Image Processing Reference
In-Depth Information
Segmentation in Video Data
A video sequence is in principle a sequence of images. The methods presented in
the previous chapters therefore apply equally well to a video sequence as an im-
age. We simply process one image at a time. There are, however, two differences
between a video sequence and an image. First, working with video allows us to con-
sider temporal information and hence segment objects based on their motion. This is
discussed below in Sect. 8.2 . Moreover, temporal information is the cornerstone of
tracking , which is described in the next chapter. Second, video acquisition and im-
age acquisition may not be the same, and that can have some consequences. Below,
this is discussed.
Video Acquisition
A video camera is said to have a certain framerate . The framerate is a measure for
how many images the camera can capture per second and is measured in Hertz (Hz).
The framerate depends on the number of pixels (and the number of bits per pixel)
and the electronics of the camera. Usually the framerate is geared toward a certain
transmission standard like USB, Firewire, Camera Link, etc. Each of these standards
has a certain bandwidth , which is the amount of data that can be transmitted per
second. With a fixed bandwidth we are left with a choice between high resolution
of the image and a high framerate. When one goes up the other one goes down.
In the end the desired framerate and resolution will always depend on the concrete
Say we have a system including a camera with a framerate of 20 Hz. This means
that a new image is captured every 50 ms. But it also means that the image pro-
cessing algorithms can spend a maximum of 50 ms per image. To underline this we
often talk about two framerates; one for the camera and one for the image processing
algorithms. The overall framerate of a system is the smallest of the two framerates.
Another important factor in video acquisition is compression. Very often the
captured video sequence needs to be compressed in order to insure a reasonable
framerate/resolution. The more the video is compressed, the higher the framer-
ate/resolution, but the worse the quality of the decompressed video. The question
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