Patent Description:
Digital video communication and storage applications are implemented by a wide range of digital devices, e.g. digital cameras, cellular radio telephones, laptops, broadcasting systems, video teleconferencing systems, etc. 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 contradicting parameters: compression efficiency and computational complexity. Video coding standards, such as ITU-T H. <NUM>/AVC or ITU-T H. <NUM>/HEVC, provide a good tradeoff between these parameters. For that reason support of video coding standards is a mandatory requirement for almost any video compression application.

The state-of -the-art video coding standards are based on partitioning of a source picture into video coding blocks (or short blocks). Processing of these blocks depend on their size, spatial position and a coding mode specified by an encoder. Coding modes can be classified into two groups according to the type of prediction: intra- and inter-prediction modes. Intra-prediction modes use pixels of the same picture (also referred to as frame or image) to generate reference samples to calculate the prediction values for the pixels of the block being reconstructed. Intra-prediction is also referred to as spatial prediction. Inter-prediction modes are designed for temporal prediction and uses reference samples of previous or next pictures to predict pixels of the block of the current picture. After a prediction stage, transform coding is performed for a prediction error that is the difference between an original signal and its prediction. Then, the transform coefficients and side information are encoded using an entropy coder (e.g., CABAC for AVC/H. <NUM> and HEVC/H. The recently adopted ITU-T H. <NUM>/HEVC standard (ISO/IEC <NUM>-<NUM>:<NUM>, "Information technology - High efficiency coding and media delivery in heterogeneous environments - Part <NUM>: High efficiency video coding", November <NUM>) declares a set of state-of-the-art video coding tools that provide a reasonable tradeoff between coding efficiency and computational complexity. An overview on the ITU-T H. <NUM>/HEVC standard has been given by <NPL>.

Further prior art is disclosed in the following documents. The document "<NPL>" discloses an intra-prediction involving the generation of certain sample values. These values are a result of subpixel interpolation. For predicting a pixel of a block, this document suggests the use of different subpixel positions and hence these sample values vary depending their position in the current block.

Furthermore, <CIT> discloses a method for intra prediction of a current video coding block of a frame of a video signal, the current video coding block comprising a plurality of pixels, each pixel being associated with a pixel value, with the method comprising generating on the basis of a plurality of primary reference pixel values a plurality of secondary reference pixel values, wherein the plurality of primary reference pixel values are associated with a plurality of primary reference pixels located in neighbouring video coding blocks of the current video coding block and wherein each of the secondary reference pixel values is generated on the basis of two or more of the primary reference pixel values. Further, this document also discloses intra predicting the pixel values of the pixels of the current video coding block on the basis of the plurality of primary reference pixel values and the plurality of secondary reference pixel values.

Similarly to the ITU-T H. <NUM>/AVC video coding standard, the HEVC/H. <NUM> video coding standard provides for a division of the source picture into blocks, e.g., coding units (CUs). Each of the CUs can be further split into either smaller CUs or prediction units (PUs). A PU can be intra- or inter-predicted according to the type of processing applied for the pixels of PU. In case of inter-prediction, a PU represents an area of pixels that is processed by motion compensation using a motion vector specified for a PU. For intra prediction, the adjacent pixels of neighbor blocks are used as reference samples to predict a current block. A PU specifies a prediction mode that is selected from the set of intra-prediction modes for all the transform units (TUs) contained in this PU. A TU can have different sizes (e.g., 4x4, 8x8, 16x16 and 32x32 pixels) and can be processed in different ways. For a TU, transform coding is performed, i.e. the prediction error is transformed with a discrete cosine transform or a discrete sine transform (in the HEVC/H. <NUM> standard, it is applied to intra-coded blocks) and quantized. Hence, reconstructed pixels contain quantization noise (which can become apparent, for example, as blockiness between units, ringing artifacts along with sharp edges, etc.) that in-loop filters such as DBF, SAO and ALF try to suppress. The use of sophisticated prediction coding (such as motion compensation and intra-prediction) and partitioning techniques (e.g., QT for CUs and PUs as well as RQT for TUs) allows to significantly reduce the redundancy in PUs.

The prediction tools which led to the prosperous application of these video coding standards can be roughly distinguished into inter and intra prediction tools. While intra prediction solely relies on information which is contained in the current picture, inter prediction employs the redundancy between different pictures to further increase the coding efficiency. Therefore, in general intra prediction requires higher bitrates than inter prediction to achieve the same visual quality for typical video signals.

Nevertheless, intra coding is an essential part of all video coding systems, because it is required to start a video transmission, for random access into ongoing transmissions and for error concealment. In the HEVC standard, however, only one adjacent row/column of pixels is used as a prediction basis for the currently processed video coding block (which in case of HEVC is referred to as coding unit or CU). Furthermore, in case of intra prediction based on an angular prediction, only one direction can be applied per CU. Due to these limitations, high bit rates are required for the residuals of intra coded CUs.

Thus, there is a need for devices and methods for video coding, which allow increasing the coding efficiency for intra prediction.

Therefore, the object of the present invention is to provide an apparatus and method for intra prediction of a current video block, and an encoding apparatus and decoding apparatus which include such an intra prediction apparatus, which allow increasing the coding efficiency for intra prediction.

This object is solved by the attached independent claims and further embodiments and improvements of the invention are listed in the attached dependent claims. Hereinafter, up to the "brief description of the drawings", expressions like ". aspect according to the invention", "according to the invention", or "the present invention", relate to technical teaching of the broadest embodiment as claimed with the independent claims. Expressions like "implementation", "design", "optionally", "preferably", "scenario", "aspect" or similar relate to further embodiments as claimed, and expressions like "example", ". aspect according to an example", "the disclosure describes", or "the disclosure" describe technical teaching which relates to the understanding of the invention or its embodiments, which, however, is not claimed as such.

First of all, it should be noted that the following disclosure employs a plurality of terms which, in embodiments, have the following meaning: Slice - a spatially distinct region of a picture that is independently encoded/decoded. Slice header - Data structure configured to signal information associated with a particular slice. Video coding block (or short block) - an MxN (M-column by N-row) array of pixels or samples (each pixel/sample being associated with at least one pixel/sample value), or an MxN array of transform coefficients. Coding Tree Unit (CTU) grid - a grid structure employed to partition blocks of pixels into macro-blocks for video encoding. Coding Unit (CU) - a coding block of luma samples, two corresponding coding blocks of chroma samples of an image that has three sample arrays, or a coding block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax used to code the samples. Picture Parameter Set (PPS) - a syntax structure containing syntax elements that apply to zero or more entire coded pictures as determined by a syntax element found in each slice segment header. Sequence Parameter Set (SPS) - a syntax structure containing syntax elements that apply to zero or more entire coded video sequences as determined by the content of a syntax element found in the PPS referred to by a syntax element found in each slice segment header.

Video Parameter Set (VPS) - a syntax structure containing syntax elements that apply to zero or more entire coded video sequences. Prediction Unit (PU) - a prediction block of luma samples, two corresponding prediction blocks of chroma samples of a picture that has three sample arrays, or a prediction block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax used to predict the prediction block samples. Transform Unit (TU) - a transform block of luma samples, two corresponding transform blocks of chroma samples of a picture that has three sample arrays, or a transform block of samples of a monochrome picture or a picture that is coded using three separate color planes and syntax used to predict the transform block samples. Supplemental enhancement information (SEI) - extra information that may be inserted into a video bit-stream to enhance the use of the video. Luma - information indicating the brightness of an image sample. Chroma - information indicating the color of an image sample, which may be described in terms of red difference chroma component (Cr) and blue difference chroma component (Cb).

According to a first aspect the invention, the invention provides an apparatus for intra prediction of a current video coding block of a frame of a video signal, wherein the current video coding block comprising a plurality of pixels and each pixel is associated with at least one pixel value (also referred to as sample value). The apparatus comprises: a reference pixel unit configured to generate on the basis of a plurality of primary reference pixel values a plurality of secondary reference pixel values, wherein the plurality of primary reference pixel values are associated with a plurality of primary reference pixels located in neighboring video coding blocks of the current video coding block, wherein the reference pixel unit is configured to generate each of the secondary reference pixel values on the basis of two or more of the primary reference pixel values; and an intra prediction unit configured to intra predict the pixel values of the pixels of the current video coding block on the basis of the plurality of primary reference pixel values and the plurality of secondary reference pixel values.

Thus, an improved apparatus for video coding is provided, which allows increasing the coding efficiency for intra prediction.

In a first implementation form of the apparatus according to the first aspect of the invention, the plurality of primary reference pixels are located in a row of pixels directly above the current video coding block and in a column of pixels to the left of the current video coding block.

The current video coding block may in particular but not claimed be rectangular, in particular quadratic.

In a second implementation form of the apparatus according to the first aspect of the invention, the invention provides that the plurality of secondary reference pixel values are associated with a plurality of secondary reference pixels located in further neighboring video coding blocks of the current video coding block, wherein the further neighboring video coding blocks are not the neighboring video coding blocks providing the primary reference pixel values providing the plurality of primary reference pixels. The apparatus will thus be particularly efficient.

In a third implementation form of the apparatus according to the first aspect of the invention or any one of the first and second implementation form thereof, the reference pixel unit is further configured to determine for each secondary reference pixel value of a subset of the plurality of secondary reference pixel values a first component of the secondary reference pixel value on the basis of directional intra prediction and a second component of the secondary reference pixel value on the basis of an interpolation prediction between a first secondary reference pixel value and a second secondary reference pixel value and to combine the first component of the secondary reference pixel value and the second component of the secondary reference pixel value to generate the secondary reference pixel value, wherein the first secondary reference pixel value and the second secondary reference pixel value are not part of the subset of the plurality of secondary reference pixel values. The apparatus will thus be particularly efficient.

As used herein but not claimed interpolation prediction is based on interpolation methods for predicting a set of unknown values in given positions using a set of a known values. Interpolation consists in selecting a pre-defined function that approximates a set of known values and calculating values of this function at the positions of the unknown target values. Typical functions used for interpolation are linear, spline or cubic, and could be applied to the whole set of known values or have different parameters for different subsets of known values. The latter case is known as piecewise interpolation.

As used herein but not claimed directional intra prediction is based on propagating boundary values inside a block to be predicted, so that each pixel value of the predicted block is calculated by projecting the position of that pixel onto a set of reference pixels in the specified direction. In case the projected position is a fractional, lies between pixel positions, sub-pixel interpolation prediction between neighboring pixels can be applied.

In a fourth implementation form of the apparatus according to the third implementation form of the first aspect according to the invention, the reference pixel unit is configured to use a directional mode of the H. <NUM> standard, the H. <NUM> standard, or a standard evolved from one of these standards for determining the first component of the secondary reference pixel value on the basis of directional intra prediction.

In a fifth implementation form of the apparatus according to the third or fourth implementation form of the first aspect according to the invention, the reference pixel unit is further configured to determine the first secondary reference pixel value on the basis of the primary reference pixel values of primary reference pixels neighboring the first secondary reference pixel and the second secondary reference pixel value on the basis of the primary reference pixel values of primary reference pixels neighboring the second secondary reference pixel. The apparatus will thus be particularly efficient.

In a sixth implementation form of the apparatus according to the fifth implementation form of the first aspect according to the invention, the reference pixel unit is configured to determine the first secondary reference pixel value prsg[<NUM>] and the second secondary reference pixel value prsg[<NUM>N] on the basis of the following equations: <MAT> <MAT> wherein N denotes the linear size of the current video coding block. The apparatus will thus be particularly efficient.

In a seventh implementation form of the apparatus according to any one of the third to sixth implementation form of the first aspect according to the invention, the reference pixel unit is configured to determine the second component pgrad[k] of the secondary reference pixel value on the basis of an interpolation prediction between the first secondary reference pixel value prsg[<NUM>] and the second secondary reference pixel value prsg[<NUM>N] on the basis of the following equation: <MAT> with <MAT>. The apparatus will thus be particularly efficient.

In an eighth implementation form of the apparatus according to any one of the third to seventh implementation form of the first aspect according to the invention, the reference pixel unit is configured to combine the first component pint[k] of the secondary reference pixel value and the second component pgrad[k] of the secondary reference pixel value to generate the secondary reference pixel value prs[k] on the basis of the following equation: <MAT> wherein wgrad[k] + wint[k] = <NUM> and <NUM> ≤ wgrad[k],wint[k] ≤ <NUM>. The apparatus will thus be particularly efficient.

In a ninth implementation form of the apparatus of the eighth implementation form of the first aspect according to the invention, the reference pixel unit is configured to adjust the weights wgrad[k] and/or wint[k] depending on the direction, on the index k and/or on the size of the current video coding block. The apparatus will thus be particularly efficient.

In a tenth implementation form of the apparatus according to the first aspect according to the invention as such or any one of the preceding implementations forms thereof, the intra prediction unit is configured to intra predict the pixel values of the pixels of the current video coding block on the basis of the plurality of primary reference pixel values and the plurality of secondary reference pixel values on the basis of the following equation: <MAT> wherein p[x, y] denotes the pixel value of the pixel of the current video coding block having the coordinates x,y, w<NUM> denotes a first weight, prs<NUM> denotes a primary reference pixel value, w<NUM> denotes a second weight, and prs<NUM> denotes a secondary reference pixel value. The apparatus will thus be particularly efficient.

In an eleventh implementation form of the apparatus according to the tenth implementation form of the first aspect according to the invention, the intra prediction unit is configured to intra predict the pixel values of the pixels of the current video coding block on the basis of the plurality of primary reference pixel values and the plurality of secondary reference pixel values on the basis of the following equation: <MAT> wherein drs<NUM> denotes the distance from the primary reference pixel associated with the primary reference pixel value prs<NUM> to the pixel of the current video coding block having the coordinates x, y, drs<NUM> denotes the distance from the secondary reference pixel associated with the secondary reference pixel value prs<NUM> to the pixel of the current video coding block having the coordinates x, y, and D denotes the distance from the primary reference pixel associated with the primary reference pixel value prs<NUM> to the secondary reference pixel associated with the secondary reference pixel value prs<NUM>, i.e. D = drs<NUM> + drs<NUM>. The apparatus will thus be particularly efficient.

According to a second aspect the invention, the invention provides an encoding apparatus for encoding a current video coding block of a frame of a video signal, wherein the current video coding block comprises a plurality of pixels and each pixel is associated with a pixel value. The encoding apparatus comprises an intra prediction apparatus according to the first aspect as such or any one of the implementation forms thereof for providing a predicted video coding block; and an encoding unit configured to encode the current video coding block on the basis of the predicted video coding block.

According to a third aspect the invention, the invention also provides a decoding apparatus for decoding an encoded video coding block of a frame of a video signal, wherein the encoded video coding block comprises a plurality of pixels and each pixel is associated with a pixel value. The decoding apparatus comprises: an intra prediction apparatus according to the first aspect as such or any one of the implementation forms thereof for providing a predicted video coding block; and a restoration unit configured to restore a video coding block on the basis of an encoded video coding block and the predicted video coding block.

According to a fourth aspect the invention, the invention provides a method for intra prediction of a current video coding block of a frame of a video signal, wherein the current video coding block comprises a plurality of pixels and each pixel is associated with a pixel value. The method comprises the steps of: generating on the basis of a plurality of primary reference pixel values a plurality of secondary reference pixel values, wherein the plurality of primary reference pixel values are associated with a plurality of primary reference pixels located in neighboring video coding blocks of the current video coding block and wherein each of the secondary reference pixel values is generated on the basis of two or more of the primary reference pixel values; and intra predicting the pixel values of the pixels of the current video coding block on the basis of the plurality of primary reference pixel values and the plurality of secondary reference pixel values.

Although not claimed, the method according to the fourth aspect of the invention can be performed by the intra-prediction apparatus according to the first aspect of the invention. Further features of the method according to the fourth aspect of the invention result directly from the functionality of the intra-prediction apparatus according to the first aspect of the invention and its different implementation forms.

According to a fifth aspect according to the invention, the invention provides a computer program comprising program code for performing the method according to the fourth aspect when executed on a computer.

The invention can be implemented, although not claimed, in hardware, in software, or in a combination of hardware and software.

In the various figures, identical reference signs will be used for identical or at least functionally equivalent features.

In the following description, reference is made to the accompanying drawings, which form part of the disclosure, and in which are shown, by way of illustration, specific aspects in which the present invention may be placed. It is understood that other aspects may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, as the scope of the present invention is defined be the appended claims.

<FIG> shows a schematic diagram illustrating an intra prediction apparatus <NUM> according to an embodiment.

The intra prediction apparatus <NUM> is configured to intra predict a current video coding block of a frame of a video signal, wherein the current video coding block comprises a plurality of pixels and each pixel is associated with a pixel value.

The intra prediction apparatus <NUM> comprises a reference pixel unit <NUM> configured to generate on the basis of a plurality of primary reference pixel values a plurality of secondary reference pixel values, wherein the plurality of primary reference pixel values are associated with a plurality of primary reference pixels located in already predicted neighboring video coding blocks of the current video coding block. The reference pixel unit <NUM> is configured to generate each of the secondary reference pixel values on the basis of two or more of the primary reference pixel values.

Moreover, the intra prediction apparatus <NUM> comprises an intra prediction unit <NUM> configured to intra predict the pixel values of the pixels of the current video coding block on the basis of the plurality of primary reference pixel values and the plurality of secondary reference pixel values for providing a predicted current video coding block. In an embodiment, the plurality of primary reference pixels are located in a row of pixels directly above the current video coding block and in a column of pixels to the left of the current video coding block.

In a not claimed embodiment, the current video coding block is a quadratic video coding block or a rectangular video coding block.

According to the invention, the plurality of secondary reference pixel values are associated with a plurality of secondary reference pixels located in further neighboring video coding blocks of the current video coding block, wherein the further neighboring video coding blocks are not the neighboring video coding blocks providing the plurality of primary reference pixels.

Further embodiments of the intra prediction apparatus <NUM> will be described further below.

<FIG> shows a schematic diagram illustrating an encoding apparatus <NUM> according to an embodiment and a decoding apparatus <NUM> according to an embodiment.

The encoding apparatus <NUM> is configured to encode a current video coding block of a frame of a video signal, wherein the current video coding block comprises a plurality of pixels and each pixel is associated with a pixel value. The encoding apparatus <NUM> comprises the intra prediction apparatus <NUM> shown in <FIG> for providing a predicted video coding block and an encoding unit <NUM> configured to encode the current video coding block on the basis of the predicted video coding block and providing the encoded current video coding block, for instance, in the form of a bitstream. Further embodiments of the encoding apparatus <NUM> will be described further below. In an embodiment, the encoding apparatus <NUM> could be implemented as a hybrid encoder, as defined, for instance, in the HEVC standard, and could comprise further components, such as an entropy encoder.

The decoding apparatus <NUM> is configured to decode the encoded video coding block of a frame of a video signal, which is contained in the bitstream provided by the encoding apparatus <NUM>, wherein the encoded video coding block comprises a plurality of pixels and each pixel is associated with a pixel value. The decoding apparatus <NUM> comprises the intra prediction apparatus <NUM> shown in <FIG> for providing a predicted video coding block and a restoration unit <NUM> configured to restore a video coding block on the basis of the encoded video coding block and the predicted video coding block. Further embodiments of the decoding apparatus <NUM> will be described further below. In an embodiment, the decoding apparatus <NUM> could be implemented as a hybrid decoder, as defined, for instance, in the HEVC standard, and could comprise further components, such as a decoding unit for providing a residual video coding block on the basis of the encoded video coding block.

<FIG> shows a schematic diagram illustrating a method <NUM> for intra prediction of a current video coding block of a frame of a video signal according to an embodiment, wherein the current video coding block comprises a plurality of pixels and each pixel is associated with a pixel value.

The intra prediction method <NUM> comprises a step <NUM> of generating on the basis of a plurality of primary reference pixel values a plurality of secondary reference pixel values, wherein the plurality of primary reference pixel values are associated with a plurality of primary reference pixels located in neighboring video coding blocks of the current video coding block and wherein each of the secondary reference pixel values is generated on the basis of two or more of the primary reference pixel values.

Moreover, the intra prediction method <NUM> comprises a step <NUM> of intra predicting pixel values of the pixels of the current video coding block on the basis of the plurality of primary reference pixel values and the plurality of secondary reference pixel values.

Further embodiments of the intra prediction method <NUM> will be described further below.

As will be described in the following in the context of <FIG>, in an embodiment, the reference pixel unit <NUM> of the intra prediction apparatus <NUM> is further configured to determine for each secondary reference pixel value of a subset of the plurality of secondary reference pixel values a first component of the secondary reference pixel value on the basis of directional intra prediction and a second component of the secondary reference pixel value on the basis of an interpolation prediction between a first secondary reference pixel value and a second secondary reference pixel value and to combine the first component of the secondary reference pixel value and the second component of the secondary reference pixel value to generate the secondary reference pixel value, wherein the first secondary reference pixel value and the second secondary reference pixel value are not part of the subset of the plurality of secondary reference pixel values.

<FIG> shows a schematic diagram of an exemplary current video coding block illustrating several aspects of the intra prediction apparatus <NUM> and the intra prediction method <NUM> according to an embodiment, in particular the relationship between primary reference pixels, secondary reference pixels and intra predicted pixels. The corresponding processing steps implemented in the intra prediction apparatus <NUM> and the intra prediction method <NUM> according to an embodiment are shown in <FIG>.

In <FIG> the grey square of pixels represents the exemplary currently processed video coding block. For the exemplary current video coding block shown in <FIG> the primary reference pixels are the pixels in the row of pixels above the current video coding block and the pixels in the column of pixels to the left of the current video coding block. Thus, in the embodiment shown in <FIG>, the primary reference pixels belong to neighboring video coding blocks, which already have been intra predicted, i.e. processed by the intra prediction apparatus <NUM>. In <FIG> the primary reference pixels in the row above the current video coding block are indexed from <NUM> to 2N and the primary reference pixels in the column of pixels to the left of the current video coding block are indexed from <NUM> to -2N.

<FIG> illustrates as an example the case, where the intra prediction apparatus <NUM> intra predicts the pixel value of the pixel of the currently processed video coding block, i.e. the currently processed pixel, which is identified in <FIG> by a darker shade of grey. For the intra prediction mode having an exemplary direction of <NUM>° assumed in <FIG> the reference pixel unit <NUM> is configured to determine the primary reference pixel prs<NUM> associated with the currently processed pixel. Moreover, the secondary reference pixel prs<NUM> on the "opposite side" of the currently processed pixel is determined (this is also illustrated in the processing steps <NUM>, <NUM> and <NUM> of <FIG>). The position of the secondary reference pixel prs<NUM> depends on the intra-prediction mode, the size of the block to be predicted and the position of the currently processed pixel being predicted. If this position does not coincide with a primary (i.e. already predicted) reference pixel (see also processing step <NUM> of <FIG>), the corresponding secondary reference pixel value will be determined as follows.

The primary reference pixel prs<NUM> and/or the secondary reference pixel prs<NUM> might not be located at integer pixel position and, therefore, may require a sub-pixel interpolation process, e.g. as defined by HEVC standard (see also processing steps <NUM>, <NUM>, <NUM> and <NUM> of <FIG>).

In a next stage, the intra prediction unit <NUM> of the apparatus <NUM> is configured to intra predict the pixel value of the currently processed pixel on the basis of the primary reference pixel value prs<NUM> and the secondary reference pixel value prs<NUM>.

In an embodiment, the intra prediction unit <NUM> of the apparatus <NUM> is configured to intra predict the pixel value of the currently processed pixel as a weighted sum of the primary reference pixel value prs<NUM> and the secondary reference pixel value prs<NUM>, i.e.: <MAT> wherein p[x, y] denotes the pixel value of the currently processed pixel located at the coordinates x,y, w<NUM> denotes a first weight and w<NUM> denotes a second weight.

In the embodiment shown in <FIG>, the intra prediction unit <NUM> is configured to determine the first and the second weight w<NUM>, w<NUM> on the basis of the distance drs<NUM> between the primary reference pixel prs<NUM> and the currently processed pixel, the distance drs<NUM> between the secondary reference pixel prs<NUM> and the currently processed pixel and the distance D between the primary reference pixel prs<NUM> and the secondary reference pixel prs<NUM>, i.e. D = drs<NUM> + drs<NUM> (see also processing steps <NUM> and <NUM> in <FIG>). More specifically, in an embodiment the intra prediction unit <NUM> is configured to intra predict the pixel value of the currently processed pixel on the basis of the following equation: <MAT>.

The embodiment shown in <FIG> uses the generation of secondary reference samples, i.e. pixel values, for the unknown sides of the currently processed video coding block. In the HEVC standard, for instance, the unknown sides are the right side and the bottom side of the currently processed video coding block.

<FIG> shows the intra prediction modes provided by the HEVC/H. <NUM> standard, including a planar mode (the intra-prediction mode index is <NUM>), DC mode (the intra-prediction mode index is <NUM>), and <NUM> directional modes (the intra-prediction mode index ranges from <NUM> to <NUM>). In the following embodiments of the intra prediction apparatus <NUM> and the intra prediction method <NUM> will be described, which can use one or more of the directional modes shown in <FIG> to generate a secondary reference pixel value on the basis of at last two primary reference pixel values.

<FIG> shows a schematic diagram illustrating a general concept implemented in the reference pixel unit <NUM> of an apparatus <NUM> according to an embodiment for generating the secondary reference pixels. The reference pixel unit <NUM> according to embodiments of the invention uses a combination of two components, namely gradually interpolated pixel values or components and directionally predicted pixel values or components, i.e. pixels predicted on the basis of a directional prediction, as provided, for instance, by the <NUM> directional modes defined in the HEVC/H. <NUM> standard.

As illustrated in <FIG>, according to embodiments of the invention these two components can be computed independently and combined in order to obtain the secondary reference samples prs<NUM> directly or by means of an interpolation of these values. According to embodiments of the invention, directionally predicted values are calculated the same way as if these pixels would belong to the block being predicted, i.e. the pixels "inside" of the currently processed video coding block. The combination of these two components can be performed in different ways.

In an embodiment, the reference pixel unit <NUM> is configured to take a weighted sum of the first component, i.e. the directionally predicted pixel value, and the second component, i.e. the gradually interpolated pixel value, for generating a secondary reference pixel value on the basis of the following equation: <MAT> wherein wgrad[k] + wint[k] = <NUM> and <NUM> ≤ wgrad[k], wint[k] ≤ <NUM> and k denotes the index for identifying the secondary reference pixel values. For instance, in figure the index k runs from <NUM> (secondary reference pixel to the left in the bottom row) to 2N (secondary reference pixel at the top in the row on the right side of the currently processed video coding block). In an embodiment, the weights wgrad[k],wint[k] can have the value <NUM>. In an embodiment, the weights wgrad[k],wint[k] can depend on the size of the current video coding block and/or the selected directional mode. In an embodiment, the weight wgrad[k] can have the values provided in the following table (wherein the weight wint[k] can be derived from the relation wgrad[k] + wint[k] = <NUM> and the numbers defining the respective angular mode range are indexes identifying different directional modes):.

In another embodiment, the reference pixel unit <NUM> is configured to generate the secondary pixel values prs[k] by blending (which can include non-linear operations) the first components, i.e. the directionally predicted pixel values, and the second components, i.e. the gradually interpolated pixel values.

<FIG> shows a diagram illustrating intra-prediction processing steps implemented in the intra-prediction apparatus <NUM> according to the invention, which is based on a two-stage process <NUM> for generating the gradually interpolated pixel values.

In a first processing stage <NUM>, the reference pixel unit <NUM> is configured to determine the secondary pixel values of a first and second (or last) secondary reference pixels, wherein the first and the last secondary reference pixels are those secondary reference pixels that are adjacent to the primary reference pixels. For the exemplary scenario shown in <FIG> the first secondary reference pixel (being associated with a pixel value prs[<NUM>]) is the most left pixel in the bottom row and the last secondary reference pixel (being associated with a pixel value prs[<NUM>N]) is the top pixel in the row on the right hand side of the currently processed video coding block.

In an embodiment, the reference pixel unit <NUM> is configured to determine the first secondary reference pixel value prsg[<NUM>] and the second secondary reference pixel value prsg[<NUM>N] on the basis of the following equations: <MAT> wherein prs denotes the pixel value of a respective primary reference pixel and wrs denotes a primary reference pixel weight.

The second processing stage <NUM> of the process shown in <FIG> can be done in different ways. According to the invention, the reference pixel unit <NUM> of the apparatus <NUM> is configured to generate the gradually interpolated pixel values, i.e. the respective second component for generating the respective secondary reference pixel value prs[k] using linear interpolation. In an embodiment, the reference pixel unit <NUM> of the apparatus <NUM> is configured to determine the value of a step size on the basis of the following equation: <MAT> and to use this value for the step size s to compute the gradually interpolated values: <MAT>.

In another embodiment, the reference pixel unit <NUM> of the apparatus <NUM> is configured to define an average pixel value of the first and the last of the gradually interpolated pixels, for instance, on the basis of the following equation: <MAT>.

In an embodiment, which is illustrated on the basis of the exemplary video coding block shown in <FIG>, the reference pixel unit <NUM> of the apparatus <NUM> is configured to determine a corresponding secondary reference pixel for this average pixel value by projecting of the point located at the middle of the currently processed video coding block to the positions of the secondary pixels. The coordinates of the point located at the middle of the currently processed video coding block can be expressed as follows: <MAT> wherein W and H denote the width and height of the currently processed video coding block, respectively. For this embodiment, the second processing stage <NUM> shown in <FIG> becomes dependent on the intra-prediction mode used for predicting the first components, i.e. the directionally predicted pixel values, of the secondary reference pixel values, because the interpolation is performed taking into account the position of the secondary reference pixel associated with the average pixel value. In further embodiments, two different step size values could be used by the reference pixel unit <NUM> to perform linear interpolation between the points, namely: <MAT>.

In further embodiments, the reference pixel unit <NUM> can be configured to use instead of a linear interpolation a <NUM>-nd or higher order interpolation for determining the gradually interpolated pixel values in the interval (prsg[<NUM>], prsg[<NUM>N]).

<FIG> illustrates an algorithm implemented in the reference pixel unit <NUM> according to further embodiments for generating the secondary reference pixel values and/or as an alternative to the processing step <NUM> shown in <FIG>.

In a first processing step <NUM> of the algorithm shown in <FIG> a size S of the currently processed video coding block to be predicted and an intra prediction mode IIPM are selected. In the next processing step <NUM> is directionally predicted pixels Pu are generated for the unknown reference sample sides using the intra prediction mode IIPM selected in processing step <NUM>. In an embodiment, the reference pixel unit <NUM> is configured to provide one or more conventional intra prediction mechanisms (e.g. the conventional intra prediction mechanisms defined in the standard H. <NUM>) for selection and use in processing step <NUM> of <FIG>. In embodiments of the invention the processing step <NUM> can include a filtering or no filtering of the primary reference pixel values used for generating the secondary reference pixel values.

After the secondary reference pixels have been directionally generated, the reference pixel unit <NUM> of the apparatus can be configured to filter these secondary reference pixels by a filter Fdp in a processing step <NUM>, wherein the reference pixel unit <NUM> can be configured to select the filter Fdp according to the size S of the currently processed video coding block, i.e. the block to be predicted, and/or the selected intra prediction mode IIPM. (see processing step <NUM> in <FIG>). In embodiments of the invention, the Fdp filter applied in processing step <NUM> could differ from the one optionally applied to the primary reference samples in processing step <NUM>.

In an embodiment, the reference pixel unit <NUM> can be configured to select the filter Fdp in processing step <NUM> to be stronger than the filters specified in the H. <NUM> standard to filter known reference samples. However, it is possible to apply different filters Fdp, including but not limiting to FIR, IIR, non-linear or median filters. These filters may provide different effects including blurring, de-ringing or sharpening.

In the following sections further embodiments of the encoding apparatus <NUM> and the decoding apparatus <NUM> will be described, including the signaling between the apparatus <NUM> and the decoding apparatus <NUM> as implemented by embodiments of the invention. As will be appreciated, embodiments of the invention do not require a special signaling at the side of the decoding apparatus <NUM> and, therefore, do not increase the complexity of bitstream parsing operations.

<FIG> shows a processing scheme <NUM> implemented in the decoding apparatus <NUM> according to an embodiment based on the HEVC standard.

In a first processing step <NUM> the index of the intra prediction mode IIPM is parsed from the bitstream. Thereafter, in processing step <NUM> a decision is taken depending on whether the decoded intra prediction mode is a directional intra prediction mode. In the case the signaling scheme is applied in the context of HEVC video coding, the intra prediction mode is directional when IIPM is greater than <NUM>. Embodiments of the invention can make use of the planar mode as well. In such a case, this condition can be written as IIPM is not equal to <NUM>.

For directional (and possibly planar) intra prediction modes the value of the flag "idw_dir_mode_PU_flag" is parsed from the bitstream in processing step 1105b. According to embodiments of the invention this flag is introduced into the bitstream to code whether to apply the proposed mechanism to the prediction unit (a set of transform units). In an embodiment, the value of this flag is assigned to <NUM> for non-directional (DC and PLANAR) intra prediction modes in step 1105a. In processing step <NUM>, TUs belonging to a PU are determined, and for each TU a decision is taken (processing step <NUM>) to use either a conventional prediction scheme (processing step 1111b) or the distance-weighted prediction (processing step 1111a), as provided by embodiments of the invention, for obtaining the predicted signal. The decision for a TU in processing step <NUM> is taken on the basis of the value of the flag "idw_dir_mode_PU_flag", which has been determined in processing steps 1105a and 1105b.

<FIG> shows a processing scheme <NUM> implemented in the encoding apparatus <NUM> according to an embodiment based on the HEVC standard.

The processing scheme <NUM> starts in a processing step <NUM> by selecting an intra prediction mode out of the set of candidate intra prediction modes for the given PU. Then, the flag "idw_dir_mode_PU_flag" is assigned to a value of <NUM> (see processing step <NUM>), which means that distance-weighted directional prediction (DWDIP) is not applied within the PU. For each TU of the PU a rate-distortion cost (RD-cost) is estimated (see processing steps <NUM>, <NUM>, <NUM>). A PU coding cost could be estimated by summing up the RD-costs for the TUs and adding signaling costs (see processing step <NUM>).

If the intra-prediction mode picked up from candidate intra prediction modes list is not directional, there are no further calculations: the RD-cost for the given PU and intra-prediction mode is determined (see processing steps <NUM> and 1215b). Otherwise, similar operations (see processing steps 1215a, <NUM>, <NUM>, <NUM> and <NUM>) are performed for the case when the flag "idw_dir_mode_PU_flag" is set to <NUM>, i.e. DWDIP is enabled for the given PU. The decision by the encoding apparatus <NUM> about which value of the flag "idw_dir_mode_PU_flag" should be used can be made by comparing RD-costs (see processing step <NUM>).

Claim 1:
An apparatus (<NUM>) for intra prediction of a current video coding block of a frame of a video signal, the current video coding block comprising a plurality of pixels, each pixel associated with a pixel value, the apparatus (<NUM>) comprising:
a reference pixel unit (<NUM>) configured to generate on the basis of a plurality of primary reference pixel values a plurality of secondary reference pixel values, wherein the plurality of primary reference pixel values are associated with a plurality of primary reference pixels located in neighboring video coding blocks of the current video coding block and wherein the plurality of secondary reference pixel values are associated with a plurality of secondary reference pixels located in further neighboring video coding blocks of the current video coding block opposite the primary reference pixels,
wherein the reference pixel unit (<NUM>) is configured to generate each of the secondary reference pixel values on the basis of two or more of the primary reference pixel values by the reference pixel unit (<NUM>) being configured
to firstly determine (<NUM>) secondary pixel values of a first and second secondary reference pixel wherein the first and the second secondary reference pixels are those secondary reference pixels that are adjacent to the primary reference pixels, the first secondary reference pixel being the left-most pixel in the bottom row below the currently processed video block and the second secondary reference pixel being the top pixel in the column on the right hand side of the currently processed video block, and
secondly, to determine the secondary pixel values of the other secondary reference pixels in the bottom row and the column on the right hand side of the currently processed block by linear interpolation using said first and second secondary reference pixel values; and
an intra prediction unit (<NUM>) configured to intra predict the pixel values of the pixels of the current video coding block on the basis of the plurality of primary reference pixel values and the plurality of secondary reference pixel values.