Note: Descriptions are shown in the official language in which they were submitted.
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Method for the coding of picture signals
This application is a divisional of Canadian
patent application serial number 2,108,778 filed on April
14, 1992.
The invention relates to a method for the coding
of picture signals.
BACKGROUND OF THE INVENTION
This invention relates to a system for coding
image signals such as by means of a DCT (Discrete Cosine
Transform), for example.
A transformation circuit for facilitating an 8*8
or a 2*(4*8) DCT transformation is described in DE 36 42
664. Switching between an 8*8 and a 2*(4*8) DCT may be
accomplished in response to the state of a logic level on a
control line.
SUMMARY OF THE INVENTION
An object of the invention is to provide a system
for coding image signals by means of a codec suitable for
processing both progressively scanned and interlace scanned
image signals.
In a system according to the present invention,
before coding with a hybrid coder which can process blocks
of progressively scanned picture elements (pixels), line
sections from respective blocks of interlace scanned picture
elements within two vertically superimposed blocks are
arranged such that only line sections from one field of an
image signal are contained within each of these blocks.
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Image motion is detected and the line sections are re-sorted
within the superimposed blocks in the presence of dynamic
image content.
According to a method for hybrid coding of image
signals proposed by ISO-MPEG (International Organisation for
Standardization, Motion Picture Expert Group) under Standard
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Proposal number ISO 11172, progressively scanned input signals
are DCT processed in blocks, whereby respective blocks of 8*8
picture elements are coded or decoded and a sequence of inter-
frame coded images is replaced as regular intervals by intra-
frame coded images. The effectiveness of the coding is also a
function of the relatively high spatial correlation of picture
elements within such blocks. If interlaced source signals are
to be processed by such a hybrid decoded, coding effectiveness
decreases if dynamic image content or the data rate required
for coding increases. This results because every second line
derives from a block having different phases of motion, and
correlation of picture elements within such a block decrease.
In contrast, coding effectiveness is maintained in the presence
of a static image. With a dynamic image, image lines
associated with a first field from two superimposed 8*8 picture
element blocks are now combined into a first 8*8 block, and
lines associated with a corresponding second field from these
two superimposed 8*8 picture element blocks are combined into a
second 8*8 block, and are applied in this form to the hybrid
coder.
Due to such reorganisation of the input signals, it
is not necessary to switch between 8*8 and a 2*(4*8) DCT
transformation in the hybrid coder as in DE 36 42 664.
Instead, an 8*8 DCT can also be advantageously performed for a
dynamic image.
A motion detector indicates whether a static of
dynamic image is present, and re-sorting or addressing of the
lines is done accordingly. Such motion information may be
added to the coded data for the respective block by means of a
bit per block or double block. During decoding, the
corresponding lines are arranged in the original sequence
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whereby the motion information is evaluated. According to
the MPEG standard, four luminance picture element blocks
arranged in the shape of a square are combined into a
macroblock. Advantageously, two of the superimposed blocks
of such a macroblock form a pair in the above-mentioned
sense. Accordingly, one bit per macroblock can indicate the
resorting.
The invention may be summarized according to one
aspect as method for the coding of picture signal pixel
blocks having a predetermined size, including a transform
using a hybrid coder, wherein the original picture signal
has interlace format, the method including the steps: a) in
a first mode, re-sorting or re-addressing line sections of
the pixel blocks within two vertically adjacent square shape
pixel blocks each having said predetermined size, so that
one of these two pixel blocks contains only line sections
from one field type of said interlace format picture signal
and the other block contains only line sections from the
other field type of said interlace format picture signal, in
order to achieve a high spatial correlation between pixels
in the re-sorted or re-addressed pixels blocks; b) in a
second mode, not carrying out such re-sorting or re-
addressing of line sections of the pixel blocks having said
predetermined size, in order to keep a high spatial
correlation between pixels in the pixels blocks; c) DCT
transforming the square shape pixel blocks of said
predetermined size resulting from steps a) or b); d) coding
the picture signal, thereby adding to the picture signal
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additional items of information indicating whether a re-
sorting or re-addressing of the line sections according to
step a) has taken place in corresponding pixel blocks;
wherein in each case four square shape luminance blocks of
said predetermined size are combined in a square shape
macroblock and two block pairs from said vertically adjacent
pixel blocks are included in such macroblock, each one of
said additional items of information in each case being
valid in common for the four luminance blocks included in a
macroblock.
According to another aspect the invention provides
method for the decoding of transformed and coded picture
signal pixel blocks, wherein the pixel blocks of an original
interlace format picture signal were coded in a hybrid coder
and thereby a modified processing took place in a first
mode, and wherein in a first mode re-sorting or re-
addressing line sections of the pixel blocks within two
vertically adjacent square shape pixel blocks each having
said predetermined size took place so that one of these two
pixel blocks contains only line sections from one field type
of said interlace format picture signal and the other block
contains only line sections from the other field type of
said interlace format picture signal, in order to achieve a
high spatial correlation between pixels in the re-sorted or
re-addressed pixels blocks, and wherein in a second mode
such re-sorting or re-addressing of line sections of the
pixel blocks having said predetermined size was not carried
out in order to keep a high spatial correlation between
pixels in the pixel blocks, the method including the
decoding steps: - inverse transforming coded square shape
blocks of a predetermined size to provide corresponding
square shape pixel blocks having said predetermined size;
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square shape pixel blocks having said predetermined size;
- evaluating additional items of information that are
included in the coded picture signal to be decoded, said
additional items of information indicating whether said
first or said second mode was carried out in the coding;
- in the case of said additional items of information
indicating that said first mode is to be carried out for
corresponding pixel blocks, re-sorting or re-addressing
pixel block line sections within two vertically adjacent
square shape pixel blocks each having said predetermined
size, so that within each of these two pixel blocks
alternating line sections from both field types of said
interlace format picture signal are contained, wherein one
of these two pixel blocks when input for said inverse
transforming contained only line sections from one field
type of said interlace format picture signal and the other
block when input for said inverse transforming contained
only line sections from the other field type of said
interlace format picture signal; - in the case of said
additional items of information indicating that said second
mode is to be carried out, not re-sorting or re-addressing
the line sections in corresponding pixel blocks, wherein in
each case four square shape luminance blocks of said
predetermined size are combined in a square shape macroblock
and two block pairs from said vertically adjacent pixel
blocks are included in such macroblock, each one of said
additional items of information in each case being valid in
common for the four luminance blocks included in a
macroblock.
According to a further aspect the invention
provides a coded digital signal containing coded data for
picture signal pixel blocks having a predetermined size, the
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transform in a hybrid coder, wherein the original picture
signal data input to said coding had interlace format, the
coded digital signal having the following properties: a) in
a first mode for square shape macroblock data contained in
said coded digital signal, line section data of the pixel
blocks within two vertically adjacent square shape pixel
blocks each having said predetermined size are in a re-
sorted format, so that one of these two pixel blocks
contains line section data from only one field type of said
interlace format original picture signal and the other block
contains line section data from only the other field type of
said interlace format original picture signal, in order to
achieve a high spatial correlation between pixels in the re-
sorted pixels blocks; b) in a second mode for square shape
macroblock data contained in said coded digital signal, no
such re-sorted format of line section data of the pixel
blocks having said predetermined size is present, in order
to keep a high spatial correlation between pixels in the
pixels blocks; c) the data for the square shape pixel blocks
of said predetermined size according to features a) or b)
are transformed coded data, wherein said coded digital
signal contains in addition items of information indicating
whether a re-sorted format according to feature a) is
present in corresponding macroblocks, and wherein in each
case four square shape luminance blocks of said
predetermined size form a macroblock and two block pairs
from said vertically adjacent pixel blocks are included in
such macroblock, each one of said additional items of
information in each case being valid in common for the four
luminance blocks included in such macroblocks of said coded
digital signal.
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BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the location of image lines
within blocks for static (a) and dynamic (b) images.
FIG. 2 is a block diagram codec apparatus in
accordance with the invention.
FIGS. 3 and 4 are flow diagrams respectively
depicting encoder and decoder processing methods in
accordance with the principles of the invention.
FIG. la and FIG. lb respectively show two
superimposed blocks of luminance or chrominance picture
elements in the x-y plane. For simplicity of illustration,
the blocks each have a size of 4*4 picture elements instead
of a size of 8*8 picture elements. In general, the blocks
could also have a size of (2*n)*(2*m) where n = 1, 2, 3,
. . ., m = 1, 2, 3, . . ., instead of 8*8. The two digit
numbers respectively mark the spatial position of a picture
element. The first digit of this number represents the
block number, the second, the line number within a block.
The picture elements of the known hybrid coder
which are to be coded or decoded in progressively scanned
form are arranged in accordance with FIG. la. This likewise
applies for picture elements having static picture content
for interlace scanned picture elements. Before the coding
in the case of dynamic picture content, the lines of two
superimposed blocks are interchanged in accordance with FIG.
lb and after the decoding, they are re-arranged in
accordance with FIG. la.
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FIG. 2 shows a hybrid coder 25 corresponding to
the aforesaid Standard Proposal. Interlace scanned picture
signals from
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picture n are supplied to the input 21 and thence arrive in a
picture store 22 and a movement detector 24. The items of data
(two superimposed blocks) of picture n-1 needed by the movement
detector 24 and the line sections of the respective two blocks
involved are read out from the picture store 22 into a block
store 23, from which the hybrid coder is able to select 8*8
blocks on each occasion. The picture elements for static picture
content corresponding to Fig. 1a and those for dynamic picture
content corresponding to Fig. lb are buffer stored in the block
store 23.
The movement detector can be realised in accordance with various
known methods. For example, the absolute value differences of
picture elements from blocks having the same spatial position of
picture n and picture n-1 may be formed for each block or double
block that has to be coded. Alternatively, movement vectors
(e.g. for two superimposed blocks on each occasion) formed by
the hybrid coder 25 can be used instead of the movement
detector. If the instantaneous sum of these absolute value
differences and/or the amount of the corresponding movement
vectors for this block or these blocks exceeds a predetermined
threshold (i.e. dynamic picture content is involved), the
picture elements corresponding to Fig. lb, otherwise those
corresponding to Fig. la, are buffer stored in the block store
23.
The re-sorting may be undertaken in accordance with the
following listing:
DO.y = 1, N/2
DO x = 1,N
Houtl(x~Y) - Binl(x~2*Y-1)
Houtl(x~Y) - Binl(x~2*Y)
ENDDO
ENDDO
DO y = 1,N/2
DO x = 1,N
Boutl(x~y+N/2) - Bin2(x~2*y-1)
Houtl(x~Y+N/2) - Bin2(x~2*Y)
ENDDO
ENDDO,
4
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wherein, Bins is the block located in the higher position and N
is an even number.
FIG. 3 is a flow chart illustrating a method as
described above in accordance with the principles of the
5 invention. In method step 30 an input signal is evaluated to
determine if it exhibits interlaced or progressive scan form.
A progressive scan signal is transformed and coded without
further processing at step 32 via node 31. If an interlaced
signal is detected at step 30, the interlaced signal is
evaluated at step 34 to determine if it contains motion. If it
does not, the interlaced signal is coupled via step 36 without
rearranging its original line structure to step 32 where the
interlaced signal is transformed and coded. If step 34 senses
that the interlaced signal contains motion, the processing of
step 36 is controlled so as to rearrange the line structure of
the interlaced signal (as previously discussed). The
interlaced signal with rearranged line structure is transformed
and subsequently coded by step 32. In step 38 a control signal
indicating a rearranged line structure when an interlaced
signal with motion is detected is provided to the coding
function in step 32. The coding function in step 32 may
provide a motion vector to motion detection step 34 to indicate
a motion condition for rearranging the line structure of an
interlaced signal. Picture and block storage steps as may be
required to facilitate the process illustrated by FIG. 3 have
been discussed previously in connection with FIG. 2 and have
not been shown to simplify FIG. 3.
FIG. 4 is a flowchart illustrating decoder processing
steps associated with the coding process discussed in
connection with FIG. 3. An input signal transform coded as
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discussed previously is decoded and inverse transformed by step
40. Before being applied to an output, the decoded signal is
processed by a step 42, which rearranges the line structure
back to an original structure if the signal exhibits an
interlaced line format with motion. For this purpose step 44
determines if the input signal exhibits an interlaced line
structure. If it does, step 46 determines if the interlaced
signal contains motion. If motion is detected, a control
signal is provided to step 42 to effect rearranging of the
lines of the interlaced signal back to an original structure.
