PICTURE DATA ENCODING METHOD

Abstract: When transformed and quantized blocks of picture data are encoded, a block being constituted by a matrix of coefficients, the events within each block to be encoded comprise firstly a run indicating the number of zero-coefficients in the block between a current non-zero coefficient and the previous non-zero coefficient, and secondly, the size of the current non-zero coefficient of the block. Each event is in accordance with the invention completed by adding a third parameter which indicates whether the current non-zero coefficent is the last non-zero coefficent of the block. Hence, each event is represented as a three-dimensional quantity.

Description: PICTURE DATA ENCODING METHOD The present invention relates to a method for encoding picture data, and more specifically encoding of transformed and quantized blocks of picture data, optionally groups of < * -* such blocks, and said method is primarily of the type appearing from the preamble of the enclosed patent claim 1. Lately digital compression techniques have been put into use in order to reduce the bit consumption in the represen¬ tation of single pictures or picture sequences (live pictures) . These techniques are used e.g. in picture tele¬ phony. The picture telephony encoding technique itself is standardized in the international telecommunication organiza¬ tion CCITT. The standardizing work was conducted by an expert group under CCITT SG XV WP XV/1, and the resulting recommenda¬ tion has the number J.261. Also within the international standardizing organization ISO work has been done to provide standards for encoding both live pictures and single pictures (still pictures) . The groups responsible for these activities have designations ISO/IEC JTC1/SC2/WG11 (for live pictures) and ISO/IEC JTC1/SC2/WG10 (for still pictures) . The methods which are usually called MTEG and JPEG, are described in respectively ISO/IEC JTC1 Committee Draft 10918 and ISO/IEC JTC1 Committee Draft 11172. A common feature for all these techiques is that blocks of picture values are processed. By applying a suitable transformation to the block, the result will be a block in the transform plane where the energy content is concentrated toward a block corner. Usually a discrete cosine transform (DCT) is used. All the three above mentioned method use DCT. The transformed blocks are quantized, and the quantized values are thereafter encoded with a so-called "variable length code". < While the quantization introduces errors in the recon¬ structed signal (i.e. is irreversible), the variable length type encoding is a reversible process. This means that it is possible to reconstruct excactly that which has been encoded with a variable length code. The purpose of using encoding of the variable length type, is to reduce the data amount trans¬ mitted, without changing the information content. Events of small probability are encoded with many bits, while events of great probability are encoded using few bits. The so-called "Huffman encoding" is an example of encoding of the variable length type. It is a condition for improvement when the variable length coding is used, that different events occur with different probabilities. Thus, DCT transformed blocks of picture data are suitable for the variable length type encoding. It is possible to use one or several input values to the variable length code. When several values are used, the case is what can be called a multi-dimensional variable length code. In the coding technique which is standardized in CCITT, and in the encoding methods now presented for standardization within the ISO, two-dimensional variable length codes are used. The quantized transform coefficients in the transformed block are processed in a certain consecutive order. One input value is the level of the quantized transform coefficient, while the second value is the number of transform coefficients equal to zero since the last encoded (non-zero) coefficient. Thus, only non-zero coefficients are encoded in this case. (Encoding in this manner, i.e. only encoding values which are different from a given value, and indicating "travel length" in the block since the last encoded value, is termed "run length coding". ) When the last non-zero coefficient in the block has been coded, a codeword is. transmitted (the codeword is here termed EOB, "end of block") to signal that no more non-zero coefficients remain in the block. The use of EOB is suitable because it is often very many zero coefficients after the last non-zero coefficient in a block, unless the block is very small. The enclosed fig. 1 shows the consecutive order for processing coefficients in a block, as it is conducted according to the CCITT standard algorithm for picture tele¬ phony. Further, fig. 2 shows a block containing quantized coefficients and the resulting events to be encoded when the CCITT standard algorithm is used. The transition from EVENT to a variable length codeword can be done merely by using a look-up table. The values of RUN and LEVEL are entered in a table, and provide the codeword for this event. This is illustrated in a schematical manner in the enclosed fig. 3. a) Transforming picture data blocks using DCT, b.) quan¬ tizing the transform coefficients and c) variable length encoding them as here described, are using three techniques which can be used as respective single techniques. However, a) and b) are methods which when applied to picture data, provide a result which is rather well suited for method c) . The method in accordance with the invention is directed to encoding transformed and quantized blocks of picture data, optionally groups of such blocks, where a block is constituted by a matrix of coefficients, and where the energy content in a block is increasing toward a block corner. The events within each block or group of blocks to be encoded, comprise (a) a run indicating the number of zero-coefficients in the block/group between a current non-zero coefficient and the previous non-zero coefficient, where the run direction in the block/group is predetermined, and (b) the size of the current non-zero coefficient in the group/block. Each event is listed in a series of successive events, and the particular event is thereafter re-encoded via a look¬ up table to a variable length code. The method is particu¬ larly characterized in that each event is completed by (c) a third parameter indicating whether the current non-zero coefficient is the last non-zero coefficient in the block, said third parameter having only two possible values, each event thereby being represented as a three-dimensional quantity. In one embodiment of the invention, the run direction within the block follows the CCITT standard algorithm for block coefficient encoding. When a group of blocks is to be encoded, this is in a favourable embodiment of the invention done by arranging the group as a matrix of all coefficients in each block up to the last non-zero coefficient in the block, in a column, and with juxtaposed columns block by block. In the last mentioned case one uses preferentially a run direction within the group matrix which is simple row by row, while ensuring that group matrix positions without any coefficient do not count when calculating the run value. A closer study will now be made regarding the particular features of the invention, while referring to the enclosed drawings which indicate example embodiments of the method in accordance with the invention. Fig. 1 shows, as previously mentioned, an example of a consecutive order for encoding coefficients in accordance with the CCITT, fig. 2 shows an example of two-dimensional variable length coding used in CCITT, fig. 3 shows in a schematical manner the use of a look-up table for codewords with a two-dimensional variable length code, fig. 4 shows three-dimensional variable length coding with several blocks, and fig. 5 shows in a schematical manner the use of a look-up table for codewords with a three-dimensional variable length code. The new variable length code will use three input values; 1. the value of the the transform coefficient (which is different from zero) , 2. the run length (the number of zero coefficients since the last non-zero coefficient) , 3. a parameter to show whether the coefficient is the last non-zero coefficient in the block or not. Related to the standard algorithms which have been described above, the values indicated in items 1 and 2 above can be found in a corresponding manner. The difference is that the codeword for the last non-zero coefficient is integrated in the variable length code itself, i.e. it does not appear as a separate codeword. The purpose hereof is to enable a better adaptation of the codeword length for different events, to the probability that a particular event should occur. (Please note: An event is here defined as a combination of the values 1, 2 and 3 above.) Hence it should be possible to transmit the same information using a lesser amount of data. When looking at figs. 1 and 2, the following sequence of events will be found to need encoding with a three-dimensional variable length code: EVENT = (run, level, not last non-zero coeff. (I)/ last non-zero coeff. (S) ) EVENTS = (0,3,1) (1,2,1) (7,1,S) The application which has been described above, is based upon execution of the encoding for one block at a time. Another application is based upon encoding several blocks at the same time. One faces the same starting point as before, but in this case there is more than one block which have been transformed and quantized, and which are to be encoded one at a time. One may visualize that all coefficients in a block are laid down in a consecutive order one after the other (i.e. from coefficient no. 1 to cofficient no. 64 for the situation appearing in fig. 1) . This is done for all blocks which are to be encoded simultaneously. It is assumed that only blocks containing one or several non-zero coefficients are to be encoded. It is also necessary to have knowledge of the number of these "non-zero blocks". Fig. 4 shows an example of six blocks, and wherein the highest number of a non-zero coeffi¬ cient is 5 (block 2). Thereafter the three-dimensional variable length code is used, and encoding is executed in consecutive order with coef¬ ficient no. 1 in block 1-6, thereafter coefficient no. 2 in block 1-6 etc. Coefficient no. 2 in block 4 is the last non¬ zero coefficient in block 4. The input values to the variable length code for this coefficient will be (run = 2, level = 1,S (the last non-zero coefficient) ) . The information regarding the fact that this was the last non-zero coefficient of block 4, is stored. When encoding coefficients with a higher number, block 4 will not be taken into consideration, i.e. block 4 will then not contribute to an increase of the RUN value. If fig. 4 is used to examplify this situation, the following events will be the ones to be encoded: EVENT = (run, level, not last non-zero coeff. (I)/ last non-zero coeff. (S) ) EVENTS = (0,5,1) (0,7,1) (0,3,1) (0,1,1) (0,4,1) (0,8,1) (0,2,1) (2,1,S) (1,3,1) (1,3,1) (0,2,S) (0,2,S) (1,1,S) (0,1,1) (0,2,S) (0,1,S) The codewords are found by use of a look-up table, and in this connection it is referred to the schematical representation of fig. 5. The three input parameters, i.e. run, level and "I/S" parameter, produce one single output value which represents the three-dimensional event.

Claims: P A T E N T C L A I M S 1. A method for encoding transformed and quantized blocks of picture data, optionally groups of such blocks, where a block is constituted by a rcatrix of coefficients, and the energy content of a block is increasing toward a block corner, the events within each block or group of blocks to be encoded, comprising (a) a run indicating the number of zero-coefficients in the block/group between a current non-zero coefficient and the previous non-zero coefficient, the run direction in said block/group being predetermined, and (b) the size of said current non-zero coefficient in said group/block, each event being entered in a series of successive events and the respective event thereafter being re-encoded to a variable length code by using a look-up table, c h a r a c t e r i z e d in that each event is completed by adding (c) a third parameter stating whether said current non¬ zero coefficient is the last non-zero coefficient of the block, said third parameter having only two possible values, each event thereby being represented as a three-dimensional quantity. 2. The method of claim 1, wherein a block is to be encoded, c h a r a c t e r i z e d in that the run direction within said block follows the CCITT standard algorithm for block coeffecient encoding. 3. The method of claim 1, wherein a group of blocks is to be encoded, c h a r a c t e r i z e d in that said group is arranged as a matrix of all coefficients in each block up to the last non¬ zero coefficient in the block, in a column, with juxtaposed columns block by block. 4. The method of claim 3, c h a r a c t e r i z e d in that the run direction within said group matrix is simple row by row, positions in said group matrix without any coefficient not being taken into con¬ sideration when calculating the run value.

Inventors: BJOENTEGAARD GISLE, BJOENTEGAARD, GISLE
Applicants: TELEDIREKTORATETS FORSKNINGSAV, TELEDIREKTORATETS FORSKNINGSAVDELING
Application year: 1993
Main classification: H03M7
IPC codes: G06T9, H03M7, H04N1, H04N7