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Grokking The Gimp
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Subsections

     
6.4 Sharpening

Due to a whole range of issues, the acquisition and digitization of an image can often produce a slightly blurred or out-of-focus result. Furthermore, the ensuing processing to improve tonal range and contrast can also deteriorate the crispness of the image. To remedy this, it is desirable to sharpen things up. Sharpening is usually applied at the end of the processing sequence after all other modifications have been made.

In the GIMP, there are two tools for sharpening an image; they are both located in the menu Image:Filters/Enhance. These tools are called Sharpen and Unsharp Mask. Although the underlying principles of the two are the same, I prefer Unsharp Mask because it has several parameters that have intuitive meanings and that provide more control over the sharpening process. The remainder of this section describes in detail how to use Unsharp Mask.

          
6.4.1 The Unsharp Mask Concept

Before knowing the sharp, little grasshopper, you must first become one with the unsharp. Wheeew...that sounds like a bad episode of Kung Fu. I'm expecting David Carradine to peak out from behind my monitor any moment now! The truth is, though, that Unsharp Mask, as arcane and counter-intuitive as its name may sound, is an excellent tool for sharpening. The principle of the Unsharp Mask and the ins and outs of this special filter are covered in this section.

Figure  6.32

  
Figure 6.32: Understanding the Unsharp Mask
Figure 6.32

illustrates the principle of the Unsharp Mask. The upper black line graphed in the top part of the figure represents pixel values as a function of pixel index. For example, this line could represent the color values along the row of an image. The graph shows a transition from a lower to a higher pixel value at the middle of the graph. Visually, if you were looking at this row of pixels in an image, it would appear as an edge between regions of constant value.

The red line in the upper graph of Figure  6.32 illustrates how the pixel values change if the row of pixels is blurred. The resulting red curve is a smoothed version of the original. If you viewed the row of pixels corresponding to this smooth curve, it would appear much less sharp than the pixels for the black curve. So, in essence, the red curve is an unsharp version of the black. Now, subtract the unsharp version from the original and the result is the lower black line in the top graph of Figure  6.32. You could say that this result is just the original curve with its unsharp component masked out.

This is the interpretation of what the unsharp mask does, but how does this sharpen the image? The answer is given by the lower graph of Figure  6.32, which shows the original black line representing the row of pixel values. The graph also shows a red curve, which is the original curve and the associated unsharp masked version added to it. As you can see, just before the edge, there is now a dip in pixel value at the low side of the transition and a peak on the high side. Thus, the result is that the edge has been made sharper.

Grokking The Gimp
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