[Magick-bugs] Documentation bug in -unsharp

[Harald Hanche-Olsen]

The documentation for the -unsharp option says

amount:    The percentage of the difference between the original and the blur
           image that is added back into the original (default 1.0).

However, a bit of experimentation reveals that the parameter must be a
fraction, not a percentage. The difference is a factor of 100 ... The
provided default 1.0 is a hint in this direction: The effect of a 1%
unsharp mask would be just about impossible to discern.

Also, I think it would be nice if this part was clarified:

  The image is convolved with a Gaussian operator of the given radius
  and standard deviation (sigma).

Mathematically, I can't make sense of that. A Gaussian has two
parameters: The mean and the standard deviation. The mean really
should be zero here, or the unsharp mask would be offset with respect
to the image. I am guessing that a Gaussian convolution kernel of
standard deviation sigma is truncated at the given radius before the
convolution, but that is only a guess. It would make sense of the
requirement that the radius be larger than sigma, however.

- Harald