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Subject: Re: [OM] Homepage
From: John Hermanson <omtech@xxxxxxxxx>
Date: Wed, 17 Mar 1999 08:44:13 -0500
As others have reminded me, some of my lab work at the Germaine School
of Photography in NYC involved masking in 4X5.  Now I see the
connection.  Thanks.

John

Kenneth Sloan wrote:
> 
> > Someone once said (maybe on this group), that when you scan an image,
> > you should do it so you only have to scan it once, then use this "raw"
> > or "master" image for all future images, whether it be downsizing or
> > creating a thumbnail.
> > And YES, there is a huge improvement in  resized image quality when I
> > use the "unsharp"  filter.  It really sharpens it (why do they call it
> > "unsharp"?)
> 
> For shame!  When I teach this to Computer Science nerds I always tell
> them that photography folk have been doing this for years - and have
> lots of darkroom techniques that seem "unusual" in the digital world.
> 
> The technique is called "unsharp masking".  Start with a slightly
> out-of-focus negative.  Make an even MORE out-of-focus POSITIVE
> transparency.  Now...print the sandwich and what do you get?  A sharper
> print.  Why?  Because the print is a blend of the original image (which
> contains high and low frequency energy) and the unsharp mask (which
> contains very little high frequency energy, but lots of low frequency
> energy).  Reversing the sense of the two image means that you have:
> 
>     Image - UnsharpMask = "sharper" image
> 
> In the digital world, you can either do this directly:
> 
>     Blur <Image >UnsharpMask
>     Blend Image -UnsharpMask > SharpImage
> 
> Or...you can combine both operations by convolving with a kernel which
> is a combination of "blur" and "subtract", viz:
> 
>         -1 -1 -1     0 0 0     1 1 1
>         -1  9 -1  =  0 1 0  -  1 1 1
>         -1 -1 -1     0 0 0     1 1 1
> 
> (the above is just an example...please don't pick at the details, or
> I'll write more...you have been warned!)
> 
> I like to create the unsharp mask and then play with the amount of
> Blend.  I use a function:
> 
>     Blend Image1 Image2 t >SharpImage
> 
> where 't' is the weight of Image1 and (1-t) is the weight of Image2.
> This allows everything from image mixing to image SUBTRACTION (which is
> what we want when Image2 is an UnsharpMask).
> 
> And...yes - I have found that when my son scans prints on his cheapo
> scanner, it helps to run the sampled image through an "UnsharpMask"
> step. Use with moderation, though!  You can take out the low frequency
> information, but nothing can put back in the high frequency information
> that you DIDN'T GET when you scanned originally.  Better to make sure
> that you get the best possible original scan!
> 
> --
> Kenneth Sloan                                            sloan@xxxxxxx
> Computer and Information Sciences                       (205) 934-2213
> University of Alabama at Birmingham                 FAX (205) 934-5473
> Birmingham, AL 35294-1170   http://www.cis.uab.edu/info/faculty/sloan/
>

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