GIMP digital-photo-noise-reduction questions

[Jamon Camisso]

Walter Dnes wrote:
>   I've gotten myself a new Pentax K-X camera, and I'm trying out various
> settings.  On a camera forum, people have suggested that a program like
> GIMP or imagemagick can do a better job of noise reduction than the
> built-in de-noiser firmware.  I've turned off the de-noiser, and taken
> sample shots at all ISO settings, which I've uploaded JPEGS of to the
> "initial" folder at...
>> http://www.mediafire.com/?sharekey=ebf53a21c2247895d0d290dca69ceb5cf761ff1a36003591ef687f17504eae2e
> 
>   There are a few noise-removal options I'm aware of...
>   - a basic de-noiser setting in the UFRAW plugin
>   - GMIC plugin
>   - maybe others
> 
>   And because the noise-removal softens the picture, I need to use
> 
>   - filters => enhance => sharpen (or unsharp mask)
> 
>   They all seem to take some numeric parameters, which I don't
> understand.  Can someone point me to tutorials for these or other
> noise-removal options?  Under linux, of course <G>.
> 

The UFRaw denoise is a pared down version of gimp's wavelet denoise 
plugin I think. I tend to denoise with wavelet denoise in the gimp, 
though it's rather complicated and every image requires different 
settings. There's also GREYCstoration, another gimp plugin that is well 
regarded.

The image you uploaded looks good at iso 1600  (haven't checked beyond 
that) and I think that the K-X is known for it's excellent noise 
characteristics throughout it's iso range. Once you hit iso 800/1600 or 
beyond, on any camera, there will be noise that you can reduce using 
those plugins.

My workflow usually works like this:
1) shoot everything raw
2) open in ufraw, adjust everything possible there (apart from noise). 
check the auto exposure adjustments, auto saturation etc. just to get a 
feel for the image, then tweak or reset things accordingly.
3) export to the gimp, whereupon I check the auto levels again to see 
what the gimp thinks. reset or adjust accordingly.
4) do some healing to get rid of dust, dead pixels etc., resize, correct 
for distortions, rotate and whatever else.
5a) denoise with wavelet denoise if applicable.
5b) unsharp mask (it should always be the last step). Anywhere from 
25/0.25 to 4/0.40 on my K20D works well - try using 3/0.30 as a baseline 
and see what you think on your K-X (usm is sensor and resolution 
dependent, e.g. more megapixels = more usm).

Keep in mind thoughh that noise can be a great part of the image and can 
be used creatively - I've tried to do that in these 2 photos:
http://www.flickr.com/photos/jamonation/3184501781/
http://www.flickr.com/photos/jamonation/4110205713/

My strategy with my K20D is to not worry about it, or use a tripod. 
Also, noise is much less apparent in a print than on a screen when pixel 
peeping ;)

My $0.02, hope it helps. http://www.pentaxforums.com/forums/ will be a 
great spot to frequent as well, plenty of Linux folk around there too.

Jamon
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