequency characteristics causing the motion blur. In this case, at least one interpolation image signal interpolated between frames of a display image is motion-adaptively created from the previous and subsequent frames, and the created interpolation image signals are interpolated between the frames and are sequentially displayed.
FIG. 44 is a block diagram of an outline configuration of an FRC drive display circuit in a conventional liquid crystal display device and, in FIG. 44, the FRC drive display circuit includes an FRC portion 100 that converts the number of frames of the input image signal by interpolating the image signals subjected to the motion compensation process between frames of the input video signal, an active-matrix liquid crystal display panel 103 having a liquid crystal layer and an electrode for applying the scan signal and the data signal to the liquid crystal layer, and an electrode driving portion 104 for driving a scan electrode and a data electrode of the liquid crystal display panel 103 based on the image signal subjected to the frame rate conversion by the FRC portion 100.
The FRC portion 100 includes a motion vector detecting portion 101 that detects motion vector information from the input image signal and an interpolation frame generating portion 102 that generates interpolation frames based on the motion vector information acquired by the motion vector detecting portion 101.
In the above configuration, for example, the motion vector detecting portion 101 may obtain the motion vector information with the use of a block matching method, a gradient method, etc., or if the motion vector information is included in the input image signal in some form, this information may be utilized. For example, the image data compression-encoded with the use of the MPEG format includes motion vector information of a moving image calculated at the time of encoding, and this motion vector information may be acquired.
FIG. 45 is a view for explaining a frame rate conversion process by the conventional FRC drive display circuit shown in FIG. 44. The FRC portion 100 generates interpolation frames (gray-colored images in FIG. 45) between frames with the motion compensation using the motion vector information output from the motion vector detecting portion 101 and sequentially outputs the generated interpolation frame signals along with the input frame signals to perform a process of converting the frame rate of the input image signal from 60 frames per second (60 Hz) to 120 frames per second (120 Hz).
FIG. 46 is a view for explaining an interpolation frame generation process of the motion vector detecting portion 101 and the interpolation frame generating portion 102. The motion vector detecting portion 101 uses the gradient method to detect a motion vector 105 from, for example, a frame #1 and a frame #2 shown in FIG. 45. That is, the motion vector detecting portion 101 obtains the motion vector 105 by measuring a direction and an amount of movement in 1/60 second between the frame #1 and the frame #2. The interpolation frame generating portion 102 then uses the obtained motion vector 105 to allocate an interpolation vector 106 between the frame #1 and the frame #2. An interpolation frame 107 is generated by moving an object (in this case, an automobile) from a position of the frame #1 to a position after 1/120 second based on the interpolation vector 106.
By performing the motion-compensated frame interpolation process with the use of the motion vector information to increase a display frame frequency in this way, the display state of the LCD (the hold-type display mode) can be made closer to the display state of the CRT (the impulse-type display mode) and the image quality deterioration can be improved which is due to the motion blur generated when displaying a moving image.
In the motion-compensated frame interpolation process, it is essential to detect the motion vectors for the motion compensation. For example, the block matching method, the gradient method, etc., are proposed as representative techniques for the motion vector detection. In the gradient method, the motion vector is detected for each pixel or small block between two consecutive frames and this is used to interpolate each pixel or small block of the interpolation frame between two frames. That is, an image at an arbitrary position between two frames is interpolated at an accurately compensated position to convert the number of frames.
Patent Document 1: Specification of Japanese Patent No. 3295437
Non-Patent Document 1: Ishiguro Hidekazu and Kurita Taiichiro, “Consideration on Motion Picture Quality of the Hold Type Display with an octuple-rate CRT”, IEICE Technical Report, Institute of Electronics, Information and Communication Engineers, ETD96-4 (1996-06), p. 19-26
Non-Patent Document 2: Yamauchi Tatsuro, “TV Standards Conversion”, Journal of the Institute of Television Engineers of Japan, Vol. 45, No. 12, pp. 1534-1543 (1991)