Bill Richardson wrote:

> --I have been playing with high pass sharpening as a substitute for
> USM. It seems very effective and less destructive. I would
> appreciate knowing what others have learned about this technique.

More on high pass can be found here:

http://members.ozemail.com.au/~binaryfx/PSTV_links.html#H (scroll down to the high pass links)


Here is what I recently wrote at the Applied Colour Theory list:

http://groups.yahoo.com/group/colortheory/

"I have played with this for a few years now, since I first read an article
on this in a graphics magazine (DesignGraphics.com.au - Issue #50 1999).

I have not found this to be a replacement for USM - but it does make a handy addition in some cases. I have found that it can bump out a little more appearance of detail in some images which have been sharpened with USM to a satisfactory amount - when used with care. You can also introduce luminosity blends into the method.

>

Noise amplification, excessive edge halo width (too high high pass
settings). Just some of the regular things as with USM but as you have less
control with high pass you have to take more care.

The basic operation of USM is done by subtracting a duped blurred copy of the original from the original. The image is made sharper by removing low frequency data from the image so you can see the high frequency data. So one way to think of USM is that it removes blur so you can see the existing sharpness of the image.

On the other hand - High Pass removes low frequency data and only passes
through higher frequencies. When we sharpen via high pass methods we are adding existing sharpness back on top of the image.

Both methods amplify local edge contrast - in similar but different ways.
USM sharpens with built in noise suppression, then when you factor in
thresholding as well there is added noise control. High Pass seeks out all
the noise and edges in the image and then you add them back to the original.

So this is why a high pass and USM of the same approx overall effect will
always deliver a very slightly sharper image via high pass.

There are some basic steps than can be done when high pass sharpening:

* Prefilter with deskeckle or minor g/blur before high passing (less sharp
but less noisy)
* High Pass and then despeckle or minor g/blur (more sharp than above but
less noisy than using no noise suppression)
* Or combine both of the above methods of pre & post smoothing using smaller
amounts via fades
* Desaturate the high pass blend layer after high pass filtering
* Introduce luminosity blends (see my other recent post for more on this
subject)
* Introduce variable halo intensity (see my other recent post for more on this
subject)
* Blend modes from soft light, overlay, hard light, linear light or perhaps
vivid light
* Various opacity settings
* Try smaller high pass values to start and watch for halo issues as you
increase size

So if noise is a concern - then USM or other methods may be better. If you
don't have to worry about noise or are after a little more pop - then high
pass methods might be better, although the halo is different."

There are other methods of sharpening, as mentioned here:

http://members.ozemail.com.au/~binaryfx/PSTV_links.html#S (scroll down to sharpening and USM).

http://members.ozemail.com.au/~binaryfx/PSTV_convolcorner.html

http://members.ozemail.com.au/~binaryfx/PSTV_sharpcontrol.html

Regards,

Stephen Marsh.