[magick-users] Problem using convert - unsharp mask

[Glenn Randers-Pehrson]

At 01:32 AM 9/27/02 +0200, Thomas Kaiser wrote:
>On Thu, 26 Sep 2002 18:50:09 -0400, Glenn Randers-Pehrson wrote:
>
>> Looking at the source for image.c where it decodes the "-unsharp" option,
>> it appears that the comandline utilities actually provide
>> 
>>    -unsharp <radius>x<sigma>+<amount>+<threshold>
>
>Ah... interesting...
>
>> Try that and let me know if it works for you.
>
>Not in the way, I would expect.
>
>Comparing the results of
>
>    convert -unsharp 1.2x1+4+0 test test1.2x1+4+0
>
>and
>
>    convert -unsharp 30x1+4+0 test test30x1+4+0
>
>results in no significant differences but the latter takes approx. 50 times
>longer to complete.

That is not surprising.  A radius of 30 involves on the order of 61x61
input pixels in the convolution of each output pixel.  A radius of 1.2
involves 3x3 or 5x5 pixels.

>Please can anybody give me any hints, what 'sigma' means?

It describes the relative weight of pixels as a function of their distance
from the center of the convolution kernel.  For small sigma, the outer
pixels have little weight.

Glenn