Today I'm going to teach you how to go from this cap (by ohlookcaps!) to this! Note that the end result of this icon is quite saturated, but that's not necessarily the end result of this process.
Program: GIMP, shown on a mac but it should work absolutely the same way on a PC.
Translatable: Yes, though I'm not sure why you'd want to.
"High pass" is a filter in photoshop that can be used for sharpening with more precision than the standard sharpening filters that come with the program. If you'd like to see how it works, check out this tutorial, which includes a gif before-and-after comparison.
Unfortunately, GIMP does not come with this feature. There are plugins to imitate it, but in general my poor laptop can't take anymore unnecessary downloads, so I learned to mimic it with layers instead. Let's move to the pretty pretty pictures, shall we?
Open your image in GIMP and crop it. This is not the most inspired crop in the world, so you're lucky it's not for a cropping tutorial. Anyhow, crop, but don't resize yet! This method can be harsh on your image so you want to have the extra buffer that resizing gives.
Once you've got your crop how you want it, duplicate the layer twice. Turn the visibility on the very bottom layer off by clicking the eye next to the layer. Now go back and select the top layer.
Now, while you have that layer selected, go to Filters -> Gaussian Blur. That will bring up the dialog you see above. The only setting we're playing with today is the box with the numbers in it. Now, the higher your number the less intense the sharpening effect of the "high pass" will be. Kind of backwards, but I do not question the computer gods. As this image is already fairly sharp, I set it on 100 in both boxes; with images that are less well-defined, I might set it on 50 or even 25.
Once you have your image blurred, set that layer on "grain extract." It will make this neat gray effect you see here:
Congrats! You have now done a fake high pass. Now what?
Well, in order for this to do any good we need to change the blending mode to 'overlay' or 'soft light,' but if we do that right now we'll no longer have the high pass effect. In order to keep it, we're going to right-click the layer and select 'merge visible.' The resulting layer will be on 'normal' blend mode; change it to either overlay or soft light, then make the bottom layer visible again.
Tada! Your new image is sharp, crisp, and lovely. You can, of course, stop here, resize, and go about your coloring business; however, I still wanted to play a bit so I'll take you through the rest of what I did. If you're done, skip to the last step for some notes on resizing.
I went to Layer --> Create a new layer from visible. This creates a flat layer to color on without getting rid of the original, in case you mess up or want to play with opacity later. After that, I went to Color --> Hue/Saturation and cranked up the saturation a bit. That looked nice, but still not quite what I wanted.
I then duplicated the layer twice and set both to multiply, and then duplicated it again and set the top one to screen. This is a good way of brightening your image without losing color in general, as the multiply hyper-saturates your color and the screen washes it out again; the end result is a more vivid version of your current colors. I wish I'd taken a cap of this, but alas it is late and I am stupid.
I went to layer --> create new layer from visible again and did another slight increase in the saturation. Then it was time to resize!
Tada! Icon size. Once you resize, you may notice that your image looks sharper than you'd like. If this happens, create a new layer from visible again and do a Guassian blur set at 0.5. And then you're done. :)
These are not finished icons, please don't use them
I hope this is useful to someone! If anything's unclear, feel free to ask, and I'd love to see your results. ♥ Banah