Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
  • H04N19/102—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or selection affected or controlled by the adaptive coding
  • H04N19/103—Selection of coding mode or of prediction mode
  • H04N19/105—Selection of the reference unit for prediction within a chosen coding or prediction mode, e.g. adaptive choice of position and number of pixels used for prediction
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
  • H04N19/102—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or selection affected or controlled by the adaptive coding
  • H04N19/103—Selection of coding mode or of prediction mode
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
  • H04N19/102—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or selection affected or controlled by the adaptive coding
  • H04N19/12—Selection from among a plurality of transforms or standards, e.g. selection between discrete cosine transform [DCT] and sub-band transform or selection between H.263 and H.264
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
  • H04N19/134—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the element, parameter or criterion affecting or controlling the adaptive coding
  • H04N19/157—Assigned coding mode, i.e. the coding mode being predefined or preselected to be further used for selection of another element or parameter
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/10—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding
  • H04N19/169—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding
  • H04N19/17—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object
  • H04N19/176—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using adaptive coding characterised by the coding unit, i.e. the structural portion or semantic portion of the video signal being the object or the subject of the adaptive coding the unit being an image region, e.g. an object the region being a block, e.g. a macroblock
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
  • H04N19/00—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals
  • H04N19/60—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using transform coding
  • H04N19/61—Methods or arrangements for coding, decoding, compressing or decompressing digital video signals using transform coding in combination with predictive coding

Definitions

• the present invention relates generally to the field of image processing, and more specifically to the encoding and decoding of digital images and digital image sequences.
• the encoding / decoding of digital images applies in particular to images from at least one video sequence comprising:
• the present invention applies similarly to the coding / decoding of 2D or 3D type images.
• the invention may especially, but not exclusively, apply to video coding implemented in current AVC and HEVC video encoders and their extensions (MVC, 3D-AVC, MV-HEVC, 3D-HEVC, etc.), and to corresponding decoding.
• MVC Motion Picture Codon Coding
• 3D-AVC 3D-AVC
• MV-HEVC 3D-HEVC
• 3D-HEVC 3D-HEVC
• residue block also called residue of prediction, corresponding to the original block minus a prediction.
• the prediction can be omitted, the residual block then being equivalent to the original block.
• the residue blocks are transformed using a transform mathematical operation and then quantized using a mathematical quantization operation, for example of the scalar type.
• a mathematical quantization operation for example of the scalar type.
• the mathematical operation of transform will be called later 'transformed' and the mathematical operation of quantification will be called later 'quantification'.
• Coefficients are obtained at the end of the quantization step. They are then browsed in a reading order that depends on the coding mode that was chosen. In the HEVC standard, for example, the reading order is dependent on the prediction made and can be done in the "horizontal", “vertical” or “diagonal” order.
• a one-dimensional list of coefficients is obtained.
• the coefficients of this list are then encoded in bits by an entropy coding whose purpose is to code the coefficients without loss.
• bits obtained after entropy coding are written in a signal or data stream which is intended to be transmitted to the decoder.
• such a signal comprises:
• the decoding is done image by image, and for each image, block by block. For each block, the corresponding elements of the stream are read. Inverse quantization and inverse transform of block coefficients are performed to produce the decoded prediction residue. Then, the prediction of the block is calculated and the block is reconstructed by adding the prediction to the decoded prediction residue.
• DCT discrete Cosine Transform
• DST discrete sinus transforms
• such a transform can be of separable type or not separable.
• a first Al transform is applied to a residue block X of K pixels which are organized in the form of an MxN matrix, where Al is a matrix of data of size MxM and M, N are natural numbers greater than or equal to 1.
• a first transformed block Al.x is obtained at the end of the application of this first transform.
• a transposition operation t is then applied to the transformed block Al.x.
• a transposed block (Al.x) f is obtained.
• the transformed block X obtained according to this second case is similar to the transformed block X obtained according to the first case, to a close transposition.
• the corresponding inverse transform makes it possible to obtain the residue block x using the following calculation:
• AI "1 and Ac " 1 represent the respective inverse transforms of the Al and Ac transforms. They make it possible to obtain the values of the residue block x from the values of the transformed block X.
• the matrices AI "1 and Ac " 1 are commonly called inverse matrices of Al and Ac respectively, in the case where the matrices are chosen orthogonal. correspond to the matrices transposed of Al and Ac respectively.
• the inverse transform consists in multiplying the transformed block X by the inverse matrix A "1 of A which can be the transpose of A, when A is orthogonal. This inverse transform makes it possible to obtain the following residue block x:
• Non-separable mode depend transforms for Intra coding in HEVC Adrià Arrufat et al. VCIP 2014 "to increase the number of transforms by proposed modes of prediction.
• a prediction mode is selected, a transform is selected from among the 16 transforms stored for this prediction mode, according to a predetermined coding performance criterion, such as the debit-distortion criterion well known to those skilled in the art.
• a predetermined coding performance criterion such as the debit-distortion criterion well known to those skilled in the art.
• the amount of transforms to be added in order to implement such an adaptation has an impact on the memory resources to be used both by the coder who must store the transforms for each of the predictions considered, and by the decoder which must also know the transforms to apply the transform inverse of that applied to the coding.
• the amount of memory increases linearly with the number of transforms to be provisioned by prediction mode and becomes significant compared to the amount of storage required for current encoders, such as for example coders.
• HEVC for which the amount of memory dedicated to the storage of transforms is about 1 kb.
• an object of the present invention relates to a method of coding at least one image cut into blocks, implementing, for a current block to code the image:
• Such a coding method is remarkable in that when storing the set of transform operations associated with the selected prediction mode, the number of transform operations contained in this set is different from the number of transform operations contained in the set. in a set of transform operations that is stored in association with at least one other predetermined prediction mode of the plurality of predetermined prediction modes.
• the number of transform operations in each of said two sets contains in common at least one identical transform operation.
• Such an arrangement further reduces the memory resources of the encoder for storing the transform matrices.
• At least two sets of transform operations could contain in common:
• a single transform operation for example a DCT type transform
• the set of transform operations associated with the selected prediction mode is stored in association with at least one other prediction mode of the plurality of predetermined prediction modes.
• Such an arrangement makes it possible to further reduce the memory resources of the encoder for storing the transform matrices.
• the invention proposes different ways of determining the number of transform operations according to the prediction mode, so as to optimize the compromise "memory resources-coding performance”.
• the number of transform operations that is determined in the case of a prediction mode associated with a vertical or horizontal prediction direction is greater than the number of transform operations that is determined in the case a prediction mode associated with an oblique prediction direction.
• the number of transform operations which is determined in the case of a prediction mode for which the prediction is calculated by averaging more than two pixels of an edge of the current block, is greater or equal to the number of transform operations that is determined for any other prediction mode.
• the number of transform operations that is determined in the case of a prediction mode that has been previously selected, in the plurality of predetermined prediction modes, as the prediction mode the more likely is greater than the number of transform operations that is determined in the case of a prediction mode that has not been previously selected as the most likely prediction mode.
• the number of transform operations contained in the set of stored transform operations in association with the selected prediction mode is determined according to the amount of information representative of the prediction mode. selected.
• the invention also relates to a device for encoding at least one image divided into blocks, comprising a processing circuit which, for a current block to be coded with the image, is arranged for:
• the coding device is remarkable in that the processing circuit is arranged to store the set of transform operations associated with the selected prediction mode, the number of transform operations contained in this set being different from the number of transform operations contained in a set of transform operations that is stored in association with at least one other predetermined prediction mode of the plurality of predetermined prediction modes.
• Such a coding device is particularly suitable for implementing the aforementioned coding method.
• the invention also relates to a method for decoding a data signal representative of at least one image divided into blocks, implementing, for a current block to be decoded:
• a prediction mode of the current block to be decoded this prediction mode belonging to a plurality of predetermined prediction modes
• Such a decoding method is remarkable in that when storing the set of transform operations associated with the determined prediction mode, the number of transform operations contained in this set is different from the number of transform operations contained in the set. in a set of transform operations that is stored in association with at least one another predetermined prediction mode of the plurality of predetermined prediction modes.
• the number of transform operations in each of the two sets contains in common at least one identical transform operation.
• the set of transform operations associated with the determined prediction mode is stored in association with at least one other prediction mode of the plurality of predetermined prediction modes.
• the number of transform operations which is determined in the case of a prediction mode associated with a vertical or horizontal prediction direction is greater than the number of transform operations which is determined in the case of a prediction mode associated with an oblique prediction direction.
• the number of transform operations which is determined in the case of a prediction mode for which the prediction is calculated by averaging more than two pixels of an edge of the current block, is greater than or equal to the number of transform operations that is determined for any other prediction mode.
• the number of transform operations which is determined in the case where the predetermined prediction mode has been previously selected, in the plurality of predetermined prediction modes, as the most predictive mode of prediction. likely is greater than the number of transform operations that is determined in the case where the determined prediction mode has not been previously selected as the most probable prediction mode.
• the number of transform operations contained in the set of stored transform operations in association with the determined prediction mode is determined according to the amount of information representative of the prediction mode. determined.
• the invention also relates to a device for decoding a data signal representative of at least one image divided into blocks, comprising a processing circuit which, for a current block to be decoded, is arranged for:
• a mode of prediction of the current block to be decoded this prediction mode belonging to a plurality of predetermined prediction modes,
• a transform operation to the data representative of the residue block, such a transform operation belonging to a set of transform operations that is previously stored in association with the determined prediction mode, reconstructing the current block using a predictor block obtained at the end of the prediction and data obtained following the transformation operation.
• the decoding device is remarkable in that the processing circuit is arranged to store the set of transform operations associated with the determined prediction mode, the number of transform operations contained in this set being different from the number of transform operations contained in a set of transform operations that is stored in association with at least one other predetermined prediction mode of the plurality of predetermined prediction modes.
• Such a decoding device is particularly suitable for implementing the aforementioned decoding method.
• the invention also 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.
• This program can use any programming language, and be in the form of source code, object code, or intermediate code between source code and object code, such as in a partially compiled form, or in any other form desirable shape.
• the invention also relates to a computer-readable recording medium on which a computer program is recorded, this program comprising instructions adapted to the implementation of one of the coding or decoding methods according to the invention. as described above.
• the invention also relates to a computer-readable recording medium on which a computer program is recorded, this program comprising instructions adapted to the implementation of the coding or decoding method according to the invention, as described. above.
• the recording medium may be any entity or device capable of storing the program.
• the medium may include storage means, such as a ROM, for example a CD ROM or a microelectronic circuit ROM, or a magnetic recording means, for example a USB key or a hard disk.
• the recording medium may be a transmissible medium such as an electrical or optical signal, which may be conveyed via an electrical or optical cable, by radio or by other means.
• the program according to the invention can be downloaded in particular on an Internet type network.
• the recording medium may be an integrated circuit in which the program is incorporated, the circuit being adapted to execute or to be used in the execution of the aforementioned coding or decoding method.
• FIG. 1 represents the steps of the coding method according to the invention
• FIG. 2 represents an embodiment of a coding device according to the invention
• FIG. 3 represents the steps of a method for determining a variable number of transforms by prediction mode, according to one embodiment of the invention
• FIG. 4 represents a table illustrating the number of transforms determined by prediction mode, after successive addition of sixteen transforms according to the determination method of FIG. 3;
• FIG. 5 represents a comparison diagram between the compromise - memory resources / coding performance - as obtained following the implementation of the iterative process of FIG. 3, and that obtained with a number of identical transforms by prediction mode,
• FIG. 6A represents a first example of grouping of various Intra prediction modes according to the symmetry of their corresponding angles
• FIG. 6B represents a second example of grouping of various Intra prediction modes according to the symmetry of their corresponding angles
• FIG. 7 represents a graph which illustrates the relationship between thirty-three Intra HEVC prediction modes and their corresponding angular directions
• FIG. 8 represents an example of grouping of different modes of prediction Inter according to the symmetry of their corresponding angles
• FIG. 9 represents a table illustrating the number of transforms determined by group of prediction modes, after successive addition of sixteen transforms according to a variant of the determination method of FIG. 3,
• FIG. 10 represents an embodiment of a decoding device according to the invention.
• FIG. 1 1 represents the main steps of the decoding method according to the invention.
• the coding method according to the invention is for example implemented in a software or hardware way by modifications of an encoder initially conforming to any one of the current or future video coding standards.
• the coding method according to the invention is represented in the form of an algorithm comprising steps C0 to C9 as represented in FIG.
• the coding method according to the invention is implemented in a coding device CO represented in FIG.
• such an encoder device comprises:
• an input ENT_C to receive a current image to be encoded
• a processing circuit CT_C for implementing the coding method according to the invention, the processing circuit CT_C containing:
• an output SOR_C for delivering a coded stream containing the data obtained at the end of the coding of the current image.
• the code instructions of the computer program PG_C are for example loaded into a RAM memory, MR_C, before being executed by the processing circuit CT_C.
• the coding method shown in FIG. 1 applies to any current image IC j fixed or part of a sequence of L images IC-i,
• a current image IC j is partitioned into a plurality of blocks Bi, B 2 , B ,, ..., B s (1 ⁇ i ⁇ S), for example of size MxM pixels, where M is a natural integer greater than or equal to 1.
• Such a partitioning step is implemented by a partitioning software module MP_C shown in FIG. 2, which module is controlled by the processor PROC_C.
• block means coding unit (coding unit). This last terminology is notably used in the standard HEVC "ISO / IEC / 23008-2 Recommendation ITU-T H.265 High Efficiency Video Coding (HEVC)”.
• HEVC High Efficiency Video Coding
• such a coding unit groups together sets of pixels of rectangular or square shape, also called blocks, macroblocks, or sets of pixels having other geometrical shapes.
• Said blocks Bi, B 2 , B ,, ..., B s are intended to be coded according to a predetermined order of travel, which is for example of the lexicographic type. This means that the blocks are coded one after the other, from left to right. Other types of course are of course possible. Thus, it is possible to cut the image IC j into several subimages called slices and to independently apply a division of this type on each sub-image. It is also possible to code not a succession of lines, as explained above, but a succession of columns. It is also possible to browse the rows or columns in one direction or the other.
• Each block can also be divided into sub-blocks which are themselves subdividable.
• the coder CO selects as current block a first block to be coded B, of the image IC j , such as for example the first block B-.
• the current block B is predicted by known Intra and / or Inter prediction techniques.
• the block B is predicted with respect to at least one predictor block according to a prediction mode MP S selected from among a plurality of predetermined prediction modes MP 0 , MP-1, ..., MP V ,. .., MP R where 0 ⁇ v ⁇ R + 1 and 0 ⁇ s ⁇ R + 1.
• block B is predicted with respect to a plurality of candidate predictor blocks.
• Each of the candidate predictor blocks is a block of pixels that has already been encoded or encoded and decoded.
• Such predictor blocks are stored beforehand in the buffer MT_C of the coder CO as represented in FIG. 2.
• an optimal predictor block BP op t is obtained following a putting into competition of said predetermined prediction modes, for example by minimizing a distortion rate criterion that is well known to the human being. job.
• the BP op t block is considered as an approximation of the current block B ,.
• the information relating to this prediction is intended to be written in a signal or data stream to be transmitted to a decoder. Such information includes in particular the type of prediction (Inter or Intra), and if appropriate, the selected prediction mode MP S , the partitioning type of the current block if the latter has been subdivided, the reference image index and the displacement vector used in the case where an Inter prediction mode has been selected. This information is compressed by the CO encoder.
• the data relating to the current block B are compared with the data of the predictor block BP op t. More precisely, during this step, the difference between the predictor block obtained BP op t and the current block B i is conventionally calculated.
• a set of data, called residual block ⁇ , is then obtained at the end of step C4.
• the steps C3 and C4 are implemented by a predictive coding software module PRED_C shown in FIG. 2, which module is controlled by the processor PROC_C.
• a transform of the residue block Br 1 can be for example:
• a direct transform such as, for example, a discrete cosine transform of the DCT type
• a direct transform such as, for example, a discrete sinus transform of the DST type
• Said transform belongs to a set of transforms which, during a prior storage step C0 shown in FIG. 1, is stored in association with the selected prediction mode MP S , in the buffer memory MT_C of FIG. during this storage step: the predetermined prediction mode MP 0 is stored in association with a set of transforms containing a number NB 0 of transforms,
• the predetermined prediction mode MP is stored in association with a set of transforms containing a number NB of transforms
• the predetermined prediction mode MP V is stored in association with a set of transforms containing a number NB V of transforms,
• the predetermined prediction mode MP R is stored in association with a set of transforms containing a number NB R of transforms.
• the number of transforms, noted NB a which is contained in the set of transforms associated with the prediction mode MP a is different from the number of transforms, noted NB b , which is contained in a set of stored transforms in association with the prediction mode MP b .
• the number of transforms in each of said two sets contains in common at least one identical transform operation.
• the number of transforms in each of said two sets may for example contain in common:
• more than two sets of transforms may contain in common at least one identical transform operation.
• the sets of transforms respectively associated with each of these thirty-five prediction directions may contain in common at least one identical transform operation.
• the set of transforms associated with said selected prediction mode MP S and containing a number NB S of transforms is stored in association with at least one other prediction mode, denoted MP U , of said plurality of predetermined prediction modes MP 0 , MPi, ..., MP V , ..., MP R , with 0 ⁇ u ⁇ R + 1.
• the aforementioned determination step C5 consists of:
• the complexity is defined for example by counting the number of mathematical operations (addition, multiplication, binary shift) involved for calculating the transform of the coefficients of the residual block.
• the residue block Br is transformed using the transform T s> k or T s> k *.
• Such an operation is performed by a transform software module MTR_C, as represented in FIG. 2, which module is controlled by the processor PROC_C.
• a transformed Bt block is obtained.
• Step C7 the data of the transformed block Bt are quantized according to a conventional quantization operation, such as, for example, a scalar or vector quantization.
• a block Bq of quantized coefficients is then obtained.
• Step C7 is performed by means of a quantization software module MQ_C as represented in FIG. 2, which module is controlled by the processor PROC_C.
• step C7 is represented following the transforming step C6.
• step C7 can be integrated in step C6 which is then implemented with integers including the quantization factor.
• step C8 the data of the block Bq 1 is encoded.
• Such coding is, for example, entropic coding of CABAC type ("Context Adaptive Binary Arithmetic Coder" in English) or else an entropy coding of arithmetic type or Huffman type.
• CABAC Context Adaptive Binary Arithmetic Coder
• entropy coding of arithmetic type or Huffman type.
• Step C8 is implemented by an encoding software module MC_C shown in FIG. 2, which module is controlled by the processor PROC_C.
• the signal or data flow F which contains:
• the IDX index can be written in the stream, for example in the form of a binary code.
• Such an arrangement is for example implemented if the transform T s> k or T Sik * is a transform which is common to the R + 1 sets of transforms respectively associated with the R + 1 predetermined prediction modes MP 0 , MP-i, ..., MP V , ..., MP R.
• the IDX index will then be set to 1 or 0, for example 1. If, on the other hand, the transform T s> k or T s> k * is a transform which is not common to the R + 1 sets of transforms respectively associated with the R + 1 predetermined prediction modes MP 0 , MP-i, ..
• the IDX index will contain a first bit of value 0, followed by an additional codeword representative of the transform T s> k or T s> k * which has been selected in the number NB S of transforms contained in the set of transforms associated with the prediction mode MP S selected.
• the first bit of value 0 is for example coded using a CABAC encoder.
• the additional code word may be coded on a fixed length if the number of transforms NB S is a power of 2.
• the additional codeword may also be coded on a variable length code if the number of transforms NB S is a power of 2 or not.
• the step C9 is implemented by a software module MCF data signal construction, as shown in Figure 2, which module is controlled by the PROC_C processor.
• the data signal F is then delivered via the output SOR_C of the coder CO of FIG. 2, then transmitted by a communication network (not shown) to a remote terminal.
• the data signal F furthermore comprises certain information encoded by the coder CO, such as the type of prediction (Inter or Intra) applied to the step C3, and, if appropriate, the prediction mode selected.
• a decoded residue block BDn is then obtained.
• the decoded block BDi is then constructed by adding to the optimal predictor block BP opt the decoded residue block BDn.
• the decoded block BD is the same as the decoded block obtained at the end of the decoding process of the image IC j which will be described later in the description.
• the decoded block BD is thus made available for use by the coder CO of FIG. 2.
• n max of transforms that can be associated with a given intra prediction mode is determined.
• n max 16
• this number may or may not include at least one HEVC transform common to each of the 35 intra ipm prediction modes of the HEVC standard.
• step ST2 the number of transforms for each of the thirty-five intrapm prediction modes of the HEVC standard is initialized to zero.
• Each prediction mode has at this step only one common transform of the type of that of HEVC (DCT or DST) and zero additional transform.
• DCT or DST DCT
• Such a step is summarized in the table TB1 of FIG. 4, whose first column lists the 35 prediction modes intra ipm 0 to ipm 34 and the first line lists the first fifteen iterations, during each of which is added a transformed. Following the first column of Table TB1 are also listed, for each of the first fifteen iterations:
• BDRate the coding performance obtained, that is to say the percentage of flow reduction obtained relative to a HEVC encoder.
• a first iteration is carried out 1, during which a first transform is added.
• step ST4 shown in FIG. 3 for each of the thirty-five intra prediction modes, calculation of the coding performance and the storage capacity to be used in relation to the added transform are performed.
• the obtained coding performances that is to say the gain in bit rate, are denoted Ri t , x and the corresponding storage capacity for storing the added transform is denoted Mj tjX .
• step ST6 shown in FIG 3 it is proceeded to select the intra prediction mode IPM x for which the A_X report presents the most favorable value, that is to say the rate decrease the more important for added memory resources as low as possible.
• the intra prediction mode IPM x for which the A_X report presents the most favorable value, that is to say the rate decrease the more important for added memory resources as low as possible.
• the ratio a_0 which is the most favorable and which corresponds to the prediction mode intra 0.
• the index of this mode is indicated at the bottom of the third column of the table in association with:
• the CO encoder of FIG. 2 is updated with the transform added during this first iteration.
• the table TB1 shows how, over the first fifteen iterations carried out, the bit rate gain (BDRate) is improved as a function of the iterations, as the memory resources (ROM) increase.
• the storage capacity of prior art encoders that use a fixed number of transforms per prediction mode is 140 kb, whereas the storage capacity of the encoders according to the using a variable number of transforms per prediction mode is 60.4 kb.
• the reduction of the storage capacities of the encoders according to the invention with respect to the coders of the state of the art can thus be estimated at 57% for the gain in target bit rate of 2.8%.
• the coding method according to the invention advantageously makes it possible to obtain high coding performances, with a limited impact on the storage capacity of the transforms compared to that used in the coding methods of the prior art.
• an embodiment of the invention implemented in the case of a HEVC encoding of Intra type, and in which the set of transforms associated with an intra prediction mode selected in step C3 of FIG. 1 is previously stored in association with at least one other prediction mode of said plurality of predetermined prediction modes.
• the optimal predictor block BP op t obtained is associated with an optimal Intra prediction direction which is, for example, the direction DPI 2 2-
• the transform (if unique) or the set of transforms associated with the Intra DPI22 prediction direction is previously associated, during the step CO of FIG. 1, with another direction.
• Intra prediction provided that this other Intra prediction direction is symmetrical with respect to the Intra DPI 2 2 prediction direction. In this way, two-fold fewer transforms are stored at the CO encoder, and correspondingly, at the level of the decoder which will be described later in the following description, the advantage being a reduction of the memory encoder side and decoder.
• two Intra prediction directions with symmetries are associated with the same transform or set of transforms.
• two intra-angular prediction directions each having identical angle deviations with respect to a vertical (or horizontal) direction are associated with the same transform or set of transforms.
• an Intra-angular prediction direction having an angle of 30 ° with respect to a horizontal axis AH is associated with an Intra-Angular prediction direction presenting symmetrically an angle of -30 ° with respect to this axis. horizontal.
• these two directions of prediction Intra correspond respectively to directions DPI 4 and DP 6 .
• an Intra Angular prediction direction having an angle of -60 ° with respect to a horizontal axis is associated with an Intra Angular prediction direction having symmetrically an angle of -120 ° with respect to a vertical axis AV.
• these two prediction directions Intra correspond respectively to the directions DPI 2 o and DPI 32 .
• the transform or the plurality of transforms associated with the Intra DPI22 prediction direction selected in step C3 is previously associated with three other Intra prediction directions, provided that these other Intra prediction directions are symmetrical with respect to the Intra DPI 2 2 prediction direction.
• the advantage being a reduction of the memory encoder and decoder side.
• Intra prediction directions with symmetries are associated with the same transform or set of transforms.
• intra-angular prediction directions each having identical angles with respect to a vertical, horizontal and diagonal axis are associated with the same transform or the same set of transforms.
• an Intra Angular prediction direction having an angle of 30 ° with respect to the horizontal axis AH is associated with:
• these four Intra prediction directions correspond respectively to the directions DPI 4 , DPI 6 , DPI 2 o and DPI 32 .
• the table below represents nine groups G 1 to G 9 of Intra HEVC prediction directions to which may be associated, in the buffer MT_C of the coder CO of FIG. 2, the same transform (if unique) or a set of transforms, as determined according to the invention.
• Group Index directions may be associated, in the buffer MT_C of the coder CO of FIG. 2, the same transform (if unique) or a set of transforms, as determined according to the invention.
• 8 and DPI 34 , as illustrated in Figure 7, are grouped by three and are associated with the same transform or the same set of transforms.
• step C40 of at least one displacement.
• data of the residual block Br as obtained at the end of step C4 of FIG.
• each piece of data considered is moved inside the residue block while keeping its nearest neighbors.
• the step C40 is implemented by a calculation software module CAL_C as represented in FIG. 2, which module is controlled by the processor PROC_C.
• a type of data displacement is a transposition, namely an exchange of the row and column coordinates of a data item of the current residue block.
• a type of data displacement is a mirror, namely an exchange of columns or rows of the current residue block. Each piece of data considered in the current residue block Br, is thus moved inside the residue block while keeping its nearest neighbors.
• a type of data displacement is a combination of the transposition and the mirror, that is to say: either the application of a displacement of transposition type followed by the application of a mirror-like displacement to the data of the current residue block,
• the type of data displacement in the residual block ⁇ is a function of the prediction mode MP S selected.
• an Intra prediction direction is associated with one of the eight types of displacement, according to its grouping mode. For example :
• the intra-DPI 2 2 and DPI 6 prediction directions associated with the same transform or the same set of transforms are themselves respectively associated with the type 0 and type 5 mirror rotations as represented in FIG. 8,
• the intra-DPI 4 and DPI 6 prediction directions associated with the same transform or the same set of transforms are themselves respectively associated with the type 0 and type 2 mirror rotations as represented in FIG. 8,
• the intra-DPI 2 o and DPI 32 prediction directions associated with the same transform or the same set of transforms are themselves respectively associated with the type 0 and type 1 mirror rotations as represented in FIG. 8,
• the intra-DPI 4 , DPI 6 , DPI 2 o and DPI 32 prediction directions associated with the same transform or the same set of transforms are they are respectively associated with mirror rotations of type 6, type 4, type 0 and type 1, as shown in FIG.
• bit rate gain (BDRate) is improved as a function of the iterations, as the memory resources (ROM) increase, when at least two prediction modes are associated with the same set of transforms.
• Table TB2 shows a significant reduction in memory resources dedicated to storing transforms of the order of 20% compared to the coding method that does not use such a grouping of prediction modes.
• Table TB2 shows a significant reduction in memory resources dedicated to storing transforms of the order of 20% compared to the coding method that does not use such a grouping of prediction modes.
• the gain being evaluated in terms of storage capacity for targeted coding performances is presented the table below which gives a comparative of the capacities fixed storage units present in a coder of the prior art using the same number of transforms by prediction mode and variable storage capacities present in the encoder according to the invention, and the reduction of storage capacity obtained with the capacity of the variable storage.
• the storage capacity of the encoders that use a fixed number of transforms per prediction mode is 17.5 kb
• the storage capacity of the encoders according to the invention that use a variable number of transforms per prediction mode or set of prediction modes is only 5.3 kb.
• the reduction of the storage capacities of the encoders according to the invention with respect to the coders of the state of the art can thus be estimated at 70% for the target rate gain of 1.7%.
• the iterative determination method of the number of transforms by prediction mode or by group of prediction modes can be replaced by a method of automatically determining said number of transforms.
• the automatic determination method may be necessary when, in certain coding contexts, the CO encoder of FIG. 2 is forced to operate at a given point of complexity. It is then necessary, during the coding, to reduce the number of transforms in competition to limit the choice investigations by the coder of the optimal transform, during step C5 of FIG.
• the number of transforms which is determined in the case of a prediction mode associated with a vertical or horizontal prediction direction is made greater than the number of transforms which is determined in the case of a prediction associated with an oblique prediction direction.
• the person skilled in the art knows that among the thirty-five intra prediction modes available, the modes 0 (Planar) and 1 (DC) are the most smoothed modes because the prediction according to these two modes is calculated by averaging more than two pixels from an edge of the current block. As a result, the Planar and DC modes are associated, during the step C0 of FIG. 1, with a greater number of transforms than the other intra-HEVC prediction modes.
• the modes 10 and 26 are used to predict patterns of the image which are horizontal or vertical. Since such patterns are frequently found in nature (eg vertical trees, poles, horizon lines, etc.), they must be associated with a higher number of transforms than the number of transforms associated with the intra-prediction modes. obliques.
• the number of transforms which is determined in the case of a prediction mode which has been previously selected, in the plurality of predetermined prediction modes MP 0 , MP-i,. . . , MPv, ..., M PR, as the most probable prediction mode is greater than the number of transforms that is determined in the case of a prediction mode that has not been previously selected as a mode prediction most likely.
• MPMs of the "Most Probable Modes"
• the modes 0 (Planar), 1 (DC) and 26 (vertical) are the prediction modes assigned by default when establishing the list.
• these three modes of intra prediction are therefore previously associated, during the storage step C0 of FIG. 1, with a greater number of transforms than the number of transforms associated with the other intra-HEVC prediction modes.
• the number of transforms contained in the set of transforms stored in association with each of the predetermined prediction modes MP 0 , MP-i,. . . , MP V , ..., MP R is determined according to the amount of information representative of each of these prediction modes.
• the most probable intra prediction modes such as 0 (Planar), 1 (DC) and 26 (vertical) will be reported on less bits than the other modes. According to the invention, it is therefore appropriate to associate them with a higher number of transforms than the other intra prediction modes, because these three modes will be chosen more frequently than the others and consequently, there will be a greater variety of predictor blocks to test with these three modes.
• the number of transforms, which is stored in association with this prediction mode during step C0 of FIG. is larger than the average of the transforms over all thirty-five intra prediction modes.
• the decoding method according to the invention is for example implemented in a software or hardware way by modifications of such a decoder.
• the decoding method according to the invention is represented in the form of an algorithm comprising steps D0 to D9 as represented in FIG. According to this embodiment, the decoding method according to the invention is implemented in a decoding device or decoder DO represented in FIG.
• such a decoder device comprises:
• CT_D processing circuit for implementing the decoding method according to the invention, the CT_D processing circuit containing:
• an output SOR_D for delivering a reconstructed current image containing the data obtained at the end of the decoding according to the method of the invention.
• the code instructions of the computer program PG_D are for example loaded into a RAM memory, MR_D, before being executed by the processing circuit CT_D.
• the decoding method represented in FIG. 11 applies to a signal or data flow F representative of a current image ICj to be decoded which is fixed or which belongs to a sequence of images to be decoded.
• information representative of the current image ICj to be decoded is identified in the data signal F received at the input ENT_D of the decoder DO and as delivered at the end of the coding method of FIG.
• step D1 it is carried out in the determination signal F residues encoded blocks associated with each of the blocks Bi, B 2, Bi, ..., B s previously coded according to the above-mentioned lexicographic order.
• Such a determination step D1 is implemented by a flow analysis software identification module MI_D, as shown in FIG. 10, which module is controlled by the PROC_D processor.
• the blocks Bi, B 2 , B ,,..., B s to be decoded have, for example, a square shape and are for example of size MxM pixels where M is a natural integer greater than or equal to 1.
• Each block to be decoded can also be divided into sub-blocks which are themselves subdividable.
• the decoder DO of FIG. 10 selects, as current block B, to be decoded, the first block which has been coded at the end of the coding method of FIG. .
• step C40 for moving data of the residual block Br has been implemented at the coding .
• step D3 information relating to the prediction type of the current block B, as implemented during coding in step C3 of FIG. 1, which has been written in the signal, is determined.
• step D3 are determined:
• the index of the predictor block BP op t denoted IBP op t
• the type of partitioning of the current block B if the latter has been partitioned.
• Such a decoding step D3 is implemented by a decoding module MD_D shown in FIG. 10, which module is controlled by the processor PROC_D.
• the predictive decoding of the current block to be decoded is carried out using the IBP index opt of the block predictor that was decoded during the aforementioned step D3.
• Each of the candidate predictor blocks is a block of pixels that has already been decoded.
• Step D4 is implemented by an inverse prediction software module PRED "1 _D, as shown in FIG. 10, which is controlled by the PROC_D processor.
• step D5 which is implemented in the case where it is the set of data associated with the block Bq, or Bmq, coded which was obtained at the end of the
• dequantization of this set of data is carried out according to a conventional dequantization operation which is the inverse operation of the quantization implemented during the quantization step C7 of FIG.
• a set of current dequantized coefficients BDq, or a set of dequantized modified current coefficients BDmq, is then obtained at the end of step D5.
• Such a dequantization step is for example of scalar or vector type.
• Step D5 is performed by means of an inverse quantization software module MQ "1 _D, as shown in FIG. 10, which module is controlled by the PROC_D processor.
• the decoder DO of FIG. 10 carries out the determination of a transform of the current dequantized coefficient set BDq, or the set of dequantized modified current coefficients BD m , as obtained in step D5 above.
• a transform is an inverse transform from that determined at the end of step C5 of FIG. 1, such as, for example:
• a direct transform such as, for example, a discrete cosine transform of the DCT type
• a direct transform such as, for example, a discrete sinus transform of the DST type, -an optimized block transform distortion rate as presented in the publication "Rate-distortion optimized! transform competition for intra coding in HEVC ", Adrià Arrufat, Philippe Pierrick, Olivier Deforges, IEEE VCIP, Dec 2014,
• Said transform belongs to a set of transforms which, during a prior storage step D0 represented in FIG. 11, is stored in association with the prediction mode MP S , in the buffer memory MT_D of FIG. during this storage step:
• the predetermined prediction mode MP 0 is stored in association with a set of transforms containing a number NB 0 of transforms,
• the predetermined prediction mode MPi is stored in association with a set of transforms containing a number NB of transforms
• the predetermined prediction mode MP V is stored in association with a set of transforms containing a number NB V of transforms;
• the predetermined prediction mode MP R is stored in association with a set of transforms containing a number NB R of transforms.
• the number of transforms, noted NB a which is contained in the set of transforms associated with the prediction mode MP a is different from the number of transforms transformed, noted NB b , which is contained in a set of transforms stored in association with the prediction mode MP b .
• the number of transforms in each of said two sets contains in common at least one identical transform.
• the number of transforms in each of said two sets may for example contain in common:
• more than two sets of transforms may contain in common at least one identical transform operation.
• the sets of transforms respectively associated with each of these thirty-five prediction directions may contain in common at least one identical transform operation.
• the set of transforms, associated with said prediction mode MP S determined in the aforementioned step D3 and containing a number NB S of transforms, is stored in the buffer memory MT_D of the FIG. 10, in association with at least one other prediction mode, denoted MP U , of said plurality of predetermined prediction modes MP 0 , MP 1 ..., MP V , ..., MP R , with 0 ⁇ u ⁇ R + 1.
• step D7 the inverse transform of the transform T s> k or T s> k * is applied to the set of dequantized coefficients current BDq, or to the set of current dequantized coefficients BD mq ,, as obtained in step D5 above.
• a current decoded residue block BDr is obtained at the end of step D7.
• Such an operation is performed by an inverse transform software module MTR "1 _D, as shown in FIG. 10, which module is controlled by the processor PROC_D.
• step D5 is represented before the inverse transformation application step D7.
• step D5 can be integrated in step D7 which is then implemented with integers including the dequantization factor.
• the current block B is reconstructed, by adding to the decoded residue block BDn, obtained at the end of the aforementioned step D7, the predictor block BP op. t that was obtained at the end of the aforementioned step D4.
• a current decoded block BD is obtained.
• Step D8 is implemented by a software module CAL1_D shown in FIG. 10, which module is controlled by the processor PROC_D.
• step D9 said current decoded block BD is written in a decoded image ID j .
• Such a step is implemented by an image reconstruction URI software module as shown in FIG. 10, said module being controlled by the PROC_D processor.
• the decoding steps which have just been described above are implemented for all the blocks Bi, B 2 , B ,,..., B s to be decoded from the current image IC j considered, in a predetermined order which is for example the lexicographic order.
• the decoding method implements, according to a first embodiment of the invention, a method for determining the number of transforms for each predetermined prediction mode. , in the case of an Intra type HEVC decoding.
• FIGS. 3, 6A, 6B, 7 to 11 an embodiment of the invention implemented in the case of Intra-type HEVC decoding, and in which the set of FIG. Transforms associated with an intra prediction mode determined in step D3 of FIG. 11 are previously stored in association with at least one other prediction mode of said plurality of predetermined prediction modes.
• the optimal predictor block BP op t obtained is associated with an optimal Intra prediction direction which is, for example, the direction DPI 22.
• the transform (if unique) or the set of transforms associated with the direction of prediction Intra DPI 22 is previously associated, during the step DO of Figure 1 1, to another direction of prediction Intra , provided that this other Intra prediction direction is symmetrical with respect to the Intra DPI prediction direction 22 - In this way, two times fewer transforms are stored at the decoder DO, the advantage being a reduction of the memory decoder side.
• two Intra prediction directions with symmetries are associated with the same transform or set of transforms.
• two intra-angular prediction directions each having identical angles with respect to a vertical (or horizontal) direction are associated with the same transform or set of transforms.
• an Intra-angular prediction direction having an angle of 30 ° with respect to a horizontal axis AH is associated with an Intra-Angular prediction direction presenting symmetrically an angle of -30 ° with respect to this axis. horizontal.
• these two prediction directions Intra correspond respectively to directions DPI 4 and DP 6 .
• an Intra Angular prediction direction having an angle of -60 ° with respect to a horizontal axis is associated with an Intra Angular prediction direction having symmetrically an angle of -120 ° with respect to a vertical axis AV.
• these two prediction directions Intra correspond respectively to the directions DPI 2 o and DPI 32 .
• the transform or the plurality of transforms associated with the direction of Intra DPI prediction 2 2 determined in step D3 is previously associated with three other prediction directions
• Intra provided that these other prediction directions Intra are symmetrical with respect to the Intra DPI 2 2 prediction direction. In this way, four times fewer transforms are stored at the decoder DO, the advantage being a reduction of the decoder side memory.
• Intra prediction directions with symmetries are associated with the same transform or set of transforms.
• intra-angular prediction directions each having identical angles with respect to a vertical, horizontal and diagonal axis are associated with the same transform or the same set of transforms.
• an Intra Angular prediction direction having an angle of 30 ° with respect to the horizontal axis AH is associated with:
• these four prediction directions Intra correspond respectively to the directions DPI 4 , DPI-
• the table below represents nine groups G 1 to G 9 of Intra HEVC prediction directions to which may be associated a single transform (if unique) or a set of transforms, as determined according to the invention.
• 45 ° multiple angular directions (45 °, -45 ° and -135 °) as shown in FIG. 6B, corresponding respectively to the prediction directions DPI 2 , DPI 8 and DPI 34 , as illustrated in FIG. 7, are grouped by three and are associated, in the buffer MT_D of the decoder DO of Figure 10, the same transform or the same set of transforms.
• the step D70 is implemented by a calculation software module CAL2_D as represented in FIG. 10, which module is controlled by the processor PROC_D.
• a type of data displacement is a transposition, the reverse of that performed at the coding, namely an exchange of the row and column coordinates of a data block residue modified decoded BDmn current.
• a type of data displacement is a mirror, the reverse of that carried out at the coding, namely an exchange of the columns or lines of the current decoded modified residual block BDmn.
• Each data considered in the current decoded modified residue block BDmn is thus moved inside the latter, while keeping its nearest neighbors.
• a type of data displacement is a combination of the inverse transposition and the inverse mirror, that is to say:
• the type of data displacement in the current decoded modified residual block BDmn is a function of the prediction mode MP S determined in the aforementioned step D3.
• an Intra prediction direction determined at the end of the step D3 of FIG. 11 is associated with one of the eight types of inverse displacement, according to the group to which it belongs.
• the directions of intra prediction DPI22 and DPI 6 associated with the same transform or the same set of transforms are themselves respectively associated with the inverse mirror rotations respectively of the type 0 and type 5 mirror rotations as represented in FIG. 8,
• the intra-DPI 4 and DP 6 prediction directions associated with the same transform or the same set of transforms are themselves respectively associated with the inverse mirror rotations respectively of the type 0 and type 2 mirror rotations as represented in FIG. 8 ,
• the iterative determination method of the number of transforms by prediction mode or by group of prediction modes can be replaced by a method of automatic determination of said number of transforms.
• the automatic determination method may be necessary when, in certain decoding contexts, the decoder DO of FIG. 10 is constrained to operate at a given point of complexity.
• This given point of complexity can be indicated by the encoder CO of FIG. 2, by a given configuration information for a portion of an image, an entire image or for a sequence of images. It can comprise a maximum number of transforms identifier or a ratio of the number of transforms relative to the number of transformations available to the decoder.
• the decoder DO searches in the buffer memory MT_D of FIG. 10 only in the number of transforms associated with FIG. said configuration information signaled, the transform selected at the coding and corresponding to the IDX index determined in step D3.
• the number of transforms which is determined in the case of a prediction mode associated with a vertical or horizontal prediction direction is made greater than the number of transforms which is determined in the case of a prediction associated with an oblique prediction direction.
• the person skilled in the art knows that among the thirty-five intra prediction modes available, the modes 0 (Planar) and 1 (DC) are the most smoothed modes because the prediction according to these two modes is calculated by averaging more pixels than the edges of the current block. Consequently, the Planar and DC modes are associated, during the step D0 of FIG. 11, with a greater number of transforms than the other intra-HEVC prediction modes.
• modes 10 and 26 are used to predict image patterns that are horizontal or vertical. Since such patterns are frequently found in nature (eg vertical trees, poles, horizon lines, etc.), they must be associated with a higher number of transforms than the number of transforms associated with the intra-prediction modes. obliques.
• the number of transforms that is determined in the case of a prediction mode that has been previously selected, in the plurality of predetermined prediction modes MP 0 , MP-i, ..., MPv, ..., M PR, as the most probable prediction mode is greater than the number of transforms that is determined in the case of a prediction mode that has not been previously selected as a prediction mode. more likely.
• MPMs of the "Most Probable Modes"
• the modes 0 (Planar), 1 (DC) and 26 (vertical) are the prediction modes assigned by default when establishing the list.
• these three intra prediction modes are therefore previously associated, during the step of storage OD of Figure 1A, to a number of transforms higher than the number of transforms associated with the other modes of intra-HEVC prediction.
• the number of transforms contained in the set of transforms stored in association with each of the predetermined prediction modes MP 0 , MP-i,. . . , MP V , ..., MP R is determined according to the amount of information representative of each of these prediction modes.
• the most probable intra prediction modes such as 0 (Planar), 1 (DC) and 26 (vertical) will be reported on less bits than the other modes. According to the invention, it is therefore appropriate to associate them with a higher number of transforms than the other intra prediction modes, because these three modes will be chosen more frequently than the others and consequently, there will be a greater variety of predictor blocks to test with these three modes.
• the number of transforms, which is stored in association with this prediction mode during the step D0 of FIG. 1, is larger than the average of the transforms on all thirty-five intra prediction modes.

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