Median Filter, Negative Values = PSP's Clarify Filter?

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DJJohnson
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Median Filter, Negative Values = PSP's Clarify Filter?

Post by DJJohnson » Thu 02 Nov 2006 07:15

I was exploring some of the unique filters in PL32. When using Filter / Other / Median, if I used a large radius of 20 to 50, but then used a negative intensity of -40% to -80%, the effect was remarkably similar to Paint Shop Pro's "Clarify" filter. If I over-exaggerate the intensity's negative values more than -150% or so, up to -500%, then my sample image looks like it has the very same artifacts as when Paint Shop Pro's Clarify filter is applied too strongly or too many times.

Is this all that PSP's Clarify filter is? It sure looks that way.

One huge difference, the options in PL32's Median filter allow for a much nicer fine-tuning of this effect.

DJJohnson
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Post by DJJohnson » Thu 02 Nov 2006 21:27

Never-mind. They are not the same. Experimenting further I found out that PSP's "Clarify" filter is nothing more than a Gaussian Blur done with negative values. PL32 sure is nice for experimenting with values outside of usual constraints.

Using side-by-side comparison photos of an over-exaggerated Clarify effect from PSP (as a base example to clearly see what it does to an image), I can exactly duplicate the effect in PL32 by using Gaussian Blur, HIS space, de-select H & S channels (use only I), with a pixel blur size of ~50, and negative intensity values of -150% to -500%. Using intensity values of -10% to -60% will exactly duplicate the standard use of PSP's Clarify effect.

When using Gaussian Blur or Median Filter for this "Clarify" effect it needs to be applied to a luminance channel only. In the filter's options use the HIS color-space and adjust for the I channel. Or Lab using only the L channel.

By playing around I think I just found something nicer than PSP's "Clarify" -- using PL32's Median filter in reverse. You just have to watch for any obvious wide halo-artifacts. Increase the size and decrease the intensity accordingly to avoid them. What settings work best are very subject dependent and image size (pixel resolution) dependent.

I had heard reports in the past that PhotoShop users were envious of PSP's Clarify filter at one time. Now they don't have to be, knowing how simply it is done. There's some humor in there, in that reverse-engineering the "Clarify" filter's effect only required playing with reverse values. :-)

lutz
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Thanks for the tip - unsharp masking

Post by lutz » Tue 07 Nov 2006 01:57

Hello DJ,

thanks for the tip, these filters are worth playing with. I am not perfectly convinced yet regarding the "clarify" thing.
The idea of negative blurring sounds mathematically interesting - could it be that one of the negative blurring effects is in the end similar to unsharp-masking?

DJJohnson
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Post by DJJohnson » Tue 07 Nov 2006 13:45

Lutz, someone on another forum was testing what the Clarify filter does in PSP by applying the filter too many times. I used their "before" and "after" images to test this.

Here's the original image:

Image
(Before)

Here is what happens when they used PSP's "Clarify" too strong and too many times. I got similar results by applying it in PSP with a Strength setting of 5, applied 3 times.

Image
(After over-applying PSP's "Clarify")

Here is the image I can get from PL32 if I use its Gaussian Blur filter with a Size of 60, applied to the "I" (Intensity) channel in HIS color-model, and with an Intensity of -300%.

Image
(After over-applying PL32's Gaussian
Blur with negative values)

I did notice one thing a little different this time. In order to get the shadows and highlights to be nearly identical to the PSP "after" photo, I had to use the Histogram adjustment on the original image first, and use output settings of 15% for shadows and 75% for lights. Otherwise using PL32's Gaussian Blur in reverse would blow-out the lows and highs.

Notice in particular, the large halo on the left side of the woman's bathing-suit. That effect could be obtained no other way in PL32, nor in PSP. You can make those halos identical depending on what settings you use in both programs. This is why I believe that's all that this "Clarify" filter is. Granted, it appears to get nearly exact duplication that you have to apply a histogram correction first to compress the lows and highs that get blown-out by using a negative Gaussian Blur at those strengths. Even so, they both have the same effect when over-applied. This leads me to believe they are even more the same when used properly.

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Hoogo
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Re: Thanks for the tip - unsharp masking

Post by Hoogo » Wed 08 Nov 2006 08:30

lutz wrote:could it be that one of the negative blurring effects is in the end similar to unsharp-masking?
If I remember it correctly that is just the way sharpening is done in analogue photography. An unsharp positive and the original negative were put together and developed. And digitaly you do not use a negative and add, but use the positive and substract.