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CPP100
SECTION 9
CPP1OPS
25/10/06
www.snellwilcox.com
Version 1 Issue 6
9.6
Median Filtering
Introduction
Median filters are non-linear filters that are often used to reduce impulse noise such as bit errors and satellite
drop-out and sparkle. They operate by rank ordering an odd number of points selected from an aperture
comprised of surrounding pixels in the same video field (known as 2D median) or some combination of current
and adjacent fields\frames (3D median). The samples with the largest variation end up at the extremes of the
list with the median value in the middle. Impulse noise will tend to end up at one or other of the extremes of
the sorted list and is therefore rejected. Prefix uses a combination of 2D and 3D median filters which have
been arranged such that full spatial resolution is maintained on static images by ensuring that the central pixel
appears in the majority of points.
With moving textures, the temporal points are less correlated than the spatial points and the median output is
derived from the points within the spatial aperture thereby automatically cross-fading between temporal and
spatial filters. Problems can still occur with some moving textures and hence Prefix contains a matched filter
algorithm, which evaluates spatial and temporal gradients for each pixel to determine if it has the
characteristics of impulse noise
The median filter has six settings which control both the strength and aperture set.
Min 1. This setting is used to conceal short length dropouts and sparkle that have a high contrast.
Min 2. This setting is used to conceal medium length dropouts and sparkle that have a medium-high contrast.
Med 3. This setting is used to conceal medium length dropouts and sparkle that have a medium contrast.
Med 4. This setting is used to conceal medium length dropouts and sparkle that have a medium-low contrast.
Med 5. This setting is used to conceal long dropouts and sparkle that have a low contrast.
Med 6. This setting is used to conceal long dropouts and sparkle that have a very low contrast.
Median Filter Aperture
A subset of the points shown in the above aperture may be used in the median filter depending on the strength
selected. In addition to the points shown in the aperture drawing above, horizontally adjacent pixels are used
in the gradient analysis. The result of the gradient analysis determines whether or not the suspect pixel is
replaced by the result of the median.