Topic: Variable Frequency

Frequency has been on my mind quite a bit lately. Basically ever since I did Silly Difference Trick and Lum Frequencies. Actually, maybe even before that when I went on that medium frequency ramble. Can't really remember - but it has been on my mind for awhile.

If you use Gaussian Blur, you are using a specific frequency. Same with High Pass. With those two, it's easy to divide a photo into two frequencies. Back in my ramble, I showed how to take it a step further and divide a photograph into three frequencies.

Now, I have a problem with using set frequencies across an entire photograph. That being that not all frequencies are equal across a photograph. A good example of this is sharpening details around eyes and mouth, but not so much across cheeks and forehead. Of course, a lot of retouchers will mask the different frequencies.

We have Gauss and High Pass. The beauty is that one can be turned into the other. We also have Lens Blur, which can do true variable Gauss. If we have true variable Gauss, then we also have true variable High Pass. This means that we can also sharpen in a variable manner in the *exact* same way that Lens Blur does variable blurring.

It's truly a thing of beauty, but it doesn't end there.

If we have variable Gauss and variable High Pass, then we also have variable medium frequencies.

Did you catch that?

Repeat: We can do variable medium frequencies.

You can say, I want this part to be 15-20, this other part to be 5-30, and this part to be 0-10. And we can do so with perfect blending. We can do so without having to chop it up and mask.

Now, going back and forth between Gauss and High Pass also applies to medium frequencies. Once you have the medium frequencies, you can enhance or blast. Rather than say that you want to enhance 5-10, you can say get rid of 5-10.

All it takes is a little painting. Well, and Lens Blur. But imagine being able to be a master of frequencies with just a little painting.

Okay, I think I'm going to bust.

The General Technique

- start with some photograph
- copy to new layer
- do something to it, like Mosaic or Crosshatch or whatever
- set blending mode to Linear Light @ 50%
- invert
- favorite method to copy merged and paste
- invert this layer
- turn off the previously inverted layer
- get crazy

What you are left with is the difference. You can now use the difference. I prefer Linear Light @ 50% because it's mathematically correct, but feel free to mix it up.

Using that technique, you can go back and forth between Gauss and High Pass.

Take it another step further.

- copy photograph twice to two new layers
- do something different to each layer
- go through the Linear Light @ 50% thing

This is how you get medium frequencies. Gauss one layer at a decent radius, High Pass another layer at a smaller radius, and then go through the LL@50% thing.

This general technique is also good for a few other things. By extracting the difference in operations, you can tweak further in new ways. You can even rock back-and-forth in a manner. But I'll leave this particular thing to you.

Now, let's mix in Lens Blur.

- start with some photograph
- new Alpha1 channel
- give Alpha1 a simple, random gradient
- new Alpha2 channel
- give Alpha2 a simple, random gradient
- copy photograph to new layer
- use Alpha1 to Lens Blur
- copy original
- use Alpha2 to Lens Blur
- go through LL@50% thing

Once you copy merge and paste that, set final result to LL@50% and turn off both Lens Blurred layers. What you will be left with are variable medium frequencies. Depending, maybe even just variable blurring and variable high pass without the medium frequences.

Instead of just two different gradients, feel free to mix it up with Clouds/Difference Clouds or what have you.

You can use the same operation to blur variably, sharpen variably, and do medium frequencies variably - all in one go. Perfect blending, too.

Now, the trick is in controlling it. Or doing it in a meaningful manner. This gurus just happens to have just the ticket.

Okay, let's do a little warm-up exercise with variable sharpening. The most common method of sharpening is using frequencies. There are several ways of manipulating frequency. The aspect we will be manipulating is the frequency itself as opposed to amplitude.

Here is the image:



Just some guy that I got from a stock photograph CD. I've used this guy before and he's better for this than Biker Chic. Can I get a moment of silence for Biker Chic?

...

Thank you.

Now, we can go back-n-forth between Gauss and High Pass. Since we have Lens Blur, which can do variable Gauss, we'll be using that to get variable High Pass. That is, the higher the blur radius, the higher the sharpening radius.

So I spent a few minutes just painting some greyscale. I don't want the beard sharpened with too much of a radius, so it gets a lower value of grey. Other parts of his face I want to sharpen at a higher radius, so it gets a higher value of grey. I want his hair line just a tad, so it gets a very low shade of grey. Just for fun, I get the tip of his collar a high radius and then kind of faded it to black the closer it gets to his neck. Didn't want to sharpen the background at all, so that's all pure black.

I very quickly used the Smudge tool to so some crude blending. Very quick-n-sleazy.

Here is my final blur mask:



Chucked it into an Alpha channel. Loaded up Lens Blur and used the Alpha. Made sure my settings were to blur white and not blur black. Used a radius of 25. This is what I get after Lens Blur:



As you can see, that is the DOF effect that Lens Blur was intended for.

Once done with Lens Blur, go through the Linear Light @ 50% thing. What am I left with? A single sharpening layer to play with:



Now it's just a matter of blending modes and what-not.

Keep in mind that what was manipulated was frequency - not amplitude.

(Edited by warjournal on 06-11-2006 00:50)

Wow. You make my head hurt but in a very nice way. =)

I really appreciate the examples. Sometimes , make that most times, when I'm trying to follow your "stuff" I'm wondering "is that what it should look like?" but with the examples I can see if I've screwed up. Thanks.

Oh... and my condolences.... Biker Chic.

Long live Biker Chic !

The examples DO help.

Next bit in a day or two. Yes, with examples. Sometimes I just gotta get the text out and then go back for some proper tutelage.

"Don't cross the streamers."
- Ghostbusters

Variable Medium

Did you warm up your noodle by trying the variable sharpening? I hope so. But even if you didn't, gonna forge ahead with variable medium. Even though this next example is about variable medium, gonna also do a bit of regular sharpening. Yes, mixing frequencies like this is possible.

In order to get medium frequencies, you have to blur twice. Then you use the LL@50% thing to get the medium frequencies. To get variable medium frequencies, this means painting two different blur masks. Believe me, this isn't easy. Even though I've had this on my mind for a long time, this little conundrum has held me back from talking about these things. While I don't mind being mind-numbing, this is a bit too mind-numbing.

Then, just the other day, I was doing some channel mixing. Then it occurred to me: why not paint both blur maps at the same time using RGB channels? Disco!

R channel = BlurMask1
G channel = BlurMask2
B channel = whatever, 128 for me

Just keep your colour palette open and set to RGB. Very easy to guesstimate the frequency work with 2 sliders sitting right there. When done painting your 2d blur mask, just copy both channels in the Channels palette and get busy with Lens Blur.

In doing all of this, several different things can go in either direction. Invert one, and the result flips. Invert two things, and the results are back. Going to lay down some rules for this exercise. Going to be doing this Ghostbuster style.

- Low frequency blurring gets a higher radius than high frequency blurring, or both get the same radii. I usually use the same radius for both Lens Blur steps.

- When painting your blur mask, G can not be higher than R. Or, if you prefer, R must be equal to or greater than G.

In other words, don't cross the streamers. When you do, you reverse the polarity. And since we are dealing with several different things, you can reverse the polarity several different times. Tasty, but not right now.

Time for our image. Meet Panama Kid:



He's yet another shining example of good photographs that can be found on a cheap, royalty-free, stock photograph CD. The best $5 I've ever spend. Seriously.

Looking that the photograph, I can see some different frequencies. PK's hair and his cat are high frequency. The background is a rather tight low frequency. His body has the lowest frequencies.

With Colour palette open and armed with Paintbrush, time to think about what frequencies I want to enhance.

Sharpen hair and cat:
R = 48
G = 0
Since G=0, those number will pure sharpen.

Medium body:
R = 163
G = 69
This will enhance contrast without much sharpening at all. I guess you could say medium frequency right in the middle.

Background:
R = 255
G = 41
These number will enhance contrast and sharpen just a tad more than his body. Well, not really sharpen, but be tighter. Also be broader since R is maxed out.

And topped off with a bit of Smudge tool:



Yeah, quick-n-sleazy.

Okay, the more I want to blur, the higher the value of R. The higher the value of R, the lower the frequency.

The more high frequency I want to subtract from the low frequency, the higher the G value. When G = 0, there is nothing to subtract.

Can you guess what happens when R = G?

I love this stuff.

Once done painting, copy R and G in the Channels palette. Copy the photograph twice, and each get Lens Blur at the same radius using each channel in turn. Then it's off to do the LL@50% thing. And this is what I'm left with:



Look at PK's hair and his cat. Very tell-tale High Pass. But look at how much softer the background is in comparison.

Oh, man. That is truly a thing of beauty.

Just for fun, here he is with the final result set to LL@50% over the original:



Beautiful.

After playing with it for a bit, and maybe recording an Action to help, start breaking the Ghostbusters' rule. You just might surprise yourself.

That's about it for now.