Pulse or digital communications – Bandwidth reduction or expansion – Television or motion video signal
Reexamination Certificate
2001-02-13
2004-08-17
Lee, Young (Department: 2613)
Pulse or digital communications
Bandwidth reduction or expansion
Television or motion video signal
Reexamination Certificate
active
06778606
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a motion vector estimation apparatus in a digital video coding system and a method thereof, and more particularly to a motion estimation method and apparatus for, prior to a motion vector estimation operation, determining whether to perform it and selectively performing the motion vector estimation operation in accordance with the determined result, thereby reducing the amount of power consumed.
2. Description of the Prior Art
Generally, video signal compression coding and decoding can desirably reduce the capacity of memory necessary for storing image information as well as transmit the image information over a low-rate channel. In this regard, such compression coding and decoding techniques occupy a very important part of the multimedia industry requiring a variety of image applications such as image storage, image transmission, etc.
FIG. 1
is a block diagram showing the construction of a conventional digital video coding system. For the efficient video compression coding, there is generally used a method for estimating a motion vector with reference to a previous frame for the coding of a current frame, performing a motion compensated prediction operation using the estimated motion vector and coding the resulting prediction error. With reference to
FIG. 1
, the conventional video coding system comprises a transform unit
20
for performing a transform operation for a frame difference between an input current frame and a motion compensated prediction frame obtained by a motion compensation predictor
10
, a quantizer
30
for quantizing transform coefficients from the transform unit
20
for data compression, a variable length coder
40
for performing a variable length coding (VLC) operation for the transform coefficients quantized by the quantizer
30
, a dequantizer
50
and an inverse transform unit
60
. In this coding system, the frame difference is reconstructed by the dequantizer
50
and inverse transform unit
60
and applied to the motion compensation predictor
10
, which then adds it to the motion compensated prediction frame to obtain a reconstructed frame. On the other hand, the motion compensation predictor
10
performs a motion vector estimation operation using the input current frame and the previous frame and finds the prediction frame using an estimated motion vector. The motion compensation predictor
10
also performs a motion compensated prediction operation and transfers the estimated motion vector to the variable length coder
40
, which then variable length codes and transmits it together with the transform coefficients quantized by the quantizer
30
. An image information bit stream output from the variable length coder
40
is transmitted to a receiver or a multiplexer for its multiplexing with other signals.
In a general video coding method and system, motion prediction and compensation operations are not performed on a frame basis, but in the unit of a predetermined number of picture elements or pixels (M pixels in the horizontal direction and N pixels in the vertical direction, typically indicated by M×N pixels). This group of pixels is typically called a macroblock. It is generally prescribed that the macroblock be sized with 16 pixels in the horizontal direction and 16 pixels in the vertical direction (referred to hereinafter as “16×16”). In the present invention, although the size of the macroblock is not limited to a specific value, it will be described as 16×16 as an example for the convenience of description. A motion vector is two-dimensional information indicative of the quantity of motion of an object in the previous and current frames on two-dimensional X-Y coordinates. Namely, the motion vector consists of a transversal motion value and a longitudinal motion value.
FIG. 2
is a detailed block diagram of the motion compensation predictor
10
in the conventional digital video coding system of FIG.
1
. As shown in this drawing, the motion compensation predictor
10
includes a motion compensation unit
11
for performing a motion compensation operation, a motion estimation unit
12
for performing a motion vector estimation operation, and a previous image reconstruction unit
13
for obtaining a reconstructed version of the previous frame to be used to obtain the motion compensated prediction frame. The previous image reconstruction unit
13
obtains a reconstructed version of the coded frame by adding the reconstructed version of the frame difference transferred from the inverse transform unit
60
to a previous reconstructed frame stored therein. The reconstructed frame obtained by the previous image reconstruction unit
13
is then used as an input to the motion estimation unit
12
for the motion estimation of the subsequent input frame. The motion estimation unit
12
performs the motion vector estimation operation for the coding of the current input frame on the basis of the output frame from the previous image reconstruction unit
13
and the current input frame and outputs the resulting motion vector, which is then transferred to the variable length coder
40
for its variable length coding. The variable length coder
40
can code input values with a much smaller number of bits than fixed length coding (FLC) by assigning a smaller number of bits to a value with a higher generation frequency and a larger number of bits to a value with a lower generation frequency, respectively. On the other hand, the motion vector output from the motion estimation unit
12
is applied to the motion compensation unit
11
, which then performs the motion compensation operation using the applied motion vector and thus finally produces the prediction frame.
As an alternative, the previous original frame may be used as an input frame for the motion estimation instead of the previous reconstructed frame. In this case, a memory for storing the previous original frame must be provided in the construction of FIG.
2
. It should be noted that the use of the previous original frame as the reference frame is valid only for the motion estimation and the previous reconstructed frame is always used as the input to the motion compensation unit. The reason is that a decoder is able to perform a decoding operation on the basis of the previous reconstructed frame. Hereinafter, both the frames will be referred to as reference frames.
A motion vector has a close correlation with the surrounding blocks or macroblocks because of image characteristics. Accordingly, the coding efficiency can be increased by variable length coding a difference vector between a current motion vector and a motion vector predicted using motion vectors of the surrounding blocks or macroblocks, rather than directly variable length coding the current motion vector. The reason is that the difference vector is 0 or a value approximate to 0 at a significantly high frequency.
FIG. 3
is a view showing the position of a macroblock to be currently coded and the positions of the surrounding macroblocks used for the prediction of a motion vector of the macroblock to be currently coded. Generally, a predicted motion vector can be determined using adjacent motion vectors in the following manner. That is, the predicted motion vector is determined as the median value of motion vectors of three macroblocks, or the left macroblock MB_A, the upper macroblock MB_B and the upper right macroblock MB_C, around a macroblock (MB_X) to be currently coded. A difference vector between the determined predicted motion vector and the current motion vector is obtained and then variable length coded. This method is typically used in ITU-T H.263 and ISO/IEC MPEG-4.
FIG. 4
is a detailed block diagram of the motion estimation unit
12
in the motion compensation predictor
10
of FIG.
2
. For motion vector estimation, there have generally been proposed a full search method, pyramidal search method, three-step search method, four-step search method, etc., which have different computational complexities. P
Chung Jae-Won
Kang Hyun Soo
Kim Kyeong Joong
Hyundai Curitel Inc.
Lee Young
Merchant & Gould P.C.
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