GB2496195A - Entropy encoding video data using reordering patterns - Google Patents

Abstract

Video data compression apparatus in which block based video data is reordered and entropy encoded comprises a selector for selecting a reordering pattern from a set of two or more candidate reordering patterns, for use in reordering video data for encoding, at least some of the reordering patterns being arranged so as to interleave video data from plural colour channels and/or plural blocks to form a composite data block. A data scanner then changes the order of the video data for encoding according to the selected reordering pattern so as to generate reordered video data and an entropy encoder for entropy-encoding the reordered video data.

Description

VIDEO DATA COMPRESSION AND DECOMPRESSION

This invention relates to video data compression and decompression.

There are several video data compression and decompression 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.

Entropy, in the present context, can be considered as representing the information content of a data symbol or series of symbols. The aim of entropy encoding is to encode a series of data symbols in a]ossless manner using (ideally) the smallest number of encoded data bits which are necessary to represent the information content of that series of data symbols. In practice, entropy encoding is used to encode the quantised coefficients such that the encoded data is smaller (in terms of its number of bits) then the data size of the original quantised coefficients, A more efficient entropy encoding process gives a smaller output data size for the same input data size.

An important part of the entropy encoding process used in video data compression relates to the order in which the quantised coefficients are presented for encoding.

Typically, a data scanning process is applied to the quantised coefficients. The purpose of the scanning process is to reorder the quantised frequency-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 (which encode runs or successive sequences of zeros by a small number of data bits defining the length of the run) 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 quantisod, according to a "scanning order" so that (a) all of the cDefficients are selected once as part of the scan, and (b) the scan tends to provide the desired reordering.

In practical terms, The output of the frequency domain transformation stage typically comprises a set of frequency domain coefficients which vary according to the horizontal and vertical spatial frequencies which they represent in the original image block. There is generally a so-called "DC" coefficients which represents the average (DC) value of the samples in the original image block, together with a succession of coefficients representing respective permutations of low or high horizontal and vertical spatial frequency ranges.

The way in which these coefficients are ordered for transmission to the data scanning process is of course arbitrary, but for convenience the coefficients are often considered to form a data array with the DC coefficient in a top-left corner of the array, increasing horizontal spatial frequency represented in a left-to-right direction in the array and increasing vertical spatial frequency represented in a top-to-bottom direction in the array. Under this representation, a data scanning process which has been found to provide useful results is a so-called zigzag scan, which starts with the DC coefficient and then proceeds through the remaining coefficients.

one by one, in a zigzag fashion. An example of a zigzag scan is illustrated schematically in Figure 16 of the accompanying drawings. The scanning pattern would mean that the first two coefficients scanned after the DC coefficient would be those representing: (a) zero vertical spatial frequency and the lowest horizontal spatial frequency range; and (b) zero horizontal spatial frequency and the lowest vertical spatial frequency range, respectively. After that, the scan proceeds so that successive diagonals (in a lower-left to upper-right direction) of the array of coefficients are scanned, one coefficient at a time.

The zigzag scan is considered advantageous because, for many normal types of image, and in particular images which have been captured from real scenes, most of the information content tends to lie in the DC and low frequency coefficients. Ft is often the case that many or all of the higher frequency coefficients are zero. This is particularly the case in systems such as the proposed "High Efficiency Video Coding" (HEVC) system in which residual image data (that is to say, data representing the difference between an actual image and a predicted version of that image) is encoded. So, by scanning the DC and lower frequency coefficients first, the non-zero values can tend to be gathered together and the zero values can also tend to be gathered together. As mentioned above, this can lead to a more efficient entropy encoding process.

This invention provides video data compression apparatus in which block based video data is reordered and entropy encoded, the apparatus comprising: a selector for selecting a reordering pattern from a set of two or more candidate reordering patterns, for usa in reordering video data for encoding, at least some of the reordering patterns being arranged so as to interleave video data from plural coFour channels andlor plural blocks to form a composite data block; a data scanner for changing the order of the video data for encoding according to the selected reordering pattern so as to generate reordered video data; and an entropy encoder for entropy-encoding the reordered video data.

The invention recognises that potentially improved efficiency of the entropy encoding process can be obtained by shuffling or intei-leaving frequency domain data from two or more colour channels and/or two or more blocks. This is because of an empiricaBy observed similarity between nearby blocks of data (particularly along an image feature direction) and between different colour channels relating to the same block of data.

Further respective aspects and features of the invention are defined in the appended claims.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 schematically illustrates an audio/video (NV) 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; Figure 5 provides a schematic overview of a video data compression and decompression apparatus; Figure 6 schematically illustrates the generation of predicted images; Figure 7 schematically illustrates a largest coding unit (LCU); Figure 8 schematically illustrates a set of four coding units (CU); Figures 9 and 10 schematically illustrate the coding units of Figure 8 sub-divided into smaller coding units; Figure 11 schematically illustrates an array of prediction units (PU); Figure 12 schematically illustrates an array of transform units (TU); Figure 13 schematically illustrates a partially-encoded image; Figure 14 schematically illustrates a set of possible prediction directions; Figure 15 schematically illustrates a set of prediction modes; Figure 16 schematically illustrates a zigzag scan; Figure 17 schematically illustrates a CABAC entropy encoder; Figure 18 schematically illustrates a CAVLC entropy encoding process; Figure 19 schematically illustrates a set of data relating to a 4:4:4 video signal; Figure 20 schematically illustrates a set of data relating to a 4:2:2 video signal; Figures 21 and 22 schematically illustrate an A,0] interleave mode; Figures 23 and 24 schematically illustrate a [CD] interleave mode; Figures 25 and 26 schematically illustrate a [OF] interleave mode; Figures 27 and 28 schematically illustrate a [OTV] interleave mode; Figures 29 and 30 schematically illustrate a [OH] interleave mode; Figures 31 and 32 schematicaRy illustrate an [A,F] interleave mode; Figures 33 and 34 schematically illustrate an [A,V] interleave mode; Figures 35 and 36 schematically illustrate an [AIHI interleave mode; Figure 37 schematically illustrates a data flag indicating an interleave mode; Figure 38 schematically illustrates an interleave mode selector; and Figure 39 schematically illustrates a d&1nterleaver.

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 invention.

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 data 1 0 storage media by which sucfl software and/or firmware are stored or otherwise provided, are considered as embodiments of the present invention.

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

An input audio/video signal 10 is supplied to a video data compression apparatus 20 which compresses at least the video component of the audio/video signal 10 for transmission along a transmission route 30 such as a cable, an optical fibre, a wireless link or the like. The compressed signal s processed by a decompression apparatus 40 to provide an output audio/video signal 50. For the r&urn 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, on]y 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 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 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 store 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 storing that signal, are considered as embodiments of the present invention.

Figure 4 schematically illustrates a video camera using video data compression. In Figure 4, and 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. It will be appreciated that many existing techniques may be used for audio data compression in conjunctiDn 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 5 provides a schematic overview of a video data compression and decompression apparatus.

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

One reason why a residual image signal is generated is as follows-The data coding techniques to be described, that is to say the techniques which wifl be applied to the residual image signal, tends to work more efficiently when there is Jess "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 Es 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 330 is supplied to a transform unit 340 which generates a discrete cosine transforni (DCT) representation of the residual image data. The DCT technique 1 5 itself is well known and will not be described in detail here. There are however aspects of the techniques used in the present apparatus which will be described in more detail below, in particular relating to the selection of different blocks of data to which the OCT operation is applied. These will be discussed with reference to Figures 7-12 below.

The output of the transform unit 340, which is to say, a set of OCT coefficients for each transformed block of image data, is supplied to a quantiser 350. Various quantisation techniques are known in the field of ddeo 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 genera[ 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. Techniques for selecting a scanning order will be described below. 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) 370. Again, various types of entropy encoding may be used. Two examples which wil! be described below are variants of the so-called CABAC (Context Adaptive Binary Arithmetic Coding) system and variants of the so-called CAVLC (Context Adaptive Variable-Length Coding) system. In general terms, CABAC is considered to provide a better efficiency, and in some studies has been shown to provide a 10-20% reduction in the quantity of encoded output data for a comparable image quality compared to CAVLC. However, CAVLC is considered to represent a much lower level of complexity (in terms of its implementation) than CABAC. The CABAC technique will be discussed with reference to Figure 17 below, and the CAVLC technique will be discussed with reference to Figures 18 and 19 below.

Note that the scanning process and the entropy encoding process are shown as separate processes, but in fact can be combined or treated together. That is to say, the reading of data into the entropy encoder can take place in the scan order. Corresponding considerations apply to the respective inverse processes to be described below.

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

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

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

The entropy encoding process carried out by the entropy encoder 370 is considered 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, the return path can be implemented before the entropy encoding stage. Indeed, the scanning process carried out by the scan unit 360 is also considered lossless, but in the present embodiment the return path 390 is from the output of the quantiser 350 to the input of a complimentary inverse quantiser 420.

In general terms, an entropy decoder 410, the reverse scan unit 400, an inverse quantiser 420 and an inverse transform unit 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 wifi continue through the compression process; the process to decompress an input compressed video signal will be discussed separately below.

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

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

Turning now to the process applied to 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. In straightforward terms, the output 460 of the adder 450 forms the output decompressed video signal 480. In practice, further filtering may be applied before the signal is output.

Figure 6 schematicafly illustrated the generation of predicted images, and in particular the operation of the image predictor 320.

There are two basic modes of prediction: so-caRed 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, En the present embodiments the choice between Entra-and inter-encoding can be made on a block-by-block basis, though in other embodiments of the invention 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 Figure 6. 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 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 encoder within the encoded output datastream, h-riage 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 prediction1 in the intra-encoding system, is made on the basis of image biocks received as part of the signal 460, which is to say, the prediction is based upon encoded-decoded image blocks in order that exactly the same prediction can be made at a decompression apparatus. However, data can be derived from the input video signal 300 by an intra-mode selector 520 to control the operation of the iritra-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.

The processing applied to the signal 460 will now be described. Firstly, the signal is filtered by a fitter unit 560. This involves applying a "deblocking" filter to remove or at Feast tend to reduce the effects of the block-based processing carried out by the transform unit 340 and subsequent operations. Also, an adaptive loop filter is 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 fitter coefficients to use is included as part of the encoded output datastream.

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

In embodiments of the invention, 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 600 (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 610. 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 largest coding unit (LCU) 700, which represents a square array of 64 x 64 samples. Here, the discussion relates to luminance samples. Depending on the chrominance mode, such as 4:4:4, 4:2:2, 4:2:0 or 4:4:4:4 (GBR plus key data), there will be differing numbers of corresponding chrominance samples corresponding to the luminance block.

Li Three basic types of blocks will be described: coding units, prediction units and transform units. In general terms, the recursive subdividing of the LCUs 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 LCU may be subdivided into so-called coding units (CU). Coding units are always square and have a size between 8x8 samples and the full size of the [CU 700. The coding units can be arranged as a kind of tree structure, so that a first subdivision may take place as shown in Figure 8, giving coding units 710 of 32x32 samples; subsequent subdivisions may then take place on a selective basis so as to give some coding units 720 of 16x16 samples (Figure 9) and potentially some coding units 730 of 8x8 samples (Figure 10). Overall, this process can provide a content-adapting coding tree structure of CU blocks, each of which may be as large as the [CU or as small as 8x8 samples. Encoding of the output video data takes place on the basis of the coding unit structure, Figure 11 schematically illustrates an array of prediction units (PU). A prediction unit is a basic unit for carrying information relating to the image prediction processes, or in other words the additional data added to the entropy encoded residual image data to form The output video signal from the apparaWs of Figure 5. In general, prediction units are not restricted to being square in shape. They can take other shapes, in particular rectangular shapes forming half of one of the square coding units, as long as the coding unit is greater than the minimum (8x8) size. The aim is to allow the boundary of adjacent prediction units to match (as closely as possible) the boundary of real objects in the picture, so that different prediction parameters can be applied to different real objects. Each coding unit may contain one or more prediction units.

Figure 12 schematically iflustrates an array of transform units (TU). A transform unit is a basic unit of the transform and quantisation process. Transform units are always square and can take a size from 4x4 up to 32x32 samples. Each coding unit can contain one or more transform units. The acronym SDIP-P in Figure 12 signifies a so-called short distance intra-prediction partition. In this arrangement only one dimensional transforms are used, so a 4xN block is passed through N transforms with input data to the transforms being based upon the previously decoded neighbouring blocks and the previously decoded neighbouring lines within the current SDIF'-P.

The intra-prediction process will now be discussed. In general terms, intra-prediction involves generating a prediction of a current block (a prediction unit) of samples from previously-encoded and decoded samples in the same image. Figure 13 schematically illustrates a partially encoded image 800. Here, the image is being encoded from top-left to bottom-right on an LCU basis. An example LCU encoded partway through the handling of the whole image is shown as a block 810. A shaded region 820 above and to the left of the block 810 has already been encoded. The intra-image prediction of the contents of the block 810 can make use of any of the shaded area 820 but cannot make use of the unshaded area below that.

The block 810 represents an LCU; as discussed above, for the purposes of iritra-irnage prediction processing, this may be subdivided into a set of smaller prediction units. An example of a prediction unit 830 is shown within the LGU 810.

The intra-image prediction takes into account samples above and/or to the left of the current LGU 810. Source samples, from which the required samples are predicted, may be located at different positions or directions relative to a current prediction unit within the LCU 810. To decide which direction is appropriate for a current prediction unit, the results of a trial prediction based upon each candidate direction are compared in order to see which candidate direction gives an outcome which is closest to the corresponding block of the input image. The candidate direction giving the closest outcome is selected as the prediction direction for that prediction unit.

The picture may also be encoded on a "slice" basis. In one example, a slice is a horizontally adjacent group of LCUs. But in more general terms, the entire residual image could form a slice, or a slice could be a single LCU, or a slice could be a row of LCUs, and so on.

Slices can give some resilience to errors as they are encoded as independent units. The encoder and decoder states are completely reset at a slice boundary. For example, intra-prediction is not carried out across slice boundaries; slice boundaries are treated as image boundaries for this purpose.

Figure 14 schematically illustrates a set of possible (candidate) prediction directions. The full set of 34 candidate directions is available to a prediction unit of 8x8, 16x16 or 32x32 samples. The special cases of prediction unit sizes of 4x4 and 64x64 samples have a reduced set of candidate directions available to them (17 candidate directions and 5 candidate directions respectively). The directions are determined by horizontal and vertical displacement relative to a current block position, but are encoded as prediction modes", a set of which is shown in Figure 15. Note that the so-called DC mode represents a simple arithmetic mean of the surrounding upper and left-hand samples.

Figure 16 schematically illustrates a zigzag scan, being a scan pattern which may be applied by the scan unit 360. In Figure 16, the pattern is shown for an example block of SxS DCT coefficients, with the DC coefficient being positioned at the top left position 840 of the block, and increasing horizontal and vertical spatial frequencies being represented by 1.0 coefficients at increasing distances downwards and to the right of the top-left position 840.

Note that in some embodiments, the coefficients may be scanned in a reverse order (bottom right to top left using the ordering notation of Figure 6). Also it should be noted that in some embodiments, the scan may pass from left to right across a few (for example between one and three) uppermost horizontal rows, before carrying out a zig-zag of the remaining coefficients.

Figure 17 schematically illustrates the operation of a CABAC entropy encoder.

The CABAC encoder operates in respect of binary data, that is to say, data represented by only the two symbols 0 and 1. The encoder makes use of a so-called context modelling process which selects a context or probability model for subsequent data on the basis of previously encoded data, The selection of the context is carried out in a deterministic way so that the same determination, on the basis of previously decoded data, can be performed at the decoder without the need for further data (specifying the context) to be added to the encoded datastream passed to the decoder.

Referring to Figure 17, input data to be encoded may be passed to a binary converter 900 if it is not already in a binary form; if the data is already in binary form, the converter 900 is bypassed (by a schematic switch 910). In the present embodiments, conversion to a binary form is actually carried out by expressing the quantised DCT coefficient data as a series of binary "maps", which will be described further below.

The binary data may then be handled by one of two processing paths, a "regular" and a "bypass" path (which are shown schematically as separate paths but which, in embodiments of the invention discussed b&ow, could in fact be implemented by the same processing stages, just using slightly different parameters). The bypass path employs a so-called bypass coder 920 which does not necessarily make use of context modelling in the same form as the regular path.

In some examples of CABAC coding, this bypass path can be selected if there is a need for particularly rapid processing of a batch of data, but in the present embodiments two features of so-called "bypass" data are noted: firstly, the bypass data is handled by the CABAC encoder (950, 960), just using a fixed context model representing a 50% probability; and secondly, the bypass data r&ates to certain categories of data, one particular example being coefficient sign data. Otherwise, the regular path is selected by schematic switches 930, 940. This Involves the data being processed by a context modeller 950 followed by a coding engine 960.

The entropy encoder shown in Figure 17 encodes a block of data (that is, for example, data corresponding to a block of coefficients relating to a block of the residual image) as a single value if the block is formed entirely of zero-valued data. For each block that does not fall into this category, that is to say a block that contains at least some non-zero data, a "significance map" is prepared. The significance map indicates whether, for each position in a block of data to be encoded, the corresponding coefficient in the block is non-zero. The significance map data: being in binary form, is itself CABAC encoded. The use of the significance map assists with compression because no data needs to be encoded for a coefficient with a magnitude that the significance map indicates tc be zero. Also, the significance map can include a special code to indicate the final non-zero coefficient in the block, so that all of the final high frequency I trailing zero coefficients can be omitted from the encoding. The significance map is followed, in the encoded bitstream, by data defining the va]ues of the non-zero coefficients specified by the significance map.

Further levels of map data are also prepared and are CABAC encoded. An example is a map which defines, as a binary value (1 = yes, 0 = no) whether the coefficient data at a map position which the significance map has indicated to be ton-zero actually has the value of one'. Another map specifies whether the coefficient data at a map position which the significance map has indicated to be non-zero" actually has the value of "two". A further map indicates, for those map positions where the significance map has indicated that the coefficient data is "non-zero", whether the data has a value of "greater than two". Another map indicates, again for data identified as "non-zero", the sign of the data value (using a predetermined binary notation such as 1 for ÷, 0 for -] or of course the other way around).

In embodiments of the invention, the significance map and other maps are generated from the quantised DCT coefficients, for example by the scan unit 360, and is subjected to a zigzag scanning process (or a scanning process selected from zigzag, horizontal raster and vertical raster scanning according to the intra-prediction mode) before being sublected to CABAC encoding.

In general terms, CABAC encoding involves predicting a context, or a probability model, for a next bit to be encoded: based upon other previously encoded data. If the next bit is the same as the bit identified as "most likely" by the probability model, then the encoding of the information that "the next bit agrees with the probability model" can be encoded with great efficiency. It is less efficient to encode that "the next hit does not agree with the probability model", so the derivation of the context data is important to good operation of the encoder. The term "adaptive" means that the context or probability models are adapted, or varied during encoding, in an attempt to provide a good match to the (as yet uncocied) next data.

Using a simple analogy, in the written English Language, the letter "U" is relatively uncommon. But in a letter position immediately after the letter "Q, it is very common indeed.

So, a probability model might set the probability of a "U as a very low value, but if the current letter is a "0", the probability model for a U" as the next letter could be set to a very high probabiuty value.

CABAC encoding is used, in the present arrangements, for at least the significance map and the maps indicating whether the non-zero values are one or two. Bypass processing -which in these embodiments is identical to CABAC encoding but for the tact that the probability model is fixed at an equal (0.5:0.5) probabIlity distribution of Is and Os, is used tar at least the sign data and the map indicating whether a value is >2. For those data positions identified as >2, a separate so-called escape data encoding can be used to encode the actual value of the data. This may include a Golomb-Rice encoding technique.

The CABAC context modelling and encoding process is described in more detail in WD4: Working Draft 4 of High-Efficiency Video Coding, JCTVC-F803_dS, Draft ISO/1EC 23008-HEVC; 201x(E) 2011-10-28.

Figure 18 schematically illustrates a CAVLC entropy encoding process.

As with CABAC discussed above, the entropy encoding process shown in Figure 18 follows the operation of the scan unit 360. It has been noted that the non-zero coefficients in the transformed and scanned residual data are often sequences of ±1. The CAVLC coder indicates the number of high-frequency ±1 coefficients by a variable referred to as "trailing is" (Tis). For these non-zero coefficients, the coding efficiency is improved by using different (context-adaptive) variable length coding tables.

Referring to Figure 18, a first step 1000 generates values "coeft_token' to encode both the total number of non-zero coefficients and the number of trailing ones. At a step 1010, the sign bit of each trailing one is encoded in a reverse scanning order. Each remaining nonzero coefficient is encoded as a rilevelui variable at a step 1020, thus defining the sign and magnitude of those coefficients. At a step 1030 a variable total_zeros is used to code the total number of zeros preceding the last nonzero coefficient. Finally, at a step 1040, a variable run_before is used to code the number of successive zeros preceding each non--zero coefficient in a reverse scanning order. The coUected output of the variables defined above forms the encoded data.

As mentioned above, a default scanning order for the scanning operation carried out by the scan unit 360 is a zigzag scan is illustrated schematically in Figure 16. In other arrangements, four blocks where intra-image encoding is used, a choice may be made between zigzag scanning, a horizontal raster scan and a vertical raster scan depending on the image prediction direction (Figure 15) and the transform unit (TU) size.

However, in embodiments of the present invention, different scanning orders can be employed. The choice between scanning orders can be made in various different ways, instances of which will be described below. For example, a choice may be made according to the prediction direction (mode) established for intra-coding, as discussed above with reference to the set of modes illustrated in Figure 15. Another example relates to an arrangement in which the scan order depends upon properties of the motion vectors derived by the motion estimator 550 of Figure 6. The reason that directional information is relevant is that the different scan orders can give different efficiencies of the subsequent entropy encoding process, in dependence upon the direction or orientation of image features in the blocks to be compressed.

The techniques which will now be described as their basis in a recognition from empirical tests that they can be a correlation between the quantised data generated in respect of different colour components of a video signal. As discussed above, one aim of the scanning process carried out by the scan unit 360 is to reorder the quantised data so as, as far as is possible, non-zero data values are grouped together and zero-valued data vahJes are also grouped together. The present techniques recognise that this operation can potentially be enhanced by interleaving data representing different colour components and/or different spatial positions during the scanning process.

Before various possible interleave patterns are discussed, the basic data structure of two example video signals wiH be described. However, it will of course be understood thai the present techniques are applicable to other video signal formats (such as the 4:2:0, 4:2:1, 4:1:1, 4:1:0, 3:1:1 formats in ROB [GBR], YObOr or 4:4:4:4).

Figure 19 schematically illustrates a set of data relating to a 4:4:4 GBR video signal.

Each of the green (0), blue (B) and red (R) components have a corresponding sample (shown as a small square 1100 in Figure 19) at each pixel position in the image. There are therefore the same number of 0, B and R samples. The examples are shown arranged as blocks of 4x4 samples but can actually be any TU or PU size. Note that although generic terminology is used here, in embodiments of the invention the interleaving and scanning is applied to quantised DOT coefficients, and in particular embodiments to binary map data (such as significance map data) derived from those quantised DOT coefficients.

in the absence of interleaving, the scanning order applied by the scan unit 360 might be arranged to apply a scanning pattern to the samples contained in a first block GO of green samples, then to scan a second block Si, then 02, then 03, followed by a first block BO of blue samples, then Bi, then B2, then B3, followed by RU, Ri, R2. R3 and so on to the next green block. As an alternative, it could be GO, 01, 02, RU, BO. Ri, Si Similarly, Figure 20 schematically illustrates a set of data relating to a 4:2:2 YObOr video signal comprising luminance (Y) samples and two sets of colour difference samples Ob and Cr.

The notation 4:2:2 indicates that there are half as many samp}es in each of the colour difference signals as the number of samples in the luminance signal. As before, in the absence of interleaving, the samples would be read out in a scanning order by the scan unit 360, possibly based on a simple order of blocks such as VO, Yl, Y2, Y3, ChO. CM, Cr0, Cr1. But an important feature is to consider that in the absence of interleaving, coefficients from each block would be scanned before considering a next block.

Arrangements will now be described by which the example data shown in Figure 19 or Figure 20 can be interleaved according to its channel (where the channel indicates G, B or S in the case of Figure 19, or in the case of Figure 20 the channel indicates Y, Cb or Cr) and/or its spatial position. Here, spatial interleaving implies that samples from different ones of the numbered blocks are interleaved with one another.

The outcome of the interleaving process in each case is a so-called "superblock' which is a one-dimensional vector containing interleaved coefficient data in the correct order for processing by the entropy encoder. Accordingly, the interleaving patterns described here may be combined with the sort of scanning patterns which are generally used for intra-block scanning, such as zig-zag scanning as mentioned above. Such a combination could be implemented as foFlows: whenever the interleaving process takes (reads, interleaves) a next coefficient from a certain block) it takes that coefficient from the next block position which would be applicable to a zig-zag or other scan of that block. So, within a block, coefficients may be read in the appropriate intra-block order, but they Will of course have coefficients from other blocks interleaved between them, In another alternative, the zig-zag or other order can be imposed on eth interleaving order, so that a first coefficient is taken from a first colour or spatial interleaved block at a first zig-zag position, then a next coefficient is taken from another block at a second zig-zag position, and so on. The interleaving process can be carried out by the scan unit 360, so in this respect the scan unit 360 may be considered as representing an interleaver as part of its functionality.

A notation will be used to define the interleaving patterns to be described. This notation comprises two indicators within square brackets [X,Y), where the first indicator, X, indicates the nature of any channel interleaving appLied to the data, and the second indicator, V indicates the nature of any spatial interleaving applied to the data. The example interleaving modes to be discussed are as follows: 0,0) -No interleave (described with reference to Figures 19 and 20) [A,O] -All channel interleave, no spatial interleave (Figures 21 and 22) [CO] -Chrominance-only channel interleave, no spatial interleave (Figures 23 and 24) [OF] -No channel interleave, Full spatial interleave (Figures 25 and 26) [0,V] -No channel interleave, Vertical-aligned spatial interleave (Figures 27 and 28) [OH) -No channel interleave, Horizontal-aligned spatial interleave (Figures 29 and 30) [A,F) -All channel interleave, Full spatial interleave (Figures 31 and 32) [AN] -All channel interleave. Vertical-aligned spatial interleave (Figures 33 and 34) [A,H] -All channel interleave, Horizontal-Signed spatial interleave (Figures 35 and 36) In each of the schematic illustrations provided and discussed below, only the first few coefficients of each superblock are shown, for clarity of the diagram.

Figures 21 and 22 schematically illustrate an [A,0] interleave mode.

Here, there is no spatial interleaving in luminance, so that in Figure 21 a first superblDck (superbiock 0)15 generated from the coefficient blocks GO, BO, RO, a second superblock (superblock 1) is generated ftom the coefficient blocks 01, 31, RI, and so on. A superblock is created by concatenating successive 0, B and R coefficients as shown. In the case of Figure 22, the interleaving comprises concatenating coefficients from the same block positions in Y0, Yl, CbO and Cr0, before moving onto the next block position, and so on.

Figures 23 and 24 schematically illustrate a [C,O] interleave mode. Here once again, there is no spatial interleaving. Also, interleaving takes place only in respect of the chrominance channels (noting that in the GBR format of Figure 23, 0 is by far the dominant IS colour channel so it is considered, for the purposes of this discussion, not to be a "chrominance" channel, whereas the less dominant channels B and R are considered, only for the purposes of this discussion, as chrorninance" channels). Accordingly, in Figure 23 a first superbiock (superblock 0) is generated from the coefficient blocks BO, RO, a second superblock (superblock I) is generated from the coefficient blocks 81, Ri, and so on. A superblock is created by concatenating successive B and R coefficients as shown. In the case of Figure 24, the interleaving comprises concatenating coefficients from the same block positions in CbO and Cr0, before moving onto the next block position, and so on.

Figures 25 and 26 schematically illustrate a [OF] interleave mode. This mode involves a full spatial interleave but no channel interleaving. So, each superbiock is derived only from a single component (6, B, R. Y, Gb, Cr) of the respective signal, but the super blocks represent an interleave combination of sub-blocks at different spatial positions, as shown. In particular, it will be seen that the first four coefficients of (for example) the G channel superbiock are formed as the top-len coefficients of each of the sub blocks 00, Gi, G2, G3. The next four coefficients in the superbiock represent the second-left coefficient of each of the sub blocks, and so on. A similar arrangement applies in Figure 26.

Figures 27 and 28 schematically illustrate a [0,V] interleave mode. As before, this mode involves a full spatial interleave but no channel interleaving. The spatial interleaving is vertically aligned. So, each superblock is derived only from a single component (6, B, R, Y, Cb, Cr) of the respective signal, but the super blocks represent an interleaved combination of sub-blocks at different spatial positions, as shown. The sub-blocks GO and 62 (being vertically aligned in respect of their corresponding image positions) are interleaved for form a first superbiock, the sub-blocks Gi and G3 are interleaved to form a second superblock, and so on. In particular, it will be seen that the first two coefficients of (for example) the C channel superbiock are formed as the top-left coefficients of the sub blocks GO, (32. The next two coefficients in the superblock represent the second-left coefficient of each of the sub blocks GO, G2, and so on, A similar arrangement applies in Figure 28.

Figures 29 and 3D schematically illustrate a [D,H} interleave mode. As before, this mode involves a full spatial interleave but no channel Interleaving. The spatial interleaving is horizontally aligned. So, each superblock is derived only from a single component ((3, B, ft Cb, Cr) of the respective signal, but the super blocks represent an interleaved combination of sub-blocks at different spatial positions, as shown. The sub-blocks GO and Cl (being horizontally aligned in respect of their corresponding image positions) are interleaved for form a first superblock, the sub-blocks (32 and G3 are interleaved to form a second superbiock, and so on. In particular, it will be seen that the first two coefficients of (for example) the G channel superblock are formed as the top-left coefficients of the sub blocks GO, Cl. The next two coefficients in the superblock represent the second-left coefficient of each of the sub blocks GO, Cl, and so on. A similar arrangement applies in Figure 30.

Figures 31 and 32 schematically illustrate an [A,F] interleave mode. This mode involves a full spatial interleave of all of the channels of the video signal. The interleave patterns are best seen by an examination of the drawings, but it will be seen, for example, that the first four (3 coefficients of the superblock in Figure 31, which represents the first, fourth, seventh and tenth superblock coefficients, are the toplett coefficients of the sub blocks GO, Cl, G2, (33 respectively. A similar arrangement applies in Figure 32.

Figures 33 and 34 schematically illustrate an A,V] interleave mode. This arrangement involves the interleaving of all channels but in a vertically aligned spatial fashion.

Figures 35 and 36 schematically illustrate an [A,H] interleave mode. Similarly, this arrangement involves the interleaving of all channels but in a horizontally aligned spatial fashion.

Figure 37 schematically illustrates a data flag indicating an interleave mode. Although in some embodiments the data flag could be implemented simply as (for example) a four-bit indicator where one of the values represented by the indicator represents no interleave", the example shown schernaticafly in Figure 37 uses a single bit 1400 to indicate whether interleaving is implemented or not. For example, a value of 0 could indicate "no interleaving" and a value of 1 could indicate "interleaving. This arrangement means that the indication of which interleaving mode is in use (implemented here as a four bit value 1410) can be omitted, to save on data overhead, when interleaving is not in use, The data flag shown in Figure 37 may be included as part of the output video signal 380. Here, it is assumed that both the encoder and the decoder have access to data defining the different interleave patterns represented by different respective values of the mode indicator 1410.

Figure 38 schematicalty illustrates an interleave mode selector. A trial encoder 1420 carries out a trial encoding of a set of current blocks of data according to candidate interleave patterns stored in a pattern store (memory) 1430. A selector 1440 selects the best interleave pattern on the basis of the lowest number of bits of encoded data which are (or would be) generated using that interleave pattern, and generates the appropriate mode indicator (Figure 37) specifying the selected interleave mode. An interleaver 1450, which may in fact form part of the functionality of the scan unit 360, interleaves the data (or not, if" no interleave" is selected) according to the selected pattern.

If a trial encoding is not carried out, an interleaving pattern can be selected by an analysis of the image data itself, and in particular (a) the presence vertical or horizontal image features (which would indicate that a [X,V] or [X,H] mode should be used, respectively) or the absence of vertical or horizontal image features (which would indicate that a [X,0j or [X,FJ mode should be used), and (b) the image similarity between all colour channels of the block (which would indicate that a [A,Y] mode should be used), or the image similarity between the chrominance channels (which would indicate that a [C,Y] mode should be used, or the image dissimilarity between the colour channels (which would indicate that a [0,Y] mode should be used). Here, X and Y indicate a variable which is independent of the particular situation under discussion.

Alternatively, a X,VJ or [X,H} mode could be selected if the intraprediction direction and/or the predominant motion vectors associated with the prediction of a current image block are in the vertical or horizontal direction (within a threshold amount), respectively. If prediction mode 2 (DC) is selected, then [X, F] could be selected; otherwise, fX,0J could be selected.

Figure 39 schematically illustrates a de-interleaver. A mode detector 1460 receives the mode indicator of Figure 37 and, from it, detects whether interleaving has been carried out and, if so, which mode or pattern was used. If no interleaving has been performed, a de-interleaver 1470 is bypassed by a bypass path 1480. Otherwise, the mode detector controls the de-interleaver 1472 apply an inverse interleave operation according to the appropriate pattern stored in a pattern store 1430' which, in the case of an apparatus carrying out compression and decompression, may of course be the same as the pattern store 1430 of Figure 38.

Claims (1)

1. <claim-text>CLAIMS1. Video data compression apparatus in which block based video data is reordered and entropy encoded, the apparatus comprising: a selector for selecting a reordering pattern from a set of two or more candidate reordering patterns, for use in reordering video data for encoding, at least some of the reordering patterns being arranged so as to interleave video data from plural colour channels and/or plural blocks to form a composite data block: a data scanner for changing the order of the video data for encoding according to the 1 0 selected reordering pattern so as to generate reordered video data; and an entropy encoder for entropy-encoding the reordered video data.</claim-text> <claim-text>2. Apparatus according to claim 1, comprising: an image predictor for generating a predicted version of a current image of an input video signal: a combiner for combining the current image with the predicted version of that image so as to generate a residual image: a frequency domain converter for generating a frequency domain representation of the residual image on an image black by image block basis, a block of frequency domain representation comprising respective f!equency domain coefficients in respect of each colour channel of each image block of the residual image, the frequency domain representation forming the video data for encoding.</claim-text> <claim-text>3. Apparatus according to claim 2, comprising a quantiser for quantising the frequency domain data before the data is reordered by the data scanner.</claim-text> <claim-text>4. Apparatus according to any one of the preceding claims, in which the selector is configured to select a reordering pattern in dependence upon one or more parameters used by the image predictor in generating the predicted version of the current image.</claim-text> <claim-text>5. Apparatus according to claim 4, in which the one or mare parameters comprise a prediction direction relating to an intra-image prediction.</claim-text> <claim-text>6. Apparatus according to claim 4, in which the one or mare parameters comprise a motion direction indicative of image motion detected between the current image and another image.</claim-text> <claim-text>Apparatus according to any one of claims 1 to 3, in which the selector is configured to carry out one or more trial compressions using different respective reordering patterns, and to select a reordering pattern which the trial compression indicates will give the lowest output data quantity.</claim-text> <claim-text>8. Apparatus according to any one of claims 1 to 3, in which the selector is configured to select a reordering pattern based on an analysis of an image feature direction of one or more nearby blocks.</claim-text> <claim-text>9. Apparatus according to claim 7 or claim 8, comprising a data flag generator for generating data, to be associated with the compressed output video signal, indicative of which reordering pattern was selected by the selector.</claim-text> <claim-text>10. Apparatus according to any one of the preceding claims, in which at least one of the candidate reordering patterns interleave data from all of the colour components of the video data for encoding.</claim-text> <claim-text>11. Apparatus according to any one of the preceding claims, in which at least one of the candidate reordering patterns interleave data from a subset at the colour components of the video data for encoding 12. Apparatus according to claim 10 or claim 11, in which at Least one of the candidate reordering patterns which interleaves data from plural colour channels also interleaves data from plural blocks.13. Video data decompression apparatus comprising: an entropy decoder for entropy-decoding an input compressed video signal to generate composite data blocks of reordered video data for decoding; a selector for selecting a reordering pattern from a set of two or more candidate reordering patterns, for use in ordering the video data, at east sonic of the reordering patterns being arranged so as to deinterleave a composite data block to generate video data from plural colour channels and/or plural blocks; a data scanner for changing the order of the reordered video data according to the selected reordering pattern so as to generate blocks of ordered video data from respective colour channels.14. Apparatus according to claim 13, in which the selector is configured to select a reordering pattern in dependence upon data forming part of the compressed video signal.15. Apparatus according to claim 13, in which the selector is configured to select a reordering pattern in dependence upon data associated with the compressed video signal, specifying a reordering pattern.16. Apparatus according to claim 13, in which the selector is configured to select a reordering pattern in dependence upon data specifying parameters to be applied by the image predictor in generating the predicted version of the current image to be decompressed.17. A video data compression method in which block based video data is reordered and entropy encoded, the method comprising the steps of: selecting a reordering pattern from a set of two or more candidate reordering patterns, for use in reordering video data for encoding, at least some of the reordering patterns being arranged so as to interleave video data from plural colour channels and/or plural blocks to form a composite data block; changing the order of the video data for encoding according to the selected reordering pattern so as to generate reordered video data; and entropyencoding the reordered video data.18, Video data generated by the method of claim 17, 19. A storage medium which stores video data according to claim 18.20. A video data decompression method comprising the steps of: entropy-decoding an input compressed video signal to generate composite blocks of reordered video data for decoding; selecting a reordering pattern from a set of two or more candidate reordering patterns, for use in ordering the video data, at least some of the reordering patterns being arranged so as to deinterleave a composite data block to generate video data from plural colour channels and/or plural blocks; changing the order of the reordered video data according to the selected reordering pattern so as to generate blocks of ordered video data from respective colour channels, 21. Computer software which, when executed by a computer, causes the computer to carry out the method of claim 17 or claim 20.22. A non-transitory storage medium on which computer software according to claim 21 is stored.23. Video data capture, fransrnission and/or storage apparatus comprising apparatus according to any one of claims ito 16</claim-text>