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when, why, and how the high pass filter

Apr 5, 2013 10:28 PM

i have a good grasp of the arcane nuances of unsharp mask,

but nothing on high pass filter,  So can someone offer just a

summary of the thinking of when we select that tool, and

what parameters to consider to what end.  Thanks a lot

best,
Steve z

 
Replies
  • Currently Being Moderated
    Apr 6, 2013 1:17 AM   in reply to Steve Zeeeee

    when we select that tool

    »Tool« has a specific meaning in Photoshop, »High Pass« is a »Filter«.

     

    For an explanation of the connection to Gaussian Blur check out:

    http://en.wikipedia.org/wiki/High-pass_filter

     

    High Pass can not only be used for sharpening but can also helpful for extracting patterns from images.

    Edit: I should have clarified that I meant Patterns intended for use as Linear Light Pattern Overlays to allow painting on a Layer while maintaing a sctructure on the paint strokes to better fit in existing images.

     
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  • Currently Being Moderated
    Apr 6, 2013 7:48 AM   in reply to Steve Zeeeee

    Jeff Schewe, in his new book “The Digital Negative” (Peachpit Press) describes a use of the High Pass filter to adjust midtone contrast -- bring up midtone contrast details -- somewhat like the Clarity adjustment in camera raw, but “you have a lot more control over the parameters.”  He applies a High Pass to a layer copy of an image with blend mode set to Overlay and using Blend If options to constrain effect to the middle range of levels in the image. (pg 210-212)

     
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    Apr 6, 2013 12:30 PM   in reply to Steve Zeeeee

    The high pass filter is a tool for finding edges...from that, you can adjust the edge width to increase or decrease the radius of the edges. Everything that is not an "edge" turns into a middle grays. Depending on the blending, this give an opportunity to modify edge or non-edge values. Overlay lightens the lights while darkening the darks and does nothing at 50% grays In the case of high pass, this means the light side of the edges gets lighter and the dark edges darker. This can have a sharpening effect similar to USM without the threashold control.

     

    If the HP radius is small, the effect is sharpening...if the radius is larger, it's more of a tone /contrast control. If you constrain the effects using Blend If adjustments, you can impact midtone contrast as mentioned by Robert.

     
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