Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Patent
1998-07-16
2000-12-19
Lee, Richard
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
348699, H04N 732
Patent
active
061635758
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention generally relates to video compression. More precisely, the invention relates to a method of coding an estimated motion field and for generating motion information in a video sequence.
BACKGROUND OF THE INVENTION
The motion compensated prediction is a key element of majority of video coding schemes. FIG. 1 is a schematic diagram of an encoder for compression of video sequences using motion compensation. Essential elements in the encoder are a motion compensated prediction block 1, a motion estimator 2 and a motion field coder 3. The operating principle of the motion compensating video coders is to compress the prediction error E.sub.n (x,y), which is a difference between the incoming frame I.sub.n (x,y) being coded called the current frame and a prediction frame I.sub.n (x, y), wherein: compensated prediction block 1 and is built using pixel values of the previous, or some other already coded frame denoted I.sub.n-1 (x, y), called a reference frame, and the motion vectors of pixels between the current frame and the reference frame. Motion vectors are calculated by the motion field estimator 2 and the resulting vector field is then coded in some way before applying to the predictor block 1. The prediction frame is then: vector of pixel in location (x, y) in the current frame, whereas .DELTA.x(x,y) and .DELTA.y(x,y) are the values of horizontal and vertical displacement of this pixel. Set of motion vectors of all pixels in the current frame I.sub.n (x,y) is called motion vector field. The coded motion vector field is also transmitted as motion information to the decoder.
In the decoder, FIG. 2, pixels of the current frame I.sub.n (x,y) are reconstructed by finding the pixels' predictions I.sub.n (x, y) in the reference frame I.sub.n-1 (x, y). The motion compensated prediction block 21 generates the prediction frame using the received motion information and the reference frame I.sub.n-1 (x, y) (in this picture the reference frame is the same as the current frame). In the prediction error decoder 22 decoded prediction error E.sub.n (x,y) is then added with the prediction frame, the result being the original current frame I.sub.n.
The general object of the motion compensated (MC) prediction is to minimize amount of information which needs to be transmitted to the decoder. It should minimize the amount of prediction error measured, e.g., as the energy of E.sub.n (x,y), and minimize the amount of information needed to represent motion vector field.
The document H. Nguen, E. Dubois, "Representation of motion information for image coding". Proc. Picture Coding Symposium '90, Cambridge, Mass., Mar. 26-18, 1990, pages 841-845, gives a review of motion field coding techniques. As a rule of the thumb reduction of prediction error requires more sophisticated motion field, i.e., more bits must be spent on its encoding. Therefore the overall goal of the video encoding is to encode as compactly as possible the motion vector field keeping at the same time the measure of prediction error as low as possible.
The motion field estimation block 1, FIG. 1, calculates motion vectors of all the pixels of a given segment which minimize some measure of prediction error in this segment, for example square prediction error. Motion field estimation techniques differ both in the model of the motion field and in the algorithm for minimisation of the chosen measure of prediction error.
Due to very large number of pixels in the frame it is not efficient to transmit a separate motion vector for each pixel. Instead, in most of the video coding schemes the current frame is divided into larger image segments so that all motion vectors of the segment can be described by few parameters. Image segments can be square blocks, e.g. 16.times.16 pixels blocks are used in codecs in accordance with international standard ISO/IEC MPEG-1 or ITU-T H.261, or they can comprise of completely arbitrarily shaped regions obtained for instance by a segmentation algorithm. In practice segments include at least few te
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Karczewicz Marta
Nieweglowski Jacek
Lee Richard
Nokia Mobile Phones Limited
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