GB2614271A

GB2614271A - Picture data encoding and decoding

Info

Publication number: GB2614271A
Application number: GB2118871.9A
Authority: GB
Inventors: Mark Keating Stephen; James Sharman Karl; Richard Browne Adrian
Original assignee: Sony Group Corp
Current assignee: Sony Group Corp
Priority date: 2021-12-23
Filing date: 2021-12-23
Publication date: 2023-07-05
Also published as: GB202118871D0

Abstract

Encoding (Fig. 12) and decoding (Fig. 13) of data representing either: i) video data stream at a data rate; ii) still picture, comprising; in response to parameter data associated with encoded data, indicating encoding level from a plurality of encoding levels; each coding level defining a maximum luminance picture size and minimum compression ratio; at least some encoding levels being applicable to encoding a video data stream; at least some encoding levels being applicable to encoding a still image. For a given encoding level applicable to video data: encoding video stream so that data rate is no greater when video is compressed by minimum compression ratio; decoding using buffer 1300 capable of storing video up to a size of one of the video pictures as compressed by minimum compression ratio. For a given encoding level applicable to still image: encoding comprises still picture so that data quantity of luminance is no greater than maximum luminance picture size as compressed by the minimum compression ratio; decoding using buffer 1300 capable of storing a portion of luminance component up to the maximum luminance picture size as compressed by the minimum compression.

Description

PICTURE DATA ENCODING AND DECODING

BACKGROUND Field

This disclosure relates to picture data encoding and decoding, for example being applicable to video pictures and/or still pictures.

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 picture encoding and/or decoding 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 picture data. A corresponding decoding or decompression technique is applied to recover a reconstructed version of the original picture data.

SUMMARY

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

The present disclosure provides apparatus comprising: a picture data encoder configured selectively to encode input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and (ii) a still picture having a still picture size, and to generate respective output data; the picture data encoder being responsive to encoding constraints defined by parameter data associated with the output data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size and a minimum compression ratio, at least some of the encoding levels being applicable to encoding of a video data stream and at least some of the encoding levels being applicable to encoding of a still picture; in which, for a given encoding level applicable to encoding of a video data stream, the picture data encoder is configured to encode successive video pictures of a video stream so that a data rate of the respective output data is no greater than a data rate of the respective video data stream as compressed by the minimum compression ratio defined by the given encoding level; and in which, for a given encoding level applicable to encoding of a still picture, the picture data encoder is configured to encode a still picture so that a data quantity of the respective luminance output data is no greater than a data quantity defined by the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

The present disclosure also provides a method comprising: encoding an input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and (ii) a still picture having a still picture size, to generate respective output data, in response to parameter data associated with the output data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size and a minimum compression ratio, at least some of the encoding levels being applicable to encoding of a video data stream and at least some of the encoding levels being applicable to encoding of a still picture; in which, for a given encoding level applicable to encoding of a video data stream, the encoding step comprises encoding successive video pictures of a video stream so that a data rate of the respective output data is no greater than a data rate of the respective video data stream as compressed by the minimum compression ratio defined by the given encoding level; and in which, for a given encoding level applicable to encoding of a still picture, the encoding step comprises encoding a still picture so that a data quantity of the respective luminance output data is no greater than a data quantity defined by the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

The present disclosure also provides apparatus comprising: a picture data decoder configured selectively to decode input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and (ii) a still picture having a still picture size and to generate decoded data; and a coded picture buffer to buffer successive portions of input data and to provide a portion to the picture data decoder for decoding, the coded picture buffer having a coded picture buffer size; the picture data decoder being responsive to parameter data associated with the input data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size of the decoded data and a minimum compression ratio, at least some of the encoding levels being applicable to input data representing a video data stream and at least some of the encoding levels being applicable to input data representing a still picture; in which, for operation in accordance with a given encoding level applicable to a video data stream, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of the input data up to a size of one of the video pictures as compressed by the minimum compression ratio defined by the given encoding level; and in which, for operation in accordance with a given encoding level applicable to encoding of a still picture, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of a luminance component of the input data up to the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

The present disclosure also provides a method comprising: buffering a portion of input data in a coded picture buffer, the coded picture buffer having a coded picture buffer size, the input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and CID a still picture having a still picture size; providing a buffered portion of the input data for decoding; decoding the input data to generate decoded data; and the decoding step being responsive to parameter data associated with the input data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size of the decoded data and a minimum compression ratio, at least some of the encoding levels being applicable to input data representing a video data stream and at least some of the encoding levels being applicable to input data representing a still picture; in which, for operation in accordance with a given encoding level applicable to a video data stream, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of the input data up to a size of one of the video pictures as compressed by the minimum compression ratio defined by the given encoding level; and in which, for operation in accordance with a given encoding level applicable to encoding of a still picture, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of a luminance component of the input data up to the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

Further aspects and features are defined by the appended claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the present technology.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 schematically illustrates an audio/video (AN) data transmission and reception system using video data compression and decompression; Figure 2 schematically illustrates a video display system using video data decompression; Figure 3 schematically illustrates an audio/video storage system using video data compression and decompression; Figure 4 schematically illustrates a video camera using video data compression; Figures 5 and 6 schematically illustrate storage media; Figure 7 provides a schematic overview of a video data compression and decompression apparatus; Figure 8 schematically illustrates a predictor; Figure 9 schematically illustrates the use of parameter sets; Figure 10 schematically illustrates a decoding apparatus; Figure 11 schematically illustrates an encoding apparatus; and Figures 12 and 13 are schematic flowcharts illustrating respective methods.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

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

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

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

Note that in all of the present examples, unless the technical context requires otherwise, references to video encoding and decoding (and to corresponding video data signals and video processing apparatus) can also apply to still picture encoding and decoding (and to corresponding still picture signals and still picture processing apparatus). Still picture encoding and decoding is performed according to a still picture encoding level, which in the examples here defines an intra-image coding process.

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 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 (DOT) representation of blocks or regions of the residual image data. The DOT technique itself is well known and will not be described in detail here. Note also that the use of 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 DOT coefficients for each transformed block of image data, is supplied to a quantiser 350. Various quantisation techniques are known in the field of video data compression, ranging from a simple multiplication by a quantisation scaling factor through to the application of complicated lookup tables under the control of a quantisation parameter. The general aim is twofold. Firstly, the quantisation process reduces the number of possible values of the transformed data. Secondly, the quantisation process can increase the likelihood that values of the transformed data are zero.

Both of these can make the entropy encoding process, to be described below, work more efficiently in generating small amounts of compressed video data.

A data scanning process is applied by a scan unit 360. The purpose of the scanning process is to reorder the quantised transformed data so as to gather as many as possible of the non-zero quantised transformed coefficients together, and of course therefore to gather as many as possible of the zero-valued coefficients together. These features can allow so-called run-length coding or similar techniques to be applied efficiently. So, the scanning process involves selecting coefficients from the quantised transformed data, and in particular from a block of coefficients corresponding to a block of image data which has been transformed and quantised, according to a "scanning order" so that (a) all of the coefficients are selected once as part of the scan, and (b) the scan tends to provide the desired reordering. One example scanning order which can tend to give useful results is a 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 35 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 On the same (intra) or another (inter) image). By virtue of the units 310 or 450, the difference between the predicted block and the actual block is encoded or applied so as to encode or decode the block respectively.

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

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 I-frame encoding, however, which involves encoding the whole image by intra-encoding, in the present embodiments the choice between intra-and inter-encoding can be made on a block-by-block basis, though in other embodiments the choice is still made on an image-by-image basis.

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

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

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

For inter-image prediction, a motion compensated (MC) predictor 540 uses motion information such as motion vectors derived by a motion estimator 550 from the input video signal 300. Those motion vectors are applied to a processed version of the reconstructed image 460 by the motion compensated predictor 540 to generate blocks of the inter-image prediction.

Accordingly, the units 530 and 540 (operating with the estimator 550) each act as detectors to detect a prediction direction in respect of a current block to be predicted, and as a generator to generate a predicted block of samples (forming part of the prediction passed to the units 310 and 450) according to other samples defined by the prediction direction.

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

Firstly, the signal may be filtered by a so-called loop filter 565. Various types of loop filters may be used. One technique involves applying a "deblocking" filter to remove or at least tend to reduce the effects of the block-based processing carried out by the transform unit 340 and subsequent operations. A further technique involving applying a so-called sample adaptive offset (SAO) filter may also be used. In general terms, in a sample adaptive offset filter, filter parameter data (derived at the encoder and communicated to the decoder) defines one or more offset amounts to be selectively combined with a given intermediate video sample (a sample of the signal 460) by the sample adaptive offset filter in dependence upon a value of:(i) the given intermediate video sample; or (ii) 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 On 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 relating to the Versatile Video Coding standards and proposals at the priority date of the present application is provided by the following: * "VVC operation range extensions (Draft 4)" * JVET-W2005-v1, F Bossen, B Bross, T Ikai, D Rusanovskyy, G Sullivan, Y-K Wang; * "VVC operation range extensions (Draft 5)" JVET-X2005-v1, F Bossen, B Bross, T lkai, D Rusanovskyy, G Sullivan, Y-K Wang; * "Versatile Video Coding (Draft 8)", JVET-Q2001-vE, B. Bross, J. Chen, S. Liu and Y-K. Wang.

* ITU-T H.266 ("SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS; Infrastructure of audiovisual services -Coding of moving video -Versatile video coding", August 2020; * "Versatile Video Coding, Editorial Refinements on Draft 10", Bross et al, JVET-T2001-v2 All of these are also incorporated herein in their 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.

Coded Picture Buffer Video decoding specifications (and, indirectly, corresponding encoders) may be defined to include a so-called coded picture buffer (CPB). At the decoder side, the incoming video stream is stored to the CPB in a timely manner and is read from the CPB for decoding. The specifications assume that an entire picture can be read from the CPB in a single (theoretically instantaneous) operation. At the encoder side, encoded data may similarly be stored to a CPB (theoretically at least as a single instantaneous operation to write an entire encoded picture) before being output to the output encoded video stream. However, it is at the decoder side where the CPB is defined.

The CPB can itself be defined by various aspects, including the data rate at which encoded data enters the CPB, the size of the CPB itself and potentially any delay which applies to removal of data from the CPB (defining in turn the time needed to fill the CPB such that an entire picture can be removed as discussed above).

These parameters are important to avoid CPB over filling (running out of space) and CPB underrun (running out of data to provide to the next stage of processing). Examples of the use of a CPB will be discussed below.

Parameter Sets and Encoding Levels 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 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-T2001-v2 referenced above, various levels are defined from 1 to 6.3.

In the discussion below, unless the technical context dictates otherwise, the term "encoding level" can be taken to refer to a respective permutation of profile, level and tier as defined by the parameter set data.

Example implementation An example implementation will now be described with reference to the drawings.

Figure 9 schematically illustrates the use of video parameter sets and sequence parameter sets as discussed above. In particular, these form part of the hierarchy of parameter sets mentioned earlier such that multiple sequence parameter sets 900, 910, 920 may reference a video parameter set 930 and in turn be referenced themselves by respective sequences 902, 912, 922. In the example embodiments, level information applicable to the respective sequence is provided in the sequence parameter sets.

However, in other embodiments it will be appreciated that the level information could be provided in a different form or a different parameter set.

Similarly, although the schematic representation of Figure 9 shows the sequence parameter sets being provided as part of the overall video data stream 940, the sequence parameter sets (or other data structure carrying the level information) could instead be provided by a separate communication channel. In either case, the level information is associated with the video data stream 940.

Example implementations also relate to the encoding and decoding of still pictures. In such cases, references to input video data should be interpreted as references to an input still picture; references to encoded video data should be interpreted as references to an encoded still picture; and references to decoded video data should be interpreted as references to a decoded still picture.

Still picture encoding and decoding is defined by still picture profiles which, in a similar manner to video data profiles, define parameters of the encoding and decoding process. Infra-image encoding and decoding is used in the handling of still pictures.

Example operations -decoder Figure 10 schematically illustrates aspects of a decoding apparatus configured to receive input data such as an input (encoded) video data stream 1000 (or data representing a still picture) and to generate and output a decoded video data stream 1010 (or a decoded still picture) using a decoder 1020 of the form discussed above with reference to Figure 7. For clarity of the present explanation, the control circuitry or controller 343 of Figure 7 is drawn separately to the remainder of the decoder 1020.

Within the functionality of the controller or control circuitry 343 is a parameter set (PS) detector 1030 which detects, from appropriate fields of the input video data stream 1000, the various parameter sets 1035 including the VPS, SPS and PPS. The parameter set detector 1030 derives information from the parameter sets including the level as discussed above. This information is passed to the remainder of the control circuitry 343. Note that the parameter set detector 1030 could decode the level or could simply provide the encoded level to the control circuitry 343 for decoding.

The control circuitry 343 is also responsive to one or more decoder parameters 1040 defining at least, for example, a level which the decoder 1020 is capable of decoding.

The control circuitry 343 detects whether or not, for the given or current input video data stream 1000, the decoder 1020 is capable of decoding that input video data stream or encoded still picture and controls the decoder 1020 accordingly. The control circuitry 343 can also provide various other operating parameters to the decoder 1020 in response to information obtained from the parameter sets detected by the parameter set detector 1030.

Figure 10 also shows the use of a coded picture buffer (CPB) 1025 to buffer the input video data stream 1000 or encoded still picture before it is provided to the decoder 1020 for decoding. The control circuitry 343 controls parameters of the CPB 1025 in accordance with base parameters of the decoder 1040 and parameters derived from the parameter set decoder 1030.

Using techniques to be described below, Figure 10 therefore provides an example of apparatus comprising: a picture data decoder 1020 configured selectively to decode input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and 00 a still picture having a still picture size and to generate decoded data; and a coded picture buffer 1025 to buffer successive portions of input data and to provide a portion to the picture data decoder for decoding, the coded picture buffer having a coded picture buffer size; the picture data decoder being responsive to parameter data 1035 associated with the input data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size of the decoded data and a minimum compression ratio, at least some of the encoding levels being applicable to input data representing a video data stream and at least some of the encoding levels being applicable to input data representing a still picture; in which, for operation in accordance with a given encoding level applicable to a video data stream, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of the input data up to a size of one of the video pictures as compressed by the minimum compression ratio defined by the given encoding level; and in which, for operation in accordance with a given encoding level applicable to encoding of a still picture, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of a luminance component of the input data up to the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

Example operations -encoder In a similar way, Figure 11 schematically illustrates aspects of an encoding apparatus comprising an encoder 1100 of the type discussed above with reference to Figure 7, for example. The control circuitry 343 of the encoder is drawn separately for clarity of the explanation. The encoder acts upon an input video data stream 1110 or an input still picture to generate output data such as an output encoded video data stream 1120 or an output encoded still picture under the control of the control circuitry 343 which in turn is responsive to encoding parameters 1130 including a definition of an encoding level to be applied.

The control circuitry 343 also includes or controls a parameter set generator 1140 which generates parameter sets 1145 including, for example, the VPS, SPS and PPS to be included within the output encoded video data stream, with the SPS carrying level information encoded as described above.

In a similar manner to that described above, Figure 11 also shows the use of a coded picture buffer (CPB) 1125 to buffer the encoded video data stream 1120 or encoded still picture as generated by the encoder 1100. The control circuitry 343 controls parameters of the CPB 1125 in accordance with the encoding parameters 1130 and these parameters are communicated to the output encoded video data stream 1120 or output encoded still picture (and therefore to an eventual decoding stage) by the parameter set generator 1140.

Using techniques to be described below, Figure 11 therefore provides an example of apparatus comprising: a picture data encoder 1100 configured selectively to encode input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and 00 a still picture having a still picture size, and to generate respective output data; the picture data encoder being responsive to encoding constraints defined by parameter data 1145 associated with the output data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size and a minimum compression ratio, at least some of the encoding levels being applicable to encoding of a video data stream and at least some of the encoding levels being applicable to encoding of a still picture; in which, for a given encoding level applicable to encoding of a video data stream, the picture data encoder is configured to encode successive video pictures of a video stream so that a data rate of the respective output data is no greater than a data rate of the respective video data stream as compressed by the minimum compression ratio defined by the given encoding level; and in which, for a given encoding level applicable to encoding of a still picture, the picture data encoder is configured to encode a still picture so that a data quantity of the respective luminance output data is no greater than a data quantity defined by the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

Background to examples

Tables 136 and 137 of the draft VVC specification referenced above define various video profiles and are reproduced here in the form in which they are specified by the latest draft JVET-X2005v1 (defining range extensions) at the priority date of the present application: Table 136-Tier and level limits for the video profiles CD MaxLumaSr DI tn a r 0 CO cy Z x x el fsamniesiseci.---. -71 7:1 ri: 93 -9 CI 13 71: -N -, a FIL CO ID' z g CD 93 LA 71 (1) V) (1) -0 0

- D

CNI

g CID Main tier M. 9). it 9:2 1- CD = CD -% CD -N -.I 1.0 552 960 128 - 2 2 2.0 3 686 400 1 500 2 2 2.1 7 372 800 3 000 2 2 3.0 16 588 800 6 000 - 2 2 3.1 33 177 600 10 000 2 2 4.0 66 846 720 12 000 30 000 4 4 4.1 133 693 440 20 000 50 000 4 4 5.0 267 386 880 25 000 100 000 6 4 5.1 534 773 760 40 000 160 000 8 4 5.2 1 069 547 520 60 000 240 000 8 4 6.0 1 069 547 520 60 000 240 000 8 4 6.1 2 139 095 040 120 000 480 000 8 4 6.2 4 278 190 080 240 000 800 000 8 4 6.3 4 812 963 840 320 000 1 600 000 8 4 Table 137-Specification of CpbVcIFactor, CpbNalFactor, FormatCapabilityFactor and MinCrScaleFactor Profiles CpbVcIFactor CpbNalFactor FormatCapability Factor MinCrScaleFactor Main 10, Multilayer Main 1 000 1 100 1.875 1.00 Main 10 Still Picture 1 000 1 100 n.a. n.a.

Main 10 4:4:4, Multilayer Main 10 4:4:4 2 500 2 750 3.750 0.75 Main 104:4:4 Still 2 500 2 750 n.a. n.a.

Picture Main 12 1 200 1 320 1.875 1.00 Main 12 Infra 2 400 2 640 1.875 1.00 Main 12 Still Picture 2 400 2 640 n.a. n.a.

Main 124:4:4 3 000 3 300 3.750 0.75 Main 124:4:4 Infra 6 000 6 600 3.750 0.75 Main 124:4:4 Still 6 000 6 600 n.a. n.a.

Picture Main 164:4:4 4 000 4 400 6.000 0.75 Main 164:4:4 Infra 8 000 8 800 6.000 0.75 Main 164:4:4 Still 8 000 8 800 n.a. n.a.

Picture Here, "n.a" signifies "not applicable" or in other words, not defined. In the existing specification, a minimum compression ratio referred to here as MinCr, is defined as follows: MinCr = (MinCrBase * MinCrScaleFactor)/HBrfactor.

MinCrBase is defined by Table 136. MinCrScaleFactor is defined by Table 137.

The variable HbrFactor is defined in clause A.4.2 as follows: * If the bitstream is indicated to conform to the Main 10, Main 104:4:4, Multilayer Main 10, or Mulfilayer Main 104:4:4 profile, HbrFactor is set equal to 1.

* Otherwise, when the bitstream is indicated to conform to the Main 12, Main 12 Infra, Main 12 4:4:4, Main 12 4:4:4 Infra, Main 16 4:4:4, or Main 16 4:4:4 Infra profile, the following applies: o If the bitstream is indicated to conform to the Main tier, HbrFactor is set equal to 1.

o Otherwise (the bitstream is indicated to conform to the High tier), HbrFactor is set equal to 2 This combination of definitions provides a definition of MinCr for video data encoding and decoding in which, for a given encoding level applicable to encoding or decoding of a video data stream, successive video pictures of a video stream are encoded (or received as data to be decoded) such that a data rate of the respective encoded video data is no greater than a data rate of the respective input or original video data stream as compressed by the minimum compression ratio MinCr defined by the given encoding level. The CPB provided at the encoder and at the decoder is sized appropriately to be able to store a portion representing a single picture of such an encoded data stream.

However, the combination of definitions set out above does not provide a definition of MinCr for still image data encoding and decoding. The relevant entries are shown as "n.a" (not applicable, or not defined) in the table reproduced above.

It is proposed here that a minimum compression ratio MinCr is in fact defined for still picture encoding and decoding, and in particular is defined by the still picture profiles discussed above. It is proposed that defining such a MinCr may potentially be useful for encoder and decoder design and in particular to assist in the definition of maximum required sizes for coded picture buffers and the like.

Table 135 is defined in JVET X2005-v1 as follows: Table 135-General tier and level limits r general_level_idc value* Max luma picture size MaxLumaPs (samples) -I 8(1). Max slices per AU MaxSlicesPerAu Max # of tiles MaxTilesPerAu Max # of tile columns MaxTileCols CD i713 -' ,.., D)N DJ < ip- 74:60w X (DX 0)Z-cZn 0 a 0 13 -a m -71 CO co w 0 pi, -6 -Main tier I. = z-

CD -,

to 16 36 864 350 - 16 1 1 2.0 32 122 880 1 500 - 16 1 1 2.1 35 245 760 3 000 20 1 1 3.0 48 552 960 6 000 - 30 4 2 3.1 51 983 040 10 000 40 9 3 4.0 64 2 228 224 12 000 30 000 75 25 5 4.1 67 2 228 224 20 000 50 000 75 25 5 5.0 80 8 912 896 25 000 100 000 200 110 10 5.1 83 8 912 896 40 000 160 000 200 110 10 5.2 86 8 912 896 60 000 240 000 200 110 10 6.0 96 35 651 584 80 000 240 000 600 440 20 6.1 99 35 651 584 120 000 480 000 600 440 20 6.2 102 35 651 584 180 000 800 000 600 440 20 6.3 105 80 216 064 240 000 1 600 000 1 000 990 30 *The evel numbers in this table are in the form of "majorNum.minorNum", and the value of general_level_idc for each of the levels is equal to majorNum * 16 + minorNum * 3.

It is also proposed here to define this according to the maximum luma picture size, MaxLumaPs, of the level (as defined in Table 135) rather than according to the input picture size, PicSizeMaxInSamplesY (defined in clause A.4.1). In other words, in which, for a given encoding level applicable to encoding or decoding of a still picture, the size limit applicable to the encoded still picture data (generated by an encoding process at the encoding side or provided for decoding at the decoding side) resulting from this definition is: data quantity a MaxLumaPs/MinCr rather than data quantity <= PicSizeMaxInSamplesY/MinCr or in other words, a data quantity of the respective encoded still picture is no greater than a data quantity defined by the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

Note that this is defined as a limit for the luminance component; the CPB stores luma and chroma data and hence is larger for 444 profiles (as defined by CbpVcIFactor, FormatCapabilityFactor etc. in table 137). *** A potential advantage of doing this is that choosing a higher level to encode a given actual size of picture would allow a lower compression ratio to be used. In other words, because MaxLumaPs tends to increase with increasing level, assuming a given PicSizeMaxInSamplesY: If PicSizeMaxInSamplesY is such that the still picture can be encoded by a given profile ("profile A") having a given MaxLumaPs and MinCr, then the maximum encoded data quantity using profile A would be: data quantit YProfile A <= MaxLumaPsProfile AMA inCrprofile A However, by instead encoding that same image by a higher profile, ("profile B") having a potentially higher MaxLumaPs but a comparable MinCr, the maximum encoded data quantity using profile B would be: data quantity " Profile B MaxLumaPsProfile B/MinCrprofile In other words, a potentially greater data quantity is allowed, or in other words the effective compression of the image can be less harsh by using the higher profile. (If the data quantity was defined with respect to PicSizeMaxInSamplesY, then apart from any differences in MinCr between profiles, the data quantity limit would be the same as between the profiles.

It is possible for an encoder to select a profile (as between A and B discussed above, for example, where A and B are both capable of handling the input still picture size), for example based upon a test encode (and/or decode) process and optionally an assessment of the actual compression ratio achieved (against the MinCr threshold). For example, that if a still picture to be encoded wasn't complex enough to be hitting the MinCr limit and could in fact be encoded at a lower level without breaching the MinCr condition applicable to that lower level, then the encoder could choose not to use such a high level and select the lower level to encode that picture. (A parallel situation with moving pictures might be where the encoder is given the option of using high tier to guarantee there would be a bit-rate high enough for the operator's chosen quality level but the encoder could decide to generate a main tier bitstream if the encoder is encoding a sequence that is in fact (for example, as detected by a test encode and/or decode) easy to compress.

In other examples, this can be handled as an operator's decision rather than an encoder's decision. For example, if an operator wanted to encode a complex picture at very high quality the operator would decide to do it at a level that can support it, and by doing so would have to accept that lower-spec decoders wouldn't necessarily be able to decode that picture.

The data quantity upper limit defined above also defines a minimum size of the CPB at the encoder and decoder. This can either be a fixed hardware feature of the encoder or decoder (in which case the fixed nature of the CPB indirectly defines which profiles and levels the encoder or decoder is capable of supporting) or can be a feature configurable by the control circuitry 343 so that in response to encoding or decoding according to a particular profile and level, the control circuitry configures the CPB to provide at least the required buffer size.

In terms of proposed changes to the VVC specification to achieve these outcomes, the following are proposed as further amendments to the text of JVET-X2005-v1, with additions to the text of JVET-X2005-v1being underlined: In clause A.4.2, replace the following: The variable HbrFactor is defined as follows: If the bitstream is indicated to conform to the Main 10, Main 104:4:4, Multilayer Main 10, or Multilayer Main 104:4:4 profile, HbrFactor is set equal to 1.

with the following: The variable HbrFactor is defined as follows: If the bitstream is indicated to conform to the Main 10, Main 104:4:4, Multilayer Main 10, or Multilayer Main 104:4:4, Main 10 Still Picture, Main 10 4:4:4 Still Picture Main 12, Main 12 4:4:4, Main 16, or Main 164:4:4 profile, HbrFactor is set equal to 1.

Otherwise, if the bitstream is indicated to conform to Main 12, Main 12 Infra Main 12 Still Picture Main 124:4:4, Main 124:4:4 lntra, Main 124:4:4 Still Picture Main 164:4:4, Main 16 4:4:4 Intra or Main 164:4:4 Still Picture HbrFactor is set equal to 1 for Main tier or equal to 2 for High tier.

In clause A.4.2, add the following: j) The difference between consecutive CPB removal times of AUs n and n -1 (with n greater than 0) shall satisfy the constraint that the number of tiles in AU n is less than or equal to Min( Max( 1, MaxTilesPerAu * 120 * ( AuCpbRemovalTime[ n] -AuCpbRemovalTime[ n -1 ) ), MaxTilesPerAu), where MaxTilesPerAu is the value specified in Table 135 that apply to 30 AU n.

Bitstreams conforming to the Main 10 Still Picture, Main 104:4:4 Still Picture, Main 12 Still Picture, Main 12 4:4:4 Still Picture or Main 164:4:4 Still Picture profile at a specified tier and level shall obey the following constraint for each bitstream conformance test as specified in Annex C: -The sum of the NumBytesInNalUnit variables for AU 0 shall be less than or equal to FormatCapabilityFactor * MaxLumaPs MinCr, where FormatCapabilityFactor and MaxLumaPs are the values specified in Table 135 and Table 137, respectively, that apply to AU 0.

In Table 137, keep the entries for FormatCapabiltyFactor and MinCrScaleFactor for Still Picture profiles, with 12-bit and 16-bit Still Picture profiles using the same values as Infra profiles: Profiles CpbVcIFactor CpbNalFactor FormatCapability MinCrScale Factor Factor Main 10, Main 10 Still 1 000 1 100 1.875 1.00 Picture, Multilayer Main Main 104:4:4, Main 10 2 500 2 750 3.750 0.75 4:4:4 Still Picture, Multilayer Main 104:4:4 Main 12 1 200 1 320 1.875 1.00 Main 12 Intra, Main 12 2 400 2 640 1.875 1.00 Still Picture Main 12 4:4:4 3 000 3 300 3.750 0.75 Main 124:4:4 Infra, 6 000 6 600 3.750 0.75 Main 124:4:4 Still Picture Main 16 4:4:4 4 000 4 400 6.000 0.75 Main 164:4:4 Infra, 8 000 8 800 6.000 0.75 Main 164:4:4 Still Picture It is noted that levels which do not use the words "Still Picture" in the above tables refer to operation in accordance with a given encoding level applicable to a video data stream.

Levels which do use the words "Still Picture" refer to operation in accordance with a given encoding level applicable to encoding of a still picture. Although still picture levels and video data levels may be seen as separate, a level such as the that represented by the fourth row of Table 137 reproduced just above may alternatively be considered to be applicable to video coding (main 12 intra) and to still picture coding (main 12 still picture), albeit that the way in which the MinCr constraint is applied in each case may be different as described here.

At the encoder side, for a video data level, a constraint applied is that the data rate of the respective output data is no greater than a data rate of the respective video data stream as compressed by the minimum compression ratio defined by that encoding level. The CPB at both the encoder and the decoder side is defined to be big enough to hold a portion (such as data representing a whole image) of such an encoded data stream.

Similarly, for a still picture level, at the encoder side a constraint applied is that the data quantity of the respective luminance component of the output data is no greater than a data quantity defined by the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by that level. Again, the CPB at both the encoder and the decoder side is defined to be big enough to hold a portion (such as data representing a whole image) of such an encoded picture. Note that by defining the constraint with respect to luminance data this can also indirectly define the overall data quantity for (chroma + luma) data for that picture, as discussed above.

In other words, in order for a decoder to be able to operate at a given one of these levels level, one constraint on the decoder is that it can provide a CPB of such a minimum size.

A minimum compression ratio constraint is defined for each level but the way in which that constraint is applied to the process at the encoder and decoder can therefore be different between otherwise similar still picture and video coding levels.

Encoded picture data Picture data encoded by any of the techniques disclosed here is also considered to represent an embodiment of the present disclosure. A non-transitory machine-readable storage medium carrying such picture data is also considered to represent an embodiment of the present disclosure.

Summary Methods

Figure 12 is a schematic flowchart illustrating a method comprising: encoding (at a step 1200) an input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and OD a still picture having a still picture size, to generate respective output data, in response to parameter data associated with the output data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size and a minimum compression ratio, at least some of the encoding levels being applicable to encoding of a video data stream and at least some of the encoding levels being applicable to encoding of a still picture; in which, for a given encoding level applicable to encoding of a video data stream, the encoding step comprises encoding successive video pictures of a video stream so that a data rate of the respective output data is no greater than a data rate of the respective video data stream as compressed by the minimum compression ratio defined by the given encoding level; and in which, for a given encoding level applicable to encoding of a still picture, the encoding step comprises encoding a still picture so that a data quantity of the respective luminance output data is no greater than a data quantity defined by the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

Figure 13 is a schematic flowchart illustrating a method comprising: buffering (at a step 1300) a portion of input data in a coded picture buffer, the coded picture buffer having a coded picture buffer size, the input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and 00 a still picture having a still picture size; providing (at a step 1310) a buffered portion of the input data for decoding; decoding (at a step 1320) the input data to generate decoded data; and the decoding step being responsive to parameter data associated with the input data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size of the decoded data and a minimum compression ratio, at least some of the encoding levels being applicable to input data representing a video data stream and at least some of the encoding levels being applicable to input data representing a still picture; in which, for operation in accordance with a given encoding level applicable to a video data stream, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of the input data up to a size of one of the video pictures as compressed by the minimum compression ratio defined by the given encoding level; and in which, for operation in accordance with a given encoding level applicable to encoding of a still picture, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of a luminance component of the input data up to the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

General Matters 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 picture data encoder configured selectively to encode input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and 00 a still picture having a still picture size, and to generate respective output data; the picture data encoder being responsive to encoding constraints defined by parameter data associated with the output data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size and a minimum compression ratio, at least some of the encoding levels being applicable to encoding of a video data stream and at least some of the encoding levels being applicable to encoding of a still picture; in which, for a given encoding level applicable to encoding of a video data stream, the picture data encoder is configured to encode successive video pictures of a video stream so that a data rate of the respective output data is no greater than a data rate of the respective video data stream as compressed by the minimum compression ratio defined by the given encoding level; and in which, for a given encoding level applicable to encoding of a still picture, the picture data encoder is configured to encode a still picture so that a data quantity of the respective luminance output data is no greater than a data quantity defined by the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

2. The apparatus of clause 1, comprising: a coded picture buffer configured to buffer successive portions of the output data generated by the picture data encoder, the coded picture buffer having a coded picture buffer size of at least a minimum size defined by the encoding level applicable to the output data.

3. The apparatus of clause 2, in which each portion is a portion of data representing an entire picture.

4. The apparatus of any one of the preceding clauses, in which each encoding level defines at least a base minimum compression ratio and a scaling factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio and the scaling factor.

5. The apparatus of clause 4, in which each encoding level defines at least a high bit rate factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio, the scaling factor and the high bit rate factor.

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

7. A method comprising: encoding an input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and (ii) a still picture having a still picture size, to generate respective output data, in response to parameter data associated with the output data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size and a minimum compression ratio, at least some of the encoding levels being applicable to encoding of a video data stream and at least some of the encoding levels being applicable to encoding of a still picture; in which, for a given encoding level applicable to encoding of a video data stream, the encoding step comprises encoding successive video pictures of a video stream so that a data rate of the respective output data is no greater than a data rate of the respective video data stream as compressed by the minimum compression ratio defined by the given encoding level; and in which, for a given encoding level applicable to encoding of a still picture, the encoding step comprises encoding a still picture so that a data quantity of the respective luminance output data is no greater than a data quantity defined by the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

8. The method of clause 7, comprising: buffering the encoded output data in a coded picture buffer configured to buffer successive portions of the output data generated by the encoding step, the coded picture buffer having a coded picture buffer size of at least a minimum size defined by the encoding level applicable to the output data.

9. The method of clause 8, in which each portion is a portion of data representing an entire picture.

10. The method of any one of clauses 7 to 9, in which each encoding level defines at least a base minimum compression ratio and a scaling factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio and the scaling factor.

11. The method of clause 10, in which each encoding level defines at least a high bit rate factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio, the scaling factor and the high bit rate factor.

12. Computer software which, when executed by a computer, causes the computer to carry out the method of any one of clauses 7 to 11.

13. A machine-readable non-transitory storage medium which stores the computer software of clause 12.

14. Apparatus comprising: a picture data decoder configured selectively to decode input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and (ii) a still picture having a still picture size and to generate decoded data; and a coded picture buffer to buffer successive portions of input data and to provide a portion to the picture data decoder for decoding, the coded picture buffer having a coded picture buffer size; the picture data decoder being responsive to parameter data associated with the input data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size of the decoded data and a minimum compression ratio, at least some of the encoding levels being applicable to input data representing a video data stream and at least some of the encoding levels being applicable to input data representing a still picture; in which, for operation in accordance with a given encoding level applicable to a video data stream, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of the input data up to a size of one of the video pictures as compressed by the minimum compression ratio defined by the given encoding level; and in which, for operation in accordance with a given encoding level applicable to encoding of a still picture, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of a luminance component of the input data up to the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

15. The apparatus of clause 14, in which the portions are portions of data representing entire pictures.

16. The apparatus of clause 14 or clause 15, in which each encoding level defines at least a base minimum compression ratio and a scaling factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio and the scaling factor.

17. The apparatus of clause 16, in which each encoding level defines at least a high bit rate factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio, the scaling factor and the high bit rate factor.

18. Video storage, capture, transmission or reception apparatus comprising the apparatus of any one of clauses 14 to 17.

19. A method comprising: buffering a portion of input data in a coded picture buffer, the coded picture buffer having a coded picture buffer size, the input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and 00 a still picture having a still picture size; providing a buffered portion of the input data for decoding; decoding the input data to generate decoded data; and the decoding step being responsive to parameter data associated with the input data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size of the decoded data and a minimum compression ratio, at least some of the encoding levels being applicable to input data representing a video data stream and at least some of the encoding levels being applicable to input data representing a still picture; in which, for operation in accordance with a given encoding level applicable to a video data stream, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of the input data up to a size of one of the video pictures as compressed by the minimum compression ratio defined by the given encoding level; and in which, for operation in accordance with a given encoding level applicable to encoding of a still picture, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of a luminance component of the input data up to the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.

20. The method of clause 19, in which the portions are portions of data representing entire pictures.

21. The method of clause 19 or clause 20, in which each encoding level defines at least a base minimum compression ratio and a scaling factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio and the scaling factor.

22. The method of clause 21, in which each encoding level defines at least a high bit rate factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio, the scaling factor and the high bit rate factor.

23. Computer software which, when executed by a computer, causes the computer to carry out the method of any one of clauses 19 to 22.

24. A machine-readable non-transitory storage medium which stores the computer software of clause 23.

25. A data signal representing one or more video or still pictures, the data signal being encoded by the method of any one of clauses 7 to 11.

26. A machine-readable non-transitory storage medium which stores the data signal of clause 25. 15

Claims

CLAIMS1. Apparatus comprising: a picture data encoder configured selectively to encode input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and (ii) a still picture having a still picture size, and to generate respective output data; the picture data encoder being responsive to encoding constraints defined by parameter data associated with the output data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size and a minimum compression ratio, at least some of the encoding levels being applicable to encoding of a video data stream and at least some of the encoding levels being applicable to encoding of a still picture; in which, for a given encoding level applicable to encoding of a video data stream, the picture data encoder is configured to encode successive video pictures of a video stream so that a data rate of the respective output data is no greater than a data rate of the respective video data stream as compressed by the minimum compression ratio defined by the given encoding level; and in which, for a given encoding level applicable to encoding of a still picture, the picture data encoder is configured to encode a still picture so that a data quantity of the respective luminance output data is no greater than a data quantity defined by the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.
2. The apparatus of claim 1, comprising: a coded picture buffer configured to buffer successive portions of the output data generated by the picture data encoder, the coded picture buffer having a coded picture buffer size of at least a minimum size defined by the encoding level applicable to the output data.
3. The apparatus of claim 2, in which each portion is a portion of data representing an entire picture.
4. The apparatus of claim 1, in which each encoding level defines at least a base minimum compression ratio and a scaling factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio and the scaling factor.
5. The apparatus of claim 4, in which each encoding level defines at least a high bit rate factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio, the scaling factor and the high bit rate factor.
6. Video storage, capture, transmission or reception apparatus comprising the apparatus of claim 1.
7. A method comprising: encoding an input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and (ii) a still picture having a still picture size, to generate respective output data, in response to parameter data associated with the output data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size and a minimum compression ratio, at least some of the encoding levels being applicable to encoding of a video data stream and at least some of the encoding levels being applicable to encoding of a still picture; in which, for a given encoding level applicable to encoding of a video data stream, the encoding step comprises encoding successive video pictures of a video stream so that a data rate of the respective output data is no greater than a data rate of the respective video data stream as compressed by the minimum compression ratio defined by the given encoding level; and in which, for a given encoding level applicable to encoding of a still picture, the encoding step comprises encoding a still picture so that a data quantity of the respective luminance output data is no greater than a data quantity defined by the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.
8. The method of claim 7, comprising: buffering the encoded output data in a coded picture buffer configured to buffer successive portions of the output data generated by the encoding step, the coded picture buffer having a coded picture buffer size of at least a minimum size defined by the encoding level applicable to the output data.
9. The method of claim 8, in which each portion is a portion of data representing an entire picture.
10. The method of claim 7, in which each encoding level defines at least a base minimum compression ratio and a scaling factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio and the scaling factor.
11. The method of claim 10, in which each encoding level defines at least a high bit rate factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio, the scaling factor and the high bit rate factor.
12. Computer software which, when executed by a computer, causes the computer to carry out the method of claim 7.
13. A machine-readable non-transitory storage medium which stores the computer software of claim 12.
14. Apparatus comprising: a picture data decoder configured selectively to decode input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and 00 a still picture having a still picture size and to generate decoded data; and a coded picture buffer to buffer successive portions of input data and to provide a portion to the picture data decoder for decoding, the coded picture buffer having a coded picture buffer size; the picture data decoder being responsive to parameter data associated with the input data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size of the decoded data and a minimum compression ratio, at least some of the encoding levels being applicable to input data representing a video data stream and at least some of the encoding levels being applicable to input data representing a still picture; in which, for operation in accordance with a given encoding level applicable to a video data stream, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of the input data up to a size of one of the video pictures as compressed by the minimum compression ratio defined by the given encoding level; and in which, for operation in accordance with a given encoding level applicable to encoding of a still picture, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of a luminance component of the input data up to the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.
15. The apparatus of claim 14, in which the portions are portions of data representing entire pictures.
16. The apparatus of claim 14, in which each encoding level defines at least a base minimum compression ratio and a scaling factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio and the scaling factor.
17. The apparatus of claim 16, in which each encoding level defines at least a high bit rate factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio, the scaling factor and the high bit rate factor.
18. Video storage, capture, transmission or reception apparatus comprising the apparatus of claim 14
19. A method comprising: buffering a portion of input data in a coded picture buffer, the coded picture buffer having a coded picture buffer size, the input data representing one of (i) a video data stream defining successive video pictures at a video stream data rate and 00 a still picture having a still picture size; providing a buffered portion of the input data for decoding; decoding the input data to generate decoded data; and the decoding step being responsive to parameter data associated with the input data, the parameter data indicating at least an encoding level selected from a plurality of encoding levels, each encoding level defining at least a maximum luminance picture size of the decoded data and a minimum compression ratio, at least some of the encoding levels being applicable to input data representing a video data stream and at least some of the encoding levels being applicable to input data representing a still picture; in which, for operation in accordance with a given encoding level applicable to a video data stream, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of the input data up to a size of one of the video pictures as compressed by the minimum compression ratio defined by the given encoding level; and in which, for operation in accordance with a given encoding level applicable to encoding of a still picture, the coded picture buffer size is configured so that the coded picture buffer is capable of storing a portion of a luminance component of the input data up to the maximum luminance picture size of the given encoding level as compressed by the minimum compression ratio defined by the given encoding level.
20. The method of claim 19, in which the portions are portions of data representing entire pictures.
21. The method of claim 19, in which each encoding level defines at least a base minimum compression ratio and a scaling factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio and the scaling factor.
22. The method of claim 21, in which each encoding level defines at least a high bit rate factor, the minimum compression ratio being dependent upon a combination of the base minimum compression ratio, the scaling factor and the high bit rate factor.
23. Computer software which, when executed by a computer, causes the computer to carry out the method of claim 19.
24. A machine-readable non-transitory storage medium which stores the computer software of claim 23.
25. A data signal representing one or more video or still pictures, the data signal being encoded by the method of claim 7.
26. A machine-readable non-transitory storage medium which stores the data signal of claim 25.