RU2808075C1 - Method for image coding and decoding, coding and decoding device and corresponding computer programs - Google Patents

Abstract

FIELD: coding/decoding images.

SUBSTANCE: medium is proposed for decoding a section (Bi) of an image divided into sections, as well as a method comprising: receiving a set of coefficients encoded by a context-adaptive binary arithmetic coding (CABAC) type encoder; decoding a set of coefficients representing a block of residual data of the portion, wherein the first non-zero coefficient does not contain any sign value; applying the function to the set of coefficients to generate sign data, wherein applying the function to the set of coefficients comprises: calculating the sum of non-zero coefficients from said set of coefficients; calculation of the parity of the mentioned sum of coefficients. A sign is assigned to the first non-zero coefficient based on parity, wherein an even value of said sum of coefficients means that the sign of the first non-zero coefficient is positive, and an odd value of said sum of coefficients means that the sign of the first non-zero coefficient is negative.

EFFECT: increasing the accuracy of image coding/decoding.

3 cl, 4 dwg

Description

Field of technology to which the invention relates

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

The invention is thus particularly intended for encoding video data embodied in current (MPEG, H.264, etc.) or future video encoders (ITU-T/VCEG (H.265) or ISO/MPEG (HEVC) ).

State of the art

Modern video encoders (MPEG, H.264, etc.) use a representation in the form of video sequence blocks. The images are divided into macroblocks, each macroblock itself being divided into blocks, and each block or macroblock is encoded using intra-image or inter-image prediction. Thus, some images are encoded using spatial prediction (intra prediction) while other images are encoded using temporal prediction (inter prediction) with respect to one or more reference encoded-decodable images using motion compensation, as is known to one skilled in the art. this field of technology.

For each block, a residual block, also called prediction residual, corresponding to the original block reduced during prediction, is encoded. The residual blocks are transformed using a discrete cosine transform (DCT) type transform, then quantized using, for example, a scalar type quantization. At the end of the quantization step, coefficients are obtained, some of which are positive and others negative. They are then run through, usually in a zigzag reading order (as in the JPEG standard), thus allowing a significant number of zero coefficients to be used at high frequencies. At the end of the above-mentioned run, a one-dimensional list of coefficients is obtained, which can be called the “quantized remainder”. The coefficients from this list are then encoded using entropy coding.

Entropy coding (for example, arithmetic or Huffman-type coding) is performed as follows:

- the information element is subjected to entropy encoding to indicate the position of the last non-zero coefficient in the list,

- for each coefficient located before the last non-zero coefficient, an element of information is entropy encoded to indicate whether this coefficient is equal to zero or not,

- for each non-zero coefficient specified previously, entropy encode an element of information to indicate whether this coefficient is equal to one or not,

- for each coefficient that is not zero and not equal to one and which was located before the last non-zero coefficient, entropy encode the amplitude information element (the absolute value of the coefficient whose value was halved),

- for each non-zero coefficient, the sign assigned to it is coded as “0” (for the “+” sign) or as “1” (for the “-” sign).

According to H.264 technology, for example, when a macroblock is divided into blocks, a data signal corresponding to each block is transmitted to a decoder. This signal contains:

- quantized balances contained in the list mentioned above,

- elements of information representing the encoding mode used, in particular:

• prediction mode (intra-frame prediction, inter-frame prediction, default prediction, which produces a prediction for which information is not passed to the decoder (called "skip");

• information specifying the type of prediction (orientation, reference image, etc.);

• type of division into sections;

• conversion type, for example, 4×4 DCT, 8×8 DCT, etc.;

• traffic information, if necessary;

• etc.

Decoding is performed picture by picture, and for each picture macroblock by macroblock. For each section of the macroblock, the corresponding flow elements are read. Inverse quantization and inverse transformation of the block coefficients are performed to obtain the decoded prediction residual. The prediction of the portion is then calculated, and the portion is reconstructed by adding the prediction to the decoded prediction remainder.

Encoding within a frame or between frames, as a result of competition with each other, as embodied in the H.264 standard, is therefore based on arranging various elements of encoding information, such as those mentioned above, so that they compete to select the best mode, then is, a mode that optimizes the encoding of the region in question, in accordance with a given performance criterion, for example, the cost of random distortion, as is well known to one skilled in the art.

Information elements representing the selected encoding mode are contained in the data signal transmitted by the encoder to the decoder. The decoder is thus configured to identify a coding mode selected in the encoder and then apply a prediction that corresponds to that mode.

In "Data Hiding of Motion Information in Chroma and Luma Samples for Video Compression", J.-M. Thicssc, J. Jung and M. Antonini, International workshop on multimedia signal processing, 2011 presents a data hiding method that is implemented during video data compression.

More specifically, it is proposed to avoid including in a signal to be transmitted to a decoder at least one contention index generated from a plurality of contention indices to be transmitted. Such an index is, for example, an MVComp index that represents an information element for identifying a motion vector predictor used for an inter-frame prediction block. Such an index, which can take the value 0 or 1, is not included directly in the coded data element signal, but is transmitted by means of the parity of the sum of the quantized remainder coefficients. A connection is formed between the parity of the quantized remainder and the MVComp index. As an example, an even quantized remainder value is associated with an MVComp index value of 0, while an odd quantized remainder value is associated with an MVComp index value of 1. Two cases may arise. In the first case, if the parity of the quantized remainder already matches the parity of the MVComp index to be transmitted, the quantized remainder is encoded in the usual way. In the second case, if the parity of the quantized remainder differs from the parity of the MVComp index to be transmitted, the quantized remainder is modified so that its parity is the same as that of the MVComp index. Such modification involves gradually increasing or decreasing one or more coefficients of the quantized remainder by an odd value (e.g. +1, -1, +3, -3, +5, -5, etc.) and maintaining only the modification that optimizes the given criterion, in this case, the previously mentioned random distortion cost.

In the decoder, the MVComp index is not read into the signal. The decoder is content with simply the usual definition of the remainder. If the value of such remainder is even, the MVComp index is set to 0. If the value of this remainder is odd, the MVComp index is set to 1.

In accordance with the technology that was presented above, the coefficients that are modified are not always chosen optimally, so that the applied modification leads to disturbances in the signal transmitted to the decoder. Such violations inevitably have a negative impact on the efficiency of video data compression.

Disclosure of the Invention

One of the objects of the invention is to overcome the disadvantages of the prior art mentioned above.

Thus, the object of the present invention relates to a method for encoding at least one image divided into sections, such a method implements the following steps:

- predicting data elements of the current section as a function of at least one reference section already encoded and then decoded, outputting the predicted section;

- determining a set of residual data elements by comparing data elements related to the current site and the predicted site, and the residual data elements are respectively associated with various digital information elements for which entropy coding is required,

- generate a signal containing encoded elements of information.

The method according to the invention is characterized in that it implements, before the signal generation step, the following steps:

- determine from a certain set of residual data elements a nodeset containing residual data elements suitable for modification,

- calculating the value of a function representing the residual data elements of said specific subset,

- compare the calculated value with the value of at least one of the elements of digital information,

- depending on the result of the comparison, modifying or not modifying at least one of the residual data elements in the subset is performed,

- in case of modification, entropy coding of said at least one element of the modified residual data is performed.

This sequence allows the data hiding technology to be applied to a reduced set of residual data elements, and within this set of residual data elements are suitable for modification.

In accordance with the invention, it should be understood that the expression "residual data elements" means data elements for which the application of modification does not result in desynchronization between the encoder and the decoder.

Thus, in accordance with the invention, the residual data elements that are intended to be modified are selected much more reliably than in the previously cited prior art, thereby allowing better image reconstruction quality to be obtained in the decoder.

In addition, the ability to modify a reduced number of residual data elements allows for faster encoding.

In a particular embodiment, the steps that follow the step of determining a subset of residual data elements are implemented only if a given criterion is satisfied depending on the residual data elements suitable for modification.

This sequence further allows the encoder to rationally determine whether or not to apply hiding technology to the data. This definition step has the advantage of being applied only to a limited set of residual data elements suitable for modification. Thus, it is ensured that the data hiding technology is applied in a much more appropriate manner than in the previously cited prior art, in particular, for the best selected number of residual data elements for which it is precisely determined that, after modification of these data elements, a violation of the signal generated as a result of such modification, will not have a negative impact on the quality of image reconstruction in the decoder.

In another particular embodiment, a given determination criterion depends on the result of a comparison between the number of residual data elements suitable for modification and a given number.

This sequence improves the compression performance of the arithmetic encoder while effectively reducing signaling costs. In particular, such a sequence makes it possible to accurately detect the number of residual data elements from which it would be appropriate to apply data hiding technology so that the latter does not lead to high levels of disturbance in the signal intended for transmission to the decoder.

In yet another particular embodiment, if a plurality of digital information items are considered during the comparison step, the latter results in comparing the calculated value of a function representing the residual data items of a particular subset with the value of a function representing the plurality of digital information items.

This sequence optimizes the performance of the arithmetic encoder while optimizing the reduction of signaling costs, since it ensures that several elements of digital information are hidden in the signal intended for transmission to the decoder.

In yet another particular embodiment, at least one digital information element corresponds to the sign of the residual data element.

A sign is a particularly valuable piece of information that needs to be hidden due to the fact that the occurrence of a positive or negative sign is equally likely. Also, given that the character is necessarily encoded in one bit, it thus becomes possible, by hiding this information, to save one bit in the signal for transmission to the decoder, thereby significantly reducing the cost of signaling. The reduction in such cost will be all the greater when it is possible, according to the invention, to hide a plurality of characters, and therefore a plurality of bits.

Accordingly, the invention also relates to a device for encoding at least one image divided into sections, such a device comprising:

means for predicting data elements of the current portion as a function of at least one reference portion already encoded, followed by decoding to provide the predicted portion,

- means for determining a set of residual data elements suitable for comparing data elements related to the current section and the predicted section, the residual data elements being associated, respectively, with various digital information elements for which entropy coding is required,

- a means of generating a signal containing elements of encoded information.

Such an encoding device is characterized in that it contains, before the generation means, processing means, which is configured to:

- determine, from a certain set of residual data elements, a subset containing residual data elements suitable for their modification,

- calculate the value of a function representing elements of the residual data of a certain subset,

- compare the calculated value with the value of at least one of the elements of digital information,

- modify or not modify said at least one of the residual data elements of a certain subset depending on the result of the modification,

and means for entropy encoding at least one element of the modified residual data in the event of modification by the processing means.

Accordingly, the invention also relates to a method for decoding a data signal representing at least one partitioned image that has been previously encoded, comprising the step of obtaining, by entropy decoding of the signal data, digital data elements associated with residual data elements that are related to at least to at least one previously encoded area.

This decoding method is characterized by the fact that it contains the following steps:

- determine, from the residual data elements, a subset containing residual data elements configured to be modified during previous encoding,

- calculating the value of a function representing elements of the residual data of said specific subset,

- from the calculated value, the value of at least one element of digital information is determined, which differs from those obtained by entropy decoding.

In a particular embodiment, the steps that follow the step of determining a subset of residual data elements are implemented only if a given criterion is satisfied depending on the residual data elements to be modified.

In another particular embodiment, a predetermined determination criterion depends on the result of a comparison between the number of residual data elements that could be modified and a predetermined number.

In yet another particular embodiment, a plurality of values associated, respectively, with a plurality of digital data elements that differ from those obtained as a result of entropy decoding are obtained from said calculated value.

In yet another particular embodiment, said at least one digital information element corresponds to the sign of the residual data element.

Accordingly, the invention also relates to a data signal decoding apparatus representing at least one division of an image into portions that have been previously encoded, comprising means for obtaining, by entropy decoding of signal data elements, digital information elements associated with residual data elements, with respect to at least at least one previously encoded section.

Such a decoding device is characterized in that it contains processing means that are configured to:

- determine, from said residual data elements, a subset containing residual data elements that may have been modified during the previous encoding,

- calculate the value of a function representing elements of the residual data of a certain subset,

- from the calculated value, obtain the value of at least one element of digital information, which differs from those obtained as a result of entropy decoding.

The invention also relates to a computer program including instructions for executing steps of the encoding or decoding method presented above when the program is executed by a computer.

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

Yet another aspect of the invention is a computer readable recording medium including computer program instructions as mentioned above.

The recording medium may be any object or device capable of storing a program. For example, such a medium may include storage media such as ROM, such as a CD-ROM or microelectronic ROM, or magnetic recording media, such as a floppy disk or hard disk.

Alternatively, such a recording medium may be a transmission medium such as an electrical or optical signal, which may be transmitted via an electrical or optical cable, a radio link, or other means. The program according to the invention can in particular be downloaded via a network such as the Internet.

Alternatively, such a recording medium may be an integrated circuit in which a program is embedded, a circuit capable of executing the method in question or being used in executing the latter.

The encoding apparatus, decoding method, decoding apparatus and computer programs mentioned above exhibit at least the same advantages as those provided by the encoding method according to the present invention.

Brief description of drawings

Other features and advantages will become apparent from reading the two preferred embodiments described with reference to the drawings, in which:

in fig. 1 shows the main steps of the encoding method in accordance with the invention,

in fig. 2 shows an embodiment of a decoding device in accordance with the invention,

in fig. 3 shows the main steps of the decoding method in accordance with the invention,

in fig. 4 shows an embodiment of a decoding device in accordance with the invention.

Detailed description of the coding part

An embodiment of the invention will be described below in which the encoding method according to the invention is used to encode a sequence of images in accordance with a bit stream close to that obtained by encoding in accordance with the H.264/MPEG-4 AVG standard. In this In an embodiment, the encoding method according to the invention is, for example, implemented in software form or in hardware form by modifications to an encoder originally compliant with the H.264/MPBG-4 AVC standard. The encoding method according to the invention is presented in the form of an algorithm including steps C1 to C40 shown in FIG. 1.

According to an embodiment of the invention, the encoding method according to the invention is embodied in an encoding device or CO encoder, an embodiment of which is shown in FIG. 2.

According to the invention, before the actual encoding step, the image sequence image IE to be encoded in a given order is divided into a plurality of Z portions B ₁ , B ₂ , ..., B _i , ... B _z , as shown in FIG. 2.

It should be noted that in the sense of the invention, the term "section" means an encoding module. This latter terminology is particularly used in the HEVC/H.265 standard, a draft of which, for example, is presented in a document available at the following Internet address:

<http://phenix.int-evry.fr/jct/doc_end_user/current_document.php?id=3286>

In particular, such groups of encoding modules together establish rectangular or square shaped pixels, also called blocks, macroblocks or sets of pixels, which have other geometric shapes.

In the example shown in FIG. 2, the sections mentioned are blocks that are square in shape and all of which are the same size. Depending on the image size, which is not necessarily a multiple of the block size, the last blocks on the left and the last blocks on the bottom do not need to be square. In an alternative embodiment, the blocks may, for example, be rectangular in shape and/or may not be aligned with each other.

Each block or macroblock can, in addition, itself be divided into subblocks, which themselves can then be subdivided.

This separation is performed using the PSO separation module shown in FIG. 2, which uses, for example, a division algorithm that is well known in itself.

After said division step, each of the current portions B _i (where i is an integer such as 1≤i≤Z) of said image IE is encoded.

In the example shown in FIG. 2, such encoding is applied sequentially for each of the blocks B ₁ - B _Z of the current image IE. The blocks are encoded, for example, according to a scan, such as a raster scan, which is known to one skilled in the art.

The encoding according to the invention is implemented in the encoding software module MS_CO of the CO encoder, as shown in FIG. 2.

During step C1 shown in FIG. 1, the MS_CO encoding module according to FIG. 2 selects, as the current block B ₁ , the first block B ₁ intended for encoding the current image IE. As shown in FIG. 2, it is the first block to the left of the IE image.

During step C2 shown in FIG. 1, perform predictive coding of the current block B ₁ using known intra- and/or inter-frame prediction technologies, during which prediction is performed using predictive coding of block B ₁ with respect to at least one previously encoded and decoded block. Such prediction is performed using the FRED CO prediction software module, as shown in FIG. 2.

It goes without saying that other intra-prediction modes proposed in the H.264 standard are possible.

For the current block B ₁ , it is also possible to perform inter-frame predictive coding, during which the current block is predicted with respect to a block from a previously encoded and decoded image. Other types of forecasting may also be provided. Among the prediction options available for the current block, the optimal prediction is selected in accordance with random distortion criteria known to one skilled in the art.

Said predictive encoding step enables predictive encoding of a block Bp ₁ that is an approximation of the current block B ₁ . Information elements related to such predictive coding will be included in the signal for transmission to the decoder. Such information elements include, in particular, the type of prediction (inter- or intra-frame), and, if necessary, the intra-prediction mode, the block or macroblock division types if the latter have been subdivided, the reference image index and the motion vector used in the inter-prediction mode . Such information elements are compressed using a CO encoder.

During the next step C3, shown in FIG. 1, the prediction module PRED_CO compares the data elements related to the current block B ₁ with the data elements of the predicted block Bp ₁ . More specifically, during this stage, usually a Bp block; predictions are subtracted from the current block B ₁ to obtain the residual block Br ₁ .

During the next step C4, shown in FIG. 1, the residual block Br ₁ is transformed in accordance with a conventional direct transform operation, such as, for example, the discrete cosine transform type DCT, to obtain the transformed block Bt ₁ . This operation is performed by the conversion software module MT_CO, as shown in FIG. 2.

During the next step C5 shown in FIG. 1, the transformed block Bt ₁ is quantized in accordance with a conventional quantization operation such as, for example, scalar quantization. A block Bq ₁ of quantized coefficients is then obtained. This step is performed using the quantization software module MQ_CO, as shown in FIG. 2.

During the next step C6 shown in FIG. 1, the quantized coefficients of block Bq ₁ are scanned in a given order. In the example presented, such a sweep is a regular zigzag sweep. This step is performed using the readout software module ML_CO as shown in FIG. 2. At the end of step C6, a one-dimensional list E ₁ =(ε1, ε2, ..., εL) of coefficients is obtained, usually known as the “quantized remainder”, where L is an integer greater than or equal to 1. Each of the coefficients in the list E ₁ is interconnected with various elements of digital information for which entropy coding is supposed to be performed. Such digital information elements are described below by way of example.

Suppose that in the presented example, L = 16, and that the list E ₁ contains the following sixteen coefficients: E ₁ = (0, +9, -7, 0, 0, +1, 0, -1, +2, 0, 0, +1, 0, 0, 0, 0).

In this case:

- for each coefficient located before the last non-zero coefficient in the list E ₁ , an element of digital information, such as a bit, must be entropy encoded to indicate whether that coefficient is zero or not: if this coefficient is zero, a bit will be encoded, for example value 0, while if the coefficient is not zero, the bit will be encoded with the value 1;

- for each non-zero coefficient +9, -7, +1, -1, +2, +1, an element of digital information, such as a bit, must be entropy encoded to indicate whether the absolute value of the coefficient is equal to one: if it is equal to 1, then, for example, a bit value with a value of 1 will be encoded, while if it is not 1, a bit with a value of 0 will be encoded;

- for each non-zero coefficient and for which the absolute value is not equal to one, and which is located before the last non-zero coefficient, such as coefficients with values +9, -7, +2, an amplitude information element (the absolute value of the coefficient from which the value two has been subtracted ) are subjected to entropy coding;

- for each non-zero coefficient, the sign assigned to it is encoded using a digital information element such as a bit, for example set to '0' (for a "+" sign) or set to '1' (for a "-" sign).

Below, with reference to FIG. 1, specific encoding steps in accordance with the invention will be described.

In accordance with the invention, it is determined that entropy coding of at least one of the above-mentioned information elements should be eliminated. For reasons explained above in the description, in a preferred embodiment, it is determined that entropy encoding should not be performed on at least one sign of one of said coefficients in the list E ₁ .

As an alternative example, in particular, it would be possible to determine the need for entropy encoding of at least the least significant bit of the binary representation of the amplitude of the first non-zero coefficient in said list E ₁ .

To this end, during step C7 shown in FIG. 1, select the number of characters to be hidden when performing a later entropy encoding step. Such a step is performed using the MTR_CO processing software module as shown in FIG. 2.

In a preferred embodiment, the number of characters to be hidden is one or zero. Moreover, according to said preferred embodiment, it is intended to hide the sign of the first non-zero coefficient that is intended to be hidden. Therefore, in the presented example, the sign of the coefficient ε2=+9 is hidden.

In an alternative embodiment, the number of characters that must be hidden is either zero, one, two, three, or more.

According to a preferred embodiment, in stage C7, during the first sub-step C71 shown in FIG. 1, sublist SE ₁ containing coefficients suitable for their modification, ε'1, ε'2, ..., ε'M, where M<L, are determined from the mentioned list E ₁ . Such coefficients will be referred to below in the description as modified coefficients.

In accordance with the invention, a coefficient is modifiable if modification of its quantized value does not lead to desynchronization in the decoder when processing this modified coefficient by the decoder. Thus, the MTR_CO processing module is initially configured so that it does not perform modification:

- a zero coefficient or coefficients located before the first list of non-zero coefficients in such a way that the decoder does not affect the value of the hidden sign in this or these zero coefficients,

- and for reasons of computational complexity, zero coefficient or coefficients located after the last non-zero coefficient.

In the example presented, at the end of substep C71, a sublist SE ₁ is obtained such that SE ₁ =(9, -7, 0, 0, 1, 0, -1, 2, 0, 0, 1). Consequently, eleven modified coefficients are obtained.

During the next sub-step C72 shown in FIG. 1, the MTR_CO processing module proceeds to compare the number of modified coefficients with the specified threshold value TSIG. In a preferred embodiment, TSIG has a value of 4.

If the number of modified coefficients is less than the threshold value TSIG, then during step C20 shown in FIG. 1, a conventional entropy encoding of the coefficients in the list E ₁ is performed, such as is performed, for example, in the CAVAC encoder indicated by the reference CE_CO in FIG. 2. For this purpose, the sign of each non-zero coefficient in the list E ₁ is subjected to entropy coding.

If the number of modified coefficients is greater than the threshold TSIG, then during step C8 shown in FIG. 1, the MTR_CO processing module calculates a function value f that represents the coefficients in the sublist SE ₁ .

In the preferred embodiment, in which it is intended to hide only one character in the signal destined for the decoder, the function f represents the parity of the sum of the coefficients in the sublist SE ₁ .

During step C9 shown in FIG. 1, the MTR_CO processing module checks whether the parity of the value of the sign to be hidden matches the parity of the sum of the coefficients in the sublist SE ₁ , in accordance with a predetermined condition in the CO encoder.

In the proposed example, said convention is such that a positive sign is associated with a bit with a value of zero, while a negative sign is associated with a bit with a value of one.

Therefore, according to the condition adopted in the CO encoder according to the invention, the sign is positive, which corresponds to the value of the encoding bit equal to zero, and if the sum of the coefficients in the sublist SE] is even, then step C20 is performed to entropy encode the coefficients in the said above list E[, except for the sign of the coefficient ε2.

If, still in accordance with the convention adopted in the CO encoder according to the invention, the sign is negative, corresponding to an encoding bit value of one, and if the sum of the coefficients in the sublist SE ₁ is odd, then step C20 is also performed for the entropy coding the coefficients in the above-mentioned list E ₁ , with the exception of the sign of the coefficient ε2.

If, according to the convention adopted in the CO encoder according to the invention, the sign is positive, which corresponds to an encoding bit value of zero, and if the sum of the coefficients in the sublist SE ₁ is odd, then during step C10 of FIG. . 1, at least one modified coefficient in the sublist SE ₁ is modified.

If, still in accordance with the convention adopted in the CO encoder according to the invention, the sign is negative, corresponding to an encoding bit value of one, and if the sum of the coefficients in the sublist SE ₁ is even, then also in step C10 modify at least at least one modified coefficient in sublist SE ₁ .

Such a modification operation is performed using the MTR_CO processing module of FIG. 2.

In an exemplary embodiment in which SE ₁ =(+9, -7, 0, 0, +1, 0, -1, +2, 0, 0, +1), the total sum of f coefficients is 5, and therefore , she's odd. In order for the decoder to be able to reconstruct the positive sign assigned to the first non-zero coefficient ε2=+9, without the CO encoder passing this coefficient to the decoder, the parity of the sum must become even. Consequently, the MTR CO processing module tests, during said ethane C10, various modifications of the coefficients in the sublist SE ₁ , all with the aim of changing the parity of the sum of the coefficients. In a preferred embodiment, +1 or -1 is added to each modified coefficient, and a modification is selected from among these resulting modifications.

In a preferred embodiment, this selection results in an optimal prediction, in accordance with a performance criterion, which, for example, is a random distortion criterion, as is well known to one skilled in the art. This criterion is expressed by equation (1), presented below:

(1) J=D+λR

where D represents the distortion between the original macroblock and the reconstructed macroblock, R represents the encoding cost in bits of encoding information elements, and λ represents the Lagrange multiplier, the value of which may be fixed before encoding.

In the proposed example, the modification that results in optimal prediction, according to the random distortion criterion mentioned above, is adding the value 1 to the second coefficient -7 in the sublist SE ₁ .

Therefore, at the end of step C10, the modified sublist receives the modified sublist SEm ₁ =(+9, -6, 0, 0, +1, 0, -1, +2, 0, 0, +1).

It should be noted that during this stage, certain modifications are prohibited. Thus, if the first non-zero coefficient ε2 had a value of +1, it would be impossible to add -1 to it, since it would still be equal to zero, and thus would lose its characteristic of being the first non-zero coefficient in the list F ₁ . The decoder must then associate the decoded sign (by calculating the parity of the sum of the coefficients) with another coefficient, and a decoding error would occur.

During step C11 shown in FIG. 1, the processing module MTR_CO performs the corresponding modification in the list E ₁ . Then the following modified list Em ₁ =(0, +9, -6, 0, 0, +1, 0, -1, +2, 0, 0, +1, 0, 0, 0, 0) is obtained.

Then, in step C20, to entropy encode the coefficients in the above list, Em ₁ is performed except for the sign of the coefficient ε2, which is the "+" sign of the coefficient 9 in the present example, and this sign is hidden in the parity of the sum of the coefficients.

It should be noted that the set of coefficient amplitudes in the list E ₁ , or in the modified list Em ₁ , is encoded before the set of characters, with the exception of the sign of the first non-zero coefficient ε2, which is not encoded, as explained above.

During the next step C30 shown in FIG. 1, the MC_CO encoding module in FIG. 2 tests whether the current encoding block is the last IB image block.

If the current block is the last image block IE, then during step C40 shown in FIG. 1, the encoding method ends.

If this is not the case, the next block Bi is selected, which is then encoded in accordance with the previously mentioned raster scan order by repeating steps C1 to C20, for 1≤i≤Z.

After performing entropy encoding of all blocks B ₁ -B _Z , a signal F is constructed representing, in binary form, the encoded blocks.

The construction of the binary signal F is implemented in the stream construction software module CF, as shown in FIG. 2.

Stream F is then transmitted via a data network (not shown) to the remote terminal. The latter includes a decoder, which will be described in more detail later in this description.

Next, referring generally to FIG. 1, another embodiment of the invention will be described.

This other embodiment differs from the previous one only in the number of coefficients to be hidden, which is either 0 or N, where N is an integer such as N≥2.

To this end, the comparison sub-step C72 mentioned above is replaced by the comparison sub-step C72a represented by the dotted line in FIG. 1, during which the set of modified coefficients is compared with several predetermined threshold values 0<TSIG_1<TSIG_2<TSIG_3..., such that if the set of modified coefficients is between TSIG_N and TSIG_N+1, N characters are expected to be hidden.

If the set of modified coefficients is smaller than the first threshold value TSIG_1, then during the above-mentioned step C20, normal entropy encoding of the coefficients in the list E ₁ is performed. For this purpose, the sign of each non-zero coefficient in the list E ₁ is entropy encoded.

If the set of modified coefficients is between thresholds TSIG_N and TSIG_N+1, then during step C8 of FIG. 1, the MTR_CO processing module calculates a function value f that represents the coefficients in the sublist E ₁ .

In this other embodiment, since the decision at the encoder is to hide N characters, the function f represents the modulo 2 ^N sum of the coefficients in the sublist SE ₁ . It is assumed, in the proposed example, that N=2, the two signs to be hidden are the first two signs of the first two non-zero coefficients, respectively, that is, ε2 and ε3.

During the next step C9, shown in FIG. 1, the processing module MTR_CO checks whether a configuration of N characters, that is, 2 ^N possible configurations, matches the remainder value of module 2 ^N of the sum of the coefficients in the sublist SE ₁ .

In the proposed example, where N=2, there are 2 ² =4 different sign configurations.

These four configurations correspond to a convention in the CO encoder, which convention is defined, for example, as follows:

- a remainder equal to zero corresponds to two consecutive positive signs: +, +;

- a remainder equal to one corresponds to successive positive signs and negative signs: +, -;

- a remainder equal to two corresponds to successive negative signs and positive signs: -, +;

- a remainder equal to three corresponds to two consecutive negative signs:

If the configuration of N characters corresponds to the value of the modulo 2 ^N of the sum of the coefficients in the sublist SE ₁ , then step C20 for entropy encoding the coefficients in the above-mentioned list E ₁ is performed, with the exception of the signs of the coefficient ε2 and coefficient ε3, and these signs are hidden in the parity modulus sum 2 ^N coefficients.

If this is not the case, then step C10 is performed to modify the at least one modified coefficient in the sublist SE ₁ . Such modification is performed using the MTR_CO processing module in FIG. 2 in such a way that the modulus of the remainder 2 ^N of the sum of the modified coefficients in the sublist SE ₁ reaches the value of each of the two signs that must be hidden.

During the previously mentioned step C11, the processing module MTR_CO performs a corresponding modification of the list E ₁ . Thus, a modified list Em ₁ is obtained.

Next, step C20 is performed to entropy encode the coefficients in the above-mentioned list Em ₁ except for the sign of the coefficient ε2 and the sign of the coefficient ε3, and these signs are hidden in the parity of the modulus 2 sum of ^{the N} coefficients.

Detailed description of the decoding part

An embodiment of the decoding method according to the invention will be described below, in which the decoding method is implemented in software form or in hardware form by modifying a decoder that originally complies with the H.264/MPEG-4 AVC standard.

The decoding method according to the invention is presented in the form of an algorithm including steps D1 to D12 shown in FIG. 3.

According to an embodiment of the invention, the decoding method according to the invention is implemented in a decoding device or a DO decoder, as shown in FIG. 4.

During the preliminary stage, which is not shown in FIG. 3, in the received data signal F, sections B ₁ - B _Z are identified, which were previously encoded by the CO encoder. In a preferred embodiment, said sections are blocks that are square in shape and all have the same size. Depending on the size of the image, which does not need to be an integer multiple of the size of the blocks, the last blocks on the left and the last blocks on the bottom do not need to be square. In an alternative embodiment, the blocks may be, for example, rectangular and/or may not be aligned with each other.

Each block or macroblock can furthermore itself be divided into subblocks, which themselves can be subdivided.

This identification is performed using the flow analysis software module EX_DO, as shown in FIG. 4.

During step D1 shown in FIG. 3, EX_DO module in FIG. 4 is selected as the current block B ₁ , the first block B ₁ is for decoding. This choice consists, for example, of placing the read pointer in the signal F, at the beginning of the data elements of the first block B ₁ .

Decoding is then performed on each of the selected encoding blocks.

In the example shown in FIG. 3, such decoding is applied sequentially to each of the coded blocks B ₁ to B _Z . The blocks are decoded, for example, in accordance with the raster scan, as is well known to one skilled in the art.

The decoding according to the invention is implemented in the decoding software module MD_DO of the DO decoder, as shown in FIG. 4.

During step D2 shown in FIG. 3, perform first entropy decoding of the first selected current block B ₁ . This operation is performed using the entropy decoding module DE_DO shown in FIG. 4, for example, type SAVAS. During this step, the DE_DO module performs entropy decoding of digital information elements corresponding to the amplitude of each of the encoded coefficients in the E ₁ list or the modified Em ₁ list. At this stage, only the signs of the coefficients in the list E ₁ or in the modified list Em ₁ are not decoded.

During step D3 shown in FIG. 3, define a plurality of characters for which it is possible to hide them during the previous entropy encoding step C20. Such step D3 is performed by the processing software module MTR_DO, as shown in FIG. 4. Step D3 is similar to step C7 mentioned above to determine the number of characters to be hidden.

In a preferred embodiment, the number of hidden characters is one or zero. Moreover, according to the mentioned preferred embodiment, the sign of the first non-zero coefficient is hidden. In the example presented, therefore, it represents the positive sign of the coefficient ε2=+9.

In an alternative embodiment, the number of hidden characters is zero, one, two, three, or more.

According to a preferred embodiment of step D3, during the first sub-step D31 shown in FIG. 3, a sublist containing the coefficients ε'1, ε'2, ε'M, where M<L, which allow their modification during encoding, are determined from the mentioned list E ₁ or from the modified list Em ₁ .

This determination is made in the same way as in the above-mentioned encoding step C7.

Similar to the MTR_CO processing module mentioned above, the MTR_DO processing module is initially configured to modify:

- zero coefficient or coefficients located before the first non-zero coefficient,

- and for reasons of calculation complexity, a zero coefficient or coefficients located after the last non-zero coefficient.

In the example presented, at the end of substep D31, there is a sublist SEm ₁ such as SEm ₁ =(9, -6, 0, 0, 1, 0, -1, 2, 0, 0, 1). Consequently, eleven coefficients are obtained that could be modified.

During the next sub-step D32 shown in FIG. 3, the MTR_DO processing module proceeds to compare the number of coefficients that could be modified with a given threshold value TSIG. In a preferred embodiment, TSIG has a value of 4.

If the number of coefficients that could be modified is less than the threshold TSIG, then during step D4 shown in FIG. 3, perform the usual entropy decoding for all signs of the coefficients in the list E ₁ . Such decoding is performed using a CABAC decoder, indicated by reference value DE_DO in FIG. 4. For this purpose, the sign of each non-zero coefficient in the list E ₁ is subjected to entropy decoding.

If the number of coefficients that can be modified is greater than the threshold TSIG, then during said step D4, normal entropy decoding is performed on all the signs of the coefficients in the list Em ₁ except for the sign of the first non-zero coefficient ε2.

During step D5 shown in FIG. 3, the processing unit MTR_DO calculates a function value f that represents the coefficients in the sublist SEm to determine whether the calculated value is even or odd.

In the preferred embodiment, where only one sign is hidden in the signal F, the function f represents the parity of the sum of the coefficients in the sublist SEm ₁ .

According to the convention used in the CO encoder, which is the same as in the DO decoder, an even value of the sum of coefficients in the sublist SEm ₁ means that the sign of the first non-zero coefficient in the modified list Em ₁ is positive, while an odd one the value of the sum of coefficients in the sublist SEm ₁ means that the sign of the first non-zero coefficient in the modified list Em ₁ is negative.

In an exemplary embodiment in which SEm ₁ =(+9, -6, 0, 0, +1, 0, -1, +2, 0, 0, +1), the total sum of the coefficients is 6, and therefore, is even. Therefore, at the end of step D5, the MTR_DO processing module deduces from it that the hidden sign of the first non-zero coefficient ε2 is positive.

During step D6 shown in FIG. 3, and using all the reconstructed digital information elements during steps D2, D4 and D5, the quantized coefficients of block Bq ₁ are reconstructed in a given order. In the present example, it is a reverse zigzag scan with respect to the zigzag scan performed during the above-mentioned encoding step C6. This step is performed using the readout software module ML_DO, as shown in FIG. 4. More specifically, the ML_DO module continues processing so as to include the coefficients of the list E ₁ (one-dimensional) in the block Bq ₁ (two-dimensional), using the above-mentioned reverse zigzag scanning order.

During step D7 shown in FIG. 3, the quantized residual block Bq ₁ is subjected to inverse quantization according to a conventional inverse quantization operation, which is the inverse quantization operation performed in the above-mentioned encoding step C5, to obtain the decoded dequantized block BDq ₁ . This step is performed using the inverse quantization software module MDQ_DO, as shown in FIG. 4.

During step D8 shown in FIG. 3, inverse transform is performed on the dequantized block BDq ₁ , which is the inverse operation of the forward transform performed in encoding in the above-mentioned step C4. Thus, a decoded residual block BDr ₁ is obtained. This operation is performed using the inverse transform software module MTI_DO, as shown in FIG. 4.

During step D9 shown in FIG. 3, predictive decoding of the current block B ₁ is performed. Such predictive decoding is typically performed using known intra- and/or inter-frame prediction techniques, during which block B ₁ is predicted with respect to at least one previously decoded block. Such an operation is performed using the predictive decoding module PRED_DO, as shown in FIG. 4.

It goes without saying that other intra-prediction modes are possible, as proposed in the H.264 standard.

During this step, decoding prediction is performed using the decoded syntax elements in the previous step, and in particular containing the type of prediction (inter-frame or intra-frame), and, if necessary, the mode of intra-frame prediction, the type of block division or macroblock, if the latter has been divided , reference image index and motion vector, which are used in inter-prediction mode.

The above-mentioned predictive decoding step produces a predicted block BP ₁ .

During step D10 shown in FIG. 3, the decoded block BD ₁ is constructed by adding the decoded residual block BDr ₁ to the prediction block BP ₁ . This operation is performed using the reconstruction software module MR_DO shown in FIG. 4.

During step D11 shown in FIG. 3, the MD_DO decoding module checks whether the current decoded block is the last block identified in the F signal.

If the current block is the last block in signal F, then during step D12 shown in FIG. 3, the decoding method ends.

If this is not the case, select the next block B ₁ as the block to be decoded, in accordance with the raster scan order mentioned above, by repeating steps D1 to D10, for 1≤i≤Z.

Next, with reference to FIG. 3, another embodiment of the invention will be described.

This other embodiment differs from the previous one only in the number of hidden coefficients, which is neither 0 nor N, where N is an integer such as N≥2.

To this end, the previously mentioned comparison sub-step D32 is replaced by the sub-step D32a represented by the dotted line in FIG. 3, during which the number of coefficients that could be modified is compared with several specified threshold values 0<TSIG_1<TSIG 2<TSIG3 ..., such that if the set of said coefficients is in the range from TSIG_N to TSIG_N+1, there may be N characters are hidden.

If the number of said coefficients is less than the first threshold value TSIG_1, then during the previously mentioned step D4, normal entropy decoding of all coefficient signs in the list E ₁ is performed. For this purpose, the sign of each non-zero coefficient in the list E ₁ is subjected to entropy decoding.

If the number of said coefficients is between the threshold TSIG_N and TSIG_N+1, then during the previously mentioned step D4, normal entropy decoding of all coefficient signs in the list E ₁ is performed, except for the N corresponding signs of the first non-zero coefficients in said modified list Em ₁ , said N the signs are hidden.

In another embodiment, the processing module MTR_DO calculates, during step D5, a function value f that represents the modulo 2 ^N remainder of the sum of the coefficients in the sublist SEm ₁ . In the proposed example it is N=2.

The MTR_DO of the processing module therefore outputs from it a configuration of two hidden characters that are assigned to each of the first two non-zero coefficients ε2 and ε3, respectively, in accordance with the convention used in the encoding.

After reconstructing these two signs, perform steps D6 - D12 described above.

It goes without saying that the embodiments that have been described above have been presented solely by way of example and not by way of limitation, and that many modifications can easily be made by one skilled in the art without thereby departing from the scope of the invention.

Thus, for example, according to a simplified embodiment with respect to that shown in FIG. 1, the CO encoder could be configured to hide at least N' given characters, where N'>1, instead of either zero, one or N given characters. In this case, comparison step C72 or C72a could be omitted. Accordingly, according to a simplified embodiment with respect to that shown in FIG. 3, the DO decoder could be configured to reconstruct N' given characters instead of either zero, one, or N given characters. In this case, comparison step D32 or D32a could be eliminated.

In addition, the determination criterion applied in the encoding step C72 or in the decoding step D32 may be replaced by another type of criterion. To this end, instead of comparing a set of modified coefficients or a set of coefficients that have been modified with a threshold value, the MTR_CO or MTR_DO processing module could apply a selection criterion that is a function of the sum of the amplitudes of the coefficients that are modified or could be modified, respectively. or the number of zeros present among the coefficients that are modified and that may have been modified, respectively.

Claims (13)

1. A computer-readable recording medium storing a program configured to implement an encoding method comprising the following steps: determining a sublist from a list of coefficients of the quantized residual block, the sublist containing coefficients that are suitable for modification by identifying a first non-zero coefficient and a last non-zero coefficient, with zero coefficients located before the first non-zero coefficient and zero coefficients located after the last non-zero coefficient , are modifiable; compare the number of modified coefficients with a given threshold value equal to four; if the number of modified coefficients is less than or equal to a threshold value of four, the coefficients of the list are encoded, the entropy encoding containing the sign of each non-zero coefficient in the list; if the number of modified coefficients exceeds a threshold value of four, the parity value of the sum of coefficients in the sublist is calculated; check whether the individual sign to be hidden corresponds to the parity of the sum of the coefficients in the sublist, in accordance with the agreement, while: if the sign is positive and the sum of the coefficients in the sublist is even, the coefficients in the list are encoded except for the sign of the coefficient, which is hidden in the evenness of the sum of the coefficients; if the sign is negative and if the sum of the coefficients in the sublist is odd, the coefficients in the list are encoded except for the sign of the coefficient, which is hidden in the evenness of the sum of the coefficients; if the sign is positive and the sum of the coefficients in the sublist is odd, at least one modified coefficient in the sublist is modified and the coefficients in the list are encoded except for the sign of the coefficient, which is hidden in the parity of the sum of the coefficients; And if the sign is negative and the sum of the coefficients in the sublist is even, at least one modified coefficient in the sublist is modified and the coefficients in the list are encoded except for the sign of the coefficient, which is hidden in the parity of the sum of the coefficients, and in this case, the sign of the first non-zero coefficient is subject to concealment. 2. The medium according to claim 1, characterized in that the medium is one of a storage medium such as a ROM, a CD-ROM, a microelectronic circuit ROM, a magnetic recording medium such as a floppy disk or a hard disk. 3. The recording medium according to claim 1, characterized in that the recording medium is an integrated circuit.