GB2567863A

GB2567863A - Image data encoding and decoding

Info

Publication number: GB2567863A
Application number: GB1717686.8A
Authority: GB
Inventors: Mark Keating Stephen; James Sharman Karl; Kimlee Miri Philippe Magali
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 2017-10-27
Filing date: 2017-10-27
Publication date: 2019-05-01
Also published as: WO2019081930A1; GB201717686D0

Abstract

Image encoding comprises intra-image prediction of samples of a current region using one or more of the group of reference samples of the same image in dependence upon a prediction direction defined by a selected prediction mode. For at least one or more prediction modes a first subset of samples to be predicted comprising one or more peripheral samples in the current region a reference position amongst the reference samples different to a position pointed to by the prediction direction defined by the selected prediction mode is detected. For a second, remaining, subset of samples to be predicted a reference position amongst the reference samples which is pointed to by the prediction direction defined by the selected prediction mode is detected. The alternative reference sample may be that pointed to by a different direction and may be closer to the reference sample nearest to a given sample to be predicted. Also claimed is a corresponding decoding method.

Description

IMAGE DATA ENCODING AND DECODING

BACKGROUND

Field

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/MPEG-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 χ 96 to 7680 χ 4320 pixels resolution, roughly equivalent to bit rates ranging from 128kbit/s to 800M bit/s.

SUMMARY

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 (A/V) 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;

Figures 14 and 15 schematically illustrate a reference sample projection process;

Figure 16 schematically illustrates a predictor;

Figures 17 and 18 schematically illustrate the use of projected reference samples;

Figures 19 to 24 schematically illustrate example prediction directions;

Figures 25a to 25e schematically illustrate examples of difference data;

Figure 26 schematically illustrates a mode selector and intra-image predictor;

Figure 27 schematically illustrates a delta memory arrangement;

Figures 28 and 29 schematically illustrate block subsets; and

Figures 30 and 31 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.

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.

A combination of Figure 2 and 4 may provide a video capture apparatus comprising an image sensor 180 and encoding apparatus 190, decoding apparatus 110 and a display 120 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 1020% 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: socalled 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 l-frame encoding in other video compression techniques. In contrast to l-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 encodeddecoded 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 nonsquare 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 belowleft of thatTU.

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 the 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 (and the image decoder is configured to detect such data).

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.

Figures 14 and 15 schematically illustrate a previously proposed reference sample projection process.

In Figures 14 and 15, a block or region 1400 of samples to be predicted is surrounded by linear arrays of reference samples from which the intra prediction of the predicted samples takes place. The reference samples 1410 are shown as shaded blocks in Figures 14 and 15, and the samples to be predicted are shown as unshaded blocks. Note that an 8x8 block or region of samples to be predicted is used in this example, but the techniques are applicable to variable block sizes and indeed block shapes.

As mentioned, the reference samples comprise at least two linear arrays in respective orientations with respect to the current image region of samples to be predicted. For example, the linear arrays may be an array or row 1420 of samples above the block of samples to be predicted and an array or column 1430 of samples to the left of the block of samples to be predicted.

As discussed above with reference to Figure 13, the reference sample arrays can extend beyond the extent of the block to be predicted, in order to provide for prediction modes or directions within the range indicated in Figures 10-12. Where necessary, if previously decoded samples are not available for use as reference samples at particular reference sample positions, other reference samples can be re-used at those missing positions. Reference sample filtering processes can be used on the reference samples.

A sample projection process is used to project at least some of the reference samples to different respective positions with respect to the current image region, in the manner shown in Figures 14 and 15. In other words, in embodiments, the projection process and circuitry operates to represent at least some of the reference samples at different spatial positions relative to the current image region, for example as shown in Figures 14 and 15. Thus at least some reference samples may be moved (for the purposes at least of defining an array of reference samples from which samples are predicted) with respect to their relative positions to the current image region. In particular, Figure 14 relates to a projection process performed for modes which are generally to the left of the diagonal mode (18 in Figure 11) mainly modes

2.. . 17, and Figure 15 schematically illustrates a reference sample projection carried for modes

19.. .34, namely those generally above the block to be predicted (to the right of the diagonal mode 18). The diagonal mode 18 can be assigned to either of these two groups as an arbitrary selection, such as to the group of modes to the right of the diagonal. So, in Figure 14, when the current prediction mode is between modes 2 and 17 (or their equivalent in a system such as that of Figure 12 having a different number of possible prediction modes), the sample array 1420 above the current block is projected to form additional reference samples 1440 in the left hand column. Prediction then takes place with respect to the linear projected array 1450 formed of the original left hand column 1430 and the projected samples 1440. In Figure 15, for modes between 18 and 34 of Figure 11 (or their equivalent in other sets of prediction modes such as those shown in Figure 12), the reference samples 1500 in the left hand column are projected so as to extend to the left of the reference samples 1510 above the current block. This forms a projected array 1520.

One reason why projection of this nature is carried out is to reduce the complexity of the intra prediction process, in that all of the samples to be predicted are then referencing a single linear array of reference samples, rather than referencing two orthogonal linear arrays.

Figure 16 schematically illustrates a previously proposed prediction circuitry 600 arranged to carry out the projection process of Figures 14 and 15, specifically by providing projection circuitry 1610 configured to carry out a projection process on the reference samples currently selected for a block of region to be predicted. The projected reference samples are stored in a buffer 1620 to be accessed by an intra predictor 1630 to generate predicted samples from the projected reference samples. The projection process is carried out according to the prediction mode associated with the current block to be predicted, using the techniques discussed in connection with Figures 14 and 15.

In embodiments, the same projection process is carried out in the decoder and in the encoder, so that the predicted samples are the same in each instance.

Possible variations in operation between the use of prediction modes which will be referred to as “straight modes” and prediction modes which will be referred to as “curved modes” will now be discussed.

As further background, Figures 17 and 18 schematically illustrate an example technique by which samples 1900 of a current region 1910 or block to be predicted, are predicted from reference samples 1920. In this example, the reference samples have been projected into a linear array using the techniques described with reference to Figures 14-16 above.

A system of (x, y) coordinates is used for convenience, to allow individual reference or predicted sample positions to be identified. In the example of Figure 17, x coordinates are shown by a row 1930 of numbers, and y coordinates are shown by a column 1940 of numbers. So, each reference or predicted sample position has an associated (x, y) designation using the coordinate system.

In the example of Figure 17, a generally vertical mode (for example, a mode which is more vertical than horizontal) 1950, such as mode 23 in the designation of Figure 11, noting that a different mode number could be used if the set of modes shown in Figure 12 were employed, has been selected for prediction of samples 1900 of the block or region 1910. As discussed above with reference to Figures 14-16, such a generally vertical prediction mode is handled by the circuitry of Figure 16 by projecting the left column of reference samples into an extension 1960 of the reference samples above the block or region 1910.

Each of the samples to be predicted 1900 is predicted as follows. For each sample to be predicted, there is an associated (x, y) location such as a location (0, 5) for a sample 1970 or a location (0, 4) for a sample 1972. These two samples are used purely by way of example and the same technique applies to each of the samples 1900 to be predicted.

The sample positions of the samples 1970, 1972 to be predicted are mapped according to the direction 1950 associated with the current prediction mode to respective locations or reference positions 1974, 1976 among the reference samples. This mapping may be carried out using an expression such as that shown below, noting that this is a linear expression with respect to the coordinate system (x, y):

For horizontal modes 2-17 in the notation of Figure 11:

predicted value (x. y) = {1 -f(p)} χ ref [y+i(p)] + f(p) ^x ref [y+i(p)+1 ] with p =A χ (x+1)

For vertical modes 18-34 in the notation of Figure 11:

predicted value (x. y) = {1 -f(p)} χ ref [x+i(p)] + f(p) ^x ref [x+i(p)+1 ] with p = A χ (y+1) and where i(p)=floor(p), is the value p rounded down (towards negative infinity) to the nearest integer, f(p)=p-i(p) represents the fractional part of the value p.

A is an angle parameter indicating the angle of the current mode. To illustrate, for example, for a horizontal or vertical line, A would be 0; for a 45° diagonal line, A would be ±1.

Those skilled in the art would appreciate that integer approximations can be used to simplify the linear equations, for example, representing the angle parameter A as a fractional fixed-precision number. In HEVC, the angles have an accuracy of 5 fractional bits.

So, for example, each sample to be predicted is associated with a coordinate position within the current region; and the intra-image predictor is configured to detect the reference position for a given sample to be predicted as a function of the coordinate position of the given sample to be predicted, the function depending upon the selected prediction mode.

In example arrangements, the reference position 1974, 1976 is detected to an accuracy or resolution of less than one sample, which is to say with reference to the reference sample locations (-5, -1)...(15, -1), a fractional value is used for the x coordinate of the reference position within the projected set of reference samples 1920. For example, the reference position could be detected to a resolution of 1/32 of a sample separation, so that the x coordinate of the reference positions 1974, 1976 is identified to that resolution. The y coordinate of the reference position is -1 in each case, but this is in fact irrelevant to the calculations that then take place, which relate to interpolation along the x axis of the reference samples 1920.

The prediction of the predicted values 1970, 1972 is an interpolation of the value applicable to the detected x coordinate of the reference sample position 1974, 1976, for example as described above in the formulae shown earlier.

A similar arrangement is shown schematically in Figure 18, except that a generally horizontal prediction mode, for example a prediction mode which is more horizontal than vertical, such as (for example) mode 14 of the set shown in Figure 11 (or a corresponding number for a similar mode in the set shown in Figure 12) having a prediction direction 2000 is used. The selection of the particular prediction mode applies to the whole of the block or region 2010 of samples 2020 to be predicted and the particular example here is chosen purely for the purposes of the present explanation.

In the case of a generally horizontal mode, as discussed above, the projection circuitry shown in Figure 16 projects those reference samples from above the block or region 2010 to form an extension 2030 of reference samples to the left of the region. Once again, the derivation of two example samples to be predicted, samples 2032, 2034, is shown, such that the sample position 2032, 2034 are mapped using the direction 2000 into reference positions 2036, 2038 amongst the set of reference samples 2040. Once again, a similar (x, y) coordinate system is used and the reference positions 2036, 2038 are expressed to a 1/32 sample resolution in the y-direction. The x coordinate of the reference sample positions is -1 but this is irrelevant to the process which follows. The sample values of the samples to be predicted are obtained in the manner described above.

In these arrangements, the intra predictor 530 provides an example of a detector configured to detect the reference position as an array position, with respect to an array of the reference samples, pointed to by the prediction direction applicable to the current sample to be predicted; and a filter configured to generate the predicted sample by interpolation of the array of reference samples at the detected array position. The detector may be configured to detect the array position to an accuracy of less than one sample such as 1/32 sample.

The intra mode selector 520 the selector may be configured to perform at least a partial encoding to select the prediction mode.

Figures 19 to 24 schematically illustrate example prediction directions.

In particular, these drawings will be used to represent example embodiments in which a so-called non-uniform prediction direction can be used. The representations of Figures 19 to 24 are with respect to unprojected reference samples (the reference samples 2100 being shown schematically as shaded squares surrounding samples 2110 to be predicted at an upper side and a left side), but it will be appreciated that the projection process discussed above can be employed with all of these arrangements.

Figures 19 and 20 relate to a horizontal prediction mode defining a horizontal prediction direction 2120 with respect to an array, or group, or region 2130 of samples 2110 to be predicted. The example shown is of an 8x8 array of samples to be predicted, but the present techniques are applicable to various different array sizes and shapes, not necessarily square arrays. So, generally speaking, the current region 2130 simply comprises an array of two or more rows and two or more columns of samples to be predicted.

The situation shown in Figure 19 represents a previously proposed intra prediction process, so that in accordance with the direction 2120, samples 2110 are predicted according to reference positions in the left hand column 2140 of reference samples. So, considering (by way of example only) a second row 2150 of samples to be predicted, each of the samples to be predicted in the second row 2150 is derived from a reference position 2160 in the set of reference samples.

Similar conditions apply to each of the rows of samples in the arrangement of Figure 19 using a horizontal prediction direction 2120.

However, considering, for example, a particular sample 2170 to be predicted, the sample 2170 is in fact spatially much closer to a reference sample 2180 than to the reference sample 2190 which would be used for prediction under the previously proposed scheme using the horizontal prediction direction 2120. Similarly, a sample 2175 to be predicted is closer to a reference sample 2185 than it is to the reference sample 2190.

Therefore, in the example of a horizontal prediction direction 2120, a variation of the prediction technique of Figure 19 can be used.

Referring to Figure 20, and considering first a top row 2200 of samples to be predicted, as discussed earlier each of these is generally closer to the reference sample above that sample to be predicted than it is to a sample 2190 from which that sample would be predicted under the previously proposed horizontal prediction arrangement shown in Figure 19. Therefore, for at least some of the samples of the top row, namely a group of samples 2205, instead of using a horizontal prediction direction, these samples are predicted using vertical prediction, from the directly adjacent reference sample above that sample to be predicted.

For the samples 2210 of the top row 2200, while the samples are close to the reference samples immediately above them, they are also not very far from the reference sample 2190. This is particularly the case for the top left sample to be predicted, a sample 2215. So, for these samples 2210, rather than using a purely vertical prediction direction, a prediction direction 2220, 2225 is used which is diagonal, and in this example progressively more diagonal (that is to say, having a larger horizontal component, or further from the vertical direction used for other samples of the top row) for the top left sample than for the next sample along. This progressive change in direction takes into account the fact that these samples to be predicted of the top row are close to the reference sample 2190 as well as being close to their vertically adjacent reference samples.

Considering next a second row 2230 of samples to be predicted, a left-most sample 2240 is in fact closest to a reference sample 2235 which would be used under horizontal prediction. So, a horizontal prediction mode is used for the left-most sample to be predicted 2240. Progressing from there towards the right of the row 2230, diagonal prediction is used to reflect the fact that the samples to be predicted are close to the reference sample 2235 as well as to the reference samples vertically above them. Once again, a prediction direction 2245 having a greater horizontal component can be used for the second sample 2250 from the left, and then a slightly more vertically oriented diagonal prediction direction 2255 can be used for other samples of the row 2230. The slightly more horizontal direction 2245 reflects the fact that the second sample 2250 is not far from the reference sample 2235 which would otherwise be used under horizontal prediction.

For the remaining rows 2260 of samples to be predicted, the prediction direction 2120 as set by the current prediction mode is used.

This arrangement therefore provides an example in that, for at least one or more prediction modes (for example, the horizontal prediction mode, though in connection with Figures 21 to 24 other example prediction modes will be discussed using this technique), for a first subset of samples to be predicted, the first subset comprising one or more peripheral samples in the current region 2130, (the subset including the rows 2200 and 2230, except for the sample 2240) the intra-image predictor is configured to detect a reference position among the reference samples (indicated by the head of the respective arrow and discussed above) different to a position (directly left in the column 2140 of reference samples as shown in Figure 19) pointed to by the prediction direction defined by the selected prediction mode. For a second, remaining, subset of samples to be predicted (2260, 2240) the intra-image predictor is configured to detect a reference position amongst the reference samples pointed to by the prediction direction defined by the selected prediction mode.

In the examples shown, the reference samples 2100 represent sample positions above and to the left of the current region 2130. In example arrangements, such as that shown in Figures 19 and 20, for a prediction mode defining a generally horizontal direction, the first subset of samples can include at least one sample of a top row 2200 of samples to be predicted. In fact, in some examples such as that shown in Figure 20, the first subset can comprise at least a whole row of samples spatially closest to the reference samples, such as the row 2200. Optionally, the first subset may comprise further peripheral samples, such as the row 2230 excluding the sample 2240.

In Figures 21 and 22, an example will be discussed for a prediction mode defining a generally vertical direction, in which the first subset of samples includes at least one sample of a left-most column of samples to be predicted. In fact, in the example of Figure 22 to be discussed below, the first subset can comprise at least a column of samples spatially closest to the reference samples. In fact, in the example of Figure 22, most of the samples of a second left-most column are also included in the first subset.

The term “generally horizontal” and the term “generally vertical” can refer to prediction modes having associated directions which (respectively) have a greater horizontal than vertical component, or a greater vertical than horizontal component. So, “generally horizontal” could refer, in the notation of Figure 11 or its equivalent in the case of Figure 12, to the modes 2-17, and “generally vertical” could refer to the modes 19-34. The mode 18, being a diagonal mode, could be treated separately or could be arbitrarily joined to one or both of these groups. In an alternative definition, “generally horizontal” could refer to the horizontal mode 10 plus or minus a predetermined number of modes such as (say) three modes, which is to say modes 7-13, or plus or minus five modes, which is to say modes 5-15 and so on. Similarly, “generally vertical” could refer to modes 26 plus or minus a predetermined number of modes such as (say) three modes or five modes.

Turning to Figure 21, a previously proposed prediction process using a vertical prediction direction 2265 is shown, in which each sample to be predicted 2270 is derived according to a respective reference position in a reference sample of a top row 2275 directly above that sample to be predicted.

In contrast, in embodiments of the present disclosure such as that shown in Figure 22, samples in a left-most column of samples to be predicted 2270 are derived from the reference sample immediately to their left (for the lower six samples to be predicted), and the upper two samples of the column 2270 are derived using a diagonal prediction direction to take into account the fact that they are spatially close to the reference sample 2275 which would otherwise be used in vertical prediction as well as being spatially close to reference samples to the left of the samples to be predicted.

In a second left-most column 2280, samples except for a top sample 2285 are derived by a diagonal prediction, with the prediction direction having a greater vertical component when used for a second sample 2290 from the top than the diagonal prediction direction used for other samples in the column 2280.

A third example arrangement will be discussed with reference to Figures 23 and 24, in which a diagonal prediction direction 2300 is used. Here, purely for clarity of the diagram, the prediction direction appropriate to each sample 2310 to be predicted of the current group or block or array 2320 of samples to be predicted is indicated by the intersection of a dot (·) at the centre of that sample position and an arrow pointing to a corresponding reference position within the reference samples 2330. As before, Figure 23 shows a previously proposed arrangement and Figure 24 shows an arrangement according to embodiments of the present disclosure. It is noted that the prediction direction 2300 (for example a prediction mode 14 in the notation of Figure 11, or an equivalent in the notation of Figure 12, is “generally horizontal” which is to say its horizontal component is greater than its vertical component. Therefore, in this example, the prediction directions associated with an upper most row 2400 of Figure 24 are modified with respect to the directions shown in the previously proposed arrangement of Figure 23. In this example, a 45 degree diagonal prediction direction 2410 is used for each sample in the uppermost row 2400 of Figure 24, and the direction drawn for Figure 23 is used for other samples in the array.

In general terms, an inspection of Figures 19 to 24 will show that for a given sample in the first subset, the reference position is displaced, with respect to a reference position pointed to by the prediction direction defined by the selected prediction mode, so as to be closer to reference sample spatially nearest to the given sample. So, in Figure 20 the reference position for the top row of samples is moved nearer to (or as a limit, coincident with) the sample vertically above. In Figure 22 the reference position for the left column of samples is moved nearer to (or as a limit, coincident with) the sample horizontally to the left. In Figure 24 the reference position for the top row of samples is moved nearer to (or as a limit, coincident with) the sample vertically above.

Figures 25a-25e schematically represent examples of so-called difference data.

One example way of implementing the techniques discussed above is for the intra predictor to access difference data. So, a prediction mode (a “base” prediction mode) defining a particular prediction direction can be established for a block or region, and then difference data is applied to some sample positions within the block of samples to be predicted, to define a potentially different way of predicting values for those samples.

The examples of Figures 25a-25e relate to an 8x8 block or region, but as discussed above, these techniques are applicable to various different block sizes and/or shapes.

The examples are associated with respective prediction directions (drawn in the diagrams just below the lower right corner or the block of difference data in each case), namely:

• Figure 25a: a horizontal prediction direction (for example mode 10 in the notation of Figure 11 or the equivalent in the notation of Figure 12);

• Figure 25b: a generally horizontal diagonal prediction direction such as mode 14 in the notation of Figure 11 or the equivalent in the notation of Figure 12;

• Figure 25c: a diagonal prediction direction such as mode 18 in the notation of Figure 11 or the equivalent in the notation of Figure 12;

• Figure 25d: a vertical prediction direction such as mode 26 in the notation of Figure 11 or the equivalent in the notation of Figure 12;

• Figure 25e: a generally vertical direction such as mode 22 in the notation of Figure 11 or the equivalent in the notation of Figure 12.

In operation, the intra-image predictor is configured to access the difference data, defining one or more differences applicable to each sample of the first subset of samples to be 20 predicted, between a respective reference position for use in predicting each sample and a respective reference position pointed to by a prediction direction defined by the selected prediction mode. There can be a set of difference data for each prediction mode, or for subgroups of prediction modes (so that, for example, the data of Figure 25e could apply to modes 19...25 and the data of Figure 25b could apply to modes 17... 11, and so on). More generally, two or more sets of difference data applicable to respective groups of one or more prediction modes are provided. Separate difference data can be provided for each block size and shape, for example.

Starting with Figure 25a, the difference data provides a set of “deltas” d1...d5. A delta value of 0 indicates that the prediction direction associated with the “base” mode (that is to say, the mode selected, from the available set of prediction modes, for prediction of the current region of the current image, should be used. Where a delta value is provided, this can indicate, for example:

• A signed difference to be added to the “A” value in the formula given above; or • A replacement “A” value for use in respect of that sample; or • A second prediction direction, such that the predicted sample value is an average or mean of a prediction obtained using the prediction direction defined by the base mode (the value “A” associated with the block) and a prediction obtained using the second prediction direction defined by the delta value.

Therefore, it will be appreciated that at least the following alternate ways of applying a different prediction to the first subset of predicted samples are disclosed:

• change the prediction direction so as to select and use a different reference position; or • detect a base reference position (corresponding to the direction defined by the base prediction mode) and another direction defined by the difference or other data, and combine the results.

The difference data could be as simple as a flag which says “use vertical” or “use horizontal” in either of these embodiments. Or the difference data could be expressed so as to have three possible values (plus a null or no change value), for example as:

0: use base mode direction

1: use vertical

2: use horizontal

3: use diagonal

Or the difference data could define the required direction, either as an absolute value or as a change to the base mode direction, to a greater resolution, allowing a wider variation of directions to be used.

The skilled person will also appreciate that there are many other ways of defining differences in prediction applicable to some samples which will achieve the same effect as those discussed above.

Purely by way of example, the difference values d1 ...d5 in Figure 25a corresponds to the various alternate prediction directions shown in Figure 20 so, the sample to be predicted at the position 2240 in Figure 20 has a horizontal prediction direction in Figure 20 and a delta value of 0 in Figure 25a. The delta value d1 corresponds to an indication that the diagonal prediction direction 2225 of Figure 20 should be used, and so on.

Note that although the same variables (such as d1) are used in other diagrams, this does not imply that the variable names represent the same difference values.

In Figure 25b, the generally horizontal diagonal nature of the prediction direction means that delta values are provided for the top two rows and the left hand column of sample positions.

In Figure 25c, a 45 degree diagonal mode (for example, mode 18), delta values are provided for the top row and left column of sample positions.

Figure 25d corresponds to the arrangement shown in Figure 22, in which delta values of the left two columns except for the left sample position 2285 are provided.

Figure 25e has delta values for a top row and left two columns.

Referring to Figure 26, which schematically illustrates an intra mode selector 2600 similar in function to the intra mode selector 520 and acting as an example of a controller configured to select, from a set of prediction directions, a prediction mode for prediction of a current region of a current image; and an intra predictor 2610 similar in function to the intra predictor 530 and acting as an example of an intra-image predictor configured to predict samples of the current region with respect to one or more of the group of reference samples of the same image in dependence upon a prediction direction, defined by the selected prediction mode, between a current sample to be predicted and a reference position amongst the reference samples. The intra predictor and the intra mode selector access difference data which can be stored in a delta memory 2620.

In the case of an encoding apparatus, the intra mode selector determines which mode is to be used and provides the selected mode as data 2605 to the intra predictor and also as data 2615 for encoding in the encoded data stream. The intra predictor, in response to the selected mode, generates predicted samples 2630 from reference samples 2635, and in doing so accesses delta values from the delta memory which, for example, are added by an adder 2640 to the “A” values associated with the base mode determined by intra mode selector. In this way, in some examples the intra-image predictor is configured to predict samples in the first subset from the respective detected reference position (one obtained using the difference data). In other examples as discussed above, the intra-image predictor is configured to predict samples in the first subset in dependence upon both of the detected reference position and the position pointed to by the prediction direction defined by the selected prediction mode.

As discussed above, for at least one or more prediction modes, for a first subset of samples to be predicted, the first subset comprising one or more peripheral samples in the current region, such as at least one sample of a row or column spatially closest to the reference samples, the intra-image predictor is configured to detect a reference position amongst the reference samples different to a position pointed to by the prediction direction defined by the selected prediction mode; and for a second, remaining, subset of samples to be predicted, the intra-image predictor is configured to detect a reference position amongst the reference samples pointed to by the prediction direction defined by the selected prediction mode, the first and second subsets can be identified by null values in the difference data (indicating “no change from the base mode direction”) indicating second subset samples, or the absence of difference data indicating second subset values, or by explicit flags.

As part of its operation, the intra mode selector may carry out one or more trial encoding processes, in which case it would use the delta values in the delta memory, accessible by a connection 2650.

In the case of a decoding apparatus, a mode 2660 is detected from the encoded data by the intra mode selector 2600. Associated with the select mode will be information stored in the delta memory 2620. Note that the contents of the delta memory are the same at the encoder and the decoder.

Figure 27 provides a schematic example of a delta memory, in which a memory controller 2700 accesses multiple instances 2710 of sets of delta values, for example in the form shown in Figures 25a...25e.

Having said this, it is noted that in the case of a 8x8 block, the lower right six rows and columns, which is to say all of the block other than the top two rows and the top two columns, are always 0 and so data does not need to be explicitly stored for these sample positions. Instead, the instances 2710 can comprise data representing the top two rows 2800 of Figure 28 and the left two columns 2810 in the case of an 8x8 block, or in another example, such as a 4x4 block, data 2900 of Figure 29 defining just the top row and 2910 defining just the left column.

Figure 30 is a schematic flowchart representing an image encoding method comprising: selecting (at a step 3000), from a set of prediction directions, a prediction mode for prediction of a current region of a current image; and intra-image predicting (at a step 3010) samples of the current region with respect to one or more of the group of reference samples of the same image in dependence upon a prediction direction, defined by the selected prediction mode, between a current sample to be predicted and a reference position amongst the reference samples;

in which the intra-image predicting step comprises:

for at least one or more prediction modes, for a first subset of samples to be predicted, the first subset comprising one or more peripheral samples in the current region, detecting (at a step 3020) a reference position amongst the reference samples different to a position pointed to by the prediction direction defined by the selected prediction mode; and for a second, remaining, subset of samples to be predicted, detecting (at a step 3030) a reference position amongst the reference samples pointed to by the prediction direction defined by the selected prediction mode.

Figure 31 is a schematic flowchart representing an image decoding method comprising: selecting (at a step 3100), from a set of prediction directions, a prediction mode for prediction of a current region of a current image; and intra-image predicting (at a step 3110) samples of the current region with respect to one or more of the group of reference samples of the same image in dependence upon a prediction direction, defined by the selected prediction mode, between a current sample to be predicted and a reference position amongst the reference samples;

in which the intra-image predicting step comprises:

for at least one or more prediction modes, for a first subset of samples to be predicted, the first subset comprising one or more peripheral samples in the current region, detecting (at a step 3120) a reference position amongst the reference samples different to a position pointed to by the prediction direction defined by the selected prediction mode; and for a second, remaining, subset of samples to be predicted, detecting (at a step 3130) a reference position amongst the reference samples pointed to by the prediction direction defined by the selected prediction mode.

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 nontransitory 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 aspects and features are defined by the following numbered clauses:

1. An image encoding apparatus comprising:

a controller configured to select, from a set of prediction directions, a prediction mode for prediction of a current region of a current image; and an intra-image predictor configured to predict samples of the current region with respect to one or more of the group of reference samples of the same image in dependence upon a prediction direction, defined by the selected prediction mode, between a current sample to be predicted and a reference position amongst the reference samples;

in which:

for at least one or more prediction modes, for a first subset of samples to be predicted, the first subset comprising one or more peripheral samples in the current region, the intra-image predictor is configured to detect a reference position amongst the reference samples different to a position pointed to by the prediction direction defined by the selected prediction mode; and for a second, remaining, subset of samples to be predicted, the intra-image predictor is configured to detect a reference position amongst the reference samples pointed to by the prediction direction defined by the selected prediction mode.

2. Apparatus according to clause 1, in which the intra-image predictor is configured to predict samples in the first subset from the respective detected reference position.

3. Apparatus according to clause 1 or clause 2, in which the intra-image predictor is configured to predict samples in the first subset in dependence upon both of the detected reference position and the position pointed to by the prediction direction defined by the selected prediction mode.

4. Apparatus according to any one of the preceding clauses, in which:

the current region comprises an array of two or more rows and two or more columns of samples; and the first subset of samples comprises at least one sample of a row or column spatially closest to the reference samples.

5. Apparatus according to clause 4, in which, for a given sample in the first subset, the reference position is displaced, with respect to a reference position pointed to by the prediction direction defined by the selected prediction mode, so as to be closer to reference sample spatially nearest to the given sample.

6. Apparatus according to clause 5, in which:

the reference samples represent sample positions above and to the left of the current region;

for a prediction mode defining a generally horizontal direction, the first subset of samples includes at least one sample of a top row of samples to be predicted; and for a prediction mode defining a generally vertical direction, the first subset of samples includes at least one sample of a left-most column of samples to be predicted.

7. Apparatus according to clause 6, in which the first subset comprises at least a row or column of samples spatially closest to the reference samples.

8. Apparatus according to any one of the preceding clauses, in which the intra-image predictor is configured to access difference data, defining one or more differences applicable to each sample of the first subset of samples to be predicted between a respective reference position for use in predicting each sample and a respective reference position pointed to by a prediction direction defined by the selected prediction mode.

9. Apparatus according to clause 8, in which the difference data defines differences applicable to one or more upper rows of samples and one or more left-most columns of samples.

10. Apparatus according to clause 8, in which two or more sets of difference data applicable to respective groups of one or more prediction modes are provided.

11. Apparatus according to any one of the preceding clauses, in which the intra-image predictor comprises:

a filter configured to generate a predicted sample by interpolation of the reference samples with respect to the respective reference position.

12. Apparatus according to any one of the preceding clauses, in which the intra-image predictor is configured to detect the reference position to an accuracy of less than one sample.

13. Apparatus according to any one of the preceding clauses, in which:

each sample to be predicted is associated with a coordinate position within the current region; and the intra-image predictor is configured to detect the reference position for a given sample to be predicted as a function of the coordinate position of the given sample to be predicted, the function depending upon the selected prediction mode.

14. Apparatus according to any one of the preceding clauses, in which the controller is configured to perform at least a partial encoding to select the prediction mode.

15. Apparatus according to any one of the preceding clauses, in which the controller is configured to encode data identifying the prediction mode selected for each region of the image.

16. Video storage, capture, transmission or reception apparatus comprising apparatus according to any one of the preceding clauses.

17. An image decoding apparatus comprising:

in which:

18. Apparatus according to clause 17, in which the intra-image predictor is configured to predict samples in the first subset from the respective detected reference position.

19. Apparatus according to clause 17 or clause 18, in which the intra-image predictor is configured to predict samples in the first subset in dependence upon both of the detected reference position and the position pointed to by the prediction direction defined by the selected prediction mode.

20. Apparatus according to any one of clauses 17 to 19, in which:

21. Apparatus according to clause 20, in which, for a given sample in the first subset, the reference position is displaced, with respect to a reference position pointed to by the prediction direction defined by the selected prediction mode, so as to be closer to reference sample spatially nearest to the given sample.

22. Apparatus according to clause 21, in which:

23. Apparatus according to clause 22, in which the first subset comprises at least a row or column of samples spatially closest to the reference samples.

24. Apparatus according to any one of clauses 17 to 23, in which the intra-image predictor is configured to access difference data, defining one or more differences applicable to each sample of the first subset of samples to be predicted between a respective reference position for use in predicting each sample and a respective reference position pointed to by a prediction direction defined by the selected prediction mode.

25. Apparatus according to clause 24, in which the difference data defines differences applicable to one or more upper rows of samples and one or more left-most columns of samples.

26. Apparatus according to clause 24, in which two or more sets of difference data applicable to respective groups of one or more prediction modes are provided.

27. Apparatus according to any one of clauses 17 to 26, in which the intra-image predictor comprises:

28. Apparatus according to any one of clauses 17 to 27, in which the intra-image predictor is configured to detect the reference position to an accuracy of less than one sample.

29. Apparatus according to any one of clauses 17 to 28, in which:

30. Apparatus according to any one of clauses 17 to 29, in which the controller is configured to detect encoded data identifying the prediction mode selected for each region of the image.

31. Video storage, capture, transmission or reception apparatus comprising apparatus according to any one of clauses 17 to 30.

32. An image encoding method comprising:

selecting, from a set of prediction directions, a prediction mode for prediction of a current region of a current image; and intra-image predicting samples of the current region with respect to one or more of the group of reference samples of the same image in dependence upon a prediction direction, defined by the selected prediction mode, between a current sample to be predicted and a reference position amongst the reference samples;

in which the intra-image predicting step comprises:

for at least one or more prediction modes, for a first subset of samples to be predicted, the first subset comprising one or more peripheral samples in the current region, detecting a reference position amongst the reference samples different to a position pointed to by the prediction direction defined by the selected prediction mode; and for a second, remaining, subset of samples to be predicted, detecting a reference position amongst the reference samples pointed to by the prediction direction defined by the selected prediction mode.

33. Computer software which, when executed by a computer, causes the computer to carry out a method according to clause 32.

34. A machine-readable non-transitory storage medium which stores software according to clause 33.

35. A data signal comprising coded data generated according to the method of clause 32.

36. An image decoding method comprising:

in which the intra-image predicting step comprises:

37. Computer software which, when executed by a computer, causes the computer to carry out a method according to clause 36.

38. A machine-readable non-transitory storage medium which stores software according to clause 37.

39. A video capture apparatus comprising an image sensor and the encoding apparatus of any one of clauses 1-15, decoding apparatus of any one of clauses 17-30 and a display to which the decoded images are output.

Claims

1. An image encoding apparatus comprising:

in which:

2. Apparatus according to claim 1, in which the intra-image predictor is configured to predict samples in the first subset from the respective detected reference position.

3. Apparatus according to claim 1, in which the intra-image predictor is configured to predict samples in the first subset in dependence upon both of the detected reference position and the position pointed to by the prediction direction defined by the selected prediction mode.

4. Apparatus according to claim 1, in which:

5. Apparatus according to claim 4, in which, for a given sample in the first subset, the reference position is displaced, with respect to a reference position pointed to by the prediction direction defined by the selected prediction mode, so as to be closer to reference sample spatially nearest to the given sample.

6. Apparatus according to claim 5, in which:

7. Apparatus according to claim 6, in which the first subset comprises at least a row or column of samples spatially closest to the reference samples.

8. Apparatus according to claim 1, in which the intra-image predictor is configured to access difference data, defining one or more differences applicable to each sample of the first subset of samples to be predicted between a respective reference position for use in predicting each sample and a respective reference position pointed to by a prediction direction defined by the selected prediction mode.

9. Apparatus according to claim 8, in which the difference data defines differences applicable to one or more upper rows of samples and one or more left-most columns of samples.

10. Apparatus according to claim 8, in which two or more sets of difference data applicable to respective groups of one or more prediction modes are provided.

11. Apparatus according to claim 1, in which the intra-image predictor comprises:

12. Apparatus according to claim 1, in which the intra-image predictor is configured to detect the reference position to an accuracy of less than one sample.

13. Apparatus according to claim 1, in which:

14. Apparatus according to claim 1, in which the controller is configured to perform at least a partial encoding to select the prediction mode.

15. Apparatus according to claim 1, in which the controller is configured to encode data identifying the prediction mode selected for each region of the image.

16. Video storage, capture, transmission or reception apparatus comprising apparatus according to claim 1.

17. An image decoding apparatus comprising:

in which:

18. Apparatus according to claim 17, in which the intra-image predictor is configured to predict samples in the first subset from the respective detected reference position.

19. Apparatus according to claim 17, in which the intra-image predictor is configured to predict samples in the first subset in dependence upon both of the detected reference position and the position pointed to by the prediction direction defined by the selected prediction mode.

20. Apparatus according to claim 17, in which:

21. Apparatus according to claim 20, in which, for a given sample in the first subset, the reference position is displaced, with respect to a reference position pointed to by the prediction direction defined by the selected prediction mode, so as to be closer to reference sample spatially nearest to the given sample.

22. Apparatus according to claim 21, in which:

23. Apparatus according to claim 22, in which the first subset comprises at least a row or column of samples spatially closest to the reference samples.

24. Apparatus according to claim 17, in which the intra-image predictor is configured to access difference data, defining one or more differences applicable to each sample of the first subset of samples to be predicted between a respective reference position for use in predicting each sample and a respective reference position pointed to by a prediction direction defined by the selected prediction mode.

25. Apparatus according to claim 24, in which the difference data defines differences applicable to one or more upper rows of samples and one or more left-most columns of samples.

26. Apparatus according to claim 24, in which two or more sets of difference data applicable to respective groups of one or more prediction modes are provided.

27. Apparatus according to claim 17, in which the intra-image predictor comprises:

28. Apparatus according to claim 17, in which the intra-image predictor is configured to detect the reference position to an accuracy of less than one sample.

29. Apparatus according to claim 17, in which:

30. Apparatus according to claim 17, in which the controller is configured to detect encoded data identifying the prediction mode selected for each region of the image.

31. Video storage, capture, transmission or reception apparatus comprising apparatus according to claim 17.

32. An image encoding method comprising:

in which the intra-image predicting step comprises:

33. Computer software which, when executed by a computer, causes the computer to carry out a method according to claim 32.

34. A machine-readable non-transitory storage medium which stores software according to claim 33.

35. A data signal comprising coded data generated according to the method of claim 32.

36. An image decoding method comprising:

in which the intra-image predicting step comprises:

37. Computer software which, when executed by a computer, causes the computer to carry out a method according to claim 36.

38. A machine-readable non-transitory storage medium which stores software according to claim 37.