Magick++ Image::unsharpmask usage question

[Bob-O-Rama]

Hi,

I'm looking for some guidance on using Image::unsharpmask(). So I'm doing something like:

Image my_image;
my_image.read( "somefile.jpg" );
my_image.unsharpmask( radius, sigma, amount, threshold );

And I see no difference at all after the function returns. I'm not sure if I'm picking poor values, or if its just not working, or what. Are there any gotchas to using USM or can someone provide sample, known good, value for the parameters? I tried picking large values, that in Photoshop, would result in very visible changes in the image, and the IM ISM function happily returns an image that appears identical to the original.

Any advice is appreciated.

-- Bob