GB2599476A

GB2599476A - Data encoding and decoding

Info

Publication number: GB2599476A
Application number: GB2109458.6A
Authority: GB
Inventors: James Sharman Karl; Richard Browne Adrian; Mark Keating Stephen
Original assignee: Sony Group Corp
Current assignee: Sony Group Corp
Priority date: 2020-10-05
Filing date: 2021-06-30
Publication date: 2022-04-06
Also published as: WO2022073789A1; GB2599447A; GB202015766D0; GB202109458D0

Abstract

A method of encoding successive data items comprises encoding a given data item using a series of one or more data sets (e.g. Significance maps, Greater-than-one maps) each data set representing a respective range of values of the data items. The series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and for at least some of the data items, selecting the first threshold value to be greater than 1. Preferably the data sets comprising the significance indicator, and zero or more further data sets selected from a lost consisting of: one or more greater-than-m data sets indicating whether a given data item is greater than a value of m, m being greater than the first threshold value. The data sets may also contain a parity data set. Preferably the threshold value is an integer equal to 2n where n is an integer of zero or more. Preferably the data items are multi bit data words.

Description

DATA ENCODING AND DECODING

BACKGROUND Field

This disclosure relates to 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 systems, such as video or image 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.

In some examples, the entropy encoding process can involve generating one or more "data sets" (such as a significance map, a greater than one map, a greater than two map and/or other data sets) to describe a block of coefficients, with any excess values which cannot be encoded by the significance maps alone being encoded as so-called escape values. The coding of an escape value can On some examples) be performed by generating a first portion (for example, a unary or truncated unary coded portion such as a prefix) and a non-unary coded second portion (such as a suffix) having a length, in bits, dependent upon a second portion size value.

SUMMARY

The present disclosure addresses or mitigates problems arising from this processing. The present disclosure provides a method of encoding successive data items, the method comprising: encoding a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and for at least some of the data items, selecting the first threshold value to be greater than 1.

The present disclosure also provides an encoded data signal generated by such a method and a non-transitory machine-readable storage medium which stores such an encoded data signal.

The present disclosure also provides a method of decoding successive data items, the method comprising: decoding a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and for at least some of the data items, selecting the first threshold value to be greater than 1.

The present disclosure also provides computer software which, when executed by a computer, causes the computer to perform any of such methods, and a non-transitory machine-readable storage medium which stores such computer software.

The present disclosure also provides apparatus for encoding successive data items, the apparatus comprising: encoding circuitry configured to encode a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and control circuitry configured, for at least some of the data items, to select the first threshold value to be greater than 1.

The present disclosure provides apparatus for decoding successive data items, the apparatus comprising: decoding circuitry configured to decode a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and control circuitry configured, for at least some of the data items, to select the first threshold value to be greater than 1.

The present disclosure also provides video data capture, transmission, display and/or storage apparatus comprising any of such apparatus.

Further 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 transform-skip mode; Figures 10a, 10b, lla and llb schematically illustrate respective scanning directions; Figure 12 is a schematic diagram illustrating an encoding apparatus; Figure 13 is a schematic diagram illustrating a decoding apparatus; Figure 14 schematically represents an example of encoding a data word; Figures 15 to 19 are schematic flowcharts illustrating respective methods; Figures 20 and 21 schematically represent respective examples of encoding a data Figures 22a, 22b, 23a, 23b, 24a, 24b, 25 and 26 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. In this example, the data values to be encoded or decoded represent image data.

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 or image data compression and decompression apparatus, for encoding and/or decoding image data representing one or more images.

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 form 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 predicted 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.

Therefore, encoding (using the adder 310) involves predicting an image region for an image to be encoded; and generating a residual image region dependent upon the difference between the predicted image region and a corresponding region of the image to be encoded. In connection with the techniques to be discussed below, the ordered array of data values comprises data values of a representation of the residual image region. Decoding involves predicting an image region for an image to be decoded; generating a residual image region indicative of differences between the predicted image region and a corresponding region of the image to be decoded; in which the ordered array of data values comprises data values of a representation of the residual image region; and combining the predicted image region and the residual image region.

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 DOT technique itself is well known and will not be described in detail here.

Note also that the use of DOT 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. In other examples a so-called "transform-skip" mode can selectively be used in which no transform is applied.

Therefore, in examples, an encoding and/or decoding method comprises predicting an image region for an image to be encoded; and generating a residual image region dependent upon the difference between the predicted image region and a corresponding region of the image to be encoded; in which the ordered array of data values (to be discussed below) comprises data values of a representation of the residual image region.

The output of the transform unit 340, which is to say On an example), a set of DOT 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 diagonal order such as a so-called upright diagonal scanning order.

The scanning order can be different, as between transform-skip blocks and transform blocks (blocks which have undergone at least one spatial frequency transformation).

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, whether the compressed data was transformed or transform-skipped or the like, provides a compressed output video signal 380.

However, a return path 390 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, so 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 (and its inverse) 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 (although this may be subject to so-called loop filtering and/or other filtering before being output -see below). This forms one input to the image predictor 320, as will be described below.

Turning now to the decoding 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 (subject to the filtering processes discussed below). In practice, further filtering may optionally be applied (for example, by a loop filter 565 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 On 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 (as filtered by loop filtering; see below), 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 may be filtered by a so-called loop filter 565. Various types of loop filters may be used. One technique 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 further technique involving applying a so-called sample adaptive offset (SAO) filter may also be used. In general terms, in a sample adaptive offset filter, filter parameter data (derived at the encoder and communicated to the decoder) defines one or more offset amounts to be selectively combined with a given intermediate video sample (a sample of the signal 460) by the sample adaptive offset filter in dependence upon a value of:(i) the given intermediate video sample; or 00 one or more intermediate video samples having a predetermined spatial relationship to the given intermediate video sample.

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 loop filter unit 565 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 may be 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 SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS Infrastructure of audio-visual services -Coding of moving video High efficiency video coding Recommendation ITU-T H.265 12/2016. Also: High Efficiency Video Coding (HEVC) algorithms and Architectures, Editors: Madhukar Budagavi, Gary J. Sullivan, Vivienne Sze; chapter 3; ISBN 978-3-319-06894-7; 2014 which are incorporated herein in their respective entireties 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 or arrays (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 On at least the present examples) that each sample of an image is allocated to one, and only one, such block or region.

Transform-skip mode Figure 9 schematically illustrates a so-called transform-skip mode. In this mode, blocks of samples, for example rectangular encoding blocks or arrays of samples such as so-called transform units (TUs) are assigned a transform-skip' mode indicator, for example by a part of the functionality of the controller 343. When the transform-skip indicator is set, as shown by the schematic bypass path 900 in Figure 9, the transform unit 340 (in the encoding path) and the inverse transform unit 430 (in the decoding path of the encoding side or in a decoder) is bypassed so that no spatial frequency transform is applied to the samples in that particular block.

The transform-skip mode is selectable by the controller 343, alongside a possible selection, of, DOT, DST or another transform mode, in dependence upon properties of the block in question, properties of nearby blocks, trial (full or partial) encodings or the like. Generally, the aim of the selection algorithm executed by the controller 343 is to improve the efficiency of the encoding of the block in question.

In some previously proposed example arrangements, transform-skip mode was restricted to 4x4 block sizes or smaller. In more recent examples, this restriction has been relaxed and the transform-skip mode can be selectively applied to larger blocks. The transform-skip mode can be applied to a TU even when the TU is actually processed as multiple (smaller) sub-TUs.

Figures 10a and 11a schematically illustrate respective scanning directions, with Figure 10a providing an example applicable to a 4x4 transform-skip block and Figure 11a providing an example applicable to a so-called transform block, which is to say a block for which transform-skip mode was not enabled and so the block has undergone a spatial frequency transform (or more than one frequency transform) by the transform unit 340 during encoding.

Referring to Figure 10a, in the case of transform-skip blocks, the scan order is in this example a diagonal order from the top left ("1") to lower right ("16"). In contrast, as shown in Figure 11a, in the case of a transform block (as an example of an encoding block) the scan order is a diagonal order from the lower right to top left. Note that the scan order in use makes little substantive difference to the techniques to be discussed below other than in terms of which coefficients or samples are available "already encoded" or "already decoded" for use in the derivation of encoding parameters for subsequent samples or coefficients.

In the case of larger blocks, a similar scan order can be used, or sub-blocks of (for example) 4x4 coefficients such as sub-TUs can be scanned as shown, with a predetermined pattern being used to scan each sub-block in order.

In general terms, the blocks of samples or coefficients may be considered as groups of data values (or, once encoded, groups of encoded data values), each having an associated encoding order On other words, the scan order as illustrated by the examples of Figures 10a and 11a).

Referring to Figures 10b and 11 b, in at least some examples of processing to be discussed below, reference is made to samples or coefficients (which may for convenience be referred to below by the single generic term "coefficients" even in the case of transform-skip blocks) which have been encoded or decoded before encoding or decoding of a given coefficient 1000, 1100. In the case of transform-skip, the previously handled coefficients relevant to at least some of the techniques discussed below will be coefficients in a predetermined grouping or pattern as shown in shaded form (one box for each coefficient), which are above and left of the given coefficient 1000. In the case of non-transform-skip operation, a similar predetermined pattern or grouping is used but here the previously handled coefficients relevant to at least some of the techniques discussed below are coefficients (shown shaded) below and to the right of the given coefficient 1100. As mentioned, these coefficients will be referred to by at least some techniques below. If any coefficient in the relevant shaded group is unavailable, for example because it would fall into another block or a not-yet encoded or not-yet-decoded block, that coefficient is simply omitted from the respective process and where necessary, the result of the process may be normalised to the reduced number of coefficients. (For example, normalisation is relevant to a process deriving a mean of the shaded coefficients but is not relevant to a process detecting a maximum or minimum of the shaded coefficients).

Data sets and escape codes The entropy encoding stage (for example, performed by the entropy encoder 370, with the inverse process being performed by the entropy decoder 410) involves encoding the scanned quantized transform coefficients (with the scan applied by the scan unit 360 being accorded to the examples shown in Figures 10 and 11 for transform-skip and transform blocks respectively).

The entropy encoding is arranged to encode the values as one or more so-called data sets along with escape codes for remaining values not encoded by the data sets.

To generate the data sets, the data values to be encoded are handled in the encoding order (for example the scan order). The data sets generated in respect of a block of samples such as a 4x4 block or a 4x4 (or other) sub-portion of a larger block are selectable as zero or more of a group of candidate data sets (which is to say, the data sets could be capable of being enabled, just not enabled for a particular coefficient or sub-TU or other block or group), where the group of candidate data sets may include one or more of the following.

Note that these parameters relate to previously proposed arrangements and are provided here by way of comparison with example embodiments to be discussed below.

* Significance map (Sig) which indicates the position of so-called "significant" coefficients or samples, which is to say non-zero coefficients or samples. A significance flag indicating a non-zero value is coded for each coefficient position in the block.

* greater than 1 flag (GT1) which indicates whether the absolute value is greater than 1 for each significant coefficient. In some examples for a 4x4 block, the flag is sent only for the first 8 significant coefficients in the encoding order; in other examples, it can be sent for each significant coefficient. In other examples the GT1 flag is always sent if the significance flag is sent.

* "Value & 1" flag or parity flag is effectively the least significant bit (LSB) at this stage (where & signifies a logical AND operation).

* greater than 2 flag (GT2) which indicates if the coefficient absolute value is greater than 2 up to and including the first coefficient in the scanning order with this property.

Note that the flag is sent only for coefficients larger than 1 as indicated by the GT1 flag.

In some examples, after the occurrence of the first coefficient which is greater than 2 in the scanning order, the GT2 flag is not further sent. However, in at least some of the present examples being discussed, the GT2 flag may be sent at each calculated occurrence or at least up to a particular number of occurrences per coefficient for a sub-TU (such as up to four occurrences, the number of allowable occurrences being selectable for a sub-TU as discussed below).

In some examples, for Non-TS (non-transform skip) operation, the GT2 flag is coded whenever the GT1 flag is set. The limit of 4 applies to TS (transform skip) and may be applied up to 4 times to each coefficient or until it is not set. Thus if the first GT2 flag is not set then the value is 1 or 2 (depending on parity), if the first is set and the second is not set the value is 3 or 4, and if the first two are set by the third is not the value is 5 or 6 and so on.

* Coefficient sign which is provided for the significant coefficients. The absolute coefficient value (ABS(COEFF)) is modified in response to each coding pass of the above arrangement and the modified value is used in the next pass. The modification is: * At the generation of the significance map, subtract 1; * At the generation of the GT1 map, subtract 1; * At the generation of the value & 1 flag, divide by 2.

This provides an example in which, when one or more data sets are selected, encoding by a first technique comprises modifying the data value after encoding by a given data set to account for values which can be encoded by that data set.

In other words, a coefficient for which one GT2 flag has been generated in fact has a minimum value of 4, as shown in the following example in which each data set is shown in turn, with the following column indicating a remaining value (Val) to be encoded after the modification mentioned above: Val (i/p) Sig Val Gt1 Val Val&1 Val Gt2 0 0 1 1 0 0 2 1 1 1 0 0 0 0 3 1 2 1 1 1 0 0 4 1 3 1 2 0 1 1 1 4 1 3 1 1 1 6 1 5 1 4 0 2 1 Escape codes Escape codes are used to encode the remaining absolute level, which is to say level information which has not been encoded by the data sets outlined above. Because of the effective subtraction of 4 discussed above, in an arrangement in which one G12 flag is always sent when it is applicable, the remaining absolute level needs to be encoded only for "coeff -4". Where more than one GT2 flag is sent for a coefficient, the offset is increased to represent the increasing contribution to the value represented by each successive GT2 flag.

The remaining absolute level is encoded by an escape code, for example, comprising a first portion and a non-unary coded second portion. The second portion may have a length, in bits, dependent on a second portion size value defined by a so-called Rice parameter.

Such an arrangement may be referred to as a Golomb-Rice code in which a value to be encoded is considered as two portions (the first and second portions mentioned above). The first portion is the result of the division of the value to be encoded by M, where M = 2b, and the second portion is the remainder for example b least significant bits of the value to be encoded.

In the discussion provided here, the parameter b is referred to as the second portion size value. In examples, the quotient or first portion is encoded using unary coding and is followed by the remainder encoded using, for example, truncated binary encoding. Note that if M = 1, then this coding is equivalent to unary coding.

In example embodiments the first portion is a prefix and the second portion is a suffix.

For example, the first portion may comprise a unary encoded value. For example, the first portion may comprise a truncated unary value. Note however that the terms "first" and "second" are simply identifiers and do not necessarily imply any requirement for the first portion to precede the second portion in an encoding or transmission order.

Apparatus for coding and decoding data sets and escape codes Figure 12 schematically illustrates an example encoding apparatus.

Referring to Figure 12, data values 1200 are received in the scanning order from the scan unit 360. A generator 1210 generates the data sets described above, namely the significance map, the GT1 flag, the LSB or parity (val & 1) flag and the GT2 flag. These are provided to an output unit 1220 for output to the encoded data stream An encoder 1230 encodes the escape codes. The encoder 1230 is responsive to a detector 1240 which detects whether there are any remaining absolute values to be encoded, and also to a generator 1250 which generates the second portion size value; this processing being performed using previously proposed techniques.

In other words, detection is made as to whether the current data value exceeds the maximum which can be encoded by the data sets in use.

The generator 1250 can generate a second portion size value 1265 in respect of each data value 1200, whether or not that data value requires an escape code. Alternatively, a second portion size value may be generated only in respect of data values which do in fact 10 need an escape code.

The encoder 1230 then performs the escape code encoding as discussed above and provides the escape codes to the output unit 1220 for output to the data stream.

In the case that a remaining value is too large to be entirely represented by an escape code (which is to say, the value is larger than the maximum value which can be represented by an escape code using the prevailing parameters), a further type of code (an escape-escape code) may be provided.

Referring to Figure 13, a decoding apparatus comprises a data set decoder 1300 configured to decode the data sets (up to the GT2 flag) discussed above. A detector 1310 detects whether any escape codes or values are provided and, if so, passes these to an escape code decoder 1320 which applies an inverse operation to the escape code encoding, namely a Golomb-Rice decoding. In order to do this, the decoder 1320 makes use of second portion size value information 1330 provided by a generator 1340 which corresponds in function to the generator 1250.

The decoder 1320 decodes the escape code(s) using the second portion size value information 1330 and outputs the combination of the decoded data sets with any decoded escape codes as decoded data 1370 which forms the output of the process and which also forms the input 1360 to the generator 1340.

Examples of encoding and decoding techniques In contrast to the use of the significance data discussed above, example embodiments make use of a different or modified significance flag, to be explained below. This is applicable to various types of data words, but in these examples the data items are multi-bit data words (for example, p-bit words). They may be video data items representing a video signal and in some examples the significance flag or indicator is configured to indicate whether or not an absolute value of the given data item has a value of at least a first threshold value.

In the previously proposed examples discussed above, the significance flag related to whether the absolute value of a coefficient exceeded zero (or in other words, is greater than or equal to one). In example embodiments, the threshold against which the significance test is carried out can be greater than 1, for example the test may be >=2" where n is an integer potentially greater than 0. This addresses a potential issue when video data relating to high bit depth video signals (for example, bit depth > 10 bits) are in use, because the likelihood of coefficients generated from such data passing the previously proposed significance test (>=1) is potentially higher than such likelihood would be for lower bit depth video data. This changed likelihood can render the previously proposed significance test less useful for high bit depth video data.

In some examples, the value of n can be adaptive and, at in some examples at various times within the processing of the video data, may be 0 (which would provide a significance test equivalent to the previously proposed test) but the adaptive nature of n allows significance tests based on higher thresholds to be applied. In general, the threshold value for the significance test may be an integer equal to 2n where n is an integer of zero or more.

In other examples, n can be constrained to be at least 1.

In example embodiments, the significance flag is generated to indicate whether the absolute value of a coefficient is greater than or equal to 2, where n is an integer greater than zero. For example, if n=7 then the significance test is effectively shifted left On a more significant direction) to be a test of whether a coefficient is greater than or equal to 128. This is represented schematically by Figure 14 which illustrates a 16 bit data value, each box representing a bit position, with the most significant bit (MSB) drawn to the left and the least significant bit (LSB) drawn to the right. The significance test is applied at n=7, which is to say "is the value greater than or equal to 27?" In some situations, a per-block allowance of context coded bins for coefficient coding may be exhausted. For example, this may provide for the coding of (say) 28 flags for a given 4x4 sub-block. In this situation (once the available flags have been used) the significance flag is not coded and the value in the upper bits may be coded using another mechanism, for instance Rice coding.

The lowest 7 bits are shaded in Figure 14 and are referred to in the following discussion as "insignificant". Note that this is simply a term to distinguish them for the purposes of the present discussion and does not represent any assessment of their real importance in any particular system.

Unshaded bit positions in Figure 14 represent bit positions above the threshold of the significance test. If the significance test at 2" is not passed, then there are no bits in this range to be encoded. However, if the significance test is passed such that the data word is at least 2n then further coding of higher order bits may be required. Bits above the threshold which require or may require encoding are referred to (again, purely for the purposes of the discussion without any suggestion as to real importance) as "significant" bits or values.

This arrangement leads to various considerations, each of which will be discussed in more detail below. These are: (a) how to code "significant" values; (b) how to code "insignificant" values; (c) how to determine the value of n.

Regarding (a) and (b), this can provide an example in which, for a p-bit data item for which n is greater than zero, the technique comprises encoding a first portion of the data item represented by the least significant n bits by a first encoding technique; and encoding a second portion of the data item represented by the most significant (p-n) bits by a second encoding technique. In some examples, the first encoding technique may be different to the second encoding technique.

(a) Coding significant values Referring to Figure 15, in some examples, coding of the significant bits is conducted using techniques similar to those of the previously proposed systems discussed above but from a starting point of the significance threshold of 2".

At a step 1500, the significance flag is coded based on the test of whether the absolute value of the coefficient is greater than or equal to 2".

If the coefficient is significant, then at a step 1510 the GT1 flag is encoded as the Boolean result: GT1 = ((absolute value of coefficient) >> n) >1 Here "» p" represents a right shift by p bits. If GT1 = true, then at a step 1520 the parity flag is encoded as parity = ((absolute value of coefficient) » n) & 1 At a step 1530, the GT2 flag is coded as: GT2 = (((absolute value of coefficient) » n) -parity) >2 If the GT2 flag is set, then at a step 1540 any remainder is coded as escape codes, for example using Rice coding, where: remainder = (((absolute value of coefficient) » n) -4) » 1 At a step 1550 any insignificant bits are coded using techniques to be discussed below.

Another example arrangement is shown schematically in Figure 16.

Here, the coding can be performed just using Rice coding (as though escape values are being coded). Or, optionally, the significance flag can first be coded at a step 1600 (corresponding to the step 1500 discussed above). Any significant bits or 0 if the significance flag is not coded are then encoded by Rice coding at a step 1610, and at a step 1620 any insignificant bits are encoded using techniques to be discussed below.

Coding the significance flag as discussed above and coding zero or more further flags provides an example of encoding using one or more data sets comprise: a significance data set comprising the significance indicator; and zero or more further data sets selected from the list consisting of: one or more greater-than-m data sets indicating whether a given data item is greater than a respective value of m, m being greater than the first threshold value; and a parity data set indicating a value of a least significant bit of a data item.

Therefore, in some examples, the second encoding technique comprises generating one or more of the further data sets. In examples, the second encoding technique comprises, for any excess value of the given data item, the excess value being an amount by which the given data item exceeds a maximum value which can be encoded by the significance data set and the one or more further data sets, encoding the excess value as an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size.

(b) Coding insignificant values Various options are presented in the following description. Note that there may be "insignificant" bits (indicating portions of the word value represented by the n LSBs) whatever the result of the significance test. Therefore arrangements need to be provided to allow these n bits to be encoded whether or not the significance test is passed (potentially however using a different LSB encoding technique depending on the outcome of the significance test). Example arrangements are discussed with respect to Figures 17-19. Figure 20 schematically illustrates the n LSBs to be encoded.

Referring to Figure 17, a first possibility is simply to encode (at a step 1700) any data represented by the n LSBs using equiprobable (EP) CABAC bins.

Referring to Figure 18, the n LSBs can be encoded using secondary flags, which is to say similar flags to the previously proposed system, for example a second significance flag (a test of greater than or equal to 1) plus one or more subsequent flags such as GT1, parity, GT2 and the like. In some examples, G12 may be not used. By not coding G12 the maximum number of flags coded is kept to 4 (primary and secondary significance + GT1 + parity). This changes the offset in the calculation for the remainder from 4 to 2. Values of the n LSBs above the limit which can be encoded by the secondary flags may be coded as escape codes. In Figure 18, the flags are generated at a step 1800 and the n LSBs are encoded at a step 1810.

In Figure 19, a second significance flag is again used (and indeed is generated at a step 1900) and the n LSBs are encoded at a step 1910. A difference with respect to Figure 18 is that the second significance flag is not representative of a test of whether the absolute coefficient value is >=1, but rather of whether the absolute coefficient value is >=2(n-m), where m is a predetermined integer less than or equal to n.

If n=1 then m = 1. Otherwise, if n>1, then m<n.

For example, where n = 7 as in this example, m may equal 2 so that the secondary significance test is implemented 2 bits to the right (a less significant direction) relative to the primary significance test, as shown schematically in Figure 20.

Here, the second significance test may be expressed as a test of: (absolute coefficient value » max (n-m,0)) > 0 Although, as mentioned above, the n LSBs have to be encoded in some form whether or not the second significance test is passed. However, the approach or technique used to encode them can differ in dependence upon the outcome of the second significance test.

In an example, in Figure 21, where On this example) m = 2 and n=7, and numbering the bits from the LSB as bit 0, then the following technique may be used: If the second significance test is passed, then apply (i) and 00 below: (i) code bits 6 and 5 as: * second significance flag (though note that if n = 1 or m=1 then this is all that must be coded -there is no need for GT1 or parity); * GT1 flag as (absolute coefficient value >> max (n-m,0)) > 1 * if GT1 is set and n>1, code parity as (absolute coefficient value >> max (n-m,0)) &1 OD If n> 2 then code remaining LSBs (bits 0-4 in this example) as EP CABAC bins.

If the second significance test is not passed, then apply (i) and (ii) below: (i) if n>2, code remaining LSBs as EP CABAC bins 00 note that if n=1 or 2, the bits are already coded using the flags above so there is no need for further coding of the LSBs.

Therefore, in at least some examples, the first encoding technique comprises encoding at least some of the least significant n bits by equiprobable binary arithmetic coding. In examples, the first encoding technique comprises generating a second significance indicator in respect of a second threshold value lower than the first threshold value.

(c) How to determine the value of n The value of n can be signalled in the bit stream, derived by corresponding or symmetrical processes at the encoder and decoder sides, or predetermined. In some examples, the value of n can be adaptive, for example using equivalent techniques at the encoder and the decoder side so that the same adaptation can be performed equivalently at both sides, for example by the controller 343 in each case. Multiple values of n may be signalled, for example by component, component type, block size, whether transform or transform skip or the like. The value could be signalled once per sequence, once per slice, once per CTU, once per image or the like.

Figure 22a provides a schematic overview of such a process at the encoder side, in which a value of n is generated at a step 2200, the value of n (or a representation of it) is signalled, for example in a picture header or parameter set at a step 2210 and one or more coefficient values (for example those representing a whole picture) are encoded at a step 2220 using the generated value of n, before control returns to the step 2200 where a next value of n is generated.

In Figure 22b, the equivalent process at the decoder side is shown in which, at a step 2230, the decoder receives a signalled value of n (as provided at the step 2210) and at a step 2240 decodes one or more coefficient values (for example those representing a whole picture) before returning control to the step 2230.

By way of comparison, Figures 23a and 23b schematically illustrate steps at the encoder and decoder side respectively in which there is no need to signal the value of n (or other parameter) as between the encoder and decoder. In particular, in Figure 23a, at a step 2300, a value of n is generated locally (that is to say, at the encoder side) and is used to encode one or more coefficient values at a step 2310. Similarly, at the decoder side, the value of n is also generated locally from information available to the decoder at a step 2320 and at a step 2330 one or more coefficient values are decoded using that value of n.

An example of the step 2200 shown in Figure 24a is that the value of n is expressed as an offset amount (for example, +p or -p, where p is an integer greater than zero) relative to a Rice parameter generated for the encoding of any escape codes relevant to the current coefficient. The offset amount can be predetermined. In Figure 24a, a step 2400 concerns generating the Rice parameter (which may well be performed anyway as a separate part of the encoding process) and a step 2410 involves generating n as a fixed or predetermined offset from the generated Rice parameter.

A further example is illustrated schematically in Figure 24b. Here the value of n is based on a significance metric. At a step 2420 values are initiated, namely: n = 0 (n is the value used to define the significance test as discussed above); s = 0 (s is a count of significant coefficients, which is to say the number of coefficients which have passed the significance test defined by the prevailing n); c = 0 (c is a count of the number of coefficients coded); and min_c = 8 (min_c is a threshold lower limit on the value c before any variation is made to n at a step 2440 to be discussed below).

As each coefficient is coded, c and Of appropriate) s are incremented by 1 at a step 2430 Then, at a step 2440, n is adaptively varied. The process may for example be as follows: if c < min_c then take no action else if s/c > upper_threshold then n = n +1; double the prevailing value of min_c and reset s, c to 0; else if sic < lower_threshold then n = n -1 (subject to a constraint that it never goes below zero) and double the value of min_c and reset s, c to 0; In other words, c has to equal at least min_c (which doubles each time n is changed) for any change to n to take place.

When n is changed, whether as an increment or a decrement, min_c may be changed (for example doubled) and s and c are both reset to zero.

upperthreshold and lowerthreshold may be, for example [0.8; 0.6] for low n (for example below 5). For higher n, an example is [ 0.6, 0.4].

If neither of the threshold tests for sic is passed then no change is made to n or min_c.

The process can be reset (the step 2420) at the start of coding a slice or coding tree unit, CTU (or other block).

These techniques provide examples of adaptively varying the first threshold value in response to data values of encoded data items, for example in which the adaptive variation comprises varying the first threshold value in response to a proportion (such as sic) of encoded data items for which the significance indicator indicates that those data items had a value of at least the first threshold value.

In other examples, n can be reset to 1 or another integer higher than zero and constrained so as not to go below 1 (or that other integer) in the adaptive process discussed above.

Possible changes to the meaning of the Significant Block or Sub-Block Flag The significant sub-block flag may be taken to indicate that at least one value in the sub-block passes the new significance test (do any coefficients have absolute value >= 2"?) This provides an example of encoding (or at the decoding side, decoding) an indicator, for a block of the data items, indicating whether any data item in the block of data items has an absolute value of at least the first threshold value.

The shift by n bits can be performed on each coefficient or at the end of processing a block or sub-block. Applying the shift at the end of processing allows equivalent processing to be used (compared to that used in previously proposed systems) within the handling of a block or sub-block, for example any processes dependent upon neighbouring values within the block or sub-block. In examples, if the significant sub-block flag is not set, no further coding of the significant proportion of each coefficient is done for the sub-block and the significant proportion of each coefficient in the sub-block is set to zero. In such cases, just the LSBs are coded as the insignificant values. The insignificant values can be EP-encoded for example at the end of the block or sub-block, and then combined back into the block at decoding after the rest of the processing has been performed. By keeping them together in this way, the CABAC bitstream can be potentially better aligned.

This therefore provides an example of grouping the least significant bits for a block of data items, encoded by equiprobable binary arithmetic coding, together in an output data stream, after the encoded first portions of the data items of that block of data items.

At the decoding side, this arrangement provide an example of, for a block of data items: decoding the first portions of the data items of that block of data items using a value of n = 0; applying a bit shift of n bits to the decoded first portions of the data items; decoding the second portions of the data items for that block of data items; and combining the decoded second portions of the data items with respective bit-shifted first portions generated by the applying step.

The significant block flag may be taken to indicate that at least one value in the block passes the new significance test (abs value >= 2n). If the significant block flag is not set, no further coding of the significant proportion of each coefficient is done for the block and the significant proportion of each coefficient in the block is set to zero. If the significant block flag is set, sub-blocks are coded. The significant block flag is not coded (and assumed to be set) if n is zero.

Further Examples

As a further example, a pseudo Rice parameter may be generated. This assumes that a history store is provided, as in GB 2015486.0 and GB 2009255.7 (the contents of each of which are hereby incorporated by reference). Assume that the history stored contains an array of counters (split potentially as before) called say shiftCounters which are updated for each coefficient. In the above cases this was limited in some examples to non-zero coefficients but here updated for all.

shiftCounter = select one of shiftCounters shift = shiftCounter / divisor; if (abs(value) >= (3 « shift)) { shiftCounter += increment; } else if ((abs(value) << 1) < (1 << shift)) { shiftCounter = std::max(0, count -decrement); For this proposal generally better to underestimate n hence decrement > increment in the referenced arrangements. When coding use n = shift as calculated above.

Worked example -significance test Shift Tests for flags 1 Significance GT1 012 Parity Val >= 1 Val >= 2 Val > 2 (Val & 1) != 0 1 Val >= 2 Val >= 4 Val > 4 (Val & 2) != 0 2 Val >zic 4 Val >= 8 Val > 8 (Val & 4) != 0 1 3 Val >= 8 Val >= 16 Val > 16 (Val & 8) != 0 1 4 Val >= 16 Val >= 32 Val > 32 (Val & 16) != 0 Val >= 32 Val >= 64 Val > 64 (Val & 32) != 0 6 Val >= 64 Val >= 128 Val > 128 (Val & 64) I= 0 7 Val >i=ii 128 Val >= 256 Val > 256 (Val & 128) != 0 Worked example, coding 85 (1010101) Shift Flags (For each flag add 1 except 012 add 2) Value coded! ! Value coded Insignificant Significance i 011 012 Panty with flags I with rice value to code i 0 1 1 1 1 5 « 0 = 5 I 40 « 1 = 80 0 (0 bits) 1 1 i 1 1 0 4 « 1 = 8 i 19 « 2 = 78 1(1 bit) 2 I1 I 1 I 5 « 2 = 20 i 8 « 3 = 64 1 (2 bits) 3 / 1 1 0 4 « 3 = 32 I « 4 = 48 5 (3 bits) I 3 4 1l 1 1 1 5 « 4 = 80! 0 5(4 bits) ! 1 1 0 0 2 « 5 = 64 1 NA -1- 21 (5 bits) 6 1 0 NA NA 1 « 6 = 64 I NA 21 (6 bits) 7 0 NA NA NA 0 NA 85 (7 bits) ! l Note that the "value coded with rice" is the value on left of the equation. The contribution of this value towards the total is given on the right. Parity is responsible for the shift of 1.

Worked example of coding insignificant bits, for value 85 (1010101) 1. Just 7 LSBs 1010101 2. Secondary significance -Significance = 1, GT1 = 1, GT2 = 1, Parity = 1, Code 40 as rice - Significance = 1, GT1 = 1, Parity = 1, Code 41 as rice * Not using GT2 to keep total flags (including first significance at 4) 3. Offset secondary significance (m = 2) - Significance (>32) = 1, GT1 (>64) = 1, Parity (& 100000) = 0, Code 5 lowest LSBs 10101 This formulation only requires GT1 to be signalled if significance is 1, parity is signalled only if both significance and GT1 are 1 - GT1 (>64) = 1, Parity (& 100000) = 0, Code 5 lowest LSBs 10101 * This formulation requires both flags to be signalled -Basically using CABAC context bins to encode top two LSBs 4. Offset secondary significance (m = 1) - Significance (>64) = 1, Code 6 lowest LSBs 010101 25 * Basically using CABAC context bin to encode top LSB

Summary Apparatus

The apparatus of Figure 12, operating in accordance with the techniques described here, and for example in association with Figures 7 and 8, provides an example of apparatus for encoding successive data items, the apparatus comprising: encoding circuitry 1210, 1240, 1230 configured to encode a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and control circuitry 343, 1250 configured, for at least some of the data items, to select the first threshold value to be greater than 1.

The apparatus of Figure 13, operating in accordance with the techniques described here, and for example in association with Figures 7 and 8, provides an example of apparatus for decoding an input data signal representing successive data items, the apparatus comprising: decoding circuitry 1300, 1310, 1320 configured to decode a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and control circuitry 343, 1340 configured, for at least some of the data items, to select the first threshold value to be greater than 1.

Note that in example arrangements the significance indicator mentioned above may be a first flag in a potential succession of flags (significance, GT1, GT2 and the like). A further significance flag may also be used as discussed here.

It is noted that although Figures 12 and 13 refer to the encoding circuitry and the decoding circuitry as two respective units in each case, the first and second circuitries in each instance could be implemented as a single common circuitry having both respective functions. As discussed above, it will be noted that any of these apparatuses may be implemented as video data capture, transmission, display and/or storage apparatus comprising such apparatus.

Summary Methods

Figure 25 is a schematic flowchart illustrating a method of encoding successive data items, the method comprising: encoding (at a step 2500) a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and for at least some of the data items, selecting (at a step 2510) the first threshold value to be greater than 1.

Figure 26 is a schematic flowchart illustrating a method of decoding successive data items, the method comprising: decoding (at a step 2600) a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and for at least some of the data items, selecting (at a step 2610) the first threshold value to be greater than 1.

Any one or more of the above encoding techniques may be implemented by the apparatus of Figures 7 and/or Band/or 12.

Any one or more of the above decoding techniques may be implemented by the apparatus of Figures 7 and/or Band/or 13.

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.

It will be appreciated that the above description for clarity has described embodiments with reference to different functional units, circuitry and/or processors. However, it will be apparent that any suitable distribution of functionality between different functional units, circuitry and/or processors may be used without detracting from the embodiments.

Described embodiments may be implemented in any suitable form including hardware, software, firmware or any combination of these. Described embodiments may optionally be implemented at least partly as computer software running on one or more data processors and/or digital signal processors. The elements and components of any embodiment may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units. As such, the disclosed embodiments may be implemented in a single unit or may be physically and functionally distributed between different units, circuitry and/or processors.

Although the present disclosure has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in any manner suitable to implement the technique.

Respective aspects and features are defined by the following numbered clauses: 1. A method of encoding successive data items, the method comprising: encoding a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and for at least some of the data items, selecting the first threshold value to be greater than 1.

2. The method of clause 1, in which the one or more data sets comprise: a significance data set comprising the significance indicator; and zero or more further data sets selected from the list consisting of: one or more greater-than-m data sets indicating whether a given data item is greater than a respective value of m, m being greater than the first threshold value; and a parity data set indicating a value of a least significant bit of a data item.

3. The method of clause 2, in which the threshold value is an integer equal to 2n where n is an integer of zero or more.

4. The method of clause 3 in which the data items are multi-bit data words.

5. The method of clause 4, in which, for a p-bit data item for which n is greater than zero, the method comprises: encoding a first portion of the data item represented by the most significant (p-n) bits by a second encoding technique; and encoding a second portion of the data item represented by the least significant n bits by a first encoding technique.

6. The method of clause 5, in which the first encoding technique is different to the second encoding technique.

7. The method of clause 6, in which the first encoding technique comprises encoding at least some of the least significant n bits by equiprobable binary arithmetic coding.

8. The method of clause 7, in which the first encoding technique comprises generating a second significance indicator in respect of a second threshold value lower than the first threshold value.

9. The method of any one of clauses 5 to 8, in which the second encoding technique comprises generating one or more of the further data sets.

10. The method of clause 9, in which the second encoding technique comprises, for any excess value of the given data item, the excess value being an amount by which the given data item exceeds a maximum value which can be encoded by the significance data set and the one or more further data sets, encoding the excess value as an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size.

11. The method of any one of the preceding clauses, comprising the step of adaptively varying the first threshold value in response to data values of encoded data items.

12. The method of clause 11, in which the step of adaptively varying comprises varying the first threshold value in response to a proportion of encoded data items for which the significance indicator indicates that those data items had a value of at least the first threshold value.

13. The method of any one of the preceding clauses, in which the data items are video data items representing a video signal 14. The method of any one of the preceding clauses, in which the significance indicator is configured to indicate whether or not an absolute value of the given data item has a value of at least the first threshold value.

15. The method of clause 7 or any one of clauses 8 to 14 as dependent upon clause 7, comprising encoding an indicator, for a block of the data items, indicating whether any data item in the block of data items has an absolute value of at least the first threshold value.

16. The method of clause 15, comprising grouping the least significant bits for a block of data items, encoded by equiprobable binary arithmetic coding, together in an output data stream, after the encoded first portions of the data items of that block of data items.

17. Computer software which, when executed by a computer, causes the computer to perform the method of any one of the preceding clauses.

18. A non-transitory machine-readable storage medium which stores the computer software of clause 17.

19. An encoded data signal generated by the method of any one of clauses 1 to 16.

20. A non-transitory machine-readable storage medium which stores the encoded data signal of clause 19.

21. A method of decoding successive data items, the method comprising: decoding a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and for at least some of the data items, selecting the first threshold value to be greater than 22. The method of clause 21, in which the one or more data sets comprise: a significance data set comprising the significance indicator; and zero or more further data sets selected from the list consisting of one or more greater-than-m data sets indicating whether a given data item is greater than a respective value of m, m being greater than the first threshold value; and a parity data set indicating a value of a least significant bit of a data item.

23 The method of clause 22, in which the threshold value is an integer equal to 2n where n is an integer of zero or more.

24 The method of clause 23, in which the data items are multi-bit data words.

The method of clause 24, in which, for a p-bit data item for which n is greater than zero, the method comprises: decoding a first portion of the data item represented by the most significant (p-n) bits by a second decoding technique; and decoding a second portion of the data item represented by the least significant n bits by a first decoding technique.

26. The method of clause 25, in which the first decoding technique is different to the second decoding technique.

27. The method of clause 26, in which the first decoding technique comprises decoding at least some of the least significant n bits by equiprobable binary arithmetic coding.

28. The method of clause 27, in which the first decoding technique comprises generating a second significance indicator in respect of a second threshold value lower than the first threshold value.

29. The method of any one of clauses 25 to 28, in which the second decoding technique comprises generating one or more of the further data sets.

30. The method of clause 29, in which the second decoding technique comprises, for any excess value of the given data item, the excess value being an amount by which the given data item exceeds a maximum value which can be decoded by the significance data set and the one or more further data sets, decoding the excess value as an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size.

31. The method of any one of clauses 21 to 30, comprising the step of adaptively varying the first threshold value in response to data values of decoded data items.

32. The method of clause 31, in which the step of adaptively varying comprises varying the first threshold value in response to a proportion of decoded data items for which the significance indicator indicates that those data items had a value of at least the first threshold value.

33. The method of any one of clauses 21 to 32, in which the data items are video data items representing a video signal 34. The method of any one of clauses 21 to 33, in which the significance indicator is configured to indicate whether or not an absolute value of the given data item has a value of at least the first threshold value.

36. The method of clause 27 or any one of clauses 28 to 35 as dependent upon clause 27, comprising decoding an indicator, for a block of the data items, indicating whether any data item in the block of data items has an absolute value of at least the first threshold value.

37. The method of clause 36, comprising, for a block of data items: decoding the first portions of the data items of that block of data items using a value of n = 0; applying a bit shift of n bits to the decoded first portions of the data items; decoding the second portions of the data items for that block of data items; and combining the decoded second portions of the data items with respective bit-shifted first portions generated by the applying step.

38. Computer software which, when executed by a computer, causes the computer to perform the method of any one of clauses 21 to 37.

39. A non-transitory machine-readable storage medium which stores the computer software of clause 38.

40. Apparatus for encoding successive data items, the apparatus comprising: encoding circuitry configured to encode a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and control circuitry configured, for at least some of the data items, to select the first threshold value to be greater than 1.

41. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of clause 40.

42. Apparatus for decoding successive data items, the apparatus comprising: decoding circuitry configured to decode a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and control circuitry configured, for at least some of the data items, to select the first threshold value to be greater than 1.

43. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of clause 42.

Claims

CLAIMS1. A method of encoding successive data items, the method comprising: encoding a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and for at least some of the data items, selecting the first threshold value to be greater than 2. The method of claim 1, in which the one or more data sets comprise: a significance data set comprising the significance indicator; and zero or more further data sets selected from the list consisting of one or more greater-than-m data sets indicating whether a given data item is greater than a respective value of m, m being greater than the first threshold value; and a parity data set indicating a value of a least significant bit of a data item.3. The method of claim 2, in which the threshold value is an integer equal to 2n where n is an integer of zero or more.The method of claim 3 in which the data items are multi-bit data words.5. The method of claim 4, in which, for a p-bit data item for which n is greater than zero, the method comprises: encoding a first portion of the data item represented by the most significant (p-n) bits by a second encoding technique; and encoding a second portion of the data item represented by the least significant n bits by a first encoding technique.6. The method of claim 5, in which the first encoding technique is different to the second encoding technique.7. The method of claim 6, in which the first encoding technique comprises encoding at least some of the least significant n bits by equiprobable binary arithmetic coding.8. The method of claim 7, in which the first encoding technique comprises generating a second significance indicator in respect of a second threshold value lower than the first threshold value.9. The method of claim 5, in which the second encoding technique comprises generating one or more of the further data sets.10. The method of claim 9, in which the second encoding technique comprises, for any excess value of the given data item, the excess value being an amount by which the given data item exceeds a maximum value which can be encoded by the significance data set and the one or more further data sets, encoding the excess value as an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size.11. The method of claim 1, comprising the step of adaptively varying the first threshold value in response to data values of encoded data items.12. The method of claim 11, in which the step of adaptively varying comprises varying the first threshold value in response to a proportion of encoded data items for which the significance indicator indicates that those data items had a value of at least the first threshold value.13. The method of claim 1, in which the data items are video data items representing a video signal.14. The method of claim 1, in which the significance indicator is configured to indicate whether or not an absolute value of the given data item has a value of at least the first threshold value.15. The method of claim 7, comprising encoding an indicator, for a block of the data items, indicating whether any data item in the block of data items has an absolute value of at least the first threshold value.16. The method of claim 15, comprising grouping the least significant bits for a block of data items, encoded by equiprobable binary arithmetic coding, together in an output data stream, after the encoded first portions of the data items of that block of data items.17. Computer software which, when executed by a computer, causes the computer to perform the method of claim 1.18. A non-transitory machine-readable storage medium which stores the computer software of claim 17.19. An encoded data signal generated by the method of claim 1.20. A non-transitory machine-readable storage medium which stores the encoded data signal of claim 19.21. A method of decoding successive data items, the method comprising: decoding a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and for at least some of the data items, selecting the first threshold value to be greater than 1.22. The method of claim 21, in which the one or more data sets comprise: a significance data set comprising the significance indicator; and zero or more further data sets selected from the list consisting of: one or more greater-than-m data sets indicating whether a given data item is greater than a respective value of m, m being greater than the first threshold value; and a parity data set indicating a value of a least significant bit of a data item.23. The method of claim 22, in which the threshold value is an integer equal to 2n where n is an integer of zero or more.24. The method of claim 23, in which the data items are multi-bit data words.25. The method of claim 24, in which, for a p-bit data item for which n is greater than zero, the method comprises: decoding a first portion of the data item represented by the most significant (p-n) bits by a second decoding technique; and decoding a second portion of the data item represented by the least significant n bits by a first decoding technique.26. The method of claim 25, in which the first decoding technique is different to the second decoding technique.27. The method of claim 26, in which the first decoding technique comprises decoding at least some of the least significant n bits by equiprobable binary arithmetic coding.28. The method of claim 27, in which the first decoding technique comprises generating a second significance indicator in respect of a second threshold value lower than the first threshold value.29. The method of claim 25, in which the second decoding technique comprises generating one or more of the further data sets 30. The method of claim 29, in which the second decoding technique comprises, for any excess value of the given data item, the excess value being an amount by which the given data item exceeds a maximum value which can be decoded by the significance data set and the one or more further data sets, decoding the excess value as an escape code comprising a first portion and a non-unary coded second portion having a length, in bits, dependent upon a second portion size.31. The method of claim 21, comprising the step of adaptively varying the first threshold value in response to data values of decoded data items.32. The method of claim 31, in which the step of adaptively varying comprises varying the first threshold value in response to a proportion of decoded data items for which the significance indicator indicates that those data items had a value of at least the first threshold value.33. The method of claim 21, in which the data items are video data items representing a video signal.34. The method of claim 21, in which the significance indicator is configured to indicate whether or not an absolute value of the given data item has a value of at least the first threshold value.36. The method of claim 27, comprising decoding an indicator, for a block of the data items, indicating whether any data item in the block of data items has an absolute value of at least the first threshold value.37. The method of claim 36, comprising, for a block of data items: decoding the first portions of the data items of that block of data items using a value of n = 0; applying a bit shift of n bits to the decoded first portions of the data items; decoding the second portions of the data items for that block of data items; and combining the decoded second portions of the data items with respective bit-shifted first portions generated by the applying step.38. Computer software which, when executed by a computer, causes the computer to perform the method of claim 21 39. A non-transitory machine-readable storage medium which stores the computer software of claim 38.40. Apparatus for encoding successive data items, the apparatus comprising: encoding circuitry configured to encode a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and control circuitry configured, for at least some of the data items, to select the first threshold value to be greater than 1.41. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of claim 40.42. Apparatus for decoding successive data items, the apparatus comprising: decoding circuitry configured to decode a given data item using a series of one or more data sets, each data set representing a respective range of values of the data items; the series of one or more data sets comprising at least a significance indicator to indicate whether or not the given data item has a value of at least a first threshold value; and control circuitry configured, for at least some of the data items, to select the first threshold value to be greater than 1.43. Video data capture, transmission, display and/or storage apparatus comprising the apparatus of claim 42.