DATA ENCODING AND DECODING
CROSS REFERENCE TO RELATED APPLICATIONS
The present application claims the benefit of the earlier filing date of GB1 3 17041 .0 filed
in the United Kingdom Intellectual Property Office on 25 September 2013, the entire contents of
which application are incorporated herein by reference.
Field of the Disclosure
This disclosure relates to data encoding and decoding.
Description of the 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, are neither expressly or impliedly admitted as
prior art against the present disclosure.
There are several video data compression and decompression systems which involve
transforming video data into a frequency domain representation, quantising the frequency
domain coefficients and then applying some form of entropy encoding to the quantised
coefficients.
The transformation into the spatial frequency domain at the encoder side corresponds to
an inverse transformation at the decoder side. Example transformations include the so-called
discrete cosine transformation (DCT) and the so-called discrete sine transformation (DST). In
some examples the transformations are carried out by matrix-multiplying an array of input
samples (derived from the video data to be coded) by a matrix of transformation coefficients to
generate frequency-transformed data. Frequency-transformed data is converted back to
sample data, from which output video data can be derived, by matrix-multiplying an array of the
frequency-transformed data by a matrix of inverse-transformation coefficients.
Some standards and draft standards, such as the so-called High Efficiency Video
Coding (HEVC) standards, define encoding and decoding modes in which a spatial frequency
transformation is not in fact used. These are sometimes referred to as "transform-skip" or
"trans-quant skip" ("trans-quant" being an abbreviation for "transformation and quantisation")
modes.
Summary
An aspect of this disclosure is defined by claim 1.
Further respective aspects and features are defined in the appended claims.
It is to be understood that both the foregoing general description and the following
detailed description are exemplary, but not restrictive of, the present disclosure.
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 of embodiments, 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 4a schematically illustrates a video camera using video data compression;
Figure 4b schematically illustrates an example video camera in more detail;
Figure 4c schematically illustrates another example video camera;
Figures 4d and 4e schematically illustrate data carriers;
Figure 5 provides a schematic overview of a video data compression and
decompression apparatus;
Figure 6 schematically illustrates the generation of predicted images;
Figure 7 schematically illustrates a largest coding unit (LCU) ;
Figure 8 schematically illustrates a set of four coding units (CU) ;
Figures 9 and 10 schematically illustrate the coding units of Figure 8 sub-divided into
smaller coding units;
Figure 11 schematically illustrates an array of prediction units (PU) ;
Figure 1 schematically illustrates an array of transform units (TU) ;
Figure 13 schematically illustrates the use of scaling lists;
Figure 14 schematically illustrates a part of an encoder and/or decoder;
Figure 15 schematically illustrates an array of samples;
Figure 16 schematically illustrates an array of frequency-separated coefficients;
Figure 17 schematically illustrates a trend within the array of Figure 16 ;
Figure 18 schematically illustrates a reverse scan ;
Figure 19 schematically illustrates a trend within a scaling list;
Figure 20 schematically illustrates an image aberration ;
Figure 21 is a schematic flowchart illustrating part of an encoding operation ;
Figure 22 is a schematic flowchart illustrating part of a decoding operation ;
Figure 23 schematically illustrates a part of an encoder and/or decoder;
Figure 24 is a schematic flowchart illustrating a part of an encoding and/or decoding
operation ;
Figure 25 schematically illustrates a part of an encoder and/or decoder; and
Figure 26 is a schematic flowchart illustrating a part of an encoding and/or decoding
operation.
Description of the Embodiments
Referring now to the drawings, Figures 1-4e are provided to give schematic illustrations
of apparatus or systems making use of the compression and/or decompression apparatus to be
described below in connection with embodiments.
All of the data compression and/or decompression apparatus is to be described below
may be implemented in hardware, in software running on a general-purpose data processing
apparatus such as a general-purpose computer, as programmable hardware such as an
application specific integrated circuit (ASIC) or field programmable gate array (FPGA) or as
combinations of these. In cases where the embodiments are implemented by software and/or
firmware, it will be appreciated that such software and/or firmware, and non-transitory machinereadable
data storage media by which such software and/or firmware are stored or otherwise
provided, are considered as embodiments.
Figure 1 schematically illustrates an audio/video data transmission and reception system
using video data compression and decompression.
An input audio/video signal 10 is supplied to a video data compression apparatus 20
which compresses at least the video component of the audio/video signal 10 for transmission
along a transmission route 30 such as a cable, an optical fibre, a wireless link or the like. The
compressed signal 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 might be provided as (for example) a so-called
set top box (STB), noting that the expression "set-top" does not imply a requirement for the box
to be sited in any particular orientation or position with respect to the display 120; it is simply a
term used in the art to indicate a device which is connectable to a display as a peripheral
device.
Figure 3 schematically illustrates an audio/video storage system using video data
compression and decompression. An input audio/video signal 130 is supplied to a compression
apparatus 140 which generates a compressed signal for storing by a store device 150 such as a
magnetic disk device, an optical disk device, a magnetic tape device, a solid state storage
device such as a semiconductor memory or other storage device. For replay, compressed data
is read from the store device 150 and passed to a decompression apparatus 160 for
decompression to provide an output audio/video signal 170.
It will be appreciated that the compressed or encoded signal, and a storage medium or
data carrier storing that signal, are considered as embodiments. Reference is made to Figures
4d and 4e described below.
Figure 4a schematically illustrates a video camera using video data compression. In
Figure 4a, and image capture device 180, such as a charge coupled device (CCD) image
sensor and associated control and read-out electronics, generates a video signal which is
passed to a compression apparatus 190. A microphone (or plural microphones) 200 generates
an audio signal to be passed to the compression apparatus 190. The compression apparatus
190 generates a compressed audio/video signal 2 1 to be stored and/or transmitted (shown
generically as a schematic stage 220).
The techniques to be described below relate primarily to video data compression. It will
be appreciated that many existing techniques may be used for audio data compression in
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 1-4e) 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 4b schematically illustrates an example video camera apparatus 183 in more
detail. Those features numbered in common with Figure 4a will not be described further.
Figure 4b is an example of the camera of Figure 4a (in the case that the unit 220 of Figure 4a
provides a storage capability) in which the compressed data are first buffered by a buffer 221
and then stored in a storage medium 222 such as a magnetic disk, an optical disk, flash
memory, a so-called solid-state disk drive (SSD) or the like. Note that the arrangement of Figure
4b can be implemented as a single (physical) unit 182.
Figure 4c schematically illustrates another example video camera in which, in place of
the storage arrangement of Figure 4b, a network interface 223 is provided in order to allow the
compressed data to be transmitted to another unit (not shown). The network interface 223 can
also allow for incoming data to be received by the video camera, such as control data. Note
that the arrangement of Figure 4b can be implemented as a single (physical) unit 183.
Figures 4d and 4e schematically illustrate data carriers, for example for use as the
storage medium 222 and carrying compressed data which has been compressed according to
the compression techniques described in the present application. Figure 4d shows a schematic
example of a removable non-volatile storage medium 225 implemented as solid state memory
such as flash memory. Figure 4e shows a schematic example of a removable non-volatile
storage medium 226 implemented as a disk medium such as an optical disk.
Figure 5 provides a schematic overview of a video data compression and
decompression apparatus.
Successive images of an input video signal 300 are supplied to an adder 310 and to an
image predictor 320. The image predictor 320 will be described below in more detail with
reference to Figure 6. The adder 3 10 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 projected images.
One reason why a residual image signal is generated is as follows. The data coding
techniques to be described, that is to say the techniques which will be applied to the residual
image signal, tends to work more efficiently when there is less "energy" in the image to be
encoded. Here, the term "efficiently" refers to the generation of a small amount of encoded data;
for a particular image quality level, it is desirable (and considered "efficient") to generate as little
data as is practicably possible. The reference to "energy" in the residual image relates to the
amount of information contained in the residual image. If the predicted image were to be
identical to the real image, the difference between the two (that is to say, the residual image)
would contain zero information (zero energy) and would be very easy to encode into a small
amount of encoded data. In general, if the prediction process can be made to work reasonably
well, the expectation is that the residual image data will contain less information (less energy)
than the input image and so will be easier to encode into a small amount of encoded data.
The residual image data 330 is supplied to a transform unit 340 which generates a
discrete cosine transform (DCT) representation of the residual image data. The DCT technique
itself is well known and will not be described in detail here. There are however aspects of the
techniques used in the present apparatus which will be described in more detail below.
Note that in some embodiments, a discrete sine transform (DST) is used instead of a
DCT. In other embodiments, no transform might be used. This can be done selectively, so that
the transform stage is, in effect, bypassed, for example under the control of a "transform-skip"
command or mode.
The output of the transform unit 340, which is to say, a set of transform 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 work more efficiently in generating
small amounts of compressed video data.
A controller 345 controls the operation of the transform unit 340 and the quantiser 350
(and their respective inverse units), according to techniques to be discussed further below.
Note that the controller 345 may also control other aspects of the operation of the apparatus of
Figure 5.
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 zigzag scanning order.
The scanned coefficients are then passed to an entropy encoder (EE) 370. Again,
various types of entropy encoding may be used. Two examples 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 (or processing of data by) the entropy encoder can take place in the scan order.
Corresponding considerations apply to the respective inverse processes.
The output of the entropy encoder 370, along with additional data, for example defining
the manner in which the predictor 320 generated the predicted image, provides a compressed
output video signal 380.
However, a return path is also provided because the operation of the predictor 320 itself
depends upon a decompressed version of the compressed output data.
The reason for this feature is as follows. At the appropriate stage in the decompression
process a decompressed version of the residual data is generated. This decompressed residual
data has to be added to a predicted image to generate an output image (because the original
residual data was the difference between the input image and a predicted image). In order that
this process is comparable, as between the compression side and the decompression side, the
predicted images generated by the predictor 320 should be the same during the compression
process and during the decompression process. Of course, at decompression, the apparatus
does not have access to the original input images, but only to the decompressed images.
Therefore, at compression, the predictor 320 bases its prediction (at least, for inter-image
encoding) on decompressed versions of the compressed images.
The entropy encoding process carried out by the entropy encoder 370 is considered to
be "lossless", which is to say that it can be reversed to arrive at exactly the same data which
was first supplied to the entropy encoder 370. So, the return path can be implemented before
the entropy encoding stage. Indeed, the scanning process carried out by the scan unit 360 is
also considered lossless, but in the present embodiment the return path 390 is from the output
of the quantiser 350 to the input of a complimentary inverse quantiser 420.
In general terms, an entropy decoder 410, the reverse scan unit 400, an inverse
quantiser 420 and an inverse transform unit 430 provide the respective inverse functions of the
entropy encoder 370, the scan unit 360, the quantiser 350 and the transform unit 340. For now,
the discussion will continue through the compression process; the process to decompress an
input compressed video signal corresponds to the return path of the compression process and
so a decoding apparatus or method corresponds to the features or operation of the decoding
path of the encoder described here.
In the compression process, the scanned coefficients are passed by the return path 390
from the quantiser 350 to the inverse quantiser 420 which carries out the inverse operation of
the scan unit 360. An inverse quantisation and inverse transformation process are carried out
by the units 420, 430 to generate a compressed-decompressed residual image signal 440.
The image signal 440 is added, at an adder 450, to the output of the predictor 320 to
generate a reconstructed output image 460. This forms one input to the image predictor 320.
Turning now to the process applied to a received compressed video signal 470, the
signal is supplied to the entropy decoder 4 10 and from there to the chain of the reverse scan
unit 400, the inverse quantiser 420 and the inverse transform unit 430 before being added to the
output of the image predictor 320 by the adder 450. In straightforward terms, the output 460 of
the adder 450 forms the output decompressed video signal 480. In practice, further filtering may
be applied before the signal is output.
Accordingly, Figure 5 provides an example of video data decoding apparatus operable
to decode an array of encoded video data values, the apparatus comprising: a dequantiser
configured to dequantise the array of encoded video data values by applying a dequantisation
parameter to each data value so as to generate corresponding dequantised data values; and an
inverse frequency transformer configured to apply an inverse frequency transform to the
dequantised data values. Figure 5 also provides an example of video data encoding apparatus
operable to encode an array of input video data values in a transform-skip mode and optionally
a non transform-skip mode (discussed further below), the apparatus comprising: a frequency
transformer configured to apply a frequency transform to the input video data values to generate
an array of frequency transformed input data values; and a quantiser configured to quantise the
frequency transformed input data values by applying a quantisation parameter to each
frequency transformed input data value so as to generate corresponding quantised data values.
Note that the term "optionally" is used to indicate that the apparatus may be operable in a
transform-skip mode and a non-transform-skip mode, with the selection being made on a block
by block (such as TU-by-TU) or other basis, or may be an apparatus which is not operable in
the non-transform-skip mode.
Figure 6 schematically illustrates the generation of predicted images, and in particular
the operation of the image predictor 320.
There are two basic modes of prediction: so-called intra-image prediction and so-called
inter-image, or motion-compensated (MC), prediction.
Intra-image prediction bases a prediction of the content of a block of the image on data
from within the same image. This corresponds to so-called l-frame encoding in other video
compression techniques. In contrast to l-frame encoding, where the whole image is intraencoded,
in the present embodiments the choice between intra- and inter- encoding can be
made on a block-by-block basis, though in other embodiments the choice is still made on an
image-by-image basis.
Motion-compensated prediction makes use of motion information which attempts to
define the source, in another adjacent or nearby image, of image detail to be encoded in the
current image. Accordingly, in an ideal example, the contents of a block of image data in the
predicted image can be encoded very simply as a reference (a motion vector) pointing to a
corresponding block at the same or a slightly different position in an adjacent image.
Returning to Figure 6, two image prediction arrangements (corresponding to intra- and
inter-image prediction) are shown, the results of which are selected by a multiplexer 500 under
the control of a mode signal 510 so as to provide blocks of the predicted image for supply to the
adders 310 and 450. The choice is made in dependence upon which selection gives the lowest
"energy" (which, as discussed above, may be considered as information content requiring
encoding), and the choice is signalled to the encoder within the encoded output data stream.
Image energy, in this context, can be detected, for example, by carrying out a trial subtraction of
an area of the two versions of the predicted image from the input image, squaring each pixel
value of the difference image, summing the squared values, and identifying which of the two
versions gives rise to the lower mean squared value of the difference image relating to that
image area.
The actual prediction, in the intra-encoding system, is made on the basis of image
blocks received as part of the signal 460, which is to say, the prediction is based upon encodeddecoded
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.
The processing applied to the signal 460 will now be described. Firstly, the signal is
filtered by a filter unit 560. This involves applying a "deblocking" filter to remove or at least tend
to reduce the effects of the block-based processing carried out by the transform unit 340 and
subsequent operations. Also, an adaptive loop filter is applied using coefficients derived by
processing the reconstructed signal 460 and the input video signal 300. The adaptive loop filter
is a type of filter which, using known techniques, applies adaptive filter coefficients to the data to
be filtered. That is to say, the filter coefficients can vary in dependence upon various factors.
Data defining which filter coefficients to use is included as part of the encoded output data
stream.
The filtered output from the filter unit 560 in fact forms the output video signal 480. It is
also buffered in one or more image stores 570; the storage of successive images is a
requirement of motion compensated prediction processing, and in particular the generation of
motion vectors. To save on storage requirements, the stored images in the image stores 570
may be held in a compressed form and then decompressed for use in generating motion
vectors. For this particular purpose, any known compression / decompression system may be
used. The stored images are passed to an interpolation filter 580 which generates a higher
resolution version of the stored images; in this example, intermediate samples (sub-samples)
are generated such that the resolution of the interpolated image is output by the interpolation
filter 580 is 8 times (in each dimension) that of the images stored in the image stores 570. The
interpolated images are passed as an input to the motion estimator 550 and also to the motion
compensated predictor 540.
In embodiments, a further optional stage is provided, which is to multiply the data values
of the input video signal by a factor of four using a multiplier 600 (effectively just shifting the
data values left by two bits), and to apply a corresponding divide operation (shift right by two
bits) at the output of the apparatus using a divider or right-shifter 610. So, the shifting left and
shifting right changes the data purely for the internal operation of the apparatus. This measure
can provide for higher calculation accuracy within the apparatus, as the effect of any data
rounding errors is reduced.
The way in which an image is partitioned for compression processing will now be
described. At a basic level, and image to be compressed is considered as an array of blocks of
samples. For the purposes of the present discussion, the largest such block under consideration
is a so-called largest coding unit (LCU) 700 (Figure 7), which represents a square array of 64 x
64 samples. Here, the discussion relates to luminance samples. Depending on the chrominance
mode, such as 4:4:4, 4:2:2, 4:2:0 or 4:4:4:4 (GBR plus key data), there will be differing numbers
of corresponding chrominance samples corresponding to the luminance block.
Three basic types of blocks will be described: coding units, prediction units and
transform units. In general terms, the recursive subdividing of the LCUs allows an input picture
to be partitioned in such a way that both the block sizes and the block coding parameters (such
as prediction or residual coding modes) can be set according to the specific characteristics of
the image to be encoded.
The LCU may be subdivided into so-called coding units (CU). Coding units are always
square and have a size between 8x8 samples and the full size of the LCU 700. The coding units
can be arranged as a kind of tree structure, so that a first subdivision may take place as shown
in Figure 8, giving coding units 710 of 32x32 samples; subsequent subdivisions may then take
place on a selective basis so as to give some coding units 720 of 16x16 samples (Figure 9) and
potentially some coding units 730 of 8x8 samples (Figure 10). Overall, this process can provide
a content-adapting coding tree structure of CU blocks, each of which may be as large as the
LCU or as small as 8x8 samples. Encoding of the output video data takes place on the basis of
the coding unit structure.
Figure 11 schematically illustrates an array of prediction units (PU). A prediction unit is a
basic unit for carrying information relating to the image prediction processes, or in other words
the additional data added to the entropy encoded residual image data to form the output video
signal from the apparatus of Figure 5. In general, prediction units are not restricted to being
square in shape. They can take other shapes, in particular rectangular shapes forming half of
one of the square coding units, as long as the coding unit is greater than the minimum (8x8)
size. The aim is to allow the boundary of adjacent prediction units to match (as closely as
possible) the boundary of real objects in the picture, so that different prediction parameters can
be applied to different real objects. Each coding unit may contain one or more prediction units.
Figure 1 schematically illustrates an array of transform units (TU). A transform unit is a
basic unit of the transform and quantisation process. Transform units are always square and
can take a size from 4x4 up to 32x32 samples. Each coding unit can contain one or more
transform units. The acronym SDIP-P in Figure 12 signifies a so-called short distance intraprediction
partition. In this arrangement only one dimensional transforms are used, so a 4xN
block is passed through N transforms with input data to the transforms being based upon the
previously decoded neighbouring blocks and the previously decoded neighbouring lines within
the current SDIP-P.
In the arrangements discussed above, quantisation is applied to the frequency
separated coefficients, for example as a division of the coefficient magnitudes (the coefficient
signs being preserved separately) in a transform unit (TU) by a quantisation divisor qStep
derived from a quantisation parameter applicable to that block, Qp, according to:
qStep = baseQStep[Qp % 6] « int (Qp/6)
Here, the percentage sign % indicates a modulo function, so that A % B is equal to the
remainder when A is divided by B. The « sign indicates a left bit-shift by the value given after
the « sign, so that, for example, « 3 represents a left shift by three bit positions. The variable
baseQStep is a function of an index, varying between 0 and 5, represented by Qp % 6. So, a
(potentially) different value of baseQStep applies according to the relevant index Qp % 6. The
function int indicates an integer value.
For ease of calculation, the reciprocal of baseQStep is pre-calculated in some
embodiments of an encoder, and left-shifted by a number of bits such as 14 bits, to give values
of inverseQStep which are then multiplied by the coefficient magnitudes. A reason for this is that
multiplication operations are more straightforward to implement in some arrangements than
division operations.
This gives a combined operation as follows:
output = ((input x inverseQStep) + rounding) » qBits
where qBits = 14 + int (Qp/6) + transformShift
Here, transformShift represents a compensation for any additional shifting imposed by
the transform process.
In some embodiments, so-called scaling lists are used as a mechanism to further vary
the degree of quantisation applicable to different coefficients in the TU. Each coefficient position
is associated with a corresponding entry in the scaling list. The scaling list value is used to alter
the quantisation divisor. In some embodiments, a scaling list value of decimal 16 corresponds to
no change, which is to say that it leaves the divisor unchanged. In some embodiments, the
scaling list values are used to modify the value inverseQStep used in the equations above, so
that:
inverseQStep = (inverseQStep « 4) / scaling list value
A variable "rounding" was introduced above. Before the right-shift by qBits, a value is
added in order to round the final result. Rounding can be applied by a number of methods such
as rounding to the nearest integer or rounding to the next higher integer.
Figure 13 schematically illustrates the use of scaling lists as discussed above.
Transformed coefficients 800 are received by the quantiser 350 from the transform unit 340. An
appropriate scaling list 810 (for example, selected to correspond to the block size or other
parameters of the image compression) is used.
As mentioned above, there is a one-to-one correspondence between entries in the
scaling list 8 10 and transformed coefficients. An address generator 820 provides addresses so
as to look up each transformed coefficient and the corresponding scaling list entry. These are
passed to a unit schematically referred to as a divider 830, along with other quantisation
parameters (such as those discussed above). The divisor 830 carries out the calculation is
shown above using the scaling list entries and the other parameters to generate quantised data
840. Note that the divider may in fact carry out a multiplication operation based on reciprocals of
quantisation parameters, but the overall effect is the same as the division discussed above.
Figure 14 schematically illustrates a part of an encoder and/or decoder, and in particular
corresponds to a portion of the arrangement shown in Figure 5 and discussed above.
Corresponding scaling lists are provided to the quantisation unit 350 and to the inverse
quantisation unit 420 (which may be part of the return path of the encoder or may be part of a
stand-alone decoder). At the decoding stage, the appropriate scaling list may be selected as a
predetermined function of other encoding parameters or in response to data in or associated
with the compressed data stream which defines the appropriate scaling list to use.
Figure 15 schematically illustrates an array of samples. These in fact represent samples
of the residual image 330 discussed above, and are drawn at their appropriate relative spatial
positions, which is to say that a sample 900 lies at the upper left corner of an array of 4 x 4
adjacent samples, and a sample 910 lies at the bottom right corner of such an array, with
respect to positions of those samples within an image.
It will of course be appreciated that the 4 x 4 array of Figure 15 is just an example; the
techniques and attributes discussed here can apply to arrays of various different sizes such as
8 x 8, 16 x 16, 32 x 32 and so on. Indeed, although a 4 x 4 array is illustrated in Figures 15-19
primarily to make the drawings simpler and easier to understand, in some embodiments the
array sizes for which the techniques to be described below are applied are 8 x 8 , 16 x 16 and 32
x 32 only. In other words, in those embodiments, the techniques to be described below are not
in fact applied to 4 x 4 arrays. Nevertheless, it will be appreciated that a description of a 4 x 4
array is used here for clarity of the explanation. Note that the array size can refer to the size of
an array of encoded data values (that is, in this context, data values at the input to a
dequantiser stage) or an array of frequency transformed input data values (that is, in this
context, data values at the input to a quantiser stage).
The samples of Figure 15 are processed by the transform unit 340 to generate
frequency-separated coefficients. Figure 16 schematically illustrates an array of such
frequency-separated coefficients. Here, the position of a coefficient within the array represents
the spatial frequencies corresponding to that coefficient. By convention, a so-called DC
coefficient 920 occupies the upper-left array position. Moving towards the right within the array
of Figure 16 indicates an increasing horizontal frequency component, and moving towards the
bottom of the array of Figure 16 indicates an increasing vertical frequency component. Note that
although it is just a convention to represent the array of coefficients in this manner (rather than,
for example, having the DC coefficient in the bottom-right corner), the ordering of the
coefficients is technically relevant to other parts of the process. One reason for this is
schematically illustrated by a broken-line arrow 930 in Figure 16, which indicates that moving
from the bottom-right to the top-left positions within the array of Figure 16 corresponds to a
generally increasing importance of the coefficients to image reconstruction. That is to say, in
order to reconstruct the array or block of samples of Figure 15, the most important one of the
coefficients of Figure 16 is the DC coefficient 920, followed in terms of importance by the lower
horizontal and vertical frequency coefficients.
In general terms, this trend of importance can also correspond to a trend in terms of
coefficient magnitude, in that the magnitude of the DC coefficient can tend to be the largest
within the set of coefficients derived from a block of samples. Figure 17 schematically illustrates
such a trend within the array of Figure 16, in which smaller values tend to be towards the lower
right of the array and larger values tend to be towards the upper left of the array. Of course, a
specific individual array of coefficients may differ from this general trend.
One way in which the trends discussed above are technically relevant relates to the
entropy encoding stage carried out by the entropy encoder 370 in Figure 5 . A general principle
applied to this type of entropy encoding is that it operates more efficiently by dealing with
smaller-magnitude data values within a block before dealing with larger-magnitude data values.
Accordingly, in the case that frequency separated coefficients are subjected to entropy
encoding (whether or not an intervening quantisation stage is used) a so-called "reverse scan"
pattern is used to select data in an appropriate order for entropy encoding. Figure 18
schematically illustrates an example of a reverse scan. This example relates to a so-called
reverse-diagonal scan which starts with the coefficient 940 in the lower-right corner of the array
of coefficients and progresses up to the DC coefficient 920 according to the diagonal scanning
pattern shown. This pattern means that, generally speaking, smaller value coefficients are
encoded before larger value coefficients.
The trend in importance also relates to the scaling list values, and indeed this is one
purpose of the scaling list, to allow for different quantisation to be applied at different coefficient
positions within an array. In general terms, as shown schematically in Figure 19, scaling list
values (one value or entry for each array position in the video data to be quantised or
dequantised) tend to be smallest at the array position corresponding to the DC coefficient, and
tends to get larger with increasing horizontal and vertical spatial frequency. Note that a smaller
scaling list value corresponds to a less-harsh quantisation.
These techniques can operate successfully in respect of frequency-transformed data.
However, if a frequency-transform is not used, for example in a so-called "transform-skip" mode
(in which, as the name suggests, frequency transformation is not employed) then the data within
an array of values to be quantised will not exhibit the trends in either importance or numerical
value discussed above, and nor will any variation in the harshness of quantisation be smoothed
out across the reconstructed image data by an inverse transform process. Instead, the scaling
lists simply cause a periodic pattern of variable quantisation to be imposed on the image. In a
transform-skip block, less harsh quantisation will be applied to the upper left portion and more
harsh quantisation will be applied to the lower right portion, but in terms of the reconstructed
image, all of the image positions within the transform-skip block are equally significant and they
all corresponds to respective image positions in the reconstructed image. So, as a schematic
sample, Figure 20 schematically shows a type of aberration which may be seen if scaling lists
are used with data which has not been frequency transformed. Each block exhibits a higher
image quality towards its upper left corner, leading to a "fish scale" effect as illustrated.
Various options are provided within the present embodiments to address this problem.
For example, in some embodiments, in a transform-skip mode, the use of scaling lists is
inhibited or prevented so that the quantisation or dequantisation values are independent of the
position of each data value within the array. For example, a decoder may comprise a detector
(for example, implemented by the controller 345) configured to detect whether a transform-skip
mode is applicable to the array of encoded video data values; in which: if the detector detects
that the transform-skip mode is not applicable to the array of encoded video data values, the
detector controls the dequantiser to apply dequantisation parameters which may vary between
data values in the array of encoded data values according to the position of each data value
within the array, and controls the inverse frequency transformer to apply an inverse frequency
transform to the dequantised data values; and if the detector detects that the transform-skip
mode is applicable to the array of encoded video data values, the detector controls the
dequantiser to apply dequantisation parameters which, for each array of encoded data values,
are independent of the position of each data value within the array, and controls the inverse
frequency transformer not to apply an inverse frequency transform to the dequantised data
values. On the encoder side, if the transform-skip mode is not applicable to the array of input
video data values, the controller 345 controls the frequency transformer to apply the frequency
transformation and controls the quantiser to apply quantisation parameters which may vary
between data values in the array of frequency transformed input data values according to the
position of each frequency transformed input data value within the array; and if the transformskip
mode is applicable to the array of encoded video data values, the controller 345 controls
the frequency transformer not to apply the frequency transformation and controls the quantiser
to quantise the input video data values by applying quantisation parameters which, for each
array of input data values, are independent of the position of each input data value within the
array.
In some embodiments, instead of using a scaling list representing different values
corresponding to different coefficient positions, either a scaling list is used comprising all the
same value (that is to say, each scaling list entry in the scaling list has the same numerical
value) or a single value is provided in place of a scaling list, and that single value is used in
place of the scaling list entry at the appropriate calculation for each coefficient. For example,
such a single value (or similar) can be used in a transform-skip mode. As a potential
modification of these embodiments, the single value (whether sent or stored as an individual
value or as multiple instances of the same value in an equivalent of a scaling list) may be a
different single value dependent upon, for example, array size or video channel.
An example of a scaling list comprising all the same, identical to one another, value is
shown below:
Such a transform-skip mode scaling list may be selected from a set of two or more
different transform-skip mode scaling lists (for example, by the controller 345) according to one
or more encoding parameters of the encoded video data. Alternatively, the transform-skip
mode scaling list may be provided in a data stream containing the array of encoded data values.
Alternatively the controller may derive the value of the transform scaling list from one or more
values of the scaling list which would be applicable for use in respect of that array of data
values if the transform-skip mode were not selected, such as one or more values at respective
preselected locations in that scaling list.
First, as background to the discussions which follow, Figure 2 1 is a schematic flowchart
illustrating part of an encoding operation, and Figure 22 is a schematic flowchart illustrating a
corresponding part of a decoding operation (features of which also apply to the reverse
decoding path of an encoder).
Referring to Figure 2 1, if, at a step 1000, a transform-skip mode is enabled (that is to
say, such a mode is allowed within the context of the current encoding operation) then at a step
101 0 a "transform-skip enabled" flag is associated with the video data stream. In one example,
such a flag may be provided once per frame. If the transform-skip enabled flag is set, then the
encoder will provide (and the decoder will look for) a TU-by-TU flag indicating whether
transform-skip has been applied to that TU. If the transform-skip enabled flag is not set, then the
encoder will not provide (and the decoder will not look for) the TU-by-TU flag.
Note that in some encoders, the transform-skip mode may be compulsory under the
design parameters of that particular encoder. In other encoders, a detector (for example,
implemented by the controller 345) may be configured to detect whether a transform-skip mode
is applicable to a current array of input video data values.
At a step 1020, the controller 345 determines, for each TU, whether or not to use
transform-skip. In some examples, this can be by testing the transform-skip mode and other
modes using transforms, and selecting a mode in dependence upon an assessment of a cost
function, which may relate to one or more of the data quantity generated in respect of that mode
and the data errors generated in respect of that mode. If the transform-skip mode is selected
then control passes to a step 1030 at which the TU is encoded using the transform-skip mode,
and then to a step 1040 at which the TU-by-TU flag discussed above is set to indicate that the
transform-skip mode has been used.
On the other hand, if at the step 1000 the transform-skip mode is not enabled, or if at the
step 1020 the transform-skip mode is not selected for a particular TU, then encoding of a TU
takes place at a step 1050 using a frequency transform.
Both the steps 1040 and 1050 pass control to a final step 1060 at which the output data
stream is formed using the encoded data and, if appropriate, the flags and/or the scaling list
values and/or single values.
The corresponding decoder operations are schematically illustrated in Figure 22. A step
1100 detects whether the transform-skip mode is enabled by examining the most recent
instance of the transform-skip enabled flag. If the transform-skip mode is enabled, then at a step
1110 the TU-by-TU flag is checked for each TU. If the skip mode is selected for a particular TU
then the TU is decoded at a step 1120 without the use of an inverse transform. If, however,
either the skip mode is not selected for a TU, or (at the step 1100) the skip mode is not enabled,
then an inverse transformation is applied at a step 1130 before control is passed to the step
1120.
Figure 23 schematically illustrates a part of an encoder and/or decoder, in a similar way
to Figure 14. Indeed, many of the parts are identical and will not be described further here.
A control signal, for example from the controller 345, is provided to a multiplexer 1200
and a schematic switch 1210. The control signal indicates whether transform-skip mode is
applicable to the current TU.
If transform-skip mode is applicable, then the schematic switch 1210 is closed so as to
route data around the transform unit 340 or, in other words, to prevent the transform unit 340
from applying a frequency transform to the current block of data. Similarly, the multiplexer 1200
is controlled so that instead of a scaling list 1220, a single data value applicable to all data array
positions is provided or substituted to the quantiser 350 for use in place of the scaling list.
Accordingly, use of the scaling list is inhibited.
The single data value may be provided in a data stream containing the array of encoded
data values. Alternatively, the controller 345 may derive the single data value from one or more
values of the scaling list which would be applicable for use in respect of that array of encoded
video data values if the transform-skip mode were not selected. Purely by way of example, the
single value may be made equal to the scaling list value applicable to the top left (DC) array
value, or may be an average such as a mean of the top left four scaling list values.
Alternatively the single data value may be selected from a set of two or more different single
data values according to one or more encoding parameters of the encoded video data. For
example, the single value may be selected in dependence on a block size such as a TU size.
On the other hand, if applicable to the TU, then the schematic switch 1210 is opened so
that the transform unit 340 is enabled, and the multiplexer 1200 is controlled to pass the scaling
list 1220 rather than the single value 1230.
Similarly, at the decoding path, a corresponding schematic switch 1240 controls
operation or bypass of the inverse transform unit 430, again under the control of a control signal
from the controller 345. Another schematic multiplexer 1250 is provided so as to pass either a
scaling list 1220 or a single value 1230 applicable to the current TU to the inverse quantiser
420. As before, in a transform-skip mode, the inverse transform unit 430 is not used and the
single value 1230 is passed to the inverse quantiser 420. In a non-transform-skip mode, the
scaling list 1220 is used and the inverse transform unit 430 is enabled.
These operations are summarised by a schematic flowchart of Figure 24. If, at a step
1300, a transform-skip mode is applicable, then at a step 1310 a single value is used in place of
the scaling list for that TU. On the other hand, if at the step 1300 the transform-skip mode is not
applicable, then at a step 1320 a scaling list is used.
A similar arrangement is shown in Figure 25 and corresponding features already
described in respect of Figure 23 will not be described again. A difference between Figure 25
and Figure 23 is that the control signal from the controller 345 controls a modified quantiser 350'
and inverse quantiser 420' either to use the scaling list 1220 (in a non-transform-skip mode) or
not to use the scaling list (in a transform-skip mode), so that the use of scaling lists is thereby
inhibited. Here, not using the scaling list means simply implementing the calculations discussed
above in order to carry out quantisation, but without the contribution to (that is to say, the
variation of, sample to sample) the calculations made by the scaling list variable. The term "not
using the scaling list" in the previous sentence can of course encompass using a single value in
place of the scaling list as discussed above.
This operation is summarised in the schematic flowchart of Figure 26. If, at a step 1400,
a transform-skip mode is applicable, then at a step 1410 a scaling list is not used for that TU.
On the other hand, if at the step 1400 the transform-skip mode is not applicable, then at a step
1420 a scaling list is used.
It will be understood that although in the above discussions, the transform-skip mode is
selected on a TU-by-TU basis, the selection can be made in respect of larger blocks such as
CUs or even slices or images.
Data Signals
It will be appreciated that data signals generated by the variants of coding apparatus
discussed above, and storage or transmission media carrying such signals, are considered to
represent embodiments of the present disclosure.
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.
It will be apparent that numerous modifications and variations of the present disclosure
are possible in light of the above teachings. It is therefore to be understood that within the
scope of the appended claims, the technology may be practiced otherwise than as specifically
described herein.
Further respective features of the present disclosure are defined by the following
numbered clauses:
1. Video data decoding apparatus operable to decode an array of encoded video data
values, the apparatus comprising:
a dequantiser configured to dequantise the array of encoded video data values by
applying a dequantisation parameter to each data value so as to generate corresponding
dequantised data values;
an inverse frequency transformer configured to apply an inverse frequency transform to
the dequantised data values; and
a detector configured to detect whether a transform-skip mode is applicable to the array
of encoded video data values; in which:
if the detector detects that the transform-skip mode is not applicable to the array of
encoded video data values, the detector controls the dequantiser to apply dequantisation
parameters which may vary between data values in the array of encoded data values according
to the position of each data value within the array, and controls the inverse frequency
transformer to apply an inverse frequency transform to the dequantised data values; and
if the detector detects that the transform-skip mode is applicable to the array of encoded
video data values, the detector controls the dequantiser to apply dequantisation parameters
which, for each array of encoded data values, are independent of the position of each data
value within the array, and controls the inverse frequency transformer not to apply an inverse
frequency transform to the dequantised data values.
2. Apparatus according to clause 1, in which the dequantiser is configured to use
dequantisation parameters dependent upon respective entries in a scaling list, there being one
entry in the scaling list for each array position of the encoded video data.
3. Apparatus according to clause 1 or clause 2, in which the detector is operable to inhibit
use, by the dequantiser, of the scaling list in the case that the detector detects that that the
transform-skip mode is applicable to the array of encoded video data values.
4. Apparatus according to clause 2, in which the dequantiser is configured to use a
transform-skip mode scaling list in which all scaling list values are identical to one another in the
case that the detector detects that that the transform-skip mode is applicable to the array of
encoded video data values.
5. Apparatus according to clause 4, in which the transform-skip mode scaling list is
selected from a set of two or more different transform-skip mode scaling lists according to one
or more encoding parameters of the encoded video data.
6. Apparatus according to clause 4, in which the transform-skip mode scaling list is
provided in a data stream containing the array of encoded data values.
7. Apparatus according to clause 4, the decoding apparatus being operable to derive the
transform-skip mode scaling list values from one or more values of the scaling list which would
be applicable for use in respect of that array of encoded video data values if the transform-skip
mode were not selected.
8. Apparatus according to clause 2, in which the dequantiser is configured to substitute a
single data value in place of the scaling list values in the case that the detector detects that that
the transform-skip mode is applicable to the array of encoded video data values.
9. Apparatus according to clause 8, in which the single data value is provided in a data
stream containing the array of encoded data values.
10. Apparatus according to clause 8, the decoding apparatus being operable to derive the
single data value from one or more values of the scaling list which would be applicable for use
in respect of that array of encoded video data values if the transform-skip mode were not
selected.
11. Apparatus according to clause 8, in which the single data value is selected from a set of
two or more different single data values according to one or more encoding parameters of the
encoded video data.
12. Apparatus according to any one of the preceding claims, in which the array of encoded
data values is selected from the list consisting of:
an 8 x 8 array;
a 16 x 16 array; and
a 32 x 32 array.
13. Video data encoding apparatus operable to encode an array of input video data values
in a transform-skip mode and optionally a non transform-skip mode, the apparatus comprising:
a frequency transformer configured to apply a frequency transform to the input video
data values to generate an array of frequency transformed input data values;
a quantiser configured to quantise the frequency transformed input data values by
applying a quantisation parameter to each frequency transformed input data value so as to
generate corresponding quantised data values; and
a controller;
if the transform-skip mode is not applicable to the array of input video data values, the
controller controls the frequency transformer to apply the frequency transformation and controls
the quantiser to apply quantisation parameters which may vary between data values in the array
of frequency transformed input data values according to the position of each frequency
transformed input data value within the array; and
if the transform-skip mode is applicable to the array of encoded video data values, the
controller controls the frequency transformer not to apply the frequency transformation and
controls the quantiser to quantise the input video data values by applying quantisation
parameters which, for each array of input data values, are independent of the position of each
input data value within the array.
14. Apparatus according to clause 13, comprising a detector configured to detect whether a
transform-skip mode is applicable to the array of input video data values.
15. Apparatus according to clause 13, in which the quantiser is configured to use
quantisation parameters dependent upon respective entries in a scaling list, there being one
entry in the scaling list for each array position of the data to be quantised.
16. Apparatus according to clause 15, in which the detector is operable to inhibit use, by the
quantiser, of the scaling list in the case that the detector detects that that the transform-skip
mode is applicable to the array of input video data values.
17. Apparatus according to clause 15, in which the quantiser is configured to use a
transform-skip mode scaling list in which all scaling list values are identical to one another in the
case that the detector detects that that the transform-skip mode is applicable to the array of
input video data values.
18. Apparatus according to clause 17, in which the transform-skip mode scaling list is
selected from a set of two or more different transform-skip mode scaling lists according to one
or more encoding parameters of the input video data.
19. Apparatus according to clause 17, in which the transform-skip mode scaling list is
provided in a data stream containing the array of encoded data values.
20. Apparatus according to clause 17, the encoding apparatus being operable to derive the
transform-skip mode scaling list values from one or more values of the scaling list which would
be applicable for use in respect of that array of input video data values if the transform-skip
mode were not selected.
2 1. Apparatus according to clause 15, in which the quantiser is configured to substitute a
single data value in place of the scaling list values in the case that the detector detects that that
the transform-skip mode is applicable to the array of input video data values.
22. Apparatus according to clause 2 1 , in which the single data value is provided in a data
stream containing the array of encoded data values.
23. Apparatus according to clause 2 1, the encoding apparatus being operable to derive the
single data value from one or more values of the scaling list which would be applicable for use
in respect of that array of input video data if the transform-skip mode were not selected.
24. Apparatus according to clause 2 1 , in which the single data value is selected from a set
of two or more different single data values according to one or more encoding parameters of the
input video data.
25. Apparatus according to any one of clauses 13 to 24, in which the array of frequency
transformed input data values is selected from the list consisting of:
an 8 x 8 array;
a 16 x 16 array; and
a 32 x 32 array.
26. Video data capture, storage, transmission and/or recording apparatus comprising
apparatus according to any one of the preceding clauses.
27. A video data decoding method for decoding an array of encoded video data values, the
method comprising:
detecting whether a transform-skip mode is applicable to the array of encoded video
data values;
if it is detected that the transform-skip mode is not applicable to the array of encoded
video data values, dequantising the array of encoded video data values by applying
dequantisation parameters which may vary between data values in the array of encoded data
values according to the position of each data value within the array, and applying an inverse
frequency transform to the dequantised data values; and
if it is detected that the transform-skip mode is applicable to the array of encoded video
data values, dequantising the array of encoded video data values by applying dequantisation
parameters which, for each array of encoded data values, are independent of the position of
each data value within the array, and not applying an inverse frequency transform to the
dequantised data values.
28. A video data encoding method for encoding an array of input video data values, the
method comprising:
if a transform-skip mode is not applicable to the array of input video data values,
frequency transforming the input video data and quantising the resulting array of frequency
transformed input video data values by applying quantisation parameters which may vary
between data values in the array of frequency transformed input data values according to the
position of each frequency transformed input data value within the array; and
if a transform-skip mode is applicable to the array of encoded video data values,
quantising the input video data values by applying quantisation parameters which, for each
array of input data values, are independent of the position of each input data value within the
array.
29. Computer software which, when executed by a computer, causes the computer to
implement the method of clause 27 or clause 28.
30. A non-transitory machine-readable storage medium on which computer software
according to clause 29 is stored.
CLAIMS
1. Video data decoding apparatus operable to decode an array of encoded video data
values, the apparatus comprising:
a dequantiser configured to dequantise the array of encoded video data values by
applying a dequantisation parameter to each data value so as to generate corresponding
dequantised data values;
an inverse frequency transformer configured to apply an inverse frequency transform to
the dequantised data values; and
a detector configured to detect whether a transform-skip mode is applicable to the array
of encoded video data values; in which:
if the detector detects that the transform-skip mode is not applicable to the array of
encoded video data values, the detector controls the dequantiser to apply dequantisation
parameters which may vary between data values in the array of encoded data values according
to the position of each data value within the array, and controls the inverse frequency
transformer to apply an inverse frequency transform to the dequantised data values; and
if the detector detects that the transform-skip mode is applicable to the array of encoded
video data values, the detector controls the dequantiser to apply dequantisation parameters
which, for each array of encoded data values, are independent of the position of each data
value within the array, and controls the inverse frequency transformer not to apply an inverse
frequency transform to the dequantised data values.
2. Apparatus according to claim 1, in which the dequantiser is configured to use
dequantisation parameters dependent upon respective entries in a scaling list, there being one
entry in the scaling list for each array position of the encoded video data.
3. Apparatus according to claim 2 , in which the detector is operable to inhibit use, by the
dequantiser, of the scaling list in the case that the detector detects that that the transform-skip
mode is applicable to the array of encoded video data values.
4. Apparatus according to claim 2 , in which the dequantiser is configured to use a
transform-skip mode scaling list in which all scaling list values are identical to one another in the
case that the detector detects that that the transform-skip mode is applicable to the array of
encoded video data values.
5. Apparatus according to claim 4 , in which the transform-skip mode scaling list is selected
from a set of two or more different transform-skip mode scaling lists according to one or more
encoding parameters of the encoded video data.
6. Apparatus according to claim 4 , in which the transform-skip mode scaling list is provided
in a data stream containing the array of encoded data values.
7. Apparatus according to claim 4 , the decoding apparatus being operable to derive the
transform-skip mode scaling list values from one or more values of the scaling list which would
be applicable for use in respect of that array of encoded video data values if the transform-skip
mode were not selected.
8. Apparatus according to claim 2 , in which the dequantiser is configured to substitute a
single data value in place of the scaling list values in the case that the detector detects that that
the transform-skip mode is applicable to the array of encoded video data values.
9. Apparatus according to claim 8 , in which the single data value is provided in a data
stream containing the array of encoded data values.
10. Apparatus according to claim 8 , the decoding apparatus being operable to derive the
single data value from one or more values of the scaling list which would be applicable for use
in respect of that array of encoded video data values if the transform-skip mode were not
selected.
11. Apparatus according to claim 8 , in which the single data value is selected from a set of
two or more different single data values according to one or more encoding parameters of the
encoded video data.
1 . Apparatus according to claim 1, in which the array of encoded data values is selected
from the list consisting of:
an 8 x 8 array;
a 16 x 16 array; and
a 32 x 32 array.
13. Video data encoding apparatus operable to encode an array of input video data values
in a transform-skip mode and optionally a non transform-skip mode, the apparatus comprising:
a frequency transformer configured to apply a frequency transform to the input video
data values to generate an array of frequency transformed input data values;
a quantiser configured to quantise the frequency transformed input data values by
applying a quantisation parameter to each frequency transformed input data value so as to
generate corresponding quantised data values; and
a controller;
if the transform-skip mode is not applicable to the array of input video data values, the
controller controls the frequency transformer to apply the frequency transformation and controls
the quantiser to apply quantisation parameters which may vary between data values in the array
of frequency transformed input data values according to the position of each frequency
transformed input data value within the array; and
if the transform-skip mode is applicable to the array of encoded video data values, the
controller controls the frequency transformer not to apply the frequency transformation and
controls the quantiser to quantise the input video data values by applying quantisation
parameters which, for each array of input data values, are independent of the position of each
input data value within the array.
14. Apparatus according to claim 13, comprising a detector configured to detect whether a
transform-skip mode is applicable to the array of input video data values.
15. Apparatus according to claim 13, in which the quantiser is configured to use quantisation
parameters dependent upon respective entries in a scaling list, there being one entry in the
scaling list for each array position of the data to be quantised.
16. Apparatus according to claim 15, in which the detector is operable to inhibit use, by the
quantiser, of the scaling list in the case that the detector detects that that the transform-skip
mode is applicable to the array of input video data values.
17. Apparatus according to claim 15, in which the quantiser is configured to use a transformskip
mode scaling list in which all scaling list values are identical to one another in the case that
the detector detects that that the transform-skip mode is applicable to the array of input video
data values.
18. Apparatus according to claim 17, in which the transform-skip mode scaling list is
selected from a set of two or more different transform-skip mode scaling lists according to one
or more encoding parameters of the input video data.
19. Apparatus according to claim 17, in which the transform-skip mode scaling list is
provided in a data stream containing the array of encoded data values.
20. Apparatus according to claim 17, the encoding apparatus being operable to derive the
transform-skip mode scaling list values from one or more values of the scaling list which would
be applicable for use in respect of that array of input video data values if the transform-skip
mode were not selected.
2 1. Apparatus according to claim 15, in which the quantiser is configured to substitute a
single data value in place of the scaling list values in the case that the detector detects that that
the transform-skip mode is applicable to the array of input video data values.
22. Apparatus according to claim 2 1, in which the single data value is provided in a data
stream containing the array of encoded data values.
23. Apparatus according to claim 2 1, the encoding apparatus being operable to derive the
single data value from one or more values of the scaling list which would be applicable for use
in respect of that array of input video data if the transform-skip mode were not selected.
24. Apparatus according to claim 2 1, in which the single data value is selected from a set of
two or more different single data values according to one or more encoding parameters of the
input video data.
25. Apparatus according to claim 13, in which the array of frequency transformed input data
values is selected from the list consisting of:
an 8 x 8 array;
a 16 x 16 array; and
a 32 x 32 array.
26. Video data capture, storage, transmission and/or recording apparatus comprising
apparatus according to claim 1.
27. A video data decoding method for decoding an array of encoded video data values, the
method comprising:
detecting whether a transform-skip mode is applicable to the array of encoded video
data values;
if it is detected that the transform-skip mode is not applicable to the array of encoded
video data values, dequantising the array of encoded video data values by applying
dequantisation parameters which may vary between data values in the array of encoded data
values according to the position of each data value within the array, and applying an inverse
frequency transform to the dequantised data values; and
if it is detected that the transform-skip mode is applicable to the array of encoded video
data values, dequantising the array of encoded video data values by applying dequantisation
parameters which, for each array of encoded data values, are independent of the position of
each data value within the array, and not applying an inverse frequency transform to the
dequantised data values.
28. A video data encoding method for encoding an array of input video data values, the
method comprising:
if a transform-skip mode is not applicable to the array of input video data values,
frequency transforming the input video data and quantising the resulting array of frequency
transformed input video data values by applying quantisation parameters which may vary
between data values in the array of frequency transformed input data values according to the
position of each frequency transformed input data value within the array; and
if a transform-skip mode is applicable to the array of encoded video data values,
quantising the input video data values by applying quantisation parameters which, for each
array of input data values, are independent of the position of each input data value within the
array.
29. A non-transitory machine-readable storage medium on which is stored computer
software which, when executed by a computer, causes the computer to implement the method
of claim 27.
30. A non-transitory machine-readable storage medium on which is stored computer
software which, when executed by a computer, causes the computer to implement the method
of claim 28.