On Mon, 22 Mar 1999 13:54:14 -0600, Ken Norton <image66@xxxxxxx> jammed all
night, and by sunrise was overheard remarking:
> > Image - UnsharpMask = "sharper" image<<
> >I suppose that our "darkroom specialists" will have some point of view about
> >that, and probably some practical experience...
> However, I have done a lot of double exposure images where I take a picture
> of a flower, for example, where the first exposure is dead-on sharp. The
> second exposure is completely out of focus. The results occasionally are
> suprising in that these double exposed pictures can end up sharper than a
> single exposure. Is this effectively an "unsharp mask"?
No -- you have to subtract the "fuzzy" image. The principle is simple:
you have a positive or negative (call it N) and a corresponding negative
or positive (call it ~N). You make ~N "fuzzy" -- you do this (whether you
know it or not) by running a low pass filter over it, killing some degree
of high frequency information. So what's left, naturally, is low
frequency information. Then you add the negative (same as subtracting a
positive) this to get your result, where K is an intensity (eg, you
probably don't apply the fuzzy mask at the same intensity as the
original):
R = N + K * L(~N)
What happens here is that some (depending on L() and K) of the low
frequency information in N is removed (note that N + ~N = 0), which tends
to emphasize the high frequency information. This makes the picture look
sharper.
If you have a photo editor on an computer, you can play around with this.
Practically any photo editor or paint program will let you do it
manually; fancier ones like Photoshop and PhotoImpact have this function
built-in.
--
Dave Haynie | V.P. Technology, Met@box AG | http://www.metabox.de
Be Dev #2024 | NB851 Powered! | Amiga 2000, 3000, 4000, PIOS One
< This message was delivered via the Olympus Mailing List >
< For questions, mailto:owner-olympus@xxxxxxxxxxxxxxx >
< Web Page: http://Zuiko.sls.bc.ca/swright/olympuslist.html >
|