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Selective Sharpening in GIMP - Michael Bates shows us how to sharpen up our images in Gimp.
It can be hard to ensure the same level of detail across the entirety of an image at times depending on the circumstances under which the photograph was taken. Here the lower section of the photograph goes out of focus, and unfortunately this includes part of the main feature which is the leaf. We could just leave this photograph as it is; however we are able to return some sharpness to this area using software like GIMP in a few easy steps.
Step 1. Open your image and duplicate the background layer by right-clicking it and selecting Duplicate. Then, double-click the new layer's name so that you can change the name to avoid any confusion and let you know this is the sharpened version of the image.
Step 2. Now we are going to select the area we want to sharpen using the Freehand Select Tool (the Lasso, press ‘F' in GIMP to select it). Now draw around the area that you want to sharpen with the left mouse button held down and release it once you have drawn a complete loop. If you include an area you do not want, draw a second loop around this whilst holding down the Control key to remove it from your existing selection, you can also add areas to your selection by holding the Shift key and drawing around a section of the image.
Step 3. If your selection is part of a larger object rather than an item in its own right you will have to take a moment here to ensure the sharpened areas will blend into the image a bit better. The best way to do this is to open Select, Feather and add a few pixels of feathering to your selection, basically meaning that instead of a hard edge your selection now comes in gradually along all its edges.
Step 4. The first thing we want to do before we sharpen the image is to enhance some of the detail so this area looks better when it is sharpened. For this it 's best to open Filters, Enhance, Unsharp Mask. This tool creates the illusion that your image has more detail by finding areas of contrast and increasing it locally by making the edge of a dark area even darker and the light area next to it have an almost white edge. The Radius setting controls the size of the white and black areas created either side of a contrasting edge, the Amount changes how drastic the lightening and darkening effect is and the Threshold determines what severity the contrast must be before it is altered by the tool. Here I'm trying to recover any detail I can so I've set the threshold to 0 and the amount to 0.60 which is reasonably high (even though the slider goes much further). The settings you use will depend on the surface you are working with, the resolution of the image and how detail of the area you are trying to match; you don't want the area you sharpened looking more detailed than the original image. If you apply this tool and don't like the effect press Control and Z to undo it then Control, Shift and F to reshow the dialogue box of the last filter you used.
Step 5. Keeping the selection you created active open Filters, Enhance, Sharpen to add some traditional sharpening without increasing the contrast of the image. Remember not to overdo it otherwise your image won't look believable.
Step 6. Press Control, Shift and A to deselect the area you sharpened and have a look along the edge that went through the image to look for areas that give away your edit. Here I found an area that shows the transition between the original image and my sharpened region quite clearly, so I'm going to have to fix this next.
Step 7. Right-click the Sharpened layer and select Add Layer Mask, leave the window that opens at the default settings and press Add.
Step 8. Select the Paintbrush tool (P in GIMP) and change the settings so they look similar to mine; the only one you MUST have the same is the brush tip, which I have set to a soft-edged brush so as to avoid creating a crisp edge anywhere in my edit. Then, with black as your foreground paint colour, paint over the problem areas in the Layer Mask; this will make these areas transparent, allowing the original image to show through from underneath the sharpened layer. Once you have sorted this you will be finished, with a much improved image as your reward.