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Subject: Re: [OM] Homepage
From: Kenneth Sloan <sloan@xxxxxxxxxxx>
Date: Tue, 16 Mar 1999 22:47:19 -0600
> 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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