Information Technology Reference
In-Depth Information
Figure 5.44:
Effectively objects exist through continuous time, but are sampled for display purposes. A video object
plane (VOP) is a temporal sample of an object. VOPs correspond to pictures of MPEG-1 and MPEG-2.
Figure 5.45:
VOPs can be encoded using the same intra- and inter-coding approach of MPEG-1 and MPEG-2.
Figure 5.46 shows how video objects are handled at the decoder which effectively contains a multi-layer
compositing stage. The shape of each object is transmitted using
alpha data
which is decoded to produce a key
signal. This has the effect of keying the texture data into the overall picture. Several objects can be keyed into the
same picture. The background may be yet another video object or a still texture object which can be shifted by
vectors in the case of pans etc.
