VF - Why Sean, Why?!?

05 September 2010

After yesterday's marathon session of technobabble and math, it's only fitting that you should be rewarded with an entry today which which will be more intuitive and directly beneficial to your workflow. Now, that said, after how precipitously readership dropped off yesterday (I believe in light of the length of the post), I won't be going into such excruciating detail today. Instead I'll make broad strokes and incorporate a few external sources, asking that you tell me where you need more information for a subsequent update.

One of the basic principles of retouching which I try to impart to people is how important it is to isolate those portions of an image which you want to work on. Sometimes that takes the form of a simple selection; sometimes it's a complex mask; sometimes a color-based selection; sometimes an operation on a channel; and other times, it's a frequency-based operation. Among the things which the last category allow us to do are what you came back for today:
  • Sharpening:

    I'm sure that many of you are familiar with the idea of "High Pass Sharpening", a technique which has been around the internet for about as long as I've been using Photoshop (a long time). In fact, this technique is just what it advertises - amplifying the high frequency portions of the image (by running a highpass filter on a copy of the image) in order to accentuate the detail.

    As it's normally done, though, this technique uses the PS filter naively and so it discards some tonal detail which might otherwise be retained and selectively enhanced. My personal preference when using variants of HP sharpening is to clip a Curves adjustment layer to the high-frequency layer. This allows one to tune the sharpening effect in the highlight and shadow areas separately and achieve just the level of sharpening desired.

  • Detail Enhancement:

    Often mistaken for the singular solution to the "Dave Hill look" (sorry Dave), use of large-radius HP filters to enhance local detail is just an expanded version of the sharpening discussed above (alternatively known as HiRaLoAm). In this case, we're just selecting a larger swatch of frequencies to enhance, resulting in that larger 'gritty' look [Calvin Hollywood is another big fan of these techniques].

    Again, though, it's important to use a revised technique vs. simply running a naive HP filter so that you can retain full contrast in the detail - otherwise, what's the point? Also note that, while Linear Light is the way which we blend the frequencies back in, other blend modes are sometimes preferable artistically (beware that some come with side-effects, especially Hard Light, Pin Light, and Vivid Light).

  • Stray Hair Removal:

    One of the neat facts about frequencies is that certain types of photographed objects (or their details) tend to 'live' within certain frequency bands. Hair, for example, is a very fine detail, and so tends to exist only in higher frequencies. We can use that fact to our advantage by performing a separation as we've previously discussed, and then simply using the healing or cloning brush on the high-frequency layer to remove the hair with no trace that it had ever been there. [And yes, while the healing brush often works for this on the full-frequency image, experienced retouchers know that no tool is perfect and there are situations in which it gets very confused by the larger context of the image.]

  • Skin Smoothing:

    This will be the longest component discussion we have today, but one which has also been the most popular. To start, please take a minute to go read byRo's classic writeup on frequency separation for use in skin retouching over at RetouchPro. He calls it the "quick de-grunge technique".

    Go read it now and we'll resume when you get back.

    Pretty impressive for how quickly he did that (real world execution can be Natalia Taffarel, Gry Garness, and Christy Schuler. [Oh, and BTW, as of this writing, only one of those three very talented ladies knows what you've already learned - that's how elite your efforts thus far have made you :).].

  • Skin Retouching & Beyond:

    While the above is a brilliant, easy technique, it's actually only just the beginning. What if, instead of simply removing those image frequencies (applying a bandstop), we worked on the "grunge" frequencies with the healing and cloning tools like we talked about doing to remove stray hairs? I won't bore you with detail in this post, suffice that this creates an incredibly believable result without taking as long as conventional methods.

    Even better, this can be used on both layers in order to remove unsightly features (skin folds) by healing or cloning on each of the layers - in the high-frequency you can focus on patching in good texture, while in the low-frequency you're able to focus on getting the overall shape right. [As a bonus, because the low-frequency layer has no detail to it, you don't have to be quite so precise as when working on a single (full-frequency) image].

  • Whatever Else You Come Up With:

    Seriously - the above are just some of the everday (formerly) difficult tasks in retouching which can be streamlined by incorporating an understanding of visual frequencies. But by no means is that list conclusive. As we'll discuss in tomorrow's post, the underlying techniques which we've been covering are limited only by your creative application of them.
Until tomorrow...

P.S. I did promise you some automation, didn't I? We'll get into a heavy discussion tomorrow, but for now here is a set of actions which perform each of the techniques discussed yesterday. Each assumes that you are in the bit depth it identifies itself with, and that you are running it from the topmost layer. If you are in a single layer document, you will get an error message shortly after running it - this is normal and you should just click "Continue". If you will only be using single-layered documents, you can avoid the message by disabling the "Copy Merge" step. These actions will create all needed duplicate layers for you, and you can turn off the instruction dialogs at any time by unchecking them in the actions panel. Finally, while I have had no difficulty with them, I make no warrant that they will work for you, nor do I warrant that it will not mess up your files. Use them at your own risk.


Chris said...

Great write up Sean! Just when I thought I had a question as I was reading some parts, you had an answer. Still looking to better understand bandstop and bandpass filter and how it is incorporated in a work flow such as in retouching. I only use the HPsucks in 8 bit.

Again, very thorough blog.

eos550d said...

After I read the tutorial by byRo I understand now how this frequency separation can leads to.

If I understand correctly and suppose we use byRo's notation that

B +G +D =O

Then to get a sharp image and not too bother about the grunge then we can just minus bit of B from B.

Is my thinking right?

Sean said...

@eos550d - Actually, traditional sharpening would do it a bit differently, taking B + G + D + (scalar) D.

By amplifying D, we enhance the local contrast, making the image aesthetically sharper.

Matt Duke said...

Just found your blog (via Mayhem). Trying to wrap my mind around all the geeky goodness!

So I gave byRo's QDG technique a spin (way cool). Then followed with your suggestion of working touch up the individual layers. Is it my imagination or does clone, patch, etc. work kinda funny when dealing with the base image and the inverted HP layer in linear light blend mode? Seemed like I was getting unexpected results at first - oddly blended details. Switched to cloning at lower opacities which seemed to work better. [I tried intuiting an inverted HP layer in linear light blending mode but it made my brain hurt]

Thanks for posting!!!

Sean said...

@Matt -- Through CS5 (here's hoping CS6 changes it), you can only heal / clone intuitively in the "Normal" blend mode - otherwise it tries to sample from all of them, and you just end up with a big mess. It can be tricky to train your eyes to 'see' what you want to work on, but it can be done and can save you a lot of time.