GB2577337A

GB2577337A - Image data encoding and decoding

Info

Publication number: GB2577337A
Application number: GB1815557.2A
Authority: GB
Inventors: Mark Keating Stephen; James Sharman Karl
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 2018-09-24
Filing date: 2018-09-24
Publication date: 2020-03-25
Also published as: GB201815557D0

Abstract

Intra-prediction of image data using a Position Dependent intra Prediction Combination (PDPC) mode in which a prediction of a current sample of a current image region has a weighted dependency upon filtered and non-filtered reference samples. The PDPC mode is selectively enabled or disabled in response to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion. For example, the PDPC mode may be disallowed based on the current region size. The current sample is of a current image, of a plurality of regions of an image and is predicted with respect to one or more corresponding reference samples of the same image. The PDPC mode may be selectively enabled or disabled by a detector 1920 which controls whether the PDPC mode is used 1940 or disallowed 1900.

Description

IMAGE DATA ENCODING AND DECODING

BACKGROUND OF THE INVENTION Field of the Invention This disclosure relates to image data encoding and decoding.

Description of Related Art

The "background" description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, is neither expressly or impliedly admitted as

prior art against the present disclosure.

There are several video data encoding and decoding systems which involve transforming video data into a frequency domain representation, quantising the frequency domain coefficients and then applying some form of entropy encoding to the quantised coefficients. This can achieve compression of the video data. A corresponding decoding or decompression technique is applied to recover a reconstructed version of the original video data.

Current video codecs (coder-decoders) such as those used in H.264/M PEG-4 Advanced Video Coding (AVC) achieve data compression primarily by only encoding the differences between successive video frames. These codecs use a regular array of so-called macroblocks, each of which is used as a region of comparison with a corresponding macroblock in a previous video frame, and the image region within the macroblock is then encoded according to the degree of motion found between the corresponding current and previous macroblocks in the video sequence, or between neighbouring macroblocks within a single frame of the video sequence.

High Efficiency Video Coding (HEVC), also known as H.265 or MPEG-H Part 2, is a proposed successor to H.264/MPEG-4 AVC. It is intended for HEVC to improve video quality and double the data compression ratio compared to H.264, and for it to be scalable from 128 x 96 to 7680 x 4320 pixels resolution, roughly equivalent to bit rates ranging from 128kbit/s to 800Mbit/s.

SUMMARY OF THE INVENTION

The present disclosure addresses or mitigates problems arising from this processing. Respective aspects and features of the present disclosure are defined in the appended claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present technology. BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 schematically illustrates an audio/video (AN) data transmission and reception system using video data compression and decompression; Figure 2 schematically illustrates a video display system using video data decompression; Figure 3 schematically illustrates an audio/video storage system using video data compression and decompression; Figure 4 schematically illustrates a video camera using video data compression; Figures 5 and 6 schematically illustrate storage media; Figure 7 provides a schematic overview of a video data compression and decompression apparatus; Figure 8 schematically illustrates a predictor; Figure 9 schematically illustrates a partially-encoded image; Figure 10 schematically illustrates a set of possible intra-prediction directions; Figure 11 schematically illustrates a set of prediction modes; Figure 12 schematically illustrates another set of prediction modes; Figure 13 schematically illustrates an intra-prediction process; Figure 14 schematically illustrates an intra predictor including a reference sample filter; Figures 15A and 15B schematically illustrate a PDPC process; Figure 16 is a schematic flowchart illustrating a PDPC process; Figure 17 schematically illustrates a hierarchical block division; Figure 18 is a schematic flowchart illustrating a method; Figure 19 schematically illustrates a part of an encoder; Figure 20 schematically illustrates a part of a decoder; and Figures 21 and 22 are schematic flowcharts illustrating respective methods.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, Figures 1-4 are provided to give schematic illustrations of apparatus or systems making use of the compression and/or decompression apparatus to be described below in connection with embodiments of the present technology. Obviously, numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

All of the data compression and/or decompression apparatus to be described below may be implemented in hardware, in software running on a general-purpose data processing apparatus such as a general-purpose computer, as programmable hardware such as an application specific integrated circuit (ASIC) or field programmable gate array (FPGA) or as combinations of these. In cases where the embodiments are implemented by software and/or firmware, it will be appreciated that such software and/or firmware, and non-transitory data storage media by which such software and/or firmware are stored or otherwise provided, are considered as embodiments of the present technology.

Figure 1 schematically illustrates an audio/video data transmission and reception system using video data compression and decompression.

An input audio/video signal 10 is supplied to a video data compression apparatus 20 which compresses at least the video component of the audio/video signal 10 for transmission along a transmission route 30 such as a cable, an optical fibre, a wireless link or the like. The compressed signal is processed by a decompression apparatus 40 to provide an output audio/video signal 50. For the return path, a compression apparatus 60 compresses an audio/video signal for transmission along the transmission route 30 to a decompression apparatus 70.

The compression apparatus 20 and decompression apparatus 70 can therefore form one node of a transmission link. The decompression apparatus 40 and decompression apparatus 60 can form another node of the transmission link. Of course, in instances where the transmission link is uni-directional, only one of the nodes would require a compression apparatus and the other node would only require a decompression apparatus.

Figure 2 schematically illustrates a video display system using video data decompression. In particular, a compressed audio/video signal 100 is processed by a decompression apparatus 110 to provide a decompressed signal which can be displayed on a display 120. The decompression apparatus 110 could be implemented as an integral part of the display 120, for example being provided within the same casing as the display device.

Alternatively, the decompression apparatus 110 maybe provided as (for example) a so-called set top box (STB), noting that the expression "set-top" does not imply a requirement for the box to be sited in any particular orientation or position with respect to the display 120; it is simply a term used in the art to indicate a device which is connectable to a display as a peripheral device.

Figure 3 schematically illustrates an audio/video storage system using video data compression and decompression. An input audio/video signal 130 is supplied to a compression apparatus 140 which generates a compressed signal for storing by a store device 150 such as a magnetic disk device, an optical disk device, a magnetic tape device, a solid state storage device such as a semiconductor memory or other storage device. For replay, compressed data is read from the storage device 150 and passed to a decompression apparatus 160 for decompression to provide an output audio/video signal 170.

It will be appreciated that the compressed or encoded signal, and a storage medium such as a machine-readable non-transitory storage medium, storing that signal, are considered as embodiments of the present technology.

Figure 4 schematically illustrates a video camera using video data compression. In Figure 4, an image capture device 180, such as a charge coupled device (CCD) image sensor and associated control and read-out electronics, generates a video signal which is passed to a compression apparatus 190. A microphone (or plural microphones) 200 generates an audio signal to be passed to the compression apparatus 190. The compression apparatus 190 generates a compressed audio/video signal 210 to be stored and/or transmitted (shown generically as a schematic stage 220).

The techniques to be described below relate primarily to video data compression and decompression. It will be appreciated that many existing techniques may be used for audio data compression in conjunction with the video data compression techniques which will be described, to generate a compressed audio/video signal. Accordingly, a separate discussion of audio data compression will not be provided. It will also be appreciated that the data rate associated with video data, in particular broadcast quality video data, is generally very much higher than the data rate associated with audio data (whether compressed or uncompressed). It will therefore be appreciated that uncompressed audio data could accompany compressed video data to form a compressed audio/video signal. It will further be appreciated that although the present examples (shown in Figures 1-4) relate to audio/video data, the techniques to be described below can find use in a system which simply deals with (that is to say, compresses, decompresses, stores, displays and/or transmits) video data. That is to say, the embodiments can apply to video data compression without necessarily having any associated audio data handling at all.

Figure 4 therefore provides an example of a video capture apparatus comprising an image sensor and an encoding apparatus of the type to be discussed below. Figure 2 therefore provides an example of a decoding apparatus of the type to be discussed below and a display to which the decoded images are output.

Figures 5 and 6 schematically illustrate storage media, which store (for example) the compressed data generated by the apparatus 20, 60, the compressed data input to the apparatus 110 or the storage media or stages 150, 220. Figure 5 schematically illustrates a disc storage medium such as a magnetic or optical disc, and Figure 6 schematically illustrates a solid state storage medium such as a flash memory. Note that Figures 5 and 6 can also provide examples of non-transitory machine-readable storage media which store computer software which, when executed by a computer, causes the computer to carry out one or more of the methods to be discussed below.

Therefore, the above arrangements provide examples of video storage, capture, transmission or reception apparatuses embodying any of the present techniques.

Figure 7 provides a schematic overview of a video data compression and decompression apparatus.

A controller 343 controls the overall operation of the apparatus and, in particular when referring to a compression mode, controls a trial encoding processes by acting as a selector to select various modes of operation such as block sizes and shapes, and whether the video data is to be encoded losslessly or otherwise. The controller is considered to part of the image encoder or image decoder (as the case may be). Successive images of an input video signal 300 are supplied to an adder 310 and to an image predictor 320. The image predictor 320 will be described below in more detail with reference to Figure 8. The image encoder or decoder (as the case may be) plus the intra-image predictor of Figure 8 may use features from the apparatus of Figure 7. This does not mean that the image encoder or decoder necessarily requires every feature of Figure 7 however.

The adder 310 in fact performs a subtraction (negative addition) operation, in that it receives the input video signal 300 on a "+" input and the output of the image predictor 320 on a " input, so that the predicted image is subtracted from the input image. The result is to generate a so-called residual image signal 330 representing the difference between the actual and projected images.

One reason why a residual image signal is generated is as follows. The data coding techniques to be described, that is to say the techniques which will be applied to the residual image signal, tend to work more efficiently when there is less "energy" in the image to be encoded. Here, the term "efficiently" refers to the generation of a small amount of encoded data; for a particular image quality level, it is desirable (and considered "efficient") to generate as little data as is practicably possible. The reference to "energy" in the residual image relates to the amount of information contained in the residual image. If the predicted image were to be identical to the real image, the difference between the two (that is to say, the residual image) would contain zero information (zero energy) and would be very easy to encode into a small amount of encoded data. In general, if the prediction process can be made to work reasonably well such that the predicted image content is similar to the image content to be encoded, the expectation is that the residual image data will contain less information (less energy) than the input image and so will be easier to encode into a small amount of encoded data.

The remainder of the apparatus acting as an encoder (to encode the residual or difference image) will now be described. The residual image data 330 is supplied to a transform unit or circuitry 340 which generates a discrete cosine transform (DCT) representation of blocks or regions of the residual image data. The DCT technique itself is well known and will not be described in detail here. Note also that the use of DCT is only illustrative of one example arrangement. Other transforms which might be used include, for example, the discrete sine transform (DST). A transform could also comprise a sequence or cascade of individual transforms, such as an arrangement in which one transform is followed (whether directly or not) by another transform. The choice of transform may be determined explicitly and/or be dependent upon side information used to configure the encoder and decoder.

The output of the transform unit 340, which is to say, a set of DCT coefficients for each transformed block of image data, is supplied to a quantiser 350. Various quantisation techniques are known in the field of video data compression, ranging from a simple multiplication by a quantisation scaling factor through to the application of complicated lookup tables under the control of a quantisation parameter. The general aim is twofold. Firstly, the quantisation process reduces the number of possible values of the transformed data. Secondly, the quantisation process can increase the likelihood that values of the transformed data are zero. Both of these can make the entropy encoding process, to be described below, work more efficiently in generating small amounts of compressed video data.

A data scanning process is applied by a scan unit 360. The purpose of the scanning process is to reorder the quantised transformed data so as to gather as many as possible of the non-zero quantised transformed coefficients together, and of course therefore to gather as many as possible of the zero-valued coefficients together. These features can allow so-called run-length coding or similar techniques to be applied efficiently. So, the scanning process involves selecting coefficients from the quantised transformed data, and in particular from a block of coefficients corresponding to a block of image data which has been transformed and quantised, according to a "scanning order" so that (a) all of the coefficients are selected once as part of the scan, and (b) the scan tends to provide the desired reordering. One example scanning order which can tend to give useful results is a so-called up-right diagonal scanning order.

The scanned coefficients are then passed to an entropy encoder (EE) 370. Again, various types of entropy encoding may be used. Two examples are variants of the so-called CABAC (Context Adaptive Binary Arithmetic Coding) system and variants of the so-called CAVLC (Context Adaptive Variable-Length Coding) system. In general terms, CABAC is considered to provide a better efficiency, and in some studies has been shown to provide a 10- 20% reduction in the quantity of encoded output data for a comparable image quality compared to CAVLC. However, CAVLC is considered to represent a much lower level of complexity (in terms of its implementation) than CABAC. Note that the scanning process and the entropy encoding process are shown as separate processes, but in fact can be combined or treated together. That is to say, the reading of data into the entropy encoder can take place in the scan order. Corresponding considerations apply to the respective inverse processes to be described below.

The output of the entropy encoder 370, along with additional data (mentioned above and/or discussed below), for example defining the manner in which the predictor 320 generated the predicted image, provides a compressed output video signal 380.

However, a return path is also provided because the operation of the predictor 320 itself depends upon a decompressed version of the compressed output data.

The reason for this feature is as follows. At the appropriate stage in the decompression process (to be described below) a decompressed version of the residual data is generated. This decompressed residual data has to be added to a predicted image to generate an output image (because the original residual data was the difference between the input image and a predicted image). In order that this process is comparable, as between the compression side and the decompression side, the predicted images generated by the predictor 320 should be the same during the compression process and during the decompression process. Of course, at decompression, the apparatus does not have access to the original input images, but only to the decompressed images. Therefore, at compression, the predictor 320 bases its prediction (at least, for inter-image encoding) on decompressed versions of the compressed images.

The entropy encoding process carried out by the entropy encoder 370 is considered (in at least some examples) to be "lossless", which is to say that it can be reversed to arrive at exactly the same data which was first supplied to the entropy encoder 370. So, in such examples the return path can be implemented before the entropy encoding stage. Indeed, the scanning process carried out by the scan unit 360 is also considered lossless, but in the present embodiment the return path 390 is from the output of the quantiser 350 to the input of a complimentary inverse quantiser 420. In instances where loss or potential loss is introduced by a stage, that stage may be included in the feedback loop formed by the return path. For example, the entropy encoding stage can at least in principle be made lossy, for example by techniques in which bits are encoded within parity information. In such an instance, the entropy encoding and decoding should form part of the feedback loop.

In general terms, an entropy decoder 410, the reverse scan unit 400, an inverse quantiser 420 and an inverse transform unit or circuitry 430 provide the respective inverse functions of the entropy encoder 370, the scan unit 360, the quantiser 350 and the transform unit 340. For now, the discussion will continue through the compression process; the process to decompress an input compressed video signal will be discussed separately below.

In the compression process, the scanned coefficients are passed by the return path 390 from the quantiser 350 to the inverse quantiser 420 which carries out the inverse operation of the scan unit 360. An inverse quantisation and inverse transformation process are carried out by the units 420, 430 to generate a compressed-decompressed residual image signal 440.

The image signal 440 is added, at an adder 450, to the output of the predictor 320 to generate a reconstructed output image 460. This forms one input to the image predictor 320, as will be described below.

Turning now to the process applied to decompress a received compressed video signal 470, the signal is supplied to the entropy decoder 410 and from there to the chain of the reverse scan unit 400, the inverse quantiser 420 and the inverse transform unit 430 before being added to the output of the image predictor 320 by the adder 450. So, at the decoder side, the decoder reconstructs a version of the residual image and then applies this (by the adder 450) to the predicted version of the image (on a block by block basis) so as to decode each block. In straightforward terms, the output 460 of the adder 450 forms the output decompressed video signal 480. In practice, further filtering may optionally be applied (for example, by a filter 560 shown in Figure 8 but omitted from Figure 7 for clarity of the higher level diagram of Figure 7) before the signal is output.

The apparatus of Figures 7 and 8 can act as a compression (encoding) apparatus or a decompression (decoding) apparatus. The functions of the two types of apparatus substantially overlap. The scan unit 360 and entropy encoder 370 are not used in a decompression mode, and the operation of the predictor 320 (which will be described in detail below) and other units follow mode and parameter information contained in the received compressed bit-stream rather than generating such information themselves.

Figure 8 schematically illustrates the generation of predicted images, and in particular the operation of the image predictor 320.

There are two basic modes of prediction carried out by the image predictor 320: so-called intra-image prediction and so-called inter-image, or motion-compensated (MC), prediction. At the encoder side, each involves detecting a prediction direction in respect of a current block to be predicted, and generating a predicted block of samples according to other samples (in the same (intra) or another (inter) image). By virtue of the units 310 or 450, the difference between the predicted block and the actual block is encoded or applied so as to encode or decode the block respectively.

(At the decoder, or at the reverse decoding side of the encoder, the detection of a prediction direction may be in response to data associated with the encoded data by the encoder, indicating which direction was used at the encoder. Or the detection may be in response to the same factors as those on which the decision was made at the encoder). Intra-image prediction bases a prediction of the content of a block or region of the image on data from within the same image. This corresponds to so-called I-frame encoding in other video compression techniques. In contrast to I-frame encoding, however, which involves encoding the whole image by intra-encoding, in the present embodiments the choice between intra-and inter-encoding can be made on a block-by-block basis, though in other embodiments the choice is still made on an image-by-image basis.

Motion-compensated prediction is an example of inter-image prediction and makes use of motion information which attempts to define the source, in another adjacent or nearby image, of image detail to be encoded in the current image. Accordingly, in an ideal example, the contents of a block of image data in the predicted image can be encoded very simply as a reference (a motion vector) pointing to a corresponding block at the same or a slightly different position in an adjacent image.

A technique known as "block copy" prediction is in some respects a hybrid of the two, as it uses a vector to indicate a block of samples at a position displaced from the currently predicted block within the same image, which should be copied to form the currently predicted block.

Returning to Figure 8, two image prediction arrangements (corresponding to intra-and inter-image prediction) are shown, the results of which are selected by a multiplexer 500 under the control of a mode signal 510 (for example, from the controller 343) so as to provide blocks of the predicted image for supply to the adders 310 and 450. The choice is made in dependence upon which selection gives the lowest "energy" (which, as discussed above, may be considered as information content requiring encoding), and the choice is signalled to the decoder within the encoded output data-stream. Image energy, in this context, can be detected, for example, by carrying out a trial subtraction of an area of the two versions of the predicted image from the input image, squaring each pixel value of the difference image, summing the squared values, and identifying which of the two versions gives rise to the lower mean squared value of the difference image relating to that image area. In other examples, a trial encoding can be carried out for each selection or potential selection, with a choice then being made according to the cost of each potential selection in terms of one or both of the number of bits required for encoding and distortion to the picture.

The actual prediction, in the intra-encoding system, is made on the basis of image blocks received as part of the signal 460, which is to say, the prediction is based upon encoded-decoded image blocks in order that exactly the same prediction can be made at a decompression apparatus. However, data can be derived from the input video signal 300 by an intra-mode selector 520 to control the operation of the intra-image predictor 530.

For inter-image prediction, a motion compensated (MC) predictor 540 uses motion information such as motion vectors derived by a motion estimator 550 from the input video signal 300. Those motion vectors are applied to a processed version of the reconstructed image 460 by the motion compensated predictor 540 to generate blocks of the inter-image prediction.

Accordingly, the units 530 and 540 (operating with the estimator 550) each act as detectors to detect a prediction direction in respect of a current block to be predicted, and as a generator to generate a predicted block of samples (forming part of the prediction passed to the units 310 and 450) according to other samples defined by the prediction direction.

The processing applied to the signal 460 will now be described. Firstly, the signal is optionally filtered by a filter unit 560, which will be described in greater detail below. This involves applying a "deblocking" filter to remove or at least tend to reduce the effects of the block-based processing carried out by the transform unit 340 and subsequent operations. A sample adaptive offsetting (SAO) filter may also be used. Also, an adaptive loop filter is optionally applied using coefficients derived by processing the reconstructed signal 460 and the input video signal 300. The adaptive loop filter is a type of filter which, using known techniques, applies adaptive filter coefficients to the data to be filtered. That is to say, the filter coefficients can vary in dependence upon various factors. Data defining which filter coefficients to use is included as part of the encoded output data-stream.

The filtered output from the filter unit 560 in fact forms the output video signal 480 when the apparatus is operating as a decompression apparatus. It is also buffered in one or more image or frame stores 570; the storage of successive images is a requirement of motion compensated prediction processing, and in particular the generation of motion vectors. To save on storage requirements, the stored images in the image stores 570 may be held in a compressed form and then decompressed for use in generating motion vectors. For this particular purpose, any known compression / decompression system may be used. The stored images are passed to an interpolation filter 580 which generates a higher resolution version of the stored images; in this example, intermediate samples (sub-samples) are generated such that the resolution of the interpolated image is output by the interpolation filter 580 is 4 times (in each dimension) that of the images stored in the image stores 570 for the luminance channel of 4:2:0 and 8 times (in each dimension) that of the images stored in the image stores 570 for the chrominance channels of 4:2:0. The interpolated images are passed as an input to the motion estimator 550 and also to the motion compensated predictor 540.

The way in which an image is partitioned for compression processing will now be described. At a basic level, an image to be compressed is considered as an array of blocks or regions of samples. The splitting of an image into such blocks or regions can be carried out by a decision tree, such as that described in Bross et al: "High Efficiency Video Coding (HEVC) text specification draft 6", JCTVC-H1003_d0 (November 2011), the contents of which are incorporated herein by reference. In some examples, the resulting blocks or regions have sizes and, in some cases, shapes which, by virtue of the decision tree, can generally follow the disposition of image features within the image. This in itself can allow for an improved encoding efficiency because samples representing or following similar image features would tend to be grouped together by such an arrangement. In some examples, square blocks or regions of different sizes (such as 4x4 samples up to, say, 64x64 or larger blocks) are available for selection. In other example arrangements, blocks or regions of different shapes such as rectangular blocks (for example, vertically or horizontally oriented) can be used. Other non-square and non-rectangular blocks are envisaged. The result of the division of the image into such blocks or regions is (in at least the present examples) that each sample of an image is allocated to one, and only one, such block or region.

The intra-prediction process will now be discussed. In general terms, intra-prediction involves generating a prediction of a current block of samples from previously-encoded and decoded samples in the same image.

Figure 9 schematically illustrates a partially encoded image 800. Here, the image is being encoded from top-left to bottom-right on a block by block basis. An example block encoded partway through the handling of the whole image is shown as a block 810. A shaded region 820 above and to the left of the block 810 has already been encoded. The intra-image prediction of the contents of the block 810 can make use of any of the shaded area 820 but cannot make use of the unshaded area below that.

In some examples, the image is encoded on a block by block basis such that larger blocks (referred to as coding units or CUs) are encoded in an order such as the order discussed with reference to Figure 9. Within each CU, there is the potential (depending on the block splitting process that has taken place) for the CU to be handled as a set of two or more smaller blocks or transform units (TUs). This can give a hierarchical order of encoding so that the image is encoded on a CU by CU basis, and each CU is potentially encoded on a TU by TU basis. Note however that for an individual TU within the current coding tree unit (the largest node in the tree structure of block division), the hierarchical order of encoding (CU by CU then TU by TU) discussed above means that there may be previously encoded samples in the current CU and available to the coding of that TU which are, for example, above-right or below-left of that TU.

The block 810 represents a CU; as discussed above, for the purposes of intra-image prediction processing, this may be subdivided into a set of smaller units. An example of a current TU 830 is shown within the CU 810. More generally, the picture is split into regions or groups of samples to allow efficient coding of signalling information and transformed data. The signalling of the information may require a different tree structure of sub-divisions to that of the transform, and indeed that of the prediction information or the prediction itself. For this reason, the coding units may have a different tree structure to that of the transform blocks or regions, the prediction blocks or regions and the prediction information. In some examples such as HEVC the structure can be a so-called quad tree of coding units, whose leaf nodes contain one or more prediction units and one or more transform units; the transform units can contain multiple transform blocks corresponding to luma and chroma representations of the picture, and prediction could be considered to be applicable at the transform block level. In examples, the parameters applied to a particular group of samples can be considered to be predominantly defined at a block level, which is potentially not of eth same granularity as the transform structure.

The intra-image prediction takes into account samples coded prior to the current TU being considered, such as those above and/or to the left of the current TU. Source samples, from which the required samples are predicted, may be located at different positions or directions relative to the current TU. To decide which direction is appropriate for a current prediction unit, the mode selector 520 of an example encoder may test all combinations of available TU structures for each candidate direction and select the prediction direction and TU structure with the best compression efficiency.

The picture may also be encoded on a "slice" basis. In one example, a slice is a horizontally adjacent group of CUs. But in more general terms, the entire residual image could form a slice, or a slice could be a single CU, or a slice could be a row of CUs, and so on. Slices can give some resilience to errors as they are encoded as independent units. The encoder and decoder states are completely reset at a slice boundary. For example, intra-prediction is not carried out across slice boundaries; slice boundaries are treated as image boundaries for this purpose.

Figure 10 schematically illustrates a set of possible (candidate) prediction directions. The full set of candidate directions is available to a prediction unit. The directions are determined by horizontal and vertical displacement relative to a current block position, but are encoded as prediction "modes", a set of which is shown in Figure 11. Note that the so-called DC mode represents a simple arithmetic mean of the surrounding upper and left-hand samples. Note also that the set of directions shown in Figure 10 is just one example; in other examples, a set of (for example) 65 angular modes plus DC and planar (a full set of 67 modes) as shown schematically in Figure 12 makes up the full set. Other numbers of modes could be used.

In general terms, after detecting a prediction direction, the systems are operable to generate a predicted block of samples according to other samples defined by the prediction direction. In examples, the image encoder is configured to encode data identifying the prediction direction selected for each sample or region of the image.

Figure 13 schematically illustrates an intra-prediction process in which a sample 900 of a block or region 910 of samples is derived from other reference samples 920 of the same image according to a direction 930 defined by the intra-prediction mode associated with that sample. The reference samples 920 in this example come from blocks above and to the left of the block 910 in question and the predicted value of the sample 900 is obtained by tracking along the direction 930 to the reference samples 920. The direction 930 might point to a single individual reference sample but in a more general case an interpolated value between surrounding reference samples is used as the prediction value. Note that the block 910 could be square as shown in Figure 13 or could be another shape such as rectangular.

Figure 14 provides a more detailed representation of the intra predictor 530 of Figure 8. In particular, the intra predictor 530 comprises a reference sample filter 1600 and a prediction unit 1610 which acts to predict a current sample of a current region of an image with respect to one or more reference samples of the same image according to a prediction direction between the current sample and a reference position amongst the reference samples.

The reference sample filter applies a filtering operation to at least some of the reference samples corresponding to or applicable to the current predicted region. Here, the reference samples "corresponding to" or "applicable to" the current region are those which are for use, in an intra prediction process, for prediction of samples of the current region. These can include a "full set" of reference samples such as the samples 920 of Figure 13, even though the actual use of the full set of reference samples, for a particular image region having a particular associated prediction mode or direction, may require only some of the full set of reference samples. For example, if the prediction mode is substantially "vertical", reference samples to the left of the current region may not be used in that particular instance, but can still be considered part of the full set of reference samples. Note that techniques have been proposed to repeat or extrapolate reference samples to reference sample positions in the full set which are not available, because samples at those positions have not yet been decoded.

Alternatively the reference samples applicable to a current region could encompass those reference samples which, according to the particular prediction mode in use for that current region, are used in prediction of samples at sample positions in the current region.

The reference sample filter operates under the control of the controller 343. As before, other operations of the controller 343 (such as controlling the operation of the prediction unit 1610) are not shown in Figure 14. The controller 343 controls aspects of the operation of the reference sample filter such as: the type of filtering to be used in respect of reference samples corresponding to a particular predicted region; and filter coefficient data defining the operation of the selected type of filter. In some examples, the controller 343 can control the reference sample filter 1600 not to act on the current set of reference samples (either by bypassing the reference sample filter 1600 or by controlling the reference sample filter 1600 to apply a "dummy" filtering operation which makes no difference to the reference samples for the current predicted region).

The prediction unit operates under the control of the intra mode selector 520 as discussed above.

The prediction unit 1610 as described provides an example of an intra-image predictor configured to predict a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image according to a prediction direction between the current sample and a reference position amongst the reference samples. The filter 1600 provides an example of a reference sample filter to selectively apply the selected filter to at least some of the reference samples corresponding to the current region. The controller 343 acts, in this context, as a selector configured to select a filter, of a set of two or more filters each defined by the same filtering operation and a respective different set of filter parameter data, to be applied to reference samples applicable to a current image region. At the encoder side, each filter of the set of two or more filters may be selectable by the selector according to a respective different set of selection criteria dependent upon properties of the image. For example, the properties may include one or more aspects such as: block size, colour component (luma or not, for example), whether a secondary transform is being used, number of non-zero coefficients and the like. At the decoder side, each filter of the set of two or more filters may be selectable by the selector in response to an indication of filter selection in data input to the apparatus.

Various types of filtering have been proposed for the operation of the reference sample filter 1600. For example, the so-called HEVC system employs different reference sample filtering arrangements such as a so-called "strong" filter or a 3-tap filter when a certain set of conditions (which may be referred to as Mode Dependant Intra Smoothing ("MDIS") conditions are met so as to indicate that filtering is to be applied. If filtering is selected under the MDIS conditions, then a set of conditions applicable to the strong filtering are tested, in order to select between the strong filter and the 3-tap filter.

The 3-tap filter uses two neighbouring reference samples such that a particular reference sample is replaced by a filtered version generated from that reference sample and the immediate neighbours. The so-called strong smoothing process uses a subset of reference samples (so-called corner samples) and a linear interpolation process.

In the so-called JEM (Joint Exploration Model) system of the Joint Video Exploration Team (JVET) of ITU-T and ISO/IEC MPEG such as in the JEM 6.0 proposal, various filtering tools have been proposed, one of which is called position dependant intra prediction combination (PDPC). PDPC will be discussed in detail here.

The controller 343, in this aspect of its operation, acts upon one or more input parameters 1620. In the case of an encoder, various forms of these input parameters will be discussed below by way of examples. Also in the case of an encoder, the controller 343 generates and encodes in the data stream one or more flags 1630 to indicate the nature of the filtering performed. In the case of a decoder, the controller 343 is responsive to the one or more flags as the inputs 1620, and output flags 1630 are not generated.

Figure 14 provides an example of an image data encoding (or decoding -see note below) apparatus comprising: an intra-image predictor 1610 to predict a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image; a reference sample filter 1600 o selectively filter to at least some of the reference samples corresponding to the current image region; in which the intra-image predictor is operable in a position dependent intra prediction combination (PDPC) mode in which a prediction of a current sample of the current image region has a weighted dependency upon: one or more corresponding reference samples, as filtered by the reference sample filter; and the one or more corresponding reference samples as not filtered by the reference sample filter; a PDPC mode selector 343; 1920, 2010 (see below) to selectively enable or disable the operation of the PDPC mode, the PDPC mode selector being responsive to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion.

The decoder can make these decisions in its own right, or can in other examples be controlled by information provided by the encoder indicating whether PDPC is to be applied to a block.

Figures 15A and 15B schematically illustrate sample relationships for a PDPC filtering tool. The operation of the tool will be discussed further below with reference to Figures 18 and 19.

In Figures 15A and 15B, reference samples are shown as shaded blocks 1700 and samples to be predicted as shown as unshaded blocks 1710. A coordinate system [x, y] is used to identify individual reference samples and predicted samples.

Two forms of the reference samples are shown: unfiltered reference samples r[x, y] and filtered reference samples s[x, y] which are equivalent to r[x, y] smoothed by a low-pass filter. A prediction q[x, y] represents an intra prediction based on the filtered reference samples s[x, y]. An output prediction p[x, y] combines weighted elements of r[x, y] with q[x, y]. The weights used to obtain p[x, y] and s[x, y] can depend upon the intra prediction mode specified by the intra mode selector 520.

Figure 16 is a schematic flowchart illustrating the process discussed above.

At a step 1900, the unfiltered reference samples r[x, y] are filtered to generate s[x, y]. At a step 1910, a weighted average of r[x, y] and s[x, y] is generated and provided to a step 1920 of which a prediction q[x, y] is produced. At a step 1930 a weighted combination of r[x, y] and q[x, y] is generated to provide a predicted output p[x, y].

This provides an example in which, in the PDPC mode, the intra-image predictor is configured to generate a prediction of a current sample of the current image region according to a weighted combination of: one or more corresponding reference samples, as filtered by the reference sample filter; and a prediction value derived from the one or more corresponding reference samples as not filtered by the reference sample filter.

Example arrangements to be discussed below involve allowing or disallowing the use of PDPC in dependence upon a detection of one or more properties of a block to be encoded, such as an detection of whether the block to be encoded is of at least a certain size. For example, the threshold size could be 8 luminance samples, so that PDPC is for example disallowed if any dimension of a block is under 8 samples. This provides an example of making such a decision on the basis of a predetermined property of the current image region comprising a region size of the current image region. In some examples, this can give improved encoding results in respect of image content (such as so-called screen content material) for which the use of very small blocks is potentially more prevalent. With regard to the threshold, this provides an example in which the image regions are rectangular regions having a width and a height; and the predetermined criterion is whether each of the width and the height of the current image region have at least a respective predetermined size.

As background to some of the examples, some aspects of an encoding block structure will first be described.

In general, in the present examples, all three components (such as Y,Cb,Cr) may be encoded, for example using an identical block structure, although different block structures for the different components may be employed. A CU refers to a coding unit; a PU refers to a prediction unit and defines the parameters for the prediction unit; and a TU refers to a transform unit, or in other words the block of data that is to be transformed The HEVC system uses a system of a so-called quad tree (having a root node and a set of leaf nodes) of CUs, where each leaf CU represents one or more PUs. Each leaf CU represents the root of a transform quad tree.

The JEM system uses QTBT (quad tree binary tree), in that each root CU splits down by a quad tree. Each leaf then optionally splits further with a binary tree. Each of those leaves then represents a PU, CU, TU without distinction.

As an example, Figure 17 schematically illustrates a so-called root coding unit (CU) 2800, shown as a region 2810 on the right hand side of Figure 17. A quad tree division of the root CU 2800 produces four sub CUs 2820, one of which 2830 is shown as being divided by a further four-way division into smaller CUs 2840, one of which 2850 is split by a binary division into coding units 2860.

This arrangement provides an example in which a current image region is a sub-region of a larger image region in a hierarchy of image regions.

Figure 18 schematically illustrates an example technique by which PDPC can be allowed for selection or disallowed for selection in dependence upon a current block size.

The example of Figure 18 uses the tree-based decision (whether quad tree or binary tree) of Figure 17, in which a higher level block 2900 such as a root block 2800 or the blocks 2830, 2850 is partitioned using the example processes shown in Figure 17 at a step 2910. If further partitioning is required at a step 2920 then control returns to this step 2910. Properties of the resulting partitioned block (which becomes the current block for encoding purposes) are detected and compared to one or more conditions at a step 2930. For example, the condition may be whether the block size (or one dimension of the block) exceeds a threshold size such as 8x8 samples (for square blocks) or 8 pixels (where a condition is applied to one dimension). The outcome of this test is used to control whether or not PDPC is enabled, so that if the block property meets the condition (for example, larger than the threshold size) then PDPC is enabled for selection at a step 2940. If the block property fails to meet the condition then PDPC is disallowed for selection at a step 2950.

It will be appreciated that conditions can be imposed in either polarity, so that, for example, a small block may fail a condition expressed as "does the block have at least a threshold size?", or may pass an entirely equivalent condition expressed with the other polarity as for example "is the block smaller than a threshold size?". These two expressions of conditionality are entirely equivalent. Similarly, entirely equivalent formulations can be established with reference to a threshold such as a threshold block size. For example, a test of "are one or more dimensions of the block smaller than 9 samples?" is equivalent to a test of "are one or more dimensions of the block 8 samples or fewer?". In other words, a condition of "x<=N" is entirely equivalent to a test of "x < (N + delta)".

The allowance 2940 of PDPC does not itself imply that PDPC will be selected for use in respect of a block. The selection by the controller 343 of whether to use PDPC applies other criteria which may be known criteria for the selection of PDPC. For example, the PDPC mode selection is configured to detect whether to select the PDPC mode for the current region when the predetermined property of the current region meets a predetermined criterion, in response to data indicating whether an encoding efficiency improvement is predicted to be achieved using PDPC. However, the disallowance of PDPC at the step 2950 does, for that block, imply that PDPC is not used for that block, whatever the results of any other PDPC selection criteria and tests may be.

Figure 19 schematically illustrates a part of an encoder having an intra-image predictor 1900 operable in a PDPC mode or not in a PDPC mode according to control data 1910 received from a PDPC mode selector 1920. The PDPC mode selector is responsive to a detector 1930 which performs at least a detection of whether a current image region meets a predetermined criterion as discussed above. An example as discussed of the predetermined criterion is whether the current region or block has at least a minimum block size. If not, then the PDPC mode selector disallows the use of PDPC for that block.

Another optional aspect of the detector 1930 is to detect, for other blocks (which do meet the criterion) whether or not the PDPC should be used, for example by detecting whether an encoding efficiency improvement is predicted to be achieved using PDPC.

The PDPC mode selector 1920 generates and provides, in association with the encode data stream, identifying data 1940 indicating, for a particular block or region, whether PDPC has been used.

In Figure 20, at the decoder side, a detector 2000 is responsive to the identifying data 1940 to control a PDPD mode selector 2010 to indicate to an intra-image predictor 2020 whether or not to use a PDPC. As before, the PDPC mode selector can disallow the use of PDPC for block that fail to meet the predetermined criterion. Alternatively, the control of PDPC usage at the decoder can be simply according to what identifying data 1940 received from the encoder side.

Figure 21 is a schematic flowchart illustrating an image data encoding method comprising: predicting (at a step 2100) a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image; selectively filtering (at a step 2110) at least some of the reference samples corresponding to the current image region; in which the predicting step is operable in a position dependent intra prediction combination (PDPC) mode in which a prediction of a current sample of the current image region has a weighted dependency upon: one or more corresponding reference samples, as filtered by the reference sample filter; and the one or more corresponding reference samples as not filtered by the reference sample filter; and selectively enabling or disabling (at a step 2120) the PDPC mode in response to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion.

Figure 22 is a schematic flowchart illustrating an image data decoding method comprising: predicting (at a step 2200) a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image; selectively filtering (at a step 2210) at least some of the reference samples corresponding to the current image region; in which the predicting step is operable in a position dependent intra prediction combination (PDPC) mode in which a prediction of a current sample of the current image region has a weighted dependency upon: one or more corresponding reference samples, as filtered by the reference sample filter; and the one or more corresponding reference samples as not filtered by the reference sample filter; and selectively enabling or disabling (at a step 2220) the PDPC mode in response to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion.

In so far as embodiments of the disclosure have been described as being implemented, at least in part, by software-controlled data processing apparatus, it will be appreciated that a non-transitory machine-readable medium carrying such software, such as an optical disk, a magnetic disk, semiconductor memory or the like, is also considered to represent an embodiment of the present disclosure. Similarly, a data signal comprising coded data generated according to the methods discussed above (whether or not embodied on a non-transitory machine-readable medium) is also considered to represent an embodiment of the present disclosure.

It will be apparent that numerous modifications and variations of the present disclosure are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended clauses, the technology may be practised otherwise than as specifically described herein.

Respective example embodiments are disclosed by the following numbered clauses: 1. Image data encoding apparatus comprising: an intra-image predictor to predict a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image; a reference sample filter to selectively filter to at least some of the reference samples corresponding to the current image region; in which the intra-image predictor is operable in a position dependent intra prediction combination (PDPC) mode in which a prediction of a current sample of the current image region has a weighted dependency upon: one or more corresponding reference samples, as filtered by the reference sample filter; and the one or more corresponding reference samples as not filtered by the reference sample filter; a PDPC mode selector to selectively enable or disable the operation of the PDPC mode, the PDPC mode selector being responsive to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion.

2. Apparatus according to clause 1, in which the predetermined property of the current image region comprise a region size of the current image region.

3. Apparatus according to clause 2, in which: the image regions are rectangular regions having a width and a height; and the predetermined criterion is whether each of the width and the height of the current image region have at least a respective predetermined size.

4. Apparatus according to any one of the preceding clauses, in which the reference sample filter comprises a low pass filter.

5. Apparatus according to any one of the preceding clauses, in which the current image region is a sub-region of a larger image region in a hierarchy of image regions.

6. Apparatus according to any one of the preceding clauses, in which the intra-image predictor is configured to generate a prediction of a current sample according to a prediction direction between the current sample and a reference position amongst the reference samples.

7. Apparatus according to any one of the preceding clauses, in which, in the PDPC mode, the intra-image predictor is configured to generate a prediction of a current sample of the current image region according to a weighted combination of: one or more corresponding reference samples, as filtered by the reference sample filter; and a prediction value derived from the one or more corresponding reference samples as not filtered by the reference sample filter.

8. Apparatus according to clause 1, in which the PDPC mode selector is configured to detect whether to select the PDPC mode for the current region when the predetermined property of the current region meets a predetermined criterion, in response to data indicating whether an encoding efficiency improvement is predicted to be achieved using PDPC.

9. Apparatus according to any one of the preceding clauses, in which the PDPC mode selector is configured to generate indicator data, to be provided to an image data decoding apparatus, indicating whether or not the PDPC mode has been used in respect of a current image region.

10. Image data decoding apparatus comprising: an intra-image predictor to predict a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image; a reference sample filter to selectively filter to at least some of the reference samples corresponding to the current image region; in which the intra-image predictor is operable in a position dependent intra prediction combination (PDPC) mode in which a prediction of a current sample of the current image region has a dependency upon: one or more corresponding reference samples, as filtered by the reference sample filter; and the one or more corresponding reference samples as not filtered by the reference sample filter; a PDPC mode selector to selectively enable or disable the operation of the PDPC mode, the PDPC mode selector being responsive to indicator data provided by an image data encoding apparatus, indicating whether the PDPC mode should be used, and also to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion.

11. Video storage, capture, transmission or reception apparatus comprising apparatus according to any one of clauses 1 to 9.

12. Video storage, capture, transmission or reception apparatus comprising apparatus according to clause 10.

13. An image data encoding method comprising: predicting a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image; selectively filtering at least some of the reference samples corresponding to the current image region; in which the predicting step is operable in a position dependent intra prediction combination (PDPC) mode in which a prediction of a current sample of the current image region has a weighted dependency upon: one or more corresponding reference samples, as filtered by the reference sample filter; and the one or more corresponding reference samples as not filtered by the reference sample filter; and selectively enabling or disabling the PDPC mode in response to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion.

14. Computer software which, when executed by a computer, causes the computer to carry out a method according to clause 13.

15. A machine-readable non-transitory storage medium which stores software according to clause 14.

16. A data signal comprising coded data generated according to the method of clause 13.

17. An image data decoding method comprising: predicting a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image; selectively filtering at least some of the reference samples corresponding to the current image region; in which the predicting step is operable in a position dependent intra prediction combination (PDPC) mode in which a prediction of a current sample of the current image region has a weighted dependency upon: one or more corresponding reference samples, as filtered by the reference sample filter; and the one or more corresponding reference samples as not filtered by the reference sample filter; and selectively enabling or disabling the PDPC mode in response to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion.

18. Computer software which, when executed by a computer, causes the computer to carry out a method according to clause 17.

19. A machine-readable non-transitory storage medium which stores software according to clause 18.

Claims

CLAIMS1. Image data encoding apparatus comprising: an intra-image predictor to predict a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image; a reference sample filter to selectively filter to at least some of the reference samples corresponding to the current image region; in which the intra-image predictor is operable in a position dependent intra prediction combination (PDPC) mode in which a prediction of a current sample of the current image region has a weighted dependency upon: one or more corresponding reference samples, as filtered by the reference sample filter; and the one or more corresponding reference samples as not filtered by the reference sample filter; a PDPC mode selector to selectively enable or disable the operation of the PDPC mode, the PDPC mode selector being responsive to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion.
2. Apparatus according to claim 1, in which the predetermined property of the current image region comprise a region size of the current image region.
3. Apparatus according to claim 2, in which: the image regions are rectangular regions having a width and a height; and the predetermined criterion is whether each of the width and the height of the current image region have at least a respective predetermined size.
4. Apparatus according to claim 1, in which the reference sample filter comprises a low pass filter.
5. Apparatus according to claim 1, in which the current image region is a sub-region of a larger image region in a hierarchy of image regions.
6. Apparatus according to claim 1, in which the intra-image predictor is configured to generate a prediction of a current sample according to a prediction direction between the current sample and a reference position amongst the reference samples.
7. Apparatus according to claim 1, in which, in the PDPC mode, the intra-image predictor is configured to generate a prediction of a current sample of the current image region according to a weighted combination of: one or more corresponding reference samples, as filtered by the reference sample filter; and a prediction value derived from the one or more corresponding reference samples as not filtered by the reference sample filter.
8. Apparatus according to claim 1, in which the PDPC mode selector is configured to detect whether to select the PDPC mode for the current region when the predetermined property of the current region meets a predetermined criterion, in response to data indicating whether an encoding efficiency improvement is predicted to be achieved using PDPC.
9. Apparatus according to claim 1, in which the PDPC mode selector is configured to generate indicator data, to be provided to an image data decoding apparatus, indicating whether or not the PDPC mode has been used in respect of a current image region.
10. Image data decoding apparatus comprising: an intra-image predictor to predict a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image; a reference sample filter to selectively filter to at least some of the reference samples corresponding to the current image region; in which the intra-image predictor is operable in a position dependent intra prediction combination (PDPC) mode in which a prediction of a current sample of the current image region has a dependency upon: one or more corresponding reference samples, as filtered by the reference sample filter; and the one or more corresponding reference samples as not filtered by the reference sample filter; a PDPC mode selector to selectively enable or disable the operation of the PDPC mode, the PDPC mode selector being responsive to indicator data provided by an image data encoding apparatus, indicating whether the PDPC mode should be used, and also to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion.
11. Video storage, capture, transmission or reception apparatus comprising apparatus according to claim 1.
12. Video storage, capture, transmission or reception apparatus comprising apparatus according to claim 10.
13. An image data encoding method comprising: predicting a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image; selectively filtering at least some of the reference samples corresponding to the current image region; in which the predicting step is operable in a position dependent intra prediction combination (PDPC) mode in which a prediction of a current sample of the current image region has a weighted dependency upon: one or more corresponding reference samples, as filtered by the reference sample filter; and the one or more corresponding reference samples as not filtered by the reference sample filter; and selectively enabling or disabling the PDPC mode in response to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion.
14. Computer software which, when executed by a computer, causes the computer to carry out a method according to claim 13.
15. A machine-readable non-transitory storage medium which stores software according to claim 14.
16. A data signal comprising coded data generated according to the method of claim 13.
17. An image data decoding method comprising: predicting a current sample of a current image region, of a plurality of regions of an image, with respect to one or more corresponding reference samples of the same image; selectively filtering at least some of the reference samples corresponding to the current image region; in which the predicting step is operable in a position dependent intra prediction combination (PDPC) mode in which a prediction of a current sample of the current image region has a weighted dependency upon: one or more corresponding reference samples, as filtered by the reference sample filter; and the one or more corresponding reference samples as not filtered by the reference sample filter; and selectively enabling or disabling the PDPC mode in response to a predetermined property of the current region to disallow the use of the PDPC mode for the current region when the predetermined property of the current region fails to meet a predetermined criterion.
18. Computer software which, when executed by a computer, causes the computer to carry out a method according to claim 17.
19. A machine-readable non-transitory storage medium which stores software according to claim 18.