Re: [APML] High Pass Filter technique (WAS:New TP images)

["Mike Cook, AF9Y" <mwcook@concentric.net>]

Wei-Hao,

    Thanks for one of the most informative posts that I have seen in years.  Although I 
no longer do film, this type of post makes me glad that I've stayed on the APML.

Mike Cook
AF9Y


Date sent:      	Mon, 16 May 2005 14:08:43 -1000
From:           	Wei-Hao Wang <whwang@gmail.com>
To:             	Discussion of Film Astrophotography <astro-photo@seds.org>
Subject:        	Re: [APML] High Pass Filter technique (WAS:New TP images)
Send reply to:  	Wei-Hao Wang <whwang@gmail.com>,
	Discussion of Film Astrophotography <astro-photo@seds.org>
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> Hi Alan,
> 
> In photoshop, maybe you can try to run a high pass filter over an 
> image to see what you get.  
> 
> Basically what happens is, photoshop (Gaussian) blurs the image 
> and subtract the blurred image from the original.  In other words, the 
> high pass filter does exactly the opposite thing as Gaussian blur.
> If you duplicate an image into two copies, Gaussian blur one, high 
> pass the other one (both using the same radius), and add the blurred
> and high passed image together, you will get an image identical to 
> the original one.  This tells us that any image can be subdivided into
> two components, a high passed component and a blurred component.
> 
> The simplest application of a high passed image is adding it back to the
> orignal.  This is identical to a 100% strength unsharp mask.  The whole 
> idea of unsharp mask is to change the weights of the high passed 
> component and the blurred component.  Standard photoshop unsharp
> mask increases the weight of the high passed component.  The "true 
> unsharp mask" decreases the weight of the blurred component.  Indeed,
> there is no difference between these two.
> 
> Of course, you can use the high passed images in many different ways,
> using the layer function or the apply image function.
> 
> Hope this helps.
> 
> Cheers,
> Wei-Hao
> 
> On 5/13/05, Alan Voetsch <alanv12952@yahoo.com> wrote:
> > Hey Stuart,
> > 
> > --- Stuart Heggie <Stuart.J.Heggie@sympatico.ca> wrote:
> > > I have been helped a lot by being taught how
> > > to use
> > > the High Pass Filter technique. I tried it on your jpeg and it made a
> > > big improvement ( I think! ).
> > 
> > Do you have any further advice or suggestions on how to use this tool?
> > 
> > Thanks,
> > Alan
> > 
> 
> 
> -- 
> ________________________________________________________________
> Wei-Hao Wang  :)
> 
> Institute for Astronomy at University of Hawaii
> 
> Address:                       
> 2680 Woodlawn Drive         Personal Website:
> Honolulu, HI 96822             http://www.ifa.hawaii.edu/~wang
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Mike Cook, AF9Y
http://www.af9y.com
mwcook@cris.com




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