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

1. Field of the invention

The field of the invention is that of the 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 sequences of images occupy a lot of space in terms of memory, which requires, when these images are transmitted, to compress them in order to avoid congestion problems on the network used for this transmission.

Many video data compression techniques 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 prediction of pixels of a current image with respect to other pixels belonging to the same image (intra prediction) or to a previous or following 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 blocks of pixels are then predicted using information already reconstructed, corresponding to the previously coded/decoded blocks 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, called predictor block, and 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 cosines). The coefficients of the transformed residual block are then quantized, then coded by entropy coding and transmitted to the decoder, which can reconstruct the current block by adding this residual block to the predictor block.

The decoding is done frame by frame, and for each frame, block by block. For each block, the corresponding elements of the stream 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 (i.e. the predictor block) to the decoded residual block.

In US9253508, a DPCM (Differential Pulse Code Modulation) coding technique for coding blocks in Intra mode is inserted into an HEVC coder. Such a 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 residual in the case of lossless coding, thus 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 following pixel. Indeed, this technique requires reconstructing a set of pixels (line/column of the block for example) to predict another set of pixels. In other words, at each prediction and reconstruction of a part of the block, several pixels of the block are predicted and reconstructed.

Moreover, in US Pat. No. 9,253,508, it is not described how to make intra-classical prediction modes as defined in the HEVC standard for example and the DPCM prediction mode cohabit.

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

3. Disclosure of 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. Such a decoding method comprises, for at least one block of the image, called the current block:

- the decoding of information indicating a coding mode of the current block from among at least a first coding mode and a second coding mode age, the second coding mode being a coding mode according to which the current block is decoded via, 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 previously decoded block of the image,

- the decoding of a prediction residue associated with said pixel,

- the reconstruction of said pixel from the prediction of said pixel obtained and the decoded prediction residue associated with said pixel,

- the decoding of the current block according to the coding mode indicated by the decoded information,

- when the coding mode of the current block corresponds to a coding mode distinct from the second coding mode, the application of at least one processing method to the reconstructed current block,

- when the coding mode of the current block corresponds to the second coding mode, the deactivation of the application of said at least one processing method to the reconstructed current block for at least one pixel of said current block.

Thus, according to the invention, the application of processing to a reconstructed block is not carried out in the case of a block decoded according to a coding mode using a prediction of the pixels from previously reconstructed pixels of the same block. According to this coding mode, the prediction residue associated with each pixel is not transformed. The processing methods aim to improve the quality of the reconstructed pixel blocks, for example by reducing the effects of discontinuities between blocks due to the coding of prediction residue with a transform (deblocking filter), or by correcting the value each pixel (also known as SAO for Sample Adaptive Offset). According to the invention, the second coding mode does not use a transform of the prediction residue since the prediction residue associated with each pixel must be available immediately to reconstruct the pixel and it can be used for the prediction of the following pixels of the current block. Thus, when the current block is decoded according to the second coding mode, it is not necessary to reduce the discontinuities at the borders of this current block. Similarly, when the current block is decoded according to the second coding mode, the value of each pixel is coded individually using a prediction residue associated with each pixel. It is therefore not necessary to correct the value of each pixel.

The processing methods applied to the reconstructed blocks generally require the transmission of parameters at the block level. Deactivating these processing methods for the blocks coded according to the second coding mode thus allows a gain in throughput. Moreover, the decoding process can be substantially accelerated since these processing methods are not applied for these blocks.

The invention also relates to a method for coding a stream of coded data representative of at least one image divided into blocks. Such a 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 from among at least a first coding mode and a second coding mode, the second coding mode being a coding mode according to which the current block is coded via, 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 previously decoded block of the image,

- the coding of a prediction residue associated with said pixel obtained from the prediction of said pixel,

- the reconstruction of said pixel from the decoded prediction residue associated with said pixel and the prediction of said pixel,

- the coding of the current block according to the coding mode indicated by the coded information,

- when the coding mode of the current block corresponds to a coding mode distinct from the second coding mode, the application of at least one processing method to the reconstructed current block,

- when the coding mode of the current block corresponds to the second coding mode, the deactivation of the application of said at least one processing method to the reconstructed current block for at least one pixel of said current block.

According to a particular embodiment of the invention, the processing method is deblocking filtering applied to the pixels of the reconstructed current block which are located at the border of the reconstructed current block with a reconstructed neighboring block in the image. According to this particular embodiment of the invention, the processing method corresponds to a "deblocking" filter conventionally applied at block boundaries to reduce the effects of discontinuities between blocks.

According to a particular embodiment of the invention, when the current block is decoded or coded according to the second coding mode:

- the application of unblocking filtering

ge to the current reconstructed block is deactivated for all the pixels of the current reconstructed block,

- when the current block is decoded or coded according to a coding mode distinct from the second coding mode:

- the deblocking filtering is applied to a pixel of the reconstructed current block if said pixel is located on a border of said reconstructed current block with a neighboring reconstructed block in the image and if said neighboring block is decoded or coded according to a distinct coding mode of the second coding mode.

According to this particular embodiment of the invention, the deblocking filtering is applied only to the pixels at the boundary of two blocks which are both coded or decoded according to coding modes distinct from the second coding mode. In other words, for a current block coded or decoded according to a coding mode distinct from the second coding mode, the deblocking filtering is deactivated for the pixels of the reconstructed current block which are located on the border with a neighboring block coded or decoded according to the second coding mode.

According to another particular embodiment of the invention, when the current block is decoded or coded according to the second coding mode:

- the application of the deblocking filtering to the reconstructed current block is deactivated for a pixel of the reconstructed current block if said pixel is located on a border of said reconstructed current block with a neighboring block in the image and if said neighboring block is decoded or coded according to the second coding mode, and

- the deblocking filtering is applied to a pixel of the reconstructed current block, if said pixel is located on a border of said reconstructed current block with a neighboring block reconstructed in the image and if said neighboring block is decoded or coded according to a coding mode distinct from the second coding mode.

According to this other particular embodiment of the invention, the deblocking filtering is applied to the pixels situated at the boundary of two blocks, at least one of the blocks of which is coded or decoded according to a coding mode distinct from the second coding mode. On the other hand, the deblocking filtering is deactivated for the pixels situated at the border of two blocks which are both coded or decoded according to the second coding mode.

This particular embodiment of the invention makes it possible to smooth the block effects for the blocks coded or decoded according to the first coding mode or any other coding mode distinct from the second coding mode, even when these are close to a reconstructed block which has been coded or decoded according to the second coding mode.

According to another particular embodiment of the invention, the processing method is a method of correcting at least one pixel of the reconstructed current block by adding to the reconstructed value of said pixel a value obtained from information encoded in the data stream or decoded from the data stream. According to this particular embodiment of the invention, the processing method corresponds to the so-called SAO method which has been integrated into the HEVC compression standard.

According to a particular embodiment of the invention, when the current block is decoded or coded according to the second coding mode, the application of said correction method to the reconstructed current block is deactivated for all the pixels of the reconstructed current block.

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 comprise the different 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 detailed in greater detail.

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

- decoding information indicating a coding mode of the current block from among at least a first coding mode and a second coding mode, the second coding mode being a coding mode according to which the current block is decoded via, 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 previously decoded block of the image,

- the decoding of a prediction residue associated with said pixel,

- the reconstruction of said pixel from the prediction of said pixel obtained and the decoded prediction residue associated with said pixel,

- decode the current block according to the coding mode indicated by the decoded information,

- when the coding mode of the current block corresponds to a coding mode distinct from the second coding mode, applying at least one processing method to the reconstructed current block,

- when the coding mode of the current block corresponds to the second coding mode, deactivated ver the application of said at least one processing method to the current block reconstructed for at least one pixel of said 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 may of course include the different 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 detailed in greater detail.

The coding device comprises in particular a processor configured for, for at least one block of the image, called current block:

- coding information indicating a coding mode of the current block from among at least a first coding mode and a second coding mode, the second coding mode being a coding mode according to which the current block is coded via, 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 previously decoded block of the image,

- the coding of a prediction residue associated with said pixel obtained from the prediction of said pixel,

- the reconstruction of said pixel from the decoded prediction residue associated with said pixel and the prediction of said pixel,

- coding the current block according to the coding mode indicated by the coded information,

- when the coding mode of the current block corresponds to a coding mode distinct from the second coding mode, applying at least one processing method to the reconstructed current block,

- when the coding mode of the current block corresponds to the second coding mode, deactivating the application of said at least one processing method to the reconstructed current block for at least one pixel of said current block.

According to a particular embodiment of the invention, such an encoding 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 previously, 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 recorded on a computer-readable medium.

This program may 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 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 medium may comprise a storage means such as a memory. On the other hand, the recording media may correspond to 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 in particular be downloaded from an Internet-type network.

Alternatively, the recording media may 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 method 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 a simple illustrative and non-limiting example, and the appended drawings, among which:

- Figure 1 shows steps of the coding method according to a particular embodiment of the invention,

- Figure 2 illustrates an example of position of neighboring blocks of a current block to determine an intra prediction mode according to a particular embodiment of the invention,

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

- Figure 4 shows steps of the decoding method according to a particular embodiment of the invention,

- Figures 5A and 5B illustrate blocks of reconstructed pixels on which a post-processing is applied or not to the pixels according to the coding mode of the block to which the pixels belong, according to particular embodiments of the invention,

- Figure 6 shows the simplified structure of an encoding device suitable for implementing the encoding method according to any one of the particular embodiments of the invention,

- Figure 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.

5. Description of an embodiment of the invention

5.1 General principle

Processing carried out after the decoding of an image, also known as post-processing, is integrated into video coding standards in order to improve the quality of the reconstructed images. For example, such post-processing can be the application of a deblocking filter or a so-called SAO post-processing for Sample Adaptive Offset in English.

The deblocking filtering makes it possible to erase, after decoding of each block, the discontinuity which exists between each block and to which the human eye is very sensitive. On the other hand, SAO processing makes it possible to individually modify the value of each pixel of a decoded block.

These two post-processing methods have been integrated, for example, into the HEVC compression standard and provide gains in compression and quality.

However, according to the invention, a new mode of coding the blocks of an image, called ILR coding for In-Loop Residual in English, makes it possible to advantageously dispense with these two post-processing methods.

Concerning the effects of discontinuities between blocks, they are mainly due to the coding of the classical residue in the form of a transform. Since the basis vectors of the transform are at the block scale, they do not allow individual control of the pixel values ​​along the block boundary. On the other hand, the ILR coding mode makes it possible to code the intensity of each pixel independently of the others. There are therefore no observed block effects, and filtering is unnecessary.

Similarly, SAO processing aims to correct the individual value of certain pixels after conventional coding. However, the ILR coding mode which will be described later, already makes it possible to code the value of each pixel individually. There is therefore no need for SAO processing for these pixels.

On the other hand, it is necessary for the coder to be able to choose for each block to be coded between a conventional coding mode or an ILR coding mode in order to optimize the cost of coding the video sequence.

The general principle of the invention is therefore to make it possible to activate or not the application of a post-processing method to a reconstructed block depending on whether the block has been coded/decoded according to the ILR coding mode.

There are several advantages to activating deblocking and/or SAO processing only for blocks coded by a classic coding method, i.e. a coding mode other than ILR. Indeed, this makes it possible to reduce the throughput to be transmitted because no syntax element relating to the deblocking filter or to the SAO processing is transmitted for the blocks coded/decoded according to the ILR coding mode. This also makes it possible to substantially accelerate the decoding, since the blocks coded/decoded according to the ILR coding mode do not need to apply the deblocking and/or SAO processing steps which are quite cumbersome to implement in terms of calculations and memory resources.

5. 2 Examples of implementation

FIG. 1 presents steps of the coding method according to a particular embodiment of the invention. For example, a sequence of images l is coded; l2, ..., lNb in the form of an 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 later in relation to FIG. 6.

A sequence of images h, l2, ..., lNb, Nb being the number of images of the sequence to be coded, is supplied as input to the coding method. The coding method outputs a coded data stream STR representative of the sequence of images supplied as input.

In known manner, the coding of the sequence of images h, l2, ..., l Nb is done image by image, according to a previously established coding order known to the coder. For example, the images can be coded in the temporal order h, l2, ..., lNb or according to another order, for example li, l3, l2, ·, iNb- During a step E0, an image I , to code the sequence of images, l2, ..., l Nb is divided into blocks, for example into blocks of size 32×32, or 64×64 pixels or more. Such a block can be subdivided into sub- square or rectangular blocks, for example in size 16x16, 8x8, 4x4, 16x8, 8x16, ....

During a step E1, a first block or sub-block Xb to be coded of the image I is selected according to a predetermined direction of travel of the image I. For example, it may be the first block in the lexicographical order of browsing the image.

Claims

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

- the decoding (E42) of information indicating a coding mode of the current block from among at least a first coding mode and a second coding mode, the second coding mode being a coding mode according to which the current block is decoded (E44) via, 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 previously decoded block of the image,

- the decoding of a prediction residue associated with said pixel,

- the reconstruction of said pixel from the prediction of said pixel obtained and the decoded prediction residue associated with said pixel,

- the decoding (E44, E43) of the current block according to the coding mode indicated by the decoded information,

- when the coding mode of the current block corresponds to a coding mode distinct from the second coding mode, the application of at least one processing method to the reconstructed current block,

- when the coding mode of the current block corresponds to the second coding mode, the deactivation of the application of said at least one processing method to the reconstructed current block for at least one pixel of said 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, said current block:

- the coding (E20) of information indicating a coding mode of the current block from among at least a first coding mode and a second coding mode, the second coding mode being a coding mode according to which the current block is coded (E22) via, 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 previously decoded block of the image,

- the coding (E223) of a prediction residue associated with said pixel obtained from the prediction of said pixel,

- the reconstruction of said pixel from the decoded prediction residue associated with said pixel and the prediction of said pixel,

- the coding (E21, E22) of the current block according to the coding mode indicated by the coded information,

- when the coding mode of the current block corresponds to a coding mode distinct from the second coding mode, the application of at least one processing method to the reconstructed current block,

- when the coding mode of the current block corresponds to the second coding mode, the deactivation of the application of said at least one processing method to the reconstructed current block for at least one pixel of said current block.

3. Method according to any one of claims 1 or 2, in which said processing method is a deblocking filtering applied to the pixels of the reconstructed current block which are located at the border of the reconstructed current block with a reconstructed neighboring block in the image.

4. Process according to claim 3, in which:

- when the current block is decoded or coded according to the second coding mode:

- the application of the deblocking filtering to the reconstructed current block is deactivated for all the pixels of the reconstructed current block,

- when the current block is decoded or coded according to a coding mode distinct from the second coding mode:

- the deblocking filtering is applied to a pixel of the reconstructed current block if said pixel is located on a border of said reconstructed current block with a neighboring reconstructed block in the image and if said neighboring block is decoded or coded according to a distinct coding mode of the second coding mode.

5. Process according to claim 3, in which:

- when the current block is decoded or coded according to the second coding mode:

- the application of the deblocking filtering to the reconstructed current block is deactivated for a pixel of the reconstructed current block if said pixel is located on a border of said reconstructed current block with a neighboring block in the image and if said neighboring block is decoded or coded according to the second coding mode, and

- the deblocking filtering is applied to a pixel of the reconstructed current block, if said pixel is located on a border of said reconstructed current block with a neighboring block reconstructed in the image and if said neighboring block is decoded or coded according to a coding mode distinct from the second coding mode.

6. Method according to any one of claims 1 or 2, in which said processing method is a method of correcting at least one pixel of the reconstructed current block by adding to the reconstructed value of said pixel a value obtained from information encoded in the data stream or decoded from the data stream.

7. Method according to claim 6, in which, when the current block is decoded or coded according to the second coding mode, the application of said correction method to the reconstructed current block is deactivated for all the pixels of the reconstructed current block.

8. 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:

- decoding information indicating a coding mode of the current block from among at least a first coding mode and a second coding mode, the second coding mode being a coding mode according to which the current block is decoded via, 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 previously decoded block of the image,

- the decoding of a prediction residue associated with said pixel,

- the reconstruction of said pixel from the prediction of said pixel obtained and the decoded prediction residue associated with said pixel,

- decode the current block according to the coding mode indicated by the decoded information,

- when the coding mode of the current block corresponds to a coding mode distinct from the second coding mode, applying at least one processing method to the reconstructed current block,

- when the coding mode of the current block corresponds to the second coding mode, deactivating the application of said at least one processing method to the reconstructed current block for at least one pixel of said current block.

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

- coding information indicating a coding mode of the current block from among at least a first coding mode and a second coding mode, the second coding mode being a coding mode according to which the current block is coded via, 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 previously decoded block of the image,

- the coding (E223) of a prediction residue associated with said pixel obtained from the prediction of said pixel,

- the reconstruction of said pixel from the decoded prediction residue associated with said pixel and the prediction of said pixel,

- coding the current block according to the coding mode indicated by the coded information,

- when the coding mode of the current block corresponds to a coding mode distinct from the second coding mode, applying at least one processing method to the reconstructed current block,

- when the coding mode of the current block corresponds to the second coding mode, deactivating the application of said at least one processing method to the reconstructed current block for at least one pixel of said current block.

10. Computer program comprising instructions for implementing the decoding method according to any one of claims 1 or 3 to 7 or the coding method according to any one of claims 2 to 7, when said program is executed by a processor.

1 1. Information carrier readable by a computer, and comprising instructions of a computer program according to claim 10.