Alan Horkan wrote:
> I was trying to find out more about the new "Dog" plugin but the Plugin
> Registry doesn't seem to be working.
> Anyone know what the problem is or if it is just a problem on my end?
> (I have since learned that the dog plugin is an Edge Detection plugin, and
> DOG stands for Difference of Guassians)

I don't know about the registry, but at least I can tell you about the
DoG plugin, since I wrote it.  It does edge detection using the so-called
"Difference of Gaussians" algorithm, which works by performing two
different Gaussian blurs on the image, with a different blurring radius
for each, and subtracting them to yield the result.  This algorithm
is very widely used in artificial vision (maybe in biological vision
as well!), and is pretty fast because there are very efficient methods
for doing Gaussian blurs.

The two main parameters are "inner radius" and "outer radius", which are
simply the blurring radii for the two Gaussian blurs.  It is probably
easiest to choose values using the preview, but it may help to know that
increasing the smaller radius tends to give thicker-appearing edges, and
decreasing the larger radius tends to increase the "threshold" for
recognizing something as an edge.  In most cases you will get nicer
results if the inner radius is smaller than the outer radius, but
nothing prevents you from reversing them, and in situations where you
have a light figure on the dark background, reversing them may actually
improve the result.

The "Normalize" checkbox, if checked causes the brightness range in the
result to be stretched to the widest possible.  Note that in the preview,
only the part of the image that is shown is taken into account, so with
"Normalize" checked the preview is not completely accurate.  (It is
accurate except in terms of global contrast.)

The "Invert" checkbox inverts the result, so that you see black edges
on a white background, giving something that looks more like a drawing.

Incidentally, in 2.2-pre2 this plug-in is broken because some changes
in the GIMP core made some of the things it did no longer work 
(bug #158895) -- it crashes immediately if you try to run it.  
Hopefully this will be fixed by the next release.

  -- Bill

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