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Video encoding apparatus and video decoding apparatusRelated Patent Categories: Pulse Or Digital Communications, Bandwidth Reduction Or Expansion, Television Or Motion Video Signal, Block CodingVideo encoding apparatus and video decoding apparatus description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070147514, Video encoding apparatus and video decoding apparatus. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to a video encoding apparatus and video decoding apparatus, which encode, transmit, and store video signals with high efficiency, and decode the encoded signals. BACKGROUND ART [0002] Since a video signal has a large information volume, it is a common practice to compression-encode the video signal when it is transmitted or stored. In order to encode a video signal with high efficiency, an image in units of frames is divided into blocks in units of a predetermined number of pixels (for example, M.times.N pixels (M: the number of pixels in the horizontal direction, N: the number of pixels in the vertical direction)), each divided block is orthogonally transformed to separate the spatial frequency of the image into the respective frequency components, and these frequency components are acquired as transform coefficients and are encoded. [0003] As one of video encoding methods, a video encoding method that belongs to the category called mid-level encoding is proposed in "J. Y. A. Wang et. al. "Applying Mid-level Vision Techniques for Video Data Compression and Manipulation", M.I.T. Media Lab. Tech. Report No. 263, February 1994,". [0004] In this method, if an image including a background and a subject (the subject will be referred to as an object hereinafter) is present, the background and object are separately encoded. [0005] In order to separately encode the background and object in this way, for example, an alpha-map signal as binary subsidiary video information that expresses the shape of the object and its position in a frame, is required. Note that the alpha-map signal of the background is uniquely obtained based on that of the object. [0006] As a method of efficiently encoding this alpha-map signal, binary image encoding (e.g., MMR (Modified Modified READ) encoding or the like), or line figure encoding (chain encoding or the like) are used. [0007] Furthermore, in order to reduce the number of encoded bits of the alpha-map, a method of approximating the contour of a given shape by polygons and smoothing it by spline curves (J. Ostermann, "Object-based analysis-synthesis coding based on the source model of moving rigid 3D objects", Signal Process.:Image Comm. Vol. 6 No. 2 pp. 143-161, 1994), a method of down-sampling and encoding an alpha-map, and approximating the encoded alpha-map by curves when it is up-sampled (see Japanese Patent Application No. 5-297133), and the like are known. [0008] When an image in a frame is broken up into a background and object upon encoding the image, as described above, an alpha-map signal that expresses the shape of the object and its position in the frame is required to extract the background and object. For this reason, this alpha-map information is encoded to form a bit stream together with encoded information of an image, and the bit stream is subjected to transmission and storage. [0009] However, in the method of dividing an image in the frame into a background and object, the number of encoded bits increases as compared to the conventional encoding method that simultaneously encodes an image in the frame, since the alpha-map must also be encoded, and the encoding efficiency lowers due to an increase in the number of encoded bits of the alpha-map. DISCLOSURE OF INVENTION [0010] It is an object of the present invention to provide a video encoding apparatus and video decoding apparatus, which can efficiently encode and decode alpha-map information as subsidiary video information that express the shape of the object and its position in a frame. [0011] According to the present invention, there is provided a video encoding apparatus which encodes an image together with an alpha-map as information for discriminating the image into an object area and background area, and encodes the alpha-map using relative address encoding, comprising means for encoding a symbol that represents a position of the next change pixel to be encoded relative to a reference change pixel as the already encoded change pixel using a variable-length coding table, and means for holding not less than two variable-length coding tables equivalent to the variable-length coding table, and switching the variable-length coding tables in correspondence with a pattern of the already encoded alpha-map. [0012] According to the present invention, there is provided a video decoding apparatus for decoding an encoded bit stream obtained by encoding of the encoding apparatus, comprising means for decoding the symbol using a variable-length coding table, and means for holding not less than two variable-length coding tables equivalent to the variable-encoding table, and switching the variable-length coding tables in correspondence with a pattern of the already decoded alpha-map. [0013] Furthermore, the means for switching the variable-length coding tables is means for switching the tables with reference to a pattern near the reference change pixel. [0014] The apparatus with the above arrangement is characterized in that a plurality of types of variable-length coding tables are prepared, and these variable-length coding tables are switched in correspondence with the pattern of the already encoded alpha-map, in encoding/decoding that reduces the number of encoded bits by encoding the symbol that specifies the position of a change pixel using the variable-length coding table. According to the present invention mentioned above, an effect of further reducing the number of encoded bits of the alpha-map can be obtained. [0015] According to the present invention, there is provided a binary image encoding apparatus which serves as an encoding circuit for a motion video encoding apparatus for encoding motion video signals for a plurality of frames obtained as time-series data in units of objects having arbitrary shapes, and has means for dividing a rectangle area including an object into blocks each consisting of M.times.N pixels (M: the number of pixels in the horizontal direction, N: the number of pixels in the vertical direction), and means for sequentially encoding the divided blocks in the rectangle area in accordance with a predetermined rule, having decoded value storage means for storing a decoded value near the block, image holding means (frame memory) for storing decoded signals of the already encoded frame (image frame), a motion estimation/compensation circuit for generating a motion estimation/compensation value using the decoded signals in the image holding means (frame memory), and means for detecting a change pixel as well as a decoded value near the block with reference to the decoded value storage means, whereby a reference change pixel for relative address encoding is obtained not from a pixel value in the block but from a motion estimation/compensation signal. [0016] There is also provided an alpha-map decoder having means for sequentially decoding a rectangle area including an object in units of blocks each consisting of M.times.N pixels in accordance with a predetermined rule, means for storing a decoded value near the block, image holding means (frame memory) for storing decoded signals of the already encoded frame (image frame), a motion estimation/compensation circuit for generating a motion estimation/compensation value using the decoded signals in the image holding means (frame memory), and means for detecting a change pixel as well as a decoded value near the block with reference to the decoded value storage means, whereby a reference change pixel for relative address encoding is obtained not from a pixel value in the block but from a motion estimation/compensation signal. [0017] With these circuits, the alpha-map information as subsidiary video information that represents the shape of an object and its position in a frame can be efficiently encoded and decoded. [0018] Furthermore, there is provided a video encoding apparatus having means for storing a decoded value near a block, image holding means (frame memory) for storing decoded signals of the already encoded frame (image frame), motion estimation/compensation circuit for generating a motion estimation/compensation value using the decoded signals in the image holding means (frame memory), means for detecting a change pixel as well as a decoded value near the block with reference to the decoded value storage means, and means for switching between a reference change pixel obtained from an interpolated pixel or decoded pixel value in the block and a reference change pixel for relative address encoding, whereby relative address encoded information is encoded together with switching information. [0019] There is also provided an alpha-map decoder having means for sequentially decoding a rectangle area including an object in units of blocks each consisting of M.times.N pixels in accordance with a predetermined rule, means for storing a decoded value near the block, image holding means (frame memory) for storing decoded signals of already encoded frame (image frame), a motion estimation/compensation circuit for generating a motion estimation/compensation value using the decoded signals in the image holding means (frame memory), and means for detecting a change pixel as well as a decoded value near the block with reference to the decoded value storage means, and also having means for switching between a reference change pixel obtained from an interpolated pixel or decoded pixel value in the block and a reference change pixel for relative address encoding, whereby a reference change pixel is obtained in accordance with switching information. [0020] In this case, upon relative address encoding, a process is done while switching whether a reference change pixel b1 is detected from a "current block" as a block of the currently processed image or from a "compensated block" as a block of the previously processed image in units of blocks, and the encoding side also encodes this switching information. The decoding side decodes the switching information, and can switch whether a reference change pixel b1 is detected from the "current block" or "compensated block" on the basis of the decoded switching information. In this fashion, an optimal process can be done based on the image contents in units of blocks, and encoding with higher efficiency can be attained. [0021] According to the present invention, a video encoding apparatus which divides a rectangle area including an object into blocks each consisting of M.times.N pixels (M: the number of pixels in the horizontal direction, N: the number of pixels in the vertical direction, and sequentially encodes the divided blocks in the rectangle area in accordance with a predetermined rule) so as to encode motion video signals for a plurality of frames obtained as time-series data in units of objects having arbitrary shapes, comprises alpha-map encoding means including a frame memory for storing a decoded signal of the current frame including decoded signals near the block and a decoded signal of the encoded frame in association with an alpha-map signal representing the shape of the object, means for replacing pixel values in the block by one of binary values, motion estimation/compensation means for generating a motion estimation/compensation value using a decoded signal of the already encoded frame in the frame memory, means for size-converting (up-sampling/down-sampling) a binary image in units of blocks, means for encoding a size conversion ratio as side information, and binary image encoding means for encoding binary images down-sampled in units of blocks. 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