METHOD FOR ENCODING AND DECODING IMAGES, ENCODING AND DECODING DEVICE, AND CORRESPONDING COMPUTER PROGRAMS Field of the invention The present invention pertains generally to the field of image processing, and more precisely to the coding and decoding of parameters of digital images, whether these digital images are fixed or form part of a sequence of digital images. The coding/decoding of such image parameters applies in particular to images arising from at least one video sequence comprising: - images arising from one and the same camera and following one another temporally (coding/decoding of 2D type), - images arising from various cameras oriented according to different views (coding/decoding of 3D type), corresponding components of texture and depth (coding/decoding of 3D type), - images obtained by projection of a 360° video, -etc... The present invention applies in a similar manner to the coding/decoding of parameters of images of 2D or 3D type. The invention may in particular, but not exclusively, be applied to the video coding implemented in current AVC and HEVC video coders and their extensions (MVC, 3D-AVC, MV-HEVC, 3D-HEVC, etc), and to the corresponding decoding. Background of the invention Current video coders (MPEG, H.264, HEVC, ...) use a block-wise representation of the video sequence. The images are split into blocks, which might be re-split, for example in a recursive manner as in the HEVC standard. For a current block which is to be coded, the image parameters associated with this block are coded in the form of bits with the aid of a suitable coding scheme implemented by a coder, such as for example an entropy coder, the aim of which is to code these parameters without loss. Such parameters are for example: - the residual coefficients of prediction of the pixels of the current block, - the mode of prediction of the current block (Intra prediction, Inter prediction, default prediction carrying out a prediction for which no information is transmitted to the decoder (in English "skip"), - information specifying the type of prediction of the current block (orientation, reference image, ...), - the type of splitting of the current block, - the motion information of the current block if necessary, -etc. The bits obtained after entropy coding are written to a data signal which is intended to be transmitted to the decoder. When dealing for example with residual prediction coefficients, the HEVC standard proposes an indicator called "cbf" (English abbreviation of "Coded Block Flag") that can take: - either the value 0 which is representative of zero residual coefficients for the current block to be coded, - or the value 1 which is representative of at least one non-zero residual coefficient for the current block to be coded. Once the coded 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 bits representative of the image parameters associated with the block are read, and then decoded with the aid of a decoding scheme implemented by a decoder. The drawback of such an image parameter coding technique is that the resulting binary sequence remains expensive to signal. Thus, it does not make it possible to optimize the reduction in the gain from compression of the coded data. This results in unsatisfactory compression performance. However, in certain image coding contexts, and for certain image types considered, it turns out that the coding of such parameters does not always turn out to be useful or else turns out to be somewhat ineffective, thus engendering, according to the context, either poor quality image reconstruction, or significant recourse to the calculational resources either at the coder, or at the decoder, or an overly high limitation of the gain in bitrate. For example, in medical imaging, it is not necessarily useful to code one or more coding parameters associated with blocks of the image that are situated at the periphery of the latter. According to another example, in the case of an image obtained by two-dimensional projection of a 360° video, whether this projection is of the Mercator type, equirectangular type, of the CMP (English abbreviation of "Cube Map Projection") type, etc., it is known to note that such a projection engenders deformations in certain very particular zones of the projected image. For example, as regards an image projected onto a rectangle, it is known in advance that the upper and lower zones of this image contain the most deformations than the remainder of the image. It follows from this that the coding of the parameters associated with the blocks contained in these particular zones is not efficient, additionally engendering a pointless decrease in bitrate gain. The invention is therefore aimed at optimizing the cost of signaling of the image coding parameters, by implementing a selective coding of these coding parameters which takes account, at a first level, of the characteristic of the image to be coded, and at a second level, of the disposition of the current block to be coded in this image. Subject and summary of the invention One of the aims of the invention is to remedy drawbacks of the aforementioned prior art. To this effect, a subject of the present invention relates to a method for coding at least one image split into blocks, implementing the following: - code a first syntax element associated with a characteristic of said at least one image, - for a current block to be coded associated with at least one coding parameter, code the data of the current block. Such a coding method is noteworthy in that: - if the first syntax element is coded according to a first predefined value representative of the characteristic of the image, a second syntax element representative of the value of said at least one coding parameter is coded only if the current block belongs to a predefined zone of the image, - if the first syntax element is coded according to a second predefined value representative of the characteristic of the image, the second syntax element is coded, whether or not the current block belongs to the predefined zone. Such a provision makes it possible to avoid coding, and then transmitting to the decoder, at least one syntax element representative of a coding parameter of a block, by taking account not only of the fact that such a block is situated in a specific zone of the image, for which the coding of this syntax element is not useful having regard to the current image coding context, but also of a characteristic of the image to be coded in which the current block is situated. This therefore results in a non-negligible decrease in the signaling cost, to the extent that such a provision is reproducible at the level of the decoder. According to a particular embodiment, if the current block belongs to the predefined zone of the image, the coding of the second syntax element representative of the value of said at least one coding parameter is implemented as a function of a criterion dependent on the characteristics of the current block. Such a provision makes it possible furthermore to implement an adaptive coding of at least one syntax element representative of a coding parameter of a block, according to the value of a criterion dependent on the characteristics of the current block, such as for example the size, the value of the residual pixels of this block, the type of transform applied to the residual pixels of this block if the latter has undergone a prediction, etc.... Thus, as a function of the value of this criterion, it is decided to code or to not code this syntax element. This results in a reduction in the cost of signaling which is optimized, through better adaptation of the coding to the local characteristics of the video signal. According to another particular embodiment, the characteristic of said at least one image is the type of obtaining of said at least one image. Such a provision makes it possible to reduce the cost of signaling in a selective manner according to the way in which the scene that the image represents was captured initially. For example, if the current image is a two-dimensional image, it is decided: - to not code a syntax element representative of a coding parameter of a block of this image, in the case where the latter was obtained by projection of a video captured according to a plurality of viewing angles covering for example an angle of 360° and therefore comprises zones where it is known in advance that the information density per pixel is lower than in other zones of the image, - to systematically code a syntax element representative of a coding parameter of a block of this image, in the case where the latter originates from a video comprising 2D images following one another temporally. According to yet another particular embodiment, the characteristic of said at least one image is the current coding context of the image. Such a provision makes it possible to reduce the cost of signaling in a selective manner within the framework of certain image coding applications. According to an exemplary image coding context, in the case of image coding applied to medical imaging, it is decided to apply a coding level of different quality to at least two different zones of the current image in such a way as to: - not code a syntax element representative of a coding parameter of a block of this image, in the case where this block is located at the periphery of the image, - code a syntax element representative of a coding parameter of a block of this image, in the case where this block is located at the center of the image. According to another exemplary coding context, in the case of image coding applied to a televisual stream, it is decided to apply one and the same level of coding quality to each zone of the current image. The various aforementioned modes or characteristics of embodiment may be added independently or in combination with one another, to the operations implemented in the course of the coding method such as is defined hereinabove. Correlatively, the invention relates to a device for coding at least one image split into blocks, comprising a processing circuit which is designed to: - code a first syntax element associated with a characteristic of said at least one image, - for a current block to be coded associated with at least one coding parameter, code the data of the current block. Such a coding device is characterized in that the processing circuit is designed to: - if the first syntax element is coded according to a first predefined value representative of the characteristic of the image, code a second syntax element representative of the value of said at least one coding parameter, only if the current block belongs to a predefined zone of the image, - if the first syntax element is coded according to a second predefined value representative of the characteristic of the image, code the second syntax element, whether or not the current block belongs to the predefined zone. In a corresponding manner, the invention also relates to a method for decoding a data signal representative of at least one coded image having been split into blocks, implementing the following: - decode a first syntax element contained in said data signal, said first syntax element being associated with a characteristic of said at least one image, - for a current block to be decoded associated with at least one coding parameter, decode the data of the current block. Such a decoding method is noteworthy in that: - if the first syntax element has a first predefined value representative of the characteristic of the image, a second syntax element representative of the value of said at least one coding parameter is decoded only if the current block belongs to a predefined zone of the image, - if the first syntax element has a second predefined value representative of the characteristic of the image, the second syntax element is decoded, whether or not the current block belongs to the predefined zone. According to a particular embodiment, if the current block belongs to the predefined zone of the image, the decoding of the second syntax element representative of the value of said at least one coding parameter is implemented as a function of a criterion dependent on the characteristics of the current block. According to another particular embodiment, the characteristic of said at least one image is the type of obtaining of said at least one image. According to yet another particular embodiment, the characteristic of said at least one image is the current decoding context of the image. The various aforementioned modes or characteristics of embodiment may be added independently or in combination with one another, to the operations implemented in the course of the decoding method such as is defined hereinabove. Correlatively, the invention relates to a device for decoding a data signal representative of at least one coded image having been split into blocks, comprising a processing circuit which is designed to: - decode a first syntax element contained in the data signal, the first syntax element being associated with a characteristic of said at least one image, - for a current block to be decoded associated with at least one coding parameter, decode the data of the current block. Such a decoding device is noteworthy in that the processing circuit is designed to: - if the first syntax element has a first predefined value representative of the characteristic of the image, decode a second syntax element representative of the value of said at least one coding parameter, only if the current block belongs to a predefined zone of the image, - if the first syntax element has a second predefined value representative of the characteristic of the image, decode the second syntax element, whether or not the current block belongs to the predefined zone. The invention further relates to a computer program comprising instructions for implementing one of the coding and decoding methods according to the invention, when it is executed on a computer. Such a program may use any programming language, and be in the form of source code, object code, or of code intermediate between source code and object code, such as in a partially compiled form, or in any other desirable form. Yet another subject of the invention is also aimed at a recording medium readable by a computer, and comprising computer program instructions such as is mentioned hereinabove. The recording medium may be any entity or device capable of storing the program. For example, the medium may comprise a storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, a USB key, or else a magnetic recording means, for example a hard disk. Moreover, 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 may be in particular downloaded over a network of Internet type. Alternatively, such a recording medium may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute the method in question or to be used in the execution of the latter. Brief description of the drawings Other characteristics and advantages will become apparent on reading preferred embodiments described with reference to the figures in which: - Figure 1A represents the steps of the coding method according to one embodiment of the invention, - Figure 1B represents an alternative of the coding method of Figure 1A, - Figure 2 represents a coding device according to one embodiment of the invention, - Figure 3A represents an image split into blocks according to a first embodiment, - Figure 3B represents an image split into blocks according to a second embodiment, - Figures 4A to 4D each represent examples of zones predefined in the current image, in the case of a particular obtaining of the current image, - Figure 5A represents a current image in a coding/decoding context where the coder/decoder chooses whether or not to apply a different level of coding/decoding quality to at least two zones of the current image, - Figure 5B represents an exemplary zone predefined in the current image, in the case where the coder/decoder chooses whether or not to apply a different level of coding/decoding quality to at least two zones of the current image, - Figure 6A represents the steps of the decoding method according to one embodiment of the invention, - Figure 6B represents an alternative of the decoding method of Figure 6A, - Figure 7 represents a decoding device according to one embodiment of the invention. Detailed description of the coding part An embodiment of the invention will now be described, in which the coding method according to the invention is used to code an image or a sequence of images according to a binary stream close to that obtained by a coding implemented in a coder in accordance with any one of the current or forthcoming video coding standards. In this embodiment, the coding method according to the invention is for example implemented in a software or hardware manner by modifications of such a coder. The coding method according to the invention is represented in the form of an algorithm comprising operations C1 to C9 such as are represented in Figure 1A. According to the embodiment of the invention, the coding method according to the invention is implemented in a coding device or coder CO represented in Figure 2. As illustrated in Figure 2, the coder CO comprises a memory MEM_C comprising a buffer memory MT_C, a processor PROC_C driven by a computer program PG_C which implements the coding method according to the invention. On initialization, the code instructions of the computer program PG_C are for example loaded into a RAM memory, denoted MR_C, before being executed by the processor PROC_C. The coding method represented in Figure 1A applies to any current image ICj which is fixed or else forms part of a sequence of L images ICi, ..., ICj,..., ICL(11. According to a first embodiment represented in Figure 3A, the blocks B1, B2, ..., Bu,..., Bs are obtained on completion of a single subdivision of the current image ICj into blocks of maximum size. According to this first embodiment, the blocks have a size of for example 64x64 pixels. According to a second embodiment represented in Figure 3B, the blocks B1, B2, ..., Bu,..., Bs are obtained on completion of a subdivision of the current image ICj into blocks of size less than or equal to the aforementioned maximum size. According to this second embodiment, said blocks are for example of size 64x64 and/or 32x32 and/or 16x16 and/or 8x8 pixels. As a function of the size of the image which is not necessarily a multiple of the size of the blocks, the last blocks on the left and the last blocks at the bottom might not be square. In an alternative embodiment, the blocks may be for example of rectangular size and/or not be aligned with one another. At C2, there is undertaken the coding of a first syntax element ES1 which is associated with a characteristic of the current image IQ. The syntax element ES1 is a high-level syntax element of a video sequence comprising the current image IQ. To this effect, as a function of the coding context, this element may be coded: - at the start of the coding of each image of the video sequence, - or just once at the start of the coding of a sequence of images, - or just once at the start of the coding of the video sequence. The syntax element ES1 is coded at C2: - by a first predefined value VCESI1 which is representative of a characteristic of a first type of the current image IQ, - by a second predefined value VCESI2 which is representative of a characteristic of a second type of the current image IQ. According to an example, VCES11=1 andVCESl2=0. The coding C2 is for example an entropy coding of CABAC ("Context Adaptive Binary Arithmetic Coder" in English) type or else an entropy coding of arithmetical or Huffman type. This coding is implemented by a coding device MC_C represented in Figure 2, which device is driven by the processor PROC_C. According to a first embodiment, the characteristic of the image IQ is the type of obtaining of the latter. To this effect: - if the image IQ is for example a 2D image which was obtained by two-dimensional projection of a conventional 360° video, VCES1i=1, - if the image IQ is for example a conventional 2D image of fixed type or forming part of a sequence of images following one another temporally, VCES12=0. According to a second embodiment, the characteristic of the image IQ is the choice at the coder of whether or not to apply a different level of coding quality to at least two zones of the current image to be coded. To this effect: - if the current image IQ is coded according to a first coding context, such as for example a medical imaging context, VCES1i=1, thus indicating that the central zone of the current image is coded according to a greater level of quality than that applied to the peripheral zone of the current image, - if the current image IQ is coded according to a second coding context, such as for example a televisual context, VCES12=0, thus indicating that one and the same level of coding quality is applied to the whole current image. With reference to Figure 1A, at C3, the coder CO of Figure 2 selects as current block a first block to be coded Bu of the image IQ, such as for example the first block Bi. The current block Bu is associated with at least one coding parameter. Byway of non-exhaustive examples, such a coding parameter is: • the mode of prediction (intra prediction, inter prediction, bi-prediction, default prediction carrying out a prediction for which no information is transmitted to the decoder (in English "skip"); • the type of prediction (orientation, reference image component, • the type of subdivision into sub-blocks; • the type of transform, for example 4x4 DCT, 8x8 DCT, applied to the current block or to the residual pixels of the current block if the latter has undergone a prediction; • the pixel values; • the values of residual pixels resulting from a prediction; • a filtering of the current image ICj once coded, such as for example that achieved by the "Sample Adaptive Offset" technique of the HEVC standard (technique described in the document Chin-Ming Fu; E. Alshina; A. Alshin; Yu-Wen Huang; Ching-Yeh Chen; Chia-Yang Tsai; Chih-Wei Hsu; Shaw-Min Lei; Jeong-Hoon Park; Woo-Jin Han "Sample Adaptive Offset in the HEVC Standard" Published in: IEEE Transactions on Circuits and Systems for Video Technology (Volume: 22, Issue: 12, Dec. 2012); • the index of the intra prediction mode from among a list constructed for the current block; • the index of the motion vector from among a list constructed for the current block; • an indicator of subdivision of the residual block obtained in case of prediction of the current block; • an indicator of filtering of the reference pixels that have served to predict the current block; • etc... A second syntax element ES2 is representative of the value of said at least one coding parameter. At C4, the coder CO reads the coded value VCESI1 or VCESI2 of the syntax element ES1. If at C4, the coded value of the syntax element ES1 is VCESI1 and if, at C5, the current block Bu belongs to a predefined zone of the current image IQ, it is decided at C6 not to code the syntax element ES2. To this effect, the syntax element ES2 is set to a predefined value. Such an operation is particularly suitable when the information density of the predefined zone of the image is low or very low. According to a first embodiment, if the syntax element ES2 is representative of the mode of prediction of the current block Bu and if, for example, ES2 takes: - a first predefined value VES2i indicating an inter prediction of the current block Bu, - or a second predefined value VES22 indicating an intra prediction of the current block Bu, it is decided at C6 to assign ES2 to, for example, the first predefined value VES2i according to a convention predetermined at the coder CO. According to a second embodiment, if the syntax element ES2 is representative of the value of the residual pixels of the current block Bu and if, for example, ES2 takes: - a first predefined value VES2i indicating residual pixels of zero value of the current block Bu, - or a second predefined value VES22 indicating residual pixels of non-zero value of the current block Bu, it is decided at C6 to assign ES2 to, for example, the first predefined value VES2i according to a convention predetermined at the coder CO. The predefined values VES2i and VES22 of the second syntax element ES2 are stored beforehand in a list LTS1 which is recorded in the buffer memory MT_C of the coder CO of Figure 2. During the coding of the current block Bu, only the first or the second embodiment of the syntax element ES2 is implemented. By way of alternative, the first and second embodiments are implemented. If at C4, the coded value of the syntax element ES1 is VCESI1 and if, at C5, the current block Bu does not belong to the predefined zone of the current image, the predefined value VES2i or VES22 taken by the second syntax element ES2 is coded at C7. On completion of this coding C7, as a function of the current coding context, a coded value VCES2i or VCES22 of the second syntax element ES2 is obtained. By way of non-exhaustive example, VCES2i=0 andVCES22=1. According to a first embodiment, if the syntax element ES2 is representative of the mode of prediction of the current block Bu and if, for example, ES2 takes: - a first predefined value VES2i indicating an inter prediction of the current block Bu, - or a second predefined value VES22 indicating an intra prediction of the current block Bu, the value VES2i or VES22 is coded as for example the value VCES2i=0 or VCES22=1, respectively, as a function of the current coding context. According to a second embodiment, if the syntax element ES2 is representative of the value of the residual pixels of the current block Bu and if, for example, ES2 takes: - a first predefined value VES2i indicating residual pixels of zero value of the current block Bu, - or a second predefined value VES22 indicating residual pixels of non-zero value of the current block Bu, the value VES2i or VES22 of the syntax element ES2 is systematically coded at C7, as for example the value VCES2i=0 or VCES22=1, respectively, as a function of the current coding context. Such a coding is for example an entropy coding of CABAC ("Context Adaptive Binary Arithmetic Coder" in English) type or else an entropy coding of arithmetical or Huffman type. This coding is implemented by a coding device MC_C represented in Figure 2. If at C4, the coded value of the syntax element ES1 is VCESI2, the value VES2i or VES22 of the syntax element ES2 is systematically coded at C7, as for example the value VCES2i=0 or VCES22=1, respectively, as a function of the current coding context. According to one embodiment represented in Figure 4A, for a current image ICj of height H, in the case where: - the current image IQ is for example a 2D image obtained by two-dimensional projection of a 180° or 360° video, the syntax element ES1 is coded according to the value VCES11=1, - the current image IQ is for example a conventional 2D image of fixed type or forming part of a sequence of images following one another temporally, the syntax element ESi is coded according to the value VCES12=0. Represented in Figure 4A is the current image IQ in the guise of 2D image obtained by two-dimensional projection of a 360° video. A first predefined zone, denoted Z1, of the current image IQ has a height H1 such that 0.2H1. According to a first embodiment represented in Figure 3A, the blocks B1, B2, ..., Bu,..., Bs are obtained on completion of a single subdivision of the current image IQ into blocks of maximum size. According to this first embodiment, the blocks have a size of for example 64x64 pixels. According to a second embodiment, the blocks B1, B2, ..., Bu,..., Bs are obtained on completion of a subdivision of the current image ICj into blocks of size less than or equal to the aforementioned maximum size. According to this second embodiment, said blocks are for example of size 64x64 and/or 32x32, and/or 16x16, and/or 8x8 pixels. As a function of the size of the image which is not necessarily a multiple of the size of the blocks, the last blocks on the left and the last blocks at the bottom might not be square. In an alternative embodiment, the blocks may be for example of rectangular size and/or not be aligned with one another. With reference to Figure 6A, at D4, the decoder DO of Figure 7 selects as current block to be decoded a first set of coded data DCu of the image IQ, such as for example the first set of coded data DCi associated with the first block Bi to be decoded. The set of coded data DCu of the current block Bu is associated with at least one coding parameter. By way of non-exhaustive examples, such a coding parameter is: • the mode of prediction (intra prediction, inter prediction, bi-prediction, default prediction carrying out a prediction for which no information is transmitted to the decoder (in English "skip"); • the type of prediction (orientation, reference image component, • the type of subdivision into sub-blocks; • the type of transform, for example 4x4 DCT, 8x8 DCT, applied to the current block or to the residual pixels of the current block if the latter has undergone a prediction; • the pixel values; • the values of residual pixels resulting from a prediction; • a filtering of the current image IQ once coded, such as for example that achieved by the "Sample Adaptive Offset" technique of the HEVC standard (technique described in the document Chin-Ming Fu; E. Alshina; A. Alshin; Yu-Wen Huang; Ching-Yeh Chen; Chia-Yang Tsai; Chih-Wei Hsu; Shaw-Min Lei; Jeong-Hoon Park; Woo-Jin Han "Sample Adaptive Offset in the HEVC Standard" Published in: IEEE Transactions on Circuits and Systems for Video Technology (Volume: 22, Issue: 12, Dec. 2012); • the index of the intra prediction mode from among a list constructed for the current block; • the index of the motion vector from among a list constructed for the current block; • an indicator of subdivision of the residual block obtained in case of prediction of the current block; • an indicator of filtering of the reference pixels which have served to predict the current block; • etc... A second syntax element ES2 is representative of the value of said at least one coding parameter. At D5, the decoder DO reads whether one is dealing with the coded value VCES11 or VCES12 of the syntax element ES1 which was decoded at D2. If at D5, one is dealing with the coded value VCESI1 of the first syntax element ES1 and if, at D6, the coded data DCu are associated with a current block Bu which belongs to a predefined zone of the current image IQ, the syntax element ES2 is set to a predefined value. Such an operation is particularly suitable when the information density of the predefined zone of the image is low or very low. Examples of predefined zones of the current image, according to a first embodiment, have already been described with reference to Figures 4A to 4D and will therefore not be described again here. Examples of predefined zones of the current image, according to a second embodiment, have already been described with reference to Figure 5B and will therefore not be described again here. According to a first embodiment, if the syntax element ES2 is representative of the mode of prediction of the current block Bu to be decoded and if, for example, ES2 takes: - a first predefined value VES2i indicating an inter prediction of the current block Bu, - or a second predefined value VES22 indicating an intra prediction of the current block Bu, it is decided at D7 to assign ES2 to, for example, the first predefined value VES2i according to a convention predetermined at the decoder DO and which is the same as at the coder CO. According to a second embodiment, if the syntax element ES2 is representative of the value of the residual pixels of the current block Bu and if, for example, ES2 takes: - a first predefined value VES2i indicating residual pixels of zero value of the current block Bu, - or a second predefined value VES22 indicating residual pixels of non-zero value of the current block Bu to be decoded, it is decided at D7 to assign ES2, for example to the first predefined value VES2-I, according to a convention predetermined at the decoder DO and which is the same as at the coder CO. To this effect, in a corresponding manner to the coder CO of Figure 2, the predefined values VES2i and VES22 of the second syntax element ES2 are stored beforehand in a list LTS1, in the buffer memory MT_D of the decoder DO of Figure 7. During the decoding of the current block Bu, only the first or the second embodiment of the syntax element ES2 is implemented. By way of alternative, the first and second embodiments are implemented. If at D5, one is dealing with the coded value VCESI1 of the first syntax element ES1 and if, at D6, the coded data DCu are associated with a current block Bu which does not belong to the predefined zone of the current image, the coded value VCES2i or VCES22 of the second syntax element ES2 is decoded at D8. If VCES2i=0, on completion of this decoding D8, a decoded value VES2i of the second syntax element ES2 is obtained. If VCES22=1, on completion of this decoding D8, a decoded value VES22 of the second syntax element ES2 is obtained. According to a first embodiment, if the syntax element ES2 is representative of the mode of prediction of the current block Bu and if, for example, ES2 has been coded: - according to a first predefined value VCES2i=0 indicating an inter prediction of the current block Bu, - or according to a second predefined value VCES22=1 indicating an intra prediction of the current block Bu, the decoding of VCES2i=0 makes it possible to obtain the value VES2i of the second syntax element ES2 or the decoding of VCES22=1 makes it possible to obtain the value VES22 of the second syntax element ES2. According to a second embodiment, if the syntax element ES2 is representative of the value of the residual pixels of the current block Bu and if, for example, ES2 has been coded: - according to a first predefined value VCES2i=0 indicating residual pixels of zero value of the current block Bu, - or according to a second predefined value VCES22=1 indicating residual pixels of non-zero value of the current block Bu, the decoding of VCES2i=0 makes it possible to obtain the value VES2i of the second syntax element ES2 or the decoding of VCES22=1 makes it possible to obtain the value VES22 of the second syntax element ES2. Such a decoding is for example an entropy decoding of CABAC ("Context Adaptive Binary Arithmetic Coder" in English) type or else an entropy decoding of arithmetical or Huffman type. This decoding is implemented by the decoding device MD_D represented in Figure 7. If at D5, it is the coded value VCESI2 of the first syntax element ES1 which has been decoded, the coded value VCES2i or VCES22 of the syntax element ES2 is systematically decoded at D8. With reference to Figure 6A, the data of the current block Bu are decoded at D9. Such a decoding is implemented by a block decoding device MDB_D represented in Figure 7, which device is driven by the processor PROC_D. The decoding D9 implements, at D91, a decoding of the data DCu associated with the current block Bu to be decoded and which have been coded at C8 in Figure 1A. On completion of such a decoding, is obtained a set of digital information associated with the block of quantized coefficients Bqu which was obtained at C52 in Figure 1A. The decoding D91 is implemented by the decoding device MD_D represented in Figure 7. The decoding D9 furthermore implements a dequantization D92 of the block of quantized coefficients Bqu, according to a conventional dequantization operation which is the operation inverse to the quantization C82 of Figure 1A. A current set of dequantized coefficients BDqu is then obtained. Such a dequantization is for example of scalar or vector type and is implemented by means of an inverse quantization device MQ"1_D, such as represented in Figure 7, which device is driven by the processor PROC_D. The decoding D9 furthermore implements the application D93 of a transform to the current set of dequantized coefficients BDqu which is obtained at D92. In a manner known per se, such a transform is a transform inverse to that applied to the coding at C81 in Figure 1A, such as for example a DCT, DST, DWT, LT or other transform. In a corresponding manner to the coder CO of Figure 2, these transforms form part of a list of transforms LTS2"1 which is stored beforehand in the buffer memory MT_D of the decoder DO of Figure 7. The type of transform to be applied can be determined at the decoder conventionally, by reading, in the data signal F, the index of the transform applied to the coding. According to the invention, the type of transform to be applied can be determined by the implementation of operation D5 of Figure 6A. The transform application D93 is performed by a transform calculation device MTR~1_D, such as represented in Figure 7, which device is driven by the processor PROC_D. The inverse quantization device MQ"1_D and the transform calculation device MTR"1_D are contained in a block decoding device MDB_D represented in Figure 7, which device is driven by the processor PROC_D. On completion of the decoding of the data of the current block, a current decoded block BDU is obtained. In the example described here, by data is meant the pixels of the decoded block BDU. It should however be noted that by data is also meant the pixels of a current residual block decoded in the case where a prediction of the current block Bu was implemented on coding and an inverse prediction is therefore implemented on decoding. With reference to Figure 6A, there is undertaken at D10 the writing of the current decoded block BDU into a decoded image IDj. The writing D10 is implemented by an image reconstruction device URI such as represented in Figure 7, the device URI being driven by the processor PROC_D. The decoding method which has just been described hereinabove is implemented for all the coded data of blocks DC-i, DC2, ..., DCu,..., DCs, associated respectively with the blocks B-i, B2, ..., Bu,..., Bs to be decoded of the current image ICj considered. A variant of the decoding method of Figure 6A will now be described with reference to Figure 6B. This variant differs from the decoding method of Figure 6A only through the fact that: - the decoding D8 of the value VCES2i or VCES22 of the second syntax element ES2 is not systematically implemented, - the value VCES2i or VCES22 of the second syntax element ES2 may be decoded even if the current block Bu belongs to the predefined zone of the current image IQ. To this effect, as represented in Figure 6B, in the case where the set of coded data DCu is associated with a current block Bu which belongs to the predefined zone of the current image IQ, a criterion dependent on the characteristics of the current block is examined at D50. If the criterion is fulfilled, the value VCES2i or VCES22 of the second syntax element ES2 is decoded at D8. If the criterion is not fulfilled, at D70, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTS1 of the decoder DO of Figure 7. In the case where the set of coded data DCu is associated with a current block Bu which does not belong to the predefined zone of the current image IQ, a criterion dependent on the characteristics of the current block is examined at D61. If the criterion is fulfilled, the value VCES2i or VCES22 of the second syntax element ES2 is decoded at D8. If the criterion is not fulfilled, at D71, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTS1 of the decoder DO of Figure 7. In the case where it is the coded value VCESI2 of the first syntax element ES1 which was decoded at D2, a criterion dependent on the characteristics of the current block is examined at D62. If the criterion is fulfilled, the value VCES2i or VCES22 of the second syntax element ES2 is decoded at D8. If the criterion is not fulfilled, at D72, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTS1 of the decoder DO of Figure 7. As a variant, just one or else two of the three operations D60, D61, D62 is implemented. According to an exemplary criterion, in the case where the second syntax element ES2 is representative of the size of the current block Bu to be decoded, the value VES2i or VES22 of the syntax element ES2 is compared with a predetermined size value VTP. In the case for example of the current images represented in Figures 4A to 4C, for which the coded value VCESI1 of the syntax element ES1 indicating that the current image ICj is for example a 2D image obtained by two-dimensional projection of a 180° or 360° video is decoded at D2: - if at D6 (fig.6B), the set of coded data DCu is associated with a current block Bu which belongs to a predefined zone Z1, and • if at D60, it is determined as criterion that the size of the current block is greater than a predetermined value VTP, for example 32x32, the coded value VCES2i or VCES22 of the second syntax element ES2 representative of the size of the current block Bu is decoded at D8, • otherwise, at D70, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTS1 of the decoder DO of Figure 7, - if at D6 (fig.6B), the current block Bu does not belong to the predefined zone Z1, stated otherwise belongs to the predefined zone Z2 of the current image IQ, and • if at D61, it is determined as criterion that the size of the current block is greater than a predetermined value VTP, for example 16x16, the value VCES2i or VCES22 of the second syntax element ES2 representative of the size of the current block Bu is decoded at D8, • otherwise, at D71, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTS1 of the decoder DO of Figure 7. In the case for example of a current image (not represented), for which the coded value VCESI2 of the syntax element ES1 indicating that the current image IQ is for example a conventional 2D image of fixed type or forming part of a sequence of images following one another temporally is decoded at D2: • if at D62 (fig.6B), it is determined as criterion that the size of the current block is greater than a predetermined value VTP, for example 8x8, the coded value VCES2i or VCES22 of the second syntax element ES2 representative of the size of the current block Bu is decoded at D8, • otherwise, at D72, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTS1 of the decoder DO of Figure 7. In the case of the current image IQ represented in Figure 4D, for which the coded value VCESI1 of the syntax element ES1 indicating that the current image IQ is for example a 2D image obtained by two-dimensional projection of a 360° video is decoded at D2: - if at D6 (fig.6B), the set of coded data DCu is associated with a current block Bu which belongs to the predefined zone Z1, and • if at D60, it is determined as criterion that the size of the current block is greater than a predetermined value VTP, for example 32x32, the coded value VCES2i or VCES22 of the second syntax element ES2 representative of the size of the current block Bu is decoded at D8, • otherwise, at D70, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTS1 of the decoder DO of Figure 7, - if at D6 (fig.6B), the current block Bu belongs to the predefined zone Z2 of the current image IQ, and • if at D61, it is determined as criterion that the size of the current block is greater than a predetermined value VTP, for example 16x16, the coded value VCES2i or VCES22 of the second syntax element ES2 representative of the size of the current block Bu is decoded at D8, • otherwise, at D71, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTS1 of the decoder DO of Figure 7, - if at D6 (fig.6B), the current block Bu belongs to the predefined zone Z3, and • if at D62, it is determined as criterion that the size of the current block is greater than a predetermined value VTP, for example 8x8, the value VCES2i or VCES22 of the second syntax element ES2 representative of the size of the current block Bu is decoded at D8, • otherwise, at D72, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTSi of the decoder DO of Figure 7. In the case for example of a current image represented in Figure 5B, for which the coded value VCESli of the syntax element ES1 indicating that at least two zones of the current image ICj are decoded according to a different level of decoding quality is decoded at D2: - if at D6 (fig.6B), the set of coded data DCu is associated with a current block Bu which belongs to a predefined zone Z1, and • if at D60, it is determined as criterion that the size of the current block is greater than a predetermined value VTP, for example 32x32, the coded value VCES2i or VCES22 of the second syntax element ES2 representative of the size of the current block Bu is decoded at D8, • otherwise, at D70, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTS1 of the decoder DO of Figure 7, - if at D6 (fig.6B), the current block Bu does not belong to the predefined zone Z1, stated otherwise belongs to the predefined zone Z2 of the current image IQ, and • if at D61, it is determined as criterion that the size of the current block is greater than a predetermined value VTP, for example 16x16, the value VCES2i or VCES22 of the second syntax element ES2 representative of the size of the current block Bu is decoded at D8, • otherwise, at D71, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTS1 of the decoder DO of Figure 7. In the case for example of the current image IQ represented at the bottom right in Figure 5A, for which the coded value VCESI2 of the syntax element ESi indicating that each zone of the current image IQ is decoded according to one and the same level of decoding quality is decoded at D2: • if at D62 (fig.6B), it is determined as criterion that the size of the current block is greater than a predetermined value VTP, for example 8x8, the coded value VCES2i or VCES22 of the second syntax element ES2 representative of the size of the current block Bu is decoded at D8, • otherwise, at D72, the syntax element ES2 is set, as a function of the current decoding context, directly to the predefined value VES2i or VES22 such as stored in the list LTS1 of the decoder DO of Figure 7. It goes without saying that the embodiments which have been described hereinabove have been given purely by way of wholly nonlimiting indication, and that numerous modifications may be easily made by the person skilled in the art without however departing from the scope of the invention.