MPEG video compression - The MPEG - page 176

Figure 5.47: The alpha plane is used in MPEG-4 to describe the shape of an object. Within a bounding rectangle
are three different types of macroblock: those within the object, those outside and those which are intersected by
the boundary.
Within the bounding rectangle are three kinds of macroblock. Trans- parent macroblocks are entirely outside the
object and contain no texture data. The shape data are identical over the entire block and result in a key signal
which deselects this block in the compositing process. Such shape data are trivial to compress. Opaque
macroblocks are entirely within the object and are full of texture data. The shape data are again identical over the
entire block and result in the block being keyed at the compositor. Boundary macroblocks are blocks through which
the edge of the object passes. They contain essentially all the shape information and somewhat fewer texture data
than an opaque macroblock.
5.21 Texture coding
In MPEG-1 and MPEG-2 the only way of representing an image is with pixels and this requires no name. In MPEG-
4 there are various types of image description tools and it becomes necessary to give the pixel representation of
the earlier standards a name. This is texture coding which is that part of MPEG-4 that operates on pixel-based
areas of image. Coming later than MPEG-1 and MPEG-2, the MPEG-4 texture-coding system can afford additional
complexity in the search for higher performance. Figure 5.48(a) shows the texture decoding system of MPEG- 4
and in (b) is a comparison with MPEG-2 showing that it is a refinement of the earlier technique. These refinements
are lossless in that the reduction in bit rate they allow does not result in a loss of quality.