olli_salonen

<p>Ok, I found a partial answer through experiments. Photoshops high pass filter uses the following convolution matrix (or at least very similar):</p>

<pre>

-1 0 0 0 -1

0 -1 0 -1 0

0 0 8 0 0

0 -1 0 -1 0

-1 0 0 0 -1

</pre>

<p>This is of course for a high pass filter with radius 2. There seems to be a little more noise in the image, when convolution matrix is used, so my reasoning is probably not perfect or PS uses some kind of noise reduction.</p>

<p>But THE problem is: how on earth can I use a 21x21 convolution matrix?! Could this be avoided with some math?</p>

As I'm not using PhotoShop, but GIMP (in Linux) I would like to know

what does PhotoShop's High Pass filter do. Is there a way to replicate

the effect with a convolution matrix?

For example, if I choose a radius of 10 in PhotoShop, what would be

the equivalent convolution matrix? Is 3x3 or 5x5 matrix sufficent?

What I am after, is high pass sharpening in GIMP of course...

Thanks,

Olli Salonen