Information Technology Reference
In-Depth Information
have its resolution enhanced by an additional object piece. (b) An additional object piece may extend an object to
allow a pan.
Figure 5.67(b) shows a large sprite sent to create a moving background. Just before the edge of the sprite
becomes visible, the sprite is extended by sending an update piece. Update pieces can be appended to pixel
accuracy because they are coded with offset parameters which show where the update fits with respect to the
object piece.
Figure 5.68 shows a sprite decoder. The basic sprite is decoded from an I-VOP and placed in the sprite buffer. It
does not become visible until an S-VOP arrives carrying shifting and warping instructions. Low-latency decoding
sends sprite pieces as S-VOPs which are assembled in the buffer as each sprite piece arrives.
Figure 5.68: An MPEG-4 sprite decoder. See text for details.
5.27 Wavelet-based compression
MPEG-4 introduces the use of wavelets in still picture object coding. Wavelets were introduced in Chapter 3 where
it was shown that the transform itself does not achieve any compression. Instead the picture information is
converted to a form in which redundancy is easy to find. Figure 5.69 shows three of the stages in a wavelet
transform. Each of these represents more detail in the original image, assuming, of course, that there was any
detail there to represent. In real images, the highest spatial frequencies may not be present except in localized
areas such as edges or in detailed objects. Consequently many of the values in the highest resolution difference
data will be zero or small. An appropriate coding scheme can achieve compression on data of this kind. As in DCT
coding, wavelet coefficients can be quantized to shorten their wordlength, advantageous scanning sequences can
be employed and arithmetic coding based on coefficient statistics can be used.
Figure 5.69: Three of the stages in a wavelet transform in which different levels of detail exist.
MPEG-4 provides standards for scaleable coding. Using scaleable coding means that images can be reproduced
both at various resolutions and with various noise levels. In one application a low-cost simple decoder will decode
only the low-resolution noisy content of the bitstream whereas a more complex decoder may use more or all of the
data to enhance resolution and to lower the noise floor. In another application where the bit rate is severely limited,
the early part of a transmission may be decoded to produce a low-resolution image whose resolution increases as
further detail arrives.
The wavelet transform is naturally a multi-resolution process and it is easy to see how a resolution scaleable
system could be built from Figure 5.69 by encoding the successive difference images in separate bitstreams. As
with DCT-based image compression, the actual bit rate reduction in wavelet compression is achieved by a
combination of lossy and lossless coding. The lossless coding consists of scanning the parameters in a manner
which reflects their statistics, followed by variable-length coding.
 
Search WWH ::




Custom Search