Re: [APML]: Computer processing

["Tom Krajci" <krajcit@det5wg57.barksdale.af.mil>]

> From:          Steve Bell <sb635@delphi.com>
 
> > The Unsharp Mask function in Photoshop and unsharp mask technique used
> > on film based astrophotos is not the same thing. The PS function is just
> > a form of simple sharpening. It does not perform a contrast equalization
> > like you see in the David Mailn prints.

I haven't seen enough Malin prints, but I have experimented with 
Photoshop's unsharp mask routine.  If you crank up the percentage 
settings high enough (and raise the pixel radius to about 8-12, 
maybe 20) you *do* get a contrast equalization, but the image tends 
to look "overprocessed."  I can't say if this is identical to Malin's 
stuff. . .but hey, a couple mouse clicks and a few seconds later you 
can try different settings!  

I also compared the Photoshop unsharp mask routine to my own attempts 
in Photoshop at emulating how they do it in a darkroom.  I took an 
image, made a copy of it, blurred this copy, inverted this copy, 
multiplied this inverted image's pixel values by a constant, added it 
to the original, and did a linear histogram manipulation (because 
the resulting image is pretty low in contrast.  (I even did this with 
color images after breaking them into the three color channels (red, 
green, blue)) I experimented with different size gaussian blurs, what 
constant value to multiply the inverted image by (controls how 
strongly the effect of this 'unsharp mask' is applied to the 
original), and how much of a linear histogram manipulation to apply 
to the final image.

After all this I concluded that my efforts were (at least for the 
quality of images my scanner can deliver) roughly identical to 
Photoshop's pre canned unsharp mask routine. . .and it's much easier! 
No, my analysis is only qualitative (I didn't do statistical analysis 
on noise, etc.) but it was enough to convince me that the Photoshop 
unsharp mask routine is pretty good (when used properly) in emulating 
the old darkroom methods.

I encourage others to try this and add to the discussion.

Tom Krajci

Capt Tom Krajci
B-52 Intelligence Officer
"In God we trust, all others we monitor!"
http://spur.barksdale.af.mil