EP3058737A1

EP3058737A1 - Method for encoding and decoding images, device for encoding and decoding images, and corresponding computer programmes

Info

Publication number: EP3058737A1
Application number: EP14796227.8A
Authority: EP
Inventors: Félix Henry; Bihong HUANG
Original assignee: Orange SA
Current assignee: Orange SA
Priority date: 2013-10-15
Filing date: 2014-10-13
Publication date: 2016-08-24
Also published as: CN105745928A; FR3012004A1; US20160269738A1; WO2015055937A1

Abstract

The invention concerns the encoding of at least one image (ICj) divided into blocks, characterised in that it comprises, for a current block (Bu) to be encoded, the steps of: - determining (C3) a set of candidate predictive blocks (BP1 ₁; BP12,..., BP1 V,..., BP1 Q), - for at least one candidate predictive block (BP1 V) from said set: obtaining (C4) a residual bloc representative of the difference between the candidate predictive block and the current block (Bu), identifying (C10), in said set of candidate predictive blocks, a candidate predictive block, said identification being a function of said obtained current residual block, selecting (C12a)) said at least one candidate predictive block if it is equal to the identified predictive block, - determining (C13), from the candidate predictive blocks likely to have been selected at the end of the selection step, a candidate predictive block (BP1 opt), by means of a predefined criterion (J), - encoding (C15-C17) the residual block representative of the difference between the determined candidate predictive block and the current block (Bu).

Description

METHOD FOR ENCODING AND DECODING IMAGES, DEVICE FOR ENCODING AND DECODING IMAGES AND PROGRAMS

CORRESPONDING COMPUTER FIELD OF THE INVENTION

The present invention relates generally to the field of image processing and more specifically to the coding and decoding of digital images and digital image sequences.

The encoding / decoding of digital images applies in particular to images from at least one video sequence comprising:

images coming from the same camera and succeeding each other temporally (coding / decoding of 2D type),

images from different cameras oriented according to different views (coding / decoding of 3D type),

- corresponding texture and depth components

(3D type encoding / decoding),

- etc.

The present invention applies similarly to the coding / decoding of 2D or 3D type images.

The invention may especially, but not exclusively, apply to video coding implemented in current AVC and HEVC video encoders and their extensions (MVC, 3D-AVC, MV-HEVC, 3D-HEVC, etc.), and to corresponding decoding. Prior art

The images and digital image sequences occupy a lot of space in terms of memory, which requires, when transmitting these images, to compress them in order to avoid congestion problems on the communication network used for this transmission. , the flow rate usable on it is generally limited. This compression is also desirable for storing these data.

Many techniques of video data compression are already known. Among these, many video coding techniques, in particular the HEVC technique, use techniques for spatially or temporally prediction of groups of pixel blocks of a current image relative to other groups of blocks of pixels belonging to the same image or to a previous or next image.

More precisely, according to the HEVC technique, I-images are coded by spatial prediction (intra prediction), and P and B-images are coded by temporal prediction (inter prediction) with respect to other I, P or B-encoded images. decoded using motion compensation.

For this purpose, the images are cut a first time in blocks of pixels called CTU (abbreviation of "Coded Treeblocks Unit") which are similar to the macroblocks of the H.264 standard. These blocks can then be subdivided into smaller blocks, each of these smaller blocks or each CTU block being coded by intra prediction or between images.

According to the HEVC technique, when a CTU block is subdivided into smaller blocks, a data signal, corresponding to each block, is transmitted to the decoder. Such a signal includes:

residual data which are the coefficients of the quantized residual blocks,

coding parameters which are representative of the coding mode used, in particular:

• the prediction mode (intra prediction, inter prediction, prediction by default carrying out a prediction for which no information is transmitted to the decoder ("in English" skip "));

· Information specifying the type of prediction

(orientation, reference image, ...);

• the type of subdivision;

• the type of transform, for example DCT 4x4, DCT 8x8, etc.

· Movement information if necessary

• etc.

The decoding is done image by image, and for each image, block CTU per block CTU. For each smaller block of a CTU block, the elements corresponding stream are read. Inverse quantization and inverse transformation of the coefficients of the smaller blocks are performed. Then, the prediction of each CTU block is calculated and each CTU block is reconstructed by adding the prediction to the decoded prediction residue.

Intra or inter-competition coding, as implemented in the HEVC standard, thus relies on the putting into competition of different coding parameters, such as those mentioned above, in order to select the best mode of coding, which is that is to say, that which will optimize the coding of the block considered according to a predetermined performance criterion, for example the cost rate / distortion well known to those skilled in the art.

The coding parameters relating to the selected coding mode are contained in the data stream transmitted by the coder to the decoder, in the form of identifiers generally called competition indices. The decoder is thus able to identify the coding mode selected to the encoder, then to apply the prediction according to this mode.

The bandwidth allocated to these competition indices is not negligible, since it reaches around 30%. It also tends to increase due to the ever-increasing contribution of new coding parameters such as new dimensions and / or pixel block shapes, new Intra, Inter, etc., prediction parameters.

Object and summary of the invention

One of the aims of the invention is to overcome disadvantages of the state of the art mentioned above.

For this purpose, an object of the present invention relates to a method of coding at least one image cut into blocks.

Such a coding method is remarkable in that it comprises, for a current block to be coded, the steps of:

determination, of a set of candidate predictor blocks, for at least one candidate predictor block of the aforementioned set:

Obtaining a residual block representative of the difference between the candidate predictor block and the current block, identifying, in the set of candidate predictor blocks, a candidate predictor block, such identification being a function of the current residual block obtained,

Selecting said at least one candidate predictor block if it is equal to the identified predictor block,

determining, among the candidate predictor blocks that may have been selected at the end of the selection step, a candidate predictor block, using a predetermined criterion,

coding of the residual block representative of the difference between the determined candidate predictor block and the current block.

Such an arrangement makes it possible, during the coding of an image, to avoid including in the signal to be transmitted to the decoder the indices of the predictor blocks which are used to predict the blocks of the image respectively. This results in a significant decrease in the cost of signaling, insofar as such a provision is reproducible to the decoder.

In addition, the identification of candidate predictor blocks for the prediction of the current block is particularly reliable. It results from the fact that for a current residual block considered, the characteristics of the candidate predictor blocks are very different from each other, which facilitates the final selection of the most suitable candidate predictor block during the determination step according to a predetermined criterion. .

According to a particular embodiment, the blocks of the image preceding the current block being coded in a determined order, the aforementioned identification is a function of the pixels of the previously coded image.

Such an arrangement thus makes it possible to take account of information of the image which is already available at the time of coding of the current block, thus increasing the performance of the identification of the candidate predictor blocks.

According to a preferred embodiment of the invention, the previously coded pixels of the image are located along the current block. Such an arrangement thus makes it possible to minimize the discontinuities likely to appear along the boundaries of the current block, while corresponding better to the reality of the image.

According to a particular embodiment, the predetermined criterion is the minimization of the bitrate-distortion cost of the image.

The choice of such a criterion optimizes the prediction made to the coding.

According to a particular embodiment, the current block is a block that has previously been obtained as a result of a prediction.

Such an arrangement is intended to further refine the prediction of the current block so as to obtain optimized coding performance.

The various embodiments or aforementioned embodiments can be added independently or in combination with each other, to the steps of the coding method as defined above.

The invention also relates to a device for encoding at least one image cut into blocks, such a device being remarkable in that it comprises, for a current block to be encoded:

a determination module, a set of candidate predictor blocks,

for at least one candidate predictor block of the set:

A module for obtaining a residual block representative of the difference between the candidate predictor block and the current block,

An identification module, in the set of candidate predictor blocks, of a candidate predictor block, as a function of the current residual block obtained,

A module for selecting said at least one candidate predictor block if it is equal to the identified predictor block,

a determination module, among the candidate predictor blocks that may have been selected at the end of the selection step, of a candidate predictor block, using a predetermined criterion,

a module for coding the residual block representative of the difference between the determined candidate predictor block and the current block. Such a coding device is able to implement the aforementioned coding method.

The invention also relates to a method of decoding a data signal representative of at least one image cut into blocks, such a method comprising the steps of:

determination, in the data signal, of data representative of a current residual block associated with a current block to be decoded,

- decoding of the current residual block.

The decoding method according to the invention is remarkable in that it comprises, for a current block to be reconstructed, the steps of:

determining a set of candidate predictor blocks,

identification, in the aforementioned set, of a candidate predictor block, such identification being a function of said residual current decoded block,

reconstruction of the current block using the identified predictor block and the decoded current residual block.

An advantage of such a decoding method lies in the fact that the step of identifying the predictor block capable of reconstructing the current block is reproducible at decoding. The data signal received at the decoder advantageously contains no information associated with this identified predictor block, which significantly reduces the cost of signaling this information.

In addition, the fact that the identification of the predictor block is a function of the decoded current residual block allows a reliable reconstruction of the current block. The characteristics of the candidate predictor blocks of the determined set being very different from each other, the identification of the predictor block used for the reconstruction of the current block is facilitated. This results in a decoding of the image of better quality.

According to a particular embodiment, the blocks of the image preceding the current block being decoded in a determined order, the identification of a predictor block is a function of the pixels of the previously decoded image.

According to another particular embodiment, the previously decoded pixels of the image are located along the current block. According to another particular embodiment, the decoding method further comprises a step of determining, in the data signal, information associated with a prediction of the current block, said step of reconstructing the current block being implemented. from such prediction, the identified predictor block and the current residual block determined.

Such an arrangement makes it possible to further refine the prediction so as to obtain optimized decoding performances.

The various embodiments or aforementioned embodiments can be added independently or in combination with each other, to the steps of the decoding method as defined above.

Correspondingly, the invention also relates to a device for decoding a data signal representative of at least one image cut into blocks, such a device comprising:

a module for determining, in the data signal, data representative of a current residual block associated with a current block to be decoded,

a module for decoding said current residual block.

Such a decoding device is remarkable in that it comprises, for a current block to be reconstructed:

a module for determining a set of candidate predictor blocks,

an identification module, in said set, of a candidate predictor block, said identification being a function of said decoded current residual block,

a module for reconstructing the current block using the identified predictor block and the decoded current residual block.

Such a decoding device is able to implement the aforementioned decoding method.

The invention also relates to a computer program comprising instructions for implementing one of the coding and decoding methods according to the invention, when it is executed on a computer.

This program can use any programming language, and be in the form of source code, object code, or intermediate code between source code and object code, such as in a partially compiled form, or in any other desirable form.

The invention also relates to a computer-readable recording medium on which a computer program is recorded, this program including instructions adapted to the implementation of one of the methods according to the invention, as described herein. -above.

The invention also relates to a computer-readable recording medium on which a computer program is recorded, this program comprising instructions adapted to the implementation of the coding or decoding method according to the invention, as described. above.

The recording medium may be any entity or device capable of storing the program. For example, the medium may include storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or a magnetic recording means, for example a USB key or a hard disk.

On the other hand, the recording medium may be a transmissible medium such as an electrical or optical signal, which may be conveyed via an electrical or optical cable, by radio or by other means. The program according to the invention can be downloaded in particular on an Internet type network.

Alternatively, the recording medium may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the aforementioned coding or decoding method.

The decoding method, the coding device, the decoding device, the computer programs and the corresponding recording media mentioned above have at least the same advantages as those conferred by the coding and decoding method according to the present invention. Brief description of the drawings

Other features and advantages will appear on reading preferred embodiments described with reference to the figures in which:

FIGS. 1A and 1B show steps of the coding method according to one embodiment of the invention,

FIG. 2 represents an embodiment of a coding device according to the invention able to implement the coding method of FIGS. 1A and 1B,

FIG. 3 represents an example of partitioning the current image into several blocks of pixels,

FIG. 4 represents an embodiment according to the invention of the step of identifying a candidate predictor block as a function of the current decoded residue block obtained,

FIG. 5 represents steps of the coding method according to another embodiment of the invention,

FIG. 6 represents an embodiment of a coding device according to the invention able to implement the coding method of FIG. 5;

FIG. 7 represents steps of the decoding method according to one embodiment of the invention,

FIG. 8 represents an embodiment of a decoding device according to the invention able to implement the decoding method of FIG. 7,

FIG. 9 represents steps of the decoding method according to another embodiment of the invention,

FIG. 10 represents an embodiment of a decoding device according to the invention able to implement the decoding method of FIG. 9.

Detailed description of an embodiment of the coding part

An embodiment of the invention will now be described, in which the coding method according to the invention is used to code an image. or a sequence of images according to a bit stream close to that obtained by an encoding conforming for example to the HEVC standard.

In this embodiment, the coding method according to the invention is for example implemented in a software or hardware way by modifications of an encoder initially conforming to the HEVC standard. The coding method according to the invention is represented in the form of an algorithm comprising steps C1 to C22 as represented in FIGS. 1A and 1B.

According to the embodiment of the invention, the coding method according to the invention is implemented in a coding device C01 represented in FIG.

As illustrated in FIG. 2, such an encoding device comprises a memory MEM_C01 comprising a buffer memory MT_C01, a processing unit UT_C01 equipped for example with a microprocessor μΡ and driven by a computer program PG_C01 which implements the method of coding according to the invention. At initialization, the code instructions of the computer program PG_C01 are for example loaded into a RAM memory (not shown) before being executed by the processor of the processing unit UT_C01.

The coding method shown in FIGS. 1A and 1B applies to any current image of an SQ sequence of images to be encoded.

During a first step C1 represented in FIG. 1A, in a manner known per se, a current image ICj belonging to the image sequence SQ 1d, ICj,. CM (1≤j≤M), in a plurality of blocks Bi, B ₂ , B _u ,..., B _s (1 < _u _S S), for example of size 64 × 64 pixels. Such a partitioning step is implemented by a partitioning software module MP1 shown in FIG. 2, which module is controlled by the microprocessor μΡ of the processing unit UT_C01.

The image ICj thus partitioned is shown in FIG. 3. In the example shown, the image ICj is partitioned into four blocks Bi, B ₂ , B ₃ and B ₄ .

It should be noted that for the purposes of the invention, the term "block" means coding unit (coding unit). This last terminology is especially used in the HEVC standard, for example in the document "B. Bross, W.-J. Han, J.-R. Ohm, GJ Sullivan, and T. Wiegand, "High efficiency video coding (HEVC) text specification draft 10," JCTVC-L1003 document of JCT-VC, Geneva, CH, 14-23 January 2013.

In particular, such a coding unit groups together sets of pixels of rectangular or square shape, also called blocks, macroblocks, or sets of pixels having other geometrical shapes.

Said blocks Bi, B ₂ , B _u , ..., B _s are intended to be coded according to a predetermined order of travel, which is for example of the raster scan type. This means that the blocks are coded one after the other, from left to right.

Other types of course are of course possible. Thus, it is possible to cut the image IC _j into several sub-images called slices and to independently apply a division of this type on each sub-image. It is also possible to code not a succession of lines, as explained above, but a succession of columns. It is also possible to browse the rows or columns in one direction or the other.

During a step C2 represented in FIG. 1A, the coder CO1 selects, as current block, a first block to be encoded B _u of the image IC _j , such as, for example, the first block B-.

During a step C3 represented in FIG. 1A, a set of Q candidate predictor blocks BP1 _{1 is} determined according to the invention _; BP1 _2, ..., _V BP1, ..., _Q BP1 (1 <v≤Q). Such candidate predictor blocks are for example blocks of pixels that have already been coded or not. Such blocks are stored beforehand in the MT_CO1 buffer memory of the encoder as represented in FIG. 2. In the example represented, it is in particular a predetermined number of blocks which have been coded just before the current block considered. .

Such a determination step is implemented by a determination software module DET_CO1 shown in FIG. 2, which module is driven by the microprocessor μΡ of the processing unit UT_CO1. During a step C4 shown in FIG. 1A, for a candidate predictor block BP1 _V considered, the candidate predictor block BP1 _v is subtracted from the current block B _u to produce a residue block Br _v .

During a step C5 represented in FIG. 1A, the residue block Br _{v is} transformed according to a conventional direct transformation operation such as, for example, a discrete cosine transformation of the DCT type, to produce a transformed Bt block. _v .

During a step C6 represented in FIG. 1A, the transformed block Bt _{v is} quantized according to a conventional quantization operation, such as, for example, a scalar quantization. A block of quantized coefficients Bq _v is then obtained.

The steps C4 to C6 are implemented by a predictive coding software module PRED_C01 shown in FIG. 2, which module is driven by the microprocessor μΡ of the processing unit UT_C01. The predictive coding module PRED_C01 is able to perform a predictive coding of the current block, according to conventional prediction techniques, such as for example in Intra and / or Inter mode.

During a step C7 represented in FIG. 1A, the entropic coding of the quantized coefficient block Bq _{v is carried out} . In the preferred embodiment, it is a CABAC entropic coding. Such a step consists in: a) reading the symbol (s) of the predetermined set of symbols which are associated with said current block,

b) associating digital information, such as bits, with the read symbol (s).

Such an entropy coding step is implemented by an entropic coding software module MCE1 shown in FIG. 2, which module is driven by the microprocessor μΡ of the processing unit UT_C01. The entropic coding module MCE1 is for example of the CABAC type. It may also be a Huffman coder known as such.

During a step C8 shown in FIG. 1A, the block Bq _{v is} dequantized according to a conventional operation of FIG. dequantization, which is the inverse operation of the quantization performed in step C6. A block of dequantized coefficients BDq _v is then obtained.

During a step C9 shown in FIG 1A, it is proceeded to the inverse transformation of the dequantized coefficient block BDQ _v which is the inverse operation of the direct conversion performed in step C5 above. A decoded residue block BDr _v is then obtained.

Steps C8 and C9 are implemented by a PRED ^"1 _C01 inverse predictive coding software module represented in FIG. 2, which module is driven by the microprocessor μΡ of the processing unit UT_C01.

Since for the current block B _u considered, steps C4 to C9 are repeated for each predictor block of the set of predictor blocks BP1 _1; BP1 ₂ , ..., BP1 _V , ..., BP1 _Q , Q decoded residue blocks BDr _1; BDr ₂ ,..., BDr _v ,..., BDr _Q are obtained at the end of step C9.

During a step C10 shown in FIG. 1A, the method according to the invention is used to identify, among all the Q predictor blocks BP1-I, BP1 ₂ ,..., BP1 _V ,. , BP1 _Q , at least one predictor block capable of being found at the decoding of the current block B _u . According to the invention, such an identification step is a function of the current decoded residue block BDr _v obtained.

Such an identification step is implemented by a calculation software module CAL1_C01 shown in FIG. 2, which module is controlled by the microprocessor μΡ of the processing unit UT_C01.

According to a particular embodiment shown in FIG. 4, such an identification consists, for a current decoded residue block BDr _v , in building a current decoded block BD _{V, W} by adding to the decoded current block BDr _v a candidate predictor block BP1 _w (1≤w≤Q).

In this particular mode, a criterion is applied for minimizing the difference between the decoded pixels of the current image IC _j , which are represented by dots in FIG. 4, and the pixels of the decoded block BD _{V, W} along its boundary F. This criterion is a mathematical operator denoted SM (BD _v , w, IC _j ). The operator SM (BD _V , _W , IC _j ) is in fact representative of the quadratic error along the boundary F of the decoded residual block BDr _v with the image IC _j .

It is written as follows: MS (BD _ViW , _lC _j )

N-1

= ^ BD _{V w} (0, a) - lC _j (lin-1, col + α)

α = 0

N-1

+ ^ (3D _vw (a, 0) - IC _j lin + a, col - 1)

a = Q

OR

- BD _v , w is the decoded block considered of size NxN pixels,

- BD _v , w (n, m) is the value of the pixel of the decoded residue block BD _V , _W located on the nth row and the mth column of this block,

- IC _j is the current image,

- IC _j (k, l) is the value of the pixel of the image IC _j located on the k th line and the I th column of this image, and (lin, col) are the coordinates of the decoded block BD _{v, w} in the image IC _j .

As an alternative, a simplified criterion can be used, in which the average of the pixels of the image IC _j along the boundary F and the average of the pixels of the decoded block BD _V , _W - L 'are compared. SM operator (BD _V , _W , ICj) is then written:

MS (BD _ViW , _lC _j )

N-1

abs (C _j (lin-1, col + a)

(2N - 11)

a = 0

N-1 N-1

+ IC _j Çlin + a, col - 1)) - ^ ^ BD _V

a = 0 b = 0

Where abs () represents the absolute value.

During step C10, the decoded block BDv.wmin is determined, which minimizes one of the two aforementioned criteria, such as:

Kmin = argmin _K 5 (BD _uK, PAC _j)

The block B Dv.wmin is equal to the sum of the candidate predictor block BP1 wmin and the current decoded residue block BDr _v .

During a step C1 1 represented in FIG. _1A, the candidate candidate block BP1 _{wmin is compared} with the candidate predictor block BP1 _V associated with the decoded residue block BDr _v . Such a comparison step is implemented by a calculation software module CAL2_C01 shown in FIG. 2, which module is controlled by the microprocessor μΡ of the processing unit UT_C01.

For the current block B _u considered, the steps C4 to C1 1 are repeated for each predictor block of the set of predictor blocks BP1 i, BP1 ₂ ,..., BP1 _V ,..., BP1 _Q determined in FIG. step C3.

In the case where there is identity between the block BP1 _W min and the block BP1 _V , it is proceeded, during a step C12a) shown in FIG. 1A, to the selection of the block BP1 _W min which becomes a block identified predictor.

In the case where there is no identity between the block BP1 _wmin and the block BP1 _V , the BPI block wmin is not selected as the identified predictor block.

At the end of the selection step C12a), a plurality T of identified predictor blocks BP1 i, BP1 ₂ , BP1 _Z , BP1 _T is obtained, where 1 <z≤T≤Q.

During a step C13 shown in FIG. 1B, determination is made, among all the predictor blocks BP1 -, BP1 ₂ , BP1 _Z , BP1 _T which have been obtained in step C12a), of a preferential candidate predictor block BP1 _opt , using the minimization of a predetermined criterion. Such a criterion is expressed by equation (1) below:

(1) J = D + AR where

D represents the distortion between the original current block Bi and the reconstructed block Bi, R represents the bit cost of the coding of the coding parameters used to code the block Bi and λ represents a Lagrange multiplier whose value is fixed to the coder.

According to a particularly advantageous variant from a point of view of reducing the computation time to the encoder, the predetermined performance criterion only depends on the distortion and is expressed by equation (2) below:

(2) J '= D.

The criteria J and J 'are calculated conventionally by simulation by a calculation module CAL3_C01 represented in FIG. 2, which module is driven by the microprocessor μΡ of the processing unit UT_C01. During a step C14 represented in FIG. 1B, the predictive module PRED_C01 of FIG. 2 subtracts the preferential candidate predictor block BP1 _op t from the current block B _u to produce a residue block Br _op t _u - Au during a step C15 shown in FIG. 1B, the module

PRED_C01 of FIG. 2 carries out the transformation of the residue block Br _op t _u according to a conventional direct transformation operation such as, for example, a discrete cosine transformation of the DCT type, to produce a transformed block Bt _op tu- During a step C16 shown in FIG. 1B, the module

PRED_C01 of FIG. 2 proceeds to the quantization of the transformed block Bt _op tu according to a conventional quantization operation, such as, for example, a scalar quantization. A block of quantized coefficients Bq _op t _u is then obtained.

During a step C17 represented in FIG. 1B, the entropic coding module MCE1 of FIG. 2 proceeds with the entropic coding of the quantized coefficient block Bq _op t _u -

A data flow φ which contains the encoded data of the quantized coefficient block Bq _op t _u is then delivered at the end of step C17. Such a stream is then transmitted by a communication network (not shown) to a remote terminal. This comprises the decoder D01 represented in FIG. 7. In a manner known per se, the stream φ furthermore comprises certain information encoded by the coder C01, such as the type of prediction (inter or intra), and, if appropriate, the prediction mode, the partition partitioning type if the block has been partitioned, the reference image index and the displacement vector used in the inter prediction mode.

During a step C18 shown in FIG. 1B, the PRED module ^" 1_C01 of FIG. 2 dequantizes the block Bq _op t _u according to a conventional dequantization operation, which is the inverse operation of the quantization performed in step C16 A block of dequantized coefficients BDq _op tu is then obtained.

During a step C19 represented in FIG. 1B, the PRED module ^" 1_C01 of FIG. 2 carries out the inverse transformation of the block of dequantized coefficients BDq _op t _u which is the reverse operation of the direct transformation performed in step C15 above. A decoded residue block BDr _op tu is then obtained.

During a step C20 shown in FIG. 1B, the decoded block BD _{U is constructed} by adding to the preferential candidate predictor block BP1 _op t the decoded residue block BDr _op t _u - H is noted that this last block is the same as the decoded block obtained at the end of the image decoding process IC _j which will be described later in the description. The decoded block BD _U is then stored in the buffer memory MT_C01 of FIG. 2, in order to be used by the coder C01 as a candidate predictor block of a next block to be encoded.

During a step C21 shown in FIG. 1B, the coder C01 tests whether the current block B _u that has just been coded is the last block of the image. If the current block is the last block of the image. IC _j , during a next step C22 shown in Figure 1 B, it ends the coding process.

If this is not the case, it is proceeded again to the selection step C2 of the next block to be coded according to the above-mentioned raster scan order, then the steps C3 to C21 are repeated for this next block selected .

The coding steps which have just been described above are implemented for all the blocks Bi, B ₂ , B _u ,..., B _s to be encoded of the current image IC _j considered.

DETAILED DESCRIPTION OF ANOTHER EMBODIMENT OF THE CODING PART This alternative embodiment differs from the previous embodiment in that it implements two types of prediction which will be described below with reference to FIG. The coding method according to this other embodiment is represented in the form of an algorithm comprising steps C'1 to C'9 as represented in FIG. The coding method according to this other embodiment of the invention is implemented in a coding device C02 shown in FIG. 6.

As illustrated in FIG. 6, such a coding device C02 comprises a memory MEM_C02 comprising a buffer memory MT_C02, a processing unit UT_CO2 equipped for example with a microprocessor μΡ and driven by a computer program PG_C02 which implements the method coding according to this other embodiment. At initialization, the code instructions of the computer program PG_C02 are for example loaded into a RAM (not shown) before being executed by the processor of the processing unit UT_C02.

The coding method shown in FIG. 5 applies to any current image of an image SQ sequence to be encoded.

During a step C'1 represented in FIG. 5, a current image ICj belonging to the image sequence SQ ld, ICj, ..., I CM (1≤j≤M) is partitioned. ), in a plurality of blocks Bi, B ₂ , B _u , ..., B _s (1 < _u _S S), for example of size 64 × 64 pixels. Such a partitioning step is implemented by a partitioning software module MP2 shown in FIG. 6, which module is controlled by the microprocessor μΡ of the processing unit UT_C02. Since step C'1 is identical to step C1 of FIG. 1A, it will not be described further.

As in the embodiment of FIGS. 1A and 1B, said blocks Bi, B ₂ , B _u ,..., B _s are intended to be coded according to a predetermined order of travel, which is for example of the raster scan type. . This means that the blocks are coded one after the other, from left to right.

During a step C'2 shown in FIG. 5, the coder C02 selects as the current block a first block to be encoded B _u of the image ICj, such as, for example, the first block Bi.

During a step C'3 shown in FIG. 5, in a manner known per se, the current block B _{u is} predicted by conventional intra and / or inter prediction techniques using a predictor block BP1 _is i - Such a prediction will be called "primary prediction" in the remainder of the description.

The aforementioned prediction step makes it possible to construct a residue block Br1 _u which is obtained by calculating the difference between the current block B _u and the predictor block BP1 _se .

The step C'3 is implemented by a predictive coding software module PRED1_C02 shown in FIG. 6, which module is driven by the microprocessor μΡ of the processing unit UT_C02.

During a step C'4 represented in FIG. 5, in accordance with the invention, a set of Q candidate predictor blocks BP2 _{1 is determined;} BP2 _2, ..., _V BP2, ..., BP2 _Q (1 <V≤Q). Since such a step is identical to the determination step C3 of FIG. 1A, it will not be described further.

Such a determination step C'4 is implemented by a determination software module DET_CO2 shown in FIG. 6, which module is controlled by the microprocessor μΡ of the processing unit UT_CO2.

During a step C'5 represented in FIG. 5, in accordance with the invention, a prediction of the residue block Br1 _{u is carried out} by implementing the steps C4 to C13 described above in connection with the figures 1 A and 1 B. Such a prediction will be called "secondary prediction" in the remainder of the description.

During this secondary prediction, a preferential candidate predictor block BP2 _op t is selected.

With reference to FIG. 6, step C'5 is implemented using:

a prediction software module PRED2_C02 which is identical to the module PRED_CO of FIG. 2,

an entropic coding software module MCE2_C02 which is identical to the module MCE1 of FIG. 2,

an inverse prediction software module PRED2 ^"1 _C02 which is identical to the module PRED ^{" 1} _C01 of FIG. 2,

a calculation software module CAL1_C02 which is identical to the module CAL1_C01 of FIG. 2,

a calculation software module CAL2_C02 which is identical to the module CAL2_C01 of FIG. 2. During a step C'6 represented in FIG. 5, in accordance with the invention, the effectiveness of the preferred candidate predictor block BP2 _op t which has been selected is carried out.

The test step C'6 is implemented by a calculation software module CAL4_C02 shown in FIG. 6, which module is driven by the microprocessor μΡ of the processing unit UT_C02.

In a preferred embodiment, such a test consists in verifying whether the energy of the block Br1 _u -BP2 _op t is lower than a threshold which corresponds to the value of the energy of the block Br 1 _u .

If the negative test, the coding of the current block B _u continues in a conventional manner.

If the test is positive, it means that the preferential candidate predictor block BP2 _opt is close to the current original block B _u . Therefore, the secondary prediction is applied to the current block B _u.

Negative test

In the case where the test carried out in step C'6 is negative, during a step C'610 shown in FIG. 5, the residue block Br1 _{u is} transformed according to a conventional direct transformation operation. such as for example a discrete cosine transformation of DCT type, to produce a transformed block Bt1 _u .

During a step C'61 1 shown in FIG. 5, the transformed block Bt1 _{u is} quantized according to a conventional quantization operation, such as, for example, scalar quantization. A block of quantized coefficients Bq1 _u is then obtained.

The steps C'610 and C'61 1 are implemented by the predictive coding software module PRED1_C02 shown in FIG. 6.

During a step C'612 represented in FIG. 5, the entropic coding of the quantized coefficient block Bq1 _{u is carried out} by an entropic coding software module MCE1_CO2 identical to the entropic coding software module MCE_C01 of FIG. furthermore, an indicator Id associated with the preferential candidate predictor block BP2 _op t is coded according to a first predetermined value (bit at 0 for example) to indicate that the secondary prediction has not been applied. A data flow φ1, which contains the encoded data of the quantized coefficient block Bq1 i as well as the bit at 0 of the indicator Id, is then output at the end of the step C'612. Such a stream is then transmitted by a communication network (not shown) to a remote terminal. This comprises the decoder D02 shown in FIG. 10. In a manner known per se, the stream φ1 contains information encoded by the coder C02, such as the type of prediction (inter or intra), and if appropriate, the mode prediction, the type of partitioning of a block or macroblock if the latter has been partitioned, the reference image index and the displacement vector used in the inter prediction mode.

During a step C'613 shown in FIG. 5, the block Bq1 _{u is} dequantized according to a conventional dequantization operation, which is the inverse operation of the quantization performed in step C'61. . A block of dequantized coefficients BDq1 _u is then obtained. During a step C'614 represented in FIG. 5, the inverse transformation of the dequantized coefficient block BDq1 _u is carried out which is the inverse operation of the direct transformation carried out in step C'610 above. . A decoded residue block BDr1 _u is then obtained.

The steps C'613 and C'614 are implemented by an inverse predictive coding software module PRED1 ^"1 _C02 shown in FIG. 6, which module is driven by the microprocessor μΡ of the processing unit UT_C02. is identical to the software module PRED ^"1 _C01 of Figure 2.

C7 during a step shown in Figure 5, requires the construction of the decoded block by adding the _U BD BP1 predictor _block i block decoded residue BDr1 _u. The decoded block BD _U is then stored in the buffer MT_C02 of FIG. 6, in order to be used by the coder C02 as a predictor block that is candidate for a secondary prediction of a next decoded residue block.

During a step C'8 represented in FIG. 5, the coder C02 tests whether the current block B _u that has just been coded is the last block of the image ICj.

If the current block B _u is the last block of the image IC _j , during a step C'9 represented in FIG. 5, the coding method is terminated. If this is not the case, it is proceeded again to the selection step C'2 of the next block to be coded according to the above-mentioned raster scan order, then the steps C'3 to C'6 are reiterated for that next block selected.

Positive test

If the test performed in step C'6 is positive, during a step C'620 shown in FIG. 5, the module PRED2_C02 of FIG. 6 subtracts the preferential candidate predictor block BP2 _opt from the residual block Br1 u to produce a residue block Br2 _op t _u - During a step C'621 represented in FIG. 5, the module

PRED2_C02 of FIG. 6 transforms the residue block Br2 _op t _u according to a conventional direct transformation operation such as, for example, a discrete cosine transformation of the DCT type, to produce a transformed block Bt2 _op t _u - During a step C'622 represented in FIG. 5, the module

PRED2_C02 of FIG. 6 proceeds to the quantization of the transformed block Bt2 _op tu according to a conventional quantization operation, such as, for example, a scalar quantization. A block of quantized coefficients Bq2 _op t _u is then obtained.

During a step C'623 represented in FIG. 5, the entropic coding module MCE2_CO2 of FIG. 6 proceeds with the entropic coding of the quantized coefficient block Bq2 _op t _u. In addition, the indicator Id associated with the predictor block preferential candidate BP2 _opt is coded according to a second predetermined value (1-bit, for example) to signal that the secondary prediction has been applied.

A data stream φ2, which contains the encoded data of the quantized coefficient block Bq2 _op t _u and the bit at 1 of the indicator Id, is then output at the end of the step C'623. Such a stream is then transmitted by a communication network (not shown) to the decoder D02 shown in FIG.

During a step C'624 shown in FIG. 5, the module PRED2 ^"1 _C02 of FIG. 6 carries out the dequantization of the block Bq2 _op t _u according to a conventional operation of dequantization, which is the inverse operation the quantification performed in step C'622. A block of dequantized coefficients BDq2 _op t _u is then obtained.

During a step C'625 shown in FIG. 5, the module PRED2 ^"1 _C02 of FIG. 6 carries out the inverse transformation of the dequantized coefficient block BDq2 _op t _u which is the inverse operation of the direct transformation carried out in step C'621 above A decoded residue block BDr2 _op tu is then obtained.

During the step C7 mentioned above, the decoded block BD _{U is constructed} by adding to the preferential candidate predictor block BP2 _opt the decoded residual block BDr2 _op t _u. The decoded block BD _U thus constructed is then stored. in the MT_C02 buffer memory of FIG. 6, for use by the CO 2 coder as a predictor block candidate for a secondary prediction of a next decoded residue block.

During the step C'8 represented in FIG. 5, the coder C02 tests whether the current block B _u which has just been coded is the last block of the image IC _j .

If the current block is the last block of the image IC _j , during the step C'9 represented in FIG. 5, the coding method is terminated.

If this is not the case, it is proceeded again to the selection step C'2 of the next block to be coded according to the above-mentioned raster scan order, then the steps C'3 to C'6 are reiterated for that next block selected.

Detailed description of an embodiment of the decoding part

An embodiment of the decoding method according to the invention will now be described, in which the decoding method is implemented in a software or hardware way by modifying a decoder initially conforming to the HEVC standard. The decoding method according to the invention is represented in the form of an algorithm comprising steps D1 to D1 1 as represented in FIG. 8. As illustrated in FIG. 7, a decoder D01 according to the invention comprises a memory MEM_D01 comprising a buffer memory MT_D01, a processing unit UT_D01 equipped for example with a microprocessor μΡ and driven by a computer program PG_D01 which implements the decoding method according to the invention. At initialization, the code instructions of the computer program PG_D01 are for example loaded into a RAM before being executed by the processor of the processing unit UT_D01.

The decoding method shown in FIG. 8 applies to any current image of an SQ sequence of images to be decoded.

For this purpose, information representative of the current image IC _j to be decoded is identified in the stream φ received at the decoder.

With reference to FIG. 8, the first decoding step D1 is the identification in said stream φ of the encoded data Bq _; Bq ₂ , ..., Bq _u , ... Bq _s respectively associated with the residual blocks Bn, Br ₂ , Br _u , ..., Br _s coded previously according to the above-mentioned raster scan path, according to the coding method shown in FIG. Figures 1A and 1B.

Such an identification step is implemented by an identification module MI_D01 as shown in FIG. 8, said module being driven by the microprocessor μΡ of the processing unit UT_D01.

Said blocks Bq _; Bq ₂ , Bq _u , ..., Bq _s are intended to be decoded according to a predetermined order of travel, which is for example of the sequential type, that is to say that they are intended to be decoded one after the other. other in the raster scan order where they were encoded.

Other types of course than the one just described above are of course possible and depend on the order of course chosen coding, examples of which were mentioned above.

During a step D2 shown in FIG. 8, the decoder D01 selects as the current block a first block to be encoded Bq _u of the image IC _j , such as, for example, the first block Bq-,.

During a step D3 shown in FIG. 8, the entropy decoding of the block Bq _{u is carried out} . In the preferred embodiment, it is a CABAC entropic decoding. Such a step consists of: a) reading the symbol (s) of the predetermined set of symbols which are associated with said current residual block,

b) associating digital information, such as bits, with the read symbol (s).

Such an entropy decoding step is implemented by an entropy decoding software module MDE1 shown in FIG. 7, which module is controlled by the microprocessor μΡ of the processing unit UT_D01. The entropy coding module MDE1 is for example of the CABAC type. It can also be a Huffman decoder known as such.

During a step D4 shown in FIG. 8, dequantization of the block BDq _{u is carried out} according to a conventional dequantization operation, which is the inverse operation of the quantization performed in step C16 of FIG. 1B. A decoded dequantized block BDt _u is then obtained.

During a step D5 shown in FIG. 8, the inverse transformation of the decoded dequantized block BDt _u is carried out which is the inverse operation of the direct transformation performed in step C15 of FIG. decoded residue BDr _u is then obtained.

The steps D4 and D5 are implemented by a PRED ^"1 _D01 inverse predictive decoding software module represented in FIG. 7, which module is driven by the microprocessor μΡ of the processing unit UT_D01.

During a step D6 shown in FIG. 8, a set of Q candidate predictor blocks BP1 i, BP1 ₂ ,..., BP1 _V ,..., BP1 is determined according to the invention. _Q (1 <V≤Q). Such candidate predictor blocks are, for example, blocks of pixels that have already been decoded or not. Such blocks are stored beforehand in the buffer MT_D01 of the decoder as shown in FIG. 7. In the example shown, it is in particular a predetermined number of blocks that have been decoded just before the current block. decode considered.

Such a determination step is implemented by a DET_D01 determination software module shown in FIG. 7, which module is driven by the μΡ microprocessor of the processing unit UT_D01. During a step D7 represented in FIG. 8, the identification method is carried out according to the invention, among all the Q predictor blocks BP1 _; BP1 ₂ , ..., BP1 _v , ..., BP1 Q, of a preferential candidate predictor block BP1 _op t. According to the invention, in the same way as in the aforementioned coding, such an identification step is a function of the decoded residue block BDr _u obtained.

Said identification step is implemented by a calculation software module CAL1_D01 shown in FIG. 7, which module is controlled by the microprocessor μΡ of the processing unit UT_D01.

In the same way as for the coding method described with reference to FIGS. 1A and 1B, such an identification consists, for a current decoded residue block BDr _u , of constructing a current decoded block BD _{U> W} by adding to the residual block decoded current BDr _u a candidate predictor block BP1 _W (1 <w≤Q).

In this particular mode, and correspondingly to the coding, is applied a criterion for minimizing the difference between the decoded pixels of the current image IC _j , which are represented by dots in FIG. 4, and the pixels of the decoded block BD _{U> W} located along its border F. This criterion is a mathematical operator noted SM (BD _UiW , IC _j ). The operator SM (BD _UiW , IC _j ) is in fact representative of the quadratic error along the boundary F of the decoded residual block BDr _u with the image IC _j .

It is written as follows:

MS (BD _{u> w} , IC _j )

N-l

= ^ (BD _UiW (0, a) - IC _j lin-1, col + a))

a = 0

N-l

+ ^ (BD _uw (CL, 0) - IC _j (lin + a, col - 1))

a = 0

or :

- BD _{U> W} is the decoded block considered of size NxN pixels,

- BD _u , w (n, m) is the value of the decoded residue block BD _{U> W} located on the nth row and the mth column of this block,

- IC _j is the current image,

- IC _j (k, l) is the value of the pixel of the image IC _j located on the k th line and the I th column of this image, and (lin, col) are the coordinates of the decoded block BD _{u> w} in the image IC _j . As an alternative, a simplified criterion can be used, in which the average of the pixels of the image ICj along the border F is compared with the average of the pixels of the decoded block BD _{U> W.} The operator SM (BD _UiW , ICj) is written then:

MS (BD _{u> w} , IC _j )

N-1

_(2N _ ^ ^ {iCj Çlin - 1, col + a)

a = Q

N-1 N-1

+ iq (lin + a, col - ï)) - ^ ^ (BD _UIW (a, b))

a = 0 b = 0

Where abs () represents the absolute value.

During step D7, the decoded block BDv.wmin is identified, which minimizes one of the two aforementioned criteria, such as:

wmin = argmin _w MS {BD _uw , lC _j )

The block BDv.wmin is equal to the sum of the candidate predictor block BP1 wmin and the current decoded residue block BDr _v .

At the end of step D7, the candidate predictor block BP1 _W min is considered as the preferential candidate predictor block BP1 _op t for the inverse prediction of the current decoded residue block BDr _u .

During a step D8 shown in FIG. 8, the current block B _{u is} reconstructed by adding to the decoded current residual block BDr _u the preferential candidate predictor block BP1 _op t identified in step D7.

Said step D8 is implemented by a calculation software module CAL2_DO1 represented in FIG. 7, which module is driven by the microprocessor μΡ of the processing unit UT_DO1.

A decoded block BD _U is then obtained and stored in the buffer memory MT_DO1 of FIG. 7, in order to be used by the decoder DO1 as a candidate predictor block of a next block to be decoded.

During a step D9 shown in FIG. 8, said decoded block BD _U is written in a decoded picture I Dj. Such a step is implemented by an image reconstruction software module URI1 as shown in FIG. 7, said module being driven by the microprocessor μΡ of the processing module UT DO1. During a next step D10 shown in FIG. 8, the decoder D01 tests whether the current block BD _U that has just been decoded is the last block contained in the stream φ.

If this is the case, during a step D1 1 shown in FIG. 8, the decoding method is terminated.

If this is not the case, it is proceeded, in step D2, to the selection of the next residual block to be decoded according to the above-mentioned raster scan order.

The decoding steps that have just been described above are implemented for all the Bq blocks _; Bq ₂ , Bq _u , ..., Bq _s to decode from the current image IC _j considered.

DETAILED DESCRIPTION OF ANOTHER EMBODIMENT OF THE DECODING PART

Another embodiment of the decoding method according to the invention will now be described, in which the decoding method is implemented in a software or hardware way by modifying a decoder initially conforming to the HEVC standard. The decoding method according to the invention is represented in the form of an algorithm comprising steps D 1 to D 7 as represented in FIG. 9.

As illustrated in FIG. 10, a decoder D02 according to this other embodiment of the invention comprises a memory MEM_D02 comprising a buffer memory MT_D02, a processing unit UT_D02 equipped for example with a microprocessor μΡ and controlled by a computer program PG_D02 which implements the decoding method according to the invention. At initialization, the code instructions of the computer program PG_D02 are for example loaded into a RAM memory before being executed by the processor of the processing unit UT_D02.

The decoding method shown in FIG. 9 applies to any current image of an SQ sequence of images to be decoded.

For this purpose, information representative of the current image IC _j to be decoded is identified in a data stream φ1 or φ2 received at the decoder, as delivered following the coding method of FIG. 5. With reference to FIG. 9, the first decoding step D'1 is the identification:

in said stream φ1, encoded data Bq1 1, Bq1 ₂ ,..., Bq1 _u ,... Bq1 _s (1≤u≤S) respectively associated with the residual blocks Br, Br1 ₂ , Br1 _u , ..., Br1 _s previously coded according to the above raster scan path, in the case where the primary prediction of the coding method of FIG. 5 has been implemented,

in said stream φ2, encoded data Bq2 _op ti, Bq2 _op t2, - - -, Bq2 _op tu, ... Bq2 _op ts (1 u u S S) respectively associated with the residual blocks Br2 _op ti, Br2 _opt 2, Br 2 _op t _u , - - -, Br 2 _opt s coded previously according to the above raster scan path, in the case where the secondary prediction of the coding method shown in FIG. 5 has been implemented.

Such an identification step is implemented by an identification module MI_D02 as shown in FIG. 10, said module being controlled by the microprocessor μΡ of the processing unit UT_D02.

Said blocks Bq1 _1; Bq1 ₂ , Bq1 _u , ..., Bq1 _s or Bq2 _opt i, Bq2 _opt 2,. . . , Bq2 _op tu, ... Bq2 _op ts are intended to be decoded according to a predetermined order of travel, which is for example sequential, ie the blocks are intended to be decoded one after the other in accordance with to the raster scan order where they were encoded.

During a step D 2 shown in FIG. 9, the decoder D 02 selects as a current block a first block to be coded B 1 _u or B q 2 _op t _u of the image IC _j , such as for example the first block B q 1 _u or Bq2 _op t _u -

During a step D'3 shown in FIG. 9, the index Id associated with the selected block Bq _u is read in the stream φ1 or φ2.

Such a reading step is implemented by a reader software module ML_D02 as shown in FIG. 10, said module being driven by the microprocessor μΡ of the processing unit UT_D02.

If the index Id is equal to zero, it means that the current block to be decoded has undergone a primary prediction according to the steps C'610 to C'614 of the encoding method shown in Figure 5. It is therefore the flow φ1 that the decoder D02 is intended to treat.

If the index Id is equal to one, it means that the current block to be decoded has undergone a secondary prediction according to the steps C'5 to C'625 of the coding method shown in FIG. 5. It is therefore the flow φ2 that the decoder D02 is intended to process. where ld = Q

During a step D 310 shown in FIG. 9, the entropic decoding of the block B q 1 _{u is carried out} . Such a step being identical to the aforementioned step D3, it will not be described further.

Such an entropy decoding step is implemented by an entropic decoding software module MDE1_D02 shown in FIG. 10, which module is controlled by the microprocessor μΡ of the processing unit UT_D02. The entropy coding module MDE1_D02 is for example of CABAC type. It can also be a Huffman decoder known as such.

During a step D 31 1 shown in Figure 9, the dequantization of the block BDq1 _{u is carried out} . Such a step being identical to the above-mentioned step D4, it will not be described further. A decoded dequantized block BDt1 _u is then obtained.

During a step D 312 shown in FIG. 9, the inverse transformation of the decoded dequantized block BD t 1 _{u is carried out} . Such a step being identical to the aforementioned step D5, it will not be described further. A decoded residue block BDr1 _u is then obtained.

During a step D'4 shown in FIG. 9, in a manner known per se, the current block B _{u is} reconstructed by conventional intra and / or inter prediction techniques with the aid of FIG. a predictor block BP1 _is i- Such a step consists in adding to the residual decoded current block BDr1 _{u that} the predictor block BP1 _is classically selected.

A decoded block BD _U is then obtained following the step D4 and stored in the buffer memory MT_D02 of FIG. 10, in order to be used by the decoder D02 as a candidate predictor block of a next block to be decoded. . The steps D 31 1 to D 4 are implemented by an inverse predictive decoding software module PRED1 ^"1 _D02 shown in FIG. 10, which module is driven by the microprocessor μΡ of the processing unit UT_D02.

During a step D shown in FIG. 9, said decoded block BD _U is written in a decoded picture ID _j . Such a step is implemented by an image reconstruction software module URI2 as represented in FIG. 10, said module being controlled by the microprocessor μΡ of the processing module UT_D02.

During a next step D 6 shown in FIG. 9, the decoder D02 tests whether the current block BD _U that has just been decoded is the last block contained in the stream φ1.

If this is the case, during a step D7 shown in FIG. 9, the decoding method is terminated.

If this is not the case, it is proceeded, in step D 2, to the selection of the residual block according to Bq1 _u to be decoded according to the aforementioned sequential order. The decoding method described above is then iterated for all the S blocks to be decoded.

Case where ld = 1

During a step D'320 shown in FIG. 9, the entropy decoding of the block Bq2 _op t _{u is performed.} Such a step being identical to the aforementioned step D3, it will not be described further. A decoded quantized block BDq2 _op tu is then obtained at the end of this step.

Such an entropy decoding step is implemented by an entropic decoding software module MDE2_D02 shown in FIG.

During a step D 321 shown in FIG. 9, dequantization of the block BD q 2 _op t _{u is performed.} Such a step being identical to the aforementioned step D 4, it will not be described further. A decoded dequantized block BDt2 _op t _u is then obtained.

During a step D 322 shown in FIG. 9, the reverse transformation of the decoded dequantized block BD t 2 _op t _{u is carried out.} - Such a step being identical to the aforementioned step D 5, it will not be described any further. . A decoded residue block BDr2 _op t _u is then obtained. Said block The decoded residual BDr2 _op t _u is then stored in the buffer memory MT_D02 of FIG. 10, in order to be used by the decoder D02 as a candidate predictor block of a next block to be decoded.

The steps 321 and 322 are D'implemented by a predictive decoding software module inverse Pred2 ^"1 _D02 shown in Figure 10, which module is controlled by the microprocessor of the μΡ UT_D02 processing unit.

During a step D 323 shown in FIG. 9, a set of Q candidate predictor blocks BP2 _{1 is} determined according to the invention _; BP2 _2, ..., _V BP2, ..., BP2 _Q (1 <v≤Q). Since such a step is identical to step D6 of FIG. 8, it will not be described further.

Such a determination step is implemented by a DET_D02 determination software module shown in FIG. 10, which module is driven by the microprocessor μΡ of the processing unit UT_D02.

During a step D 324 shown in FIG. 9, the method according to the invention is used to identify, among the set of Q predictor blocks BP 2 _1; BP2 ₂ ,..., BP2 _V ,..., BP2 _Q , of a preferential candidate predictor block BP2 _op t. According to the invention, in the same way as in the aforementioned coding, such an identification step is a function of the decoded residue block BDr2 _op t _z obtained.

Said identification step is implemented by a calculation software module CAL1_D02 shown in FIG. 10, which module is controlled by the microprocessor μΡ of the processing unit UT_D02.

In the same way as in step D6 of FIG. 8, the decoding method according to this other embodiment uses a criterion for minimizing the difference between the decoded pixels of the current image IC _j , which are represented by dots in FIG. 4, and the pixels of decoded block BDr1 _{u> w} located along its border F. This criterion is a mathematical operator denoted SM (BDr1 _UiW , ICj).

It is written as follows: SM (BDrl _{z> w} , IC _j )

N-1

= ^ (sDrl _zw (0, o) - IC _j (lin - 1, col + α))

α = 0

N-1

+ ^ (sDrl _zw (a, 0) - IC _j (lin + a, col - 1) J

a = Q

or :

BDr1 u is the decoded block considered of size NxN pixels,

- BDr1 _UiW (n, m) is the value of the pixel of the decoded block BDr1 _{u> w} located on the nth row and the mth column of this block,

ICj is the current image,

ICj (k, l) is the value of the pixel of the image ICj located on the kth row and the ith column of this image, and (lin, col) are the coordinates of the decoded residue block BDr1 _{u> w} in the image ICj.

As an alternative, a simplified criterion can be used in which the average of the pixels of the image ICj along the boundary F is compared with the average of the pixels of the decoded block BDr1 _{u> w} . The operator SM (BDr1 _UiW , ICj) is written then:

SM (BDrl _UiW , IC _j )

= abs (lC _j (lin - 1, col + a)

N-1 N-1

+ IC _j (lin + a, col - li) - ^ ^ (BDrl _{u> w} (a, 6))

a = 0 b = 0

Where abs () represents the absolute value.

During the step D-324, the decoded block BDr1 _v , wmin is identified which minimizes one of the two aforementioned criteria, such as:

wmin = argmin _w MS {BDrl _uw , lC _j )

The block BDr1 _v , wmin is equal to the sum of the candidate predictor block BP2 _wmin and the current decoded residue block BDr2 _op t _u -

At the end of the step D 324, the candidate predictor block BP2wmin is considered as the preferential candidate predictor block BP2 _opt for the inverse prediction of the current decoded residue block BDr2 _op t _u - During a step D 325 shown in FIG. 9, the residual current block BD r _{u is} reconstructed by adding to the residual decoded current block BD R 2 _op t _u the preferential candidate predictor block BP 2 _opt identified in step On 324.

Said step D'325 is implemented by a calculation software module CAL2_D02 shown in FIG. 10, which module is driven by the microprocessor μΡ of the processing unit UT_D02.

The aforementioned steps D 4 and D 5 are then repeated to deliver a current block BD _U. Then step D6 is again implemented to test if the current block BD _U is the last block of the image.

The decoding steps of the stream q> i (respectively φ ₂ ) which have just been described above are implemented for all the blocks Bq1 -i, Bq1 ₂ , Bq1 _u , ..., Bq _s (respectively Bq2 _opt i, Bq2 _opt 2, ..., Bq2 _opt u, ... Bq2 _opt s) to be decoded from the current image IC _j considered.

It goes without saying that the embodiments which have been described above have been given for purely indicative and non-limiting purposes, and that many modifications can easily be made by those skilled in the art without departing from the scope. of the invention.

Claims

1. Method for coding at least one image (IC _j ) cut into blocks, characterized in that it comprises, for a current block (B _u ) to be coded, the steps of:

determination (C3) of a set of candidate predictor blocks (BP1 i, BP1 ₂ , ..., BP1 _V , ..., BP1 _Q ),

for a first candidate predictor block (BP1 _V ) considered of said set:

Obtaining (C4) a residual block representative of the difference between the first candidate predictor block considered and the current block (B _u ),

Identification (C10), in said set of candidate predictor blocks, of a second candidate predictor block, said identification being a function of said obtained residual current block and said second candidate predictor block,

Selecting (C12a)) of said first candidate predictor block considered if it is equal to said identified second predictor block,

for each of the candidate predictor blocks of said set, other than said candidate candidate first predictor block, implementing the steps of obtaining, identifying and selecting,

determination (C13), among the candidate predictor blocks selected at the end of the selection step, of a candidate predictor block (BP1 _op t), using a predetermined criterion (J),

coding (C15-C17) of the residual block representative of the difference between the determined candidate predictor block and the current block (B _u ).

The coding method according to claim 1, wherein said blocks of the image preceding the current block are coded in a sequence. determined, said identification is a function of the pixels of the previously coded image.

The coding method according to claim 2, wherein said previously coded pixels then decoded from the image are located along the current block.

4. Encoding method according to any one of claims 1 to

3, wherein said predetermined criterion is the minimization of the bit rate-distortion cost of the image.

Coding method according to any one of claims 1 to

4, in which the current block is a block (Br1 _u ) which has previously been obtained as a result of a prediction.

6. Device (D01) for encoding at least one image (IC _j ) cut into blocks, characterized in that it comprises, for a current block (B _u ) to be encoded:

means (DET_C01) for determining a set of candidate predictor blocks,

for a first candidate predictor block considered of said set:

Means (PRED_C01) for obtaining a residual block representative of the difference between the first candidate predictor block considered and the current block (B _u ),

Means (CAL1_C01) identifying, in said set of candidate predictor blocks, a second candidate predictor block, as a function of said current residual block obtained and said second candidate predictor block,

Means (CAL2_C01) for selecting said first candidate predictor block considered if it is equal to said identified second predictor block, said acquisition, identification and selection means being activated for each of the candidate predictor blocks of said set, other than said candidate candidate first predictor block,

means (CAL3_C01) for determining, from among the candidate predictor blocks that have been selected by said selection means, a candidate predictor block, using a predetermined criterion,

means (MCE1) for encoding the residual block representative of the difference between the determined candidate predictor block and the current block (B _u ).

A computer program comprising program code instructions for performing the steps of the encoding method according to any one of claims 1 to 5, when said program is executed on a computer.

8. A method for decoding a data signal (φ) representative of at least one image (IC _j ) cut into blocks, said method comprising the steps of:

determination (D1) in the data signal of data representative of a current residual block associated with a current block to be decoded,

decoding (D3-D5) of said current residual block,

said decoding method being characterized in that it comprises, for a current block to be reconstructed, the steps of:

determination (D'323) of a set of candidate predictor blocks,

identification (D'324), in said set, of a candidate predictor block, said identification being a function of said residual current decoded block and said candidate predictor block,

reconstruction (D8) of the current block (B _u ) using the identified predictor block and the decoded current residual block.

9. Decoding method according to claim 8, wherein said blocks of the image preceding the current block being decoded in a sequence. determined, said identification is a function of the pixels of the previously decoded image.

The decoding method of claim 9, wherein said previously decoded pixels of the image are located along the current block.

1 1. A decoding method according to any one of claims 8 to 10, further comprising a step (D'3) of determining, in the data signal, information (Id) associated with a prediction of the current block, said step of rebuilding the current block being implemented from said prediction, said identified predictor block and the current residual block determined.

12. Device (D01) for decoding a data signal representative of at least one image (IC _j ) cut into blocks, said device comprising:

means (MI_D01) for determining, in the data signal, data representative of a current residual block associated with a current block to be decoded,

means (MDE1) for decoding said current residual block, said decoding device being characterized in that it comprises, for a current block to be reconstructed:

means (DET_D01) for determining a set of candidate predictor blocks,

means (CAL1_D01) for identifying, in said set, a candidate predictor block, said identification being a function of said residual current decoded block and said candidate predictor block,

means (CAL2_D01) for reconstructing the current block (B _u ) using the identified predictor block and the decoded current residual block.

Computer program comprising program code instructions for performing the steps of the decoding method according to any one of claims 8 to 11, when said program is executed on a computer.