EP3272122A1

EP3272122A1 - Encoding of images by vector quantization

Info

Publication number: EP3272122A1
Application number: EP16718390.4A
Authority: EP
Inventors: Félix Henry; Bihong HUANG
Original assignee: Orange SA
Current assignee: Orange SA
Priority date: 2015-02-19
Filing date: 2016-02-18
Publication date: 2018-01-24
Also published as: WO2016132074A1; CN107343391A; KR20170120634A; FR3033114A1; US20180070109A1

Abstract

The invention relates to encoding of at least one image (IC_j). Said encoding implements, for a common block (B_u) to be coded for said image: - predicting (C3) the common block according to a prediction procedure selected from among a plurality of predetermined prediction procedures ; - obtaining a predictor block (BP_opt) from the prediction; - calculating (C4) a first set of data representing a comparison between the predictor block obtained and the common block; - comparing (C5) said calculated first set with a plurality of quantization vectors; - selecting (C6) one of said vectors according to a predetermined encoding performance criterion; - encoding (C7) an index associated with the selected vector; - calculating (C8) a second set of data representing a comparison between the first calculated data set and the selected vector; and - encoding (C9) the second calculated data set. During said encoding, at least one of said quantization vectors is modified on the basis of the data from the second calculated data set.

Description

CODING IMAGES BY VECTORIAL QUANTIFICATION

Field of the invention

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

The invention can be applied in particular, but not exclusively, to the video coding implemented in the current AVC and HEVC video coders and their extensions (MVC, 3D-AVC, MV-HEVC, 3D-HEVC, etc.), as well as to the corresponding decoding.

Background of the invention

Current video encoders (MPEG, H.264, HEVC, ...) use a block representation of the images to be encoded. The images are subdivided into blocks of square or rectangular shape, which can be subdivided in turn recursively.

For at least one block considered among the different blocks obtained, a pixel prediction of the block in question is implemented with respect to prediction pixels that belong to either the same image (Intra prediction) or to one or more preceding images. a sequence of images (Inter prediction) that have already been decoded. Such prior images are conventionally referred to as reference images and are stored in memory at both the encoder and the decoder. During such a prediction, a set of data is calculated by subtracting the pixels of the considered block from the prediction pixels. The coefficients of the computed data set are then quantized after a possible mathematical transformation, for example of the discrete cosine transform (DCT) type, then coded by an entropy coder. The encoded data is written into a data signal to be transmitted to a decoder.

Said data signal comprises in particular: the type of prediction (Intra prediction, Inter prediction, prediction by default carrying out a prediction for which no information is transmitted to the decoder ("in English" skip "));

the prediction mode (prediction direction, reference image component, etc.);

- the type of subdivision into sub-blocks;

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

the pixel values, the transformed coefficient values, the amplitudes, the signs of the quantified coefficients of the pixels contained in the block or sub-block considered.

Once the data signal has been received by the decoder, the decoding is done image by image, and for each image, block by block. For each block, the corresponding elements of the data signal are read. Inverse quantization and inverse transformation of block coefficients are performed to produce a decoded residual data block. Then, the prediction of the block is calculated and the block is reconstructed by adding the prediction to the decoded residual data block.

In a manner known per se, the quantization of the coefficients of the residual data block calculated may be of scalar or vector type.

Scalar quantization uses a quantization step that is determined at the encoding from a parameter called QP (abbreviation of "Quantization Parameter").

Vector quantization consists of coding:

comparing the computed residual data set with a plurality of quantification vectors grouped in at least one dictionary,

selecting one of the quantization vectors according to a predetermined coding performance criterion, such as, for example, the bitrate-distortion compromise well known to those skilled in the art,

to code an index associated with said selected quantization vector.

The coded index is then transmitted in the data signal to the decoder which contains the same dictionary or dictionaries as the coder and which applies inverse vector quantization by decoding said transmitted index, and then determining the quantization vector associated with said decoded index.

In view of the fact that the quantization vectors are part of a predetermined dictionary, the coding of the current block by vector quantization may prove to be poorly adapted to the statistics of the video signal.

Ultimately, this results in compression performance that is not satisfactory.

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, implementing, for a current block to code the image:

a prediction of the current block according to a prediction mode selected from among a plurality of predetermined prediction modes,

a obtaining of a predictor block at the end of the prediction,

a calculation of a first set of data representative of a difference between the obtained predictor block and the current block,

a comparison of the first set of calculated data with a plurality of quantization vectors,

a selection of one of the quantization vectors according to a predetermined coding performance criterion,

an encoding of an index associated with the selected quantization vector.

The coding method according to the invention is remarkable in that it comprises:

a calculation of a second set of data representative of a difference between the first set of calculated data and the selected quantization vector, a coding of the second calculated data set in which at least one of the quantization vectors is modified according to the data of the second calculated data set.

Such a provision advantageously makes it possible to cumulate:

the benefit derived from the exploitation of the correlations between the first set of current residual data and the quantization vectors of a dictionary,

and the benefit derived from an update of at least one of said quantization vectors, said update making it possible, over time, to adapt to the modifications of the coded data signal.

The prediction of the current block is thus both more accurate and more adapted to the current video context, which has the advantage of significantly improving the compression performance of the coded data signal to be transmitted to the decoder.

In a particular embodiment, the modification of one of the quantization vectors is implemented only if the data of the second calculated data set fulfills a predetermined criterion.

Such an arrangement makes it possible to modify one of the quantization vectors only when this modification is substantial, which avoids unnecessary calculations.

In another particular embodiment, the modification of one of the quantization vectors is implemented using a parameter whose value depends on the size of the image to be encoded.

Such an arrangement makes it possible to adapt to the size of the data available to the encoder. Thus, if the image is small, the dictionary changes must be large, since the encoder has little data to capture the statistics of the video signal. On the other hand, if the image is large, it is possible to give priority to an update of small amplitude, which is more resistant to irrelevant statistical variations of the video signal, since the encoder has much more data to adapt to these changes. variations. In yet another particular embodiment, if the current image is of Intra type, the quantization vectors are each initialized to predetermined values.

Such an arrangement makes it possible to retain the independent coding characteristic of the other images that is specific to the Intra images, even if the invention proposes an update of the quantization vector dictionary which evolves as the images of the images are processed. a sequence of images considered. Indeed, it is generally desirable that the Intra-type images are coded and then decoded independently of any other image of the video stream, so that a video can be decoded during transmission (for example, to change channels on a television receiver , in the case of broadcasting an audiovisual stream). Thus, when an Intra image of this sequence must be encoded, it can not be encoded with respect to a quantization vector of a dictionary that has been updated for an image preceding this Intra image. Therefore, the invention advantageously proposes to initialize the dictionary quantization vectors to respective predetermined values to make the coding of the Intra Image independent of any other image in the sequence.

The various embodiments or embodiments mentioned above may be added independently or in combination with each other, to the operations implemented during the coding method as defined above.

Correlatively, the invention relates to a device for coding at least one image divided into blocks, comprising, for a current block to be encoded in said image:

a prediction module for predicting the current block according to a prediction mode selected from among a plurality of predetermined prediction modes, delivering a predictor block,

a calculation module for calculating a first set of data representative of a difference between the obtained predictor block and the current block,

a comparison module for comparing the first set of calculated data with a plurality of quantization vectors, a selection module for selecting one of the quantization vectors according to a predetermined coding performance criterion,

a first coding module for coding an index associated with the selected quantization vector.

Such a coding device is remarkable in that:

the calculation module is activated to calculate a second set of data representative of a difference between the first set of calculated data and the selected quantization vector, and in that it comprises a second coding module which is activated to code the second calculated data set, at least one of the quantization vectors being modified according to the data of the second calculated data set.

Correspondingly, the invention also relates to a method for decoding a data signal representative of at least one image divided into blocks, implementing, for a current block to be decoded:

- a decoding:

An index associated with a quantization vector that belongs to a set of quantization vectors,

And prediction data of the current block to be decoded, a decoding of the index,

a determination of the quantization vector associated with the decoded index,

obtaining a predictor block from the decoded prediction data.

Such a decoding method is remarkable in that it comprises:

a determination, in the data signal, of data relating to the current block to be decoded,

a decoding of the data relating to the current block to be decoded, during which at least one of the quantization vectors is modified as a function of the decoded data,

a reconstruction of the current block from the decoded data, the determined quantization vector and the obtained predictor block. The data relating to the current block to be decoded are typically the values of the pixels of this block making it possible to reconstruct this block or transformed coefficient values making it possible to reconstruct this block.

In a particular embodiment, the modification of one of the quantization vectors is implemented only if the decoded data fulfill a predetermined criterion.

In another particular embodiment, the modification of one of the quantization vectors is implemented using a parameter whose value depends on the size of the image to be decoded.

In yet another particular embodiment, if the current image is of Intra type, the quantization vectors are each initialized to predetermined values.

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.

Correlatively, the invention relates to a device for decoding a data signal representative of at least one image divided into blocks, comprising, for a current block to be decoded:

a first decoding module for decoding:

An index associated with a quantization vector which belongs to a set of quantization vectors, and prediction data of the current block to be decoded,

a calculation module for determining the quantization vector associated with the decoded index,

a prediction module for obtaining a predictor block from the decoded prediction data.

Such a decoding device is remarkable in that it comprises:

a second decoding module which is activated to decode data relating to the current block to be decoded determined in the data signal, at least one of the quantization vectors being modified as a function of the decoded data, a reconstruction module for reconstructing the current block from the decoded data, the determined quantization vector and the obtained predictor block.

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

Such a 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 another desirable form.

Another object of the invention is also a computer readable recording medium, and including computer program instructions as mentioned above.

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

On the other hand, such a 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, such a recording medium may be an integrated circuit in which the program is incorporated, the circuit being adapted to perform the coding and / or decoding method according to the invention or to be used in the execution of this and / or these. Brief description of the drawings

Other features and advantages will appear on reading a preferred embodiment described with reference to the figures in which: FIG. 1 represents the steps of the coding method according to one embodiment of the invention,

FIG. 2 represents a coding device implementing the steps of the coding method of FIG. 1,

FIG. 3 represents a decoding device according to one embodiment of the invention,

FIG. 4 represents the steps of the decoding method implemented in the decoding device of FIG. 3. Detailed description of the coding part

One 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 a coding set. implemented in an encoder according to any current or future video coding standards.

In this embodiment, the coding method according to the invention is for example implemented in a software or hardware way by modifications of such an encoder. The coding method according to the invention is represented in the form of an algorithm comprising steps C1 to C15 as represented in FIG.

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

As illustrated in FIG. 2, such a coding device comprises a memory MEM_CO comprising a buffer memory TAMP_CO, a processing unit UT_CO equipped for example with a microprocessor μΡ and driven by a computer program PG_CO which implements the method of coding according to the invention. At initialization, the code instructions of the computer program PG_CO are for example loaded into a RAM memory MR_CO before being executed by the processor of the processing unit UT_CO.

The coding method represented in Figure 1 applies to any fixed current image IC _j or part of a sequence of L IC-i images, IC _j, ..., IC _L (1≤j≤L) to code. During a step C1 shown in FIG. 1, in a manner known per se, a current image IC _{j is} subdivided into a plurality of blocks Bi, B ₂ , B _u , ..., B _s (1 <u≤S). Such a subdivision step is implemented by a partitioning software module MP_CO shown in FIG. 2, which module is driven by the microprocessor μΡ of the processing unit UT_CO. It should be noted that for the purposes of the invention, the term "block" means coding unit (coding unit). This last terminology is notably used in the standard HEVC "ISO / IEC / 23008-2 Recommendation ITU-T H.265 High Efficiency Video Coding (HEVC)".

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 B _; B ₂ , B _u , ..., B _s are intended to be coded according to a predetermined order of travel, which is for example of the lexicographic 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 subdivide the image IC _j into several subimages called slices and to independently apply a subdivision of this type on each subimage. 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.

According to one example, the blocks Bi, B ₂ , B _u , ..., B _s have a square shape and all contain K pixels, with K> 1. According to a preferred embodiment, said blocks are of size 4x4 or 8x8 pixels.

Depending on the size of the image that is not necessarily a multiple of the size of the blocks, the last blocks on the left and the last blocks on the bottom may not be square. In an alternative embodiment, the blocks may be for example of rectangular size and / or not aligned with each other.

Each block can also be divided into sub-blocks which are themselves subdividable. During a step C2 shown in FIG. 1, the coder CO selects as a current block a first block to be coded B _u of the image IC _j , such as for example the first block B-.

During a step C3 shown in FIG. 1, the current block B _{u is} predicted by known Intra and / or Inter prediction techniques. For this purpose, the block B _u is predicted with respect to at least one predictor block according to a prediction mode selected from a plurality of predetermined prediction modes MP ₀ , MP-i,. . . , MP _V , ..., MP _Q where 0 <v≤Q. In a manner known per se, the block B _u is predicted with respect to a plurality of candidate predictor blocks. Each of the candidate predictor blocks is a block of pixels that has already been encoded or encoded and decoded. Such predictor blocks are previously stored in the buffer memory TAMP_CO of the encoder CO as represented in FIG. 2.

According to an exemplary embodiment, the prediction is of Intra type which is, in a manner known per se, associated with a plurality of prediction modes each defined by a predetermined direction. In the case of the Intra prediction proposed for example in the HEVC standard, there are thirty-five possible prediction directions, which amounts to determining thirty-five candidate predictor blocks available for the prediction of the current block B _u .

At the end of the prediction step C3, an optimal predictor block BP _opt is obtained following a putting into competition of said candidate predictor blocks, for example by minimizing a distortion flow criterion well known to those skilled in the art. The BP _op t block is considered as an approximation of the current block B _u . The information relating to this prediction is intended to be written in a signal or data stream F to be transmitted to a decoder which will be described in more detail later in the description.

The step C3 is implemented by a processor or predictive coding software module PRED_CO shown in FIG. 2, which is controlled by the microprocessor μΡ of the processing unit UT_CO.

During a step C4 shown in FIG. 1, the data relating to the current block B _u is conventionally compared with the data of the _opt predictor block BP. More specifically, during this stage, he the difference between the current block B _u and the predictor block BP _op t obtained is calculated.

A first set of data, called first residue block Br _u is then obtained at the end of step C4.

Step C4 is implemented by a processor or software module

CAL1_CO calculation as shown in Figure 2, which is controlled by the microprocessor μΡ UT_CO processing unit.

During a step C5 shown in FIG. 1, a comparison is made of the first residue block Br _u with a plurality of quantization vectors. In conventional manner, the quantization vectors belong to one or more vector quantization dictionaries available to the encoder, denoted CBK, _CBK _2, ..., _W CBK. Such dictionaries are previously stored in the buffer memory TAMP_CO of the coder CO, as represented in FIG. 2.

More precisely, step C5 consists in selecting one of the said quantization vector dictionaries from the available dictionaries.

The step C5 is implemented by a processor or calculation software module CAL2_CO as represented in FIG. 2, which is controlled by the microprocessor μΡ of the processing unit UT_CO.

According to the invention, such a selection is implemented according to one and / or the other of the following elements:

the characteristics of the prediction (for example, the Intra mode selected from among the Intra modes of the HEVC standard) which has been applied to the step C3,

the frequency characteristics of the BP predictor block _op t, the size of the current block B _u ,

characteristics of the current image IC _j , such as its size or its energy.

In the preferred embodiment, W = 70, i.e.

in the case of a current block B _u of size 4x4, there is a different dictionary for each of the 35 Intra modes considered in the HEVC standard, in the case of a current block B _u of size 8x8, there is a different dictionary for each of the 35 Intra modes considered in the HEVC standard.

According to this preferred embodiment, the selected dictionary therefore depends both on the size of the current block B _u and the selected prediction mode.

The dictionary selected at the end of step C5 is denoted CBK _op t.

During a step C6 shown in FIG. 1, a selection is made of one of the quantization vectors of the CBK dictionary _op t that has been selected in step C5.

An optimal quantization vector V _opt is obtained following a competition of said quantization vectors of the CBK _op t dictionary, for example:

or by minimizing a distortion flow criterion well known to those skilled in the art,

or by minimizing the mean squared error calculated between the data of the residual block Br _u and the corresponding data of each of the quantification vectors of the dictionary CBK _op t.

The optimal quantization vector V _op t is considered as an approximation of the residual block Br _u . The information relating to this prediction is intended to be written in the aforementioned data signal F.

The step C6 is implemented by a processor or calculation software module CAL3_CO as represented in FIG. 2, which is controlled by the microprocessor μΡ of the processing unit UT_CO.

During a step C7 shown in FIG. 1, the quantization vector V _op t selected at the end of step C6 is coded.

Such a step consists in representing the index denoted IV _op t of the quantization vector V _opt in binary form. For example, if the CBK _opt dictionary to which the quantization vector V _opt belongs contains 256 quantization vectors, then the quantization vector V _op t can be represented on 8 bits, which makes it possible to precisely identify this vector among all the others. Quantification vectors of the CBK dictionary _opt . Step C7 is implemented by a binary coding processor or software module CB_CO as shown in FIG. 2, which is controlled by the microprocessor μΡ of the processing unit UT_CO.

During a C8 step shown in Figure 1, requires the comparison of data relating to the residue block Br _u to the vector data V ₀ pt. More precisely, during this step, the difference between the residue block Br _u and the vector V _opt is calculated.

A second set of data, called secondary residue block BSr _u , is then obtained at the end of step C8.

Step C8 is implemented by the processor or software module

CAL4_CO shown in Figure 2.

During a step C9 shown in FIG. 1, the data of the secondary residue block BSr _{u are} coded in accordance with the invention.

During step C9, during a substep C91, the secondary residue block BSr _{u is} transformed according to a conventional direct transformation operation to produce a transformed block BSt _u .

Sub-step C91 is implemented by a processing processor or software module MT_CO shown in FIG. 2, which is controlled by the microprocessor μΡ of the processing unit UT_CO.

The MT_CO processor is able to implement a direct transformation such as, for example, a discrete cosine transformation of the DCT (abbreviation of "Discrete Cosine Transform") type, a discrete sinus transformation of the DST (abbreviation of "Discrete Sine" type). Transform "), a discrete wavelet transform of the DWT type (abbreviation of" Discrete Wavelet Transform ").

In the course of step C9, during a sub-step C92 shown in FIG. 1, a quantization of the data of the transformed block BSt _{u is performed} to produce a quantized block BSq _u formed of coefficients quantified. Such a quantization step is for example of scalar or vector type. Sub-step C92 is performed by means of a quantization software processor or module MQ_CO as shown in FIG. 2, which module is driven by the microprocessor μΡ of the processing unit UT_CO.

In a manner known per se, during step C9, in addition, during a substep C93 shown in FIG. 1, the quantized coefficients of the block BSq _{u are} coded. Such coding is, for example, entropic coding of CABAC type ("Context Adaptive Binary Arithmetic Coder" in English) or else an entropy coding of arithmetic type or Huffman type.

Sub-step C93 is implemented by a processor or software coding module MC_CO shown in Figure 2, which module is controlled by the microprocessor μΡ of the processing unit UT_CO.

During a step C10 shown in FIG. 1, the signal or data flow F which contains:

the data coded at the end of the aforementioned step C9,

the index IV _opt of the optimal quantization vector V _opt . Step C10 is implemented by a processor or software module MCF for building a data signal, as shown in FIG. 2.

The data signal F is then transmitted by a communication network (not shown) to a remote terminal. This includes the decoder DO shown in FIG.

In a manner known per se, the data signal F furthermore comprises certain information encoded by the coder CO, such as the type of prediction (Inter or Intra) applied to the step C3, and, if appropriate, the prediction mode selected. , the index of the obtained predictor block BP _opt obtained at the end of step C3, denoted IBP _op t, the partitioning type of the current block B _u if the latter has been partitioned, the reference image index and the displacement vector used in the Inter prediction mode.

In a manner known per se, the decoding of the residual block BSr _u is then carried out. A decoded residue block BSDr _u is then obtained. It is then proceeded to the construction of the decoded block BD _U by adding to the predicted block BP _op t the decoded residue block BSDr _u . It should be noted that the decoded block BD _U is the same as the decoded block obtained at the end of the decoding process of the image IC _j which will be described later in the description. The decoded block BD _U is thus made available for use by the coder CO of FIG. 2.

In a step C1 1 shown in FIG 1 in accordance with the invention there is carried out a test that consists in checking whether an update criterion CBK _{dictionaries,} CBK _2, ..., CBK _W is filled or not.

According to a first variant, such a criterion consists in comparing the number of non-zero coefficients in the residual block BSr _u with a predetermined threshold. For example, the update criterion is considered to be filled if the number of nonzero coefficients is greater than 3.

According to a second variant, such a criterion consists in comparing the rate of coding of the residual block BSr _u with a predetermined threshold. For example, the update criterion is considered to be fulfilled if the rate of the coding of the residual block BSr _u is greater than 10 bits.

The step C1 1 is implemented by a processor or calculation software module CAL5_CO as shown in FIG. 2, which is controlled by the microprocessor μΡ of the processing unit UT_CO.

If the update criterion is fulfilled, during a step C12 shown in FIG. 1, at least one of the CBK ^ CBK ₂ , ..., CBK _W dictionaries is updated.

The step C12 is implemented by a processor or calculation software module CAL6_CO as represented in FIG. 2, which is controlled by the microprocessor μΡ of the processing unit UT_CO.

According to a first variant, step C12 consists in updating all the current dictionaries CBK-ι, CBK ₂ ,..., CBK _W.

According to a second preferred variant, only the CBK _op t dictionary is updated.

In the preferred embodiment, the CBK _op t dictionary is updated as follows.

The vector V _op t, and its neighbors contained in the CBKopt dictionary, are first considered. Neighbors are denoted V _opt -R, V _opt -R ₊ i, V _opt -

R + 2,. . . , V _op t-1, V _op t, Vopt + 1,. . . , V _op t + R-2j Vopt + R-1, V _op t + R. The parameter R which defines the number of next and preceding neighbors of the vector V _op t is predetermined, for example at the value 5. According to this configuration, the neighboring vectors which follow and precede the vector V _opt are considered.

Then vectors Z _op t _{+ n} , n belonging to <-R, + R>, are constructed for the purpose of updating, as follows:

-opt ^ + n ⁼ Vop ₊ n + alpha ^* f (n) * ((Vop, BSr + _u) -vop, _{+ n)}

or :

alpha is a predetermined parameter, for example equal to 0.1, and f (n) is a value which depends on the distance between the index n and the index opt.

For example, in the preferred embodiment:

f (n) = 0.2 ^* (5-n) / 5

The vectors Z _op t _{+ n} with n ranging from -5 to +5 are thus calculated, and replace respectively the vectors V _op t ₊ n in the CBKopt-

All current CBK-ι, CBK ₂ , ..., CBK _W dictionaries are thus updated.

In an alternative embodiment, the update parameters may be different depending on the size of the image. Indeed, if the image is small, a quick learning of the statistics of the current image ICj is necessary.

For example :

- for an image size belonging to a simple definition video SD (abbreviation of "Standard Definition"), less than 720 pixels high and less than 1280 pixels wide, an alpha parameter equal to 0.3 is retained;

- for an image size belonging to a high-definition video (HD), ie between 720 and 1080 pixels high and between 1280 and 1920 pixels wide, or greater than 1280x1920 pixels, a parameter alpha equal to 0.2 is retained;

for an image size greater than the size of an HD image, a parameter equal to 0.1 is retained. It is of course possible to update the dictionaries in other ways. For example, it is possible to apply the same type of quantization vector update as in the preferred mode. However, the update, instead of applying to the neighboring vectors of the quantization vector V _opt in the CBK _op t dictionary, applies to the vectors close to V _op t + BSDr _u in the direction of the distortion. In a variant, the update applies to vectors close to V _opt + BSDr _u not only in the CBK dictionary _opt , but in the set of current dictionaries CBK-, CBK ₂ ,..., CBK _W.

Following the aforesaid step C12, during a step C13 shown in FIG. 1, the next block of the current image IC _{j is} selected. Then the block coding steps described above are again implemented for this next block.

If at the end of the aforementioned step C1 1, the update criterion is not fulfilled, it is proceeded, during the above-mentioned step C13, to the selection of the next block of the current image IC _j . Then the block coding steps described above are again implemented for this next block.

During a step C14 shown in FIG. 1, the coder CO of FIG. 2 tests whether the current block that has been coded in accordance with the coding method described above is the last block of the current image IC _j .

If this is not the case, the above-mentioned step C12 is implemented.

If the current block is the last block of the current image IC _j , during a step C15 represented in FIG. 1, the coder CO of FIG. 2 tests whether the following current image IC _{j +} i is a picture of Intra type or not.

In the case where the following current image IC _{j +} i is of Intra type, before proceeding to the coding of the blocks of this image according to the coding method of FIG. 1, the step C12 for updating the dictionaries is implemented. artwork.

In the example shown, the quantization vectors of the dictionary CBK0 _pt are then each initialized to a respective predetermined value.

In the case where the next current image IC _{j +} i is not of Intra type, the blocks of this image are coded directly in accordance with the coding method of FIG. 1. The coding steps C1 to C15 which have just been described above are then implemented for each of the blocks Bi, B ₂ , B _u ,..., B _s to be encoded of the current image IC _j considered, in a predetermined order which is for example the lexicographic order.

Detailed description of the decoding part

An embodiment of the invention will now be described, in which the decoding method according to the invention is used to decode a signal or data stream representative of an image or a sequence of images which is suitable for be decoded by a decoder according to any of the current or future video decoding standards.

In this embodiment, the decoding method according to the invention is for example implemented in a software or hardware way by modifications of such a decoder.

The decoding method according to the invention is represented in the form of an algorithm comprising steps D1 to D15 as represented in FIG. 4.

According to this embodiment, the decoding method according to the invention is implemented in a decoding device or decoder DO represented in FIG.

As illustrated in FIG. 3, according to this embodiment of the invention, the decoder DO comprises a memory MEM_DO which itself comprises a buffer memory TAMP_DO, a processing unit UT_DO equipped for example with a microprocessor μΡ and driven by a PG_DO computer program that implements the decoding method according to the invention. At initialization, the code instructions of the computer program PG_DO are for example loaded into a RAM memory, denoted MR_DO, before being executed by the processor of the processing unit UT_DO.

The decoding method shown in FIG. 4 applies to a signal or data flow F representative of a current image IC _j to be decoded which is fixed or which belongs to a sequence of images to be decoded. For this purpose, information representative of the current image IC _j to be decoded is identified in the data signal F received at the decoder DO and as delivered at the end of the coding method of FIG.

With reference to FIG. 4, during a step D1, the quantized residue blocks BSq-1, BSq ₂ , ..., BSq _u , BSqs (1≤u≤) are identified in the signal F. S) respectively associated with the blocks Bi, B ₂ , B _u ,..., B _s encoded previously according to the aforementioned lexicographic order, during the coding step C9 of FIG. 1.

Such an identification step is implemented by a processor or MI_DO identification software module for flow analysis, as shown in FIG. 3, said module being driven by the microprocessor μΡ of the processing unit UT_DO.

Other types of course than the one just described above are of course possible and depend on the order of course chosen coding.

In the example shown, the blocks B _; B ₂ , B _u , ..., B _s to decode have a square shape and all contain K pixels, with K> 1. According to a preferred embodiment, said blocks to be decoded are of size 4x4 or 8x8 pixels.

Depending on the size of the image that is not necessarily a multiple of the size of the blocks, the first blocks at the top left of the image and the last blocks at the bottom right of the image can do not be square. In an alternative embodiment, the blocks may be for example of rectangular size and / or not aligned with each other.

Each block to be decoded can also be divided into sub-blocks which are themselves subdividable.

During a step D2 shown in FIG. 4, the decoder DO of FIG. 3 selects as the current block the first quantized block BSq _u that contains quantized data that has been coded during the substep C93 of FIG. 1.

During a step D3 shown in FIG. 4, the information relating to the prediction of the current block B _u as implemented in the coding, during step C3 of FIG. 1, is decoded. and which have been written into the data signal F. Such reconstruction information includes the type of prediction (Inter or Intra) applied in step C3, and the if necessary, the prediction mode selected, the index IB _op t of the predictor block obtained BP _op t obtained at the end of step C3, the partitioning type of the current block B _u if the latter has been partitioned, l reference image index and displacement vector used in the Inter prediction mode.

Such a decoding step D3 is implemented by the binary decoding module DB_DO shown in FIG. 3.

During a step D4 shown in FIG. 4, the predictive decoding of the current block to be decoded is carried out using the index IB _opt of the predictor block BP _op t which has been decoded during the step D3 above. For this purpose, in a manner known per se, it is proceeded, in association with the decoded index of the predictor block BP _opt , to the selection, in the buffer memory TAMP_DO of the decoder DO of FIG. 3, of the corresponding _opt BP predictor block , which is one of a plurality of candidate predictor blocks previously stored in the buffer buffer TAMP_DO. Each of the candidate predictor blocks is a block of pixels that has already been decoded.

Step D4 is implemented by a processor or software module PRED ^"1 _DO inverse prediction, as shown in Figure 3, which is controlled by the microprocessor μΡ of the processing unit UT_DO.

During a step D5 shown in FIG. 4, the index IV _opt of the optimal quantization vector V _opt which has been selected at the end of step C6 of FIG. 1 is decoded.

Such a decoding step D5 is implemented by the decoding module DB_DO of FIG. 3.

During a step D6 shown in FIG. 4, the optimal quantization vector V _opt associated with the index IV _opt decoded is determined.

The step D6 is implemented by a processor or calculation software module CAL1_DO as represented in FIG. 3, which is controlled by the microprocessor μΡ of the processing unit UT_DO.

During a step D7 shown in Figure 4, it is proceeded to the selection of a quantization vector dictionary, denoted CBK _op t, which contains the quantization vector V _opt selected in step D6. Such a dictionary belongs to a plurality of vector dictionaries of quantification available, denoted CBK-ι, CBK ₂ , ..., CBK _W. Such dictionaries are previously stored in the buffer memory TAMP_DO of the decoder DO, as represented in FIG.

Step D7 is implemented by a processor or calculation software module CAL2_DO as shown in FIG. 3, which is controlled by the microprocessor μΡ of the processing unit UT_DO.

According to the invention, correspondingly to the coding, such a selection is implemented according to one and / or the other of the following elements:

the characteristics of the prediction (for example, the Intra mode chosen from among the Intra modes of the HEVC standard), the indices of which were decoded in step D3,

- BP predictor block frequency characteristics _op t whose index IB _op t was decoded in step D3,

the size of the current block B _u to be decoded,

characteristics of the current image IC _j to be decoded, such as its size or its energy.

In the preferred embodiment, W = 70, i.e.

in the case of a current block B _u of size 4x4, there is a different dictionary for each of the 35 Intra modes considered in the HEVC standard,

- in the case of a current block B _u 8x8 size, there is a different dictionary for each of the 35 Intra modes considered in HEVC standard.

According to this preferred embodiment, the selected dictionary therefore depends both on the size of the current block B _u to be decoded and on the prediction mode whose index has been decoded in step D3.

During a step D8 shown in FIG. 4, the data of the quantized residual block BSq _{u are} decoded in accordance with the invention.

During step D8, during a substep D81 shown in FIG. 4, a decoding of the set of quantized coefficients BSq _u current is carried out. Such a decoding is for example a CABAC type entropic decoding or else an entropy decoding of arithmetic type or Huffman type.

At the end of the aforementioned sub-step D81, a set BSDq _u of digital information associated with the set of quantized coefficients BSq _u current is obtained.

Such a substep D81 decoding is implemented by an entropy decoding module MD_DO shown in Figure 3, which is controlled by the microprocessor μΡ of the processing unit UT_DO.

During step D8, during a substep D82 shown in FIG. 4, a dequantization of the digital information obtained as a result of the substep D81 is carried out, according to a conventional dequantization operation. which is the inverse operation of the quantization implemented during the quantization sub-step C92 of FIG. 1. A set of dequantized coefficients BSDt _u current is then obtained at the end of the substep D82. Such dequantization sub-step is for example of scalar or vector type.

Sub-step D82 is performed by means of a quantization software processor or module MQ ^"1 _DO shown in FIG. 3, which is controlled by the microprocessor μΡ of the processing unit UT_DO.

In step D8, the procedure, in a substep D83 shown in FIG 4, a transformation of the set of coefficients dequantized BSDT _u current, such a transformation being a direct inverse transformation. This transformation is the inverse operation of the transformation carried out in the substep C91 of FIG. At the end of the substep D83, a current decoded residue block BSDr _u is obtained.

The substep D83 is implemented by a processor or software module MT ^"1 _DO inverse transformation, as shown in Figure 3, which is controlled by the microprocessor μΡ of the processing unit UT_DO.

The processor MT ^"1 _DO is able to implement a direct inverse transformation such as, for example, a discrete inverse transform of DCT ¹ type, a discrete inverse transformation into discrete sines of the DST ¹ type, an inverse transformation into discrete wavelets of the type DWT ¹ . During a step D9 represented in FIG. 4, according to the invention, the current block B _{u is} reconstructed by adding to the decoded residue block BSDr _u , obtained at the end of the substep D83:

the optimal predictor block BP _opt which was obtained at the end of the aforementioned step D4,

and the optimal quantization vector V _op t which was obtained at the end of the aforementioned step D6.

At the end of step D9, a decoded current block BD _U is obtained.

The step D9 is implemented by a processor or calculation software module CAL3_DO shown in FIG. 3, which is controlled by the microprocessor μΡ of the processing unit UT_DO.

During a step D10 shown in FIG. 4, said decoded BD _U block is written in a decoded picture ID _j .

Such a step is implemented by a processor or software module URI image reconstruction as shown in Figure 3, said module being controlled by the microprocessor μΡ processing module UT_DO.

During a step D1 1 shown in FIG. 4, a test is performed which consists of checking whether a criterion for updating the CBK-i, CBK ₂ , ..., CBK _W dictionaries is fulfilled or not. .

According to a first variant, such a criterion consists in comparing the number of non-zero coefficients in the decoded residue block BSDr _u with a predetermined threshold. For example, the update criterion is considered to be filled if the number of nonzero coefficients is greater than 3.

According to a second variant, such a criterion consists in comparing the rate of coding of the decoded residue block BSDr _u with a predetermined threshold. For example, the update criterion is considered completed if the block coding rate decoded residue BSDr _u is greater than 10 bits.

The step D1 1 is implemented by a processor or calculation software module CAL4_DO as represented in FIG. 3, which is controlled by the microprocessor μΡ of the processing unit UT_DO.

If the update criterion is fulfilled, during a step D12 shown in FIG. 4, at least one of the CBK ^ CBK ₂ , ..., CBK _W dictionaries is updated. The step D12 is implemented by a processor or calculation software module CAL5_DO as represented in FIG. 3, which is controlled by the microprocessor μΡ of the processing unit UT_DO.

Since step D12 is identical to step C12 for updating the dictionaries as implemented in the coding with reference to FIG. 1, this step will not be described further.

Following the aforesaid step D12, during a step D13 shown in FIG. 4, the following quantized residual block of the current image IC _j to be decoded is selected. Then the decoding steps of the next quantized residue block described above are again implemented.

If at the end of the aforementioned step D1 1, the update criterion is not fulfilled, it is proceeded, during the aforementioned step D13, to the selection of the next quantized residue block of the image current IC _j to decode. Then the decoding steps of the next quantized residue block described above are again implemented.

During a step D14 shown in FIG. 4, the decoder DO of FIG. 3 tests whether the current block which has been decoded according to the decoding method described above is the last block of the current image IC _j to decode.

If this is not the case, the aforementioned step D13 is implemented.

If the current block is the last block of the current image IC _j to be decoded, during a step D15 shown in FIG. 4, the decoder DO of FIG. 3 tests whether the next current image IC _{j +} i to be decoded is an Intra type image or not.

In the case where the following current image IC _{j +} i to be decoded is of type

Intra, before decoding the blocks of this image in accordance with the decoding method of FIG. 4, the step D12 for updating the dictionaries is implemented.

In the example shown, the quantization vectors of the CBKopt dictionary are each initialized to a respective predetermined value.

In the case where the following current image IC _{j +} i to be decoded is not Intra type, the blocks of this image are decoded directly according to the decoding method of FIG. 4. The decoding steps which have just been described above are implemented for all the blocks Bi, B ₂ , B _u ,..., B _s to be decoded from the current image IC _j considered, in a predetermined order which is for example the lexicographic order.

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 encoding at least one image (IC _j ) cut into blocks, implementing, for a current block (B _u ) to be encoded of said image:

a prediction (C3) of the current block according to a prediction mode selected from among a plurality of predetermined prediction modes,

a obtaining of a predictor block (BP _op t) at the end of said prediction, a computation (C4) of a first set of data representative of a difference between the obtained predictor block and the current block,

a comparison (C5) of said first set of calculated data with a plurality of quantization vectors,

a selection (C6) of one of the quantization vectors according to a predetermined coding performance criterion,

an encoding (C7) of an index associated with said selected quantization vector,

said method being characterized in that it comprises:

a calculation (C8) of a second set of data representative of a difference between the first calculated data set and the selected quantization vector,

an encoding (C9) of the second calculated data set in which at least one of said quantization vectors is modified (C12) as a function of the data of the second calculated data set.

The coding method according to claim 1, wherein the modification of one of said quantization vectors is implemented only if the data of the second computed data set fulfills a predetermined criterion.

3. coding method according to claim 1 or 2, wherein the modification of one of said quantization vectors is implemented using a parameter whose value depends on the size of the image to be encoded.

4. Coding method according to any one of claims 1 to 3, wherein if said current image is Intra type, the quantization vectors are each initialized to predetermined values.

5. Coding device (CO) of at least one image (IC _j ) cut into blocks, comprising, for a current block (B _u ) to be coded with said image:

prediction means (PRED_CO) for predicting the current block according to a prediction mode selected from among a plurality of predetermined prediction modes (MP ₀ , MP-1, MP _V , ..., MP _Q ); , delivering a predictor block (BP _opt ),

calculation means (CAL1_CO) for calculating a first set of data representative of a difference between the obtained predictor block and the current block,

comparison means (CAL2_CO) for comparing said first set of calculated data with a plurality of quantization vectors,

selection means (CAL3_CO) for selecting one of the quantization vectors according to a predetermined coding performance criterion,

first encoding means (CB_CO) for encoding an index associated with said selected quantization vector,

said coding device being characterized in that:

said calculating means are activated to calculate a second set of data representative of a difference between the first calculated data set and the selected quantization vector, and in that it comprises second coding means (MC_CO) which are activated to encode the second calculated data set, wherein at least one of said quantization vectors is changed based on the data of the second calculated data set.

A computer program having program code instructions for performing the steps of the encoding method according to one of the any of claims 1 to 4, when said program is run on a computer.

A computer-readable recording medium on which a computer program is recorded comprising program code instructions for performing the steps of the encoding method according to any one of claims 1 to 4, when said program is executed by a computer.

8. A method for decoding a data signal (F) representative of at least one image (ICj) cut into blocks, implementing, for a current block (B _u ) to be decoded:

a decoding (D3, D5):

An index (IV _opt ) associated with a quantization vector (V _op t) which belongs to a set of quantization vectors,

And prediction data of the current block to be decoded,

a determination (D6) of said quantization vector associated with said decoded index,

obtaining (D4) a predictor block (BP _op t) from the decoded prediction data,

said method being characterized in that it comprises:

a determination (D1) in said data signal of data relating to the current block to be decoded,

a decoding (D8) of the data relating to the current block to be decoded, during which at least one of said quantization vectors is modified (D1 2) as a function of the decoded data,

- a reconstruction (D9) of the current block from the decoded data, said quantization vector and the predictor block obtained.

The decoding method according to claim 8, wherein the modification of one of said quantization vectors is implemented only if the decoded data fulfills a predetermined criterion.

10. decoding method according to claim 8 or 9, wherein the modification of one of said quantization vectors is implemented using a parameter whose value depends on the size of the image to be decoded.

1 1. A decoding method according to any one of claims 8 to 10, wherein if said current image is of Intra type, the quantization vectors are each initialized to predetermined values.

12. Device for decoding a data signal (F) representative of at least one image (IC _j ) cut into blocks, comprising, for a current block (B _u ) to be decoded:

first decoding means (DB_DO) for decoding:

And prediction data of the current block to be decoded,

calculating means (CAL1_DO) for determining said quantization vector associated with said decoded index,

prediction means (PRED ^"1 _DO) for obtaining a predictor block (BP _op t) from the decoded prediction data,

said decoding device being characterized in that it comprises:

second decoding means (MD_DO) which are activated to decode data relating to the current block to be decoded determined in the data signal, at least one of said quantization vectors being modified as a function of the decoded data,

- reconstruction means (CAL3_DO) for reconstructing the current block from the decoded data, said determined quantization vector and the predictor block obtained.

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.

A computer-readable recording medium on which a computer program is recorded 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 run by a computer.