Image analysis – Image compression or coding – Interframe coding
Reexamination Certificate
2000-09-07
2003-04-01
Couso, Jose L. (Department: 2621)
Image analysis
Image compression or coding
Interframe coding
Reexamination Certificate
active
06542643
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the coding of video sequences, and, more particularly, to a method for measuring the activity of a portion of a picture to improve the effectiveness of the buffering that is performed during the coding process, especially in low cost applications.
This is useful in digital video coders where it is necessary to evaluate the activity of a block of information in the frequency domain. Due to the importance of the MPEG standard in treating digitized video sequences, reference will be made to an MPEG2 system to illustrate the present invention.
BACKGROUND OF THE INVENTION
The MPEG (Moving Pictures Experts Group) standard defines a set of algorithms dedicated to the compression of sequences of digitized pictures. These techniques are based on the reduction of the spatial and temporal redundance of the sequence. Reduction of spatial redundance is achieved by compressing independently the single images via quantization, discrete cosine transform (DCT) and Huffman coding.
The reduction of temporal redundance is obtained using the correlation that exists between successive pictures of a sequence. Each image can be expressed locally as a translation of a preceding and/or successive image of the sequence. To this end, the MPEG standard uses three kinds of pictures; I (Intra Coded Frame), P (Predicted Frame) and B (Bidirectionally Predicted Frame). The I pictures are coded in a fully independent mode. The P pictures are coded with respect to a preceding I or P picture in the sequence. The B pictures are coded with respect to two pictures of the I or P kind, which are the preceding one and the following one in the video sequence (see FIG.
1
).
A typical sequence of pictures can be I B B P B B P B B I B . . . , for example. This is the order in which they will be viewed. Given that any P is coded with respect to the preceding I or P, and any B is coded with respect to the preceding and following I or P, it is necessary that the decoder receive the P pictures before the B pictures, and the I pictures before the P pictures. Therefore, the order of transmission of the pictures will be I P B B P B B I B B . . . .
Pictures are processed by the coder sequentially, in the indicated order, and are successively sent to a decoder which decodes and reorders them, thus allowing their successive displaying. To code a B picture it is necessary for the coder to keep in a dedicated memory buffer, called frame memory, the I and P pictures, coded and thereafter decoded, to which current B picture refers, thus requiring an appropriate memory capacity.
One of the most important functions in coding is motion estimation. Motion estimation is based on the following consideration. A set of pixels of a frame of a picture may be placed in a position of the successive picture obtained by translating the preceding one. These transpositions of objects may expose parts that were not visible before as well as changes of their shape, such as during a zooming, for example.
The family of algorithms suitable to identify and associate these portions of pictures is generally referred to as motion estimation. Such an association of pixels is instrumental to calculate a different picture removing redundant temporal information, thus making more effective the successive processes of DCT compression, quantization and entropic coding.
A typical example of a system using this method may be illustrated based upon the MPEG2 standard. A typical block diagram of a video MPEG2 coder is depicted in FIG.
1
. Such a system is made of the following functional blocks:
1) Chroma Filter Block From 4:2:2 to 4:2:0. In this block there is a low pass filter operating on the chrominance component, which allows the substitution of any pixel with the weighed sum of neighboring pixels placed on the same column and multiplied by appropriate coefficients. This allows a successive subsampling by two, thus obtaining a halved vertical definition of the chrominance.
2) Frame Ordering Block. This block is composed of one or several frame memories outputting the frames in the coding order required by the MPEG standard. For example, if the input sequence is I B B P B B P etc., the output order will be I P B B P B B . . . .
The Intra coded picture I is a frame or a semi-frame containing temporal redundance. The Predicted-picture P is a frame or semi-frame from which the temporal redundance with respect to the preceding I or P (precedingly co/decoded) has been removed. The Biredictionally predicted-picture B is a frame or a semi-frame whose temporal redundance with respect to the preceding I and successive P (or preceding P and successive P) has been removed. In both cases the I and P pictures must be considered as already coded/decoded.
Each frame buffer in the format 4:2:0 occupies the following memory space:
Standard PAL
720 × 576 × 8 for the luminance
(Y) = 3,317,760 bits
360 × 288 × 8 for the chrominance
(U) = 829,440 bits
360 × 288 × 8 for the chrominance
(V) = 829,440 bits
total Y + U + V = 4,976,640 bits
Standard NTSC
720 × 480 × 8 for the luminance
(Y) = 2,764,800 bits
360 × 240 × 8 for the chrominance
(U) = 691,200 bits
360 × 240 × 8 for the chrominance
(V) = 691,200 bits
total Y + U + V = 4,147,200 bits
3) Estimator. This is the block that removes the temporal redundance from the P and B pictures. This functional block operates only on the most energetic component, and, therefore, the richest of information of the pictures which compose the sequence to code, such as the luminance component.
4) DCT. This is the block that implements the discrete cosine transform according to the MPEG2 standard. The I picture and the error pictures P and B are divided in blocks of 8*8 pixels Y, U, and V on which the DCT transform is performed.
5) Quantizer Q. An 8*8 block resulting from the DCT transform is then divided by a quantizing matrix to reduce the magnitude of the DCT coefficients. In particular, the cosine transformed matrix of the macroblock is divided by the matrix mQuant*Quantizer_Matrix, where Quantizer_Matrix is a priori established and can vary from picture to picture. In such a case, the information associated to the highest frequencies, less visible to human sight, tends to be removed. The result is reordered and sent to the successive block.
6) Variable Length Coding (VLC). The codification words output from the quantizer tend to contain a large number of null coefficients followed by nonnull values. The null values preceding the first nonnull value are counted and the count figure forms the first portion of a codification word, the second portion of which represents the nonnull coefficient.
These pair tend to assume values more probable than others. The most probable ones are coded with relatively short words composed of 2, 3 or 4 bits while the least probable are coded with longer words. Statistically, the number of output bits is less than in the case such a criteria is not implemented.
7) Multiplexer and Buffer. Data generated by the variable length coder, the quantizing matrices, the motion vectors and other syntactic elements are assembled for constructing the final syntax contemplated by the MPEG2 standard. The resulting bitstream is stored in a memory buffer, the limit size of which is defined by the MPEG2 standard requirement that the buffer cannot be overfilled. The quantizer block Q attends to such a limit by making the division of the DCT 8*8 blocks dependent upon how far the system is from the filling limit of such a memory buffer and on the energy of the luminance component of the 16*16 source macroblock taken upstream of the motion estimation, of the prediction error generation, and the DCT transform.
8) Inverse Variable Length Coding (I-VLC). The variable length coding functions specified above are executed in an inverse order.
9) Inverse Quantization (IQ). The words output by th
Bortot Luca
Pau Danilo
Sacchi Maria Luisa
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Couso Jose L.
Jorgenson Lisa K.
Microelectronics S.r.l.
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