Looking for tips (& problems) with my new version of PaintShop Pro, I came across a forum post on how to use something called Frequency Separation -- it's something that has gotten a bit of attention the last couple/few years with PhotoShop & The GIMP, and I wanted to take a moment to briefly explain what it is & what it does...
When you open a photo or image to work on it you might start with some sort of automated optimization tool, or you might prefer to do things yourself manually, starting to work on the Levels, stretching the image color data so it goes from black to white while making sure the colors are balanced so that your image doesn't have a color tint. If there's something you don't want in the image you might use something like Inpaint, &/or if it's a scanned photo or negative you might use the dust & scratch filters & so on. Where Frequency Separation often comes into play is when you want to re-touch something with a graduated color that has some texture to it.
Most people, if they've done some image editing, are familiar with the Clone [sometimes called Stamp] tool -- it takes image data from point A & lets you use it to paint point B. At its best the Clone Tool lets you cover one solid colored area with another, solid colored part of the image -- at its worst the results don't match &/or blend in because there's nothing else in your image that really looks like the area you're trying to fill or cover. That's when/where you might use the Healing Brush or Tool -- basically a smarter clone tool, it looks at the area where you want to paint & adjusts the colors it lays down accordingly. Frequency Separation allows you to take things a step further, because like using a Blur or Noise Removal tool, the Healing brush itself can muddy things & lose detail, e.g. turning human skin into the hard painted surface of a manikin. Frequency Separation allows you to work on the color without harming the texture detail, or lets you work on just the texture leaving the colors alone.
Skipping any technical details, what Frequency Separation does is give you 2 or more layers of your image, with one layer an exact duplicate of the original, & with the layer at the other extreme looking like a flat gray colored copy with very little detail. Then the bottom layer, the one that looks like the original, is blurred. Using the Healing Brush/Tool, or any other tools like Dodge/Burn etc., on the blurred layer lets you work with color without harming texture detail -- doing the same on the gray layer lets you change or blend textures without changing the overall colors. Where you'll see it most often used [& written about] is with faces, particularly glamor shots or portraits, where it can get rid of blemishes &/or wrinkles & such. It may not always replace painting, but like most things in an image editing app, it can make it both faster & easier while requiring sometimes much less artistic ability.
Semi pros & pros use this sort of retouching because there is no such thing as a perfect human face or body, unlike what you see [& expect to see] on magazine covers. Regular folks might use it to make what the lens saw look more like what your subject sees in the mirror -- many, many people look in the mirror and see a little bit of what they wish to see, & may dislike having their picture taken because of that, because looking at your picture threatens that perception, that self-image just a bit. Used subtly this sort of re-touching can really please the person you took a picture of -- but too much, too heavy of a touch & they'll be insulted that you thought their picture needed so much work -- be warned. And guys, Never let someone close to you know you re-touched their picture in the first place.
Of the 3 apps that I've used Frequency Separation with I like The GIMP the best. There are a few scripts or plug-ins available, but I like these two:
http://registry.gimp.org/node/26975 -- http://registry.gimp.org/node/13439
While the earliest references I've found for Frequency Separation were for The Gimp, Google & most of the buzz is about Frequency Separation in PhotoShop -- here's just one quick tutorial that walks you through the steps to DIY, though there's also a P/Shop action you can download from the same site. http://omahaimageproductions.com/blog/2012/02/skin-retouching-technique-frequency-separation/
And here's the post in the Corel forum showing how to do Frequency Separation in PaintShop Pro: http://forum.corel.com/EN/viewtopic.php?f=56&t=48518 It works fine, but if you want to use a Healing Brush there isn't one in the sense of The GIMP or P/Shop -- rather than drag/paint you place the cursor, click once, place the cursor, click once & so on.
That all said, Google. The hard part is getting the needed separate layers in the 1st place -- one you've got them you can manipulate the individual layers however you like. You might use a type of blur, with or without a selection, or go the other way and apply sharpening. You can match the gray layer's color & use the air brush or paint brush to eliminate something entirely -- it's all up to you.