GB2593777A

GB2593777A - Video data encoding and decoding

Info

Publication number: GB2593777A
Application number: GB2004955.7A
Authority: GB
Inventors: Mark Keating Stephen; James Sharman Karl; Richard Browne Adrian
Original assignee: Sony Group Corp
Current assignee: Sony Group Corp
Priority date: 2020-04-03
Filing date: 2020-04-03
Publication date: 2021-10-06
Also published as: GB202004955D0

Abstract

An apparatus comprises a video data encoder configured to receive input video data in a 4:4:4 or a 4:2:2 sampling format, the input video data representing a plurality of video component channels. The video data encoder comprises: detector circuitry to detect a predetermined property of the input video data indicative of high frequency content of at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels; filter circuitry to selectively apply a filtering operation to one or more of the plurality of video component channels in response to the detected predetermined property; and encoding circuitry to encode the input video data, as selectively filtered by the filter circuitry, to generate the encoded output video data. Variable filtering (e.g., low pass filtering, LPF) may be in response to a degree to which the detected property meets a predetermined criterion, e.g. a threshold level high frequency content of at least one channel relative to other channel(s).

Description

VIDEO DATA ENCODING AND DECODING

BACKGROUND Field

This disclosure relates to video data encoding and decoding.

Description of Related Art

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

prior art against the present disclosure.

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

SUMMARY

The present disclosure addresses or mitigates problems arising from this processing.

Respective aspects and features of the present disclosure are defined in the appended claims It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present technology.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 schematically illustrates an audio/video (AN) data transmission and reception system using video data compression and decompression; Figure 2 schematically illustrates a video display system using video data decompression; Figure 3 schematically illustrates an audio/video storage system using video data compression and decompression; Figure 4 schematically illustrates a video camera using video data compression; Figures 5 and 6 schematically illustrate storage media; Figure 7 provides a schematic overview of a video data compression and decompression apparatus; Figure 8 schematically illustrates a predictor; Figures 9a and 9b schematically illustrate trial encoding outcomes for different respective chrominance subsampling formats; Figure 10 provides a summary of the data of Figures 9a and 9b along with a variance ratio; Figure 11 schematically represents the data of Figure 10 in graphical form; Figures 12a to 12c schematically illustrate noise properties of an example portion of an

example image;

Figure 13 schematically illustrates results similar to those of Figure 11 but after a filtering process has been applied to at least a portion of the chrominance data; Figure 14 schematically illustrates encoding apparatus; Figures 15 and 16 schematically illustrate optional variations of filter circuitry and Figure 17 is a schematic flowchart illustrating a method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the drawings, Figures 1-4 are provided to give schematic illustrations of apparatus or systems making use of the compression and/or decompression apparatus to be described below in connection with embodiments of the present technology.

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

Figure 1 schematically illustrates an audio/video data transmission and reception system using video data compression and decompression. In this example, the data values to be encoded or decoded represent image data.

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

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

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

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

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

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

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

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

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

A combination of Figure 2 and 4 may provide a video capture apparatus comprising an image sensor 180 and encoding apparatus 190, decoding apparatus 110 and a display 120 to which the decoded images are output.

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

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

Figure 7 provides a schematic overview of a video or image data compression (encoding) and decompression (decoding) apparatus, for encoding and/or decoding video or image data representing one or more images.

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

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

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

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

The remainder of the apparatus acting as an encoder (to encode the residual or difference image) will now be described.

The residual image data 330 is supplied to a transform unit or circuitry 340 which generates a discrete cosine transform (DCT) representation of blocks or regions of the residual image data. The DCT technique itself is well known and will not be described in detail here.

Note also that the use of DCT is only illustrative of one example arrangement. Other transforms which might be used include, for example, the discrete sine transform (DST). A transform could also comprise a sequence or cascade of individual transforms, such as an arrangement in which one transform is followed (whether directly or not) by another transform. The choice of transform may be determined explicitly and/or be dependent upon side information used to configure the encoder and decoder. In other examples a so-called "transform skip" mode can selectively be used in which no transform is applied.

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

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

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

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

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

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

However, a return path 390 is also provided because the operation of the predictor 320 itself depends upon a decompressed version of the compressed output data.

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

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

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

In the compression process, the scanned coefficients are passed by the return path 390 from the quantiser 350 to the inverse quantiser 420 which carries out the inverse operation of the scan unit 360. An inverse quantisation and inverse transformation process are carried out by the units 420, 430 to generate a compressed-decompressed residual image signal 440. The image signal 440 is added, at an adder 450, to the output of the predictor 320 to generate a reconstructed output image 460 (although this may be subject to so-called loop filtering and/or other filtering before being output -see below). This forms one input to the image predictor 320, as will be described below.

Turning now to the decoding process applied to decompress a received compressed video signal 470, the signal is supplied to the entropy decoder 410 and from there to the chain of the reverse scan unit 400, the inverse quantiser 420 and the inverse transform unit 430 before being added to the output of the image predictor 320 by the adder 450. So, at the decoder side, the decoder reconstructs a version of the residual image and then applies this (by the adder 450) to the predicted version of the image (on a block by block basis) so as to decode each block. In straightforward terms, the output 460 of the adder 450 forms the output decompressed video signal 480 (subject to the filtering processes discussed below). In practice, further filtering may optionally be applied (for example, by a loop filter 565 shown in Figure 8 but omitted from Figure 7 for clarity of the higher level diagram of Figure 7) before the signal is output.

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

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

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

(At the decoder, or at the reverse decoding side of the encoder, the detection of a prediction direction may be in response to data associated with the encoded data by the encoder, indicating which direction was used at the encoder. Or the detection may be in response to the same factors as those on which the decision was made at the encoder).

I ntra-image prediction bases a prediction of the content of a block or region of the image on data from within the same image. This corresponds to so-called l-frame encoding in other video compression techniques. In contrast to l-frame encoding, however, which involves encoding the whole image by intra-encoding, in the present embodiments the choice between intra-and inter-encoding can be made on a block-by-block basis, though in other embodiments the choice is still made on an image-by-image basis.

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

A technique known as "block copy" prediction is in some respects a hybrid of the two, as it uses a vector to indicate a block of samples at a position displaced from the currently predicted block within the same image, which should be copied to form the currently predicted 20 block.

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

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

For inter-image prediction, a motion compensated (MC) predictor 540 uses motion information such as motion vectors derived by a motion estimator 550 from the input video signal 300. Those motion vectors are applied to a processed version of the reconstructed image 460 by the motion compensated predictor 540 to generate blocks of the inter-image prediction. Accordingly, the units 530 and 540 (operating with the estimator 550) each act as detectors to detect a prediction direction in respect of a current block to be predicted, and as a generator to generate a predicted block of samples (forming part of the prediction passed to the units 310 and 450) according to other samples defined by the prediction direction.

The processing applied to the signal 460 will now be described.

Firstly, the signal may be filtered by a so-called loop filter 565. Various types of loop filters may be used. One technique involves applying a "deblocking" filter to remove or at least tend to reduce the effects of the block-based processing carried out by the transform unit 340 and subsequent operations. A further technique involving applying a so-called sample adaptive offset (SAO) filter may also be used. In general terms, in a sample adaptive offset filter, filter parameter data (derived at the encoder and communicated to the decoder) defines one or more offset amounts to be selectively combined with a given intermediate video sample (a sample of the signal 460) by the sample adaptive offset filter in dependence upon a value of:(i) the given intermediate video sample; or 00 one or more intermediate video samples having a predetermined spatial relationship to the given intermediate video sample.

Also, an adaptive loop filter is optionally applied using coefficients derived by processing the reconstructed signal 460 and the input video signal 300. The adaptive loop filter is a type of filter which, using known techniques, applies adaptive filter coefficients to the data to be filtered.

That is to say, the filter coefficients can vary in dependence upon various factors. Data defining which filter coefficients to use is included as part of the encoded output data-stream. Techniques to be discussed below relate to the handling of parameter data relating to the operation of filters. The actual filtering operations (such as SAO filtering) may use otherwise known techniques.

The filtered output from the loop filter unit 565 in fact forms the output video signal 480 when the apparatus is operating as a decompression apparatus. It is also buffered in one or more image or frame stores 570; the storage of successive images is a requirement of motion compensated prediction processing, and in particular the generation of motion vectors. To save on storage requirements, the stored images in the image stores 570 may be held in a compressed form and then decompressed for use in generating motion vectors. For this particular purpose, any known compression / decompression system may be used. The stored images may be passed to an interpolation filter 580 which generates a higher resolution version of the stored images; in this example, intermediate samples (sub-samples) are generated such that the resolution of the interpolated image is output by the interpolation filter 580 is 4 times (in each dimension) that of the images stored in the image stores 570 for the luminance channel of 4:2:0 and 8 times (in each dimension) that of the images stored in the image stores 570 for the chrominance channels of 4:2:0. The interpolated images are passed as an input to the motion estimator 550 and also to the motion compensated predictor 540.

The way in which an image is partitioned for compression processing will now be described. At a basic level, an image to be compressed is considered as an array of blocks or regions of samples. The splitting of an image into such blocks or regions can be carried out by a decision tree, such as that described in SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS Infrastructure of audiovisual services -Coding of moving video High efficiency video coding Recommendation ITU-T H.265 12/2016. Also: High Efficiency Video Coding (HEVC) Algorithms and Architectures, chapter 3, Editors: Madhukar Budagavi, Gary J. Sullivan, Vivienne Sze; ISBN 978-3-319-06894-7; 2014 which are incorporated herein in their respective entireties by reference. Further background information is provided in [1] "Versatile Video Coding (Draft 8)", JVET-02001-vE, B. Bross, J. Chen, S. Liu and Y-K. Wang, which is also incorporated herein in its entirety by reference.

In some examples, the resulting blocks or regions have sizes and, in some cases, shapes which, by virtue of the decision tree, can generally follow the disposition of image features within the image. This in itself can allow for an improved encoding efficiency because samples representing or following similar image features would tend to be grouped together by such an arrangement. In some examples, square blocks or regions of different sizes (such as 4x4 samples up to, say, 64x64 or larger blocks) are available for selection. In other example arrangements, blocks or regions of different shapes such as rectangular blocks (for example, vertically or horizontally oriented) can be used. Other non-square and non-rectangular blocks are envisaged. The result of the division of the image into such blocks or regions is (in at least the present examples) that each sample of an image is allocated to one, and only one, such block or region.

Parameter Sets, Profiles, Levels and Tiers When video data is encoded by the techniques discussed above for subsequent decoding, it is appropriate for the encoding side of the processing to communicate some parameters of the encoding process to the eventual decoding side of the processing. Given that these encoding parameters will be needed whenever the encoded video data is decoded, it is useful to associate the parameters with the encoded video data stream itself, for example (though not necessarily exclusively, as they could be sent "out of band" by a separate transmission channel) by embedding them in the encoded video data stream itself as so-called parameter sets.

Parameter sets may be represented as a hierarchy of information, for example as video parameter sets (VPS), sequence parameter sets (SPS) and picture parameter sets (PPS). The PPS would be expected to occur once each picture and to contain information relating to all encoded slices in that picture, the SPS less often (once per sequence of pictures) and the VPS less often still. Parameter sets which occur more often (such as the PPS) can be implemented as references to previously encoded instances of that parameter set to avoid the cost of re-encoding. Each encoded image slice references a single active PPS, SPS and VPS to provide information to be used in decoding that slice. In particular, each slice header may contain a PPS identifier to reference a PPS, which in turn references an SPS, which in turn references a VPS.

Amongst these parameter sets, the SPS contains example information relevant to some of the discussion below, namely data defining the so-called profile, tier and encoding level to be 15 used.

The profile defines a set of decoding tools or functions to be used Example profiles include the "Main Profile" relating to 4:2:0 video at 8 bits, and the "Main 10 Profile" allowing 10 bit resolution and other extensions with respect to the Main Profile.

The encoding level provides restrictions on matters such as maximum sample rate and picture size. The tier imposes a maximum data rate.

In the JVET (Joint Video Experts Team) proposals for versatile video coding (VVC), such as those defined (at the filing date) by the specification JVET-02001-vE referenced above, various levels are defined from 1 to 6.2.

Chrominance subsampling formats Various formats are in use for chrominance subsampling (relative to the sampling of luminance data). Examples include formats known as 4:2:0, 4:2:2 and 4:4:4. The 4:4:4 format involves no subsampling, which is to say that each chrominance channel (for example, U and V channels) has the same spatial sampling as the luminance channel. The 4:2:2 format has half the horizontal sampling rate of luminance but the same vertical sampling rate. The 4:2:0 format has half the sampling rate vertically as well as horizontally. For this reason, 4:2:0 video data contains a number of samples in each chrominance channel which is a quarter of the number in the luminance channel.

Some video data encoding and decoding specifications require that the maximum bit-rate of 4:4:4 video data should be twice the maximum bit-rate of 4:2:0 video data.

Empirical data: 4:4:4 vs 4:2:0 Various test video sequences are available or can be made available in 4:2:0 and 4:4:4 formats. For the empirical tests, trial encodings were carried out so that 4:4:4 versions were trial encoded with a current encoder (according to a test model of the VVC project referred to as VTM8.0) and the so-called Random Access (RA) bit-rates compared with their 4:2:0 versions. This was performed for a range of quantisation parameters (OP). The results are presented in Figures 9a and 9b. In these figures, a left-hand column indicates the name of the sequence (for example "Traffic") and an encoding level used in the trial. A second column indicates results for 4:2:0 encoding, indicating, for the selected level, the maximum bitrates On kilobits per second) allowable under the main tier and the high tier (as specified by the current specification table A.2), along with the actual results obtained for each trial encoding. Similar outcomes are presented for the third column relating to 4:4:4 trial encodings. The final column "bitrate ratio" indicates a ratio of (bitrate obtained for 4:4:4 version) : (bitrate obtained for 4:2:0 version).

As can be seen, although many of these 4:4:4 sequences require less than twice the bit-rate of their counterpart 4:2:0 versions, some of them require significantly more at the lower QPs.

Particular notable figures include the bitrate ratio of 6.4 for "BirdsInCage" at OP = 22 or 3.7 at OP = 27. Similarly high figures occur for the sequences "Genesis", "View", "DucksAndLegs" and "OldTownCross".

With reference to the "CrowdRun" sequence it can be seen that at OP = 22, the outcome exceeds the maximum bitrate allowable at the main tier. Similar issues are shown to occur for the sequences "DucksAndLegs", "OldTownCross" and "Seeking".

Further investigation of these sequences showed that they had a significant amount of high frequency energy in the U channel so the variance of the Y, U and V channels was calculated and compared. The results are provided in Figure 10, in which a first column indicates the sequence name, the next three columns give variance figures for the 4:4:4 version in the (Y, U, V) channels respectively, the fifth column repeats the bitrate ratio data from Figures 9a and 9b for a OP value specified by the seventh column, and the sixth column provides a ratio of variance for the U: Y channels in the 4:4:4 version. Notably, sequences which had a high bitrate ratio at OP = 22 or also shown to have a relatively high (at least compared to other sequences in the list) U: Y variance ratio. Example arrangements to be discussed below will endeavour to detect and then deal with this variance ratio by selective application of filtering.

Further details will be discussed below.

Figure 11 is a schematic graph plotting the bitrate ratio (4:4:4 vs 4:2:0) against the ratio of U variance to Y variance in the 4:4:4 sequence. Note that bitrates are shown at QP=22 and at a OP that meets the Main Tier max. bit-rate specification. This shows that the bitrate ration tends to increase with increasing U:Y variance ratio.

Figures 12a to 12d schematically illustrate an examination of a small section of the source image "BirdsInCage" and shows a chrome channel noise pattern. In particular, although the original material is in colour (in particular an RGB representation, in order to show this in monochrome for the patent drawings, Figure 12a schematically illustrates the luminance (Y) channel and Figures 12b and 12c schematically illustrate chrominance (U, V) channels.

It is postulated that the noise pattern seen in the U channel is likely due to a lack of antialiasing in a de-Bayering process at image capture. This can also be an issue in RGB data discussed below. However, an understanding of the underlying cause of the noise pattern is not required for implementation of the present disclosure.

Reducing the chroma noise For the purposes of empirical tests, an example 1,2,1 horizontal low-pass filter was applied to the U channel prior to encoding which brought the required bit-rate much closer to twice that required for 4:2:0 encoding. Example results are shown schematically in Figure 13 demonstrating that outlying (high bitrate ratio) examples have been decreased or removed. It is noted, however, that low-pass filtering all of the input video data could have a potentially detrimental effect on the chroma PSNR (peak signal to noise ratio) and hence the chroma BD (Bjcantegaard-Delta) rate but can potentially improve the luma BD rate due to the reduced bit-rate. Experiments have shown that adjusting the chroma OP offset can mitigate this chroma loss to some extent. It may also be the case that the subjective quality is not so much affected by filtering.

Selective filtering or varying application of filters In order to mitigate potential negative effects caused by filtering, example embodiments of the present disclosure apply filtering in a targeted manner such that it is applied when deemed to be needed (as the result of a detection of a predetermined property of at least one of the chrominance channels such as a degree of high-frequency content) and/or it is applied to a varying degree in response to the detection of the predetermined property. The filtering can be applied to all of the chrominance data or to a subset of channels such as only to the U channel.

Using such techniques, it is possible to arrange that the Main 4:4:4 10 profile can work with twice the bit-rate of the Main 10 profile because, by virtue of these measures, chroma noise in the source is limited.

Example operations -encoder Figure 14 schematically illustrates aspects of an apparatus comprising a video data encoder 1400 which may operate (except where indicated here) in the manner discussed above with reference to Figure 7, for example. Some components of the video data encoder, notably the control circuitry 343 of the video data encoder and an input filter 1410, are drawn separately for clarity of the explanation. The video data encoder 1400 acts upon an input video data stream 1420 to generate and output an encoded video data stream 1460 under the control of the control circuitry 343.

The video data encoder is configured to receive the input video data stream 1420 in a 4:4:4 or a 4:2:2 sampling format, the input video data comprising luminance data representing at least a luminance video channel and chrominance data representing one or more chrominance video channels.

Detector circuitry 1430 is configured to detect a predetermined property of the input video data indicative of high frequency content of the chrominance data relative to high frequency content of the luminance data.

In an illustrative example, a "variance" figure is calculated as an average absolute difference between the original data for that channel and a low pass filtered version (for example a filtered version obtained by a horizontal low pass filter having filter coefficients in the ratio of "1 21", such as IA IA IA).

More generally the high frequency content can be assessed by any technique which applies a frequency response to the data which predominates towards higher frequencies. The example given above is to subtract a low pass filtered version from the data, but an equivalent would be to apply a high pass filter to the data. The 1 2 1 filter response is just one example. More generally, any filter response which passes more at a higher frequency than a lower frequency (or the other way round if subtracted from the original signal) is suitable; a precise definition of a cut-off frequency; a half band or other arrangement would be suitable.

Optionally, the detector circuitry 1430 can detect whether (or a degree to which) the detected predetermined property meets a predetermined criterion such as a predetermined criterion indicative of at least a threshold level of the high frequency content of the chrominance data relative to the high frequency content of the luminance data. The criterion may be stored by a criteria store 1440.

The filter circuitry 1410 is configured to selectively apply a filtering operation to at least a portion of the chrominance data (for example, one or both chrominance channels) in response to the detected predetermined property. As drawn, the filter circuitry 1410 is under the control of the control circuitry 343 which itself receive information (on which to base that control) from the detector circuitry 1430. In other examples, the filter circuitry 1410 could be directly controlled by the detector circuitry 1430.

The encoder comprises encoding circuitry 1450 configured to encode the luminance data and the chrominance data, as selectively filtered by the filter circuitry, to generate the encoded output video data.

The arrangement shown in Figure 14 can involve controlling the filter circuitry 1410 either to operate or not to operate (that is to say, to pass-through or bypass the input video data stream as drawn), for example in dependence upon whether the detected predetermined property meets the criterion or not. For example, the criterion might represent a threshold amount of high frequency content in one or more of the chrominance video channels, so that the detector 1430 detects whether that threshold is met or not. In other examples, the criterion might represent a threshold ratio between high frequency content in one or more of the chrominance video channels and high frequency content in the luminance channel, again with a detection as to whether that threshold is met or not.

In other examples, however, the filter circuitry may be configured to apply the filtering operation in response to a degree to which the detected predetermined property meets the predetermined criterion. In these examples, the criterion might relate to either of the example criteria just mentioned, but rather than expressing the criterion simply in the form of a threshold with a pass/fail detection, the criterion can be applied so as to generate a variable indication of the degree to which the detected predetermined property meets the criterion. Optionally, a threshold can also be included so that, for example, no filtering operation is applied at all by the filter circuitry 1410 unless the predetermined property meets at least the threshold, but after that the degree to which the filtering operation is applied can vary, for example, linearly or according to another function, or according to a look-up table associating a degree of filter application to a degree by which the detected predetermined property meets the predetermined criterion.

In some examples the filter may be controlled according to the bit-rate of the encoded stream, potentially in conjunction with a rate control algorithm. For example, this would tie it in with any existing rate control that adjusts the quanbser or the like to meet a target bit-rate. In some examples, the filtering operation is a low pass filtering operation such as a horizontal low pass filtering operation. In the case of a single set of filter coefficients (relevant to the discussion of Figure 15 below) these could be expressed as "1 2 1" (albeit normalised) to give a suitable low pass filtering or smoothing response. In other examples, a two-dimensional low pass filter could be used, which may give a subjectively better outcome but potentially at the expense of additional line stores or other storage.

Referring to Figure 15, in some examples the filter circuitry 1410' is configured to vary a filtering operation to be applied to at least a portion of the chrominance data (for example, one or both chrominance channels) in response to the detected predetermined property. This can be achieved, for example, by providing a coefficient/parameter store or generator 1500 to provide filter coefficients and/or other parameters to the filter circuitry 1410'in response to a control input 1510 from the control circuitry 343 or the detector 1430. So, the filter circuitry 1410' and the coefficient/parameter store or generator 1500 can collectively be viewed as performing the operation of the filter circuitry in this example.

Another possible arrangement is shown in a schematic form in Figure 16. This can operate with or without the arrangement of Figure 15, for which reason the coefficient/parameter store or generator 1500 is shown in broken line in Figure 16 to indicate its optional usage. In Figure 16, a mixer 1600 generates a mixing or weighted sum or average between the output of the filter circuitry 1410 and a bypassed (non-filtered) signal 1630 according to a mixing ratio provided by a mixing parameter controller 1610 under the control of a signal 1620 from the detector 1430 and/or the control circuitry 343. Examples of suitable mixing ratios are given by the following table: Detected property Mixing ratio < first threshold 0% filtered: 100% bypassed > second threshold (higher than first threshold) 100% filtered: 0% bypassed first threshold <= detected property <= second threshold * proportion of filtered = (detected property -first threshold) / (second threshold -first threshold) * remainder unfiltered For example where the detected property is a ratio of U variance to Y variance, the first threshold could be 0.5 and the second threshold 1.0, with a linear variation (as set out in the example of the table above) between those two boundaries.

Note that although the table represents an example linear variation between lower and upper thresholds of the detected property, other (non-linear) functions could be used. Note that the term "criterion" does not have to refer to a simple threshold but can encompass multiple thresholds and/or a mapping function for a mixing ratio, for example.

Therefore the example arrangements of Figure 15 and Figure 16 represent examples in which the filter circuitry configured to vary one or more filter operation parameters in response to the detected predetermined property, selected from the list of filter operation parameters consisting of: (i) filter frequency response; and (ii) a mixing proportion between a filtered version of a portion of the chrominance data and an unfiltered version of that portion of the chrominance data. Alternate embodiment using RGB data The examples given above have related to YUV data as an example of a luminance channel and two chrominance channels. However, this represents just one example of a plurality of video component channels. Another example involves the use of respective primary colour video channels such as red, green and blue (RGB) video channels.

In a 4:4:4 RGB arrangement, the detector circuitry 1430 may be configured to detect a predetermined property of the input video data indicative of high frequency content of at least one of the red and the blue video channels relative to high frequency content of the green video channel, for example. This has the potential advantage that the green channel tends to carry more significant information in terms of its visual impact, but other permutations of channels could be used. In such an example, the filter circuitry 1410 may be configured to selectively apply the filtering operation to at least one of (for example, both of) the red and the blue video channels in response to the detected predetermined property.

Therefore, in general terms, the arrangement of Figure 14 may be referred to as providing apparatus comprising: a video data encoder 1400 configured to receive input video data in a 4:4:4 or a 4:2:2 sampling format, the input video data representing a plurality of video component channels; the video data encoder comprising: detector circuitry 1430 configured to detect a predetermined property of the input video data indicative of high frequency content of at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels; filter circuitry 1410 configured to selectively apply a filtering operation to one or more of the plurality of video component channels in response to the detected predetermined property; and encoding circuitry 1450 configured to encode the input video data, as selectively filtered by the filter circuitry, to generate the encoded output video data.

In Figure 14, the filter circuitry may be configured to apply the filtering operation to the one or more of the plurality of video component channels in response to a degree to which the detected predetermined property meets a predetermined criterion. The predetermined criterion may be indicative of at least a threshold level of the high frequency content of the at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels. The arrangements discussed above may also be generalised to refer to the filter circuitry 1430 being configured to vary a filtering operation to be applied to at least a portion of the one or more of the plurality of video component channels in response to the detected predetermined property. For example, the filter circuitry 1430 may be configured to vary one or more filter operation parameters in response to the detected predetermined property, selected from the list of filter operation parameters consisting of: (i) filter frequency response; and (ii) a mixing proportion between a filtered version of a given video component channel being filtered and an unfiltered version of the given video component channel.

As discussed above, in other examples the plurality of video component channels represent a luminance video channel and two chrominance video channels. In such arrangements the detector circuitry may be configured to detect a predetermined property of the input video data indicative of high frequency content of at least one of the chrominance video channels relative to high frequency content of the luminance video channel; and the filter circuitry may be configured to selectively apply the filtering operation to at least one of the chrominance video channels in response to the detected predetermined property. In such examples the filter circuitry may be configured to selectively apply the filtering operation to both of the chrominance video channels in response to the detected predetermined property. Interactive Option In some examples, the user can be presented with an option regarding how to handle the detection discussed above. In particular, in examples, the system can detect a predetermined property of the input video data indicative of high frequency content of at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels and, when the predetermined property is detected, output to a graphical user interface of the apparatus an option to a user to invoke, via the graphical user interface, the option of operating (or not) the filter circuitry discussed above. 4:4:4 or 4:2:2 data The examples discussed above relate to applying the filtering to 4:4:4 input data.

However, arguably there may also be advantages if the same techniques are applied to 4:2:2 video data. For example, vertical filtering or two-dimensional filtering may be applicable to 4:2:2 data, although of course horizontal filtering may be used. In the definitions given below and in the claims, reference is made to "input video data in a 4:4:4 or a 4:2:2 sampling format", but it will be appreciated that the techniques could be limited in accordance with the specific examples given above to refer just to input video data in a 4:4:4 sampling format.

Encoded video data Video data encoded by any of the techniques disclosed here is also considered to

represent an embodiment of the present disclosure.

Summary Method

Figure 17 is a schematic flowchart illustrating a method comprising: receiving (at a step 1700) input video data in a 4:4:4 or a 4:2:2 sampling format, the input video data comprising luminance data representing a plurality of video component channels; detecting (at a step 1710) a predetermined property of the input video data indicative of high frequency content of at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels; selectively applying (at a step 1720) a filtering operation to at least one or more of the plurality of video component channels in response to the detected predetermined property; and encoding (at a step 1730) the input video data, as selectively filtered by the filter circuitry, to generate the encoded output video data.

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

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

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

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

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

Respective aspects and features are defined by the following numbered clauses: 1. Apparatus comprising: a video data encoder configured to receive input video data in a 4:4:4 or a 4:2:2 sampling format, the input video data representing a plurality of video component channels the video data encoder comprising: detector circuitry configured to detect a predetermined property of the input video data indicative of high frequency content of at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels; filter circuitry configured to selectively apply a filtering operation to one or more of the plurality of video component channels in response to the detected predetermined property; and encoding circuitry configured to encode the input video data, as selectively filtered by the filter circuitry, to generate the encoded output video data.

2. The apparatus of clause 1, in which the filter circuitry is configured to apply the filtering operation to the one or more of the plurality of video component channels in response to a degree to which the detected predetermined property meets a predetermined criterion.

3. The apparatus of clause 2, in which the predetermined criterion is indicative of at least a threshold level of the high frequency content of the at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels.

4. The apparatus of any one of the preceding clauses, in which the filtering operation is a low pass filtering operation.

5. The apparatus of any one of the preceding clauses, in which the filter circuitry is configured to vary a filtering operation to be applied to at least the one or more of the plurality of video component channels in response to the detected predetermined property.

6. The apparatus of clause 5, in which the filter circuitry is configured to vary one or more filter operation parameters in response to the detected predetermined property, selected from the list of filter operation parameters consisting of: (i) filter frequency response; and (ii) a mixing proportion between a filtered version of a given video component channel being filtered and an unfiltered version of the given video component channel.

7. The apparatus of any one of the preceding clauses, in which the plurality of video component channels represent respective primary colour video channels.

8. The apparatus of clause 7, in which the primary colour video channels are red, green and blue video channels.

9. The apparatus of clause 8, in which: the detector circuitry is configured to detect a predetermined property of the input video data indicative of high frequency content of at least one of the red and the blue video channels relative to high frequency content of the green video channel; and the filter circuitry configured to selectively apply the filtering operation to at least one of the red and the blue video channels in response to the detected predetermined property.

10. The apparatus of clause 9, in which the filter circuitry is configured to selectively apply the filtering operation to both of the red and blue video channels in response to the detected predetermined property.

11. The apparatus of any one of the preceding clauses, in which the plurality of video component channels represent a luminance video channel and two chrominance video channels.

12. The apparatus of clause 11, in which: the detector circuitry is configured to detect a predetermined property of the input video data indicative of high frequency content of at least one of the chrominance video channels relative to high frequency content of the luminance video channel; and the filter circuitry is configured to selectively apply the filtering operation to at least one of the chrominance video channels in response to the detected predetermined property.

13. The apparatus of clause 12, in which the filter circuitry is configured to selectively apply the filtering operation to both of the chrominance video channels in response to the detected predetermined property.

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

15. A method comprising: receiving input video data in a 4:4:4 or a 4:2:2 sampling format, the input video data representing a plurality of video component channels; detecting a predetermined property of the input video data indicative of high frequency content of at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels; selectively applying a filtering operation to at least one or more of the plurality of video component channels in response to the detected predetermined property; and encoding the input video data, as selectively filtered by the filter circuitry, to generate the encoded output video data.

16. The method of clause 15, in which the step of selectively applying a filtering operation comprises applying the filtering operation to the one or more of the plurality of video component channels in response to a degree to which the detected predetermined property meets a predetermined criterion.

17. The method of clause 16, in which the predetermined criterion is indicative of at least a threshold level of the high frequency content of at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels.

18. The method of any one of clauses 15 to 17, in which the filtering operation is a low pass filtering operation.

19. The method of any one of clauses 15 to 18, in which the step of selectively applying a filtering operation comprises varying a filtering operation to be applied to the one or more of the plurality of video component channels in response to the detected predetermined property.

20. The method of clause 19, in which the step of selectively applying a filtering operation comprises varying one or more filter operation parameters in response to the detected predetermined property, selected from the list of filter operation parameters consisting of: (i) filter frequency response; and (ii) a mixing proportion between a filtered version of a given video component channel being filtered and an unfiltered version of the given video component channel.

21. The method of any one of clauses 15 to 20, in which the plurality of video component channels represent respective primary colour video channels.

22. The method of clause 21, in which the primary colour video channels are red, green and blue video channels.

23. The method of clause 22, in which: the detecting step comprises detecting a predetermined property of the input video data indicative of high frequency content of at least one of the red and the blue video channels relative to high frequency content of the green video channel; and the step of selectively applying a filtering operation comprises selectively applying the filtering operation to at least one of the red and the blue video channels in response to the detected predetermined property.

24. The method of clause 23, in which the step of selectively applying a filtering operation comprises selectively applying the filtering operation to both of the red and blue video channels in response to the detected predetermined property.

25. The method of any one of clauses 15 to 20, in which the plurality of video component channels represent a luminance video channel and two chrominance video channels.

26. The method of clause 25, in which: the detecting step comprises detecting a predetermined property of the input video data indicative of high frequency content of at least one of the chrominance video channels relative to high frequency content of the luminance video channel; and the step of selectively applying a filtering operation comprises selectively applying the filtering operation to at least one of the chrominance video channels in response to the detected predetermined property.

27. The method of clause 25 or 26, in which the step of selectively applying a filtering operation comprises selectively applying the filtering operation to both of the chrominance video channels in response to the detected predetermined property.

28. Computer software which, when executed by a computer, causes the computer to carry out the method of any one of clauses 15 to 27.

29. A machine-readable non-transitory storage medium which stores the computer software of clause 28.

Claims

CLAIMS1. Apparatus comprising: a video data encoder configured to receive input video data in a 4:4:4 or a 4:2:2 sampling format, the input video data representing a plurality of video component channels; the video data encoder comprising: detector circuitry configured to detect a predetermined property of the input video data indicative of high frequency content of at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component 10 channels; filter circuitry configured to selectively apply a filtering operation to one or more of the plurality of video component channels in response to the detected predetermined property; and encoding circuitry configured to encode the input video data, as selectively filtered by the filter circuitry, to generate the encoded output video data.
2. The apparatus of claim 1, in which the filter circuitry is configured to apply the filtering operation to the one or more of the plurality of video component channels in response to a degree to which the detected predetermined property meets a predetermined criterion.
3. The apparatus of claim 2, in which the predetermined criterion is indicative of at least a threshold level of the high frequency content of the at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels.
4. The apparatus of claim 1, in which the filtering operation is a low pass filtering operation.
5. The apparatus of claim 1, in which the filter circuitry is configured to vary a filtering operation to be applied to at least the one or more of the plurality of video component channels in response to the detected predetermined property.
6. The apparatus of claim 5, in which the filter circuitry is configured to vary one or more filter operation parameters in response to the detected predetermined property, selected from the list of filter operation parameters consisting of: (i) filter frequency response; and (ii) a mixing proportion between a filtered version of a given video component channel being filtered and an unfiltered version of the given video component channel.
7. The apparatus of claim 1, in which the plurality of video component channels represent respective primary colour video channels.
8. The apparatus of claim 7, in which the primary colour video channels are red, green and blue video channels.
9. The apparatus of claim 8, in which: the detector circuitry is configured to detect a predetermined property of the input video data indicative of high frequency content of at least one of the red and the blue video channels relative to high frequency content of the green video channel; and the filter circuitry configured to selectively apply the filtering operation to at least one of the red and the blue video channels in response to the detected predetermined property.
10. The apparatus of claim 9, in which the filter circuitry is configured to selectively apply the filtering operation to both of the red and blue video channels in response to the detected predetermined property.
11. The apparatus of claim 1, in which the plurality of video component channels represent a luminance video channel and two chrominance video channels. 20
12. The apparatus of claim 11, in which: the detector circuitry is configured to detect a predetermined property of the input video data indicative of high frequency content of at least one of the chrominance video channels relative to high frequency content of the luminance video channel; and the filter circuitry is configured to selectively apply the filtering operation to at least one of the chrominance video channels in response to the detected predetermined property.
13. The apparatus of claim 12, in which the filter circuitry is configured to selectively apply the filtering operation to both of the chrominance video channels in response to the detected predetermined property.
14. Video storage, capture, transmission or reception apparatus comprising apparatus according to claim 1.
15. A method comprising: receiving input video data in a 4:4:4 or a 4:2:2 sampling format, the input video data representing a plurality of video component channels; detecting a predetermined property of the input video data indicative of high frequency content of at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels; selectively applying a filtering operation to at least one or more of the plurality of video component channels in response to the detected predetermined property; and encoding the input video data, as selectively filtered by the filter circuitry, to generate the encoded output video data.
16. The method of claim 15, in which the step of selectively applying a filtering operation comprises applying the filtering operation to the one or more of the plurality of video component channels in response to a degree to which the detected predetermined property meets a predetermined criterion.
17. The method of claim 16, in which the predetermined criterion is indicative of at least a threshold level of the high frequency content of at least one of the plurality of video component channels relative to high frequency content of at least one other of the plurality of video component channels.
18. The method of claim 15, in which the filtering operation is a low pass filtering operation.
19. The method of claim 15, in which the step of selectively applying a filtering operation comprises varying a filtering operation to be applied to the one or more of the plurality of video component channels in response to the detected predetermined property.
20. The method of claim 19, in which the step of selectively applying a filtering operation comprises varying one or more filter operation parameters in response to the detected predetermined property, selected from the list of filter operation parameters consisting of: (i) filter frequency response; and (ii) a mixing proportion between a filtered version of a given video component channel being filtered and an unfiltered version of the given video component channel.
21. The method of claim 15, in which the plurality of video component channels represent respective primary colour video channels.
22. The method of claim 21, in which the primary colour video channels are red, green and blue video channels.
23. The method of claim 22, in which: the detecting step comprises detecting a predetermined property of the input video data indicative of high frequency content of at least one of the red and the blue video channels relative to high frequency content of the green video channel; and the step of selectively applying a filtering operation comprises selectively applying the filtering operation to at least one of the red and the blue video channels in response to the detected predetermined property.
24. The method of claim 23, in which the step of selectively applying a filtering operation comprises selectively applying the filtering operation to both of the red and blue video channels in response to the detected predetermined property.
25. The method of claim 15, in which the plurality of video component channels represent a luminance video channel and two chrominance video channels. 15
26. The method of claim 25, in which: the detecting step comprises detecting a predetermined property of the input video data indicative of high frequency content of at least one of the chrominance video channels relative to high frequency content of the luminance video channel; and the step of selectively applying a filtering operation comprises selectively applying the filtering operation to at least one of the chrominance video channels in response to the detected predetermined property.
27. The method of claim 26, in which the step of selectively applying a filtering operation comprises selectively applying the filtering operation to both of the chrominance video channels in response to the detected predetermined property.
28. Computer software which, when executed by a computer, causes the computer to carry out the method of claim 15.
29. A machine-readable non-transitory storage medium which stores the computer software of claim 28.