MPEG video compression - The MPEG - page 173

picture to picture which MPEG-2 would find difficult to code. Instead, if the three-dimensional object is re-created at
the decoder, rotation can be portrayed by transmitting a trivially small amount of vector data.
If the above object is synthetic, effectively the synthesis or rendering process is completed in the decoder.
However, a suitable if complex image processor at the encoder could identify such objects in natural scenes.
MPEG-4 objects are defined as a part of a scene which can independently be accessed or manipulated. An object
is an entity that exists over a certain time span. The pictures of conventional imaging become object planes in
MPEG-4. Where an object intersects an object plane, it can be described by the coding system using intra-coding,
forward prediction or bidirectional prediction.
Figure 5.43 shows that MPEG-4 has four object types. A video object is an arbitrarily shaped planar pixel array
describing the appearance or texture of part of a scene. A still texture object or sprite is a planar video object in
which there is no change with respect to time. A mesh object describes a two- or three-dimensional shape as a set
of points. The shape and its position can change with respect to time. Using computer graphics techniques, texture
can be mapped onto meshes, a process known as warping, to produce rendered images.
Figure 5.43: In MPEG-4 four types of objects are coded.
Using two-dimensional warping, a still texture object can be made to move. In three-dimensional graphic rendering,
mesh coding allows an arbitrary solid shape to be created which is then covered with texture. Perspective
computation then allows this three-dimensional object to be viewed in correct perspective from any viewpoint.
MPEG-4 provides tools to allow two- or three-dimensional meshes to be created in the decoder and then oriented
by vectors. Changing the vectors then allows realistic moving images to be created with an extremely low bit rate.
Face and body animation is a specialized subset of three-dimensional mesh coding in which the mesh represents a
human face and/or body. As the subject moves, carefully defined vectors carry changes of expression which allow
rendering of an apparently moving face and/or body which has been almost entirely synthesized from a single still
picture.
In addition to object coding, MPEG-4 refines the existing MPEG tools by increasing the efficiency of a number of
processes using lossless prediction. This improves the performance of the motion compensation and coefficient
coding allowing either a lower bit rate or improved quality. MPEG-4 also extends the idea of scaleability introduced
in MPEG-2. Multiple scaleability is supported, where a low-bit-rate base- level picture may optionally be enhanced
by adding information from one or more additional bitstreams. This approach is useful in network applications
where the content creator cannot know the bandwidth which a particular user will have available. Scaleability
allows the best quality in the available bandwidth.
Although most of the spatial compression of MPEG-4 is based on the DCT as in earlier MPEG standards, MPEG-4
also introduces wavelet coding of still objects. Wavelets are advantageous in scaleable systems because they
naturally decompose the original image into various resolutions.
5.20 Video objects
Figure 5.44 shows an example of a video object intersecting video object planes , or VOPs. At each plane, the
shape and the texture of the object must be portrayed. Figure 5.45 shows this can be done using appropriate
combinations of intra- and inter-coding as described for the earlier standards. This gives rise to I-VOPs, P-VOPs
and B-VOPs. A group of VOPs is known as a GOV.