Patent Description:
All relevant video coding standards, like AVC/H. <NUM> or HEVC/H. <NUM>, follow the so-called hybrid approach as shown, for example, in <FIG>, where predictive coding is combined with transform coding of the prediction residual. For generating the prediction signal, two possible modes are supported by these standards, namely INTRA prediction and INTER prediction. <NUM>, the decision between these two modes can be made at macroblock (16x16 luma samples) level, and in HEVC/H. <NUM> at Coding Unit (CU) level, which can be of varying size. In INTRA prediction, sample values of already reconstructed neighboring blocks of the current block can be used for generating the prediction signal. How this INTRA prediction signal is formed from the neighboring reconstructed sample values, is specified by the INTRA prediction mode. In INTER prediction, already reconstructed frames (in coding order) can be used for generating the prediction signal. For INTER prediction, in both AVC/H. <NUM> and HEVC/H. <NUM>, either uni or bi prediction is used. For uni prediction, the prediction signal is a shifted and interpolated region of a so-called reference picture. The used reference picture is specified by the reference index and the location of the (possibly interpolated) region within the reference picture is specified (relatively to the current block) by the motion vector. The motion vector itself is predictively encoded relatively to a motion vector predictor, such that only the motion vector difference has to be actually encoded. <NUM>, the motion vector predictor is selected by transmitting a motion vector predictor index. In both AVC/H. <NUM> and HEVC/H. <NUM>, motion vectors can be specified with an accuracy of a quarter pel (qpel). The process of generating such an (interpolated) prediction signal is also called motion-compensated prediction. In bi prediction, two motion-compensated prediction signals are linearly superposed (typically using a factor of <NUM> for both constituent prediction signals). Therefore, for bi-prediction two reference indices and motion vector differences (and motion vector predictor indices, in HEVC/H. <NUM>) have to be transmitted.

In order to simplify the encoding of contiguous areas having the same motion characteristics, HEVC/H. <NUM> supports the so-called MERGE mode, where prediction parameters (i.e., reference indices and motion vectors) of either locally neighboring or temporally co-located blocks can be re-used for the current block. The SKIP mode of HEVC/H. <NUM> is a particular case of MERGE, where no prediction residual is transmitted.

As an example, an intra-picture estimation/prediction of the hybrid video codec of <FIG> based on rectangular blocks of size, e.g., W x H as shown <FIG> performs as explained below.

The sample values of the current block <NUM> are predicted using reconstructed sample values of previous coded blocks in the same picture, i.e., reference samples <NUM>. Usually, reference samples <NUM> are adjacent samples left (side) and above of the current block <NUM>.

Prediction modes describe different processes for calculating/deriving the predicted sample value at a specific position in the current block <NUM> from sample values of one or more reference samples <NUM>, for instance Planar, DC and Angular prediction modes in HEVC. These modes can be grouped into those for homogeneous regions (Planar and DC prediction modes) and those for directional structures (Angular prediction mode). The intra encoding process consists of choosing or estimating the intra prediction mode, which is transmitted to the decoder in the data stream.

The angular prediction process consists of continuing/copying the reference samples <NUM> into the current block <NUM> with a given direction or angle. <FIG> shows an example of the different supported directions. These are grouped into two categories, namely horizontal (H) and vertical (V) angular prediction, where horizontal mainly uses the left (H) and vertical mainly uses the above (V) reference samples <NUM>.

The samples of the block are predicted in a line-by-line order. Therefore, rows and columns are swapped for horizontal angular prediction. Now, the line of reference samples <NUM> is always on top of the block, such that every line can be filled with a shifted version of the reference samples. The shift or offset value is determined by the prediction direction.

As shown in <FIG>, for the sample, i.e., the area to be predicted, at position (x, y) the corresponding reference sample position is <MAT> therefore the value of sample (x, y) results from interpolating two (or more) reference sample values.

The development of modern video codecs tends towards larger block sizes in combination with non-square (e.g., rectangular) block shapes. Regarding intra-picture prediction and especially the angular prediction mode described above, this might be inefficient. A close prediction of the real signal of a larger block is only possible, if the signal continues linearly in the exact direction of the angular mode over quite a few numbers of rows/columns without any larger deviations.

One obvious solution of this problem is to split the block into smaller blocks, each with an individually adapted angular prediction direction and each with an individual residuum as shown in <FIG>. However, this solution tends to result in higher signaling costs and smaller instead of larger blocks, and therefore, as a result it causes decreasing the coding efficiency.

In addition, <CIT> discloses an effective wedgelet partition coding using spatial prediction, i.e., reconstructed values of blocks preceding the current block in accordance with the coding/decoding order allow for at least a prediction of a correct placement of a starting point of the wedgelet separation line, namely by placing the starting point of the wedgelet separation line at a position of a maximum change between consecutive ones of a sequence of reconstructed values of samples of a line of samples extending adjacent to the current block along a circumference thereof. Further, a document, <NPL>" discloses a concept regarding intra prediction tools including mode coding, i.e., usage of angular intra prediction modes and offsets for coding.

As mentioned above, the available and supported prediction modes of now a days video codecs are already pretty effective in terms of keeping the prediction residual low at a reasonable amount of prediction side information necessary in order to control the prediction using these prediction modes. However, there are some deficiencies to further improve the coding efficiency using the available prediction modes, and therefore, it would be favorable to further increase the coding efficiency of block-based predictive video codecs.

It is, thus, the object of the subject-matter of the present application to provide a video codec using block-based predictive coding which has an improved coding efficiency.

This object is achieved by the subject-matter of the claims of the present application.

It is basic idea underlying the subject-matter of the present application that a coding efficiency increase is achievable by applying a prediction mode to predict a number of blocks of a picture by extrapolating a neighborhood of the respective block into the block along a direction which varies across the respective block. That is, according to the prediction mode of the present application, i.e., a contour prediction mode, the prediction direction for the current block to be predicted is changed based on the sample block which has been previously predicted. That means, it is possible to change the prediction direction direction for the current block to be predicted is changed based on the sample block which has been previously predicted. That means, it is possible to change the prediction direction within the same block between the lines of the samples of the block is composed of, and therefore, a larger block is predicted more preciously with less prediction error. The side information, in turn, may be kept low and, additionally, large block size (a coarse block granularly), is feasible, so that it is possible to reduce signaling costs even further and as a result improve the coding efficiency.

In accordance with the embodiments of the present application, the direction across the respective block in unit of lines of samples the respective block is composed of, or in unit of groups of lines of samples into which the block is partitioned, is varied based on one or more offset values indicating an offset of sample positions at which the neighborhood is to be sampled for predicting a predetermined line or predetermined group of lines relative to further sample positions at which the neighborhood is to be sampled for predicting a further line or further group of lines adjacent to the predetermined line or predetermined group of lines. That is, the prediction direction is changed for each line of samples (e.g., the line of blocks to be predicted) or each of groups of lines of samples (the group of lines of samples to be predicted), based on the determined offset value. Therefore, it is possible to improve the predict efficiency for a larger block.

In accordance with the embodiments of the present application, the offset value for each line of samples or each group of lines is determined by reading a parameter indexing one of a global offset value for the respective block, a first predetermined offset value, or a second predetermined offset value, wherein the first predetermined offset value and the second predetermined offset value indicate offsets of the sample positions relative to the further sample positions into opposite directions. For example, the offset values may be typically expressed as ΔxC(n) ∈ [-<NUM>. <NUM>] with <MAT>. In this case, the first predetermined offset value could be -<NUM> and the second predetermined offset value could be +<NUM> (or vice versa), and then, the global offset value is between the first and the second predetermined offset value. Thus, the offset value is determined by reading parameter indexing and therefore, it is possible to efficiently predict non-liner contour and as a result the coding efficiency is also improved.

Further advantages are achieved by the codec of the claims. Preferred embodiments of the present application are described below with respect to the figures, amongst which:.

The following description of the figures starts with a presentation of a description of video encoder and video decoder of a block-based predictive codec for coding pictures of a video in order to form an example for a coding framework into which embodiments for a composed prediction codec may be built in. The video encoder and video decoder are described with respect to <FIG>. Thereinafter the description of embodiments of the concept of the prediction mode of the present application are presented along with a description as to how such concepts could be built into the video encoder and decoder of <FIG> and <FIG>, respectively, although the embodiments described with the subsequent <FIG> and following, may also be used to form video encoder and video decoders not operating according to the coding framework underlying the video encoder and video decoder of <FIG> and <FIG>.

<FIG> shows an apparatus for predictively coding a video <NUM> composed of a sequence of pictures <NUM> into a data stream <NUM>. Block-wise predictive coding is used to this end. Further, transform-based residual coding is exemplarily used. The apparatus, or encoder, is indicated using reference sign <NUM>. <FIG> shows a corresponding decoder <NUM>, i.e. an apparatus <NUM> configured to predictively decode the video <NUM>' composed of pictures <NUM>' in picture blocks from the data stream <NUM>, also here exemplarily using transform-based residual decoding, wherein the apostrophe has been used to indicate that the pictures <NUM>' and video <NUM>', respectively, as reconstructed by decoder <NUM> deviate from pictures <NUM> originally encoded by apparatus <NUM> in terms of coding loss introduced by a quantization of the prediction residual signal. <FIG> and <FIG> exemplarily use transform based prediction residual coding, although embodiments of the present application are not restricted to this kind of prediction residual coding. This is true for other details described with respect to <FIG> and <FIG>, too, as will be outlined hereinafter.

The encoder <NUM> is configured to subject the prediction residual signal to spatial-to-spectral transformation and to encode the prediction residual signal, thus obtained, into the data stream <NUM>. Likewise, the decoder <NUM> is configured to decode the prediction residual signal from the data stream <NUM> and subject the prediction residual signal thus obtained to spectral-to-spatial transformation.

Internally, the encoder <NUM> may comprise a prediction residual signal former <NUM> which generates a prediction residual <NUM> so as to measure a deviation of a prediction signal <NUM> from the original signal, i.e. video <NUM> or a current picture <NUM>. The prediction residual signal former <NUM> may, for instance, be a subtractor which subtracts the prediction signal from the original signal, i.e. current picture <NUM>. The encoder <NUM> then further comprises a transformer <NUM> which subjects the prediction residual signal <NUM> to a spatial-to-spectral transformation to obtain a spectral-domain prediction residual signal <NUM>' which is then subject to quantization by a quantizer <NUM>, also comprised by encoder <NUM>. The thus quantized prediction residual signal <NUM>" is coded into data stream <NUM>. To this end, encoder <NUM> may optionally comprise an entropy coder <NUM> which entropy codes the prediction residual signal as transformed and quantized into data stream <NUM>. The prediction signal <NUM> is generated by a prediction stage <NUM> of encoder <NUM> on the basis of the prediction residual signal <NUM>" decoded into and decodable from, data stream <NUM>. To this end, the prediction stage <NUM> may internally, as is shown in <FIG>, comprise a dequantizer <NUM> which dequantizes prediction residual signal <NUM>" so as to gain spectral-domain prediction residual signal <NUM>"', which corresponds to signal <NUM>' except for quantization loss, followed by an inverse transformer <NUM> which subjects the latter prediction residual signal <NUM>‴ to an inverse transformation, i.e. a spectral-to-spatial transformation, to obtain prediction residual signal 24ʺʺ, which corresponds to the original prediction residual signal <NUM> except for quantization loss. A combiner <NUM> of the prediction stage <NUM> then recombines, such as by addition, the prediction signal <NUM> and the prediction residual signal 24ʺʺ so as to obtain a reconstructed signal <NUM>, i.e. a reconstruction of the original signal <NUM>. Reconstructed signal <NUM> may correspond to signal <NUM>'.

A prediction module <NUM> of prediction stage <NUM> then generates the prediction signal <NUM> on the basis of signal <NUM> by using, for instance, spatial prediction, i.e. intra prediction, and/or temporal prediction, i.e. inter prediction. Details in this regard are described in the following.

Likewise, decoder <NUM> may be internally composed of components corresponding to, and interconnected in a manner corresponding to, prediction stage <NUM>. In particular, entropy decoder <NUM> of decoder <NUM> may entropy decode the quantized spectral-domain prediction residual signal <NUM>" from the data stream, whereupon dequantizer <NUM>, inverse transformer <NUM>, combiner <NUM> and prediction module <NUM>, interconnected and cooperating in the manner described above with respect to the modules of prediction stage <NUM>, recover the reconstructed signal on the basis of prediction residual signal <NUM>" so that, as shown in <FIG>, the output of combiner <NUM> results in the reconstructed signal, namely the video <NUM>'or a current picture <NUM>' thereof.

Although not specifically described above, it is readily clear that the encoder <NUM> may set some coding parameters including, for instance, prediction modes, motion parameters and the like, according to some optimization scheme such as, for instance, in a manner optimizing some rate and distortion related criterion, i.e. coding cost, and/or using some rate control. As described in more details below, encoder <NUM> and decoder <NUM> and the corresponding modules <NUM>, <NUM>, respectively, support different prediction modes such as intra-coding modes and inter-coding modes which form a kind of set or pool of primitive prediction modes based on which the predictions of picture blocks are composed in a manner described in more detail below. The granularity at which encoder and decoder switch between these prediction compositions may correspond to a subdivision of the pictures <NUM> and <NUM>', respectively, into blocks. Note that some of these blocks may be blocks being solely intra-coded and some blocks may be blocks solely being inter-coded and, optionally, even further blocks may be blocks obtained using both intra-coding and inter-coding, but details are set-out hereinafter. According to intra-coding mode, a prediction signal for a block is obtained on the basis of a spatial, already coded/decoded neighborhood of the respective block. Several intra-coding sub-modes may exist the selection among which, quasi, represents a kind of intra prediction parameter. There may be directional or angular intra-coding sub-modes according to which the prediction signal for the respective block is filled by extrapolating the sample values of the neighborhood along a certain direction which is specific for the respective directional intra-coding sub-mode, into the respective block. The intra-coding sub-modes may, for instance, also comprise one or more further sub-modes such as a DC coding mode, according to which the prediction signal for the respective block assigns a DC value to all samples within the respective block, and/or a planar intra-coding mode according to which the prediction signal of the respective block is approximated or determined to be a spatial distribution of sample values described by a two-dimensional linear function over the sample positions of the respective block with deriving tilt and offset of the plane defined by the two-dimensional linear function on the basis of the neighboring samples. Compared thereto, according to inter-prediction mode, a prediction signal for a block may be obtained, for instance, by temporally predicting the block inner. For parametrization of an inter-prediction mode, motion vectors may be signaled within the data stream, the motion vectors indicating the spatial displacement of the portion of a previously coded picture of the video <NUM> at which the previously coded/decoded picture is sampled in order to obtain the prediction signal for the respective block. This means, in addition to the residual signal coding comprised by data stream <NUM>, such as the entropy-coded transform coefficient levels representing the quantized spectral-domain prediction residual signal <NUM>", data stream <NUM> may have encoded thereinto prediction related parameters for assigning to the blocks prediction modes, prediction parameters for the assigned prediction modes, such as motion parameters for inter-prediction modes, and, optionally, further parameters which control a composition of the final prediction signal for the blocks using the assigned prediction modes and prediction parameters as will be outlined in more detail below. Additionally, the data stream may comprise parameters controlling and signaling the subdivision of picture <NUM> and <NUM>', respectively, into the blocks. The decoder <NUM> uses these parameters to subdivide the picture in the same manner as the encoder did, to assign the same prediction modes and parameters to the blocks, and to perform the same prediction to result in the same prediction signal.

<FIG> illustrates the relationship between the reconstructed signal, i.e. the reconstructed picture <NUM>', on the one hand, and the combination of the prediction residual signal 24ʺʺ as signaled in the data stream, and the prediction signal <NUM>, on the other hand. As already denoted above, the combination may be an addition. The prediction signal <NUM> is illustrated in <FIG> as a subdivision of the picture area into blocks <NUM> of varying size, although this is merely an example. The subdivision may be any subdivision, such as a regular subdivision of the picture area into rows and columns of blocks, or a multi-tree subdivision of picture <NUM> into leaf blocks of varying size, such as a quadtree subdivision or the like, wherein a mixture thereof is illustrated in <FIG> where the picture area is firstly subdivided into rows and columns of tree-root blocks which are then further subdivided in accordance with a recursive multi-tree subdivisioning to result into blocks <NUM>.

The prediction residual signal 24ʺʺ in <FIG> is also illustrated as a subdivision of the picture area into blocks <NUM>. These blocks might be called transform blocks in order to distinguish same from the coding blocks <NUM>. In effect, <FIG> illustrates that encoder <NUM> and decoder <NUM> may use two different subdivisions of picture <NUM> and picture <NUM>', respectively, into blocks, namely one subdivisioning into coding blocks <NUM> and another subdivision into blocks <NUM>. Both subdivisions might be the same, i.e. each block <NUM>, may concurrently form a transform block <NUM> and vice versa, but <FIG> illustrates the case where, for instance, a subdivision into transform blocks <NUM> forms an extension of the subdivision into blocks <NUM> so that any border between two blocks <NUM> overlays a border between two blocks <NUM>, or alternatively speaking each block <NUM> either coincides with one of the transform blocks <NUM> or coincides with a cluster of transform blocks <NUM>. However, the subdivisions may also be determined or selected independent from each other so that transform blocks <NUM> could alternatively cross block borders between blocks <NUM>. As far as the subdivision into transform blocks <NUM> is concerned, similar statements are thus true as those brought forward with respect to the subdivision into blocks <NUM>, i.e. the blocks <NUM> may be the result of a regular subdivision of picture area into blocks, arranged in rows and columns, the result of a recursive multi-tree subdivisioning of the picture area, or a combination thereof or any other sort of segmentation. Just as an aside, it is noted that blocks <NUM> and <NUM> are not restricted to being quadratic, rectangular or any other shape. Further, the subdivision of a current picture <NUM> into blocks <NUM> at which the prediction signal is formed, and the subdivision of a current picture <NUM> into blocks <NUM> at which the prediction residual is coded, may not the only subdivision used for coding/decoding. These subdivision from a granularity at which prediction signal determination and residual coding is performed, but firstly, the residual coding may alternatively be done without subdivisioning, and secondly, at other granularities than these subdivisions, encoder and decoder may set certain coding parameters which might include some of the aforementioned parameters such as prediction parameters, prediction signal composition control signals and the like.

<FIG> illustrates that the combination of the prediction signal <NUM> and the prediction residual signal 24ʺʺ directly results in the reconstructed signal <NUM>'. However, it should be noted that more than one prediction signal <NUM> may be combined with the prediction residual signal 24ʺʺ to result into picture <NUM>' in accordance with alternative embodiments such as prediction signals obtained from other views or from other coding layers which are coded/decoded in a separate prediction loop with separate DPB, for instance.

In <FIG>, the transform blocks <NUM> shall have the following significance. Transformer <NUM> and inverse transformer <NUM> perform their transformations in units of these transform blocks <NUM>. For instance, many codecs use some sort of DST or DCT for all transform blocks <NUM>. Some codecs allow for skipping the transformation so that, for some of the transform blocks <NUM>, the prediction residual signal is coded in the spatial domain directly. However, in accordance with embodiments described below, encoder <NUM> and decoder <NUM> are configured in such a manner that they support several transforms. For example, the transforms supported by encoder <NUM> and decoder <NUM> could comprise:.

Naturally, while transformer <NUM> would support all of the forward transform versions of these transforms, the decoder <NUM> or inverse transformer <NUM> would support the corresponding backward or inverse versions thereof:.

In any case, it should be noted that the set of supported transforms may comprise merely one transform such as one spectral-to-spatial or spatial-to-spectral transform.

As already outlined above, <FIG> have been presented as an example where the composed-prediction concept described further below may be implemented in order to form specific examples for video encoders and decoders according to the present application. Insofar, the video encoder and decoder of <FIG> and <FIG>, respectively, represent possible implementations of the video encoders and decoders described herein below. As will be outlined in more detail below, when having the subsequently explained embodiments for composed prediction according to the present application built into the video encoder and decoder of <FIG> and <FIG>, the video encoder of <FIG> and the video decoder of <FIG> support, at least as one option, to process a block <NUM> in the manner outlined in more detail below, or even all blocks a current picture <NUM> is composed of. Thus, the embodiments described hereinafter, inter alias, refer to a video encoder which equals the encoder <NUM> of <FIG> which treats blocks <NUM> in the manner outlined in more detail below and the same applies with respect to the decoder of <FIG> which, thus, represents an example for a video decoder according to an embodiment where blocks <NUM> are treated in the manner outlined in more detail below. <FIG> and <FIG> are, however, only specific examples. A video encoder according to embodiments of the present application may, however, perform block-based encoding using the concept outlined in more detail below and being different from the encoder of <FIG> such as, for instance, in that the subdivision into blocks <NUM> is performed in a manner different than exemplified in <FIG>, or in that this encoder does not use transform prediction residual coding with coding the prediction residual, for instance, in spatial domain directly instead. Likewise, video decoders according to embodiments of the present application may perform decoding from data stream <NUM> using the composed-prediction coding concept further outlined below, but may differ, for instance, from the decoder <NUM> of <FIG> in that sub-divides picture <NUM>' into blocks in a manner different than described with respect to <FIG> and/or in that same does not derive the prediction residual from the data stream <NUM> in transform domain, but in spatial domain, for instance.

In particular, with respect to the block-subdivisioning into blocks <NUM>, it is noted that same may be done in the manner outlined with respect to <FIG> or in a different manner. A subdivisioning into transform blocks, if present, may also be done as described with respect to <FIG> or in a different manner. In particular, the subdivisioning into blocks on the one hand and into other blocks on the other hand, such as transform blocks, may be done independent from each other by separately subdividing picture <NUM> into these blocks, respectively, or in a dependent manner. For instance, one subdivision such as the subdivision into transform blocks may form an extension of the other subdivision as described above, or both subdivisions may form separate extensions of a common primary subdivision such as, for instance, the subdivision of the picture into an array of tree root blocks as described with respect to <FIG>. And such possibilities also apply for other sub-picture granularities which will be mentioned below such as with respect to the definition of certain prediction parameters, prediction modes, contribution weights or the like. Different subdivisions may be used for different ones of these entities and same may be defined independent from each other, partially independent or as extensions from one another.

As already explained, it is known that the samples of the block are predicted in a line-by-line order and therefore, rows and columns are swapped for horizontal angular prediction. The prediction mode of the present application, i.e., the contour prediction mode also needs to distinguish between horizontal and vertical contour patterns. Thus, the outline of the prediction process is similar to angular prediction, with swapping rows and columns for horizontal contour patterns and filling the block with shifted reference samples in a line-by-line order.

Contrary to the angular mode (i.e., with constant offset α), the contour mode, i.e., the prediction mode according to the present application, requires a sequence of offsets with one individual value α(y) per line, i.e., the offset value is defined for each line of the samples or each group of the lines of the samples. The length of the sequence corresponds to the height of the block for vertical and to the width for horizontal contour mode, respectively. For the sample at position (x, y) the corresponding reference sample position is defined as: <MAT> with offsets α(n) ∈ [-<NUM>,<NUM>].

This is the most general representation of the contour sequence and all alternative representations in the context of below explained offset variations can be converted to this general representation.

For coding efficiency and implementation reasons, the properties of the contour sequence can be constraint as follows:.

Regarding the prediction process, <FIG> illustrates one important difference between the angular mode and the contour mode, namely how side reference samples are accessed during line-by-line prediction, i.e., after swap for horizontal modes, reference samples <NUM> above the block <NUM> are referred to main and those left of the block to side reference samples <NUM>, respectively. That is, as shown in <FIG>, in the angular mode, reference samples <NUM> above the block are referred to samples (in the line of blocks <NUM> to be predicted). Contrary to this, as shown <FIG>, in the contour mode, both reference samples <NUM> in the above and the side of the block <NUM> are referred to samples to be predicted.

In detail, for the angular prediction as shown in <FIG>, the line of main reference samples <NUM> is extended to the left by projecting side reference samples <NUM> to the corresponding position using the direction or angle determined by offset α. Consequently, the prediction process does not have to distinguish between positive and negative reference sample positions (xr can become negative if α < <NUM>).

For the contour prediction, as shown in <FIG>, the line of main reference samples <NUM> cannot be extended by projected side reference samples <NUM>. Due to the non-linear contour sequence, there is no bijective correspondence between side and main reference sample positions, i.e., two different side reference positions map to the same main reference position or vice versa. Therefore, the contour mode's prediction process has to distinguish between positive and negative reference sample positions <NUM>. In case xr < <NUM> the side reference position yr is derived and used for prediction. Note that this would also work for the angular prediction by not extending the main reference samples, but directly using side reference position <MAT> in case xr < <NUM>. For the contour prediction, however, deriving yr is not directly possible as α(y) changes from line to line. One solution is to step backwards in the contour sequence until ∑n≤y α(n) = -x and subtract the number of steps from y. For a higher precision, a fractional last step can be applied in case a full step would lead to a value < -x. Another solution is to use the average slope of the contour sequence up to the current line as <MAT>.

Regarding contour derivation and signaling, the contour mode can either.

In case the contour sequence is estimated at the encoder <NUM>, the original signal can be used as a reference for rate-distortion RD optimization. This allows selecting the contour sequence with the lowest RD cost, but the information has to be included in the data stream. Thus, the challenging aspect is efficient signaling of the contour sequence, i.e. restricting and reducing the amount of transmitted information as much as possible. This can be achieved by applying the constraints using the integer step, multiline, relative offset or list index as explained above together with codes that are adapted to the data statistics (fixed length, truncated unary, etc.) and CABAC (Context Adaptive Binary Arithmetic Coding).

In case the contour sequence is predicted at encoder <NUM> and decoder <NUM>, no information has to be included in the data stream <NUM>, but the original signal cannot be used as a reference. For predicting the contour sequence from information that is also available at the decoder <NUM>, two types of information can be used: one is structural information like block <NUM> partitioning and angular mode direction, and the other is the reconstructed signal in the vicinity of the current block <NUM>. For the latter the area left of the side reference samples can be used as a template block <NUM>, where the reconstructed signal serves as the reference for a minimum distortion derivation of the contour sequence, which is then used to predict the current block <NUM>. However, this assumes that the texture of the current block <NUM> has the same contour than the template block.

It should be also noted that the contour mode according to the present application is implemented as an extension to the conventional angular mode and can be applied as an optional feature to all intra luma blocks. The contour sequence is estimated at the encoder <NUM>, transmitted in the data stream and applied as part of the intra prediction/reconstruction process at the decoder <NUM>.

For example, the contour mode according to the present application is embodied as follows. The contour sequence is derived by determining the offset values using the given values of ΔxD for the conventional angular mode, the set of integer offsets C is {-<NUM>, ΔxD, <NUM>}. The stored and transmitted contour sequence represents indices i(n) of list C relative to the position of ΔxD, such that i ∈ {-<NUM>, <NUM>, <NUM>} corresponds to {-<NUM>, ΔxD, <NUM>}. For efficient signaling, the set of offsets is reduced to {ΔxD, <NUM>} if ΔxD = -<NUM> and to {-<NUM>, ΔxD} if ΔxD = <NUM>, resulting in indices {<NUM>,<NUM>} and {-<NUM>,<NUM>}, respectively.

Then, the contour mode is signaled by transmitting in the data stream as follows: in case the transmitted luma mode of a block corresponds to an angular mode, first a flag is transmitted that indicates whether contour mode is applied for this block or not. For contour mode, the flag is followed by the sequence of indices i(n).

In this embodiment, multiline is not used, such that the length of the transmitted sequence of indices i(n) corresponds to the height of the block for vertical and to the width for horizontal angular modes, respectively. For every index i(n) a first bit indicated whether the value is zero or not. In the latter case a second bit indicates sign, i.e. whether the index is -<NUM> or +<NUM>. The second bit is not necessary for the special cases mentioned above, where ΔxD = ±<NUM> and only two index values are allowed.

The prediction process of this embodiment follows the explained contour mode in the above referring to <FIG>, directly applying the side reference samples using position yr derived via stepping backward in the contour sequence and fractional refinement for the last step. The edge and boundary filters known for some implementations of angular prediction are not used for contour prediction.

At the encoder <NUM> the contour sequence is derived in a line-wise process (after swapping rows and columns for horizontal angular/contour modes). The first line is predicted as described above with each of the possible offsets {-<NUM>, ΔxD, <NUM>}. The decision for one offset is made by estimating the RD cost, namely the distortion in terms of the sum of squared differences (SSD) between the resulting line of predicted sample values and the line of original sample values and the rate by adding a penalty value for offsets other than ΔxD, i.e., i(n) ≠ <NUM>, which require an additional bit for signaling the sign. The offset with the lowest RD cost value is selected for the first line and the process is successively repeated for all remaining lines as illustrated in <FIG>.

In <FIG>, the reference samples are positioned directly neighboring to the block. However, the reference samples do not necessary by as touch directly neighbor to the block. For instance, the reference samples may be positioned at one or more samples away from a border of the current block.

The contour mode according to the present application can be implemented as a separate mode or as an extension to the conventional angular mode.

In the above described embodiments, horizontal contour prediction is explained, i.e., the line of the samples comprises row and therefore, direction across the respective block from information included in the data stream is relatively horizontal. However, in case the line of samples comprises columns, then the direction across the respective block is relatively vertical.

In addition, further to the above explained embodiments, the contour mode can be for example also:.

The inventive data stream can be stored on a digital storage medium or can be transmitted on a transmission medium such as a wireless transmission medium or a wired transmission medium such as the Internet.

Depending on certain implementation requirements, embodiments of the present application can be implemented in hardware or in software.

Some embodiments according to the present application, the subject-matter comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.

Generally, embodiments of the present application can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer.

A further example of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein.

A further embodiment according to the present application comprises an apparatus or a system configured to transfer (for example, electronically or optically) a computer program for performing one of the methods described herein to a receiver.

The above described embodiments are merely illustrative for the principles of the present application. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein.

The above embodiments, inter alia, prevailed, the video decoder for supporting a prediction mode for predicting blocks of a video, wherein the video decoder is configured to predict each of the blocks by extrapolating a neighborhood of the respective block into the block along a direction which varies across the respective block.

The video decoder is configured to vary the direction across the respective block by determining an offset value for each line of samples of lines of samples the respective block is composed of, the offset value indicating an offset of sample positions at which the neighborhood is to be sampled for predicting the respective line relative to further sample positions at which the neighborhood is to be sampled for predicting a line of samples adjacent to the respective line towards the neighborhood, or for each of groups of lines of samples into which block is partitioned.

The video decoder is configured to read for each line of samples or for each group of lines a parameter indexing one of a set of offset values comprising, a global offset value for the respective block, a first predetermined offset value, and a second predetermined offset value, wherein the first predetermined offset value and the second predetermined offset value indicate offsets of the sample positions relative to the further sample positions into opposite directions.

The video decoder, wherein the set of the offset values additionally comprises a first set of one or more offset values each of which indicates an offset of the sample positions relative to the each sample positions between the first predetermined offset value and the global offset value, and/or a second set of one or more offset values each of which indicates an offset of the sample positions relative to the each sample positions between the global offset value and the second predetermined offset value.

The video decoder is configured to determine, for the respective block, the global offset value by prediction or based on a prediction parameter comprised by a data stream for the respective block.

The video decoder is configured to determine, for the respective block, the global offset value by selecting same among possible values between the first and second predetermined offset values.

Claim 1:
Video decoder (<NUM>) for supporting a prediction mode for predicting blocks (<NUM>) of a video, wherein the video decoder (<NUM>) is configured to predict each of the blocks (<NUM>) by
extrapolating a neighborhood of the respective block (<NUM>) into the block along a direction which varies across the respective block so that the direction for a current block to be predicted is changed based on a sample block which has been previously predicted,
wherein the video decoder (<NUM>) is configured
to vary the direction across the respective block (<NUM>) in unit of lines of samples the respective block is composed of, or in unit of groups of lines of samples into which the block is partitioned, based on one or more offset values indicating an offset of sample positions at which the neighborhood is to be sampled for predicting a predetermined line or predetermined group of lines relative to further sample positions at which the neighborhood is to be sampled for predicting a further line or further group of lines adjacent to the predetermined line or predetermined group of lines, and
to read for each line of samples (<NUM>) or for each group of lines a parameter indexing one of a set of offset values comprising
a global offset value for the respective block (<NUM>),
a first predetermined offset value, and
a second predetermined offset value,
wherein the first predetermined offset value and the second predetermined offset value are determined by reading the parameter indexing which is transmitted in a bitstream,
wherein the first predetermined offset value and the second predetermined offset value indicate offsets of the sample positions relative to the further sample positions into opposite directions, and
wherein the global offset value is an offset value between the first and the second predetermined offset values.