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 Post subject: Improving (maybe) High Pass filter script by RobA
PostPosted: Mon Mar 04, 2013 11:33 pm  (#1) 
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While I was already satisfied by the old version of the script I had, I stumbled upon the updated version (of aug 2012) on registry: http://registry.gimp.org/node/7385 Then I began to read the comments and someone pointed out, in 2010, how PS handled this high pass filter:
Blacklemon67 wrote:
duplicate, blur, set layer blend mode to grain extract, merge down.
This looks more (to me at least) like photoshop's high-pass. It's more defined then the other one.

Then robA considered the comment and changed his script accordingly.

Reading this comment made me want to reproduce the filter manually, but instead of 'blur' I tried 'selective gaussian blur' (blur radius 5 & max delta 50) then overlaid twice, and this manual output appeared very good to me, especially because with the filter I had side effects making some dark areas darker (visible when dark contours are close to a dark area) and creating slight halos, side effects that didn't came up with the manual way. Looking twice, It seems the output I had was, above all, softer than the filter thus it would allow smooth high pass if needed and stronger if wanted by duplicating high pass overlay layer or using another blend mode.

Picture demo with a Public Domain photo:


Attachments:
File comment: go full screen to see the differences
high pass vs high pass.jpg
high pass vs high pass.jpg [ 759.03 KiB | Viewed 632 times ]
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 Post subject: Re: Improving (maybe) High Pass filter script by RobA
PostPosted: Tue Mar 05, 2013 2:04 am  (#2) 
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anarkhya: thank you! i tried that and it worked very well on a picture where it was needed. i'll remember that selective gaussian blur thing.

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 Post subject: Re: Improving (maybe) High Pass filter script by RobA
PostPosted: Tue Mar 05, 2013 2:54 pm  (#3) 
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The manual image is best. The colours on the filter image are too bright and tend to make the bricks rather unrealistic - I've worked in the construction industry for thirty years so know a bit about brickwork.
I've tried the filter a couple of times, just playing around time, and thought it was pretty good. I'll try out this manual method next time. Thanks for the info.

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 Post subject: Re: Improving (maybe) High Pass filter script by RobA
PostPosted: Sun Mar 24, 2013 11:55 am  (#4) 
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Whatever it takes to get the result you want, is what you should use.

Mathematically, a high pass filter removes the low frequency components of the image, leaving the high frequency components (hence high-pass), which can be done by a blur (convolution) on the image.

Using the selective Gaussian blur to come up with a better overlay seem to give great results - it just isn't a straight high-pass filter, as the frequency that is eliminated varies based on the source image, rather than a single number (the blur radius).

-Rob A>

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 Post subject: Re: Improving (maybe) High Pass filter script by RobA
PostPosted: Sun Mar 24, 2013 3:53 pm  (#5) 
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Err.. So, are you saying that is not an high pass filter anymore ? Then I should learn how to make a script..


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 Post subject: Re: Improving (maybe) High Pass filter script by RobA
PostPosted: Mon Mar 25, 2013 3:44 pm  (#6) 
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anarkhya wrote:
Err.. So, are you saying that is not an high pass filter anymore ? Then I should learn how to make a script..


Right! It is a filter, and a useful one, just not a standard high pass filter.

Like, a Gaussian blur is (by definition) a low pass filter (it removed high frequency details/sharp edges). And a Selective Gaussian Blur is a low pass filter in tandem with an edge protection mechanism (it removes high frequency details, but preserves edges).

You might find through experimentation that is better to use the selective Gaussian blur repeatedly, with weaker settings than one large blur.

-Rob A>

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 Post subject: Re: Improving (maybe) High Pass filter script by RobA
PostPosted: Mon Mar 25, 2013 5:31 pm  (#7) 
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I've known about the manual way of creating a 'high-pass' effect, but have never used a 'selective gaussian blur'.

I'll have to try this out as how RobA suggested, with weaker settings and multiple 'selective gaussian bluing' than just one large blur.

Thanks! ;)

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