Information Technology Reference
In-Depth Information
At the encoder, mesh vectors are used to warp the pixels in the I-VOP to predict the new perspective. The
differential shape codes are used to predict the new shape of the object. The warped, reshaped object is then
compared with the actual object, and the result is a texture residual which will be DCT coded.
At the decoder, (b) the I-VOP is in memory and the mesh motion, the shape difference and the texture residual
arrive as a P-VOP. The mesh motion is decoded and used to warp the object pixels taken from the I-VOP memory.
The shape difference is decoded to produce a new shape around the warped pixels, resulting in a predicted object.
The texture residual is decoded and added to the predicted object to produce the final decoded object which is then
composited into the output picture along with other objects.
It should be clear that by transmitting warping vectors the pixels from the I-VOP can be used to create a very
accurate prediction of the texture of the P-VOP. The result is that the texture residual can be encoded with a very
small number of bits.
5.26 Sprites
Sprites are images or parts of images which do not change with time. Such information only needs to be sent once
and is obviously attractive for compression. However, it is important to appreciate that it is only the transmitted
sprite which doesn't change. The decoder may manipulate the sprite in a different way for each picture, giving the
illusion of change.
Figure 5.66 shows the example of a large sprite used as a background. The pixel array size exceeds the size of the
target picture. This can be sent as an I-VOP (
intra-coded video object plane
). Once the texture of the sprite is
available at the decoder, vectors can be sent to move or warp the picture. These instructions are sent as S-VOPs
(
static video object planes
). Using S-VOP vectors, a different part of the sprite can be delivered to the display at
each picture, giving the illusion of a moving background. Other video objects can then be keyed over the
background.
Figure 5.66:
A large sprite used as a background may be bigger than the screen. If shifted by vectors, the illusion
of a moving background is given.
Sending a large background sprite in one I-VOP may cause difficulty at high compression factors because the one-
off image distorts the bit rate. The solution here is
low-latency
sprite coding. Although the sprite is a single still
texture object, it need not all be transmitted at once. Instead the sprite is transmitted in
pieces
. The first of these is
an
object piece
, whereas subsequently
update pieces
may be sent.
Update pieces may coincide spatially with the object piece and result in a refinement of image quality, or may be
appended spatially to increase the size of a sprite. Figure 5.67 shows the example of an object piece which is
transmitted once but used in a series of pictures. In Figure 5.67(a) an update piece is transmitted which improves
the quality of the sprite so that a zoom-in can be performed.
Figure 5.67:
An object piece may be used in several VOPs. (a) To allow a zoom-in, part of the sprite object may
