Lum Frequencies
From TechSlop
Many moons ago, Peter Hajba wrote an article called The Power of the High Pass Filter. Sadly, it has largely disappeared from the internet. It's still out there somewhere, but you will have to look for it.
The article is about how to use High Pass to fix luminosity for tilable texture. However, and this is the great thing, he went into great depth about frequencies and why High Pass works for his technique. A true gem.
The article got me thinking about putting the ideas into my own context. As a result of much fiddling and thinking, I wrote my own little ditty. See Frequencies: High, Medium, and Low. No where near as detailed as Peter's article, but shows how to do frequency things in Photoshop using stock tools. Using those techniques, I've learned a whole new set of tricks and techniques.
Let's take a quick look at some the luminosity frequencies of a photograph:
- Low, rather vague and no detail.
- Medium, vague but some shapes can be made out.
- High, just the detail with washed out patches.
- Composite, all three frequencies put back together.
A common use for High Pass is for shaprening or adding contrast.
- Copy photo to new layer
- Filter > Other > High Pass with Radius on the low end
- Change blending mode to Overlay, Linear Light, or something.
- Other tweaks as needed, such as Opacity or something.
What that will do is boost contrast in the high detail frequency. While a good technique, it does have a drawback or two. The main drawback is that it can adversely over-saturate pixels. I'm not the only one that hates dealing with it - love the technique, but hate working around the side-affects.
Right. Time to write my own filter to do things the way I like. You know, a shortcut.
Enter: Lum Frequencies. And it looks a little something like this:
What it does is extract the Luminosity, tear it apart into three frequencies, allow you to scale the frequencies, and then puts it all back together.
- q Minor and q Major: These are basically radius sliders. Anything less than q Minor is high frequency, anything over q Major is a low frequency, and anything between the two is a medium frequency.
- High High and High Low: These are the scale sliders for the high frequencies. High High will scale the top half of the frequency, while High Low will scale the bottom half. These work as percentage sliders, so 200% will double the given half of the frequency, and 0 will have no affect.
- Medium High and Medium Low: Same, but for medium frequencies.
- Low High and Low Low: Again, same, but for low frequencies.
- Base Offset: Since frequencies are identity 128, I decided to toss this in case you don't want identity 128. Base Offset will work much like Brightness.
- Unclip High/Low: When adding frequency in the way that Lum Frequency does, it can be rather easy to go well above 255 or well below 0. This will result in clipping or blowing out. These sliders will scale the effect down for High/Low to help reduce clipping if you don't want it. Works, but could use a slightly better algorithm.
- Saturation: Even though this whole thing started because of over-saturation of pixels, I put this in just in case. This will increase or decrease saturation in a relative manner using HsY. Sometimes, however, when this slider is extremely low, some pixels will peak through. Doesn't happen that often, but it is most definitely on my Bug Hunt list.
It can be a bit of mind-bender to think in frequencies. Some weird and unexpected things can happen. For example, once the frequency hits a certain point, it may look like nothing is happening. Or you may get some 'bleed' and not know where it's coming from. Takes practice, but I think it's worth it. If you want to know where a certain frequency resides, just get crazy with the appropriate slider.
And now some simple examples.
Here is that guy in Paris:
That may look like regular contrast work, but things are divided into six chunks and are independantly tweakable. Hmmm, looks like I got a little too crazy.
Even though Lum Frequencies was originally for boosting contrast based on frequency, it is possible to scale frequencies down for some interesting smoothing effects:
And here is an artsy-fartsy that I'm fond of:
Of course, the usual caveat of losing your eyeballs. Sometimes frequency is the way to go for boosting contrast, while other times Curves or Levels would be better. Don't lose your eyeballs to thinking that frequency is a magic contrast bullet.
Download
PC/Win
Filename: TS_lumfrequencies.8bf
Filter > Tech Slop > Lum Frequencies
Supported Modes: RGB 8-bit
Version: Beta 1
Download: TS_lumfrequences.rar (http://tech-slop.serveit.org/plugs/TS_lumfrequencies.rar)
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