Contrary to the other blur plug-ins, the Selective Gaussian Blur plug-in
doesn't act on all pixels: blur is applied only if the difference
between its value and the value of the surrounding pixels is less than a
defined Delta value. So, contrasts are preserved because difference is
high on contrast limits. It is used to blur a background so that the
foreground subject will stand out better. This add a sense of depth to
the image with only a single operation.
