Image analysis – Image compression or coding – Predictive coding
Reexamination Certificate
1998-04-03
2002-04-09
Tran, Phuoc (Department: 2721)
Image analysis
Image compression or coding
Predictive coding
C382S233000
Reexamination Certificate
active
06370276
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to image predictive decoding and image predictive coding and, more particularly, to image predictive decoding methods, image predictive decoding apparatuses, image predictive coding methods, image predictive coding apparatuses, and data storage media, which are used for processing variable-size images.
BACKGROUND OF THE INVENTION
In order to store or transmit a digital image with high efficiency, it is necessary to compressively code the digital image. As a typical method for compressive coding of a digital image, there is DCT (Discrete Cosine Transformation) represented by JPEG (Joint Photographic Experts Group) and MPEG (Moving Picture Experts Group). Besides, there are waveform coding methods such as sub-band coding, wavelet coding, and fractal coding. Further, in order to eliminate a redundant signal between images, inter-image prediction using motion compensation is carried out, and a difference signal is subjected to waveform coding.
Here, an MPEG method based on motion compensation DCT will be described. Initially, an input image of one frame to be coded is divided into plural macroblocks each having the size of 16×16 pixels. Each macroblock is further divided into four blocks each having the size of 8×8 pixels, and each block of 8×8 pixels is subjected to DCT and quantization. This process is called “intra-frame coding”.
On the other hand, using a motion detecting method such as block matching, from a frame temporarily adjacent to an object frame including an object macroblock to be quantized, a prediction macroblock having the smallest error from the object macroblock is detected, and motion compensation from the past image is carried out on the basis of the detected motion, thereby to obtain an optimum prediction block. A signal showing the motion toward the prediction macroblock having the smallest error is a motion vector. An image used as a reference for generating the prediction macroblock is called a reference image, hereinafter. Thereafter, a difference between the object block and the corresponding prediction block is obtained, and this difference is subjected to DCT to obtain a DCT coefficient. The DCT coefficient is quantized, and the quantized output is transmitted or stored together with the motion information. This process is called “inter-frame coding”.
The inter-frame coding has two prediction modes: prediction from a previous image in the display order, and prediction from both of previous and future images. The former is called “forward prediction”, and the latter is called “bidirectional prediction”.
On the decoder end, after restoring the quantized DCT coefficient to the original difference signal, the prediction block is obtained on the basis of the difference signal and the motion vector, and the prediction block and the difference signal are added to reproduce the image. In this conventional technique, it is premised that the size of the reference image (an image used as a reference for generating a prediction image) is equal to the size of the object image.
In recent years, plural objects constituting an image (arbitrary shape images) are separately subjected to compressive coding and transmitted, thereby to improve the coding efficiency and to enable object by object reproduction. In coding and decoding of such arbitrary shape image, the size of the image changes very often. For example, a ball becomes smaller and smaller, till at last it disappears. Further, there is a case where the size of the image (object) becomes zero.
In ordinary predictive coding, a reference image is a reproduced image just before an object image which is currently being processed. When the size of the reference image is zero, since nothing is defined in the reference image, i.e., since the reference image has no significant image content data to be used for predictive coding, predictive coding cannot be carried out. In this case, there is no conventional way except the intra-frame coding. However, generally the intra-frame coding increases the quantity of coded data and reduces the compression efficiency. When an image disappears (image size=zero) and appears frequently in a sequence of motion picture, the coding efficiency is significantly degraded. For example, in a motion picture of flashing spotlight, when the light disappears and appears in image units, all the images of lights must be subjected to the intra-frame coding.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an image predictive decoding method, an image predictive decoding apparatus, an image predictive coding method, an image predictive coding apparatus, and a data storage medium, which can realize efficient predictive coding or decoding of a variable-size image even when the size of a reference image is zero or when the reference image is completely transparent.
Other objects and advantages of the invention will become apparent from the detailed description that follows. The detailed description and specific embodiments described are provided only for illustration since various additions and modifications within the scope of the invention will be apparent to those of skill in the art from the detailed description.
According to a first aspect of the present invention, there is provided an image predictive decoding method in which image data obtained by compressively coding a variable-size image using a prescribed method is input, a prediction image is generated using, as a reference image, at least one reproduced image which has been reproduced before an image being an object of decoding, and the object image is subjected to predictive decoding; wherein the prediction image is generated using, as a reference image, at least one reproduced image which has been recently reproduced and includes significant image content data to be referred to. In this method, when the size of the reference image (reproduced image) is zero, i.e., when the reference image is completely transparent, predictive decoding is carried out using another reproduced image of which size is not zero. Therefore, when plural objects constituting an image are subjected to compressive coding and transmitted object by object to increase the compression efficiency, it is avoided that a variable-size image which has already disappeared is used as the reference image for predictive decoding. As a result, coded data obtained by efficient compressive coding that suppresses the code quantity can be appropriately decoded.
According to a second aspect of the present invention, there is provided an image predictive decoding method in which image data obtained by compressively coding a variable-size image using a prescribed method is input, a prediction image is generated using, as a reference image, a prescribed image which has been reproduced before an image being an object of decoding, and the object image is subjected to predictive decoding; wherein, when the reproduced image used as a reference image has no significant coded data to be referred to, an image having a prescribed value as its image data is used as the prediction image. In this method, when the size of the reference image (reproduced image) is zero, i.e., when the reference image is completely transparent, predictive decoding is carried out using a prediction image having a prescribed value. Therefore, in addition to the above-mentioned effects, generation of the prediction image is facilitated.
According to a third aspect of the present invention, there is provided an image predictive decoding method in which image data obtained by compressively coding a variable-size image using a prescribed method is input, a prediction image is generated using a reference image, and an image being an object of decoding is subjected to predictive decoding; wherein the prediction image is generated using, as the reference image, at least one of two reproduced signals which have been recently reproduced, which one has significant image data to be referred to. Therefore, in the case where pl
Alavi Amir
Parkhurst & Wendel L.L.P.
Tran Phuoc
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