On Thu, 19 Jun 2003 16:46:13 -0700 (PDT)
Joel Eduardo Rodriguez Ramirez <[EMAIL PROTECTED]> wrote:

> Actually I would like to colaborate with a new filter also
> (as Bowie), but mine idea is in the direction of:
> ``Inverse Image Filtering with Conjugate Gradient''
> http://people.cornell.edu/pages/zz25/imgcg/

As far as I know nobody tried to implement conjugate gradient
filtering as a GIMP plug-in.

One of the main reasons I think is that it is not easy to give an efficient
implementation. The running time is quadratic in the number of pixels in the
image which means that it is too slow to use it on the normal sized
images. I would expect that any realistic implementation should use
the algorithm on small parts of the image and then somehow combine
the results. So, this is not a trivial plug-in to write.

Also you should not over-estimate what techniques like conjugate
gradient filtering can do. The examples that most authors give
are highly artificial. In the page that you referred to the
convolution is known exactly and there was no noise in the
blurred image. In real life this never happens.

First of all you hardly ever know the exact details of the
blurring convolution. Most deconvolution algorithms give
very disappointing results unless you have a very good approximation
of the blurring convolution.

Second, almost all images contain noise and this has disastrous effect
on the deconvolution. Most deconvolution algorithms tend to amplify
the noise to an extreme degree. For example, when using the unmodified
inverse deconvolution the end result is normally completely dominated
by noise. Although conjugate gradient filtering is not so extremely
sensitive to noise, like all deconvolution techniques its results
will deteriorate rapidly even when there are only very small errors
in the input. 

A few years ago, when I started with my own deconvolution plug-in
I examined several existing algorithms. My own conclusion was that
this is a difficult subject. Most of the best algorithms are
simply too slow for practical applications. I finally selected
FIR Wiener filtering as a practical compromise. The running
time is linear in the number of pixels in the image, and
in virtually all cases its results are much better than those
of similar plug-ins like unsharp mask or sharpen.
If you are interested you can find it at http://refocus.sourceforge.net.


Ernst Lippe
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