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Redux sharpening allows you to selectively sharpen the areas of your image where it is most needed, without introducing too much noise.

1. Duplicate the image

In GIMP:

  Ctrl -D  

2. Using the duplicate - Decompose the image to HSV

 In GIMP:

  Right Click -> Colours -> Components -> Decompose, 

  Choose HSV in the drop down list and make sure Decompose to Layers is selected  

3. Copy the "Value" layer of the HSV and paste it into the original image as a new layer.

 In GIMP:

  Select the "Value" Layer, 

  Hit Ctrl-A (select all), Ctrl-C (copy) in the Value layer. 

  Select the original image, hit Ctrl-V (paste) 

  Right click on the floating layer and choose "New Layer"  

  You can close the HSV image now if you wish. 

4. Set the pasted layer mode to Value.

 In GIMP:

  Select the Layer and choose value from the drop down "Modes" list  

5. Add a Layer Mask (White, full opacity) to your Value layer

 In GIMP:

  Right click the layer, choose "Add Layer Mask"

  Select White (full opacity) when asked 

6. Go back to your duplicate image (from step 1). Now run Edge detect on the image, choose Sobel and an amount of about 6.

 In GIMP:

  Right click -> Filters -> Edge - Detect -> Edge..., 

  Choose Sobel as the Algorithm and 6.0 for the amount  

7. Desaturate your Edge image.

 In GIMP:

  Right click -> Colours -> Desaturate 

8. Darken the desaturated Edge image, by sliding the midtones slider in the "Levels" dialogue to the right. What you are aiming for is a nice clear, sharp, white outline of your image on a pure black background.

 In GIMP:

  Right click -> Colours -> Levels  

  Move the middle triangle below the histogram to the right 

9. Optionally paint out any bits you don't want to sharpen.

10. Apply a Gaussian Blur to the image, with a radius of between 3 and 7. The larger the image the larger the value you will need. This step helps to get rid of any small areas of white you don't want.

 In GIMP:

  Right click -> Filters -> Blur -> Gaussian Blur  

11. Optionally open the levels dialogue again and darken it as in Step 8, to get a nice clear, sharp outline.

12. Make sure the layer mask is selected in the original image. Copy your Edge layer from the duplicate and paste it into the original image, into the layer mask of the Value layer.

 In GIMP:

  Copy the Edge layer by clicking on it and hitting Ctrl-A, Ctrl-C. 

  Click on the Layer Mask of the original in the Layers dialogue to select it, 

  Paste the Edge layer into it with Ctrl-V,

  Click the anchor button to anchor the floating layer into the mask  

13. Select the image part of the Value layer, ie not the Mask, and apply Unsharp Mask to taste (start out with a radius of 1 and an amount of 2)

 In GIMP:

  Click on the image (ie not the mask) of your Value layer in the Layers dialogue. 

  Right Click -> Filters -> Enhance -> Unsharp mask