Edit: Sharpening images using a tool called Unsharp Mask has been around for a while now and is intended to sharpen edges – as is the process detailed below. The method detailed here is therefore largely redundant but you may find uses for it.
This is a quick note of how to apply image sharpening to the “edges” of an image. One reason for creating this post is so that I have somewhere to find the steps involved when I need them. The steps described are for GIMP. It is likely that you can follow a similar process in other image editing software. It might already be part of how sharpening is done in other application.
Sharpening is usually one of the last things you do to an image before saving it.
With the image loaded into GIMP
- Create a duplicate layer.
- Apply sharpness to the duplicate layer. Feel free to apply more sharpness that you normally would.
- Add a layer Mask to the duplicate but select “Grayscale copy of layer” as you do it.
- Right click on the layer mask and select “Show Layer Mask”
- Go to “Filters” > Edge-Detect > Edge > OK (I haven’t seen any benefit from making any selections from the Edge dialogue)
- While viewing the layer mask go to Colors > Curves and pull “darks” down and “lights” up. i.e. increase the contrast. Part of the idea here is to make areas where you don’t need to apply sharpening black. If you do apply sharpening to some of these areas they can get “pixelly”.
- If there are areas you don’t want or need to apply sharpening you can paint those areas black on the layer.
- Go to Filters > Blur > Gaussian Blur and apply a small amount of blurring to spread the area that the sharpening will be applied over.
- Return to the image i.e. no longer show the layer mask. Right click on the layer mask and un-check “Show Layer Mask”.
- Adjust the opacity of the duplicate if required to give you the image you want.
- Right click on the duplicate and Merge down to a single image.
- Save the file. Go to File > Overwrite…
When I have more time I might add some screenshots of this process.