Unsharp Mask Filter for Virtual Dub
Version 1.2

[Place this text file in the VirtualDub plugins directory to make
it available via the Help button on the filter configuration dialog
box. The computer must have a program called Notepad available in
its search path.]

This filter implements a well-known contrast enhancement/sharpening
technique called unsharp masking. The basic idea is that a blurred version
of the original image is created and then subtracted from the original
image. The resulting image will have contrast boosts around features in
the image. The result is a very pleasing sharpening that is superior to
simple sharpening with a convolution kernel or high-pass boost.

The filter supports active preview to simplify setting configurable options.

The following options are provided:

Diameter: This option determines the size of the neighborhood around
a pixel over which a neighborhood average is determined. The pixel is
replaced with this average to implement blurring. It must be an odd number.
Running time will be increased as the diameter is increased. Small values
such as 3 and 5 are fine for most applications.

Strength: This option determines the amount of enhancement. The maximum value
is 512. This option does not affect running time.

Threshold: This option sets how different pixels must be from their
neighborhood average before they are subjected to sharpening. For example,
sometimes with strong sharpening, what should be smooth areas (such as skin)
develop undesirable features or contours. By setting  an appropriate threshold,
these areas can be excluded from the enhancement process. Thresholding slightly
decreases running time.

Edge masking: Sometimes it is desirable to exclude areas around the frame. For
example, letterboxed video has black bars above and below the video content.
Without masking, artifacts would be produced because the edges of the black
bars would be treated as strong features to be enhanced. Masking allows these
areas to be excluded, thereby eliminating the artifacts. Masking slightly
decreases running time.

Interlaced source: If the source video is interlaced then the neighborhood calculation
must be modified. In effect the frame is split into its fields and the algorithm is
applied to the fields. Check this box if your source video is interlaced.

For additional information, version updates, and other filters, please
go to the following web site:

Filters for VirtualDub
http://www.geocities.com/Colosseum/Pressbox/8699/index.html

Donald Graft
May 2, 2000
(C) Copyright 2000, All Rights Reserved