Introduction to compression - The MPEG - page 9

Figure 1.7: (a) Spatial or intra-coding works on individual images. (b) Temporal or inter-coding works on
successive images.
Figure 1.7 (a) shows that when individual pictures are compressed without reference to any other pictures, the time
axis does not enter the process which is therefore described as intra-coded (intra = within) compression. The term
spatial coding will also be found. It is an advantage of intra-coded video that there is no restriction to the editing
which can be carried out on the picture sequence. As a result compressed VTRs such as Digital Betacam, DVC
and D-9 use spatial coding. Cut editing may take place on the compressed data directly if necessary. As spatial
coding treats each picture independently, it can employ certain techniques developed for the compression of still
pictures. The ISO JPEG (Joint Photographic Experts Group) compression standards are in this category. Where a
succession of JPEG coded images are used for television, the term 'Motion JPEG' will be found. [ 6 ] [ 7 ]
Greater compression factors can be obtained by taking account of the redundancy from one picture to the next.
This involves the time axis, as Figure 1.7 (b) shows, and the process is known as inter-coded (inter = between) or
temporal compression.
Temporal coding allows a higher compression factor, but has the disadvantage that an individual picture may exist
only in terms of the differences from a previous picture. Clearly editing must be undertaken with caution and
arbitrary cuts simply cannot be performed on the MPEG bitstream. If a previous picture is removed by an edit, the
difference data will then be insufficient to re-create the current picture.
1.7.1 Intra-coded compression
Intra-coding works in three dimensions on the horizontal and vertical spatial axes and on the sample values.
Analysis of typical television pictures reveals that whilst there is a high spatial frequency content due to detailed
areas of the picture, there is a relatively small amount of energy at such frequencies. Often pictures contain
sizeable areas in which the same or similar pixel values exist. This gives rise to low spatial frequencies. The
average brightness of the picture results in a substantial zero frequency component. Simply omitting the high-
frequency components is unacceptable as this causes an obvious softening of the picture.
A coding gain can be obtained by taking advantage of the fact that the amplitude of the spatial components falls
with frequency. It is also possible to take advantage of the eye's reduced sensitivity to noise in high spatial
frequencies. If the spatial frequency spectrum is divided into frequency bands the high-frequency bands can be
described by fewer bits not only because their amplitudes are smaller but also because more noise can be
tolerated. The wavelet transform (MPEG-4 only) and the discrete cosine transform used in JPEG and MPEG-1,
MPEG-2 and MPEG-4 allow two-dimensional pictures to be described in the frequency domain and these are
discussed in Chapter 3 .
1.7.2 Inter-coded compression