GB2410643A

GB2410643A - Motion vector labelling in frequency of occurrence and run length encoding of labels

Info

Publication number: GB2410643A
Application number: GB0509287A
Authority: GB
Inventors: Michael James Knee; Violet Snell
Original assignee: Snell and Wilcox Ltd
Current assignee: Snell Advanced Media Ltd
Priority date: 2001-01-24
Filing date: 2001-01-24
Publication date: 2005-08-03
Anticipated expiration: 2021-01-24
Also published as: GB0509287D0; GB2371933A; GB2410643B; GB0101875D0; GB2371933B

Abstract

The data rate of motion vector information is reduced by labelling the vectors in descending order of frequency, and run length encoding the labels. Preferably the labels are scanned in a pattern so as to increase the expected run lengths eg, raster, boustrophedon, block-based or spiral scanning or using a space filling curve such as Peano or Hilbert scan. The frequency of occurrence of vectors may be determined separately for each picture and both horizontal and vertical components are taken in consideration in determining the frequency of occurrence of vectors.

Description

COMPRESSION OF MOTION VECTORS

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This invention relates to methods of compressing motion vectors including, for example, the motion vectors that are used in motion compensated video compression schemes such as MPEG-2 and MPEG.

One of the key features of MPEG-2 compression is the use of motion compensated prediction, in which blocks of video samples are predicted from other frames or fields that have already been encoded and decoded. Highly efficient compression is achieved by transmitting the prediction error rather than the original samples. The prediction process compensates for the motion between predicted and reference pictures by applying a motion vector to each block. It is then necessary to transmit those motion vectors as part of the compressed bitstream, because the decoder needs to reconstruct the prediction that was made in the encoder. The transmission of motion vectors incurs an inevitable overhead in bit rate. In the MPEG-2 standard, this overhead is reduced by compressing the motion vectors.

The motion vector compression algorithm in the MPEG-2 standard consists of two elements: forming differences between motion vectors in adjacent blocks and transmitting these differences using variable-length coding. This algorithm is a compromise between efficiency and the relatively limited hardware complexity available in an MPEG-2 decoder.

Another application of motion vector compression is in generating a compressed version of the Information Stream. The Information Stream is a data stream consisting of MPEG-2 coding parameters, formatted in such a way that it can be used to aid MPEG-2 re-encoding of decoded video signals in order to avoid cascading impairments. Reference is directed in this context to WO 98/03017. There is in certain cases a requirement for a compressed version of the Information Stream, because the channel capacity available may be limited. One method of reducing the bit rate of the Information Stream is to use the MPEG-2 syntax itself to transmit the coding parameters.

In this case it Is sometimes found necessary to reduce the bit rate even further, and this can be done by selectively modifying some motion vectors, as described in WO 99/38327.

It is an object of the present Invention to provide Improved techniques for the compression of motion vectors.

Accordingly, the present invention consists, In one aspect, in a process for reducing the data rate of motion vector Information, comprising labelling the motion vectors in descending order of frequency of occurrence, and run length encoding the labels Advantageously, the motion vector labels are run-length encoded using a scanning pattern designed to increase the expected run lengths.

There is also described a process for reducing the data rate of motion vector Information, In which the motion vectors are run length coded using a scanning pattern designed to Increase the expected run lengths of quantised motion vectors Advantageously, the vectors are transmitted by run-length encoding in descending order of frequency of occurrence There is further described a process for reducing the data rate of motion vector Information, in which the components of the motion vectors are quantised, the quantsed values are compressed and transmitted and the quantisation errors are additionally transmitted, whenever sufficient channel capacity is available.

Advantageously, the motion vectors are compressed using run length coding Preferably, the motion vectors are scanned using a scanning pattern designed to increase the expected run lengths of quantised motion vectors The invention will now be described by way of example, with reference to the accompanying drawings, in which figure 1 Is a block diagram Illustrating one embodiment of the invention, Figure 2 is a series of diagrams showing examples of scanning patterns for use in the present invention; and Figure 3 is a flow chart illustrating a further aspect of the invention.

The method of motion vector compression which will be described in more detail below is based on three inventive features (which will have utility alone and in other combinations): quantising the motion vectors, with transmission of the quantising error only if sufficient bandwidth remains after the quantised vectors have been transmitted, scanning the motion vectors for run-length coding in such a way as to maximise run- lengths given the two-dimensional nature of areas of constant quantised motion vectors, run-length coding of quantised motion vectors, or of labels representing them, in descending order of their frequency of occurrence.

Referring to Figure 1, the first step is to quantise the motion vectors that are to be encoded. The simplest way to do this is to remove a fixed number (for example, two) of least-significant bits from the two's-complement representation of each component of the vector. If horizontal and vertical component are each represented with 9 bits, the quantisation produce would thus reduce the vectors to 14 bits in total. The remaining 4 bits will then be compressed using the techniques described here, while the least-significant bits will be transmitted if sufficient capacity remains in the channel.

The second step is to scan the quantised motion vectors, in such a way as to try to maximise the expected run-lengths of the vectors. Possible scanning patterns include: straightforward raste m row oy row! scanning of the.tor - 4 boustrophedon scanning block-based scanning spiral scanning scanning using a space-filling curve such as the Peano or Hilbert scan Examples of these patterns are shown in Figure 2.

Motion vectors, if properly measured, will tend for most picture material to vary smoothly across and down a picture. Where vectors are appropriately O quantised, there is therefore a relatively high probability that the motion vectors of neighbouring blocks will take the same value. Run length coding along a scan path which connects both horizontally and vertically adjacent blocks, will therefore increase coding efficiency.

The third step is to run-length encode the quantised motion vectors in order of decreasing frequency of occurrence. For this purpose, the horizontal and vertical components of the vectors are taken together. The vector values are sorted according to their frequency of occurrence within the picture. The occurrences of the most frequent vector are then transmitted in the chosen scanning order using run-length coding, for example using the method shown in Figure 3. That vector is then removed from the list and the occurrences of the next most frequent vector are in turn transmitted using run-length coding.

The process is repeated until one or more of the following conditions is met, the choice being a question of configuration of the system: a specified number of most frequent vectors from the list have been processed a specified number of occurrences have been transmitted the number of vectors represented by the current element in the list falls below a certain value The remaining quantised vectors are then transmitted in the order in which they occur in the scan, either by variable-length or by fixed-length coding. In extreme c,,cumstar,ces warren the channel canacitv is nsuffcient - 5 for these vectors, they are omitted from the bitstream. However, the invention is intended for use when this problem occurs extremely rarely.

Finally, the quantising errors are transmitted in descending order of bit significance, until the capacity of the channel is reached.

It will be noted that Figure 1 shows a multiplexer at the output of the system. This does nut add to the unctior-aiity of the invention itself, but merely serves to indicate that the output is formed into a single bitstream.

The invention in its various aspects has a number of important advantages. The motion vector compression scheme is very efficient, particularly when the input vectors have high spatial correlation, as is the case when phase correlation is used as the method of motion estimation.

The method is scalable in that the most important information is transmitted first.

It should be understood that this invention has been described by way of example only and that a wide variety of modifications are possible without departing from the scope of the invention. Thus, the division of the motion vector bit rate into 14 MSB's and 4 LSB's is of course only one alternative.

The described process of separately run length coding the vectors in order of frequency, has the advantage of enabling information about the least frequent vectors to be discarded first. Other approaches exist, including direct run length coding of the quantised vectors along the chosen, scan path.

Claims

1 A process for reducing the data rate of motion vector information, comprising rebelling the motion vectors in descending order of frequency of occurrence, and run length encoding the labels

2. A process according to Claim 1, in which the motion vector labels are run-length encoded using a scanning pattern designed to increase the expected run lengths.

3. A process according to Claim 1 or Claim 2, in which the frequency of occurrence of vectors is determined separately for each picture

4. A process according to Claim 3, in which both the horizontal and vertical components of each vector are taken in consideration in the determining the frequency of occurrence of vectors.