High pass is very useful filter when it comes to the frequency extraction technique. This technique consists of separating the image into a low pass(blurred image) and a high pass(grey averaged image). When those two passes are combined using linear light blending mode (as photoshop calls it) they produce 99,999% identical image to the original.

Why it is useful? Applying this technique you can treat the texture and tonal content separately, for example to average the skin tones or to smooth the facial texture. In fact it is broadly used in texture creation and digital retouching.

To produce a low pass is easy, just apply a gaussian blur of a known radius.
To produce a high pass you may apply it as a filter using the same radius if there is one. Or you may(in 8bit) subtract the unblured image from the blured one(low pass) with an offset of 2 and scale at 128 which essentially mean that it's averaged on a middle grey.

You may find references to this technique at http://www.theartofretouching.com/blog/ ... d-low-pass

Linear light is a combination of linear dodge and linear burn and has this formula

if (Blend > ½) R = Target + 2x(Blend-½)
if (Blend <= ½) R = Target + 2xBlend - 1


Please let me know how and if this technique currently may be done in Krita.

Thank you

A kind of image produced by high pass filter, you may be familiar with it.

http://www.thesilverartifice.com/tutorials/highpass.jpg

A more detailed example of how it is done in photoshop may be found here:

http://www.youtube.com/watch?v=dMyaeZmkZD8

The currently is no high pass filter.

No auto-filter for this, but you can create the High pass layer manually:
-Duplicate it,
-Gaussian Blur 10pix on the duplicate
-Invert
-50%opacity
-Merge down

Actually, this sounds like a nice junior job for someone who wants to either get started writing a C++ filter for Krita, or maybe learn some OpenGTL and write an OpenGTL filter .

In fact the math beneath it is fairly simple, it is also broadly documented being a term ported from the signal processing science.

P.S. In photoshop I actually never use the filter but have an action that recreates the manual procedure, it is stated as being more accurate probably because photoshop's filter is some what antique.

Animtim wrote:No auto-filter for this, but you can create the High pass layer manually:
-Duplicate it,
-Gaussian Blur 10pix on the duplicate
-Invert
-50%opacity
-Merge down

Very cool works nicely, thanks for the hit

I spotted a problem with the edges of the image, in fact with your technique I managed to recreate almost perfectly the image I was trying to split. But the edges had a weird artifact http://www.pasteall.org/pic/30365 . So I tried to figure out what it was, your technique is correct the problem lies actually in the gaussian blur filter that does not take the edges into consideration. This may be related to the fact that performing the gaussian transform it considers the canvas as to be finite and not extended "infinitely" in order to produce blur in the lateral areas.

It may be actually simpler, I think krita considers the out of canvas areas as to be transparent, that is what produces the artifact. Reference to spot the problem may be found here: http://www.mediafire.com/?pr75s5b3k1aq5dm

The math behind it isn't that simply, especially if you want good results. We are already using the FFT from fftw3 in the convolution filter and that is already quite tricky.

If you want to implement that, it would of course the welcome and we could help you with the implementation.

This problem may be solved quite easily. It would be sufficient to extend the pixels that constitute the border of an image beyond the border for the length specified in the filter in order to produce an adequate result. In simple words every pixel on the border of the image should be considered as a line extending outside at the time the filter is applied, this way back alpha would not bleed inside the canvas.

An example of this may be found here: http://www.mediafire.com/?wg0bve448ow5zjl

I recently discovered how to rescue nvidia's bug at the startup of ubuntu with no gui OUUUhhH!!!, so to start programming would be kind of an overkill. I'm very pleased to hear that I may ask you for advice in case I would start programming, thank you indeed

Normally blur should repeat the border. Looks like there is some bug.

I think it has been reported here, and apparently solved

https://bugs.kde.org/show_bug.cgi?id=245870

I have reopened it.

I just posted there, test file included.

"Hello,

I get the border's black alpha bleeding into the canvas on all sides except the left one.

reference here: http://www.mediafire.com/?vem5vjb7c7yql5v

Thank you

I'm using Krita 2.4 on ubuntu 12.04 on a Lenovo w520 (linux certified) "

I spotted an old conversation regarding the problem: http://mail.kde.org/pipermail/kimagesho ... 08016.html

It said that it worked previously (as boudewijn suggested) but them broke down with the new tile engine.

As suggested applying Gaussian via filter layer does the job well, but Krita crashes when trying to merge/apply filter layer.