Digital Signal Processing Reference
In-Depth Information
16
VIDEO NOISE AND
COMPRESSION ARTIFACTS
CHAPTER OUTLINE
16.1 Salt-and-pepper Noise
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16.2 Mosquito Noise
143
16.3 Block Artifacts
144
Video or image data can contain noise: this can be introduced
by the sensor, during transmission or during storage. Imperfec-
tions on old film can cause noise when it's converted to digital
video form
this tends to be a random type of noise. Noise
reduction processing or filtering is a process that can reduce or
eliminate this effect.
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16.1 Salt-and-pepper Noise
Salt-and-pepper noise occurs when individual pixels or small
areas of the image have high contrast, or are very different in color
or intensity from their surrounding pixels or areas. This pixel or
area is visually noticeable and means that the image contains
dark and white dots
which is why it's referred to as salt-and-
pepper noise. Sources include particles of dust inside the camera
and defective CCD elements.
Filtering can be employed to counter this type of noise. A
linear filter with a low-pass characteristic can be employed to
smooth sharp transitions in both vertical and horizontal direc-
tions. This type of filtering is very effective, but it will remove, or
at least reduce the amount of, all high-frequency spatial content.
Thus sharp edges will also be smoothed out with the resultant
blurring, or loss of sharpness of the image.
Alternatively, another type of filter, known as the median filter,
is often used. A conventional digital filter is a linear combination,
or weighting of nearby pixels, for the new adjusted value for
a given pixel. The right combination of weighting, or coefficients,
results in a smoothing, or low pass, effect.
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