This guide is composed of two parts: the first one focuses on the tools, while the second one is about some general tips you may find useful. Since the program I'm using is Photoshop, I'm afraid the first part might not be fully translatable, but hopefully you can still enjoy the second half of the workshop.
BEFORE WE BEGIN, SOME GENERAL ADVICES
01. Don't sharpen your base. You don't know where the icon making process will bring you, and sometimes what looks nicely sharpened in the beginning, might turn out slightly over-sharpened in the end. Make a copy of your base and sharpen that one, so that you can always go back to the original picture if needed.
02. Over-sharpening issues may also be caused by adjustment layers (Vibrancy on high settings, deep Curves, highly contrasted Levels), multiple Soft Light layers and heavy texture use. Be careful with the sharpening when you use such techniques and tools.
03. Monitors have different settings and display edges differently. What you see as a perfectly sharpened icon might look oversharp in one monitor, or blurry in others. Keep that in mind when making your icons and receiving (or giving) criticism on that aspect
SHARPEN (Filter > Sharpen > Sharpen)
It's probably the quickest way to sharpen, but since it's kind of default - that is, you don't control the amount of sharpening applied - the results may vary and most of the times you end up with a pretty over-sharpened outcome.
UNSHARP MASK (Filter > Sharpen > Unsharp Mask)
A little less quick, but you get to control the sharpening and adapt it to the picture you're working with. There are three sliders:
Amount: it controls the intensity of the sharpening; this can also be thought of as how much contrast is added at the edges.
Radius: it affects the width of the edges to be enhanced; a smaller radius enhances smaller-scale details while a bigger radius enhances the contrast between larger areas.
Threshold: it determines how much difference there must be between two adjacent pixels before any change is made. The threshold setting can be used to sharpen more-pronounced edges, while leaving subtler edges untouched.
HIGH PASS (Filter > Other > High Pass...)
As in the Unsharp Mask, you have a Radius slider to control the width of the edges to be enhanced. A small radius affects details, a bigger one affects the contrast. But while in previous cases applying the filter on a copy of the base was just a suggestion, in this case it's a must. This is what you get when you apply the High Pass Filter:
> Radius: 1.0 Radius: 5.0
You see, you can't set a layer like that on Normal. But you can set it to Soft Light to get a pretty subtle result, or to Overlay to get a slightly bolder sharpening. In my experience, this filter preserves the quality better than the Sharpen Filter does, so I suggest you to use it on LQ images.
Radius: 1.0 > Soft Light: Overlay:
PAINT DAUBS (Filter > Artistic > Paint Daubs)
Again, you'd better use this one on a copy. You can use this filter in order to sharpen your picture by setting the Brush Size setting to 1, and the Sharpness setting between 0 and 2 (default Brush Type: Simple). You get a quite crisp outcome, so I suggest you not to use this filter on LQ pictures. Here's an example:
> Sharpness: 1 Sharpness: 2
REDUCE NOISE (Filter > Noise > Reduce Noise)
This tool is available for Photoshop CS2 and newer versions only.
There are four sliders and you can use this filter to sharpen your icons by playing with the Preserve Details and Sharpen Details settings, especially.
By lowering the Preserve Details amount you blur your picture, while by increasing the Sharpen Details amount you sharpen it. Nice combo, isn't it?
This is not a tool you can use to sharpen you base, but it's a useful option available to control the sharpening once you applied one of the previous filters. You can go to Edit > Fade Sharpen (or Fade Unsharp Mask, or Fade High Pass, or Fade Paint Daubs, depending on which you used) and soften the effect of the filter.
LOWER THE OPACITY/FILL OF THE SHARPENED LAYER INSTEAD OF USING THE FADE OPTION
I know, I say the Fade option was useful. And it is, but once you faded the effect of the filter, you can't go back. My suggestion is, play with the Opacity/Fill of the layer instead. You can lower the opacity/fill to soften the sharpening AND you can raise it again later on, should you want to. As I said, you don't know where the icon making process will bring you.
PLAY WITH THE BLENDING MODE OF THE SHARPENED LAYER
You don't have to set the sharpened layer to Normal. If you often duplicate your base and set it to Soft Light/Screen, you can also sharpen one of the duplicated layers. The sharpening is subtler or bolder depending on how many of those layers you sharpen and on their opacity.
USE LAYER MASKS
You don't have to sharpen all the canvas. You can always mask those parts that are already sharp, or grainy, or irrelevant. You can use the sharpening to draw the attention to the focal point of the icon, for example. Be careful though: over-sharpened eyes combined with blurry skin don't look good.
COMBINING BLUR AND SHARPEN
You can do that in many ways. For example, you can duplicate your base a couple of times and apply a Blur Filter (like Gaussian Blur or Surface Blur) on the first copy and a Sharpen Filter on the second one. Then all you have to do is play with the opacity - or Blending Mode - of both till you're happy with the result. Or you can blur some areas of the sharpened layer with the Blur tool. It depends on the effect you're aiming for and on the cap you're working with.
WHEN ALL ELSE FAILS (written by mm3butterfly)
So you used every sharpening technique you could think of, but still aren't satisfied with the sharpening balance of your icon. The general feel of the icon is nice, but there are specific areas you want softer. It's time for the brute-force approach.
Here's my disgustingly vibrant icon, where I used lots of Levels and Vibrance. I like the coloring, her facial features and the right side of the icon are looking good, but I'm having trouble with the hair. It's sharp, and I want it softer.
Clone stamp your image (Ctrl+Shift+Alt+E), zoom in to around 300-500%, get your Smudge Tool and set it to 10-30%, sized 3px, 5% hardness. Then carefully smudge the oversharpening away, following the pixel line.
Do you notice any difference or am I just obsessive?
You've got to be careful with this technique, since it can easily and quickly deform your subject. It's also a good trick to use on pixelly areas, left from using the Vibrancy tool, rough skin, or the infamous pixel blocks.
YOU CAN SHARPEN WHENEVER YOU WANT
There's no rule about it. You can sharpen at the very beginning, in the middle of the process, or when you're done with everything else. However, if you choose the first option, remember to check the sharpening when you're done, because you may need to lower/increase the opacity of the sharpened layer.
Credits & Useful Links
01. Sharpening 101 and Unsharp Masking @wiki for some technical details about the Unsharp Mask;
02. raiindust for the Paint Daubs trick.
03. this comment by anivad for the Reduce Noise trick.
04. Sharpening in Photoshop, a huge and comprehensive guide on sharpening.
- share your own sharpening technique, tips &tricks;
- ask for feedback on the sharpening of your icons (post a link to your graphics journal OR a link to one of your icon entries OR up to five icons)
- praise icons made by others (in which sharpening is particularly amazing).