Image Processing Reference
Again the data rate is based on 30 fps, but now each frame contains only 176
pixels. This is also part of the ITU H.261 video-conferencing standard.
As stated previously for CIF, some implementations change the number of lines
according to the geographic use. In PAL-based territories, the format is 176
144 and in
NTSC territories, the line count is reduced to 120.
CIF Versus SIF
Beware when you read specifications that describe screen sizes. The values for CIF and SIF
are often mixed up and intermingled, but they aren't the same thing. SIF stands for source
The exact sizes of these image formats depend on the underlying video standard that
the content is derived from. Table 6-2 consolidates and summarizes the range of values for
Figure 6-3 shows the relative difference between these sizes.
Prefacing with a Q quarters the area. The SQ size is smaller still, but not by an inte-
ger ratio and the scaling is not the same in X and Y. The names prefixed by 4, 9, and 16
refer to multiples of the area and are 2
3, and 4
4 scaled-up sizes.
The consequence of all the confusion over frame sizes is that your compres-
sion software may use terms like QSIF to describe a screen size, but you may
not get a picture that is the size you expect it to be. It is a good idea to spec-
ify the X and Y values for your picture size explicity and not rely on these
Table 6-2 Intermediate Formats