EP3854088A1

EP3854088A1 - Methods and devices for encoding and decoding a data stream representing at least one image

Info

Publication number: EP3854088A1
Application number: EP19774169.7A
Authority: EP
Inventors: Félix Henry; Pierrick Philippe; Mohsen ABDOLI; Gordon Clare
Original assignee: Orange SA
Current assignee: Orange SA
Priority date: 2018-09-21
Filing date: 2019-09-03
Publication date: 2021-07-28
Also published as: CN112740692A; WO2020058593A1; BR112021003210A2; US20220046287A1; US11985355B2; CN112740692B; CN118354067A; KR20210058848A; FR3086486A1; US20240244266A1; JP2022500944A

Abstract

The invention relates to a method for decoding a stream of encoded data representing at least one image that is split into blocks. For at least one block of the image, referred to as the current block, information indicating an encoding mode of the current block is decoded (E42) from the data stream. When the encoding mode of the current block corresponds to a first encoding mode, the decoding (E433) of a prediction residual associated with the current block is performed using a first group of syntax elements, while, when the encoding mode of the current block corresponds to a second encoding mode, decoding (E442) of the prediction residual associated with the current block is performed using a second group of syntax elements, which is a subset of syntax elements of said first group of syntax elements.

Description

METHODS AND DEVICES FOR ENCODING AND DECODING A DATA FLOW REPRESENTATIVE OF AT LEAST ONE IMAGE

1. Field of the invention

The field of the invention is that of coding and decoding of images or sequences of images, and in particular of video streams.

More specifically, the invention relates to the compression of images or sequences of images using a block representation of the images.

The invention can in particular be applied to image or video coding implemented in current or future coders (JPEG, MPEG, H.264, HEVC, etc. and their amendments), and to the corresponding decoding.

2. Prior Art

Digital images and image sequences occupy a lot of memory space, which means that when transmitting these images, they must be compressed to avoid congestion problems on the network used for this transmission.

Numerous techniques for compressing video data are already known. Among these, the HEVC compression standard ("High Efficiency Video Coding, Coding Tools and Specification", Matthias Wien, Signais and Communication Technology, 2015) proposes to implement a pixel prediction of a current image compared to other pixels belonging to the same image (intra prediction) or to a previous or next image (inter prediction).

More specifically, intra prediction exploits spatial redundancies within an image. To do this, the images are cut into blocks of pixels. The pixel blocks are then predicted using information already reconstructed, corresponding to the blocks previously coded / decoded in the current image according to the order of traversal of the blocks in the image.

Furthermore, conventionally, the coding of a current block is carried out using a prediction of the current block, known as the predictor block, and of a prediction residue or "residual block", corresponding to a difference between the current block and the predictor block. The residual block obtained is then transformed, for example by using a transform of the DOT type (transformed into discrete cosine). The coefficients of the transformed residual block are then quantified, then coded by an entropy coding and transmitted to the decoder, which can reconstruct the current block by adding this residual block to the predictor block.

Decoding is done image by image, and for each image, block by block. For each block, the corresponding elements of the flow are read. The inverse quantization and the inverse transformation of the coefficients of the residual block are carried out. Then the prediction of the block is calculated to obtain the predictor block and the current block is reconstructed by adding the prediction (ie the predictor block) to the decoded residual block.

In US9253508, a DPCM (for Differential Puise Code Modulation) coding technique for coding blocks in Intra mode is inserted in a HEVC coder. One such technique consists in predicting a set of pixels of an intra block by another set of pixels of the same block which have been previously reconstructed. In US9253508, a set of pixels of the intra block to be coded corresponds to a line of the block, or a column or a line and a column and the intra prediction used to predict the set of pixels is one of the intra directional predictions defined in the HEVC standard.

However, such a technique is not optimal. Indeed, the reconstruction of a set of pixels of the intra block corresponds either to the addition of a prediction residue in the case of lossless coding, therefore offering a fairly low compression rate, or to the addition a prediction residue after inverse transformation and / or inverse quantization of said other set of pixels serving as prediction. Such a technique therefore does not make it possible to predict each pixel of the intra block using a local prediction function and to reconstruct the predicted pixel before predicting a next pixel. Indeed, this technique requires to reconstruct a set of pixels (row / column of the block for example) to predict another set of pixels. In other words, each time a part of the block is predicted and reconstructed, several pixels of the block are predicted and reconstructed.

In addition, in US9253508, it is not described how to make coexistence of intra classical prediction modes as defined in the HEVC standard for example and the DPCM prediction mode.

There is therefore a need for a new coding and decoding method to improve the compression of image or video data.

3. Statement of the invention

The invention improves the state of the art. To this end, it relates to a method for decoding a stream of coded data representative of at least one image divided into blocks, the decoding method comprises, for at least one block of the image, known as the current block:

- the decoding of information indicating a coding mode of the current block,

- when the coding mode of the current block corresponds to a first coding mode, the decoding of the current block comprising:

- decoding a prediction residue associated with the current block using a first group of syntax elements,

- obtaining a prediction of the current block from reconstructed pixels of a previously decoded block, the reconstruction of said current block from the prediction obtained and from said decoded prediction residue.

- when the coding mode of the current block corresponds to a second coding mode, the decoding of the current block comprising:

decoding the prediction residue associated with the current block using a second group of syntax elements, said second group being a subgroup of syntax elements of said first group of syntax elements,

- reconstruction of said current block comprising:

- for each pixel of the current block:

obtaining a prediction of said pixel from another previously decoded pixel, said other previously decoded pixel belonging to said current block or to a block of the previously decoded image,

the reconstruction of said pixel from the prediction of said pixel obtained and from the prediction residue decoded using the second group of syntax elements.

Thus, according to the invention, at least part of the syntax elements of an existing coding mode can be used. This allows data processing to be shared, since the same processing unit can be used, and to reduce implementation costs, both at the hardware level and at the software level.

It should be noted that according to the invention, the first group of syntax elements and the second group of syntax elements are distinct. Indeed, the second group of syntax elements being a subgroup of the first group, it comprises at least one syntax element of said first group. The second group of syntax elements differs from the first group in that it does not include all of the syntax elements in the first group. Indeed, the second group of syntax elements comprises a number of syntax elements strictly less than the number of syntax elements of the first group. Thus the second group of syntax elements is a strict subgroup of the first group of syntax elements.

The invention thus makes it possible to adapt the coding of the prediction residue obtained according to the second coding mode to the statistics of the coefficients to be coded. Indeed, such a statistic differs from the statistic of the coefficients of the prediction residue obtained according to the first coding mode. Data compression is thus improved. The invention also relates to a method for coding a stream of coded data representative of at least one image divided into blocks. The coding method comprises, for at least one block of the image, called the current block:

- the coding of information indicating a coding mode of the current block,

- when the coding mode of the current block corresponds to a first coding mode, the coding of the current block comprising:

- obtaining a prediction of the current block from reconstructed pixels of a previously decoded block,

- obtaining a prediction residue associated with the current block obtained from the prediction obtained,

- the coding of said prediction residue associated with the current block using a first group of syntax elements,

- when the coding mode of the current block corresponds to a second coding mode, the coding of the current block comprising:

- for each pixel of the current block:

- obtaining a prediction residue from the prediction obtained for said pixel,

the coding of a prediction residue associated with the current block comprising the prediction residues associated with the pixels of said current block, using a second group of syntax elements, said second group being a subgroup of elements of syntax of said first group of syntax elements.

The invention also relates to a coded data stream representative of at least one image divided into blocks. The coded data stream comprises, for at least one block of the image, known as the current block:

- information indicating a coding mode of the current block,

- when the coding mode of the current block corresponds to a first coding mode, a prediction residue according to the first coding mode coded using a first group of syntax elements, the prediction residue according to the first coding mode being obtained from a prediction of the current block from reconstructed pixels of a previously decoded block,

- when the coding mode of the current block corresponds to a second coding mode, a prediction residue according to the second coding mode coded using of a second group of syntax elements, said second group being a subgroup of syntax elements of said first group of syntax elements, said prediction residue according to the second coding mode being obtained by:

- for each pixel of the current block:

- obtaining a prediction residue associated with said pixel from the prediction obtained for said pixel.

Such a data stream can be stored on any recording medium, for example a memory, or transmitted in the form of an electrical or optical signal, which can be routed via an electrical or optical cable, by radio or by other means.

According to a particular embodiment of the invention, the first group of syntax elements comprises localization syntax elements indicating the localization of a first non-zero coefficient of the prediction residue associated with said current block, according to a traversing order determined coefficients of said prediction residue, and said second group of syntax elements does not include said localization syntax elements.

When decoding or coding the prediction residue of the current block according to the first coding mode, the prediction residue is traversed from the first non-zero coefficient of the prediction residue to the last coefficient of the prediction residue according to said order of determined route.

According to the particular embodiment described here, during the decoding or coding of the prediction residue of the current block according to the second coding mode, the prediction residue is then traversed from the first coefficient of the prediction residue to the last coefficient the prediction residue according to said determined route order. This particular embodiment of the invention makes it possible to reduce the cost of coding the prediction residue according to the second coding mode when the first non-zero coefficient corresponds to the first coefficient of the prediction residue.

According to another particular embodiment of the invention, the prediction residue associated with the current block comprising at least one sub-block of coefficients, said first group of syntax elements comprises a sub-block syntax element associated with said at least one sub-block of coefficients, said sub-block syntax element indicating whether at least one coefficient of the sub-block is non-zero, and for each sub-block of coefficients of the prediction residue comprising at least one non-zero coefficient, a syntax element of significance for each coefficient of the sub-block, said element of syntax of significance indicating whether said coefficient is zero or not. According to this other particular embodiment of the invention, the second group of syntax elements comprises a significant syntax element for each coefficient of the prediction residue.

In other words, according to this particular embodiment of the invention, according to the second coding mode, the coefficients of the prediction residue associated with the current block are not grouped by sub-block and the element of sub-block syntax n is not included in the second group of syntax elements.

According to another particular embodiment of the invention, the first group of syntax elements comprises, for each non-zero coefficient of the prediction residue traversed according to a determined traversing order:

- an element of syntax indicating whether the absolute value of the coefficient is equal to 1 or not,

- for each coefficient for which the absolute value of the coefficient is not equal to 1, a syntax element indicating whether the absolute value of the coefficient is equal to 2 or not,

- for each coefficient for which the absolute value of the coefficient is neither 1 nor 2, a syntax element indicating whether the absolute value of the coefficient reduced by 3,

- a syntax element indicating whether the coefficient is positive or negative.

According to this other particular embodiment of the invention, the second group of syntax elements comprises for each non-zero coefficient of the prediction residue traveled according to a determined route order, a syntax element indicating the absolute value of the coefficient, and said syntax element indicating whether the coefficient is positive or negative.

The invention also relates to a decoding device configured to implement the decoding method according to any one of the particular embodiments defined above. This decoding device could of course include the various characteristics relating to the decoding method according to the invention. Thus, the characteristics and advantages of this decoding device are the same as those of the decoding method, and are not described in more detail.

The decoding device notably comprises a processor configured for, for at least one block of the image, called the current block:

- decode information indicating a coding mode of the current block,

- when the coding mode of the current block corresponds to a first coding mode, decode the current block via: - decoding a prediction residue associated with the current block using a first group of syntax elements,

the reconstruction of said current block from the prediction obtained and from said decoded prediction residue.

- when the coding mode of the current block corresponds to a second coding mode, decode the current block via:

- reconstruction of said current block comprising:

- for each pixel of the current block:

According to a particular embodiment of the invention, such a decoding device is included in a terminal.

The invention also relates to an encoding device configured to implement the encoding method according to any one of the particular embodiments defined above. This coding device could of course include the various characteristics relating to the coding method according to the invention. Thus, the characteristics and advantages of this coding device are the same as those of the coding method, and are not described in more detail.

The coding device notably comprises a processor configured for, for at least one block of the image, known as the current block:

- coding information indicating a coding mode of the current block,

- when the coding mode of the current block corresponds to a first coding mode, code the current block via:

- obtaining a prediction of the current block from reconstructed pixels of a previously decoded block, - obtaining a prediction residue associated with the current block obtained from the prediction obtained,

- when the coding mode of the current block corresponds to a second coding mode, code the current block via:

- for each pixel of the current block:

- obtaining a prediction residue from the prediction obtained for said pixel,

According to a particular embodiment of the invention, such a coding device is included in a terminal, or a server.

The decoding method, respectively the coding method, according to the invention can be implemented in various ways, in particular in wired form or in software form. According to a particular embodiment of the invention, the decoding method, respectively the coding method, is implemented by a computer program. The invention also relates to a computer program comprising instructions for implementing the decoding method or the coding method according to any one of the particular embodiments described above, when said program is executed by a processor. Such a program can use any programming language. It can be downloaded from a communication network and / or saved on a computer-readable medium.

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 recording medium or information medium readable by a computer, and comprising instructions of a computer program as mentioned above. The recording media mentioned above can be any entity or device capable of storing the program. For example, the media can include a storage means such as a memory. On the other hand, the recording media can correspond to a transmissible medium such as an electrical or optical signal, which can be routed via an electrical or optical cable, by radio or by other means. The program according to the invention can in particular be downloaded from a network of the Internet type.

Alternatively, the recording media can correspond to an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the process in question.

4. List of figures

Other characteristics and advantages of the invention will appear more clearly on reading the following description of a particular embodiment, given by way of simple illustrative and nonlimiting example, and of the appended drawings, among which:

FIG. 1 presents steps of the coding method according to a particular embodiment of the invention,

FIG. 2 illustrates an example of the position of the neighboring blocks of a current block for determining an intra prediction mode according to a particular embodiment of the invention,

FIG. 3 illustrates an example of the position of the reference pixels used to predict pixels of a current block according to a particular embodiment of the invention,

FIG. 4 presents steps of the decoding method according to a particular embodiment of the invention,

FIG. 5 illustrates an example of a signal comprising coded data representative of at least one block of an image according to a particular embodiment of the invention,

FIG. 6 shows the simplified structure of a coding device suitable for implementing the coding method according to any one of the particular embodiments of the invention,

FIG. 7 shows the simplified structure of a decoding device suitable for implementing the decoding method according to any one of the particular embodiments of the invention,

- Figure 8 illustrates the division into sub-blocks of a block of coefficients.

5. Description of an embodiment of the invention

5.1 General principle

The general principle of the invention is to allow the use of part of a group of syntax elements used to code a prediction residue obtained from a coding mode using a pixel prediction from '' at least one block previously reconstructed, to code a prediction residue resulting from an intra pixel based prediction, ie a prediction of the pixels of the block to be coded from pixels of the block to be coded previously reconstructed.

The invention thus makes it possible to improve the compression performance of the intra pixel based coding mode and to reduce the costs of implementing this new coding mode, in particular by making it possible to reuse part of the elements of syntax already used by a other coding mode.

5. 2 Examples of implementation

FIG. 1 presents steps of the coding method according to a particular embodiment of the invention. For example, we code a sequence of images l _; l ₂ , ..., l _N b in the form of a STR coded data stream according to a particular embodiment of the invention. For example, such a coding method is implemented by a coding device as described below with reference to FIG. 6.

A sequence of images h, l ₂ , ..., l _Nb , Nb being the number of images of the sequence to be coded, is supplied at the input of the coding method. The coding method outputs a stream of STR coded data representative of the sequence of images supplied as input.

In known manner, the coding of the sequence of images h, l ₂ , ..., l _N b is done image by image according to a coding order previously established and known to the coder. For example, the images can be coded in time order h, l ₂ , ..., l _N b or in another order, for example I 1 3, I2,, I Nb -

During a step E0, an image I, to be coded of the sequence of images, l ₂ , ..., l _Nb is cut into blocks, for example into blocks of size 32 × 32, or 64 × 64 pixels or more. Such a block can be subdivided into square or rectangular sub-blocks, for example of size 16 × 16, 8 × 8, 4x4, 16 × 8, 8 × 16, etc.

During a step E1, a first block or sub-block X _b to code of the image I is selected according to a direction of travel of the image I predetermined. For example, it can be the first block in the lexicographic order of the image.

During a step E2, the encoder will choose the coding mode for coding the current block X _b . According to the particular embodiment described here, the encoder selects the coding mode for coding the current block X _b from a first coding mode M1 and a second coding mode M2. Additional coding modes (not described here) can be used. According to the particular embodiment described here, the first coding mode M1 corresponds to the coding of the current block by intra classical prediction, for example as defined according to the HEVC standard and the second coding mode M2 corresponds to the coding by In Loop Residual prediction (ILR). During step E2, the coder can perform a bit rate / distortion optimization to determine the best coding mode for coding the current block. During this bit rate / distortion optimization, additional coding modes distinct from the first and second coding modes can be tested, for example a coding mode in inter mode. During this bit rate / distortion optimization, the coder simulates the coding of the current block X _b according to the different coding modes available in order to determine the bit rate and the distortion associated with each coding mode and selects the coding mode offering the best compromise. bit rate / distortion, for example according to the function D + 2R, where R represents the bit rate necessary to code the current block according to the coding mode evaluated, D the distortion measured between the decoded block and the original current block and l a Lagrangian multiplier, for example entered by the user or defined at the encoder.

During a step E20, information indicating the coding mode selected for the current block is coded in the data stream STR.

If the current block X _b is coded according to the first coding mode M1, the method goes to step E21 of coding the block according to M1. If the current block X _b is coded according to the second coding mode M2, the method goes to step E22 of coding the block according to M2.

The following step E21 of coding the block according to the first coding mode M1 is described below, according to a particular embodiment of the invention. According to the particular mode described here, the first coding mode corresponds to an intra classical prediction, such as that defined in the HEVC standard.

During a step E210, a quantization step ¾ is determined. For example, the quantization step ¾ can be set by the user, or calculated using a quantization parameter setting a compromise between compression and quality and entered by the user or defined by the coder. Thus, such a quantization parameter can be the parameter L, used in the rate-distortion cost function D + l .R, where D represents the distortion introduced by the coding and R the bit rate used to code. This function is used to make coding choices. Conventionally, we are looking for the way to code the image which minimizes this function.

As a variant, the quantification parameter can be QP, corresponding to the quantification parameter conventionally used in AVC or HEVC standards. Thus, in the HEVC standard, the quantification step ¾ is determined by the equation 5 ₁ = levelScale [QP% 6] “(QP / 6)) where levelScale [k] = {40, 45, 51, 57, 64 , 72} for k = 0..5. During a step E21 1, a prediction of the current block is determined using an intra-classical prediction mode. According to this intra-classical prediction, each predicted pixel is calculated only from the decoded pixels from the neighboring blocks (reference pixels) located above the current block, and to the left of the current block. How the pixels are predicted from the reference pixels depends on a prediction mode that is passed to the decoder, and which is chosen by the coder from a predetermined set of modes known to the coder and the decoder.

Thus, in HEVC, there are 35 possible prediction modes: 33 modes which interpolate the reference pixels in 33 different angular directions, and 2 other modes: the DC mode in which each pixel of the predicted block is produced from the average reference pixels, and PLANAR mode, which performs plane and non-directional interpolation. This so-called “intra classical prediction” approach is well known and also used in the ITU-T H.264 standard (where there are only 9 different modes) as well as in the experimental JEM software available at the internet address (https : // i vet. hhi .f rau nh of er. de /), where there are 67 different prediction modes. In all cases, the intra classical prediction respects the two aspects mentioned above (pixel prediction from neighboring blocks and transmission to the decoder of an optimal prediction mode).

During step E21 1, the coder therefore chooses one of the prediction modes available from the predetermined list of prediction modes. One way of choosing is, for example, to evaluate all the prediction modes and to keep the prediction mode which minimizes a cost function such as, conventionally, the bit rate-distortion cost.

During a step E212, the prediction mode chosen for the current block is coded from the neighboring blocks of the current block. FIG. 2 illustrates an example of the position of the neighboring blocks A _b and B _b of the current block X _b for coding the prediction mode of the current block X _b .

During step E212, the intra prediction mode chosen for the current block is coded using the intra prediction modes associated with the neighboring blocks.

For example, the approach described in the HEVC standard for coding the prediction mode of the current block can be used. In the example of FIG. 2, such an approach consists in identifying the intra m _A prediction mode associated with the block A _b located above the current block, and the intra m _B prediction mode associated with the block B _b located just to the left of the current block. Depending on the value of m _A and m _B , a so-called MPM (for Most Probable Mode) list, containing 3 intra prediction modes, and a so-called non-BPM list, containing the 32 other prediction modes, are created.

According to the HEVC standard, in order to code the intra prediction mode of the current block, syntax elements are transmitted:

-a binary indicator indicating whether the prediction mode to be coded for the current block is in the BPM list or not,

-if the prediction mode of the current block belongs to the MPM list, an index in the MPM list corresponding to the prediction mode of the current block is coded,

- if the prediction mode of the current block does not belong to the BPM list, an index in the non-BPM list corresponding to the prediction mode of the current block is coded. During a step E213, the prediction residue R for the current block is constructed.

During step E213, conventionally, a predicted block P is constructed as a function of the prediction mode chosen in step E21 1. Then the prediction residue R is obtained by calculating the difference for each pixel, between the predicted block P and the original current block.

During a step E214, the prediction residue R is transformed into R _T.

During step E214, a frequency transform is applied to the block of residue R so as to produce the block R _T comprising transformed coefficients. The transform could be a DCT type transform for example. It is possible to choose the transform to be used in a predetermined set of transforms E _T and to signal the transform used to the decoder.

During a step E215, the transformed residue block R _T is quantified using for example a scalar quantization of quantization step This produces the quantized transformed prediction residue block R _TQ .

During a step E216, the coefficients of the quantized block R _TQ are coded by an entropy coder. One can for example use the entropy coding specified in the HEVC standard. In this case, the coding of the coefficients of the residue R _TQ works as follows.

An order of traversal of the coefficients is determined. This order of travel is the same for the coder and the decoder. It is for example defined by default within the coder and the decoder. For example, it is a scan of the current quantized block R _TQ line by line and column by column.

Elements of syntax are transmitted to indicate the location of the first non-zero coefficient encountered according to the order of travel. These syntax elements will be called LastX and LastY (indicating the coordinates of said coefficient in the current quantized current block RTQ).

The coefficients are then traversed from said first non-zero coefficient to the last coefficient of the current quantized block R _TQ . The coefficients of the current quantized block R _TQ are grouped into sub-blocks. For example, the coefficients are grouped into 4x4 size sub-blocks contained in the current quantized block R _TQ , as illustrated in FIG. 8 showing a block of transformed prediction residue cut into sub-blocks of 4x4 coefficients. Other sizes of sub-blocks are of course possible.

For each sub-block, an element of syntax coded_sub_block_flag is transmitted, indicating whether this sub-block consists entirely of zeros or not. In other words, this syntax element takes the value 0 if all the coefficients of the sub-block are harmful and the value 1 otherwise (at least one coefficient of the subgroup is different from 0).

For each sub-block which includes at least one non-zero coefficient, an element of syntax sig_coeff_flag is transmitted for each coefficient (located after the last coefficient of the sub-block indicated by LastX and LastY according to the order of traversal determined), this syntax element indicating whether the coefficient is zero or not. Such a syntax element is not transmitted for the first non-zero coefficient identified by LastX and LastY since the coder already knows that this coefficient is non-zero.

For each non-zero coefficient, an element of syntax coeff_abs_level_greater1_flag is transmitted, indicating whether the coefficient is equal to 1 or not.

For each non-zero coefficient not equal to 1, an element of syntax coeff_abs_level_greater2_flag is transmitted, indicating whether the coefficient is equal to 2 or not. For each non-zero coefficient not equal to 1 and not equal to 2, an element of coeff_abs_level_remaining syntax is transmitted, indicating the amplitude of the coefficient reduced by 3. Finally, for each non-zero coefficient, an element of syntax coeff_sign_flag is transmitted to indicate whether the coefficient is positive or negative.

In known manner, the current block is decoded by de-quantizing the coefficients of the quantized block R _TQ , then by applying the inverse transform to the de-quantized coefficients to obtain the decoded prediction residue. The prediction is then added to the decoded prediction residue in order to reconstruct the current block and obtain its decoded version. The decoded version of the current block can then be used later to spatially predict other neighboring blocks of the image or else to predict blocks of other images by inter-image prediction.

The following step E22 of coding the block according to the second coding mode M2 is described below, according to a particular embodiment of the invention. According to the particular mode described here, the second coding mode corresponds to coding by ILR prediction.

During a step E220, a local predictor PL for the current block is determined. According to the coding mode described here, the pixels of the current block are predicted by pixels previously reconstructed from a neighboring block of the current block or of the current block itself.

Preferably, to predict, pixels are chosen which are as close as possible to the pixel to be predicted. For this reason, we speak of a local predictor. The local predictor PL can also be assimilated to a mode for predicting the current block associated with the second coding mode M2. According to this interpretation, in the particular embodiment described here, the first coding mode uses a first group of intra prediction modes, for example the intra prediction modes defined by the HEVC standard, and the second coding mode, here the ILR mode, uses a second group of prediction modes distinct from the first group of intra prediction modes.

The local predictor PL can be unique or it can be selected from a set of predetermined local predictors (second group of prediction modes). According to an alternative embodiment, 4 local predictors are defined. Thus, if X is called a current pixel to predict from the current block, A the pixel located immediately to the left of X, B the pixel located immediately to the left and above X, C the pixel located immediately above X, as illustrated in FIG. 3 showing a current block X _b . 4 local predictors PL1, PL2, PL3, PL4 can be defined as follows:

PL1 (X) = min (A, B) if C> max (A, B)

max (A, B) if C <min (A, B)

A + B-C otherwise

PL2 (X) = A

PL3 (X) = B

PL4 (X) = C

where min (A, B) corresponds to the function returning the smallest value between the value of A and the value of B and max (A, B) corresponds to the function returning the largest value between the value of A and the value of B.

During step E220, it is determined which local predictor PL to use for the current block. In other words, the same local predictor will be used for all the pixels of the current block, i.e. the same prediction function. For this, several alternative embodiments are possible. The coding of the current block with each of the predictors can be simulated (similar to an optimization for choosing a coding mode for the current block), and the local predictor which optimizes a cost function (for example, which minimizes the function D + AR, where R is the bit rate used to code the block, D is the distortion of the decoded block compared to the original block, and l is a parameter set by the user) is selected.

Or, in order to limit the complexity of selecting a local predictor for the current block, an orientation of the texture of the previously coded pixels is analyzed. For example, the pixels previously coded in the block which are located above or to the left of the current block are analyzed using a Sobel operator. If it is determined that:

- the orientation is horizontal, the local predictor PL2 is selected,

- the orientation is vertical, the local predictor PL3 is selected,

- the orientation is diagonal, the local predictor PL4 is selected,

- if no orientation emerges, the local predictor PL1 is selected.

A syntax element is coded in the STR data stream to indicate to the decoder which local predictor was used to predict the current block.

During a step E221, a quantization step d ₂ is determined. For example, the quantization step d ₂ depends on the same quantization parameter as the quantization step ^ which would be determined in step E210 if the current block was coded according to the first coding mode. During a step E222, a prediction residue R1 is calculated for the current block. To do this, once the local predictor has been chosen, for each current pixel of the current block:

the current pixel X of the current block is predicted by the local predictor PL selected, using either pixels outside the block and already reconstructed (and therefore available with their decoded value), or pixels previously reconstructed in the current block, either of the two, in order to obtain a predicted value PRED. In all cases, the predictor PL uses previously reconstructed pixels. In FIG. 3, it can be seen that the pixels of the current block situated on the first line and / or the first column of the current block will use as reference pixels (to construct the predicted value PRED) pixels external to the block and already reconstructed (pixels in gray in FIG. 3) and possibly already reconstructed pixels of the current block. For the other pixels of the current block, the reference pixels used to construct the predicted value PRED are located inside the current block;

-the difference DIFF between PRED and X is quantified in a value Q (X), by a scalar quantizer of quantization step d ₂ , by Q (X) = ScalarQuant (DIFF) = ScalarQuant (¾, X- PRED), the scalar quantifier being for example a nearest neighbor scalar quantizer such as: ScalarQuant (A, x) = floor

Q (X) is the quantized residue associated with X. It is calculated in the spatial domain, ie calculated directly from the difference between the predicted PRED value of the pixel X and the original value of X. Such a quantized residue Q (X ) for the pixel X is stored in a quantized prediction residue block R1 _Q , which will be coded later;

the decoded predicted value P1 (X) of X is calculated by adding to the predicted value PRED the de-quantized value of the quantized residue Q (X). The decoded predicted value P1 (X) of X is thus obtained by P1 (X) = PRED + ScalarDequant (¾, Q (X)). For example, the inverse nearest scalar quantization function is given by: ScalarDequant (A, x) = D x x.

The decoded predicted value P1 (X) thus makes it possible to predict possible pixels which remain to be processed in the current block. Furthermore, the block P1 comprising the decoded / reconstructed values of the pixels of the current block constitutes the predictor ILR of the current block (as opposed to the intra-classical predictor).

The sub-steps described above are performed for all the pixels of the current block, in a traversing order which ensures that the pixels used for the prediction chosen from PL1, ..., PL4 are available.

According to an alternative embodiment, the order of traversal of the current block is the lexicographic order, ie from left to right, and from top to bottom. According to another alternative embodiment, several travel orders of the current block can be used, for example:

-the lexicographic order, or

-crossing the first column from top to bottom, then the column just to its right, etc. or,

-crossing the diagonals one after the other.

According to this other variant, it is possible to simulate the coding cost associated with each of the travel orders and to choose the best travel order of the current block in the flow / distortion direction, then to code for the current block information representative of the 'chosen route order.

At the end of step E222, the quantized residue block R1 _Q has been determined. This quantized residue block R1 _Q must be coded to be transmitted to the decoder. The predictor P1 of the current block was also determined.

During a step E223, the quantized residue block R1 _Q is coded in order to transmit it to the decoder. It is possible to use any known approach, such as the method described in HEVC to code the quantized coefficients of a classical prediction residue.

According to the particular embodiment of the invention described here, the values of the quantized residue block R1 _Q are coded using an entropy coder in the STR data stream using at least part of the syntax elements used to code a prediction residue from the coding mode M1.

To encode the residue R1 _Q , a subgroup of the group of syntax elements used for the residue R _TQ is used.

Indeed, the statistic of the residue R1 _Q is different from that of the residue R _TQ It is therefore necessary to adapt the entropy coding of the prediction residue R1 _Q to this particular statistic in order to maximize the compression. However, by limiting itself to a syntax which is a subset of the syntax transmitted for an R _TQ residue, it is possible to reuse the same software or hardware component, which is of great interest from the point of view of the cost of implementation.

According to an alternative embodiment of the invention, the coding of the residue R1 _Q is done by omitting the elements of syntax LastX and LastY and by systematically traversing all the coefficients of the block of quantified prediction residue R1 _Q.

In this case, the coding of the coefficients of the residue R1 _Q works as follows.

An order of traversal of the coefficients is determined. The coefficients are traversed from the first coefficient of the quantized residue block R1 _Q to the last coefficient of the quantized residue block R1 _Q. The rest of the entropy coding of the coefficients is similar to that described in the case of the coding of a transformed prediction residue resulting from the coding mode M1.

These coefficients are grouped by sub-blocks, for example 4x4-sized sub-blocks contained in the current quantized residue block R1 _Q. Other sizes of sub-blocks are of course possible.

For each sub-block, an element of syntax coded_sub_block_flag is transmitted, indicating whether this sub-block consists entirely of zeros or not. For each sub-block which contains at least one non-zero coefficient, an element of syntax sig_coeff_flag is transmitted for each coefficient, this element of syntax indicating whether the coefficient is zero or not. For each non-zero coefficient, an element of syntax coeff_abs_level_greater1_flag is transmitted, indicating whether the coefficient is equal to 1 or not. For each non-zero coefficient not equal to 1, an element of syntax coeff_abs_level_greater2_flag is transmitted, indicating whether the coefficient is equal to 2 or not. For each non-zero coefficient not equal to 1 and not equal to 2, an element of coeff_abs_level_remaining syntax is transmitted, indicating the amplitude of the coefficient reduced by 3. Finally, for each non-zero coefficient, an element of syntax coeff_sign_flag is transmitted to indicate whether the coefficient is positive or negative.

According to another variant of the particular embodiment described here, the coding of the quantized residue R1 _Q is done by omitting the elements of syntax LastX and LastY and by systematically traversing all the coefficients of the block of quantized residue R1 _Q , and by omitting the coded_sub_block_flag element. A significance value sig_coeff_flag is therefore systematically coded for each coefficient of the quantized residue block R1 _Q.

According to this variant, the coding of the coefficients of the residue R1 _Q operates as follows. An order of traversal of the coefficients is determined. The coefficients are traversed from said first coefficient of the quantized residue block R1 _Q to the last coefficient of the quantized residue block R1 _Q according to the determined order of travel. For this purpose, for each coefficient, an element of syntax sig_coeff_flag is transmitted, this element of syntax indicating whether the coefficient is zero or not. For each non-zero coefficient, an element of syntax coeff_abs_level_greater1_flag is transmitted, indicating whether the coefficient is equal to 1 or not. For each non-zero coefficient not equal to 1, an element of syntax coeff_abs_level_greater2_flag is transmitted, indicating whether the coefficient is equal to 2 or not. For each non-zero coefficient not equal to 1 and not equal to 2, an element of coeff_abs_level_remaining syntax is transmitted, indicating the amplitude of the coefficient reduced by 3. Finally, for each non-zero coefficient, an element of syntax coeff_sign_flag is transmitted to indicate whether the coefficient is positive or negative. According to another alternative embodiment, the coding of the residue R1 _Q is carried out only using the elements of syntax coeff_abs_level_remaining and coeff_sign_flag. According to this variant, all the coefficients of the block are systematically traversed and the value of each coefficient is coded. The coding of the coefficients of the residue R1 _Q works as follows. An order of traversal of the coefficients is determined. The coefficients are traversed from the first coefficient of the quantized residue block R1 _Q to the last coefficient of the quantized residue block R1 _Q.

To this end, for each coefficient of the quantized residue block R1 _Q, an element of coeff_abs_level_remaining syntax is transmitted, indicating the amplitude of the coefficient, and for each non-zero coefficient, an element of syntax coeff_sign_flag is transmitted in order to indicate whether the coefficient is positive or negative.

It appears that according to all of the variant embodiments described above, the coding of the prediction residue R1 _Q is carried out on the basis of a group of syntax elements which is a strict subset (that is to say not equal) and not empty of the syntax elements used for the "classical" residue R _TQ .

According to a particular embodiment of the invention, it is possible to determine and code an additional prediction residue R2 from the predictor ILR obtained for the current block. The coding of an additional prediction residue R2 is however optional. It is indeed possible to simply code the current block by its predicted version P1 and the quantized residue R1 _q .

In order to code an additional prediction residue R2 for the current block, the following steps are implemented.

During a step E224, the difference R2 between the predictor P1 and the original current block X _b is calculated in order to constitute an additional residue R2: R2 = X _b -P1. The following steps correspond to the conventional steps of coding this residue R2.

During a step E225, the residue R2 is transformed using a frequency transform so as to produce the block of coefficients R2 _T.

The transform can be a DCT type transform for example. It is possible to choose the transform to be used in a predetermined set of transforms E _T2 and to signal the transform used to the decoder. In this case, the set E _T2 can be different from the set E _T , in order to adapt to the particular statistics of the residue R2.

During a step E226, the block of coefficients R2 _T is quantized, for example using a scalar quantization of quantization step d. This produces the R2 _TQ block.

The quantization step d can be set by the user, it can also be calculated using another parameter l fixing the compromise between compression and quality and entered by user or coder. For example, the quantization step d may correspond to the quantization step ¾ or be determined in a similar manner to this.

During a step E227, the coefficients of the quantized block R2 _TQ are then transmitted in a coded manner. For example, the coding specified in the HEVC standard can be used.

In known manner, the current block is decoded by de-quantizing the coefficients of the quantized block R2 _TQ , then by applying the inverse transform to the de-quantized coefficients to obtain the decoded prediction residue. The prediction P1 is then added to the decoded prediction residue in order to reconstruct the current block and to obtain its decoded version X _rec . The decoded version X _rec of the current block can then be used later to spatially predict other neighboring blocks of the image or else to predict blocks of other images by inter-image prediction.

During a step E23, it is checked whether the current block is the last block of the image to be processed by the coding method, taking into account the travel order defined above. If so, the method proceeds to coding (step E25) of the next image of the video if necessary. If not, during a step E24, the next block of the image to be processed is selected according to the path of the image defined above and the coding method goes to step E2, where the selected block becomes the current block treat.

FIG. 4 presents steps of the method of decoding a stream STR of coded data representative of a sequence of images 1 _; l ₂ , ..., l _N b to be decoded according to a particular embodiment of the invention.

For example, the STR data stream was generated via the coding method presented in relation to FIG. 1. The data stream STR is supplied at the input of a decoding device DEC, as described in relation to FIG. 7.

The decoding method decodes the image-by-image stream and each image is decoded block by block.

During a step E40, an image I, to be decoded is subdivided into blocks. Each block will undergo a decoding operation consisting of a series of steps which are detailed below. The blocks can be the same size or different sizes.

During a step E41, a first block or sub-block X _b to be decoded from the image I, is selected as the current block according to a direction of travel of the image I, which is predetermined. For example, it can be the first block in the lexicographic order of the image.

During a step E42, information indicating an encoding mode for the current block is read from the data stream STR. According to the particular embodiment described here, this information indicates whether the current block is coded according to a first coding mode M1 or according to a second coding mode M2. According to the particular embodiment described here, the first coding mode M1 corresponds to the coding of the current block by intra classical prediction, for example as defined according to the HEVC standard, and the second coding mode M2 corresponds to the coding by In Loop prediction Residual (ILR).

In other particular embodiments, the information read from the stream STR can also indicate the use of other coding modes for coding the current block (not described here).

The following step E43 of decoding the current block is described when the current block is coded according to the first coding mode M1.

During a step E430, a quantization step ¾ is determined. For example, the quantization step ¾ is determined from the quantization parameter QP read from the data stream STR or in a similar manner to what was done at the coder. For example, the quantization step ¾ can be calculated using the quantization parameter QP read from the data stream STR. For example, the QP quantization parameter can be the quantification parameter conventionally used in AVC or HEVC standards. Thus, in the HEVC standard, the quantification step ¾ is determined by the equation 5 ₁ = levelScale [QP% 6] “(QP / 6)) where levelScale [k] = {40, 45, 51, 57, 64 , 72} for k = 0..5.

During a step E431, the prediction mode used to code the current block is decoded from the neighboring blocks. For this, like what was done at the coder, the intra prediction mode chosen for the current block is decoded, using the intra prediction modes associated with the neighboring blocks of the current block.

The construction of the two MPM and non-MPM lists is strictly similar to what was done during coding. According to the HEVC standard, syntax elements of the following type are decoded:

-if the prediction mode of the current block belongs to the MPM list, an index in the MPM list corresponding to the prediction mode of the coded current block,

- if the prediction mode of the current block does not belong to the MPM list, an index in the non-BPM list corresponding to the prediction mode of the current block coded.

The binary indicator and the prediction mode index are therefore read for the current block from the STR data stream, to decode the intra prediction mode of the current block.

During a step E432, the decoder constructs a predicted block P for the current block from the decoded prediction mode. During a step E433, the decoder decodes the coefficients of the quantized block R _TQ from the data stream STR, for example using the decoding specified in the HEVC standard. In this case, the decoding of the coefficients of the residue R _TQ works as follows. A travel order of the coefficients is determined corresponding to the travel order used at the coder. LastX and LastY syntax elements indicating the coordinates of the first nonzero coefficient in the residue block R _TQ according to the determined order of traversal, are decoded. The coefficients are traversed from the first non-zero coefficient to the last coefficient of the block. To this end, these coefficients are grouped into 4x4 size sub-blocks contained in the current quantized residue block R _TQ . For each sub-block, an element of syntax coded_sub_block_flag is decoded, indicating whether this sub-block consists entirely of zeros or not. For each sub-block which contains at least one non-zero coefficient, an element of syntax sig_coeff_flag is decoded for each coefficient (located after the last coefficient of the block indicated by LastX and LastY), this element of syntax indicating whether the coefficient is zero or not. For each non-zero coefficient, an element of syntax coeff_abs_level_greater1_flag is decoded, indicating whether the coefficient is equal to 1 or not. For each non-zero coefficient and not equal to 1, an element of syntax coeff_abs_level_greater2_flag is decoded, indicating whether the coefficient is equal to 2 or not. For each non-zero coefficient not equal to 1 and not equal to 2, an element of coeff_abs_level_remaining syntax is decoded, indicating the amplitude of the coefficient reduced by 3. Finally, for each non-zero coefficient, an element of syntax coeff_sign_flag is decoded to indicate whether the coefficient is positive or negative.

During a step E434, the decoded block R _TQ is de-quantized, for example using a scalar de-quantization of quantization step This produces the block of RTQD- quantized coefficients

During a step E435, an inverse frequency transform is applied to the block of de-quantified coefficients R _T Q _D so as to produce the block of decoded prediction residue RTQDI. The transform could be a reverse DCT type transform for example. It is possible to choose the transform to be used in a predetermined set of transforms by decoding an indicator from the data stream STR.

During a step E436, the current block is reconstructed from the predicted block P obtained in step E432 and the decoded residue block R _TQ DI obtained in step E435, in order to produce the decoded current block X _rec , by X _rec = P + RTQDI-

The following step E44 describes the decoding of the current block when the current block is coded according to the second coding mode M2. During a step E440, the local predictor PL used to predict the pixels of the current block is determined. In the case where only one predictor is available, the local predictor is for example defined by default at the level of the decoder and no element of syntax needs to be read in the stream STR to determine it.

In the case where several local predictors are available, for example the predictors PL1 -PL4 described above, a syntax element is decoded from the data stream STR to identify which local predictor was used to predict the current block. The local predictor is therefore determined from this decoded syntax element.

During a step E441, the quantization step d ₂ is determined, in a similar manner to what has been done at the coder.

During a step E442, the quantized residue R1 _Q is decoded from the data stream STR. According to the particular embodiment of the invention described here, the values of the quantized residue block R1 _Q are decoded from the data stream STR using at least part of the syntax elements used to decode a prediction residue from the coding mode M1. To decode the residue R1 _Q , a subgroup of the group of syntax elements used for the residue R _TQ is used.

According to an alternative embodiment, the decoding of the residue R1 _Q is done by omitting the elements of syntax LastX and LastY and by systematically traversing all the coefficients of the block of quantized residue R1 _Q. In this case, the decoding of the coefficients of the residue R1 _Q works as follows. A travel order of the coefficients is determined, corresponding to the travel order determined by the coder. The coefficients are traversed from the first coefficient of the quantized residue block R1 _Q to the last coefficient of the quantized residue block R1 _Q. To this end, these coefficients are grouped into 4x4 size sub-blocks contained in the quantized residue block R1 _Q. For each sub-block, an element of syntax coded_sub_block_flag is decoded, indicating whether this sub-block consists entirely of zeros or not. For each sub-block which contains at least one non-zero coefficient, an element of syntax sig_coeff_flag is decoded for each coefficient, this element of syntax indicating whether the coefficient is zero or not. For each non-zero coefficient, an element of syntax coeff_abs_level_greater1_flag is decoded, indicating whether the coefficient is equal to 1 or not. For each non-zero coefficient not equal to

1, an element of syntax coeff_abs_level_greater2_flag is decoded, indicating whether the coefficient is equal to 2 or not. For each non-zero coefficient and not equal to 1 and not equal to

2, an element of coeff_abs_level_remaining syntax is decoded, indicating the amplitude of the coefficient reduced by 3.

Finally, for each non-zero coefficient, an element of coeff_sign_flag syntax is decoded in order to indicate whether the coefficient is positive or negative. According to an alternative embodiment, the decoding of the residue R1 _Q is done by omitting the elements of syntax LastX and LastY and by systematically traversing all the coefficients of the block of quantized residue R1 _Q , and by omitting the element of syntax coded_sub_block_flag and therefore in systematically decoding a value for each coefficient of the block.

In this case, the decoding of the coefficients of the residue R _TQ works as follows. An order of traversal of the coefficients is determined. The coefficients are traversed from the first coefficient of the quantized residue block R1 _Q to the last coefficient of the quantized residue block R1 _Q. For each coefficient, an element of syntax sig_coeff_flag is decoded, this element of syntax indicating whether the coefficient is zero or not. For each non-zero coefficient, an element of syntax coeff_abs_level_greater1_flag is decoded, indicating whether the coefficient is equal to 1 or not. For each non-zero coefficient not equal to

2, an element of syntax coeff_abs_level_remaining is decoded, indicating the amplitude of the coefficient decreased by 3. Finally, for each coefficient not zero, an element of syntax coeff_sign_flag is decoded in order to indicate whether the coefficient is positive or negative.

According to another alternative embodiment, the decoding of the residue R1 _Q is done only using the elements of syntax coeff_abs_level_remaining and coeff_sign_flag. In this case, the decoding of the coefficients of the residue R1 _Q works as follows. An order of traversal of the coefficients is determined. The coefficients are traversed from the first coefficient of the quantized residue block R1 _Q to the last coefficient of the quantized residue block R1 _Q. For this purpose, for each coefficient an element of syntax coeff_abs_level_remaining is decoded, indicating the amplitude of the coefficient and for each coefficient not zero, an element of syntax coeff_sign_flag is decoded in order to indicate whether the coefficient is positive or negative.

During a step E443, the quantized residue block R1 _Q is de-quantified using the quantization step d ₂ , so as to produce the de-quantized residue block R1 _QD .

During a step E444, when the de-quantized residue block R1 _QD is obtained, the predicted block P1 is constructed using the local predictor PL determined during step E440.

During step E444, each pixel of the current block is predicted and reconstructed as follows:

the current pixel X of the current block is predicted by the predictor PL selected, using either pixels outside the block and already decoded, or pixels previously reconstructed from the current block, or both, to obtain a predicted PRED value. In all cases, the predictor PL uses previously decoded pixels;

the decoded predicted value P1 (X) of the current pixel X is calculated by adding to the predicted value PRED, the de-quantified value of the prediction residue R1 _QD , such that P1 (X) = PRED + R1 QD (X).

These steps are implemented for all the pixels of the current block, in a traversing order which ensures that the pixels used for the prediction chosen from among PL1, ..., PL4 are available.

For example, the route order is the lexicographic order (from left to right, then the lines from top to bottom).

According to a particular embodiment of the invention, the predicted block P1 comprising the decoded predicted values P1 (X) of each pixel of the current block here constitutes the decoded current block X _rec .

According to another particular embodiment of the invention, it is considered here that an additional prediction residue has been coded for the current block. It is therefore necessary to decode this additional prediction residue in order to reconstruct the decoded version of the current block X _rec . For example, this other particular embodiment can be activated or not by default at the level of the coder and the decoder. Alternatively, an indicator can be encoded in the data stream with the block level information to indicate for each block encoded according to the ILR encoding mode whether an additional prediction residue is encoded. Alternatively, an indicator can be coded in the data stream with the image level or image sequence information to indicate for all the blocks of the image or of the image sequence coded according to the ILR coding mode if a additional prediction residue is coded.

When an additional prediction residue is coded for the current block, during a step E445, the coefficients of the quantized prediction residue R2 _TQ are decoded from the data stream STR, using means adapted to those implemented to the coder, for example the means implemented in a HEVC decoder.

During a step E446, the block of quantized coefficients R2 _TQ is de-quantified, for example using a scalar de-quantization of quantization step This produces the block of unquantified coefficients R2 _TQD .

During a step E447, an inverse frequency transform is applied to the block R2 _TQD so as to produce the block of decoded prediction residue R2 _{TQD |} .

The reverse transform could be a reverse DCT type transform for example.

It is possible to choose the transform to be used in a predetermined set of transforms E _T2 and to decode the information signaling the transform to be used at the decoder. In this case, the set E _T2 is different from the set E _T , in order to adapt to the particular statistics of the residue R2.

During a step E448, the current block is reconstructed by adding the predicted block P1 obtained during the step E444 to the decoded prediction residue R2 _TQDL

During a step E45, it is checked whether the current block is the last block of the image to be processed by the decoding method, taking into account the course order defined above. If so, the method proceeds to decoding (step E47) of the next image of the video if necessary. If not, during a step E46, the next block of the image to be processed is selected according to the path of the image defined above and the decoding method proceeds to step E42, the selected block becoming the current block to treat.

FIG. 5 illustrates an example of a STR signal comprising coded data representative of at least one block of an image according to a particular embodiment of the invention.

The signal STR comprises a coded indicator TY indicating for a block of an image, a coding mode for this block. When the TY indicator indicates that the block is coded according to the second coding mode, here the ILR mode, the signal then comprises quantized prediction residue values R1 _Q coded using a group of syntax elements which is a subgroup of the group of syntax elements used to code prediction residue values from the first coding mode. Such a subgroup includes elements of syntax as described in relation to FIG. 1 or 4 when the current block is coded according to the second coding mode.

The signal optionally includes coded values of quantized transformed prediction residues R2 _TQ . When several local predictors are possible for the current block, the signal also includes a coded local predictor indicator PL.

When the TY indicator indicates that the block is coded according to the first coding mode, here the intra classical prediction mode, the signal then comprises quantized transformed prediction residue values R _TQ coded using a group d elements of syntax as described in relation to FIGS. 1 or 4 when the current block is coded according to the first coding mode, a binary indicator Î _M PM indicating whether the prediction mode to be coded for the current block is in the list MPM or not, and an idx _MpM index indicating the index of the prediction mode of the current block in the corresponding list.

FIG. 6 presents the simplified structure of a COD coding device suitable for implementing the coding method according to any one of the particular embodiments of the invention. According to a particular embodiment of the invention, the steps of the coding method are implemented by computer program instructions. For this, the coding device COD has the conventional architecture of a computer and notably comprises a memory MEM, a processing unit UT, equipped for example with a processor PROC, and controlled by the computer program PG stored in MEM memory. The computer program PG includes instructions for implementing the steps of the coding method as described above, when the program is executed by the processor PROC.

On initialization, the code instructions of the computer program PG are for example loaded into a memory RAM (not shown) before being executed by the processor PROC. The processor PROC of the processing unit UT implements in particular the steps of the coding method described above, according to the instructions of the computer program PG.

FIG. 7 shows the simplified structure of a DEC decoding device suitable for implementing the decoding method according to any one of the particular embodiments of the invention.

According to a particular embodiment of the invention, the DEC decoding device has the conventional architecture of a computer and in particular comprises a MEMO memory, a UTO processing unit, equipped for example with a PROCO processor, and controlled by the PGO computer program stored in MEMO memory. The PGO computer program includes instructions for implementing the steps of the decoding method as described above, when the program is executed by the PROCO processor.

On initialization, the code instructions of the PGO computer program are for example loaded into a RAM memory (not shown) before being executed by the PROCO processor. The processor PROCO of the processing unit UTO implements in particular the steps of the decoding method described above, according to the instructions of the computer program PGO.

Claims

1. Method for decoding a coded data stream representative of at least one image, said image being divided into blocks, the decoding method comprises, for at least one block of the image, known as the current block:

- the decoding (E42) of information indicating a coding mode of the current block,

- when the coding mode of the current block corresponds to a first coding mode, the decoding (E43) of the current block comprising:

- decoding (E433) of a prediction residue associated with the current block using a first group of syntax elements,

- obtaining (E432) a prediction of the current block from reconstructed pixels of a previously decoded block,

- the reconstruction (E436) of said current block from the prediction obtained and from said decoded prediction residue.

- when the coding mode of the current block corresponds to a second coding mode, the decoding (E44) of the current block comprising:

- decoding (E442) of the prediction residue associated with the current block using a second group of syntax elements, said second group being a subgroup of syntax elements of said first group of syntax elements ,

- the reconstruction (E444) of said current block comprising:

- for each pixel of the current block:

2. Method for coding a data stream representative of at least one image, said image being divided into blocks, the coding method comprises, for at least one block of the image, known as the current block:

- the coding (E20) of information indicating a coding mode of the current block,

- when the coding mode of the current block corresponds to a first coding mode, the coding (E21) of the current block comprising:

- obtaining (E213) a prediction of the current block from reconstructed pixels of a previously decoded block, - obtaining (E213) a prediction residue associated with the current block obtained from the prediction obtained,

- the coding (E216) of said prediction residue associated with the current block using a first group of syntax elements,

- when the coding mode of the current block corresponds to a second coding mode, the coding (E22) of the current block comprising:

- for each pixel of the current block:

- obtaining (E222) a prediction of said pixel from another previously decoded pixel, said other previously decoded pixel belonging to said current block or to a block of the previously decoded image,

- obtaining (E222) a prediction residue from the prediction obtained for said pixel,

- the coding (E223) of a prediction residue associated with the current block comprising the prediction residues associated with the pixels of said current block, using a second group of syntax elements, said second group being a sub- group of syntax elements of said first group of syntax elements.

3. Method according to any one of claims 1 or 2, in which:

said first group of syntax elements comprises localization syntax elements indicating the localization of a first non-zero coefficient of the prediction residue associated with said current block, according to a determined traversing order of the coefficients of said prediction residue,

- And said second group of syntax elements does not include said localization syntax elements.

4. Method according to any one of claims 1 -3, in which:

said prediction residue associated with the current block comprising at least one sub-block of coefficients, said first group of syntax elements comprises:

an element of sub-block syntax associated with said at least one sub-block of coefficients, said element of sub-block syntax indicating whether at least one coefficient of the sub-block is non-zero, and

for each sub-block of coefficients of the prediction residue comprising at least one non-zero coefficient, an element of syntax of significance for each coefficient of the sub-block, said element of syntax of significance indicating whether said coefficient is zero or not, and - Said second group of syntax elements includes a syntax element of significance for each coefficient of the prediction residue.

5. Method according to any one of claims 1 -4, in which:

said first group of syntax elements comprises, for each non-zero coefficient of the prediction residue traveled according to a determined route order:

- for each coefficient for which the absolute value of the coefficient is neither 1 nor 2, a syntax element indicating the absolute value of the coefficient minus 3,

- a syntax element indicating whether the coefficient is positive or negative, and

said second group of syntax elements comprises for each non-zero coefficient of the prediction residue traveled according to a determined route order:

- a syntax element indicating the absolute value of the coefficient,

- said syntax element indicating whether the coefficient is positive or negative.

6. Device for decoding a coded data stream representative of at least one image, said image being divided into blocks, the decoding device comprises a processor (PROCO) configured to, for at least one block of the image, says current block:

- decode information indicating a coding mode of the current block,

- when the coding mode of the current block corresponds to a first coding mode, decode the current block via:

- reconstruction of said current block comprising:

- for each pixel of the current block: obtaining a prediction of said pixel from another previously decoded pixel, said other previously decoded pixel belonging to said current block or to a block of the previously decoded image,

7. Device for coding a data stream representative of at least one image, said image being cut into blocks, the coding device comprises a processor (PROC) configured to, for at least one block of the image, known as current block:

- coding information indicating a coding mode of the current block,

- for each pixel of the current block:

- obtaining a prediction residue from the prediction obtained for said pixel,

8. Coded data stream representative of at least one image, said image being divided into blocks, the coded data stream comprises, for at least one block of the image, known as the current block:

- information indicating a coding mode of the current block, - when the coding mode of the current block corresponds to a first coding mode, a prediction residue according to the first coding mode coded using a first group of syntax elements, the prediction residue according to the first coding mode being obtained from a prediction of the current block from reconstructed pixels of a previously decoded block,

- when the coding mode of the current block corresponds to a second coding mode, a prediction residue according to the second coding mode coded using a second group of syntax elements, said second group being a sub- group of syntax elements of said first group of syntax elements, said prediction residue according to the second coding mode being obtained by:

- for each pixel of the current block:

9. Data flow according to claim 8, in which:

said first group of syntax elements comprises localization syntax elements indicating the localization of a first non-zero coefficient of said prediction residue associated with the current block according to a travel order of the prediction residue determined,

10. Data flow according to any one of claims 8 or 9, in which:

said prediction residue comprising at least one sub-block of coefficients, said first group of syntax elements comprises:

for each sub-block of coefficients of the prediction residue comprising at least one non-zero coefficient, an element of syntax of significance for each coefficient of the sub-block, said element of syntax of significance indicating whether said coefficient is zero or not, and

- Said second group of syntax elements includes a syntax element of significance for each coefficient of the prediction residue.

1 1. Data flow according to any one of claims 8-10, in which:

- for each coefficient for which the absolute value of the coefficient is neither 1 nor 2, a syntax element indicating the absolute value of the coefficient reduced by 3, - a syntax element indicating whether the coefficient is positive or negative , and

- a syntax element indicating the absolute value of the coefficient,

12. Computer program comprising instructions for implementing the decoding method according to any one of claims 1 or 3 to 5 or the coding method according to any one of claims 2 to 5, when said program is executed by a processor.

13. Information medium readable by a computer, and comprising instructions of a computer program according to claim 12.