Compression—The Simple Version 67
A reduction in image update is the result of only small por-
tions of the picture changing. Unfortunately, if there is a signiﬁ -
cant change to the original picture such as several persons entering
the camera view, the update time will increase in direct proportion
to the amount of picture changes. Conditional compression was
originally very popular in the video conferencing arena where
only the small movements of a person’s mouth were necessary to
transmit. See Figure 4-1.
An alternative approach commonly referred to as “unconditional”
video transmission involves full frame compression. This method
grabs each frame of video independently, and the entire picture is
compressed and transmitted to the receiver regardless of changes
within the monitored area. Because this method of compression
transmits every picture in its entirety, there can be no arguments
as to its integrity.
There are two criteria by which each of the compression
techniques discussed here can be measured: the algorithm com-
plexity and amount of compression achieved. When data com-
pression is used in a data transmission application, the goal is
speed. The speed of the transmission relies on number of bits sent,
Figure 4-1 Talking Heads
68 Digital CCTV
the time it takes the encoder to generate the coded message, and
the time it takes for the decoder to recover the original data.
Intraframe or Spatial Compression
Compression is achieved by taking advantage of spatial and
temporal redundancies elementary to video. In plain English,
spatial compression reduces the amount of information within the
space of one image by removing repetitive pieces of information.
Spatial compression is used to compress the pixels of one frame
by itself or one frame within a sequence to eliminate unneeded
information within each frame.
Spatial redundancy takes advantage of the similarity in color
values shared by bordering pixels. Spatial compression, some-
times referred to as intraframe compression, takes advantage of
similarities within a video frame. Intraframe compression exploits
the redundancy within the image, known as spatial redundancy.
Intraframe compression techniques can be applied to individual
frames of a video sequence. For example, a large area of blue sky
generally does not change much from pixel to pixel.
The same number of bits is not necessary for such an area as
for an area with large amounts of detail, for example if the sky
was ﬁ lled with multi-colored hot air balloons. Spatial compression
deletes information that is common to the entire ﬁ le or an entire
sequence within the ﬁ le. It also looks for redundant information,
but instead of logging every pixel in a frame, it deﬁ nes the area
Interframe or Temporal Compression
Some compressors employ temporal compression, which makes
the assumption that frames that are next to each other look very
similar. Therefore, it is used only on sequences of images. Tempo-
ral compression, sometimes referred to as frame differencing or
interframe compression, compares a frame of video with the one