Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR CODING OR DECODING MOTION VECTORS AND CODING OR
DECODING DEVICE IMPLEMENTING THE SAID PROCESS
This is a divisional of Application Serial
No. 2,160,087 filed October 6, 1995.
The present invention relates to a process for
coding motion vectors in an image compression system. The
invention applies especially, but not uniquely, to the
coding of video images in respect of telecommunication
channels rated at less than 64 kbit/s.
The invention also relates to a decoding process,
to a coding device and to a decoding device.
An image compression device, or coder, is
described for example in Recommendation H.261 of the
Comite Consultatif International Telegraphique et
Telephonique (CCITT), especially with reference to Figure 3
("source coder") of this document.
The device is based on the processing of the image
in pixel blocks or macro-blocks. A block is composed of a
matrix of eight times eight pixels, whereas a macroblock
comprises four blocks. A macro-block can be coded in one of
two modes termed "inter" or "intra", all the blocks making
up the macro-block being coded in the same mode. In the case
of "intra" coding, the blocks are subjected to a discrete
cosine transformation, the coefficients thus obtained being
quantized, and then coded in accordance with a variable-
length code process ("VLC" coding). In the case of "inter"
coding, it is the difference between the blocks and blocks
of like position in a reference image which is subjected to
the discrete cosine transformation.
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To improve the performance of the coding in the
"inter" mode, motion compensation processes are employed. A
process of this type makes it possible to determine, in the
reference image or a window thereof, a macro-block which
will allow the effective "inter" coding of the macro-block
to be coded of the current image. Various selection criteria
exist for determining such a macro-block in the reference
image. The relative position of the reference macro-block
with respect to the position of the macro-block to be coded
in the current image is determined by what is called a
motion vector. This motion vector is transmitted in the
header of the coded macro-block.
A decoder also possesses a memory containing the
reference image. Knowing the motion vector and the
coefficients corresponding to the coded macro-block, it will
be able to restore the values of the pixels of the original
macro-block. These values may contain errors, especially a
quantization error.
The motion vectors are coded differentially with
respect to a prediction vector. This vector is equal to the
motion vector of the macro-block lying immediately to the
left of the macro-block to be coded or decoded. The
horizontal H and vertical V components of a vector are coded
separately. According to a known process, the prediction
vector is reinitialized to (0,0) in the following two cases:
if the macro-block immediately to the left of the
macro-block to be coded or decoded was coded in "intra"
mode,
if the macro-block to be coded lies on the left
edge of the image.
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According to one aspect the invention is a process
for the differential coding of a motion vector (MVO)
associated with a block (MO), the motion vector being used
for implementing an inter coding mode for the coding of an
image, another coding mode being the intra coding,
characterized in that it comprises the steps:
- of determining the components (MVOx, MVOy) of
the said motion vector (MVO),
- of determining, if a block which is among the
blocks lying to the left or lying immediately above or lying
above and to the right of the block of which a vector is to
be coded, is coded in intra mode, a prediction vector which
each component (Px, Py) is equal to the median value of a
zero value and of the corresponding components of two
candidate motion vectors, said candidate motion vectors
being the vectors associated with the two blocks coded in
inter mode among the three coded blocks lying to the left,
lying immediately above and lying above and to the right of
the block of which a vector is to be coded,
- of subtracting the components thus obtained from
the components of the vector to be coded.
According to a particular embodiment, the macro-
blocks associated with the candidate motion vectors are
macro-blocks adjacent to the macro-block of which a vector
is to be coded.
According to a particular embodiment, the macro-
blocks associated with the candidate motion vectors form
part of the macro-blocks lying above or to the left of the
macro-block of which a vector is to be coded.
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According to a particular embodiment, the macro-
blocks associated with the candidate motion vectors comprise
the macro-block lying immediately to the left, the macro-
block lying immediately above and the macro-block lying
above and to the right of the macro-block of which a vector
is to be coded.
According to another aspect the invention is a
process for the decoding of a motion vector (MVO) coded by
differential coding, characterized in that it comprises the
steps:
- of extracting, from the data received, the
differences in components (MVDx, MVDy) corresponding to this
vector (MVO),
- of determining, if a block which is among the
blocks lying to the left or lying immediately above or lying
above and to the right of the block of which a vector is to
be decoded, is coded in intra mode, a prediction vector
which each component (Px, Py) is equal to the median value
of the corresponding components of a zero vector and of
vectors associated with the two blocks, among the three
blocks lying to the left, lying immediately above and lying
above and to the right of the block of which a vector is to
be decoded, which were coded in inter mode,
- of adding the said differences in components
(MVDx, MVDy) to the components (Px, Py) of the prediction
vector.
According to a particular embodiment, the macro-
blocks associated with the candidate motion vectors are
macro-blocks adjacent to the macro-block of which a vector
is to be decoded.
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According to a particular embodiment, the macro-
blocks associated with the candidate motion vectors form
part of the macro-blocks lying above or to the left of the
macro-block of which a vector is to be decoded.
According to a particular embodiment, the macro-
blocks associated with the candidate motion vectors comprise
the macro-block lying immediately to the left, the macro-
block lying immediately above and the macro-block lying
above and to the right of the macro-block of which a vector
is to be decoded.
According to another aspect the invention is a
device for the differential coding of a motion vector (MVO)
associated with a block (MO), the motion vector being used
for implementing an inter coding mode for the coding of an
image, another coding mode being the intra coding,
characterized in that it comprises:
- means for determining the components (MVOx,.
MVOy) of the said motion vector (MVO),
- means for determining, if a block which is among
the blocks lying to the left or lying immediately above or
lying above and to the right of the block of which a vector
is to be coded, is coded in intra mode, a prediction vector
which each component (Px, Py) is equal to the median value
of a zero value and of the corresponding components of two
candidate motion vectors, said candidate motion vectors
being the vectors associated with the two blocks coded in
inter mode among the three coded blocks lying to the left,
lying immediately above and lying above and to the right of
the block of which a vector is to be coded,
- means for subtracting the components thus
obtained from the components of the vector to be coded.
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According to another aspect the invention is a
device for the decoding of a motion vector (MVO) coded by
differential coding, characterized in that it comprises:
- means for extracting, from the data received,
the differences in components (MVDx, MVDy) corresponding to
this vector (MVO),
- means for determining, if a block which is among
the blocks lying to the left or lying immediately above or
lying above and to the right of the block of which a vector
is to be decoded, is coded in intra mode, a prediction
vector which each component (Px, Py) is equal to the median
value of the corresponding components of a zero vector and
of vectors associated with the two blocks, among the three
blocks lying to the left, lying immediately above and lying
above and to the right of the block of which a vector is to
be decoded, which were coded in inter mode,
- means for adding the said differences in
components (MVDx, MVDy) to the components (Px, Py) of the
prediction vector.
According to a particular embodiment, the
compression, respectively decompression device implements
the coding, respectively decoding process.
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The invention will be better understood through
the description of a non-limiting preferred particular
example illustrated by the figures among which:
- Figure 1 is a functional diagram of an image
coder implementing the invention,
- Figure 2 represents the numbering of the macro-
blocks of an image with the CIQF format,
- Figure 3 illustrates the processing of candidate
vectors corresponding to macro-blocks lying beyond the
boundaries of the image,
- Figure 4 is a functional diagram of a decoder
implementing the process according to the invention.
A video image coder is represented in Figure 1.
This coder comprises a video input 1 feeding a motion
compensation processor 10 (associated with a reference
memory 2), a subtractor 3 and one of the inputs of a
multiplexer 4 with two inputs. The second input of the
multiplexer 4 is connected to the output of the subtractor
3. The output of the multiplexer 4 is linked to a discrete
cosine transformation processor (DCT processor 5).
In "intra" mode, the subtractor is not used. The
values of pixels are transmitted directly from the input 1
to the DCT processor 5.
In "inter" mode, it is the difference between the
signal at the input of the device and a portion of the
reference image stored in the memory 2 which is transmitted
to the DCT processor 5.
The output of the DCT processor 5 is connected to
a quantization circuit 6, the output of which is connected
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on the one hand to a dequantization circuit 7 followed by an
inverse DCT processor 8, and on the other hand to a coding
and multiplexing circuit 14. The coding circuit carries out,
in particular, the VLC coding of the motion vector
differences, the VLC coding of the DCT transform
coefficients and the assembling of the various data in
accordance with the structure required.
A block to be coded is subjected to the DCT
transformation, whether it be an "inter" or "intra" block,
that is to say one containing pixel values or differences in
pixel values respectively. The DCT coefficients obtained are
quantized and then coded in accordance with VLC coding. Such
VLC codings, as well as the associated processes (block scan
direction, coding of coefficient pairs and of lengths of
strings of zero coefficients) will not be described in
further detail since they are well known to those skilled in
the art. The coded coefficients are inserted into the video
multiplex in an appropriate manner.
The quantized coefficients are subjected to
dequantization and then to inverse DCT transformation. This
is the process to which these same data will be subjected in
the decoder, the resulting pixel values containing the same
errors, at least in part, notably the quantization errors.
In the coder, these decoded data serve to update the
reference image stored in the memory 2.
The output of the memory 2 is connected to an
input of the subtractor 3, possibly across a spatial loop
filter 9. The output of the inverse DCT processor 8 is
connected to the reference memory 2 across a two-input adder
11. The second input of the adder 11 is connected to the
output of a second multiplexer 12 with two inputs, one of
which is connected to the output of the loop filter 9, and
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the other of which is not connected up, delivering zero
pixel values. In "intra" mode, the output of the multiplexer
12 delivers the zero values. The two multiplexers 4 and 12
are controlled simultaneously by a control unit 13 which
also handles the quantizing of the coefficients, notably as
a function of the degree of fill of the video buffer
register (not illustrated).
The motion compensation processor compares macro-
blocks originating from the input 1 with macro-blocks of the
reference image, and determines on the one hand whether a
macro-block is to be coded in intra or inter mode. Such a
decision can be made by evaluating the value of an error
function as the sum of the squares of the pixel differences
in the case of the "inter" mode with respect to threshold
values.
The coding in "intra" mode is enforced from time
to time if it is not chosen for a given number of images for
one and the same macro-block. This is done in order to limit
the accumulation of reconstruction errors within the inverse
DCT transformation. For example, following recommendation
H.261 mentioned earlier, such enforcement is carried out at
least every 132 transmissions of one and the same block.
Motion compensation or estimation is employed in
particular at the level of the processor 10. There are
numerous processes for searching for the best motion vector
and as many variants thereof: exhaustive comparison of
blocks, hierarchical comparison processes etc. These
processes can be implemented in particular by
microprocessors, special-purposes processors etc.
According to the present example, the structure
for sampling the luminance of an image is 144 lines of 176
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pixels, this corresponding to a format of 11 times 9 macro-
blocks. For the chrominance and for each of the colour
components, the format is 72 lines of 88 pixels. This
corresponds to a so-called 4:1:1 luminance chrominance
structure.
This image format is also called the CIQF format
(standing for Common Intermediate Quarter Format).
In this example it will be assumed that such an
image format is available at the input 1 of the coder.
Hereafter, for the purposes of simplifying the
explanations, only the luminance will be considered.
Figure 2 gives the numbering of the luminance
macro-blocks. The actual macro-blocks of an image are
surrounded by continuous lines. Hereafter, the eight macro-
blocks which surround a current macro-block (that is to say
the one currently being processed) will be referred to as
adjacent macro-blocks. The macro-blocks adjacent to the
macro-block 13 are the macro-blocks 1, 2, 3, 12, 14, 23, 24
and 25. Again for the purposes of simplifying the
explanations, the presence of dummy macro-blocks beyond the
boundary of the image will be assumed. Macro-block A is a
macro-block adjacent to the macro-blocks 1, 12 and 23.
For each macro-block coded in "inter" mode, the
processor 10 provides a pair of components of the motion
vector associated with this block. In accordance with the
present embodiment, the components have a precision of half
a pixel and are confined to the interval [-16, +15.5].
According to the present embodiment, the circuit
14, which comprises for example a memory and a suitably
programmed microprocessor, determines a prediction vector
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for each motion vector. The components Px and Py of this
prediction vector are subtracted from the components of the
motion vector to be coded.
According to the present embodiment, the
components of the prediction vector are calculated in a
manner made explicit by Figure 3.
MO denotes the current macro-block, that is to say
the macro-block for which a prediction vector is being
determined. On the other hand, MVix and MViy denote the
first and second components respectively of the motion
vector associated with macro-block i.
M1 denotes the macro-block immediately to the left
of MO, M2 the macro-block immediately above MO, and M3 the
macro-block above and to the right of MO. The motion vectors
associated with the macro-blocks Ml to M3 are called
candidate vectors and are denoted MV1 to MV3. Figure 3a
gives the arrangement of the macro-blocks Ml to M3 around
MO.
Since a motion vector is defined only for macro-
blocks coded in "inter" mode, the components of the
candidate vectors are assumed to be zero in the following
cases:
- when a macro-block has been coded in "intra"
mode,
- when a macro-block lies beyond the boundary of
the image.
Figures 3b, 3c and 3d illustrate various cases.
The pairs (0,0) indicate the macro-blocks for which the
candidate vector is regarded as zero. The dotted line
indicates the boundaries of the image.
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Having determined the components of the three
candidate vectors, the components of the prediction vector
are calculated by choosing for each component of this vector
the median value of the corresponding components of the
candidate vectors. For example, if the candidate vectors are
respectively:
MV1 = (-2; 3), MV2 = (1; 5), MV3 = (-1; 7)
then the prediction vector will be
(Px; Py) = (-1; 5).
We then form the difference between this
prediction vector and the motion vector MVO:
MVDx = MVx-Px
MVDy = MVy-Py.
These differences are next coded by VLC coding and
introduced into the video multiplex.
Given that the calculation of the prediction
vector demands a knowledge of the vectors M1 to M3, these
vectors are held in memory by the processor 10.
During decoding, a prediction vector is determined
in a similar manner. Of course, addition of the components
of this vector with the decoded differences will be carried
out.
Figure 4 gives a functional diagram of a decoder.
The data received from a reception and demodulation system
are stored in a register 15. This register is required in
order to match the speed of transmission of the data to
their actual use. The data are demultiplexed by a
demultiplexer 16. The coefficients are decoded, undergo
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inverse quantization (circuit 17), and then an inverse DCT
transformation (circuit 18). The resulting blocks are added
(by the adder 19) to the result of the prediction in the
case of coding in "inter" mode. The prediction memory 21 is
of course updated appropriately.
A processor 20 is tasked with controlling the
quantization and in decoding the data corresponding to the
motion vectors. It carries out the VLC decoding of the data,
retains in memory the vectors required for the calculation
of the prediction vector, determines this vector and then
adds to the components thereof the decoded differences of
components. The motion vectors thus re-established are used
in a known manner.
According to an embodiment variant, motion vectors
are associated with the macro-blocks coded in "intra" mode.
This makes it possible to compensate for the transmission
errors which could corrupt the coded coefficients of this
macro-block. In this case, the corresponding candidate
vectors are not regarded as being zero.
Obviously, the present invention is not limited to
the embodiment example described, but applies to other
compression systems using motion compensation.
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