BR112018016409B1 - DECODER, METHOD TO DECODE, ENCODER, AND METHOD TO ENCODE - Google Patents

Abstract

The invention proposes a decoder for decoding the block coefficients of a video sequence from a bit stream. The decoder comprises a scan pattern list module for providing one or more predefined scan orders, a scan order generator for generating one or more scan orders, a scan order selector for selecting an order scan order for each block from the pre-defined and generated scan orders based on scan order information contained in the bit stream, a decoding module for decoding one or more coefficient vectors of each block from the bit stream. bits, a deserializer for reverse scanning, for each block, the one or more coefficient vectors of that block in accordance with the scan order selected for that block so as to obtain a matrix of coefficients. The scan order generator generates one or more scan orders according to one or more block coefficient matrices of the previously obtained video sequence.

Description

FIELD OF TECHNIQUE

[0001] The present invention generally relates to the field of video processing and to a method and apparatus for scanning order selection, specifically content-adaptive scanning order selection, and specifically relates to a decoder for decoding coefficients of blocks of a video sequence and to an encoder for encoding block coefficients of a video sequence.

BACKGROUND

[0002] Digital video storage and communication applications are implemented by a wide range of digital devices, such as digital cameras, cellular radio telephones, laptops, broadcast systems, and video teleconferencing systems. One of the most important and challenging tasks of these applications is video compression. The task of video compression is complex and is constrained by two contradictory parameters: compression efficiency and computational complexity. Video coding standards, such as ITU-T H.264/AVC or ITU-T H.265/HEVC High Efficiency Video Coding (HEVC) standard, provide a good compromise between these parameters. For this reason, support for video coding standards is a mandatory requirement for almost all video compression applications.

[0003] State-of-the-art video coding standards are based on partitioning a source image into blocks. The processing of these blocks depends on their size, spatial position and an encoding mode specified by an encoder. Coding modes can be classified into two groups according to the type of prediction: intra and interprediction modes. Intraprediction modes use pixels from the same image to generate reference samples to calculate prediction values for the pixels in the block being reconstructed. Intraprediction can also be referred to as spatial prediction. Interprediction modes are designed for temporal prediction and use reference samples from previous or next images to predict pixels from the current image block. After the prediction stage, a transform coding is performed for a prediction error which is the difference between an original signal and its prediction. Then, the transform coefficients and side information are encoded using an entropy encoder such as Context Adaptive Binary Arithmetic Coding (CABAC) for AVC/H.264 and HEVC/H.265.

[0004] Evidently, prediction errors have different statistics for a different prediction mode and strongly depend on the content to be predicted. Therefore, different transforms are beneficial from the point of view of compression performance for different cases. To increase the coding efficiency of the transform coding stage, multiple transforms can be used that have different energy compression for different specific cases.

[0005] The ITU-T H.265/HEVC standard known, for example, from ISO/IEC 23008-2:2013, "Information technology - High efficiency coding and media delivery in heterogeneous environments - Part 2: High efficiency video coding" , November 2013, declares a set of prior art video coding tools that provide a reasonable compromise between coding efficiency and computational complexity. An overview of the ITU-T H.265/HEVC standard is given in the article by Gary J. Sullivan, "Overview of the High Efficiency Video Coding (HEVC) Standard", in IEEE Transactions on Circuits and Systems for Video Technology, Vol 22, No. 12, December 2012, the entire contents of which are incorporated herein by reference.

[0006] Similar to the ITU-T H.264/AVC video coding standard, the HEVC/H.265 video coding standard provides a division of the source image into blocks, for example, coding units (CUs). Each of the CUs can be further divided into or smaller CUs or prediction units (PUs). A PU can be intra or interpredicted according to the type of processing applied to the PU pixels. In the case of interprediction, a PU represents an area of pixels that is processed by motion compensation using a motion vector specified for a PU. For intraprediction, the PU specifies the prediction mode for a set of transform units (TUs). A TU can have different sizes (e.g. 4x4, 8x8, 16x16 and 32x32 pixels) and can be processed in different modes. For a TU a transform coding is being performed, that is, the prediction error is being transformed with a discrete cosine transform or the discrete sine transform - in the HEVC/H.265 standard, it is applied to intracoded blocks - and quantized . As a result, the reconstructed pixels contain quantization noise and blocking artifacts that can influence the prediction accuracy.

[0007] In all previous video coding standards except HEVC/H.265, only one transform was used to keep the design simple. The HEVC/H.265 standard specifies two transforms for intracoding: DST for 4x4 blocks as this transform provides better energy compression for small intrapredicted blocks, and DCT for other blocks.

[0008] Furthermore, a mode-dependent coefficient scanning (MDCS) technique is adopted for the HEVC/H.265 standard. The basic idea behind this is to select a scan order subject to a selected intraprediction mode. For intracoded blocks, the scan order of a 4x4 transform block (TB) and a TB 8x8 luma is determined by the intraprediction mode, see Joel Sole, Rajan Joshi, Nguyen, Tianying Ji, Marta Karczewicz, Gordon Clare, Felix Henry , and Alberto Duenas. "Transform Coefficient Coding in HEVC," IEEE Transactions on Circuits and Systems for Video Technology, vol. 22, no. 12, December 2012, pp. 1765-1777. Each of the 35 intraprediction modes uses one of three possible scan orders: diagonal, horizontal, or vertical. A lookup table maps the intraprediction mode to one of the scans.

[0009] This tool explores the horizontal or vertical correlation of the residual depending on the intraprediction mode. For example, for a horizontal prediction mode, transform coefficient energy is binned in the first few columns, so a vertical scan results in fewer bins being entropy encoded. Similar to a vertical prediction, a horizontal scan is beneficial. Experiments show that including horizontal and vertical scans for large TBs offers little compression efficiency, so the application of these scans is limited to two smaller TBs.

[0010] Techniques similar to MDCS are considered in the document Yung-Lyul Lee, Ki-Hun Han, Dong-Gyu Sim, and Jeongil Seo. "Adaptive Scanning for H.264/AVC Intra Coding," ETRI Journal, Volume 28, Number 5, October 2006, pp. 668-671, in the document Byeong-Doo Choi, Jin-Hyung Kim, and Sung-Jea Ko. "Adaptive Coefficient Scanning Based on the Intra Prediction Mode," ETRI Journal, Volume 29, Number 5, October 2007, pp. 694-696, and in the document Md. Salah Uddin Yusuf and Mohiuddin Ahmad. "A Novel Scanning Scheme for Directional Spatial Prediction of AVS Intra Coding," International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering, Vol. 2, Issue 8, August 2013, pp. 4120-4126. The scan orders differ from those used in the HEVC/H.265 standard. Also, it is proposed to map the intraprediction modes to scan order orders.

[0011] Furthermore, in partitioning mode - as known to Bae Keun Lee, Jae Cheol Kwon, and Joo Young Kim. "Method and Apparatus for Coefficient Scan Based on Partition Mode of Prediction Unit," United States Patent Application Publication, US 2014/0314143 A1, PCT/KR2012/009373, November 8, 2012 -, and quantization parameters (QP) - as known by Thai-Ha Nguyen, Yoichi Yagasaki, and Ali Tabatabai. "QP adaptive coefficients scanning and application," United States Patent Application Publication, US 2011/0249726 A1, April 1, 2011 - can be used as features to select a scanning order not only for intra but for inter coding as well.

[0012] Thus, there is a huge variety of scan orders, which can be efficient for the same modes and TU sizes. This is the main reason why a procedure based on RDO (Rate Skew Optimization) or similar - for example, based on skew or activity analysis - can be used to make a final decision about which of the potential scan orders should be selected. .

[0013] An example of how it is possible to select a scanning order in an RDO-based procedure is known, for example, from Muhammed Zeyd Coban and Marta Karczewicz. "Adaptive scanning of transform coefficients for video coding," United States Patent Application Publication, US 2012/0099646 A1, October 17, 2011. In this case, the scanning order that provides the lowest RD cost is selected. Thus, the main advantage of this proposal is that a selected scanning order provides the highest coding gain. Therefore, the coding efficiency of this proposal is higher than for other methods if excess signaling processing is not taken into account.

[0014] This prior technique proposes two algorithms to select the scanning order providing the highest coding gain. The main distinguishing feature between these two algorithms is that different mechanisms are used to detect such TBs where this adaptive scan order selection technique is inapplicable, i.e., a standard scan order is used. In a first algorithm, the number of significant transform coefficients is compared with a threshold number. In a second algorithm, the position of the last significant transform coefficients is compared with a threshold position. In contrast to the previously described techniques, a selected scan order must be signaled for both of these cases, so that a decoder can apply a correct scan order to the received transform coefficients. Thus, the main disadvantage of proposals that use an RDO-based decision-making procedure is an excess of signaling processing.

[0015] Another way to improve compression efficiency compared to MDCS is to adapt the scanning order according to the statistics gathered for blocks that have similar parameter values such as intraprediction mode, partitioning mode, block size, etc. . instead of using scanning orders defined a priori. Consequently, initial scan orders are defined for use in cases when a statistic is not sufficient. When the number of blocks used to collect statistics exceeds a threshold, the scanning order can be adapted based on the statistics collected, see Yan Ye and Marta Karczewicz. "Adaptive coefficient scanning for video coding," United States Patent, US 8,488,668 B2, June 4, 2008. A similar proposal is known from Sridhar Srinivasan. "Adaptive coefficient scan order," European Patent Specification, EP 1679903 B1, 30.11.2005.

[0016] However, such an adaptive scan order based on statistics gathered from previously encoded or decoded blocks does not always provide a coding efficiency that is higher than predefined scan orders such as zigzag, diagonal, vertical, horizontal, clockwise and counterclockwise helix scans, etc.

SUMMARY

[0017] Having recognized the aforementioned disadvantages and problems, the present invention aims to improve the state of the art. Specifically, the object of the present invention is to provide a video decoder, a decoding method, a video encoder, and a coding method for improved coding efficiency.

[0018] The present invention specifically intends to improve coding efficiency by means of an improved scanning order for scanning a coefficient matrix for one or more coefficient vectors or for inverse scanning of one or more coefficient vectors in a coefficient matrix .

[0019] Furthermore, the present invention aims to improve coding efficiency by reducing excess signaling processing.

[0020] The aforementioned object of the present invention is achieved by the solution provided in the attached independent embodiments. Advantageous implementations of the present invention are further defined in the respective dependent embodiments.

[0021] A first aspect of the present invention provides a decoder for decoding block coefficients of a video sequence from a bit stream. The decoder comprises a scan pattern list module configured to provide one or more predefined scan orders. A scan order is also known in the art as a scan pattern. The decoder comprises a scan order generator configured to generate one or more scan orders. The decoder comprises a scan order selector configured to select a scan order for each block of predefined scan orders generated based on scan order information contained in the bit stream. The decoder comprises a decoding module configured to decode one or more coefficient vectors of each block of the bit stream. The decoder comprises a deserializer configured to inversely scan, for each block, the one or more coefficient vectors of that block in accordance with the scan order selected for that block to obtain a matrix of coefficients. The scan order generator is configured to generate the one or more scan orders depending on one or more coefficient matrices previously obtained from blocks of the video sequence.

[0022] Hereby, the present invention is advantageous in that it provides both predefined scan orders and generated scan orders adaptable to the content. By extending the set of available scan orders, the quantization rate distortion efficiency can be consequently improved.

[0023] In one form of implementation of the decoder according to the first aspect of the invention, the scan pattern list module is configured to provide one or more predefined scan orders for each of a plurality of coding parameter sets. . The scan order generator is configured to generate a scan order for each of the coding parameter sets depending on one or more previously obtained coefficient matrices associated with that coding parameter set. The scan order selector is configured to select a scan order for each block by determining a set of encoding parameters for that block, and selecting the scan order for the coefficient matrix obtained for that block from the predefined scan orders generated for the set of encoding parameters determined.

[0024] Hereby, both predefined scan orders and content-adaptive generated scan orders can be provided for each set of encoding parameters. As such, different scan orders can be generated for different sets of coding parameters, so that the set of available scan orders can be increased and adapted for each set of coding parameters. Therefore, the selected scanning order can additionally improve the rate distortion efficiency.

[0025] Specifically, each set of coding parameters may correspond, for example, to a combination of one or more prediction mode, a partition mode, one or more transform parameters, and one or more quantization parameters.

[0026] In one form of implementation of the decoder according to the first aspect of the invention, the decoder comprises a coefficient distribution estimator configured to provide, for each of the sets of coding parameters, a distribution statistic of one or more previously obtained coefficient matrices associated with that set of coding parameters. The scan order generator is configured to generate the scan order for each of the coding parameter sets depending on the distribution statistics for that coding parameter set.

[0027] Hereby, the generated scan orders can reflect a distribution statistic of previous coefficient matrices. The generated scan orders can thus provide good coding efficiency for subsequent coefficient matrices if the distribution statistics of the subsequent matrices are similar to those of the previous coefficient matrices.

[0028] In one form of decoder implementation according to the first aspect of the invention, if the scan order selected for a given block is predefined, the coefficient distribution estimator is configured not to update or is configured to reset the statistic. distribution for the set of encoding parameters of the given block.

[0029] Hereby, if the scan order selected for a given block is a predefined scan order, this means that, for the given block's set of coding parameters, the generated scan order can provide a coding efficiency which is worse than that of predefined scan orders. Therefore, the given block presents a distribution statistic that does not correspond to that of the previous coefficient matrices, so the distribution statistic of the given preference block is not used to update the distribution statistic provided by the coefficient distribution estimator. Alternatively, it may be preferable to reset the distribution statistics provided by the coefficient distribution estimator as the previous coefficient matrix statistics may no longer match the present coefficient matrix statistics.

[0030] In one form of implementation of the decoder according to the first aspect of the invention, the decoding module is configured to analyze, for each block, the encoding parameter information of the bit stream. The scan order selector is configured to determine the set of encoding parameters of a given block according to the analyzed encoding parameter information for that given block.

[0031] Hereby, the decoder can obtain information regarding the set of coding parameters and use the predefined and generated scanning orders that correspond to this set of coding parameters, so as to improve the coding efficiency.

[0032] In one form of implementation of the decoder according to the first aspect of the invention, the decoding module is configured to analyze scan order information that identifies the selected scan order of the bit stream.

[0033] Hereby, the decoder is able to identify the selected scan order of the bit stream and, depending on whether this selected scan order is predefined or generated, the decoder can correctly inverse scan the coefficient vectors and can, for example, whether or not to correctly update the distribution statistics provided by the coefficient distribution estimator.

[0034] The functions of the decoder according to the first aspect of the invention and any functions of any form of their implementation can be performed by a processor or a computer, and any of their means can be implemented as software and/or hardware in such processor or computer.

[0035] A second aspect of the present invention provides a method for decoding block coefficients of a video sequence from a bit stream. The method comprises providing one or more predefined scanning orders. The method comprises generating one or more scanning orders. The method comprises selecting a scan order for each block from the predefined scan orders generated based on scan order information contained in the bit stream. The method comprises decoding one or more coefficient vectors from each block of the bit stream. The method comprises inverse scanning, for each block, of the one or more coefficient vectors of that block in accordance with the scanning order selected for that block in order to obtain a matrix of coefficients. Generating the one or more scan orders comprises generating the one or more scan orders depending on one or more coefficient matrices previously obtained from blocks of the video sequence.

[0036] Additional features or implementations of the method according to the second aspect of the invention can perform the functionality of the decoder according to the first aspect of the invention and its different forms of implementation.

[0037] The method according to the second aspect of the invention or any of its implementation forms can be executed by a processor or a computer.

[0038] A third aspect of the present invention provides an encoder for encoding the block coefficients of a video sequence into a bit stream, the coefficients of each block being contained in a respective coefficient matrix. The encoder comprises a scan pattern list module configured to provide one or more predefined scan orders. The encoder comprises a scan order generator configured to generate one or more scan orders. The encoder comprises a scan order selector configured to select a scan order for each block of the predefined and generated scan orders. The encoder comprises a serializer configured to scan, for each block, the coefficient matrix of that block in accordance with the scan order selected for that block to obtain one or more coefficient vectors. The encoder comprises a coding module configured to encode the coefficient vectors within the bit stream. The scan order generator is configured to generate the one or more scan orders depending on one or more previously scanned coefficient matrices of blocks of the video sequence.

[0039] Hereby, the present invention is advantageous in that it provides both predefined scan orders and generated scan orders adaptable to content. By extending the set of available scan orders, the quantization rate distortion efficiency can be consequently improved.

[0040] In one form of encoder implementation according to the third aspect of the invention, the scan pattern list module is configured to provide one or more predefined scan orders for each of a plurality of encoding parameter sets. . The scan order generator is configured to generate a scan order for each of the coding parameter sets depending on one or more previously scanned coefficient matrices associated with that coding parameter set. The scan order selector is configured to select a scan order for each block by determining a set of encoding parameters for that block, and selecting the scan order for that block's coefficient matrix from the predefined scan orders generated for the set. of given coding parameters.

[0041] Hereby, both predefined scan orders and content-adaptive generated scan orders can be provided for each set of coding parameters. As such, different scan orders can be generated for different sets of coding parameters, so that the set of available scan orders can be increased and adapted for each set of coding parameters. Therefore, the selected scanning order can additionally improve the rate distortion efficiency.

[0042] Specifically, each set of coding parameters may correspond to a combination of one or more prediction mode, a partition mode, one or more transform parameters, and one or more quantization parameters.

[0043] In one form of implementation of the encoder in accordance with the third aspect of the invention, the encoder comprises a coefficient distribution estimator configured to provide, for each of the sets of encoding parameters, a distribution statistic of one or more matrices of previously scanned coefficients associated with that set of coding parameters. The scan order generator is configured to generate the scan order for each of the coding parameter sets depending on the distribution statistics for that coding parameter set.

[0044] Hereby, the generated scan orders can reflect a distribution statistic of previous coefficient matrices, so that these generated scan orders can provide good coding efficiency for subsequent coefficient matrices in the case that the statistic of distribution of the subsequent matrices is similar to that of the previous coefficient matrices.

[0045] In one form of encoder implementation according to the third aspect of the invention, if the selected scan order for a given coefficient matrix is predefined, the coefficient distribution estimator is configured not to update or is configured to reset the distribution statistics for the set of encoding parameters of the given block.

[0046] Hereby, if the scan order selected for a given block is a predefined scan order, this means that, for the set of coding parameters of the given block, the generated scan order can provide a coding efficiency which is worse than that of the predefined scan orders. Therefore, the given block presents a distribution statistic that does not correspond to that of the previous coefficient matrices, so the distribution statistic of the given preference block is not used to update the distribution statistic provided by the coefficient distribution estimator. Alternatively, it may be preferable to reset the distribution statistics provided by the coefficient distribution estimator as the previous coefficient matrix statistics may no longer match the present coefficient matrix statistics.

[0047] In one form of implementing the encoder in accordance with the third aspect of the invention, the encoder comprises a signaling module configured to add scan order information that identifies the selected scan order in the bit stream.

[0048] Hereby, it is advantageous that the decoder is able to identify the selected scan order of the bit stream and, depending on whether this selected scan order is predefined or generated, the decoder can correctly inverse scan the coefficient vectors. and can, for example, correctly update or not the distribution statistic provided by the coefficient distribution estimator.

[0049] In one form of encoder implementation according to the third aspect of the invention, the scan order selector is configured to select the scan order from predefined and generated scan orders based on a cost function.

[0050] Hereby, a procedure based on the cost function can be used to precisely select a scanning order from the generated scanning orders and the predefined scanning orders. Consequently, even if scan orders generated based on statistics gathered from previously encoded or decoded blocks do not provide better coding efficiency than predefined scan orders, scan order selection can improve coding efficiency. Specifically, the cost function may, for example, be a Rate Distortion Optimization (RDO) function.

[0051] The functions of the encoder according to the third aspect of the invention and any functions of any of its implementation forms can be performed by a processor or a computer, and any of its means can be implemented as software and/or hardware in such processor or computer.

[0052] A fourth aspect of the present invention provides a method for encoding block coefficients of a video sequence into a bit stream, the coefficients of each block being contained in a respective coefficient matrix. The method comprises providing one or more predefined scanning orders. The method comprises generating one or more scanning orders. The method comprises selecting a scan order for each block from the predefined and generated scan orders. The method comprises scanning, for each block, the coefficients of that block according to the scanning order selected for that block in order to obtain one or more coefficient vectors. The method comprises encoding the coefficient vectors within the bit stream. Generating the one or more scan orders comprises generating the one or more scan orders depending on one or more previously scanned coefficient matrices of blocks of the video sequence.

[0053] Additional features or implementations of the method according to the fourth aspect of the invention can perform the functionality of the encoder according to the third aspect of the invention and its different forms of implementation.

[0054] The method according to the fourth aspect of the invention or any of its implementation forms can be executed by a processor or a computer.

[0055] A fifth aspect of the present invention provides a decoder, preferably in accordance with the first aspect, for decoding block coefficients of a video sequence of a bit stream. The decoder comprises a decoding module configured to decode one or more coefficient vectors of each block of the bit stream. The decoder comprises a scanning module configured to reconstruct, for at least one block, hidden information from one or more coefficients of that block, the hidden information constituting or forming part of scan order information associated with that block. The decoder comprises a scan order selector configured to select a scan order for each block from a set of scan orders, preferably from a set of predefined and generated scan orders, based on scan order information associated with that block. The decoder comprises a deserializer configured to inversely scan, for each block, the one or more coefficient vectors of that block in accordance with the scan order selected for that block to obtain a matrix of coefficients.

[0056] Hereby, coding efficiency is advantageously improved in that excess signaling processing is reduced.

[0057] In one form of implementation of the decoder according to the fifth aspect of the invention, the verification module is configured to reconstruct, for at least one block, the hidden information of at least one hiding coefficient vector chosen from among the one or more coefficient vectors obtained for that block. Specifically, the one or more coefficient vectors may correspond to a respective coefficient group (CG), the hiding coefficient vector then corresponding to the coefficient group on which data hiding is performed. Hereby, reconstructing the hidden information from several hidden coefficient vectors, for example, preferably from several groups of coefficients, is advantageous in that more bits can be hidden and a larger index for the scan order information can be reconstructed.

[0058] In one form of decoder implementation according to the fifth aspect of the invention, the verification module is configured to reconstruct, for at least one block, the hidden information of the hiding coefficient vector depending on a hiding condition, the concealment condition comprising a comparison of a parameter depending on the concealment coefficient vector with a threshold. Hereby, concealment is facilitated.

[0059] In one form of implementation of the decoder according to the fifth aspect of the invention, the parameter is:

[0060] - a number of significant coefficients within the occultation coefficient vector, or

[0061] - a distance between first and last significant coefficients within the occultation coefficient vector, or

[0062] - a ratio of a maximum absolute value to a non-zero minimum absolute value of the coefficients within the hiding coefficient vector. The hiding condition is true if the parameter is above the threshold, and false if the parameter is below the threshold. Hereby, the concealment condition is advantageously provided.

[0063] In one form of implementation of the decoder according to the fifth aspect of the invention, the verification module is configured to check the hiding condition, and reconstruct the hidden information of the hiding coefficient vector if the hiding condition is true . Hereby, the scanning order can be selected without additional side information from the bit stream.

[0064] In one form of implementation of the decoder according to the fifth aspect of the invention, if the concealment condition is false, the scan order selector is configured to select a predefined default scan order. Hereby, the scanning order can be selected even if no hiding was possible on the encoder side.

[0065] In one form of implementation of the decoder according to the fifth aspect of the invention, the verification module is configured to reconstruct the hidden information of the occultation coefficient vector by applying a verification function on the occultation coefficient vector, a result of the verification function and the information is hidden. Hereby, the scanning order can be advantageously obtained from the coefficients.

[0066] In one form of implementation of the decoder according to the fifth aspect of the invention, the result of the verification function depends on coordinates (x, y), within the coefficient matrix, of the last significant coefficient of the occultation coefficient vector . The check function could, for example, be the following function: mod( x+y + R ,2) and y being the coordinates, and R being an integer value. Hereby, the result of the check function can be obtained directly from coefficient coordinates.

[0067] In one form of implementation of the decoder according to the fifth aspect of the invention, the verification module is configured to reconstruct, for a first block, hidden information from one or more coefficients of the first block, the hidden information constituting or forming part of scan order information associated with the first block, and the scan order selector is configured to select, for the first and at least one second block, a scan order based on the scan order information associated with the first block. Hereby, this form of implementation provides a "blend mode" decoding, in which the selected scan order value for the second block is reconstructed/predicted using the coefficient values of the first block without hiding anything within the coefficients of the second block. second block. For example, the first and second blocks are spatial neighbors.

[0068] Specifically, the scan order selector may be configured to select, for a group of blocks, a scan order based on the scan order information associated with the first block, wherein the group of blocks may comprise the first block and a plurality of second blocks. The block group may include multiple blocks within a block group that have at least one non-standard scan index hidden within their coefficients. Alternatively, the group of blocks may include several sequentially coded blocks, such that said coded blocks have the same scan index. Alternatively, the group of blocks may include several blocks and/or several sequentially coded blocks that have coefficients for which the concealment condition is false.

[0069] Specifically, in blend mode several options are available for the decoder to know that no scan order information is hidden in the coefficients of at least one second block. Firstly, side information can be encoded in the bit stream, for example, a flag for a group of blocks that enables or disables the use of merge mode for them. Secondly, without side information, it is proposed to apply a merge rule on the second block depending on the hiding condition and set of encoding parameters of the first block and/or the second block. In other words, a merge is applied to the second block if the hiding condition is not met for the second block, that is, the scan order for the second block is selected based on the scan order information associated with the first block. . Thirdly, a combination of the first two alternatives is proposed, said combination comprising side information in the bit stream and applying a merge depending on the hiding condition.

[0070] The functions of the decoder according to the fifth aspect of the invention and any functions of any form of their implementation can be performed by a processor or a computer, and any of their means can be implemented as software and/or hardware in such processor or computer.

[0071] A sixth aspect of the present invention provides a method, preferably in accordance with the second aspect, for decoding the block coefficients of a video sequence from a bit stream. The method comprises decoding one or more coefficient vectors from each block of the bit stream. The method comprises reconstructing, for at least one block, the hidden information of one or more coefficients of that block, the hidden information constituting or forming part of scan order information associated with that block. The method comprises selecting a scan order for each block from a set of scan orders, preferably from a set of predefined and generated scan orders, based on scan order information associated with that block. The method comprises inverse scanning, for each block, of the one or more coefficient vectors of that block in accordance with the scanning order selected for that block, thereby obtaining a matrix of coefficients.

[0072] Additional features or implementations of the method according to the sixth aspect of the invention can perform the functionality of the decoder according to the fifth aspect of the invention and its different forms of implementation.

[0073] The method according to the sixth aspect of the invention or any of its implementation forms can be executed by a processor or a computer.

[0074] A seventh aspect of the present invention provides an encoder, preferably according to the third aspect, for encoding block coefficients of a video sequence into a bit stream, the coefficients of each block being contained in a respective matrix of coefficients. The encoder comprises a scan order selector configured to select a scan order for each block from a set of scan orders, preferably from a set of predefined and generated scan orders. The encoder comprises a serializer configured to scan, for each block, the coefficient matrix of that block in accordance with the scan order selected for that block to obtain one or more coefficient vectors. The encoder comprises a hiding module configured to hide, for at least one block, scan order information that identifies the selected scan order for that block at least partially in one or more coefficients of that block (i.e., configured to hide those scan order information or part thereof in those one or more coefficients). The encoder comprises an encoding module configured to encode the coefficient vectors within the bit stream. By this, the coding efficiency can be improved by reducing signaling overhead.

[0075] In one form of encoder implementation according to the seventh aspect of the invention, the hiding module is configured to hide, for at least one block, the scan order information in at least one chosen hiding coefficient vector. among the one or more coefficient vectors obtained for that block.

[0076] In one form of encoder implementation according to the seventh aspect of the invention, the hiding module is configured to hide, for at least one block, the scan order information in the hiding coefficient vector depending on a condition of concealment, the concealment condition comprising a comparison of a parameter depending on the concealment coefficient vector with a threshold.

[0077] In one form of encoder implementation according to the seventh aspect of the invention, the parameter is:

[0078] - a number of significant coefficients within the occultation coefficient vector, or

[0079] - a distance between the first and last significant coefficients within the occultation coefficient vector, or

[0080] - a ratio of a maximum absolute value to a non-zero minimum absolute value of the coefficients within the hiding coefficient vector, wherein the hiding condition is true if the parameter is above the threshold, and false if the parameter is below the limit.

[0081] In one form of encoder implementation according to the seventh aspect of the invention, the hiding module is configured to check the hiding condition, and to hide the scan order information in the hiding coefficient vector if the condition concealment is true.

[0082] In one form of encoder implementation according to the seventh aspect of the invention, if the hiding condition is false, the scan order selector is configured to recursively select another scan order from the set of scan orders until the hiding condition for that other selected scan order is true. Thereby, selecting other scan orders is advantageous in that it is possible to perform data hiding even if the first selected scan order would imply a false hiding condition.

[0083] In one form of encoder implementation according to the seventh aspect of the invention, if the hiding condition is false and the selected scan order is a predefined standard scan order, the hiding module is configured to not hide the scan order information in the occultation coefficient vector.

[0084] In one form of encoder implementation according to the seventh aspect of the invention, the hiding module is configured to hide, for at least one block, the scan order information in the hiding coefficient vector:

[0085] - applying a check function to the occultation coefficient vector so as to obtain a result of the check function,

[0086] - determine whether the result of the verification function matches the scan order information,

[0087] - if not, modify the occultation coefficient vector so that a result of the verification function matches the scan order information.

[0088] In one form of encoder implementation according to the seventh aspect of the invention, the result of the verification function depends on coordinates (x, y), within the coefficient matrix, of the last significant coefficient of the hiding coefficient vector . The check function could, for example, be the following function: mod( x+y + R ,2)x and y being coordinates, and R being an integer value.

[0089] In a form of implementation of the encoder according to the seventh aspect of the invention, if an identical scan order is selected by the scan order selector for a first and at least a second block, the concealment module is configured to hide scan order information that identifies the identical scan order in one or more coefficients of the first block only. Hereby, encoding according to a "blending mode" is possible. In such a mode, preferably scan order information identifying identical scan order is not hidden in coefficients of the second block. In other words, the scan order value selected for the second block is reconstructed/predicted on the decoder side using the scan order information hidden in coefficients of the first block. The first and second blocks are, for example, spatial neighbors or, for example, successive blocks in the bit stream. Thus, the scan order information of the second block can be transmitted to the decoder without hiding anything within the coefficients of the second block.

[0090] Specifically, a group of blocks comprising the first block and the one or more second blocks may be a group of blocks that have at least one non-standard scan index hidden within their coefficients. Alternatively, the group of blocks may include several sequentially coded blocks, such that said coded blocks have the same scan index. Alternatively, the group of blocks may include several blocks and/or several sequentially encoded blocks that have coefficients for which the concealment condition is false.

[0091] Specifically, in merge mode several options are available for the decoder to know that no scan order information is hidden in the coefficients of at least one second block. First, the encoder adds side information to the bitstream, for example, a flag that enables or disables the use of blending mode for one or more blocks, for example, for the one or more second blocks. Second, without side information, it is proposed to apply a merge rule in the second block depending on the hiding condition and set of coding parameters of the first block and/or the second block. In other words, a merge is applied to the second block if the hiding condition is not met for the second block, that is, the scan order for the second block is selected based on the scan order information associated with the first block. . Thirdly, a combination of the first two alternatives is proposed, said combination comprising the side information in the bit stream and applying a merge depending on the hiding condition.

[0092] The functions of the encoder according to the seventh aspect of the invention and any functions of any form of their implementation can be performed by a processor or a computer, and any of their means can be implemented as software and/or hardware in such processor or computer.

[0093] An eighth aspect of the present invention provides a method, preferably according to the fourth aspect of the invention, for encoding block coefficients of a video sequence into a bit stream, the coefficients of each block being contained in a respective coefficient matrix. The method comprises selecting a scan order for each block from a set of scan orders, preferably from a set of predefined and generated scan orders. The method comprises scanning, for each block, the coefficient matrix of that block according to the selected scanning order for that block in order to obtain one or more coefficient vectors. The method comprises hiding, for at least one block, scan order information identifying the selected scan order for that block at least partially in one or more coefficients of that block. The method comprises encoding the coefficient vectors within the bit stream.

[0094] Additional features or implementations of the method according to the eighth aspect of the invention can perform the functionality of the encoder according to the seventh aspect of the invention and its different forms of implementation.

[0095] The method according to the eighth aspect of the invention or any of its implementation forms can be executed by a processor or a computer.

[0096] A ninth aspect of the present invention provides a computer program comprising program code for carrying out the method according to the second, fourth, sixth or eighth aspect of the invention, when the computer program executes on a computing device .

[0097] It should be noted that all devices, elements, units and means described in the present application can be implemented in software or hardware elements or any type of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step is to be fully formed by eternal entities not reflected in the description of a specific detailed element of this entity which performs this specific step or functionality, it should be clear to a person skilled in the art. technique that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0098] The above aspects and forms of implementation of the present invention will be explained in the following description of specific embodiments in relation to the attached drawings, in which

[0099] Figure 1 shows a video encoder according to an embodiment of the present invention.

[0100] Figure 2 shows a video decoder according to an embodiment of the present invention.

[0101] Figure 3 shows a flowchart of estimating coefficient distribution in the video decoder according to an embodiment of the present invention.

[0102] Figure 4 shows a flowchart of estimating the distribution of coefficients in the video encoder according to an embodiment of the present invention.

[0103] Figure 5 shows a flowchart of an initialization of a list of scan orders in accordance with an embodiment of the present invention.

[0104] Figure 6 shows a flowchart of implicit and explicit signaling of selected scanning order in accordance with an embodiment of the present invention.

[0105] Figures 7 and 8 show a concealment condition in accordance with an embodiment of the present invention.

[0106] Figure 9 shows a coefficient entropy decoding according to the prior art.

[0107] Figure 10 shows an embodiment of the present invention for improving coding efficiency by reducing excess signaling processing in the bit stream.

[0108] Figure 11 shows a reordering of coefficients according to the present invention.

[0109] Figure 12 shows an example of occultation conditions defined for scan orders.

DETAILED DESCRIPTION OF MODALITIES

[0110] Figure 1 shows a video encoder in accordance with an embodiment of the present invention, and specifically shows an encoder 100 for encoding block coefficients of a video sequence into a bit stream 107, the coefficients of each block being contained in a respective matrix of coefficients.

[0111] The encoder comprises a scan pattern list module 110 configured to provide one or more predefined scan orders.

[0112] The encoder comprises a scan order generator 111 configured to generate one or more scan orders.

[0113] The encoder comprises a scan order selector 109 configured to select a scan order for each block of the predefined and generated scan orders.

[0114] The encoder comprises a serializer 104 configured to scan, for each block, the coefficient matrix of that block according to the scanning order selected for that block in order to obtain one or more coefficient vectors.

[0115] The encoder comprises a coding module 106 configured to encode the coefficient vectors within the bit stream 107.

[0116] The scan order generator 111 is configured to generate the one or more scan orders depending on one or more previously scanned coefficient matrices of blocks of the video sequence.

[0117] Figure 2 shows a video decoder according to an embodiment of the present invention, and specifically a decoder 200 for decoding block coefficients of a video sequence from a bit stream 207.

[0118] The decoder comprises a scan pattern list module 210 configured to provide one or more predefined scan orders.

[0119] The decoder comprises a scan order generator 211 configured to generate one or more scan orders.

[0120] The decoder comprises a scan order selector 209 configured to select a scan order for each block of predefined scan orders generated based on SCAN_IDX scan order information contained in the bit stream.

[0121] The decoder comprises a decoding module 206 configured to decode one or more coefficient vectors of each block of the bit stream 207.

[0122] The decoder comprises a deserializer 204 configured to reverse scan, for each block, the one or more coefficient vectors of that block in accordance with the scan order selected for that block in order to obtain a matrix of coefficients.

[0123] The scan order generator 211 is configured to generate the one or more scan orders depending on one or more coefficient matrices previously obtained from blocks of the video sequence.

[0124] Specifically, Figure 1 shows an example of an encoder-side residual coding part with content-adaptive scan order selection. In the example, encoder 100 is configured to apply inter- or intra-prediction to each block of each image of the video sequence. An intraprediction mode uses pixels from the same image to generate reference samples to calculate prediction values for the pixels in the block being reconstructed. An interprediction mode is designed for temporal prediction and using reference samples from previous or next images to predict pixels of the current image block. The result of the prediction stage may be a residual signal 101 which is the difference between an original signal of a block and its prediction signal obtained through inter- or intra-prediction.

[0125] A prediction mode for obtaining the residual signal 101 may correspondingly refer to either an intra- or an inter-prediction mode. Furthermore, the prediction mode may correspond to any one of a plurality of available prediction modes or directions. For example, the plurality of available prediction modes may comprise up to 35 intra-prediction modes, which may include 33 directional modes and 2 non-directional modes, such as a direct current (DC) prediction mode and a flat prediction mode. The 33 directional modes can cover an angular range of π radians or 180 degrees. If the direction of a vertical mode is considered to be zero degrees, the 33 directional modes can be symmetrical around the -45 degree geometric axis. With respect to the prediction mode, the encoder may be configured to determine, for a given block, one of the plurality of available prediction modes based on a Rate Distortion Optimization (RDO).

[0126] A Coding Tree Unit (CTU) is the root of a video sequence coding structure of a predefined size, which contains a portion of an image (e.g., 64x64 pixels). A CTU can be partitioned into Coding Units (CUs). A CU is a basic video sequence coding structure of a predetermined size, which contains a part of an image which belongs to a CTU. A CU can be partitioned into additional CUs. A Prediction Unit (PU) is a coding structure which is the result of partitioning a CU. A PU can be partitioned into several Transformation Units (TUs) or coincide with a single TU. A PU can be intra or interpredicted according to the type of processing applied to the PU pixels. In the case of interprediction, a PU represents an area of pixels that is processed by motion compensation using a motion vector specified for a PU.

[0127] In this aspect, a partition mode may depend on the PU, and specifically on the size and/or orientation of the PU. The partition mode may correspond to any of a plurality of available partition modes. The encoder may be configured to determine, for a block or PU, one of the plurality of available partition modes based on an RDO method. For example, the partition mode can correspond to a horizontally oriented shape, in which a CU can be partitioned into 2N*N PUs or blocks. Also, the partition mode can correspond to a vertically oriented shape, in which a CU can be partitioned into N*2N PUs or blocks, or a square shape, in which a CU can be partitioned into 2N*2N PUs or blocks.

[0128] The residual signal 101 may be provided with a transform module 102 for transform encoding the residual signal 101 in order to obtain transform coefficients. Specifically, for intraprediction a PU can specify a prediction mode for a set of TUs. A TU can have different sizes - for example, 4x4, 8x8, 16x16 and 32x32 pixels - and can be processed in different modes. Examples of transforms that can be applied to a residual signal include, for example, Discrete Cosine Transform (DCT) and Discrete Sine Transform (DST).

[0129] In this regard, a transform parameter can be the size or shape of the TU, or both the size and shape. An additional transform parameter can be the transform being applied, for example DCT or DST among others.

[0130] The residual signal 101 calculated for a block of the source image is transformed by the transform module 102 and then quantized by a quantization module 103. The quantization module 103 processes the transform coefficients to obtain quantized transform coefficients (QTCs) in the form of a coefficient matrix.

[0131] In this aspect, a quantization parameter may correspond to a number of significant coefficients of the coefficient matrix, that is, the number of coefficients of the coefficient matrix that have a non-zero value.

[0132] The coefficient matrix is serialized by scanning serializer 104 in an order determined by scan order selector 109. The scanning order can be either selected from the predefined scan orders provided by the scan pattern list 110 or from the generated scan orders provided by the scan order generator 111. The scan orders provided by the scan pattern list 110 are predefined for both the encoder 100 and decoder 200 sides and thus a scan order selected from the scan pattern list 110 scanning is static, that is, it is not modified throughout the entire encoding or decoding process. Scan orders provided by scan order generator 111 are dynamic, that is, they are generated depending on one or more coefficient matrices previously obtained from blocks of the video sequence. Specifically, the generated scan orders may be generated according to a distribution statistic of coefficient matrices during the encoding or decoding process.

[0133] The one or more coefficient vectors obtained by the serializer 104 from a coefficient matrix are stored in a quantized coefficient buffer 105. The encoding module 106 may be in the form of an entropy encoding module for encoding in entropy the coefficient vectors within the bit stream 107. The scan order that is selected by the scan order selector 109 may be identified using a scan index or SCAN_IDX scan order information.

[0134] This scan index information can be explicitly added to the bitstream 107, that is, the value of the SCAN_IDX scan order information can be stored in the bitstream 107 via a dedicated variable or parameter. Alternatively, a hiding module 112 may be configured to at least partially hide the SCAN_IDX scan order information in the coefficient vectors, i.e., in the values of the coefficient matrix after serialization. For example, a hiding module 112 may correspondingly perform adjustments of the coefficients stored in the quantized coefficient temporary store 105 so that a corresponding verification module located in a verification module 212 of the decoder 200 may derive the SCAN_IDX scan order information by applying a verification function for these. The resulting adjusted coefficients or coefficient vectors stored in the quantized coefficient buffer 105 may then be entropy encoded and transmitted to the decoder 200 in the bit stream 107.

[0135] The coefficients of the one or more coefficient vectors stored in the quantized coefficient buffer 105 may then be adjusted or not, depending on whether the SCAN_IDX scan order information is explicitly or implicitly transmitted in the bit stream 107, i.e. depending on whether the scan order information is hidden or not.

[0136] Specifically, the scan pattern list module 110 is configured to provide one or more predefined scan orders for each of a plurality of coding parameter sets. The scan order generator 111 is configured to generate a scan order for each of the coding parameter sets depending on one or more previously scanned coefficient matrices associated with that coding parameter set. Scan order selector 109 is configured to select a scan order for each block:

[0137] - determining a set of coding parameters for that block, and

[0138] - selecting the scan order for the coefficient matrix of that block from the predefined and generated scan orders for the given set of coding parameters.

[0139] In this aspect, a set of encoding parameters corresponds to a specific combination of at least one or more of:

[0140] - a prediction mode,

[0141] - a partition mode,

[0142] - one or more transform parameters, and

[0143] - one or more quantization parameters.

[0144] The prediction mode, partition mode, transform parameters, and quantization parameters are generally referred to by reference 108 in Figure 1. For example, a set of encoding parameters may correspond to only one prediction mode (e.g. , a specific directional intraprediction mode), to only one partition mode (e.g., a specific orientation of the PU), to only one or more transform parameters (e.g., a specific size and/or shape of the TU), or to only one or more quantization parameters (for example, a specific number of significant coefficients from the coefficient matrix). Also, the set of encoding parameters may correspond to a specific combination of prediction mode, partition mode, transform parameters, and/or quantization parameters.

[0145] Encoder 100 may comprise a coefficient distribution estimator 113 configured to provide, for each of the sets of coding parameters, a distribution statistic of one or more matrices of previously scanned coefficients associated with that set of coding parameters. . The scan order generator 111 is configured to generate the scan order for each of the sets of coding parameters depending on the distribution statistics for that set of coding parameters.

[0146] Specifically, the coefficients stored in the quantized coefficient buffer 105 are also a source for the estimation of coefficient distribution statistics that is used by the scan order generator 111. However, before estimating the coefficient distribution, the stored coefficients are deserialized back from the coefficient vector to an array of coefficients via a deserializer 114.

[0147] The distribution statistic of a preference coefficient matrix comprises, as defined in US 2008/0310504, an indication of the probability that a coefficient value in a position of the two-dimensional coefficient matrix is zero or non-zero and by, example, comprising a count, probability, or other statistical metric associated with each of the coefficient positions of the two-dimensional coefficient matrix. A scan order generated depending on such a distribution statistic may correspondingly be a scan order that is configured to scan with higher priority those coefficient positions with a higher probability of having non-zero coefficients.

[0148] Specifically, if the selected scan order for a given block is a generated scan order, the coefficient distribution estimator 113 may be configured to update the distribution statistic for the set of encoding parameters of the given block accordingly. with distribution statistics of the coefficient matrix of the given block. The distribution statistics of the coefficient matrix of the given block may reflect whether or not the coefficient at a position in the coefficient matrix of the given block is zero or non-zero. The distribution statistic provided by the coefficient distribution estimator 113 may then, for example, be updated depending on said distribution statistic of the coefficient matrix of the given block.

[0149] Specifically, if the selected scan order for a given coefficient matrix is predefined, the coefficient distribution estimator 113 may be configured not to update or may be configured to reset the distribution statistic for the set of coding parameters. of the given block. Resetting the distribution statistic may, for example, consist of resetting said count, probability or other statistical metric.

[0150] This update, non-update, or reset of the distribution statistic performed by the coefficient distribution estimator 113 depending on whether the scan order selector 109 selected a generated or predefined scan order is reflected in Figure 1 by a dashed arrow of the scan order selector 109 for coefficient distribution estimator 113.

[0151] The decoder of the proposed modality is shown in Figure 14. The processing stages are similar to those of the encoder (see Figure 13) and are applied in reverse order. The most important difference is that the temporary storage of quantized coefficients stores the QTCs after they are deserialized, that is, as an array of QTCs but not the QTCs as one or more vectors.

[0152] Specifically, Figure 2 shows an example of a residual decoding part with content-adaptive scan order selection. In the example, decoder 200 is configured to obtain a residual signal 201 from bit stream 207. After generating, by decoding module 206 (e.g., an entropy decoding module) and deserializer 204, the coefficient matrix comprising quantized transform coefficients, a quantized coefficient buffer 205 stores said coefficient matrix. The stored coefficient matrix is then processed by an inverse quantization module 203 and an inverse transform module 202 to obtain the residual signal 201 that corresponds to the residual signal 101 in the encoder 100. Using the residual signal 201, the encoded video sequence can be reconstructed according to known techniques.

[0153] The coefficient distribution estimator 213 of the decoder 200 may be similar or identical in function to the coefficient distribution estimator 113 of the encoder 100. In the example, these differ in that the coefficient distribution estimator 213 takes as input the matrix of coefficients stored in the quantized coefficient buffer 205 while the coefficient distribution estimator 113 receives as input the coefficient matrix obtained from the deserializer 114.

[0154] The present invention proposes the combined use of predefined scanning orders and scanning orders generated on both the encoder 100 and decoder 200 sides depending on previously scanned video sequence block coefficient matrices, respectively obtained. Specifically, the generated scan orders are generated by analyzing a distribution statistic of the coefficients within the coefficient matrix, that is, by analyzing a distribution of the quantized transform coefficients.

[0155] A specific feature of the proposed invention is that the distribution statistics for the quantized transform coefficients (QTCs) are estimated more precisely taking into account the selected scanning order, as shown for example in Figures 1 and 2 by the arrow between the scan order selector 109, 209 and the coefficient distribution estimator 113, 213. This more accurate estimation may consist of, for example, updating, not updating, or resetting the distribution statistic as shown above.

[0156] The distribution of coefficients can be estimated more precisely by dividing the source data into 4 stationary periods and estimating the distribution of coefficients only within these stationary periods. A stationary period is a number of blocks that have a similar distribution of QTCs and are spatially close to each other.

[0157] In the present invention, it is specifically proposed to detect a stationary period by introducing a limit on the number of processed blocks that are signaled to have adaptively generated scan order. When the scan order is selected from the predefined scan orders provided by the scan pattern list module, there is no need to update the distribution statistics, because for this case the fact that the scan order is a predefined one indicates that the block currently processed belongs to a different stationary period.

[0158] Figure 3 shows a flowchart of estimating distribution statistics in the video decoder according to an embodiment of the present invention.

[0159] In the video decoder, the estimation 301 of the distribution statistic by the coefficient distribution estimator 213 comprises several steps.

[0160] In step 302, the prediction mode, partition mode, transform parameters, and/or quantization parameters 208 for a block are analyzed from bit stream 207. Preferably, this analysis is performed by decoding module 206 According to the analyzed information, the scanning order selector 209 preferably determines the encoding mode of that block.

[0161] In a step 303, the SCAN_IDX scan order information that identifies the selected scan order for that block is analyzed from the bit stream 207. This analysis is preferably done by the decoding module 206.

[0162] In a step 304, the scan order selector 209 selects the scan order for that block from the predefined and generated scan orders based on the analyzed scan order information SCAN_IDX.

[0163] In a step 305, it is determined whether the selected scan order is a predefined scan order provided by the scan pattern list module 210 or a generated scan order provided by the scan order generator 211.

[0164] If the scan order selected for that block is a generated one (e.g., provided by scan order generator 211), the coefficient distribution estimator 213 updates, in a step 306, the current distribution statistic. This current distribution statistic is preferably obtained from previous blocks. The current distribution statistics are updated according to the distribution statistics of the coefficient matrix obtained for that block.

[0165] If, however, the scan order selected for that block is a predefined one (e.g., provided by the scan pattern list module 210), the coefficient distribution estimator 213 processes, in one step 306, the distribution statistics for that block separately from the current distribution statistics. For example, the coefficient distribution estimator 213 may not update or may reset the current distribution statistic.

[0166] After step 306 or 307, estimation 301 of the distribution statistic ends (at step 308).

[0167] Figure 4 shows a flowchart of estimating distribution statistics in the video encoder in accordance with an embodiment of the present invention.

[0168] In the video encoder, the estimation 401 of the distribution statistic by the coefficient distribution estimator 113 comprises several steps.

[0169] In a step 402, an RDO procedure is performed by the scan order selector 109 in order to select from the predefined scan orders provided by the scan pattern list module 110 and the generated scan orders provided by the scan order generator 111, the scan order that provides the minimum rate distortion (RD) cost. In this step, the SCAN_IDX scan order information of the selected scan order is obtained.

[0170] In a step 403, the scan order that corresponds to the selected scan order information SCAN_IDX is obtained.

[0171] Additional steps 404 to 407 correspond to steps 305 to 308 of Figure 3.

[0172] As shown in Figures 3 and 4, on both the encoder and decoder sides the scan order list is initialized. This includes members of the scan pattern list module and scan orders generated by the scan order generator. Depending on whether the scan order selected by the scan order selector is generated or predefined, a decision about the distribution statistic is made. This decision may consist of updating the current distribution statistic or not, or resetting the current distribution statistic.

[0173] One of the possible ways of initializing the list of scan orders is shown in Figure 5. Figure 5 correspondingly shows a flowchart 500 of an initialization 501 of a list of scan orders in accordance with an embodiment of the present invention.

[0174] Its initialization depends on the prediction mode, partitioning mode, transform parameters and quantization parameters specified for a block being processed.

[0175] In a step 502, it is determined whether the prediction mode of that block is a flat intraprediction mode. If this is true, a scan pattern list of predefined scan orders is defined 508 to comprise a Z order and predefined diagonal scan orders. If this is false, step 503 is performed.

[0176] In step 503, it is determined whether the prediction mode of that block is a CC intraprediction mode. If this is true, a scan pattern list of predefined scan orders is defined 508 to comprise Z order and predefined diagonal scan orders. If this is false, step 504 is performed.

[0177] In step 504, it is determined whether the prediction mode of that block is a near-horizontal angular intraprediction mode. If this is true, a list of predefined scan order scan patterns is defined 509 to comprise vertical, vertical-diagonal, and helical scan orders. If this is false, step 505 is performed.

[0178] In step 505, it is determined whether the prediction mode of that block is an angular intraprediction mode close to the vertical. If this is true, a list of predefined scan order scan patterns is defined 510 to comprise horizontal, horizontal-diagonal, and helical scan orders. If this is false, step 506 is performed.

[0179] In step 506, it is determined whether the prediction mode of that block is an angular intraprediction mode close to the diagonal. If this is true, a scan pattern list of predefined scan orders is defined 511 to comprise diagonal, Z-order, and helix scan orders. If this is false, step 507 is performed. In step 507, a list of scan patterns of predefined scan orders is defined to comprise a diagonal scan order.

[0180] After defining the scan order list of predefined scan orders for that block, the scan order list is adjusted 512 using the portioning mode, transform parameters, and quantization parameters of that block.

[0181] In a step 513, the one or more generated scan orders provided by the scan order generator are added to the predefined scan orders, so that, for example, the encoder scan order selector 109 can select from Predefined and generated scan orders, the scan order that provides the minimum RD cost.

[0182] As shown in Figure 5, the predefined scan orders may depend on the prediction mode and/or partitioning mode and/or transform parameters and/or quantization parameters. For example, the most likely scan orders for DC or flat intramode cases are Z order and diagonal scan order. For angular modes, the directionality of the residual signal can be taken into account so that predefined scan orders can be selected to be orthogonal to the intramode directionality.

[0183] Later this list of predefined scan orders can be updated according to other coding parameters, such as portioning mode, transform parameters and quantization parameters. Specifically, this list can be updated according to the partitioning and transformed sizes.

[0184] For example, in interprediction mode, non-square prediction modes can also indicate the directional nature of the residual signal and the list of predefined scan orders can be updated accordingly: if a partitioning rectangle is oriented vertically then the predefined scan orders can be updated accordingly. horizontal and horizontaldiagonal scans are added to the scan list. The list of predefined scan orders is updated accordingly for horizontally oriented partitioning: in such a case, vertical and vertical-diagonal scans are added to the predefined scan orders.

[0185] For example, the list of predefined scan orders can be updated according to transform sizes. Smaller TU sizes introduce restrictions on some scanning patterns. So, for example, helix scans may be excluded from the predefined scan order list if the TU size is smaller than 8x8 pixels.

[0186] Quantization parameters (QP) can affect both the list of predefined scan orders and the signaling method. For high QP values, the number of scan patterns can be reduced. For example, helix scans may be skipped for all TU sizes because the QTCs become more aligned with the left, top, and top left of the QTC array.

[0187] An example of the dependence of the signaling scheme on QP is shown in Figure 6. Figure 6 specifically shows a flowchart 600 of implicit and explicit signaling of selected scan order in accordance with an embodiment of the present invention.

[0188] In a first step 601, a QP value is obtained. If this QP value is less than or equal 602 to a QP value for high bitrates QPHBR, then the SCAN_IDX scan order information must be entropy 603 encoded in the bitstream.

[0189] If a QP value is above the QPHBR value and below an additional QP value for low bitrates QPLBR, then the SCAN_IDX scan order information must be partially entropy encoded 606 and partially hidden within the scan coefficients. transformed quantized by the hiding module 112.

[0190] If the QP value is above the QPHBR value and above or equal to that additional QP value for low bit rates QPLBR, then the SCAN_IDX scan order information must be hidden 605 within the transform coefficients quantized by the scanning module. concealment 112.

[0191] Consequently, obfuscation becomes more expensive with increasing bitrate and is completely turned off for high bitrates. For low bitrates vice versa, hiding becomes more preferable. Otherwise, that is, for medium bitrates, both signaling mechanisms are being used. The selection of the signaling mechanism is performed for each block or TU depending on whether obfuscation can be performed for it.

[0192] According to the present invention, a decoder 200 for decoding block coefficients of a video sequence of a bit stream 207 is proposed. The decoder comprises:

[0193] - a decoding module 206 configured to decode one or more coefficient vectors from each block of the bit stream 207,

[0194] - a scanning module 212 configured to reconstruct, for at least one block, hidden information of one or more coefficients of that block, the hidden information constituting or forming part of SCAN_IDX scan order information associated with that block,

[0195] - a scan order selector 209 configured to select a scan order for each block from a set of scan orders, preferably from a set of predefined and generated scan orders, based on the scan order information SCAN_IDX associated with that block,

[0196] - a deserializer 204 configured to reverse scan, for each block, the one or more coefficient vectors of that block in accordance with the scan order selected for that block in order to obtain a matrix of coefficients.

[0197] According to the present invention, a corresponding encoder 100 for encoding block coefficients of a video sequence into a bit stream 107 is proposed, the coefficients of each block being contained in a respective coefficient matrix. The encoder comprises:

[0198] - a scan order selector 109 configured to select a scan order for each block from a set of scan orders, preferably from a set of predefined and generated scan orders,

[0199] - a serializer 104 configured to scan, for each block, the coefficient matrix of that block in accordance with the scan order selected for that block in order to obtain one or more coefficient vectors,

[0200] - a hiding module 112 configured to hide, for at least one block, the SCAN_IDX scan order information that identifies the selected scan order for that block at least partially in one or more coefficients of that block,

[0201] - an encoding module 106 configured to encode the coefficient vectors within the bit stream 107.

[0202] Coding efficiency can then be improved because excess signaling processing in the bit stream 107, 207 generated by the encoder and received by the decoder can be reduced.

[0203] Preferably, the verification module 212 is configured to reconstruct, for at least one block, the hidden information of at least one hiding coefficient vector chosen from the one or more coefficient vectors obtained for that block. Also, the hiding module 112 is configured to hide, for at least one block, the SCAN_IDX scan order information in at least one hiding coefficient vector chosen from the one or more coefficient vectors obtained for that block.

[0204] Reconstruction and concealment may depend on a concealment condition. Such a concealment condition comprises a comparison of a parameter that depends on the concealment coefficient vector with a threshold.

[0205] The parameter can be:

[0206] - a number of significant, i.e., non-zero, coefficients within the occultation coefficient vector, or

[0207] - a distance between first and last significant coefficients within the occultation coefficient vector, or

[0208] - a ratio of a maximum absolute value to a non-zero minimum absolute value of the coefficients within the hiding coefficient vector,

[0209] where the hiding condition is true if the parameter is above the threshold, and false if the parameter is below the threshold.

[0210] Figures 7 and 8 show a concealment condition in accordance with an embodiment of the present invention. Specifically, Figure 7 shows a block or TU which is divided into groups of coefficients (CGs), each coefficient vector corresponding to a group of coefficients. Figure 8 shows an example of a coefficient vector that illustrates the case that the parameter consists of the distance N between the first and last significant coefficients, that is, non-zero within the coefficient vector. If the N value is above a TSIG threshold, then the hiding condition is true and scan order information can be hidden in that coefficient vector.

[0211] Specifically, the verification module 212 may be configured to reconstruct the hidden information of the occultation coefficient vector by applying a verification function to the occultation coefficient vector, a result of the verification function being the hidden information. Also, the hiding module 112 is configured to hide, for at least one block, the SCAN_IDX scan order information in the hiding coefficient vector:

[0212] - applying a check function on the occultation coefficient vector so as to obtain a result of the check function,

[0213] - determining whether the result of the check function matches the SCAN_IDX scan order information,

[0214] - if not, modify the occultation coefficient vector so that a result of the check function matches the SCAN_IDX scan order information.

[0215] It is proposed that the result of the verification function depends on coordinates (x, y), within the matrix of coefficients, of the last significant coefficient of the concealment coefficient vector. The check function could be, for example, the following function: mod( x+y + R ,2) x and y being the coordinates, and R being an integer value.

[0216] For example, the check function may also be one of the following functions or other known check functions:

[0217] - a parity check function applied to the least significant bits of some coefficient selected from the coefficient matrix using a binary pattern, and

[0218] - a parity check function applied to a position index of the last or first non-zero coefficient or to a linear combination of the position index of the last or first non-zero coefficient.

[0219] In this aspect, Figure 9 shows an entropy 901 decoding of coefficients in accordance with the prior art, and specifically in accordance with the H.265 video coding standard. Entropy decoding 901 comprises decoding 902 X and Y coordinates of the last significant, i.e. non-zero, QTC. Then, a mask of significance values of QTCs (significance mask) is flagged 903 according to a scanning order, starting from the last significant QTC. The values and signals are analyzed 904, 905 of the bit stream only for the significant coefficient values.

[0220] With this, a length of the 1D significance mask can be derived from the scan order and coordinates of the last significant QTC. Obviously, it is not enough to just apply known data hiding methods to embed scan order indication data into QTCs, because QTCs are parsed from the bit stream using the scan order information.

[0221] Figure 10 shows an embodiment of the present invention, and specifically an embodiment for improving coding efficiency by reducing signaling overhead in the bit stream 107, 207 generated by the encoder and received by the decoder.

[0222] In the embodiment of Figure 10, a hiding of scanning order within QTCs is possible. During the encoding process, several different scan orders - indexed by SCAN_IDX - are applied during the quantization 1001 and hiding 1002 stages. Quantization procedure 1001 may utilize scanning information to estimate the number of bits (i.e., bit rate ) to encode the QTC values and to perform a rate distortion optimization by adjusting these QTC values with respect to the estimated bitrate.

[0223] If the hiding was unsuccessful 1003, that is, for example if the hiding condition is false, then another SCAN_IDX scan order is selected 1004. Preferably another scanning order is selected until the hiding is successful. If the selected scan is a standard scan order, for example, no masking is performed.

[0224] If hiding is successful or the selected scan order is that of the default, then it is possible to reorder 1005 the QTCs according to the default scan order. The next steps comprise serializing 1006 the QTCs and entropy coding 1007. Said steps 1006 and 1007 may correspond to blocks 104 and 106 of Figure 1. Serialization 1006 may also be inserted between 1001 and 1002 as is the case in Figure 1.

[0225] Hiding 1002 comprises checking the hiding condition, and also modifying the QTCs so that the hiding condition is met. This modification may include: changing the values of QTCs, setting the values of QTCs to zero, and making non-significant (zero-valued) QTCs to be significant (non-zero) ones. Of course, it is always possible to adjust the QTCs so that the concealment condition is met. However, the QTC adjustments on the encoder side are constrained by the distortion incurred by these adjustments, and reasonable adjustment constraints apply. For example, because of the quantization procedure a reasonable coefficient fit is plus or minus one. This constraint makes it possible that the concealment operation may not be performed, ie there were no possible adjustments of QTCs to meet the concealment condition (1003 evaluate to the "no" case for non-default scans).

[0226] On the decoder side, after entropy decoding 1011, hidden information is retrieved 1012 from serialized data, and therefore the hiding stage also requires information about the scanning order that will be used on the decoder side so that, these can be embedded with respect to the selected scan order. The data hiding procedure can also be performed together with quantization, as is done, for example, for Signaling Bit Hiding. However, concealment may fail for some of the QTCs because the concealment condition is not met. For example, the TU has a number of significant QTCs that is less than a threshold. In this case further processing depends on whether a scan order is that standard. If standard scanning was used, the decoder can derive it 1015 by applying the same concealment condition to QTCs analyzed during the entropy decoding stage. Otherwise, the scan order is skipped and the next scan order is being checked if available.

[0227] If default scan order is in effect, reordering the QTC stage can be skipped, because the source and target scans are the same. For the non-standard scanning orders it is proposed to perform a coefficient reordering, so that the entropy encoding and entropy decoding stages can be performed without changes. A specific implementation may use a remapping operation to avoid an unnecessary copy operation for reordering, i.e., redefining a match between 2D QTC array coordinates and 1D position within a selected scan. At the serialization/deserialization stage this remapping can provide CABAC context indices and last significant QTC coordinates for the given 1D scan position as if the QTCs were reordered.

[0228] Figure 11 shows a reordering of coefficients in accordance with the present invention, and specifically shows an example of reordering QTCs for a 4x4 block: diagonal to horizontal scan reordering.

[0229] In this regard, reference is made to the document "Transform Coefficient Coding in HEVC" by Joel Sole et al., IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS FOR VIDEO TECHNOLOGY, VOL. 22, NO. 12, DECEMBER 2012, pages 1765-1777. In Section V.B of this document, it is stated that a significance flag is encoded using a context template for each coefficient between the last in scan order (which is excluded) and the CC coefficient. For a 4 x 4 TB transform block, ie matrix of QTCs, the context depends on the position of the coefficient within the TB, as in H.264/AVC. Despite the fact that an entropy encoding is performed for both serialized and binarized data, a contest model for encoding a significance bit of a QTC is selected depending on the position of this QTC within an array of QTCs, and the significance of spatially neighboring QTCs. (right and bottom). The prior art decoder analyzes the significance bit of the bit stream using a context model, which is selected according to the QTC position for which this bit is being analyzed and the previously decoded significant QTC positions. If the scan order is unavailable in the decoder, the spatial position of QTC is undefined and thus the context model cannot be derived.

[0230] The proposed reordering stage, as shown for example in Figure 11, before the entropy encoder, ensures that the encoder and decoder will use the same scan order and the same context models for the encoded/decoded QTCs.

[0231] Figure 11 specifically describes a case when, after quantization, a matrix of QTCs was obtained (lower left part of this) with the last significant coefficient (for diagonal scanning) located at (x=1, y=2). In case of diagonal scanning 8 bits of coefficient significance must be encoded.

[0232] In order to select another scan order (e.g., by selector 109), e.g., horizontal scanning rather than a diagonal one, it may be beneficial to encode the first two lines of QTC bits plus two QTC bits from the third line (10 total QTCs). However, if these coefficients are reordered from a diagonal to a horizontal scan (as shown at the top of Figure 11), the position of the same last 8 bits of QTC must be encoded.

[0233] For the entropy coding module 106, two scans are available:

[0234] - a pattern, which would be used by the entropy decoding module 206 to analyze the bit stream

[0235] - the scan selected by scan order selector 109.

[0236] When the entropy coding module selects a context model according to the position of QTCs, it not only uses X,Y defined by the scan order selector 109, but recalculates X,Y for the corresponding position of the QTC after it is reordered.

[0237] For example, in the example of Figure 11, it is proposed to consider a case when the scan order selector 109 selected a diagonal scan and the default scan is a horizontal one. The entropy coding module 106 will encode the coefficient placed at position (x=1, y=1) (marked as "E" for diagonal scanning) using the reordered position context module (x0, y=2) (also marked as "E" for horizontal scanning), as well as neighboring coefficients will be defined not as "I" and "H", but as "F" and "I".

[0238] This reordering operation can be implemented as a remapping of X and Y coordinates of a selected scan to that pattern, i.e. to implement this reordering it is not necessary to perform a memory copy/swap operations for QTCs but just provide a lookup table for x and y coordinates for each pair (selected scan, standard scan).

[0239] Data hiding may utilize known methods to embed scan order in QTCs. However, in the present case of scan order concealment, an additional possibility is that a scan order can be specified by a condition applied to coordinates of the last significant QTC (x, y) value, for example: mod(x + y,2) = scan_idx. (C1)

[0240] In this case, the encoder can modify the position of the last significant QTC in order to indicate a scan that should be used by the decoder. The hiding condition for the described hiding method is a set of coordinates specified for a scanning order. In this regard, Figure 12 shows an example of masking conditions defined for scan orders.

[0241] In Figure 12, "X" marks indicate positions that are checked using condition (C1). It is also possible to combine this proposal with data hiding within QTC values. For example, the value retrieved from the last significant QTC position can be used as part of the hiding condition for hiding within values of QTCs.

[0242] It is known that concealment is more efficient if the host and target signals have a universal distribution. Another modality of this invention is the possibility of making the distribution of (C1) closer to universal by randomization. Specifically, an additional source of random noise R can be added: mod(x + y + R,2) = scan_idx.

[0243] The R value should be as uncorrelated with the selected scan order as possible. For example, this may be a number of neighboring TUs, a parity value of a sum of reference sample values, or a total number of encoded bits.

[0244] Thus, the invention proposes, to reduce excess signaling processing, to derive a scanning order before deserialization of QTCs, during entropy decoding. When hiding TU-related data (such as intraprediction mode, filtering flag, etc.) the information being hidden is used after the deserialization and dequantization stages. The invention thus proposes a possibility of using hidden data in QTCs before these coefficients are deserialized.

[0245] The present invention describes a quantization procedure that is applicable to the residuals obtained after the prediction and transform processes used, for example, for coding moving images. The aim of the present invention is to improve the quantization rate distortion efficiency by extending the set of selected available scan orders using different coding characteristics (eg, intraprediction mode, block size, partitioning mode) at the TU level. A specific implementation of this invention is presented as a technique that simultaneously uses mode-dependent and RDO-based scan selection mechanisms.

[0246] The present invention solves problems related to RDO-based selection of scan orders. Specifically, it solves the problem that statistically based adaptive scan orders gathered from previously encoded or decoded blocks do not always provide better coding efficiency with respect to known predefined scan patterns. Therefore, an RDO-based procedure to select only one scan among the generated content-adaptive scan orders and predefined scan patterns is proposed as a solution.

[0247] Specifically, the present invention solves the aforementioned problem of increasing the encoding efficiency of scan order selection in which both predefined scan patterns and adaptively generated scan orders are available.

[0248] The present invention in its multimedia coding modalities further provides the advantage that it can be used in many potential applications in a hybrid video coding structure that are compatible with the Joint Exploration Model (JEM) which is the foundation for the next generation video encoding standard. An additional advantage is the reduced BD rate (Bj0ntegaard Delta Rate) and subjective quality improvements compared to JEM1. An additional advantage is that a computationally complex Adaptive Multiple Transform (AMT) also known as Enhanced Multiple Transform (EMT) can be replaced by a simpler adaptive scan order selection procedure.

[0249] One aspect of the present invention is the combination of predefined scan patterns or orders with adaptively generated scan orders. Furthermore, adding a signaling mechanism to a scan order selection procedure allows you to significantly improve the process of generating adaptive scan orders to increase your coding efficiency.

[0250] The present invention has been described together with various embodiments as examples as well as implementations. However, other variations can be understood and carried out by those persons skilled in the art and who practice the claimed invention, from studies of the drawings, from this description of the independent embodiments. In the embodiments as well as in the description, the word "comprising" does not exclude other elements or stages and the indefinite article "one" or "an" does not exclude a plurality. A single element or other unit can fulfill the functions of several entities or items recited in the concretizations. The mere fact that certain measures are recited in different mutually dependent embodiments does not indicate that a combination of these measures cannot be used in an advantageous implementation.

Claims (35)

1. Decoder (200) for decoding block coefficients of a video sequence from a bit stream (207), the decoder (200) characterized in that it comprises: - a scan pattern list module (210) configured to provide one or more predefined scan orders, - a scan order generator (211) configured to generate one or more scan orders, - a scan order selector (209) configured to select a scan order for each block from the predefined scan orders and generated based on scan order information (SCAN_IDX) contained in the bit stream (207), - a decoding module (206) configured to decode one or more coefficient vectors of each block of the bit stream (207), and - a deserializer (204) configured to reverse scan, for each block, the one or more coefficient vectors of that block in accordance with the scan order selected for that block so as to obtain a matrix of coefficients, wherein the scan order generator (211) is configured to generate the one or more scan orders depending on one or more coefficient matrices previously obtained from blocks of the video sequence. 2. Decoder (200), according to claim 1, characterized by the fact that the scan pattern list module (210) is configured to provide one or more predefined scan orders for each of a plurality of sets of coding parameters, the scan order generator (211) is configured to generate a scanning order for each of the sets of coding parameters depending on one or more previously obtained coefficient matrices associated with that set of coding parameters, the scan order selector (209) is configured to select a scan order for each block by: - determining a set of coding parameters for that block, and - selecting the scan order for the coefficient matrix obtained for that block of the orders predefined and generated scan parameters for the given set of coding parameters. 3. Decoder (200), according to claim 2, characterized by the fact that it comprises: - a coefficient distribution estimator (213) configured to provide, for each of the sets of coding parameters, a coefficient distribution statistic one or more previously obtained coefficient matrices associated with that set of coding parameters, wherein the scan order generator (211) is configured to generate the scan order for each of the sets of coding parameters depending on the distribution statistic for that set of encoding parameters. 4. Decoder (200), according to claim 3, characterized by the fact that, if the selected scanning order for a given block is predefined, the coefficient distribution estimator (213) is configured not to update or is configured to reset the distribution statistics for the given block's set of encoding parameters. 5. Decoder (200), according to any one of claims 2 to 4, characterized by the fact that the decoding module (206) is configured to analyze, for each block, coding parameter information (208) of the data stream. bits (207), and the scan order selector (209) is configured to determine the set of encoding parameters of a given block according to the analyzed encoding parameter information for that given block. 6. Decoder (200), according to any one of claims 1 to 5, characterized by the fact that the decoding module (206) is configured to analyze the scan order information (SCAN_IDX) identifying the selected scan order of the bit stream (207). 7. Method for decoding block coefficients of a video sequence from a bit stream (207), the method characterized by the fact that it comprises the steps of: - providing one or more predefined scan orders, - generating one or more orders - select a scan order for each block from the predefined scan orders generated based on scan order information (SCAN_IDX) contained in the bit stream (207), - decode one or more scan coefficient vectors each block of the bit stream (207), and - inverse scanning, for each block, of the one or more coefficient vectors of that block in accordance with the scan order selected for that block in order to obtain a matrix of coefficients, in which generating one or more scan orders comprises generating the one or more scan orders depending on one or more coefficient matrices previously obtained from blocks of the video sequence. 8. Encoder (100) for encoding block coefficients of a video sequence into a bit stream (107), the coefficients of each block are contained in a respective coefficient matrix, the encoder (100) characterized by the fact that it comprises : - a scan pattern list module (110) configured to provide one or more predefined scan orders, - a scan order generator (111) configured to generate one or more scan orders, - a scan order selector scan (109) configured to select a scan order for each block from the predefined and generated scan orders, - a serializer (104) configured to scan, for each block, the coefficient matrix of that block according to the order of scan selected for that block to obtain one or more coefficient vectors, and - an encoding module (106) configured to encode the coefficient vectors within the bit stream (107), wherein the scan order generator ( 111) is configured to generate the one or more scan orders depending on one or more previously scanned coefficient matrices of blocks of the video sequence. 9. Encoder (100), according to claim 8, characterized by the fact that the scan pattern list module (110) is configured to provide one or more predefined scan orders for each of a plurality of sets of coding parameters, the scan order generator (111) is configured to generate a scan order for each of the coding parameter sets depending on one or more previously scanned coefficient matrices associated with that coding parameter set, the scan order selector (109) is configured to select a scan order for each block by: - determining a set of coding parameters for that block, and - selecting the scan order for that block's coefficient matrix from the orders predefined and generated scan parameters for the given set of coding parameters. 10. Encoder (100), according to claim 9, characterized by the fact that it comprises: - a coefficient distribution estimator (113) configured to provide, for each of the sets of coding parameters, a distribution statistic of one or more previously scanned coefficient matrices associated with that set of coding parameters, wherein the scan order generator (111) is configured to generate the scan order for each of the sets of coding parameters depending on the distribution statistic for that set of encoding parameters. 11. Encoder (100), according to claim 10, characterized by the fact that, if the selected scanning order for a given matrix of coefficients is predefined, the coefficient distribution estimator (113) is configured not to update or is configured to reset the distribution statistics for the given block's set of encoding parameters. 12. Encoder (100), according to any one of claims 8 to 11, characterized in that it comprises a signaling module configured to add scan order information (SCAN_IDX) identifying the selected scan order in the bit stream ( 107). 13. Encoder (100), according to any one of claims 8 to 12, characterized by the fact that the scan order selector (109) is configured to select the scan order from the predefined scan orders and generated with based on a cost function. 14. Method for encoding block coefficients of a video sequence into a bit stream (107), characterized by the fact that the coefficients of each block are contained in a respective coefficient matrix, and the method comprises the steps of: - provide one or more predefined scan orders, - generate one or more scan orders, - select a scan order for each block from the predefined and generated scan orders, - scan, for each block, the coefficients of that block accordingly with the scan order selected for that block so as to obtain one or more coefficient vectors, and - encoding the coefficient vectors within the bit stream (107), wherein generating one or more scan orders comprises generating the one or more further scan orders depending on one or more previously scanned coefficient matrices of blocks of the video sequence. 15. Decoder (200) for decoding block coefficients of a video sequence from a bit stream (207), the decoder (200) characterized by the fact that it comprises: - a decoding module (206) configured to decode one or plus coefficient vectors of each block of the bit stream (207), - a verification module (212) configured to reconstruct, for at least one block, hidden information from one or more coefficients of that block, the hidden information constituting or forming part of scan order information (SCAN_IDX) associated with that block, - a scan order selector (209) configured to select a scan order for each block from a set of scan orders comprising predefined scan orders and generated, based on the scan order information (SCAN_IDX) associated with that block, and - a deserializer (204) configured to reverse scan, for each block, the one or more coefficient vectors of that block according to the scan order selected for that block in order to obtain a matrix of coefficients. 16. Decoder (200), according to claim 15, characterized by the fact that the verification module (212) is configured to reconstruct, for at least one block, the hidden information of at least one chosen hiding coefficient vector among the one or more coefficient vectors obtained for that block. 17. Decoder (200), according to claim 16, characterized by the fact that the verification module (212) is configured to reconstruct, for at least one block, the hidden information of the hiding coefficient vector depending on a condition of concealment, the concealment condition comprising a comparison of a parameter depending on the concealment coefficient vector with a threshold. 18. Decoder (200), according to claim 17, characterized by the fact that the parameter is: - a number of significant coefficients within the hiding coefficient vector, or - a distance between first and last significant coefficients within the vector of hiding coefficient, or - a ratio of a maximum absolute value to a non-zero minimum absolute value of the coefficients within the hiding coefficient vector, where the hiding condition is true if the parameter is above the threshold, and false if the parameter is below the limit. 19. Decoder (200), according to claim 17 or 18, characterized by the fact that the verification module (212) is configured to check the occultation condition, and reconstruct the hidden information of the occultation coefficient vector if the hiding condition is true. 20. Decoder (200), according to any one of claims 17 to 19, characterized by the fact that, if the hiding condition is false, the scan order selector (209) is configured to select a default scan order preset. 21. Decoder (200), according to any one of claims 16 to 20, characterized by the fact that the verification module (212) is configured to reconstruct the hidden information of the hiding coefficient vector by applying a verification function to the hiding coefficient vector, a result of the verification function being the hidden information. 22. Decoder (200), according to claim 21, characterized by the fact that the result of the verification function depends on coordinates (x, y), within the coefficient matrix, of the last significant coefficient of the occultation coefficient vector , specifically the check function being the following function: mod( x+y + R ,2) x and y being the coordinates, and R being an integer value. 23. Decoder (200), according to any one of claims 15 to 22, characterized in that the verification module (212) is configured to reconstruct, for a first block, hidden information of one or more coefficients of the first block , the hidden information constituting or forming part of scan order information (SCAN_IDX) associated with the first block, and the scan order selector (209) is configured to select, for the first and at least one second block, an order based on the scan order information (SCAN_IDX) associated with the first block. 24. Method for decoding block coefficients of a video sequence from a bit stream (207), the method characterized by the fact that it comprises the steps of: - decoding one or more coefficient vectors of each block of the bit stream ( 207), - reconstruct, for at least one block, hidden information of one or more coefficients of that block, the hidden information constituting or forming part of scan order information (SCAN_IDX) associated with that block, - select a scan order for each block of a set of scan orders comprising predefined and generated scan orders, based on the scan order information (SCAN_IDX) associated with that block, and - inverse scanning, for each block, of the one or more coefficient vectors of that block according to the scanning order selected for that block, to obtain a matrix of coefficients. 25. Encoder (100) for encoding block coefficients of a video sequence into a bit stream (107), characterized by the fact that the coefficients of each block are contained in a respective coefficient matrix, and the encoder (100) comprises: - a scan order selector (109) configured to select a scan order for each block from a set of scan orders comprising predefined and generated scan orders, - a serializer (104) configured for scanning, for each block, of the coefficient matrix of that block in accordance with the scan order selected for that block so as to obtain one or more coefficient vectors, - a hiding module (112) configured to hide, for at least one block, scan order information (SCAN_IDX) identifying the selected scan order for that block at least partially in one or more coefficients of that block, and - an encoding module (106) configured to encode the coefficient vectors within the bit stream ( 107). 26. Encoder (100), according to claim 25, characterized by the fact that the hiding module (112) is configured to hide, for at least one block, the scan order information (SCAN_IDX) in at least one occultation coefficient vector chosen from the one or more coefficient vectors obtained for that block. 27. Encoder (100), according to claim 26, characterized by the fact that the hiding module (112) is configured to hide, for at least one block, the scan order information (SCAN_IDX) in the coefficient vector of concealment depending on a concealment condition, the concealment condition comprising a comparison of a parameter depending on the concealment coefficient vector to a threshold. 28. Encoder (100), according to claim 27, characterized by the fact that the parameter is: - a number of significant coefficients within the hiding coefficient vector, or - a distance between first and last significant coefficients within the vector of hiding coefficient, or - a ratio of a maximum absolute value to a non-zero minimum absolute value of the coefficients within the hiding coefficient vector, where the hiding condition is true if the parameter is above the threshold, and false if the parameter is below the limit. 29. Encoder (100), according to claim 27 or 28, characterized by the fact that the hiding module (112) is configured to check the hiding condition, and to hide the scan order information (SCAN_IDX) in the hiding coefficient vector if the hiding condition is true. 30. Encoder (100), according to any one of claims 27 to 29, characterized by the fact that, if the hiding condition is false, the scan order selector (109) is configured to recursively select another order scan order from the set of scan orders until the hiding condition for that other selected scan order is true. 31. Encoder (100), according to any one of claims 27 to 30, characterized by the fact that, if the hiding condition is false and the selected scanning order is a predefined standard scanning order, the hiding module ( 112) is configured to not hide the scan order information (SCAN_IDX) in the hiding coefficient vector. 32. Encoder (100), according to any one of claims 26 to 31, characterized in that the hiding module (112) is configured to hide, for at least one block, the scanning order information (SCAN_IDX) on the occultation coefficient vector by: - applying a scan function on the occultation coefficient vector in order to obtain a result of the scan function, - determining whether the result of the scan function matches the scan order information (SCAN_IDX) , and - if not, modify the hiding coefficient vector so that a result of the scan function matches the scan order information (SCAN_IDX). 33. Encoder (100), according to claim 32, characterized by the fact that the result of the verification function depends on coordinates (x, y), within the coefficient matrix, of the last significant coefficient of the occultation coefficient vector , specifically the check function being the following function: mod( x+y + R ,2) x and y being the coordinates, and R being an integer value. 34. Encoder (100), according to any one of claims 25 to 33, characterized by the fact that, if an identical scanning order is selected by the scanning order selector (109) for a first and at least a second block , the hiding module (112) is configured to hide scan order information (SCAN_IDX) by identifying the identical scan order in one or more coefficients of the first block only. 35. Method for encoding block coefficients of a video sequence into a bit stream (107), characterized by the fact that the coefficients of each block are contained in a respective coefficient matrix, and the method comprises the steps of: - selecting a scan order for each block from a set of scan orders comprising predefined and generated scan orders, - scanning, for each block, the coefficient matrix of that block according to the scan order selected for that mode block to obtain one or more coefficient vectors, - hide, for at least one block, scan order information (SCAN_IDX) identifying the selected scan order for that block at least partially in one or more coefficients of that block, and - encode the coefficient vectors within the bitstream (107).