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High-Pass & Gaussian – Choosing the radius
Posted by: byRo on 12-31-1969.

"If you take a look at the threads concerning sharpening / blurring / high-pass filter you will quickly discover two things: 1) The radius is very important; 2) Nobody tells you just how to get it right. Hope this will help out. "



Any advice you see with exact numbers for the radius will be accompanied by a phrase like: “..not set in stone...”, “Let’s try......”, “My favorite...” etc...etc... Which although, well meaning, doesn’t help the newcomers and still gives a lot of work to the “oldies”.




What is the radius?

The the radius is not a hard fast limit where the effects of the filter end, it is just an indication of where the effect is getting small.


I did some tests and came to the conclusion that the effect of a Gaussian blur at one radius distance away is 16% - and that’s for any radius. Which means that if I have a block of white next to a block of black and do a blur of radius 10, then at a distance of 10 pixels from the border edge of the two, I get 16% white mixed into the black in one direction and 16% black in the white in the other. At less than the radius there is more mixing and beyond the radius there is less.


Yeah, but I always use a radius of three!

The radius is specified in pixels, but the visual effect that it will have one your photo depends on the size of the image, or the parts of the image. A nose in one photo may be 50 pixels wide, in another only 10. A fixed value only holds true if you are always working with facial features at about the same pixel size. (except *1)



Let’s have a look at the information carried in a photo at three different radii. Note that we won’t be talking about one radius but instead a whole band.


The terms may be confusing – the LOW band has high pixel radii and the HIGH band has small radii. (*2)



Now it becomes clear what each band deals with:

LOW:                Overall image information, showing general light / dark areas;

MEDIUM:          Shows the images main features;

HIGH:               Edges and fine detail and texture.




The Gaussian blur and high-pass filters (they are the same thing, remember) have just one radius as a parameter, but in fact they are specifying a band. Its's just that for the high-pass filter we will get the information from the specified radius down to zero. And conversely, the Gaussian blur will give us everything from that radius and up.

Although they are very useful that way, it’s even better if we can define where to start and stop – using a smaller well-defined band.



We can use these three band-pass images to tweak the photo. The results are subtle but may give us just that “something” to make the photo special.

LOW band: The right side of the face is much darker than the right. Although, maybe, the photographer wanted it that way, it seems a little too much. As the LOW band carries the general area information we can tone down the difference by subtracting a fraction of the low band.

To subtract it entirely we would normally invert <ctrl><I> the layer and set the blending to Linear Light at 50%, but as we don’t want to eliminate the band completely, just to tweak it, we can set the blending to Soft Light and opacity 100%.


MEDIUM band: This help defines the facial features – a Soft Light blend at 100% gives more definition to the face.


HIGH band: This is almost equivalent (*3) to sharpening, as we want to improve the outlines and not the texture (*4) we will use Linear Light blending – 50% opacity looks fine here.




Original -=> Tweaked


OK, doesn't look much different here on this little picture. When you try this at full size you'll see it better.












As I said above the radius is not a well-defined measure. Usually prefer working “by the numbers” but in this case I found it better just to visualize the effects and then choose the bands that I wish to tweak. To make the work easier I set up an action (in the thread which does all the hard work setting up the bands.


Although it doesn’t look like it, this works just like the graphic equalizer on your stereo set.


All the band layers are initially set to Normal blending.


Work up from the bottom....

- Turn the layer (band) on.

- If you think the band is going to help, then add it in by changing the blending to overlay (or soft / hard / linear light). Adjust opacity to taste.

- If the band shows something you want to take out, invert it <ctrl><I> and then add it in.

- If unsure, turn it off and go to the next one


Maybe someone will like the idea and make a neat plug-in with pretty sliders (*5) - until then we’ll keep tweaking the layers.


Besides the bands, there are two other layers – one at each end (*6). The Clipping layer on top is to warn you when you have some pixel values at the limits – as you keep on adding in more bands it will happen sooner or later. The Brightness / Contrast at the bottom is the way to fix the clipping.


Have fun!


(PS: The LOW, MEDIUM, HIGH bands were 50-100, 05-10 and 0.5 – 1)


Next up:

Sharpening: The true story;

JPEG: Know thine enemy. 


Notes and observations:  

*1 Except JPEG artifacts (but that’s another tutorial)

*2 If you remember some physics, it’s the same as wavelength and frequency – radius is wavelength and the bands are frequencies. So the high-pass filter lets through everything from the chosen radius down – or in ither words from that equivalent frequency up.

*3 When we refer to sharpening usually we will be talking about the use of the chosen radius down to zero. Here we will stop short, meaning that the high-frequency noise present in the smallest radii doesn’t get sharpened (if we’re lucky, but that’s another tutorial).

*4 The Overlay / Soft Light / Hard Light blending options act by multiplying, while the Linear Light does addition. The effect is that the first three have more impact on the mid-greyish areas and the latter on the extremes (but that’s another tutorial)

*5 If you do, give me some credit, OK?

*6 If you have ever worked with audio processing, you’ll recognize these as the clipping indicator and the input level adjust.

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