"Walter Dnes" on wrote...
| On Tue, Aug 15, 2006 at 08:52:33AM +0100, Dermot Paikkos wrote
|
| > Not too sure about the unsharp/sharpen command. For a very good
| > article on the unsharp mask parameters have a look at
| >
| > http://redskiesatnight.com/Articles/IMsharpen/
|
| Thanks for the pointer. This gives me ideas where to proceed from the
| numbers that Steven gave me.
|
One think not noted by that artical is that under IM it is often better
to ignore the 'radius argument, and just provide the 'sigma' you want to
use. IM will automaticall work out an approriate 'radius'
EG: rather than -unsharp 2x1.5+1
you are better off with -unsharp 0x1.5+1
This ensures that the convolve matrix doesn't just get clipped at the
extreme radius, producing undesirable effects. Of course for large
sigma, you may like to limit the radius to for speed purposes.
The artical however does provide a formual for radius - sigma
conversions.
NOTE: sigma is actually a statistical term meaning the radius at which
90% of the effect is covered.
The radius argument above is only really used to determine the size for
the convolve matrix (rounded up to nearest integer). It really doesn't
really have a bearing on the actual operation, only is algorithmic
interpretation.
Anthony Thyssen ( System Programmer ) <[EMAIL PROTECTED]>
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``Probable impossibilites are to be preferred to improbable possibilites''
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Anthony's Home is his Castle http://www.cit.gu.edu.au/~anthony/
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