1. Field of the Invention
The present invention relates to an image-data processing apparatus which detects a part in which noise occurs in decoded image data obtained by decoding image data which is created by performing frequency transformation and lossy compression, for example, using an MPEG (Moving Picture Expert Group) method.
2. Description of the Related Art
When image data is read or transmitted via a transmission line, the image data is data-compression coded to save storage capacity or transmission line capacity. The compression-coded image data is data-decompression decoded to be restored to the original image data when it is read or received at the receiving end.
When image data is compression coded, redundant information included in the image data is removed based on statistical characteristics of the image data, whereby a reduction in the amount of data is achieved. Generally, since human vision exhibits low-pass filtering characteristics, the high frequency components of the image are roughly quantized, so that high compression-coding efficiency is obtained.
As this type of the highly efficient compression-coding method, an MPEG method is often used in which data-compression coding is performed by executing a discrete cosine transform and then restricting the high frequency components. Since data compression is performed using the frequency transformation and lossy compression in which the high frequency components are restricted, noise occurs, so-called “mosquito noise”, which is a change in the level of the luminance component and the chrominance component that are in the proximity of a high-contrast edge or a moving object. The mosquito noise degrades the quality of the restored image.
FIGS. 9A and 9B show mosquito noise of a character “A” which is displayed at the same position of a screen. In FIG. 9A, no noise occurs in the original image of the character “A” before MPEG compression. When the image is MPEG compressed and then decompression-decoded, mosquito noise N occurs due to slight level variations in the periphery of the character “A”, that is, a high contrast edge part, as shown in FIG. 9B.
When a moving image of a rotating gear shown in FIG. 10A is MPEG compression-coded and decompression-decoded, the mosquito noise N occurs due to slight level variations in the periphery of the rotating gear. In FIG. 10B, since the gear rotates, the part having the mosquito noise moves along with the edges of the gear.
Methods have been proposed for reducing the mosquito noise occurring due to decoding of the compressed image data. For example, in Japanese Unexamined Patent Application Publication No. 7-212759, a method for reducing the mosquito noise in the image signal is disclosed as follows. That is, since the mosquito noise occurs at the high contrast edge, an edge-part extracting unit 1, as shown in FIG. 11, extracts an edge part of an input image signal Sv and then a noise-reduction filter 2 filters the extracted edge part of the image signal that is output from the edge-part extracting unit 1, whereby an image signal having reduced mosquito noise is obtained.
In this conventional method shown in FIG. 11, filtering processing for rejecting the mosquito noise is performed at the edge part of an image signal regardless of the presence or absence of the mosquito noise. Therefore, the quality of the image is degraded where no mosquito noise occurs. Furthermore, when, instead of an image signal which is decompression-decoded from highly efficiently compression-coded image data, a high quality image signal Sv having no mosquito noise is input, the operation of the noise-reduction filter 2 at the edge part of the image signal Sv rather degrades the image quality thereof.
Accordingly, as shown in FIG. 12, the edge-part extracting unit 1 and the noise-reduction filter 2 are provided so that an MPEG decoding unit 3 outputs an output signal Sm only thereto. In addition, a switching circuit 4 is provided so that an input image Ss which is not processed by the MPEG decoding unit 3 bypasses the noise-reduction filter 2 and avoids undergoing the edge-part filtering performed by the noise-reduction filter 2. A user may operate this switching circuit 4.
However, when the MPEG decoded image data is input as the image signal Ss, since the input decoded image data is not processed for reducing the mosquito noise by the noise-reduction filter 2, a degraded image is obtained.
The foregoing problem arises because, without detecting a part having the noise such as the mosquito noise, the edge part of the decoded image signal obtained by decoding the image data that undergoes frequency transformation and lossy compression is assumed to be a part having the noise and then noise reduction processing is performed on the edge part.