Motion picture encoding system and motion picture decoding system

A motion picture encoding system comprising a frame-based shape motion detecting unit (24) and a frame-based shape motion compensation unit (26) which make motion compensated prediction for the shape data (2) of each alpha block included in an interlaced frame comprised of a pair of top and bottom fields so as to generate a frame-based prediction data for shape. In addition, a field-based shape motion detecting unit (28) and a field-based shape motion compensation unit (30) make motion compensated prediction for the shape data (2) of each alpha block included in each of the two fields of the frame independently so as to generate a field-based prediction data for shape. An arithmetic encoding unit (32) can inter-code the shape data using the frame-based prediction data for shape, and inter-code the shape data using the field-based prediction data for shape. The arithmetic encoding unit (32) then selects one of the coded results having the shortest code length.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a motion picture encoding system which can 
encode a series of interlaced motion picture objects with a high encoding 
efficiency, and to a motion picture decoding system which can decode a 
series of coded interlaced motion picture objects. 
2. Description of the Prior Art 
Video encoding verification model or VM of Motion Picture Expert Group 
Phase-4 (MPEG-4) which is being standardized by JTC11/SC29/WG11 of the 
ISO/IEC has been known as an example of a method of encoding shape 
information for use in a prior art motion picture encoding system. The 
contents of the video encoding VM are varying with ongoing standardization 
efforts by MPEG-4. Hereinafter, a description of the video encoding VM 
will be made assuming that the video encoding VM is the one of version 
7.0, which will be referred to as the VM. 
In the VM, a sequence of motion pictures is defined as a collection of 
motion picture objects each having an arbitrary shape with respect to time 
and space, and encoding process is carried out for each motion picture 
object. Referring now to FIG. 25, there is illustrated a diagram showing 
the structure of video data in the VM. In the VM, one specific scene of a 
motion picture is called video session or VS. Furthermore, one motion 
picture object which can vary with time is called video object or VO which 
is a component of a VS. Accordingly, a VS is defined as a collection of 
one or more VO's. 
One video object layer or VOL is a component of a VO and is comprised of a 
plurality of video object planes or VOP's. A VOL is provided with the aim 
of displaying motion pictures in a hierarchical form. An important factor 
in providing a plurality of layers for each VO with respect to time is the 
frame rate. An important factor in providing a plurality of layers for 
each VO with respect to space is the display roughness. Each VO 
corresponds to each of a plurality of objects in one scene such as each of 
conferees that joins a TV meeting or the background that can be seen 
behind the conferees. Each VOP is an image data representing the state of 
a corresponding VO at each time, which corresponds to one frame and which 
is a unit on which an encoding process is performed. 
Referring next to FIG. 26, there is illustrated a view showing an example 
of VOP's in one scene. Two VOP's, i.e., VOP1 representing a person and 
VOP2 representing a painting hung on the wall behind the person are shown 
in FIG. 26. Each VOP is constructed from a texture data showing the color 
light and dark level of each VOP and a shape data showing the shape of 
each VOP. The texture data of each pixel is comprised of an 8-bit 
luminance signal and a chrominance signal having one half the size of the 
luminance signal in both horizontal and vertical dimensions. The shape 
data of each pixel is a matrix of binary values in which each element is 
set to 1 when each element is in the interior of a VOP; otherwise, each 
element is set to 0. Each shape data has the same size as a corresponding 
luminance signal. In representation of a motion picture using VOP's, a 
conventional frame image can be formed by arranging a plurality of VOP's 
within one frame, as shown in FIG. 26. When only one VO exists in a motion 
picture sequence, each VOP is synonymous with each frame. In this case, 
each VOP has no shape data, and therefore only the texture data of each 
VOP is encoded. 
Referring next to FIG. 27, there is illustrated a block diagram showing the 
structure of a prior art VOP encoding device for use in a VM encoding 
system disclosed in ISO/IEC JTC11/SC29/WG11, MPEG97/N1642, MPEG-4 
Verification Mode Version 7.0. In the figure, reference character P1 
denotes an input VOP data, P2 denotes a shape data which is extracted from 
the input VOP data, P3 denotes a shape encoding unit which can encode the 
shape data P2, P4 denotes a shape memory which can store a local decoded 
shape data P7 furnished by the shape encoding unit P3, P5 denotes a motion 
vector of shape furnished by the shape encoding unit P3, and P6 denotes a 
coded shape data furnished by the shape encoding unit P3. 
Furthermore, reference character P8 denotes a texture data which is 
extracted from the input VOP data P1, P9 denotes a texture motion 
detecting unit which receives the texture data P8 and then detects a 
motion vector of texture P10, P11 denotes which receives the motion vector 
of texture P10 and delivers a prediction data for texture P12, P13 denotes 
a texture encoding unit which can encode the prediction data for texture 
P12, P14 denotes a coded texture data furnished by the texture encoding 
unit 13, P16 denotes a texture memory which can store the local decoded 
texture data P15 furnished by the texture encoding unit P13, and P17 
denotes a variable length encoding and multiplexing unit which can receive 
the motion vector of shape P5, the coded shape data P6, the motion vector 
of texture P10, and the coded texture data P14, and then furnishes a coded 
bitstream. 
In operation, the input VOP data P1 is divided into the shape data P2 and 
the texture data P8 first. The shape data P2 is delivered to the shape 
encoding unit P3, and the texture data P8 is delivered to the texture 
motion detecting unit P9. Then each of the shape data and the texture data 
is divided or partitioned into multiples of 16.times.16 pixels blocks and 
the encoding process is done per 16.times.16 pixels block. As shown in 
FIG. 26, each of the plurality of blocks of the shape data per which the 
shape encoding process is done is hereafter referred to as an alpha block, 
and each of the plurality of blocks of the texture data per which the 
texture encoding process is done is hereafter referred to as a macroblock. 
First, the description will be directed to the encoding process for the 
shape data. Referring next to FIG. 28, there is illustrated a block 
diagram showing the structure of the shape encoding unit P3. In the 
figure, reference character P19 denotes a shape motion detecting unit 
which can receive the shape data P2 and then detect a motion vector of 
shape P5, P20 denotes a shape motion compensation unit which can receive 
the motion vector of shape P5 and then furnish a prediction data for shape 
P21, P22 denotes an arithmetic encoding unit which can receive the 
prediction data for shape P21 and then furnish a coded shape data P23, and 
P24 denotes a shape encoding mode selecting unit which can receive the 
coded shape data P23 and then furnish a coded shape data P6. 
First, a description will be made as to the motion detection which is 
carried out for the input shape data P2. When the shape motion detection 
unit P19 receives the shape data P2 of each of a plurality of alpha blocks 
into which the shape data of the VOP has been partitioned, it detects a 
motion vector of shape P5 for each alpha block from motion vectors of 
shape of other alpha blocks around the current alpha block, which have 
been stored in the shape motion detecting unit P19, and the motion vectors 
of texture of macroblocks around the corresponding macroblock at the same 
location, which have been furnished by the texture motion detecting unit 
P9. A block matching method which has been used for detecting the motion 
vector of texture for each macroblock can be used as a method of detecting 
the motion vector of shape for each alpha block. Using the method, a 
motion vector of shape can be detected for each alpha block by searching a 
small area in the vicinity of the motion vectors of shape of other alpha 
blocks referred around the current alpha block and the motion vectors of 
texture of macroblocks around the corresponding macroblock at the same 
location as the alpha block being tested. The motion vector of shape P5 of 
each alpha block to be encoded is delivered to the variable length 
encoding and multiplexing unit P17 and is then multiplexed into a coded 
bitstream P18 as needed. 
Next, a description will be made as to the motion compensation and the 
arithmetic encoding for the shape data of each alpha block to be encoded. 
The shape motion compensation unit P20 generates and furnishes a 
prediction data for shape P21 used for the encoding process from a 
reference shape data stored in the shape memory P4 according to the motion 
vector of shape P5 determined in the above-mentioned process. The 
prediction data for shape P21, together with the shape data P2 of each 
alpha block to be encoded, is applied to the arithmetic encoding unit P22. 
The arithmetic encoding process is then done for each alpha block to be 
encoded. The arithmetic encoding method is the encoding method that can 
adapt dynamically to the frequency of occurrence of a series of symbols. 
Therefore, it is necessary to obtain the probability that the value of 
each pixel in the alpha block currently being encoded is 0 or 1. 
In the VM, the arithmetic encoding process is done in the following manner. 
(1) A pixel distribution pattern or context around the target pixel to be 
arithmetic encoded is examined. 
The context construction used in intra or intra-coding mode, that is, when 
encoding the shape data of the alpha block being decoded by using only the 
shape data within the VOP currently being encoded is shown in FIG. 29a. 
The context construction used in inter or inter-coding mode, that is, when 
encoding the shape data of the alpha block being encoded by using the 
prediction data for shape which has been extracted in the motion 
compensation process as well is shown in FIG. 29b. In the figures, the 
target pixel to be encoded is marked with `?`. In either pattern, a 
context number is computed according to the following equation. 
##EQU1## 
where Ck shows the value of a pixel in the vicinity of the pixel to be 
encoded as shown in FIGS. 29a and 29b. 
(2) The probability that the value of the target pixel to be encoded is 0 
or 1 is obtained by indexing a probability table using the context number. 
(3) The arithmetic encoding is carried out according to the indexed 
probability of the value of the target pixel to be encoded. 
The procedures mentioned above are carried out in both the intra mode and 
the inter mode. The shape encoding mode selecting unit P24 selects either 
the coded result obtained in the intra shape encoding mode or the coded 
result obtained in the inter shape encoding mode. The shape encoding mode 
selecting unit P24 selects the one having a shorter code length. The final 
coded shape data P6 thus obtained, including information indicating the 
selected shape encoding mode, is delivered to the variable length encoding 
and multiplexing unit P17 in which the shape coded data as well as the 
corresponding texture data is multiplexed into the coded bitstream P18 
according to a given syntax (or grammatical rules which coded data must 
obey). The local decoded shape data P7 of the alpha block is stored in the 
shape memory P4 and is also furnished to the texture motion detecting unit 
P9, the texture motion compensation unit P11, and the texture encoding 
unit P13. 
Next, a description will be made as to the texture encoding. After the 
texture data of the VOP to be encoded is partitioned into a plurality of 
macroblocks, the texture data P8 of a macroblock to be encoded is applied 
to the texture motion detecting unit P9. The texture motion detecting unit 
P9 then detects a motion vector of texture P10 from the texture data P8. 
In the case where the texture data P8 of the macroblock to be encoded is 
an interlaced signal, the texture motion detecting unit P9 can perform a 
frame-based motion detecting operation on each macroblock composed of 
lines from the two fields alternately, one of which contains lines each 
spatially located above the corresponding line of the other field is 
called top field and the other one of which is called bottom field, and 
perform a field-based motion detecting operation on each macroblock 
composed of lines from only one of the two fields, independently, as shown 
in FIG. 30. By using this motion detecting process, a reduction in the 
encoding efficiency due to the difference in the position of a moving 
object between the pair of two fields of an interlaced frame can be 
prevented, and therefore the efficiency of the frame-based prediction can 
be improved. 
The texture motion compensation unit P11 generates and furnishes a 
prediction data for texture P12 from a reference texture data stored in 
the texture memory P16 according to the motion vector of texture P10 of 
the macroblock to be encoded from the texture motion detecting unit P9. 
The prediction data for texture P12 is then delivered to the texture 
encoding unit P13 as well as the texture data P8. From the texture data P8 
(or intra texture data) and the difference (or inter texture data) between 
the texture data P8 and the prediction data for texture P12, the texture 
encoding unit P13 selects the one which offers a higher degree of encoding 
efficiency, and then compresses and encodes the selected data using DCT 
and scalar quantization. When the texture data P8 is an interlaced signal, 
the texture motion detecting unit P9 estimates a frame-based motion vector 
of texture for each macroblock and field-based motion vectors of texture 
for each macroblock, and then selects the one which offers a higher degree 
of encoding efficiency from all selectable texture encoding modes. 
In addition, the texture encoding unit P13 can select either frame-based 
DCT coding or field-based DCT coding in the case where the texture data P8 
is an interlaced signal. As shown in FIG. 31, in the case of the 
frame-based DCT encoding, each block is composed of lines from the pair of 
two fields, i.e., top and bottom fields, alternately, and the frame-based 
DCT encoding process is done per each 8.times.8 block. In the case of the 
field-based DCT encoding, each block is composed of lines from only one of 
the two fields, i.e., top and bottom fields, and the field-based DCT 
encoding process is done per each 8.times.8 block for each of the first 
and second fields. Accordingly, the generation of high-frequency 
coefficients in the vertical direction due to the difference in the 
position of a moving object between the two fields of an interlaced frame 
can be prevented and hence the power concentration effect can be improved. 
After the quantized DCT coefficients undergo reverse quantization, reverse 
DCT, and an addition to the reference texture data, they are written into 
the texture memory P16 as the local decoded texture data P15. The local 
decoded texture data P15 is used for predictions of the later VOP which 
will be formed later. The texture encoding mode information indicating the 
selected texture encoding mode: the intra mode, the inter mode with 
frame-based prediction, or the inter mode with field-based prediction and 
the DCT encoding mode information indicating the selected DCT encoding 
mode: the frame-based DCT encoding mode or the field-based DCT encoding 
mode included in the coded texture data P14 are delivered to the variable 
length encoding and multiplexing unit P17 and then are multiplexed into 
the coded bitstream P18 according to the given syntax. 
When the VOP to be encoded is an interlaced image, there is a difference in 
the position of a moving object between the pair of two fields of the 
interlaced image which has been caused by a difference in time between the 
two fields of the interlaced image, as previously mentioned. Therefore, in 
the prior art encoding system mentioned above, the texture encoding 
process is done by performing a switching operation between the 
frame-based encoding and the field-based encoding so as to make a 
correction to the displacement between the pair of two field pictures of 
the interlaced frame. On the other hand, predictions and encoding are 
carried out for each frame picture composed of a pair of two fields in the 
shape encoding process without any correction to the difference in the 
position of a moving object between the pair of two fields of the 
interlaced frame. Accordingly, a problem with the prior art encoding 
system is that the prediction and encoding efficiencies are relatively low 
due to the difference in the position of a moving object between the pair 
of two fields of the interlaced frame. 
SUMMARY OF THE INVENTION 
The present invention is made to overcome the above problem. It is 
therefore an object of the present invention to provide a motion picture 
encoding system capable of encoding interlaced VOP's without reducing 
prediction and encoding efficiencies. 
It is another object of the present invention to provide a motion picture 
decoding system capable of decoding a coded bitstream including a sequence 
of interlaced frames while carrying out motion prediction while making a 
correction to the difference in the position of a moving object between 
the two fields of each interlaced frame. 
In accordance with a preferred embodiment of the present invention, there 
is provided a motion picture decoding system which can decode a coded 
bitstream obtained by encoding a motion picture comprised of a sequence of 
interlaced images each having its texture data and shape data, comprising: 
a bitstream analyzer for extracting the following data from the coded 
bitstream for each of a plurality of small regions included in an 
interlaced image to be reconstructed; (1) the coded shape data, (2) shape 
encoding mode information indicating whether the coded shape data is a 
data encoded with frame-based motion compensated prediction or field-based 
motion compensated prediction, and (3) a frame-based motion vector of 
shape or field-based motion vectors of shape; a frame-based motion 
compensation unit for making a motion compensated prediction for shape of 
each of the plurality of small regions to be reconstructed according to 
the frame-based motion vector of shape so as to generate a frame-based 
prediction data for shape; a field-based motion compensation unit for 
making a motion compensated prediction for shape of each of first and 
second fields of each of the plurality of small regions to be 
reconstructed according to the field-based motion vectors of shape so as 
to generate a field-based prediction data for shape; and a decoding unit 
for decoding the coded shape data of each of the plurality of small 
regions to be reconstructed by using either the frame-based prediction 
data or the field-based prediction data, according to the shape encoding 
mode information. 
In accordance with another preferred embodiment of the present invention, 
there is provided a motion picture decoding system which can decode a 
coded bitstream obtained by encoding a motion picture comprised of a 
sequence of interlaced images each having its texture data and shape data, 
comprising: a bitstream analyzer for extracting the following data from 
the coded bitstream for each of a plurality of small regions included in 
an interlaced image to be reconstructed; (1) the coded shape data, (2) 
texture encoding mode information indicating whether the coded texture 
data is a data encoded with frame-based motion compensated prediction or 
field-based motion compensated prediction, and (3) a frame-based motion 
vector of shape or field-based motion vectors of shape; a frame-based 
motion compensation unit for making a motion compensated prediction for 
shape of each of the plurality of small regions to be reconstructed 
according to the frame-based motion vector of shape so as to generate a 
frame-based prediction data for shape; a field-based motion compensation 
unit for making a motion compensated prediction for shape of each of first 
and second fields of each of the plurality of small regions to be 
reconstructed according to the field-based motion vectors of shape so as 
to generate a field-based prediction data for shape; and a decoding unit 
for decoding the coded shape data of each of the plurality of small 
regions to be reconstructed by using either the frame-based prediction 
data or the field-based prediction data as needed, according to the 
texture encoding mode information. 
In accordance with another preferred embodiment of the present invention, 
there is provided a motion picture decoding system which can decode a 
coded bitstream obtained by encoding a motion picture comprised of a 
sequence of interlaced images each having its texture data and shape data, 
comprising: a bitstream analyzer for extracting the following data from 
the coded bitstream for each of a plurality of small regions included in 
an interlaced image to be reconstructed; (1) the coded shape data, (2) 
shape encoding mode information indicating if the coded shape data of a 
first field is a data encoded with field-based motion compensated 
prediction, (3) a field-based motion vector of shape of the first field, 
and (4) coded data of a prediction error for shape of a second field; a 
field-based motion compensation unit for making a motion compensated 
prediction for shape of the first field of each of the plurality of small 
regions to be reconstructed according to the field-based motion vector of 
shape of the first field so as to generate a field-based prediction data 
for shape of the first field; a first decoding unit for decoding the coded 
shape data of the first field of each of the plurality of small regions to 
be reconstructed by using the field-based prediction data for shape of the 
first field as needed, according to the shape encoding mode information; a 
prediction computing unit for computing a prediction value for the shape 
data of the second field of each of the plurality of small regions to be 
reconstructed by using the decoded shape data of the first field from the 
first decoding unit; and a second decoding unit for decoding the coded 
shape data of the second field of each of the plurality of small regions 
to be reconstructed by using the prediction error and the prediction value 
from the prediction computing unit. 
Preferably, the prediction computing unit includes a unit for computing a 
context number for each pixel of the shape data of the second field of 
each of the plurality of small regions to be reconstructed by using the 
decoded shape data of the first field, and a unit for computing a 
prediction value of each pixel of the shape data of the second field of 
each of the plurality of small regions to be reconstructed from the 
context number. 
In accordance with another preferred embodiment of the present invention, 
there is provided a motion picture decoding system which can decode a 
coded bitstream obtained by encoding a motion picture comprised of a 
sequence of interlaced images each having its texture data and shape data, 
comprising: a bitstream analyzer for extracting the following data from 
the coded bitstream for each of a plurality of small regions included in 
an interlaced image to be reconstructed; (1) the coded shape data, (2) 
shape encoding mode information indicating if the coded shape data of a 
first field is a data encoded with field-based motion compensated 
prediction, (3) a field-based motion vector of shape of the first field, 
and (4) coded data of a prediction error for shape of a second field; a 
field-based motion compensation unit for making a motion compensated 
prediction for shape of the first field of each of the plurality of small 
regions to be reconstructed according to the field-based motion vector of 
shape of the first field so as to generate a field-based prediction data 
for shape of the first field; a decoding unit for decoding the coded shape 
data of the first field of each of the plurality of small regions to be 
reconstructed by using the field-based prediction data for shape of the 
first field as needed, according to the shape encoding mode information; 
and a prediction computing unit for computing a prediction value of a 
decoded shape data of a second field of each of the plurality of small 
regions to be reconstructed by using the decoded shape data of the first 
field so as to generate a decoded shape data of the second field by using 
the computed prediction value. 
Preferably, the prediction computing unit includes a unit for computing a 
context number for each pixel of the shape data of the second field of 
each of the plurality of small regions to be reconstructed by using the 
decoded shape data of the first field, and a unit for computing a 
prediction value of each pixel of the shape data of the second field of 
each of the plurality of small regions to be reconstructed from the 
context number. 
In accordance with another preferred embodiment of the present invention, 
there is provided a motion picture decoding system which can decode a 
coded bitstream obtained by encoding a motion picture comprised of a 
sequence of interlaced images each having its texture data and shape data, 
comprising: a bitstream analyzer for extracting the following data from 
the coded bitstream for each of a plurality of small regions included in 
an interlaced image to be reconstructed; (1) the coded shape data, (2) 
first shape encoding mode information indicating if the coded shape data 
of a first field is a data encoded with field-based motion compensated 
prediction, (3) a field-based motion vector of shape of the first field, 
(4) second shape encoding mode information indicating whether the shape 
data of a second field is to be decoded or not, and (5) coded data of a 
prediction error for shape of the second field if the second shape 
encoding mode information indicates that the shape data of the second 
field is to be decoded; a field-based motion compensation unit for making 
a motion compensated prediction for shape of the first field of each of 
the plurality of small regions to be reconstructed according to the 
field-based motion vector of shape so as to generate a field-based 
prediction data for shape of the first field; a first decoding unit for 
decoding the coded shape data of the first field of each of the plurality 
of small regions to be reconstructed by using the field-based prediction 
data of the first field as needed, according to the shape first encoding 
mode information; a prediction computing unit for computing a prediction 
value for the shape data of the second field to be reconstructed by using 
the decoded shape data of the first field from the first decoding unit; a 
second decoding unit for decoding the coded shape data of the second field 
of each of the plurality of small regions to be reconstructed; and a unit 
for generating a decoded shape data of the second field of each of the 
plurality of small regions to be reconstructed from the prediction value 
of the second field furnished by the prediction computing unit, or by 
adding the prediction value of the second field to the shape data of the 
second field decoded by the second decoding unit, according to the second 
field encoding mode information. 
In accordance with another preferred embodiment of the present invention, 
there is provided a motion picture decoding system which can decode a 
coded bitstream obtained by encoding a motion picture comprised of a 
sequence of interlaced images each having its texture data and shape data, 
comprising: a bitstream analyzer for extracting the following data from 
the coded bitstream for each of a plurality of small regions included in 
an interlaced image to be reconstructed; (1) the coded shape data, (2) 
shape encoding mode information indicating if the coded shape data of a 
first field is a data encoded with field-based motion compensated 
prediction, (3) a field-based motion vector of shape of a first field, and 
(4) coded data of a delta vector used for adjusting a decoded shape data 
of the first field; a field-based motion compensation unit for making a 
motion compensated prediction for shape of the first field of each of the 
plurality of small regions to be reconstructed according to the 
field-based motion vector of shape of the first field so as to generate a 
field-based prediction data for shape of the first field; a first decoding 
unit for decoding the coded shape data of the first field of each of the 
plurality of small regions to be reconstructed by using the field-based 
prediction data for shape of the first field from the field-based motion 
compensation unit as needed, according to the shape encoding mode 
information; a second decoding unit for decoding the coded data of the 
delta vector so as to generate a delta vector; and a unit for generating a 
decoded data of the second field of each of the plurality of small regions 
to be reconstructed by using the decoded shape data of the first field 
from the first decoding unit and the delta vector from the second decoding 
unit. 
In accordance with another preferred embodiment of the present invention, 
there is provided a motion picture decoding system which can decode a 
coded bitstream obtained by encoding a motion picture comprised of a 
sequence of interlaced images each having its texture data and shape data, 
comprising: a bitstream analyzer for extracting the following data from 
the coded bitstream for each of a plurality of small regions included in 
an interlaced image to be reconstructed: (1) the coded shape data, (2) 
shape encoding mode information indicating whether the coded shape data is 
a data encoded with frame-based motion compensated prediction or 
field-based motion compensated prediction, (3) a frame-based motion vector 
of shape or a field-based motion vector of shape of a first field, and (4) 
a differential motion vector showing a difference between the field-based 
motion vector of shape of the first field and the field-based motion 
vector of shape of a corresponding second field; a frame-based motion 
compensation unit for making a motion compensated prediction for shape of 
each of the plurality of small regions to be reconstructed according to 
the frame-based motion vector of shape so as to generate a frame-based 
prediction data for shape; a field-based motion compensation unit for 
making a motion compensated prediction for shape of the first field of 
each of the plurality of small regions to be reconstructed according to 
the field-based motion vector of shape of the first field so as to 
generate a field-based prediction data for shape of the first field, and 
for computing a field-based motion vector of shape of the second field by 
adding the differential vector to the field-based motion vector of shape 
of the first field, and then making a motion compensated prediction for 
shape of the second field of each of the plurality of small regions to be 
reconstructed according to the field-based motion vector of shape of the 
second field, so as to generate a field-based prediction data for shape of 
the second field; and a decoding unit for decoding the coded shape data of 
each of the plurality of small regions to be reconstructed by using either 
the frame-based prediction data or the field-based prediction data of the 
first and second fields as needed, according to the shape encoding mode 
information. 
In accordance with another preferred embodiment of the present invention, 
there is provided a motion picture encoding system which can encode a 
motion picture comprised of a sequence of interlaced images each having 
its texture data and shape data, comprising: a frame-based motion 
detecting unit for detecting a frame-based motion vector of shape for each 
of a plurality of small regions into which the shape data of a current 
interlaced image to be encoded having a pair of first and second fields is 
partitioned; a frame-based motion compensation unit for making a motion 
compensated prediction according to the frame-based motion vector of shape 
so as to generate a frame-based prediction data for shape; a field-based 
motion detecting unit for detecting field-based motion vectors of shape 
for both the first and second fields of each of the plurality of small 
regions to be encoded; a field-based motion compensation unit for making a 
motion compensated prediction according to the field-based motion vectors 
of shape of the first and second fields so as to generate a field-based 
prediction data for shape; an encoding unit for inter-coding the shape 
data of each of the plurality of small regions to be encoded by using the 
frame-based prediction data for shape, and inter-coding the shape data of 
each of the plurality of small regions to be encoded by using the 
field-based prediction data for shape, so as to furnish two types of coded 
shape data; a shape encoding mode selecting unit for selecting one of the 
two types of coded shape data from the encoding unit according to a 
predetermined selection criterion and then furnishing the selected coded 
shape data, and for furnishing shape encoding mode information indicating 
the type of the selected coded shape data, i.e., a shape encoding mode 
according to which the selected coded shape data has been generated; and a 
multiplexing unit for multiplexing the shape encoding mode information and 
the selected coded shape data into a coded bitstream, and further 
multiplexing either the frame-based motion vector of shape or the 
field-based motion vectors of shape which is selected according to the 
shape encoding mode information into the coded bitstream. 
Preferably, the field-based shape motion detecting unit detects field-based 
motion vectors of shape, which are referred to as inter-image field-based 
motion vectors of shape, for both the first and second fields of each of 
the plurality of small regions to be encoded of the current image from the 
shape data of an immediately preceding image, and the field-based shape 
motion detecting unit further detects a field-based motion vector of 
shape, which is referred to as one of intra-image field-based motion 
vectors of shape, for the first field of each of the plurality of small 
regions to be encoded of the current image from the shape data of the 
immediately preceding image and, after that, detects another field-based 
motion vector of shape, which is referred to as another one of the 
intra-image field-based motion vectors of shape, for the second field of 
each of the plurality of small regions to be encoded of the current image 
from the coded shape data of the first field of the current image. 
Furthermore, the field-based shape motion compensation unit makes motion 
compensated prediction for shape of the first and second fields by using 
the intra-image field-based motion vectors of shape so as to generate an 
intra-image field-based prediction data for shape, and further makes 
motion compensated prediction for shape of the first and second fields of 
by using the inter-image field-based motion vectors of shape so as to 
generate an inter-image field-based prediction data for shape. 
In accordance with another preferred embodiment of the present invention, 
there is provided a motion picture encoding system which can encode a 
motion picture comprised of a sequence of interlaced images each having 
its texture data and shape data by using motion compensated prediction, 
comprising: a frame-based motion detecting unit for detecting a 
frame-based motion vector of shape for each of a plurality of small 
regions into which the shape data of an interlaced image to be encoded 
having a pair of first and second fields is partitioned; a frame-based 
motion compensation unit for making a motion compensated prediction 
according to the frame-based motion vector of shape to generate a 
frame-based prediction data for shape; a field-based motion detecting unit 
for detecting field-based motion vectors of shape for both the first and 
second fields of each of the plurality of small regions to be encoded; a 
field-based motion compensation unit for making a motion compensated 
prediction according to the field-based motion vectors of shape of both 
the first and second fields so as to generate a field-based prediction 
data for shape; an encoding unit for intra-coding the shape data of each 
of the plurality of small regions to be encoded, inter-coding the shape 
data of each of the plurality of small regions to be encoded by using the 
frame-based prediction data for shape, or inter-coding the shape data of 
each of the plurality of small regions to be encoded by using the 
field-based prediction data for shape according to information indicating 
a texture encoding mode according to which the corresponding texture data 
of each of the plurality of small regions to be encoded is encoded, so as 
to generate a coded shape data of each of the plurality of small regions; 
and a multiplexing unit for multiplexing the texture encoding mode 
information and the coded shape data into a coded bitstream, and further 
multiplexing either the frame-based motion vector of shape or the 
field-based motion vectors of shape which is selected according to the 
shape encoding mode information into the coded bitstream. 
In accordance with another preferred embodiment of the present invention, 
there is provided a motion picture encoding system which can encode a 
motion picture comprised of a sequence of interlaced images each having 
its texture data and shape data, comprising: a field-based motion 
detecting unit for detecting a field-based motion vector of shape for a 
first field of each of a plurality of small regions into which the shape 
data of an interlaced image to be encoded having a pair of the first field 
and a corresponding second field is partitioned; a field-based motion 
compensation unit for making a motion compensated prediction for shape of 
the first field of each of the plurality of small regions to be encoded 
according to the field-based motion vector of shape of the first field 
from the field-based motion detecting unit so as to generate a field-based 
prediction data for shape of the first field; a first encoding unit for 
intra-coding the shape data of the first field of each of the plurality of 
small regions to be encoded, and inter-coding the shape data of the first 
field of each of the plurality of small regions to be encoded by using the 
field-based prediction data for shape of the first field, so as to 
generate two types of coded shape data of the first field of each of the 
plurality of small regions and a local decoded data of the first field of 
each of the plurality of small regions to be encoded; a shape encoding 
mode selecting unit for selecting one of the two types of coded shape data 
of the first field from the first encoding unit according to a 
predetermined selection criterion and then furnishing the selected coded 
shape data of the first field, and for furnishing shape encoding mode 
information indicating the type of the selected coded shape data of the 
first field, i.e., a shape encoding mode according to which the selected 
coded shape data of the first field has been generated; a prediction 
computing unit for computing a prediction value of each pixel of the shape 
data of the second field of each of the plurality of small regions to be 
encoded by using the local decoded data of the first field furnished by 
the first encoding unit and the shape data of the second field of each of 
the plurality of small regions to be encoded; a second encoding unit for 
encoding a difference between the prediction value computed by the 
prediction computing unit and the actual value of each pixel of the shape 
data of the second field of each of the plurality of small regions to be 
encoded, and for furnishing the coded difference as a coded shape data of 
the second field; and a multiplexing unit for multiplexing the field-based 
motion vector of shape of the first field, the selected coded shape data 
of the first field, the shape encoding mode information, and the coded 
shape data of the second field obtained by the second encoding unit into a 
coded bitstream. 
Preferably, the motion picture encoding system further comprises a unit for 
enabling the prediction computing unit and the second encoding unit when 
receiving information for instructing the encoding of the difference 
between the prediction computed by the prediction computing unit and the 
actual value of each pixel of the shape data of the second field, and for 
disabling the prediction computing unit and the second encoding unit 
otherwise. The multiplexing unit also multiplexes the information for 
instructing the encoding of the difference into the coded bitstream. 
Preferably, the prediction computing unit includes a unit for computing a 
context number for each pixel of the shape data of the second field of 
each of the plurality of small regions to be encoded by using the local 
decoded shape data of the first field furnished by the first encoding unit 
and the shape data of the second field, and a unit for determining a 
prediction value of each pixel of the shape data of the second field of 
each of the plurality of small regions to be encoded from the computed 
context number. 
In accordance with another preferred embodiment of the present invention, 
there is provided a motion picture encoding system which can encode a 
motion picture comprised of a sequence of interlaced images each having 
its texture data and shape data, comprising: a field-based motion 
detecting unit for detecting a field-based motion vector of shape for a 
first field of each of a plurality of small regions into which the shape 
data of an interlaced image to be encoded having a pair of the first field 
and a corresponding second field is partitioned; a field-based motion 
compensation unit for making a motion compensated prediction for shape of 
the first field of each of the plurality of small regions to be encoded 
according to the field-based motion vector of shape of the first field 
from the field-based motion detecting unit to generate a field-based 
prediction data for shape of the first field; a first encoding unit for 
intra-coding the shape data of the first field of each of the plurality of 
small regions to be encoded, and inter-coding the shape data of the first 
field of each of the plurality of small regions to be encoded by using the 
field-based prediction data for shape, so as to generate two types of 
coded shape data and a local decoded shape data of the first field of each 
of the plurality of small regions to be encoded; a shape encoding mode 
selecting unit for selecting one of the two types of coded shape data of 
the first field from the first encoding unit according to a predetermined 
selection criterion and then furnishing the selected coded shape data of 
the first field, and for furnishing shape encoding mode information 
indicating the type of the selected coded shape data, i.e., a shape 
encoding mode according to which the selected coded shape data of the 
first field has been generated; a delta vector detecting unit for, by 
using the local decoded data of the first field furnished by the first 
encoding unit and the shape data of a second field of each of the 
plurality of small regions, detecting a delta vector used for adjusting 
the local decoded shape data of the first field to generate an 
approximation of the shape data of the second field of each of the 
plurality of small regions; a second encoding unit for encoding the delta 
vector so as to generate a coded shape data of the second field of each of 
the plurality of small regions; and a multiplexing unit for multiplexing 
the field-based motion vector of shape of the first field, the selected 
coded shape data of the first field, the shape encoding mode information, 
and the coded shape data of the second field obtained by the second 
encoding unit into a coded bitstream. 
In accordance with another preferred embodiment of the present invention, 
there is provided a motion picture encoding system which can encode a 
motion picture comprised of a sequence of interlaced images each having 
its texture data and shape data, comprising: a frame-based motion 
detecting unit for detecting a frame-based motion vector of shape for each 
of a plurality of small regions into which the shape data of an interlaced 
image having a pair of first and second fields is partitioned; a 
frame-based motion compensation unit for making a motion compensated 
prediction according to the frame-based motion vector of shape so as to 
generate a frame-based prediction data for shape; a field-based motion 
detecting unit for detecting a field-based motion vector of shape for the 
first field of each of the plurality of small regions to be encoded; a 
differential vector detecting unit for detecting a differential vector 
showing a difference between the field-based motion vector of shape of the 
first field and a field-based motion vector of shape of a corresponding 
second field of each of the plurality of small regions to be encoded by 
searching a small area in the vicinity of the field-based motion vector of 
shape of the first field, by using the shape data of the second field; a 
field-based motion compensation unit for making a motion compensated 
prediction for shape of the first field of each of the plurality of small 
regions to be encoded according to the field-based motion vector of shape 
of the first field from the field-based motion detecting unit, and for 
making a motion compensated prediction for shape of the second field of 
each of the plurality of small regions to be encoded according to a 
field-based motion vector of shape of the second field obtained by adding 
the differential vector to the field-based motion vector of shape of the 
first field, so as to generate a field-based prediction data for shape; an 
encoding unit for intra-coding the shape data of each of the plurality of 
small regions to be encoded, inter-coding the shape data of each of the 
plurality of small regions to be encoded by using the frame-based 
prediction data for shape, and inter-coding the shape data of each of the 
plurality of small regions to be encoded by using the field-based 
prediction data for shape, so as to furnish three types of coded shape 
data of each of the plurality of small regions; a shape encoding mode 
selecting unit for selecting one of the three types of coded shape data 
from the encoding unit according to a predetermined selection criterion 
and then furnishing the selected coded shape data, and for furnishing 
shape encoding mode information indicating the type of the selected coded 
shape data, i.e., a shape encoding mode according to which the selected 
coded shape data has been generated; and a multiplexing unit for 
multiplexing the frame-based motion vector of shape, the field-based 
motion vector of shape of the first field, the differential vector, the 
selected coded shape data, and the shape encoding mode information into a 
coded bitstream. 
In accordance with another aspect of the present invention, there is 
provided a method of decoding a coded bitstream obtained by encoding a 
motion picture comprised of a sequence of interlaced images each having 
its texture data and shape data, comprising the steps of: extracting the 
following data from the coded bitstream for each of a plurality of small 
regions included in an interlaced image to be reconstructed; (1) the coded 
shape data, (2) shape encoding mode information indicating whether the 
coded shape data is a data intra-coded or inter-coded, and, in the latter 
case, further indicating whether the coded shape data is a data 
inter-coded with frame-based motion compensated prediction or with 
field-based motion compensated prediction, and (2) a frame-based motion 
vector of shape if the shape encoding mode information indicates that the 
coded shape data is a data inter-coded with field-based motion compensated 
prediction, or field-based motion vectors of shape if the shape encoding 
mode information indicates that the coded shape data is a data inter-coded 
with frame-based motion compensated prediction; if the shape encoding mode 
information indicates that the coded shape data of each of the plurality 
of small regions is a data intracoded, decoding the intra-coded shape 
data; if the shape encoding mode information indicates that the coded 
shape data of each of the plurality of small regions is a data interceded 
with frame-based motion compensated prediction, making a motion 
compensated prediction for shape of each of the plurality of small regions 
to be reconstructed according to the frame-based motion vector of shape so 
as to generate a frame-based prediction data for shape, and then decoding 
the inter-coded shape data by using the frame-based prediction data for 
shape; and, if the shape encoding mode information indicates that the 
shape data of each of the plurality of small regions is a data interceded 
with field-based motion compensated prediction, making a motion 
compensated prediction for shape of each of the plurality of small regions 
to be reconstructed according to the field-based motion vectors of shape 
so as to generate a field-based prediction data for shape, and then 
decoding the inter-coded shape data by using the field-based prediction 
data for shape. 
Further objects and advantages of the present invention will be apparent 
from the following description of the preferred embodiments of the 
invention as illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
First Embodiment 
Referring next to FIG. 1, there is illustrated a block diagram showing the 
structure of a motion picture encoding system according to a first 
embodiment of the present invention. In the figure, reference numeral 1 
denotes an input VOP data, 2 denotes a shape data which is extracted from 
the input VOP data 1, 3 denotes a shape encoding unit which encodes the 
shape data 2, 4 denotes a shape memory which stores a local decoded shape 
data 8 or reference shape data furnished by the shape encoding unit 3, 5 
denotes a motion vector of shape furnished by the shape encoding unit 3, 6 
denotes a coded shape data furnished by the shape encoding unit 3, and 7 
denotes shape prediction mode information indicating a shape prediction 
mode, which is furnished by the shape encoding unit 3. 
Furthermore, reference numeral 9 denotes a texture data which is extracted 
from the input VOP data 1, 10 denotes a frame-based motion detecting unit 
which can receive the texture data 9 and then detect a frame-based motion 
vector 11, 12 denotes a frame-based motion compensation unit which can 
receive the frame-based motion vector 11 and then furnish a frame-based 
prediction data for texture 13, 14 denotes a field-based motion detecting 
unit which can receive the texture data 9 and then furnish a field-based 
motion vector 15, and 16 denotes a field-based motion compensation unit 
which receives the field-based motion vector 15 and then furnishes a 
field-based prediction data for texture 17. 
Furthermore, reference numeral 18 denotes a texture encoding unit which can 
receive the frame-based and field-based prediction data for texture 13 and 
17 and then furnish a coded texture data 19, 21 denotes a texture memory 
which can store a local decoded texture data 20 or reference texture data 
furnished by the texture encoding unit 18, and 22 denotes a variable 
length encoding and multiplexing unit which can receive the motion vector 
of shape 5, the coded shape data 6, the shape prediction mode information 
7, the frame-based and field-based motion vectors 11 and 15, and the coded 
texture data 19, and then furnish a coded bitstream 23. 
The motion picture encoding system according to the present embodiment can 
be so constructed as to encode video object planes or VOP's, like the 
prior art encoding system mentioned above. The description will be 
directed to an improvement in the encoding process by the motion picture 
encoding system when VOP's are interlaced images, which is the primary 
object of the present invention. 
First, a description will be made as to the texture data encoding. The 
texture encoding process is done using frame-based and field-based motion 
compensated prediction in which two inter texture data are generated per 
macroblock using a frame-based motion vector and field-based motion 
vectors, like the prior art encoding system. In order to explain the 
frame-based and field-based motion compensated prediction clearly, the 
motion detecting unit of the first embodiment is divided into the 
frame-based motion detecting unit 10 and the field-based motion detecting 
unit 14, and the motion compensation unit of the first embodiment is 
divided into the frame-based motion compensation unit 12 and the 
field-based motion compensation unit 16, as illustrated in FIG. 1. The 
frame-based motion detecting unit 10 and the frame-based motion 
compensation unit 12 make a frame-based prediction as shown in FIG. 30. 
The frame-based motion detecting unit 10 estimates or detects a frame-based 
motion vector 11 per each macroblock composed of lines from the two fields 
of a frame currently being encoded using a reference texture data stored 
in the texture memory 21. The frame-based motion vector 11 is furnished to 
the frame-based motion compensation unit 12 and a frame-based prediction 
data for texture 13 is read out of a corresponding part of the reference 
texture data stored in the texture memory 21. 
Similarly, the field-based motion detecting unit 14 and the field-based 
motion compensation unit 16 make a field-based prediction as shown in FIG. 
30. The field-based motion detecting unit 14 estimates a field-based 
motion vector 15 per each block composed of lines from only one of the two 
fields of the frame currently being encoded by using the reference texture 
data stored in the texture memory 21, first. As a result, two field-based 
motion vectors are determined for each macroblock comprised of a block 
composed of lines from the top field of the frame and another block 
composed of lines from the bottom field of the frame. When the field-based 
motion compensation unit 16 receives the two field-based motion vectors 15 
from the field-based motion detecting unit 14, it reads a pair of 
field-based prediction data for texture from a corresponding part of the 
reference texture data stored in the texture memory 21. The pair of 
field-based prediction data for texture for the two fields of the frame is 
mixed to generate a prediction data in the form of a frame, which is a 
final result furnished as a prediction data for texture 17 in the 
field-based prediction. 
As a consequence of the frame-based and field-based predictions, the 
frame-based prediction data for texture 13 and the field-based prediction 
data for texture 17 are generated. When the texture encoding unit 18 
receives the frame-based and field-based prediction data for texture 13 
and 17 and the texture data 9 of a macroblock to be encoded, it determines 
whether it selects either the frame-based prediction or the field-based 
prediction, and further determines whether it selects either intra or 
intra-coding mode or inter or inter-coding mode, which is the frame-based 
prediction mode or the field-based prediction mode which has been 
selected. Finally, the texture encoding unit 18 selects the one mode that 
offers the highest degree of encoding efficiency. Either the original or 
intra signal (i.e., the texture data 9) or the prediction error or inter 
signal (i.e., the difference between the frame-based prediction data for 
texture 13 or the field-based prediction data for texture 17 and the 
texture data 9) is encoded using an appropriate compression method such as 
a combination of DCT and scalar quantization. 
Texture encoding mode information indicating the selected texture encoding 
mode: the intra-coding mode, the inter-coding mode with frame-based 
prediction, or the inter-coding mode with field-based prediction, and the 
coded texture data are combined into a coded texture data 19 and then the 
coded texture data 19 is delivered to the variable length encoding and 
multiplexing unit 22. The variable length encoding and multiplexing unit 
22 then multiplexes the coded texture data 19 into a coded bitstream 23 
according to a predetermined syntax. 
Next, a description will be made as to a process for encoding an interlaced 
shape data, which is the primary object of the present invention. 
Referring next to FIG. 2, there is illustrated a block diagram showing the 
structure of the shape encoding unit 3. In the figure, reference numeral 
24 denotes a frame-based shape motion detecting unit which can receive the 
shape data 2 of each of a plurality of alpha blocks to be encoded, into 
which the input VOP shape data 1 of a current VOP included in the 
interlaced frame currently being encoded is partitioned, and then detect a 
frame-based motion vector of shape 25 for each alpha block to be encoded, 
26 denotes a frame-based shape motion compensation unit which can receive 
the frame-based motion vector of shape 25 and then furnish a frame-based 
prediction data for shape 27, 28 denotes a field-based shape motion 
detecting unit which can receive the shape data 2 of each alpha block to 
be encoded and then detect field-based motion vectors of shape 29 for each 
alpha block to be encoded, 30 denotes a field-based shape motion 
compensation unit which can receive the field-based motion vector of shape 
29 and then furnish a field-based prediction data for shape 31. 
Furthermore, reference numeral 32 denotes an arithmetic encoding unit which 
can receive the frame-based and field-based prediction data for shape 27 
and 31 and then furnish a plurality of arithmetic coded shape data 33, 34 
denotes a shape encoding mode selecting unit which can receive the 
plurality of arithmetic-coded shape data 33 and then furnish a coded shape 
data 6 and shape prediction mode information 7 indicating a shape encoding 
mode selected, and 35 denotes a local decoding unit which can receive the 
plurality of arithmetic encoded shape data 33, and the frame-based and 
field-based prediction data for shape 27 and 31, decode one of the 
plurality of arithmetic encoded shape data 33 according to the shape 
prediction mode information 7 from the shape encoding mode selecting unit 
34, and furnish the decoded shape data as a local decoded shape data 8. 
The motion vector of shape 5 in FIGS. 1 and 2 represents a frame-based 
motion vector of shape 25 or a pair of two field-based motion vectors of 
shape 29 of each alpha block to be encoded. 
Next, a description will be made as to motion predictions in the shape data 
encoding process. In the shape data encoding process, a frame-based and 
field-based motion compensated prediction is formed in order to carry out 
motion prediction while making a correction to the difference in the 
position of a moving object between the two fields of an interlaced frame, 
like the texture data encoding process. In the frame-based and field-based 
motion compensated prediction, both a frame-based prediction for shape and 
a field-based prediction for shape are made independently, as will be 
mentioned below. Referring next to FIGS. 3a and 3b, there are illustrated 
diagrams showing the frame-based and field-based motion compensated 
prediction. In the frame-based prediction for shape, a motion vector of 
shape can be detected for each of a plurality of input alpha blocks into 
which the frame currently being encoded is divided, and a prediction data 
for shape can be generated per each alpha block included in the frame by 
using the motion vector of shape, as shown in FIG. 3a. In the field-based 
prediction for shape, each of the plurality of input alpha blocks is 
divided into a first block composed of lines from the top field and a 
second block composed of lines from the bottom field. Then a motion vector 
of shape can be detected for each of the first and second blocks, and a 
pair of prediction data for shape can be generated for the first and 
second blocks or the two fields by using the pair of motion vectors of 
shape, as shown in FIG. 3b. In this case, the pair of prediction data for 
shape for the top and bottom fields of each alpha block to be encoded 
included in the frame currently being encoded is combined into a final 
field-based prediction data for shape in the form of a frame. In other 
words, the pair of prediction data for shape is mixed to generate the 
final field-based prediction data for shape. 
The frame-based shape motion detecting unit 24 and the frame-based shape 
motion compensation unit 26 as shown in FIG. 2 make the frame-based 
prediction for shape. The frame-based shape motion detecting unit 24 
estimates a frame-based motion vector of shape 25 for each alpha block to 
be encoded included in an interlaced frame by using a reference shape data 
stored in the shape memory 4. The process of detecting a motion vector of 
shape per alpha block can be done using the block matching method 
mentioned above which is applied to the shape data plane or alpha plane. 
Any procedure of searching an area surrounding the location of the alpha 
block being tested can be used. 
In FIG. 2, it is assumed that the shape encoding unit detects a motion 
vector of shape for the alpha block being tested using a prior art method 
of referring to a motion vector of texture which was detected for a 
corresponding macroblock at the same location through the frame-based 
prediction for texture, and motion vectors of shape of alpha blocks in the 
vicinity of the alpha block being tested or to be encoded, and then 
searching a small area surrounding the location of the alpha block being 
tested. Alternatively, an area surrounding the alpha block currently being 
encoded can be simply searched by using only the reference shape data. 
When the frame-based shape motion compensation unit 26 receives the 
frame-based motion vector of shape 25 of each alpha block to be encoded, 
it extracts a frame-based prediction data for shape 27 from a 
corresponding part of the reference shape data stored in the shape memory 
4. 
Similarly, the field-based shape motion detecting unit 28 and the 
field-based shape motion compensation unit 30 as shown in FIG. 2 make the 
field-based prediction for shape. By using the reference shape data stored 
in the shape memory 4, the field-based shape motion detecting unit 28 
estimates or detects a field-based motion vector of shape 29 for the first 
block of each alpha block to be encoded, which is composed of lines from 
the top field, and also estimates or detects another field-based motion 
vector of shape 29 for the second block of each alpha block to be encoded, 
which is composed of lines from the bottom field, as shown in FIG. 3b. As 
a result, the pair of motion vectors of shape 29 is created for each alpha 
block to be encoded. The pair of field-based motion vectors of shape 29 
for each alpha block to be encoded can be determined by referring to the 
motion vector of texture of a corresponding macroblock at the same 
location (i.e., the frame-based motion vector of texture 11 or the pair of 
field-based motion vectors of texture 15) which can be estimated with the 
frame-based or field-based prediction for texture, and motion vectors of 
shape of alpha blocks in the vicinity of the alpha block being tested or 
to be encoded, and then searching a small area surrounding the location of 
the alpha block being tested. 
When the field-based shape motion compensation unit 30 receives the pair of 
field-based motion vectors of shape 29, it reads a pair of field-based 
prediction data for shape 27 associated with the two fields from a 
corresponding part of the reference shape data stored in the shape memory 
4. The pair of field-based prediction data for shape of each alpha block 
which is associated with the top and bottom fields of the frame currently 
being encoded is then combined into a final field-based prediction data 
for shape 31 in the form of a frame. 
Next, a description will be made as to the steps of arithmetic encoding and 
selecting one of different shape encoding modes in the shape data encoding 
process. The arithmetic encoding unit 32 determines a code word by 
computing the probability that the value of each pixel to be coded of the 
alpha block currently being coded is 0 or 1 by using the same method as 
mentioned in the prior art. The three types of data, i.e., the shape data 
2 and the frame-based and field-based prediction data for shape 27 and 31 
are applied to the arithmetic encoding unit 32. The arithmetic encoding 
unit 32 intra-codes the shape data 2, inter-codes the shape data 2 using 
the frame-based prediction data for shape 27, and inter-codes the shape 
data 2 using the field-based prediction data for shape 31. The arithmetic 
encoding unit 32 then furnishes three types of coded data which have been 
generated in the respective shape encoding modes. 
In the present preferred embodiment, the probability that the value of the 
target pixel currently being coded is 0 or 1 is obtained by indexing a 
probability table prepared using an intra context or an inter context. An 
inter context can be computed for each of the frame-based and field-based 
prediction data for shape. Only one context can be prepared for both the 
frame-based and field-based prediction data for shape, and alternatively 
two different inter contexts can be prepared for the frame-based 
prediction data for shape and the field-based prediction data for shape, 
respectively. Furthermore, only one probability table can be prepared for 
inter mode. Alternatively, two different probability tables can be 
prepared for inter mode with frame-based prediction for shape and inter 
mode with field-based shape prediction for shape, respectively. In the 
present preferred embodiment, only one inter context is prepared, like the 
prior art encoding system. Furthermore, only a corresponding probability 
table is prepared for inter mode. Accordingly, in the arithmetic encoding 
process of the present embodiment, the process for computing probabilities 
for the field-based prediction data for shape 31 is added, as compared 
with the prior art encoding system. 
When the shape encoding mode selecting unit 34 selects one from among the 
intra or inter-coding mode, the inter or inter-coding mode with 
frame-based prediction for shape, and the inter mode with field-based 
prediction for shape, the shape encoding mode selecting unit 34 selects 
the one that offers a coded data having the shortest code length. The 
shape encoding mode selecting unit 34 furnishes the coded data which has 
been encoded in the selected mode as the coded shape data 6, and the shape 
encoding mode information 7 indicating the selected shape encoding mode. 
The motion vector of shape 5, the coded shape data 6, and the shape 
encoding mode information 7 thus obtained are then transferred to the 
variable length encoding and multiplexing unit 22, as shown in FIG. 1. The 
variable length encoding and multiplexing unit 22 then multiplexes those 
data together with the coded texture data into a coded bitstream 23 
according to a predetermined syntax. 
When the local decoding unit 35 receives the plurality of arithmetic coded 
shape data 33 from the arithmetic encoding unit 32, and the frame-based 
and field-based prediction data for shape 27 and 31, it decodes one of the 
plurality of arithmetic coded shape data 33 according to the shape 
encoding mode information 7 and then furnishes the decoded shape data as a 
local decoded shape data 8. For example, when the shape encoding mode 
information 7 indicates the inter mode with frame-based prediction for 
shape, the local decoding unit 35 decodes the shape data 33 arithmetic 
encoded in the inter mode using the frame-based prediction data for shape 
27. The local decoded shape data 8 is then stored in the shape memory 4, 
as shown in FIG. 1, and is also delivered to the frame-based motion 
detecting unit 10, the frame-based motion compensation unit 12, and the 
texture encoding unit 18 for the process for encoding the corresponding 
texture data. 
As previously mentioned, according to the first embodiment of the present 
invention, even when encoding the shape data of each interlaced frame to 
be encoded, motion predictions can be formed independently for a pair of 
the top and bottom fields of each interlaced frame, and therefore the 
encoding efficiency in the inter mode can be improved as compared with the 
prior art. 
Numerous variants may be made in the exemplary embodiment mentioned above. 
Instead of, when encoding each alpha block in the inter mode with 
field-based prediction for shape, encoding each alpha block by combining 
the pair of field-based shape prediction data for the top and bottom 
fields of each alpha block included in an interlaced frame currently being 
coded into a field-based prediction data for shape 31 in the form of a 
frame, as shown in FIG. 4a, the shape encoding unit 3 can arithmetically 
encode each alpha block per each of the top and bottom fields using each 
of the pair of field-based prediction data for shape, as shown in FIG. 4b. 
The variant can offer the same advantage as that offered by the first 
embodiment. 
While a motion picture encoding system according to another variant of the 
aforementioned first embodiment has the same structure as shown in FIGS. 1 
and 2, the field-based shape motion detecting unit 28 and the field-based 
shape motion compensation unit 30 operate differently from those of the 
first embodiment. The field-based shape motion detecting unit 28 and the 
field shape motion compensation unit 30 of this variant make an inter-VOP 
field-based prediction for shape, which corresponds to the field-based 
prediction for shape as previously explained in the first embodiment. In 
addition, they can make an intra-VOP field-based prediction for shape, 
which will be described below. 
The difference between the inter-VOP field-based prediction for shape and 
the intra-VOP field-based prediction for shape will be explained with 
reference to FIGS. 5a and 5b. In the inter-VOP field-based prediction for 
shape, a motion vector for each of the two fields can be estimated from 
the decoded shape data of the immediately preceding VOP stored in the 
shape memory 4. A shape prediction data for each of the two fields is 
extracted from the coded shape data of the immediately preceding VOP 
stored in the shape memory according to the detected motion vector of 
shape. Finally, the pair of extracted shape prediction data for both the 
top and bottom fields is mixed to generate an inter-VOP field-based shape 
prediction data in the form of frame, as shown in FIG. 5a. 
In the intra-VOP field-based prediction for shape, a prediction for the top 
field is formed using the coded shape data of the immediately preceding 
VOP, for example. Then a prediction for the bottom field is formed using 
the shape data of the encoded part of the current VOP, e.g., the shape 
data of the top field of the current VOP. Finally, the pair of shape 
prediction data for both the top and bottom fields is mixed to generate an 
intra-VOP field-based shape prediction data in the form of frame, as shown 
in FIG. 5b. 
Accordingly, the motion picture encoding system according to this variant 
of the first embodiment can offer the four different shape encoding modes: 
the intra mode, the inter mode with inter-VOP field-based prediction for 
shape, the inter mode with intra-VOP field-based prediction for shape, and 
the inter mode with frame-based prediction for shape. In either one of the 
four different shape encoding modes, the arithmetic encoding unit 32 can 
perform its encoding operation in the same manner as that mentioned in the 
exemplary embodiment mentioned above. The shape encoding mode selecting 
unit 34 then selects the one that can offer a coded shape data having the 
shortest code length from among the four different shape encoding modes. 
Both the coded shape data 6 and the shape encoding mode information 7 are 
delivered to the variable length encoding and multiplexing unit 22, and 
the variable length encoding and multiplexing unit 22 then multiplexes 
them into a coded bitstream 23 according to a predetermined syntax. 
As previously mentioned, the motion picture encoding system of this variant 
employs the same encoding method as the prior art encoding system. It is 
apparent from the above description that even when encoding an interlaced 
shape data using another encoding method other than the arithmetic 
encoding method mentioned above, the variant shown can provide the 
high-efficiency encoding process by carrying out motion prediction while 
making a correction to the difference in the position of a moving object 
between the two fields of an interlaced frame. 
As previously explained, this variant of the first embodiment of the 
present invention can offer an advantage of being able to improve the 
encoding efficiency in the inter mode by making a correction to the 
difference in the position of a moving object between the two fields of an 
interlaced frame even when encoding an interlaced shape data. 
When intra-coding an interlaced VOP by using the variant of the first 
embodiment, the encoding process can be done efficiently as follows. The 
encoding process can be done for each of the two fields of the interlaced 
VOP. For example, the shape data of one part of the VOP in one of the two 
fields can be intracoded first. Then either intra-coding or inter-coding 
with intra-VOP field-based prediction for shape can be done selectively 
for the other part of the VOP in the other field. Using this encoding 
method, the shape data of the interlaced VOP can be encoded with a smaller 
amount of codes, as compared with the case, such as using the prior art 
method, in which the shape data of each of the two fields cannot but be 
intracoded. 
In another variant of the first embodiment, instead of arithmetic encoding 
each alpha block by combining a pair of field-based shape prediction data 
for the top and bottom fields of each alpha block included in an 
interlaced frame currently being encoded into a field shape prediction 
data 31 in the form of a frame when the inter mode with inter-VOP 
field-based prediction for shape or intra-VOP field-based prediction for 
shape is selected, as shown in FIG. 4a, the shape encoding unit 3 can 
arithmetic encode each of the top and bottom fields of each alpha block 
using each of the pair of field-based shape prediction data for the top 
and bottom fields, as shown in FIG. 4b. The other variant can offer the 
same advantage as that offered by the variant mentioned above. 
Second Embodiment 
Referring next to FIG. 6, there is illustrated a block diagram showing the 
structure of a motion picture decoding system according to a second 
embodiment of the present invention. In the figure, reference numeral 36 
denotes a coded bitstream, 37 denotes a syntax analyzing and variable 
length decoding unit which can receive the coded bitstream 36 and then 
generate and furnish a coded shape data 38, a motion vector of shape 39, 
shape encoding mode information 40, a coded texture data 44, a frame-based 
motion vector 45, and field-based motion vectors 48, 41 denotes a shape 
decoding unit which can receive the coded shape data 38, the motion vector 
of shape 39, and the shape encoding mode information 40, and then generate 
and furnish a decoded shape data 43, and 42 denotes a shape memory for 
storing the decoded shape 43. 
Furthermore, reference numeral 46 denotes a frame-based motion compensation 
unit which can receive the frame-based motion vector 45 and then generate 
and furnish a frame-based prediction data for shape 47, 49 denotes a 
field-based motion compensation unit which can receive the field-based 
motion vectors 48 and then generate and furnish a field-based prediction 
data for texture 50, 52 denotes a texture decoding unit which can receive 
the coded texture data 44, the frame-based prediction data for texture 47, 
and the field-based prediction data for texture 50, and then generate and 
furnish a decoded texture data 53, and 51 denotes a texture memory for 
storing the decoded texture data 53. 
The motion picture decoding system according to the second embodiment of 
the present invention can be so constructed as to decode coded video 
object planes or VOP's. The description will be directed to an improvement 
in the decoding process by the motion picture decoding system when VOP's 
are interlaced images, which is the primary object of the present 
invention. 
First, a description will be made as to the syntax analyzing process and 
the variable length decoding process of this embodiment. When the syntax 
analyzing and variable length decoding unit 37 receives the coded 
bitstream 36, it isolates or extracts the coded shape data 38, the motion 
vector of shape 39, the shape encoding mode information 40, the coded 
texture data 44, the frame-based motion vector 45, and the field-based 
motion vectors 48 from the coded bitstream 36 applied thereto, for each of 
alpha blocks into which the coded shape data of a current VOP to be 
decoded included in an interlaced frame is partitioned. The coded shape 
data 38, the motion vector of shape 39, the shape encoding mode 
information 40 are then delivered to the shape decoding unit 41, the coded 
texture data 44 is delivered to the texture decoding unit 52, the 
frame-based motion vector 45 is delivered to the frame-based motion 
compensation unit 46, and the field-based motion vectors 48 are delivered 
to the field-based motion compensation unit 49. The coded texture data 44 
is decoded per macroblock. The coded shape data 38 is decoded per alpha 
block. 
Next, the description will be directed to motion compensation in the 
texture decoding process. The syntax analyzing and variable length 
decoding unit 37 decodes the coded information indicating the texture 
encoding mode which was selected upon the process for encoding the texture 
data and then examines the decoded mode information. When the texture 
encoding mode information indicates that the inter mode with frame-based 
prediction was selected, the syntax analyzing and variable length decoding 
unit 37 isolates a frame-based motion vector 45 for each macroblock from 
the coded bitstream 36 and then furnishes it to the frame-based motion 
compensation unit 46. The frame-based motion compensation unit 46 then 
extracts a frame-based prediction data for texture 47 from the texture 
memory 51 according to the frame-based motion vector 45 applied thereto, 
as shown in FIG. 30. On the other hand, when the texture encoding mode 
information indicates that the inter mode with frame-based prediction was 
selected, the syntax analyzing and variable length decoding unit 37 
isolates two field-based motion vectors 45 for each macroblock from the 
coded bitstream 36 and then decodes and furnishes them to the field-based 
motion compensation unit 49. The field-based motion compensation unit 49 
then extracts a pair of field-based prediction data for texture 47 for the 
pair of top and bottom field from the texture memory 51 according to the 
field-based motion vectors 48 applied thereto, and mixes the pair of 
field-based prediction data for texture 47 to generate a field-based 
prediction data for texture 50, as shown in FIG. 30. When the texture 
encoding mode information indicates that the intra mode was selected, no 
motion vector is extracted from the coded bitstream 36 and hence no motion 
compensation is carried out prior to the texture decoding process. 
Next, the description will be directed to the texture decoding process. The 
syntax analyzing and variable length decoding unit 37 extracts a coded 
texture data 44 from the coded bitstream 36 applied thereto and then 
delivers it to the texture decoding unit 52. When the texture encoding 
mode information indicates that the intra mode was selected when encoding 
the texture data, the coded texture data 44 of the macroblock to be 
decoded is the data which was obtained by encoding the intra signal of the 
corresponding macroblock. When the texture encoding mode information 
indicates that the inter mode was selected when encoding the texture data, 
the coded texture data 44 is the data which was obtained by encoding the 
prediction error or difference between the intra signal of the 
corresponding macroblock and the prediction data for texture which has 
been obtained by the above-mentioned motion compensation process. The 
texture decoding unit 52 reconverts the coded texture data 44 into a 
decoded texture data 53 by reverse quantization, reverse DCT, and so on. 
In addition, the texture decoding unit 52 adds the frame-based or 
field-based prediction data for texture 47 or 50 from the frame-based or 
field-based motion compensation unit 46 or 49 to the decoded texture data 
53 so as to generate the final decoded texture data 53 in the inter mode. 
Next, the description will be directed to the shape decoding process. 
Referring next to FIG. 7, there is illustrated a block diagram showing the 
structure of the shape decoding unit 41. In the figure, reference numeral 
54 denotes a frame-based motion vector of shape included in the motion 
vectors of shape 39, 55 denotes a frame-based shape motion compensation 
unit which can receive the frame-based motion vector of shape 54 and then 
generate and furnish a frame-based prediction data for shape 56, 57 
denotes field-based motion vectors of shape included in the motion vectors 
of shape 39, 58 denotes a field-based shape motion compensation unit which 
can receive the field-based motion vectors of shape 57 and then generate 
and furnish a field-based prediction data for shape 59, and 60 denotes an 
arithmetic decoding unit which can receive the frame-based prediction data 
for shape 56, the field-based prediction data for shape 59, the coded 
shape data 38, and the shape encoding mode information 40, and then 
generate and furnish a decoded shape data 43. 
By using the following procedures, the coded shape data will be decoded. 
(1) The shape encoding mode information is decoded. 
(2) One or more motion vectors of shape are decoded. 
(3) Motion compensation is carried out for the shape decoding process. 
(4) Arithmetic encoding including a context calculation is carried out. 
The syntax analyzing and variable length decoding unit 37 performs the 
process (1) for encoding the shape encoding mode information. The syntax 
analyzing and variable length decoding unit 37 further performs the 
process (2) for encoding one or more motion vectors of shape 39 according 
to the decoded shape encoding mode information 40. When the shape encoding 
mode information 40 represents the intra mode, no motion vector is applied 
to the shape decoding unit. When the shape encoding mode information 40 
represents the inter mode with frame-based prediction for shape, one 
frame-based motion vector of shape 54 is applied to the frame-based shape 
motion compensation unit of the shape decoding unit and it is then decoded 
for each alpha block. When the shape encoding mode information 40 
represents the inter mode with intra-VOP field-based prediction for shape 
or inter-VOP field-based prediction for shape, two field-based motion 
vectors of shape 57 are applied to the field-based shape motion 
compensation unit of the shape decoding unit and they are then decoded for 
each alpha block. 
The above-mentioned processes (3) and (4) will be described for each of the 
shape encoding modes hereinafter. When the shape encoding mode information 
40 represents the intra mode, the motion compensation process (3) is not 
carried out. The coded shape data 38 extracted by the syntax analyzing and 
variable length decoding unit 37 is applied to the arithmetic decoding 
unit 60, and the arithmetic decoding unit 60 then computes a context for 
each pixel of the alpha block currently being decoded as shown in FIG. 29a 
from only the coded shape data 38 of the alpha block, and estimates the 
probability that the value of the target pixel is 0 or 1 by referring to 
the same probability table as that used when encoding the shape data. The 
coded shape data 38 of the alpha block is thus decoded into a sequence of 
8-bit binary data each having a value of 0 or 255 in decimal. 
When the shape encoding mode information 40 represents the inter mode with 
frame-based prediction for shape, the frame-based shape motion 
compensation unit 55 shown in FIG. 7 carries out motion compensation for 
the shape data. The frame-based shape motion compensation unit 55 extracts 
a frame-based prediction data for shape 56 as shown in FIG. 3 from the 
shape memory 42 according to the frame-based motion vector of shape 
applied thereto. The frame-based shape motion compensation unit 55 then 
delivers the frame-based prediction data for shape 56 to the arithmetic 
decoding unit 60. The arithmetic decoding unit 60 computes a context 
number for each pixel of the alpha block currently being decoded from the 
coded shape data 38 extracted by the syntax analyzing and variable length 
decoding unit 37 and the frame-based prediction data for shape 56 
extracted by the frame-based shape motion compensation unit 55, as shown 
in FIG. 29b. The arithmetic decoding unit 60 then estimates the 
probability that the value of each pixel is 0 or 1 by referring to the 
same probability table as that used when encoding the shape data. The 
coded shape data 38 of the alpha block is thus decoded into a sequence of 
8-bit binary data each having a value of 0 or 255 in decimal. 
When the shape encoding mode information 40 represents the inter mode with 
inter-VOP field-based prediction for shape, the field-based shape motion 
compensation unit 58 shown in FIG. 7 carries out motion compensation for 
the shape data of the alpha block currently being decoded. The field-based 
shape motion compensation unit 58 extracts a field-based prediction data 
for shape 59 as shown in FIG. 3 from the shape memory 42 according to the 
two field-based motion vectors of shape 57 applied thereto. The 
field-based shape motion compensation unit 58 then delivers the 
field-based prediction data for shape 59 to the arithmetic decoding unit 
60. The arithmetic decoding unit 60 then computes a context number for 
each pixel of the alpha block according to a context construction as shown 
in FIG. 29b, or according to another context construction used when 
encoding the shape data, from the coded shape data 38 extracted by the 
syntax analyzing and variable length decoding unit 37 and the field-based 
prediction data for shape 59 extracted by the field-based shape motion 
compensation unit 58. The arithmetic decoding unit 60 then estimates the 
probability that the value of each pixel of the alpha block is 0 or 1 by 
referring to the same probability table as that used when encoding the 
shape data. The coded shape data 38 is thus decoded into a sequence of 
8-bit binary data each having a value of 0 or 255 in decimal. 
When the shape encoding mode information 40 represents the inter mode with 
intra-VOP field-based prediction for shape, the field-based shape motion 
compensation unit 58 shown in FIG. 7 carries out motion compensation for 
the shape data. The field-based shape motion compensation unit 58 extracts 
intra-VOP field-based prediction data for shape 59 (for simplicity, the 
same reference numeral as the inter-VOP field-based prediction data for 
shape is assigned to the intra-VOP field-based prediction data for shape) 
as shown in FIG. 5 from the shape memory 42 according to the two 
field-based motion vectors of shape 57 applied thereto. The field-based 
shape motion compensation unit 58 then delivers the intra-VOP field-based 
prediction data for shape 59 to the arithmetic decoding unit 60. The 
arithmetic decoding unit 60 computes a context number for each pixel of 
the alpha block currently being decoded according to a context 
construction as shown in FIG. 29b, or according to another context 
construction used when encoding the shape data, from the coded shape data 
38 extracted by the syntax analyzing and variable length decoding unit 37 
and the field-based prediction data for shape 59 extracted by the 
field-based shape motion compensation unit 58. The arithmetic decoding 
unit 60 then estimates the probability that the value of each pixel of the 
alpha block is 0 or 1 by referring to the same probability table as that 
used when encoding the shape data. The coded shape data 38 is thus decoded 
into a sequence of 8-bit binary data each having a value of 0 or 255 in 
decimal. 
The shape data 43 which has been reconstructed as a sequence of 8-bit 
binary data each having a value of 0 or 255 in decimal through any one of 
the above processes in the four different decoding modes is then delivered 
to both the frame-based and field-based motion compensation units 46 and 
49 for texture decoding and is also stored in the shape memory 42 for 
later alpha block decoding. 
As previously mentioned, the decoding system according to the second 
embodiment can decode an interlaced, coded shape data by carrying out 
motion compensation while making a correction to the difference in the 
position of a moving object between the two fields of an interlaced frame. 
Thus the present embodiment offers an advantage of being able to encode 
the shape data more smoothly as compared with the prior art decoding 
system. 
Numerous variants may be made in the exemplary embodiment shown. It is 
apparent that instead of the same arithmetic decoding method as that used 
in the prior art decoding system, another decoding method other than the 
arithmetic decoding method, which corresponds to an encoding method used 
when encoding the shape data, can be used by replacing the arithmetic 
decoding unit with another unit which conforms to the other decoding 
method. 
In anther variant, when the shape encoding mode information 40 represents 
the inter mode with inter-VOP field-based prediction for shape or 
intra-VOP field-based shape prediction for shape, instead of combining the 
pair of field-based prediction data for shape of the top and bottom fields 
of each alpha block to be decoded included in an interlaced frame into a 
field-based prediction data for shape 31 in the form of a frame, as shown 
in FIG. 8a, a shape prediction data can be provided independently for each 
of the top and bottom fields so as to decode the coded shape data of each 
alpha block to be decoded of the frame. This variant can offer the same 
advantage as that offered by the preferred embodiment mentioned above. The 
decoded results for the top and bottom fields are combined into a result 
in the form of frame. The reconstructed frame is then displayed. 
Third Embodiment 
Referring next to FIG. 9, there is illustrated a block diagram showing the 
structure of a motion picture encoding system according to a third 
embodiment of the present invention. In the figure, reference numeral 61 
denotes texture prediction mode information indicating a texture encoding 
mode selected. The other components in the figure are the same as those of 
the first embodiment shown in FIG. 1. 
The motion picture encoding system according to the third embodiment of the 
present invention can be so constructed as to encode video object planes 
or VOP's. The description will be directed to an improvement in the 
decoding process by the motion picture decoding system when VOP's are 
interlaced images, which is the primary object of the present invention. 
In the first embodiment, the texture encoding mode and the shape encoding 
mode are determined independently. On the contrary, in the present 
embodiment, the texture and shape encoding process can be done while the 
texture encoding mode information and the shape encoding mode information 
are determined in relation to each other. In general, when encoding an 
interlaced signal, the selection of field-based prediction upon texture 
encoding points to the conclusion that there exist a noticeable difference 
in the position of a moving object between the two fields of an interlaced 
frame. Accordingly, in this case, it is appropriate to carry out 
field-based prediction for the shape data as well. On the contrary, the 
selection of frame-based prediction upon texture encoding points to the 
conclusion that there does not exist a noticeable difference in the 
position of a moving object between the two fields of an interlaced frame. 
Accordingly, in this case, it is appropriate to carry out frame-based 
prediction for the shape data as well. From this point of view, the shape 
encoding mode is varied according to the texture prediction mode in order 
to reduce the amount of computations required for determining the shape 
encoding mode and the amount of codes included in the coded shape encoding 
mode information. 
To be more specific, the texture encoding unit 18 furnishes the texture 
encoding mode information 61 to the shape encoding unit 3, as shown in 
FIG. 9. The shape encoding unit 3 carries out a process for encoding the 
shape data 2 according to the texture encoding mode information 61. 
Referring next to FIG. 10, there is illustrated a block diagram showing the 
structure of the shape encoding unit 3. The arithmetic encoding unit 32 
does not perform the encoding process in either one of all the shape 
encoding modes. That is, the arithmetic encoding unit 32 performs the 
encoding process in only one encoding mode selected according to the 
texture encoding mode information 61 furnished by the texture encoding 
unit 18, unlike the aforementioned first embodiment of the present 
invention. 
For example, the texture encoding mode information 61 can have four 
varieties of values indicating the intra mode, the inter mode with 
frame-based prediction, the inter mode with inter-VOP field-based 
prediction, and the inter mode with intra-VOP field-based prediction. The 
arithmetic encoding unit 32 of the shape encoding unit 3 performs the 
encoding process in only one selected mode which is defined by the texture 
encoding mode information 61. The shape encoding unit 3 then furnishes a 
coded shape data 6 to the variable length encoding and multiplexing unit 
22. In the encoding system of this embodiment, the texture encoding mode 
information 61 is multiplexed into a coded bitstream. Accordingly, this 
results in eliminating a process for encoding and multiplexing the shape 
encoding mode information into the bitstream. 
As previously mentioned, the present embodiment offers an advantage of 
being able to reduce the amount of computations in the shape encoding 
process. Furthermore, since it is unnecessary to encode the shape encoding 
mode information, the amount of codes included in the coded bitstream can 
be reduced. 
In a variant, when the inter mode with field-based prediction for shape is 
selected, the encoding process can be done independently for each of the 
top and bottom fields of each alpha block to be encoded of an interlaced 
frame, instead of encoding the alpha block by combining the pair of 
field-based prediction data for shape for the top and bottom fields of the 
alpha block of the interlaced frame into a field-based prediction data for 
shape in the form of a frame by means of the shape encoding unit 3. The 
variant can offer the same advantage as that offered by the third 
embodiment mentioned above. 
Fourth Embodiment 
Referring next to FIG. 11, there is illustrated a block diagram showing the 
structure of a motion picture decoding system according to a fourth 
embodiment of the present invention. In the figure, reference numeral 62 
denotes texture encoding mode information indicating a texture encoding 
mode selected when encoding the texture data. Referring now to FIG. 12, it 
illustrates a block diagram showing the structure of the shape decoding 
unit 41. The motion picture decoding system of this embodiment differs 
from that of the second embodiment shown in FIGS. 6 and 7 in that the 
texture encoding mode information 62, instead of the shape encoding mode 
information, is applied to the shape decoding unit 41. 
The syntax analyzing and variable length decoding unit 37 decodes the 
texture encoding mode information coded per macroblock and then delivers 
the decoded texture encoding mode information 62 as the shape encoding 
mode information to the shape decoding unit 41 so that the shape decoding 
unit 41 performs a decoding process in the shape decoding mode indicated 
by the shape encoding mode information, i.e., the texture encoding mode 
information 62. The texture encoding mode information 62 can have the same 
varieties of values as the shape encoding mode information. The shape 
decoding unit 41 performs a decoding process in the same manner as that of 
the second embodiment. 
As previously mentioned, the decoding system according to the fourth 
embodiment can decode the coded shape data of an interlaced frame by 
carrying out motion compensation while making a correction to the 
difference in the position of a moving object between the two fields of 
the interlaced frame. Thus the present embodiment offers an advantage of 
being able to encode the shape data more smoothly as compared with the 
prior art decoding system. 
Numerous variants may be made in the exemplary embodiment mentioned above. 
It is apparent that instead of the same arithmetic decoding method as that 
used in the prior art decoding system, another decoding method other than 
the arithmetic decoding method, which corresponds to an encoding method 
used when encoding the shape data, can be used by replacing the arithmetic 
decoding unit with another unit which conforms to the other decoding 
method. 
In another variant, when the inter mode with inter-VOP field-based 
prediction for shape or intra-VOP field-based prediction for shape is 
selected as the shape encoding mode dependent on the texture encoding 
mode, a prediction data for shape can be generated independently for each 
of the top and bottom fields of each alpha block to be decoded so as to 
decode the coded shape data, instead of decoding the coded shape data of 
the alpha block by combining the pair of field-based prediction data for 
shape for the top and bottom fields of each alpha block to be decoded into 
a field-based prediction data for shape in the form of a frame. The 
variant can offer the same advantage as that offered by the 
above-mentioned exemplary embodiment. The decoded results for the top and 
bottom fields are combined into a decoded result in the form of a frame 
and the frame is then displayed. 
Fifth Embodiment 
Referring next to FIG. 13, there is illustrated a block diagram showing the 
structure of a motion picture encoding system according to a fifth 
embodiment of the present invention. In the figure, reference numeral 63 
denotes a motion detecting unit which for example corresponds to the 
combination of the frame-based motion detecting unit 10 and the 
field-based motion detecting unit 14 as shown in FIG. 1, 64 denotes motion 
vectors which for example correspond to the combination of the frame-based 
motion vector 11 and the field-based motion vectors 15 as shown in FIG. 1, 
65 denotes a motion compensation unit which for example corresponds to the 
combination of the frame-based motion compensation unit 12 and the 
field-based motion compensation unit 16, and 66 denotes to a prediction 
data for texture which for example corresponds to the combination of the 
frame-based prediction data for texture 13 and the field-based prediction 
data for texture 17 as shown in FIG. 1. 
The description will be directed to an improvement in the encoding process 
by the motion picture encoding system of this embodiment when VOP's are 
interlaced images, which is the primary object of the present invention. 
First, a description will be made as to the texture encoding process. In 
the present embodiment, the texture encoding process can be done in the 
same manner as the first embodiment mentioned above, for example. As 
illustrated in FIG. 13, the motion detecting unit 63 is not divided into a 
frame-based motion detecting section and a field-based motion detecting 
section, and the motion compensation unit 65 is not divided into a 
frame-based motion compensation section and a field-based motion 
compensation section. This means that what internal processes are carried 
out in the motion detecting unit 63 and the motion compensation unit 65 is 
a matter of indifference. The shape encoding unit 3 requires only a motion 
vector data, which was estimated per macroblock in the texture data 
prediction, for the motion prediction for the shape data. The motion 
vector data can be a frame-based one or a field-based one. 
Next, a description will be made as to the shape data encoding process. 
Referring next to FIG. 14, there is illustrated a block diagram showing 
the structure of the shape encoding unit 3. In the figure, reference 
numeral 67 denotes a field-based shape motion detecting unit which can 
receive the shape data 2 and then detect field-based motion vectors of 
shape 68, 69 denotes a field-based shape motion compensation unit which 
can receive the field-based motion vectors of shape 68 and then generate 
and furnish a field-based prediction data for shape 70, 71 denotes an 
arithmetic encoding unit which can receive the field-based prediction data 
for shape 70, and then arithmetic encode the shape data 2 of a first field 
of each alpha block to be encoded in both intra mode and inter mode, and 
72 denotes a shape encoding mode selecting unit which can determine one of 
the two different encoding modes that provides a coded result having a 
shorter code length, and then furnish a coded shape data 78 of the first 
field and shape encoding mode information 7 indicating the selected shape 
encoding mode. 
Furthermore, reference numeral 73 denotes a local decoded data of the first 
field furnished by the shape encoding mode selecting unit 72, 74 denotes a 
context computing unit which can receive the shape data 2 and then compute 
and furnish a context number 75 used for inter-line prediction which will 
be described below, 76 denotes a prediction determining unit which can 
receive the context number 75 and then determine a prediction or predicted 
value, 77 denotes an entropy encoding unit which can receive the 
difference between the prediction determined by the prediction determining 
unit 76 and the shape data 2 and then furnish a coded shape data 79 of a 
corresponding second field, and 101 denotes a prediction computing unit 
constructed of the context computing unit 74 and the prediction 
determining unit 76. 
In the fifth embodiment, arithmetic encoding with motion prediction is 
carried out first for one of the two fields of each alpha block to be 
encoded, which will be referred to as the first field the shape data of 
which is encoded before encoding the shape data of the other field which 
will be referred to as the second field. For the second field of each 
alpha block to be encoded, a prediction is made in the same space by using 
both the local decoded data of the first field and the coded data of alpha 
blocks in the vicinity of the current alpha block which are stored in the 
shape memory so as to arithmetic encode the prediction error of each pixel 
of the current alpha block to be encoded, which shows the difference 
between the prediction or predicted value determined by the prediction 
determining unit 76 and the actual value or shape data of each pixel. 
Next, the description will be directed to the encoding process for the 
first field. In the motion prediction for the first field, inter-VOP 
field-based prediction for shape is made. The field-based shape motion 
detecting unit 67 detects a field-based motion vector of shape 68 for the 
first field of each alpha block to be encoded of an interlaced frame. At 
that time, the field-based shape motion detecting unit 67 refers to a 
motion vector data 64 furnished by the texture encoding part of the 
encoding system. When the field-based shape motion compensation unit 69 
receives the field-based motion vector of shape 68 from the field-based 
shape motion detecting unit 67, it generates a field-based prediction data 
for shape 70. The arithmetic encoding unit 71 then arithmetic encodes the 
shape data 2 of the first field in the intra mode and further arithmetic 
encodes the shape data 2 of the first field using the field-based 
prediction data for shape 70 in the inter mode. The shape encoding mode 
selecting unit 72 selects either the coded result obtained in the intra 
mode or the coded result obtained in the inter mode. The shape encoding 
mode selecting unit 72 selects the one having a shorter code length. The 
selected arithmetic-coded result is then furnished as the coded shape data 
78 of the first field together with the shape encoding mode information 7 
indicating the selected encoding mode. The local decoded data 73 of the 
first field is furnished to the context computing unit 74 for encoding the 
second field of the alpha block currently being encoded. 
As previously mentioned in First Embodiment, entropy encoding other than 
the arithmetic encoding can be applied to the shape encoding process for 
the first field. 
Next, a description will be made as to the encoding process for the second 
field. The encoding process for the second field can be done by using the 
following procedures. 
(1) Compute a context number used for inter-line prediction. 
(2) Provide an estimate of the target pixel currently being encoded from 
the computed context number used for inter-line prediction. 
(3) Entropy encode the prediction error showing the difference between the 
actual value of the pixel and its prediction or predicted value. 
The above procedures will be described in greater detail hereinafter. 
First, the context computing unit 74 computes a context number used for 
the inter-line prediction. Referring next to FIG. 15, it illustrates a 
diagram showing the context construction for the inter-line prediction. In 
the inter-line prediction, an estimate of the target pixel currently being 
encoded is provided from the values of other pixels arranged on other 
lines above and under the line on which the target pixel is located, as 
shown in FIG. 15. The context computing 74 computes a context number 75 
for the target pixel currently being encoded in the second field, which is 
marked with `?`, from the values of other pixels c1 through c12 of FIG. 15 
in the vicinity of the target pixel according to the following equation. 
##EQU2## 
The prediction determining unit 76 then forms a prediction for the target 
pixel currently being encoded according to the computed context number 
used for the inter-line Prediction. That is, the prediction determining 
unit 76 provides an estimate of the value of the target pixel currently 
being encoded from the computed context number 75. A rule to determine an 
estimate of the value of the target pixel according to the context number 
75 must be provided in advance of making the prediction. For example, when 
the values of the other pixels c1 through c12 are all 1, that is, when the 
context number is 4095, the value of the target pixel marked with `?` is 
estimated to be 1. When the values of the other pixels c1 through c12 are 
all 0, that is, when the context number is 0, the value of the target 
pixel marked with `?` is estimated to be 0. Such a rule is predetermined. 
Predicted values are thus obtained for all the pixels in the second field 
of the alpha block currently being encoded. 
The difference between the actual value of the target pixel currently being 
encoded, which is marked with `?` in FIG. 15, and its prediction or 
predicted value determined by the prediction determining unit 76 is then 
computed. The prediction error can have any of three possible values: 1, 
0, and -1. Using context numbers for inter-line prediction computed for 
all the pixels in the second field of the alpha block, a small difference 
in the position of a moving object between the two fields of the 
interlaced frame can be corrected and hence the possibility that the 
prediction error of each pixel is 0 is increased. The entropy encoding 
unit 77 can reduce the redundancy by carrying out appropriate entropy 
encoding such as run-length encoding. The entropy encoding unit 77 
furnishes the coded result as the coded shape data 79 of the second field. 
Numerous variants may be made in the exemplary embodiment shown. It is 
apparent that another encoding method other than the arithmetic encoding 
method can be used for encoding the field to be referred to, i.e., the 
first field of each alpha block to be encoded of an interlaced frame. It 
is also clear that the context construction mentioned above as an example 
can be replaced by another context construction which is defined in the 
same manner as that used for a corresponding decoding process no matter 
how the other context construction is defined. 
As previously mentioned, the encoding system according to the fifth 
embodiment can encode an interlaced shape data by forming predictions for 
one of the pair of two fields of an interlaced frame to be encoded using 
the other field. Thus the present embodiment offers an advantage of being 
able to improve the encoding efficiency as compared with the case of 
arithmetic encoding all the pixels of each alpha block to be encoded 
similarly. 
Sixth Embodiment 
Referring next to FIG. 16, there is illustrated a block diagram showing the 
structure of a motion picture decoding system according to a sixth 
embodiment of the present invention. The motion picture decoding system is 
so constructed as to decode a coded bitstream generated by the motion 
picture encoding system according to the fifth embodiment. The motion 
picture decoding system of the sixth embodiment can extract the coded 
prediction error data of each alpha block to be decoded included in an 
interlaced frame from a coded bitstream applied thereto, form an 
inter-line prediction for each pixel in a second field of the alpha block, 
and reconstruct the frame. In FIG. 16, reference numeral 80 denotes the 
coded shape data of a first field of each alpha block to be decoded of the 
interlaced frame, which is furnished by the syntax analyzing and variable 
length decoding unit 37, 81 denotes a field-based motion vector of shape 
which is also furnished by the syntax analyzing and variable length 
decoding unit 37, 82 denotes shape encoding mode information showing a 
shape encoding mode selected upon the encoding process, which is also 
furnished by the syntax analyzing and variable length decoding unit 37, 
and 83 denotes the coded shape data of the second field of the alpha block 
to be decoded, which is also furnished by the syntax analyzing and 
variable length decoding unit 37. 
The motion picture decoding system according to the sixth embodiment of the 
present invention can be so constructed as to decode coded video object 
planes or VOP's. The description will be directed to an improvement in the 
decoding process by the motion picture decoding system when VOP's are 
interlaced images, which is the primary object of the present invention. 
The texture decoding process of this embodiment is the same as that of the 
second embodiment, and therefore the description about the texture 
decoding process will be omitted hereinafter. Accordingly, only a 
description about the shape decoding will be made hereinafter. 
First, a description will be made as to the syntax analyzing process and 
the variable length decoding process of this embodiment. When the syntax 
analyzing and variable length decoding unit 37 receives a coded bitstream 
36, it isolates the coded shape data of each of the two fields of each 
alpha block to be encoded included in an interlaced frame from the coded 
bitstream 36. The field that is extracted first from the coded bitstream 
36 is referred to as the first field, and the other field is referred to 
as the second field, like the fifth embodiment mentioned above. The coded 
shape data 80 of the first field is the code word which was arithmetic 
encoded by the motion picture encoding system of the fifth embodiment, for 
example. The coded shape data 80 of the first field is then delivered to 
the shape decoding unit 41, together with the shape encoding mode 
information 82 showing whether the intracoded shape data or the interceded 
shape data was selected as the coded shape data and the field-based motion 
vector of shape 81 when the inter mode was selected. The coded data 83 of 
the second field is the code word which was obtained by entropy encoding 
the prediction error data obtained by prediction using a local decoded 
data of the first field. The coded shape data 83 of the second field is 
delivered to the shape decoding unit 41. 
Next, the description will be directed to the shape decoding process. 
Referring next to FIG. 17, there is illustrated a block diagram showing 
the structure of the shape decoding unit 41. In the figure, reference 
numeral 84 denotes an entropy decoding unit which can decode the coded 
shape data 83 of the second field, 85 denotes a decoded shape data of the 
first field which is furnished by the arithmetic decoding unit 60, 86 
denotes a decoded shape data of the second field which is obtained by 
adding a prediction or predicted value from the prediction determining 
unit 76 to the output of the entropy decoding unit 84, and 102 denotes a 
prediction computing unit constructed of the context computing unit 74 for 
computing a context number used for inter-line prediction and the 
prediction determining unit 76. 
By using the following procedures, the coded shape data of each alpha block 
to be decoded of an interlaced frame will be decoded. 
(1) The first field is decoded with motion compensation and arithmetic 
decoding. 
(2) The second field is decoded by computing a context number used for 
inter-line prediction for each pixel in the second field, determining a 
prediction or predicted value using the context number (i.e., providing an 
estimate of the value of each pixel), and adding the decoded prediction 
error to the prediction. 
The process (1) for decoding the coded shape data of the first field is 
carried out similar to the decoding process based on motion compensation 
and arithmetic encoding as previously mentioned in the above preferred 
embodiments. The decoding process of this embodiment differs from those of 
the above-mentioned exemplary embodiments in that the decoded shape data 
85 of the first field is delivered to the prediction computing unit 102 to 
decode the second field or reconstruct the shape data of the second field. 
In the decoding process for the second field, the context computing unit 74 
computes a context number used for inter-line prediction for the target 
pixel currently being decoded. The context computing 74 computes a context 
number 75 given by the above equation (2) for the target pixel currently 
being decoded, which is marked with `?` in FIG. 15, according to the 
context construction shown in FIG. 15. 
The prediction determining unit 76 then forms an inter-line prediction for 
the target pixel currently being decoded using the computed context number 
75 in the same manner as the fifth embodiment mentioned above. The 
prediction determining unit 76 thus provides an estimate of the value of 
the target pixel currently being decoded from the computed context number 
75. The entropy decoding unit 84 decodes the coded data 83 or coded 
prediction error data of the second field extracted by the syntax 
analyzing and variable length decoding unit 37. The output of the entropy 
decoding unit 84 is then added to the prediction or predicted value of the 
target pixel currently being decoded, which has been computed by the 
prediction determining unit 76. As a result, the decoded data 86 of the 
second field is generated. 
The shape data 85 and 86 of the first and second fields, each of which has 
been reconstructed as a sequence of 8-bit binary data each having a value 
of 0 or 255 in decimal through the above processes, are delivered to the 
frame-based and field-based motion compensation units 46 and 49 for 
texture decoding and are also stored in the shape memory 42 for later 
alpha block decoding, as shown in FIG. 16. 
As previously mentioned, the decoding system according to the sixth 
embodiment can decode an interlaced, coded shape data by carrying out 
motion compensation while making a correction to the difference in the 
position of a moving object between the two fields of the interlaced 
frame. Thus the present embodiment offers an advantage of being able to 
encode the shape data more smoothly as compared with the prior art 
decoding system. 
Numerous variants may be made in the exemplary embodiment shown. For 
example, it is apparent that a process of decoding a bitstream which was 
encoded using another method other than the arithmetic coding method can 
be carried out in the decoding system by replacing the arithmetic decoding 
unit with another unit which conforms to the other method. 
Referring next to FIG. 18, there is illustrated a block diagram showing the 
structure of a shape decoding unit 41 according to a variant of the sixth 
embodiment mentioned above. In the figure, reference numeral 103 denotes a 
prediction computing unit comprised of the context computing unit 74 for 
computing a context number used for inter-line prediction for each pixel 
in the second field of each alpha block to be decoded, and the prediction 
determining unit 76 for determining a prediction or predicted value of 
each pixel using the context number computed by the context computing unit 
74. The shape decoding unit 41 of this variant differs from that of the 
sixth embodiment mentioned above in that the shape decoding unit 41 of 
this variant determines the value of each pixel in the second field using 
the computed context number without adding the prediction error to the 
prediction of each pixel when decoding the second field. Therefore, in 
this variant, the coded bitstream does not need to include the coded shape 
data of the second field. That is, the decoding system of this variant can 
reconstruct the shape data of the second field from only the coded shape 
data of the first field. 
Accordingly, the decoding system of the variant can be applied to the 
situation where the transmission bit rate is limited and therefore it is 
unnecessary to reconstruct the shape data with great accuracy. 
Furthermore, the variant can offer the same advantage as the sixth 
embodiment. 
In accordance with another variant of the sixth embodiment, there is 
provided a motion picture decoding system in which the structure of the 
motion picture decoding system according to the sixth embodiment and the 
structure of the above variant is combined. The motion picture decoding 
system of the other variant can select either the decoding process of the 
sixth embodiment or that of the above variant for the second field 
according to mode information included in a coded bitstream applied to 
thereto. Accordingly, this variant offers an advantage of being able to 
control the quality of the coded shape data dynamically according to the 
circumstances. 
Seventh Embodiment 
Referring next to FIG. 19, there is illustrated a block diagram showing the 
structure of a shape coding unit of a motion picture encoding system 
according to a seventh embodiment of the present invention. In the figure, 
reference numeral 104 denotes prediction error encoding instruction 
information for instructing the shape coding unit 3 to switch between a 
first mode in which prediction errors are encoded and a second mode in 
which prediction errors are not encoded. 
While the motion picture encoding system of the seventh embodiment has the 
same structure of that of the aforementioned fifth embodiment, they 
differs from each other in that the shape encoding unit 3 of the seventh 
embodiment has the second mode in which no prediction is done, and hence 
prediction errors are not encoded and multiplexed into a coded bitstream, 
i.e., in which no coded shape data 79 of the second field is generated, in 
addition to the first mode in which the prediction errors for the second 
field are encoded and multiplexed into a coded bitstream. Accordingly, the 
motion picture decoding system of the present embodiment can vary the 
quality of the coded shape data dynamically. The shape encoding unit 3 
comprises a switch 105 which can be turned on or off according to the 
prediction error coding instruction information 104 applied thereto, as 
shown in FIG. 19. 
When the prediction error coding instruction information 104 indicates the 
activation of the decoding and multiplexing processes for the second 
field, the switch 105 activates the prediction computing unit 101, the 
entropy encoding unit 77, and so on. When the prediction error coding 
instruction information 104 indicates the deactivation of the decoding and 
multiplexing processes for the second field, the switch 105 deactivates 
the prediction computing unit 101, the entropy encoding unit 77, and so 
on. The switching can be carried out per VOP or alpha block. The 
prediction error coding instruction information 104 is multiplexed into a 
coded bitstream per switching. 
The function of switching between the two modes of this embodiment can be 
also applied to the other motion picture encoding system of the 
aforementioned first or third embodiment. 
Accordingly, the motion picture encoding system according to the seventh 
embodiment can carry out the encoding process for the second field by 
encoding prediction errors without reducing the quality of the coded shape 
data when a sufficiently large transmission bit rate is ensured. On the 
other hand, when serious limitations are imposed to the transmission bit 
rate, the motion picture encoding system according to the present 
embodiment can encode the shape data at the expense of the quality of the 
coded shape data by eliminating the process for encoding prediction 
errors. Accordingly, the present embodiment offers an advantage of being 
able to control the quality of the coded shape data dynamically according 
to the circumstances. 
Eighth Embodiment 
Referring next to FIG. 20, there is illustrated a block diagram showing the 
structure of a shape encoding unit of a motion picture encoding system 
according to an eighth embodiment of the present invention. While the 
motion picture encoding system of this embodiment has the same structure 
as that of the above-mentioned fifth embodiment shown in FIG. 13, the 
shape encoding unit 3 of this embodiment differs from that of the fifth 
embodiment. In FIG. 20, reference numeral 87 denotes a delta vector 
detecting unit which can estimate or detect a delta vector used for 
estimating the shape data of a second field of each alpha block to be 
encoded of an interlaced frame by referring to a local decoded shape data 
of the first field of each alpha block to be encoded, and 88 denotes an 
entropy encoding unit which can encode the delta vector detected by the 
delta vector unit 87 and then furnish the coded shape data of the second 
field of each alpha block to be encoded. 
In the present embodiment, arithmetic encoding with motion prediction is 
carried out first for one of the two fields of each alpha block to be 
encoded, which will be referred to as the first field the shape data of 
which is encoded before encoding the shape data of the other field which 
will be referred to as the second field. For the second field of each 
alpha block to be encoded, a small delta vector in the horizontal 
direction is estimated by referring to the local decoded data of the first 
field of each alpha block to be encoded and the delta vector is then 
encoded as the coded shape data of the second field. The encoding method 
can be implemented for the reason that the shape data is a binary plane 
including a plurality of bits each having a value of 0 or 1 and an 
approximation of the shape data of the second field can be obtained by 
shifting the shape data of the first field in the horizontal direction 
using the horizontal small delta vector. 
The encoding process for the first field is done in the same manner as the 
fifth embodiment mentioned above. The encoding process is carried out by 
using the following procedures. 
First, a delta vector is detected in the following manner. After the 
arithmetic encoding unit 71 arithmetic encodes the shape data of the first 
field of each alpha block to be encoded using the field-based prediction 
data for shape 70, the shape encoding mode selecting unit 72 generates and 
furnishes a local decoded data 73 of the first field to the delta vector 
detecting unit 87. The delta vector detecting unit 87 determines the 
amount of shifting the input shape data 2 of the second field in the 
horizontal direction with respect to the local decoded data 73 of the 
first field so that the shifted input shape data 2 of the second field 
most approximates to the local decoded data 73 of the first field so as to 
detect or estimate a delta vector having a magnitude corresponding to the 
shifting amount. The shifting amount is defined as the number of pixels by 
which the input shape data 2 is shifted. A process for searching a small 
area is performed so as to estimate the delta vector. For example, the 
search range can extend from .+-.1 pixel to .+-.3 or 4 pixels. 
Next, entropy encoding is performed on the detected delta vector. The 
entropy encoding unit 88 performs appropriate entropy encoding such as 
Huffman coding on the delta vector using a predetermined variable length 
encoding table. 
As previously explained, the motion picture encoding system of the eighth 
embodiment can encode an interlaced shape data by substituting a delta 
vector used for estimating the shape data of one field of a frame to be 
encoded from the shape data of another field which pairs up the former 
field for the shape data of the other field. Accordingly, the present 
embodiment offers an advantage of being able to encode the shape data with 
an extremely small amount of codes, as compared with the case in which all 
the pixels of each alpha block to be encoded are arithmetic encoded 
similarly. 
In a variant, the above-mentioned encoding method according to the eighth 
embodiment can be combined with another encoding method of the other 
embodiment mentioned above so as to carry out an encoding process by 
switching between the encoding method of the eighth embodiment and the 
other encoding method per VOP or alpha block. The motion picture encoding 
system according to this variant can carry out an encoding process for the 
second field of each alpha block of an interlaced frame without reducing 
the quality of the coded shape data of the second field when a 
sufficiently large transmission bit rate is ensured. On the other hand, 
when serious limitations are imposed to the transmission bit rate, the 
motion picture encoding system of this variant can encode the shape data 
of the second field of each alpha block of the interlaced frame at the 
expense of the quality of the coded shape data of the second field by 
substituting only a delta vector for estimating the shape data of the 
second field from the shape data of the first field for the shape data of 
the second field. Accordingly, the variant offers an advantage of being 
able to control the quality of the coded shape data of each alpha block 
dynamically according to the circumstances. 
Ninth Embodiment 
A motion picture decoding system according to a ninth embodiment of the 
present invention is so constructed as to decode a coded bitstream 
generated by the motion picture encoding system according to the 
aforementioned eighth embodiment. The motion picture decoding system 
according to the ninth embodiment can encode interlaced video object 
planes or VOP's. The texture decoding process of this embodiment is the 
same as that of the second embodiment mentioned above, and therefore the 
description about the texture decoding process will be omitted 
hereinafter. Accordingly, only a description about the shape decoding will 
be made. The motion picture decoding system of this embodiment has the 
same structure as the sixth embodiment shown in FIG. 16, with exception 
that the coded shape data 83 of the second field is replaced by a coded 
delta vector 90 and the shape decoding unit 41 has a different structure. 
First, a description will be made as to the syntax analyzing process and 
the variable length decoding process of this embodiment. When the syntax 
analyzing and variable length decoding unit 37 shown in FIG. 16 receives a 
coded bitstream 36, it isolates the coded shape data of each of the two 
fields of each alpha block to be decoded from the coded bitstream 36. The 
field that is extracted first from the coded bitstream 36 is referred to 
as the first field, and the other field is referred to as the second 
field. The coded shape data 80 of the first field is the code word which 
was arithmetic encoded by the motion picture encoding system of the eighth 
embodiment of the present invention, for example. The coded shape data 80 
of the first field is then delivered to the shape decoding unit 41, 
together with the shape encoding mode information 82 showing whether the 
intracoded shape data or the interceded shape data was selected upon the 
encoding process and the motion vector of shape 81 when the inter mode was 
selected. The coded shape data of the second field is the code word or 
coded delta vector 90 which was obtained by entropy encoding a delta 
vector obtained by prediction using the local decoded data of the first 
field. 
Next, the description will be directed to the shape decoding process. 
Referring next to FIG. 21, there is illustrated a block diagram showing 
the structure of the shape decoding unit 41. In the figure, reference 
numeral 90 denotes a coded delta vector which is furnished by the syntax 
analyzing and variable length decoding unit 37, 91 denotes an entropy 
decoding unit which can decode the coded delta vector 90 and then furnish 
a decoded delta vector 92, and 93 denotes a correction data generating 
unit which can generate a decoded shape data 94 of the second field from 
the decoded shape data 85 of the first field using the delta vector 92. 
By using the following procedures, the coded shape data will be decoded. 
(1) The first field is decoded with motion compensation and arithmetic 
decoding. 
(2) The second field is decoded by generating the shape data of the second 
field using the delta vector. 
The process (1) for decoding the coded shape data of the first field is 
carried out similar to the decoding process based on motion compensation 
and arithmetic encoding as previously mentioned in the above preferred 
embodiments. Therefore the description about the process (1) will be 
omitted hereinafter. The decoded shape data 85 of the first field is 
delivered to the correction data generating unit 93 to decode the second 
field or reconstruct the shape data of the second field. 
In the decoding process for the second field, the entropy decoding unit 91 
reconstructs a delta vector 92 from the coded delta vector furnished by 
the syntax analyzing and variable length decoding unit 37. The correction 
data generating unit 93 then generates the decoded shape data 94 of the 
second field from the decoded shape data 85 of the first field using the 
delta vector 92. A method of shifting the positions of all the pixels 
included in an alpha block currently being decoded in the horizontal 
direction by one or more pixels defined by the delta vector, as shown in 
FIG. 22, can be used as the method of generating the decoded shape data 94 
of the second field from the decoded shape data 85 of the first field. Any 
pixel 100 in FIG. 22 that has entered the alpha block upon shifting the 
pixels has the same value as the adjacent pixel on the same line in the 
same alpha block. Another method can be used as the generating method. 
The decoded shape data 85 and 94 which have been reconverted into a 
sequence of 8-bit binary data each having a value of 0 or 255 in decimal 
through the above processes are delivered to the frame-based and 
field-based motion compensation units for texture decoding and are also 
stored in the shape memory 42 for later alpha block decoding. 
As previously mentioned, the motion picture decoding system according to 
the ninth embodiment can decode an interlaced, coded shape data by 
carrying out motion compensation in consideration of motions between the 
pair of two fields. Accordingly, the motion picture decoding system of the 
ninth embodiment can be applied to the situation where the transmission 
bit rate is limited and therefore it is unnecessary to reconstruct the 
shape data with great accuracy. 
In a variant of the exemplary embodiment mentioned above, the structure of 
the ninth embodiment is combined with the decoding system according to one 
of the other preferred embodiments mentioned above. The motion picture 
decoding system of this variant is so constructed as to switch between the 
decoding method using the delta vector according to the ninth embodiment 
and another decoding method for the second field of a frame currently 
being decoded according to mode information included in the coded 
bitstream. Accordingly, the motion picture decoding system can decode the 
coded bitstream while controlling the quality of the shape data 
dynamically according to the amount of change in the transmission bit 
rate. 
Tenth Embodiment 
Referring next to FIG. 23, there is illustrated a block diagram showing the 
structure of a shape encoding unit of a motion picture encoding system 
according to a tenth embodiment of the present invention. While the motion 
picture encoding system of this embodiment has the same structure as that 
of the above-mentioned first embodiment shown in FIG. 1, the shape 
encoding unit 3 of this embodiment differs from that of the first 
embodiment. In FIG. 23, reference numeral 95 denotes a differential vector 
detecting unit which can receive the shape data 2 of each alpha block to 
be encoded and then detect a differential vector 96 from the motion vector 
of shape 68 of the first field of each alpha block to be encoded. 
In the present embodiment, arithmetic encoding with motion prediction, such 
as frame-based prediction for shape, inter-VOP field-based prediction for 
shape, and intra-VOP field-based prediction for shape mentioned above in 
the exemplary embodiments shown, is carried out first for one of the two 
fields, which will be referred to as the first field the shape data of 
which is encoded before encoding the shape data of the other field which 
will be referred to as the second field. For the second field of each 
alpha block to be encoded, no motion prediction is carried out 
independently. A small area in the vicinity of the motion vector of shape 
68 of the first field is searched to estimate a differential vector 
showing the difference between the motion vector of shape of the first 
field of each alpha block to be encoded and that of the second field of 
each alpha block to be encoded. This differential vector detection can be 
implemented for the reason that there rarely exists a difference in the 
magnitudes and directions of the motion vectors of shape of the two fields 
and hence there is a strong correlation between the motion vector of one 
of the two fields and that of the other field. The amount of computations 
required for detecting a motion vector of shape for the second field and 
the amount of codes included in coded motion vector data can be reduced by 
carrying out the above-mentioned process of searching a small area around 
the motion vector of the first field so as to estimate a differential 
vector and then encoding the differential vector. 
The frame-based prediction for shape is carried out in the same manner as 
the aforementioned first embodiment. The encoding process for the first 
field is done in the same manner as the fifth embodiment. The encoding 
process for the second field is done in the same manner as the encoding 
method as mentioned above with inter-VOP field-based prediction for shape 
and arithmetic encoding, with the exception of detection of a motion 
vector for the second field. Therefore only a description of the motion 
vector detection will be made hereafter. 
The field-based shape motion detecting unit 67 detects a field-based motion 
vector of shape 68 for the first field of each alpha block to be encoded 
using a reference shape data stored in the shape memory 4. When the delta 
vector detecting unit 87 receives the field-based motion vector of shape 
68 of the first field, it searches a small area, included in the shape 
data 2 of the second field, in the vicinity of the field-based motion 
vector of shape 68 so as to detect or estimate a differential vector 96 
showing the difference between the motion vector of shape of the first 
field of each alpha block to be encoded and that of the second field of 
each alpha block to be encoded. For example, the search range can be about 
.+-.1 pixel in both the horizontal and vertical directions. The 
differential vector 96 thus estimated is furnished as a motion vector data 
of the second field, and is then added with the motion vector of shape 68 
of the first field to generate a motion vector of shape of the second 
field. The field-based shape motion compensation unit 69 then carries out 
motion compensated prediction for the second field according to the motion 
vector of shape 68 in the same manner as the motion compensated prediction 
for the first field. Finally, the field-based shape motion compensation 
unit 69 generates the pair of field-based prediction data for shape for 
the first and second fields. The arithmetic encoding unit 71 of this 
embodiment then carries out the intra-coding process, the inter-coding 
process using the frame-based prediction data for shape, and the 
inter-coding process using the field-based prediction data for shape, like 
that of the first embodiment mentioned above. The differential vector 96 
can be encoded by using an appropriate entropy encoding method. 
As previously explained, the motion picture encoding system of the tenth 
embodiment can encode an interlaced shape data by substituting a delta 
vector used for estimating a motion vector of one field of a frame to be 
encoded from a motion vector of another field which pairs up the former 
field for the motion vector of the former field of the frame. Accordingly, 
the present embodiment offers an advantage of being able to reduce the 
amount of computations required for estimating field-based motion vectors 
of shape and the amount of codes in coded motion vectors. 
Eleventh Embodiment 
Referring next to FIG. 24, there is illustrated a block diagram showing the 
structure of a shape decoding unit of a motion picture decoding system 
according to an eleventh embodiment of the present invention. The motion 
picture decoding system according to the eleventh embodiment of the 
present invention is so constructed as to decode a coded bitstream 
generated by the motion picture encoding system according to the 
aforementioned tenth embodiment. In FIG. 24, reference numeral 97 denotes 
a field-based motion vector of shape of a first field of each alpha block 
to be decoded included in an interlaced frame, and 98 denotes a delta 
vector showing the difference between the motion vector of shape of the 
first field of each alpha block to be encoded and that of a second field 
of each alpha block to be encoded, which is furnished by the syntax 
analyzing and variable length decoding unit 37. 
The motion picture decoding system according to the eleventh embodiment can 
encode interlaced video object planes or VOP's. The shape decoding process 
of this embodiment is the same as that of the second embodiment mentioned 
above with the exception of a method of decoding a coded motion vector of 
one field that will be decoded at a later time as compared with that of 
the other field, and therefore the description about only the difference 
between this embodiment and the second embodiment will be made. 
In the present embodiment, the inter-VOP field-based prediction is carried 
out by using the field-based motion vector of shape of one of the two 
fields, which will be referred to as the first field the shape data of 
which is encoded at an earlier time as compared with that of the other 
field, so that the field-based motion vector of shape of the other field 
which will be referred to as the second field is reconstructed. 
To be more specific, when the syntax analyzing and variable length decoding 
unit 37 receives a coded bitstream 36, it extracts the differential vector 
98 from the coded bitstream 36. The shape decoding unit 41 then adds the 
differential vector 98 to the field-based motion vector of shape 97 of the 
first field so as to determine the field-based motion vector of shape of 
the second field. The later decoding process is done using the same 
procedures as of the aforementioned second embodiment. 
As previously mentioned, the decoding system according to the eleventh 
embodiment can decode an interlaced, coded shape data by carrying out 
motion compensation while making a correction to the difference in the 
position of a moving object between the two fields of the interlaced 
frame. Thus the present embodiment offers an advantage of being able to 
encode the shape data more smoothly as compared with the prior art 
decoding system. 
Many widely different embodiments of the present invention may be 
constructed without departing from the spirit and scope of the present 
invention. It should be understood that the present invention is not 
limited to the specific embodiments described in the specification, except 
as defined in the appended claims.