Patent Description:
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, are neither expressly or impliedly admitted as prior art against the present disclosure.

There are several video data compression and decompression systems which involve generating predicted samples and encoding so-called residual data representing a difference between the prediction and the input data.

Some standards and draft standards, such as the so-called High Efficiency Video Coding (HEVC) standards or the Joint Exploration Model (JEM) or Future Video Coding (FVC) proposals, define encoding and decoding systems applicable to the present disclosure.

Previously proposed arrangements are disclosed by <CIT>; <CIT> and <NPL>.

An aspect of this disclosure is defined by claim <NUM>.

Further respective aspects and features 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 not restrictive of, the present disclosure.

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 of embodiments, when considered in connection with the accompanying drawings, wherein:.

Referring now to the drawings, <FIG> 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.

All of the data compression and/or decompression apparatus is 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 machine-readable data storage media by which such software and/or firmware are stored or otherwise provided, are considered as embodiments.

<FIG> schematically illustrates an audio/video data transmission and reception system using video data compression and decompression.

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

The compression apparatus <NUM> and decompression apparatus <NUM> can therefore form one node of a transmission link. The decompression apparatus <NUM> and decompression apparatus <NUM> 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.

<FIG> schematically illustrates a video display system using video data decompression. In particular, a compressed audio/video signal <NUM> is processed by a decompression apparatus <NUM> to provide a decompressed signal which can be displayed on a display <NUM>. The decompression apparatus <NUM> could be implemented as an integral part of the display <NUM>, for example being provided within the same casing as the display device. Alternatively, the decompression apparatus <NUM> might be 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 <NUM>; it is simply a term used in the art to indicate a device which is connectable to a display as a peripheral device.

<FIG> schematically illustrates an audio/video storage system using video data compression and decompression. An input audio/video signal <NUM> is supplied to a compression apparatus <NUM> which generates a compressed signal for storing by a store device <NUM> 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 store device <NUM> and passed to a decompression apparatus <NUM> for decompression to provide an output audio/video signal <NUM>.

It will be appreciated that the compressed or encoded signal, and a storage medium or data carrier storing that signal, are considered as embodiments. Reference is made to <FIG> described below.

<FIG> schematically illustrates a video camera using video data compression. In <FIG>, and image capture device <NUM>, 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 <NUM>. A microphone (or plural microphones) <NUM> generates an audio signal to be passed to the compression apparatus <NUM>. The compression apparatus <NUM> generates a compressed audio/video signal <NUM> to be stored and/or transmitted (shown generically as a schematic stage <NUM>).

The techniques to be described below relate primarily to video data compression. 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 <FIG>) 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.

<FIG> schematically illustrates an example video camera apparatus <NUM> in more detail. Those features numbered in common with <FIG> will not be described further. <FIG> is an example of the camera of <FIG> (in the case that the unit <NUM> of <FIG> provides a storage capability) in which the compressed data are first buffered by a buffer <NUM> and then stored in a storage medium <NUM> such as a magnetic disk, an optical disk, flash memory, a so-called solid-state disk drive (SSD) or the like. Note that the arrangement of <FIG> can be implemented as a single (physical) unit <NUM>.

<FIG> schematically illustrates another example video camera in which, in place of the storage arrangement of <FIG>, a network interface <NUM> is provided in order to allow the compressed data to be transmitted to another unit (not shown). The network interface <NUM> can also allow for incoming data to be received by the video camera, such as control data. Note that the arrangement of <FIG> can be implemented as a single (physical) unit <NUM>.

<FIG> schematically illustrate data carriers, for example for use as the storage medium <NUM> and carrying compressed data which has been compressed according to the compression techniques described in the present application. <FIG> shows a schematic example of a removable non-volatile storage medium <NUM> implemented as solid state memory such as flash memory. <FIG> shows a schematic example of a removable non-volatile storage medium <NUM> implemented as a disk medium such as an optical disk.

<FIG> provides a schematic overview of a video data compression and decompression apparatus.

Successive images of an input video signal <NUM> are supplied to an adder <NUM> and to an image predictor <NUM>. The image predictor <NUM> will be described below in more detail with reference to <FIG>. The adder <NUM> in fact performs a subtraction (negative addition) operation, in that it receives the input video signal <NUM> on a "+" input and the output of the image predictor <NUM> 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 <NUM> representing the difference between the actual and projected images.

One reason why a residual image signal is generated is as follows. The data coding techniques to be described, that is to say the techniques which will be applied to the residual image signal, tends 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, the expectation is that the residual image data will contain less information (less energy) than the input image and so will be easier to encode into a small amount of encoded data.

The residual image data <NUM> is supplied to a transform unit <NUM> which generates a discrete cosine transform (DCT) representation of the residual image data. The DCT technique itself is well known and will not be described in detail here.

Note that in some embodiments, a discrete sine transform (DST) is used instead of a DCT. In other embodiments, no transform might be used. This can be done selectively, so that the transform stage is, in effect, bypassed, for example under the control of a "transform-skip" command or mode.

The output of the transform unit <NUM>, which is to say, a set of transform coefficients for each transformed block of image data, is supplied to a quantiser <NUM>. 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 work more efficiently in generating small amounts of compressed video data.

A controller <NUM> controls the operation of the transform unit <NUM> and the quantiser <NUM> (and their respective inverse units), according to techniques to be discussed further below. Note that the controller <NUM> may also control other aspects of the operation of the apparatus of <FIG> and indeed the operation of the apparatus of <FIG>.

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

The scanned coefficients are then passed to an entropy encoder (EE) <NUM>. 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 <NUM>-<NUM>% 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 (or processing of data by) the entropy encoder can take place in the scan order. Corresponding considerations apply to the respective inverse processes.

The output of the entropy encoder <NUM>, along with additional data, for example defining the manner in which the predictor <NUM> generated the predicted image, provides a compressed output video signal <NUM>.

However, a return path is also provided because the operation of the predictor <NUM> 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 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 <NUM> 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 <NUM> 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 <NUM> is considered 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 <NUM>. So, the return path can be implemented before the entropy encoding stage. Indeed, the scanning process carried out by the scan unit <NUM> is also considered lossless, but in the present embodiment the return path <NUM> is from the output of the quantiser <NUM> to the input of a complimentary inverse quantiser <NUM>.

In general terms, an entropy decoder <NUM>, the reverse scan unit <NUM>, an inverse quantiser <NUM> and an inverse transform unit <NUM> provide the respective inverse functions of the entropy encoder <NUM>, the scan unit <NUM>, the quantiser <NUM> and the transform unit <NUM>. For now, the discussion will continue through the compression process; the process to decompress an input compressed video signal corresponds to the return path of the compression process and so a decoding apparatus or method corresponds to the features or operation of the decoding path of the encoder described here.

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

The image signal <NUM> is added, at an adder <NUM>, to the output of the predictor <NUM> to generate a reconstructed output image <NUM>. This forms one input to the image predictor <NUM>.

Turning now to the process applied to a received compressed video signal <NUM>, the signal is supplied to the entropy decoder <NUM> and from there to the chain of the reverse scan unit <NUM>, the inverse quantiser <NUM> and the inverse transform unit <NUM> before being added to the output of the image predictor <NUM> by the adder <NUM>. In straightforward terms, the output <NUM> of the adder <NUM> forms the output decompressed video signal <NUM>. In practice, further filtering may be applied before the signal is output.

<FIG> schematically illustrates the generation of predicted images, and in particular the operation of the image predictor <NUM>, under the control of the controller <NUM> (not itself shown in <FIG>).

There are two basic modes of prediction: so-called intra-image prediction and so-called inter-image, or motion-compensated (MC), prediction.

Intra-image prediction bases a prediction of the content of a block 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, where the whole image is intra-encoded, 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 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.

Returning to <FIG>, two image prediction arrangements (corresponding to intra- and inter-image prediction) are shown, the results of which are selected by a multiplexer <NUM> under the control of a mode signal <NUM> so as to provide blocks of the predicted image for supply to the adders <NUM> and <NUM>. 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 encoder 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.

The actual prediction, in the intra-encoding system, is made on the basis of image blocks received as part of the signal <NUM>, 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 <NUM> by an intra-mode selector <NUM> to control the operation of the intra-image predictor <NUM>.

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

The processing applied to the signal <NUM> will now be described. Firstly, the signal is filtered by a filter unit <NUM>. This involves applying a "deblocking" filter to remove or at least tend to reduce the effects of the block-based processing carried out by the transform unit <NUM> and subsequent operations. Also, an adaptive loop filter is applied using coefficients derived by processing the reconstructed signal <NUM> and the input video signal <NUM>. The adaptive loop filter is a type of filter which, using known techniques, applies adaptive filter coefficients to the data to be filtered. That is to say, the filter coefficients can vary in dependence upon various factors. Data defining which filter coefficients to use is included as part of the encoded output data stream.

The filtered output from the filter unit <NUM> in fact forms the output video signal <NUM>. It is also buffered in one or more image stores <NUM>; 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 <NUM> may be held in a compressed form and then decompressed for use in generating motion vectors. For this particular purpose, any known compression / decompression system may be used. The stored images are passed to an interpolation filter <NUM> 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 <NUM> is <NUM> times (in each dimension) that of the images stored in the image stores <NUM>. The interpolated images are passed as an input to the motion estimator <NUM> and also to the motion compensated predictor <NUM>.

In embodiments, a further optional stage is provided, which is to multiply the data values of the input video signal by a factor of four using a multiplier <NUM> (effectively just shifting the data values left by two bits), and to apply a corresponding divide operation (shift right by two bits) at the output of the apparatus using a divider or right-shifter <NUM>. So, the shifting left and shifting right changes the data purely for the internal operation of the apparatus. This measure can provide for higher calculation accuracy within the apparatus, as the effect of any data rounding errors is reduced.

The way in which an image is partitioned for compression processing will now be described. At a basic level, and image to be compressed is considered as an array of blocks of samples. For the purposes of the present discussion, the largest such block under consideration is a so-called coding tree unit (CTU) <NUM> (<FIG>), which in this example represents a square array of <NUM> x <NUM> samples. Here, the discussion relates to luminance samples. Depending on the chrominance mode, such as <NUM>:<NUM>:<NUM>, <NUM>:<NUM>:<NUM>, <NUM>:<NUM>:<NUM> or <NUM>:<NUM>:<NUM>:<NUM> (GBR plus key data), there will be differing numbers of corresponding chrominance samples corresponding to the luminance block.

In general terms, the recursive subdividing of the CTUs allows an input picture to be partitioned in such a way that both the block sizes and the block coding parameters (such as prediction or residual coding modes) can be set according to the specific characteristics of the image to be encoded.

The CTU may be subdivided into so-called coding units (CU). Coding units may be square or rectangular and have a size between a minimum size (smaller than the CTU) and the full size of the CTU <NUM>. The coding units can be arranged as a kind of tree structure, so that a first subdivision may take place as shown in <FIG>, giving coding units <NUM> of 32x32 samples; subsequent subdivisions may then take place on a selective basis so as to give some coding units <NUM> of 16x16 samples (<FIG>) and potentially some coding units <NUM> of 8x8 samples (<FIG>). Overall, this process can provide a content-adapting coding tree structure of CU blocks, each of which may be as large as the CTU or as small as 8x8 samples. Encoding of the output video data takes place on the basis of the coding unit structure.

The CUs may also represent the minimum sub-division for prediction and transform, so that the CUs are also used as prediction units (PUs) and transform units (TUs), or in other examples a different or further sub-division into PUs and/or TUs may be performed.

At least parts of the entropy encoding operations carried out by the entropy encoder <NUM> can make use of so-called CABAC (context adaptive binary arithmetic coding) techniques. The CABAC context modelling and encoding process is described in more detail in "<NPL>.

A significant feature of the CABAC system is that a particular CABAC bit stream has to be decoded by a single decoder. That is to say, the CABAC data for a particular individual bit stream is inherently serialised, because each encoded value depends on previously encoded values, and cannot be handled by multiple decoders in parallel. However, when decoding video at very high operating points (for example, high bit rates and/or high quality such as professional quality), the CABAC throughput requirement is such that it becomes difficult to implement an entropy-decoder capable of decoding the worst-case frame in a timely manner. With this and other similar throughput constraints in mind, parallel operation of data-processing systems such as HEVC video decoders has been proposed.

Various issues arise when parallel decoder operation is being contemplated. As discussed above, a particular bit stream should be decoded by a respective decoder. Techniques for addressing these requirements will now be described.

Referring to <FIG>, the CTUs are encoded in a CTU order from top left to lower right of the image. Clearly, this is just an example order and a different order could be used, but here it illustrates an order of dependency in that in an intra-image prediction arrangement, a particular transform unit (TU) <NUM>, which is predicted using reference samples <NUM> above and to the left of the TU <NUM>, may require the use of reference samples from one or more neighbouring and previously encoded (and decoded) CTUs. In the example of <FIG>, the TU <NUM>, being at an upper edge of a CTU <NUM>, relies upon reference to samples from neighbouring CTUs <NUM>, <NUM>. Similarly, if a TU is at the extreme left edge of a CTU, the TU can rely upon reference samples from a CTU to the left of the current CTU. This provides an example in which a group of references samples is disposed, with respect to the current image region above and to the left of the current image region. In examples, the group of reference samples comprises one or more linear arrays of reference samples disposed with respect to the current region, the intra-image predictor being configured to derive the predicted samples in dependence upon reference samples of the one or more linear arrays pointed to by the prediction direction.

This potential dependency between CTUs imposes a decoding order for CTUs shown schematically in <FIG>. Here, a top left CTU <NUM> is decoded first, followed by a CTU <NUM> next to the right of the CTU <NUM>. After that, a next CTU <NUM> in the top row can be decoded, but also taking into account the dependency of the type shown in <FIG>, a first CTU <NUM> of a second row can be decoded. The decoding can proceed, CTU by CTU, in the order indicated by the numbers drawn within each box (representing a respective CTU) in <FIG>. By looking at the numbers indicating the order of decoding in <FIG>, it can be seen that this arrangement can be referred to as a so-called wavefront system, in which each row can start decoding once the left hand two CTUs of the row above have been completed.

With regards to the so-called wavefront system, any advantage which it brings tends to decrease as the CTU size increases. Also, the degree of parallelism of the wavefront arrangement is dependent only upon the height of the picture not the width. In particular, the parallelism offered by this system therefore tends to increase with the square root of the number of pixels, rather than with the number of pixels itself.

With regard to <FIG>, a different example arrangement is shown in which a restriction is included with regards to intra-image prediction, such that the use of reference samples from neighbouring CTUs is disallowed altogether. This means that no CTU has a dependency, from the point of intra-image prediction, on any other CTU. This leads to a potential decoding order shown schematically in <FIG>, in which all of the CTUs <NUM> can potentially be decoded in parallel. Such an arrangement only impacts TUs at the top and/or left of a CTU, so that for larger CTUs, the impact is reduced because the proportion of TUs at the top and/or left will be lower for a larger CTU. In this arrangement, all CTUs can be decoded in parallel, although as mentioned there is no requirement that they all be coded in parallel, there is simply no dependency amongst them to prevent such decoding. However, by preventing these of neighbouring reference samples there will be a potential loss of coding efficiency, at least for the top/left TUs of a CTU, because reference samples which might otherwise have been relevant to an efficient prediction are not allowed to be used.

This discussion of <FIG> provides an example in which a prediction mode selector is configured to inhibit the selection of a prediction operation for the current region, for which at least some of the reference samples are disposed in a CTU above the current CTU.

In another alternative arrangement, the restriction mentioned above is amended so that reference samples from a CTU to the left of the current CTU can be used for intra-image prediction, but reference samples above and above right of the current CTU cannot be used. This leads to a potential decoding order shown schematically in <FIG> in which decoding can take place on a column-by-column basis amongst the CTUs <NUM>. Once again, there is a potential increase in parallelism at decoding, compared to <FIG>, but a potential decrease in coding efficiency because reference samples (which might otherwise have been relevant) are not used.

This discussion of <FIG> provides an example in which a prediction mode selector is configured to inhibit the selection of a prediction operation for the current region, for which at least some of the reference samples are disposed in a CTU to the left of the current CTU.

The use of above and left reference samples from other CTUs can be inhibited, for example in an arrangement in which the prediction mode selector is configured to inhibit the selection of a prediction operation for the current region, for which at least some of the reference samples are disposed either in a CTU above the current CTU or a CTU to the left of the current CTU.

<FIG> schematically illustrates a variation of the arrangement of <FIG>, in particular relating to the manner by which a choice is made between intra-image prediction and inter-image prediction.

An encoding efficiency detector <NUM> operates to make a basic choice as between intra-image and inter-image prediction, using conventional techniques, for example by detecting one or more metrics indicating a quantity of encoded data which would arise if each candidate technique were used and selecting the candidate technique:.

However, as part of the function of the prediction operation selector, further circuitry or modules <NUM>, <NUM> are provided.

In particular, a reference sample encoding detector <NUM>, responsive to the currently selected prediction mode from the intra mode selector <NUM> detects the type of encoding used in respect of reference samples required by the currently selected intra mode, at least in instances where the reference samples (or at least some of the reference samples) applicable to that mode are located in a neighbouring CTU. If those reference samples in a neighbouring CTU were intra-image encoded then, for the current TU, a mode controller <NUM> inhibits (or in some examples prevents) the use of intra-image mode provisionally selected by the intra selector <NUM> and forces the use of inter-image encoding. In an alternative arrangement, intra-image encoding could still be allowed, but a different intra-image mode could be enabled, being one in which reference samples from that neighbouring CTU are not used. In other words, the restriction does not prevent the use of (say) a <NUM> degree prediction when (say) the above row is unavailable. In those example cases the top row of reference samples could be replaced with other values that can be derived from the left row of reference samples. Or in another arrangement a dc or planar mode could be used instead.

The mode controller <NUM> can also act as a generator configured to generate indicator data <NUM>, for provision to an image data decoder (for example as part of or otherwise in association with the encoded data stream), indicating the prediction mode selected by the selector.

So, neighbouring CTU samples can be used as references if they themselves were obtained by inter-image encoding, which does not have the same dependency ordering within a particular image. This represents a restriction in intra-prediction, such that the use of neighbouring reference samples which themselves were intra-predicted is disallowed when they are from neighbouring CTUs. This therefore allows the intra prediction process to be performed in parallel at CTU level, as there is no dependency on other intra predicted blocks outside the current CTU. It can also reduce the loss of coding efficiency as opposed to other techniques discussed above, since more reference data is potentially available, for example neighbouring inter-predicted samples.

This provides an example of inhibiting the selection of an intra-image prediction operation for the current region for which at least some of the reference samples at one or more of the predetermined sides of the current image region are disposed in a CTU other than the current CTU and those reference samples were generated using an intra-image prediction mode; and allowing the selection of an intra-image prediction operation for the current region for which at least some of the reference samples at one or more of the predetermined sides of the current image region are disposed in a CTU other than the current CTU and those reference samples were generated using an inter-image prediction mode.

The apparatus of <FIG>, when operating in accordance with these principles and potentially using the apparatus of <FIG>, provides an example of an image encoding apparatus (<FIG>) comprising: a selector (<FIG>) configured to select a prediction operation, for prediction of samples of a current region of a current image with respect to one or more of a group of reference samples, from a set of candidate prediction operations, at least some of which define, as an intra-image prediction operation, a prediction direction between a current sample to be predicted and a group of reference samples in the same image; and an intra-image predictor <NUM> configured to derive predicted samples of a current image region from reference samples of the same image in response to selection, by the selector, of an intra-image prediction operation for the current image region; in which: the current region comprises at least a subset of a current coding tree unit (CTU) in an array of CTUs; the group of references samples is disposed, with respect to the current image region at one or more predetermined sides of the current image region; and the selector is configured to inhibit the selection of a prediction operation for the current region, for which at least some of the reference samples at one or more of the predetermined sides of the current image region are disposed in a CTU other than the current CTU.

Referring back to <FIG>, in an example in which the current image is an image of a sequence of images; the apparatus can comprise an inter-image predictor <NUM> configured to predict samples of the current image region, with respect to a reference image region of another, reference, image of the sequence of images, according to a direction vector indicating a spatial displacement of the reference image region from the current image region. Here, a set of candidate prediction operations can comprise an inter-image prediction operation.

<FIG> schematically illustrates a potential decoding order of CTUs encoded using the techniques of <FIG>, showing that all of the CTUs <NUM> can potentially be encoded or decoded in parallel.

<FIG> schematically illustrates a parallel encoding and/or decoding apparatus operating according to any one of the restrictions discussed above, in which input data (image data to be encoded in the case of an encoder or encoded image data to be decoded, in the case of a decoder) is demultiplexed by a demultiplexer <NUM> to multiple encoder and/or decoder units <NUM> for encoding or decoding in parallel, with the results being multiplexed by a multiplexer <NUM> back into an output signal (encoded data in the case of an encoder or decoded data in the case of a decoder).

The interaction between the techniques discussed above and the so-called "constrained intra" technique will now be discussed. The constrained intra mode is provided within the standards and proposals to allow, for example, the image to be refreshed periodically even if an entire intra-encoded image (an "I frame" in MPEG terminology) is not being provided. In the constrained intra mode, at least portions of the image are constrained so that they may only be derived from other samples which have themselves been intra-image encoded or encoded using intra-block-copy processing. In this way, the constrained intra mode prevents (for example, over the course of several images) erroneous data from previously processed images being propagated into subsequent images by virtue of inter-image processing. In the constrained intra mode, therefore, a source block containing reference samples for a current block must itself have been intra-image encoded and/or encoded using intra-block-copy processing.

However, constrained intra operation is not concerned with CTU boundaries. The constrained intra rule (when applied) is applied equally to all blocks in a CTU.

The present techniques of <FIG> involve an inhibition on the use of intra modes for which the reference samples span CTUs, but do not necessarily impose a requirement that inter-image encoding is used for those blocks. Therefore, the constrained intra mode of operation can still be used in combination with these techniques.

<FIG> is a schematic flowchart illustrating an image encoding method comprising:.

It will be appreciated that data signals generated by the variants of coding apparatus discussed above, and storage or transmission media carrying such signals, are considered to represent embodiments of the present disclosure. For example, Figure <NUM> schematically represents a data signal comprising coded data generated according to the method of <FIG>, the coded data comprising a succession of data portions <NUM>. <NUM> each representing a CTU and each being decodable without reference to a data portion representing a different CTU. The data portions <NUM>,,,<NUM> are represented schematically one next to another but it will be appreciated that this does not imply that they are or need to be generated serially, nor does this representation imply that they are or need to be transmitted or stored serially.

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 claims, the technology may be practiced otherwise than as specifically described herein.

Claim 1:
An image encoding apparatus comprising:
a selector (<NUM>, <NUM>) configured to select a prediction operation, for prediction of samples of a current region(<NUM>) of a current image of a sequence of images with respect to one or more of a group of reference samples, from a set of candidate prediction operations, at least some of which define, as an intra-image prediction operation, a prediction direction between a current sample to be predicted and a group of reference samples in the same image;
an intra-image predictor (<NUM>) configured to derive predicted samples of a current image region from reference samples of the same image in response to selection, by the selector, of an intra-image prediction operation for the current image region; and
an inter-image predictor (<NUM>) configured to predict samples of the current image region, with respect to a reference image region of another, reference, image of the sequence of images, according to a direction vector indicating a spatial displacement of the reference image region from the current image region;
in which:
the current region comprises at least a subset of a current coding tree unit (CTU) in an array of CTUs;
the group of references samples (<NUM>) is disposed, with respect to the current image region (<NUM>) at one or more predetermined sides of the current image region;
the set of candidate prediction operations comprises an inter-image prediction operation; and
the selector is configured:
to inhibit the selection of an intra-image prediction operation for the current region for which at least some of the reference samples at one or more of the predetermined sides of the current image region (<NUM>) are disposed in a CTU other than the current CTU and those reference samples were generated using an intra-image prediction mode; and
to allow the selection of an intra-image prediction operation for the current region for which at least some of the reference samples at one or more of the predetermined sides of the current image region (<NUM>) are disposed in a CTU other than the current CTU and those reference samples were generated using an inter-image prediction mode..