Frank van Maarseveen schreef: > On Sat, Nov 28, 2009 at 01:41:04PM +0100, Jos De Laender wrote: > >> Dear all, >> >> I'm reviewing - yes I want to understand - the wavelet denoising used in >> dcraw, reworked in ufraw, ported to gimp in some variant. >> All seem to have the same basis going back to dcraw. >> >> Forgive me if it is a stupid question, but is there actually someone >> around that understands in detail the algorithm that is used and/or has >> some reference to it ? >> > > There are no stupid questions, only stupid answers. A couple of remarks > (I'll try not to be stupid -- no guarantees ;-). > Thanks Frank. > There's no way this can be explained thoroughly on a single sheet of paper > or its electronic equivalent -- there is just too much underneath. Well, I'm a bit familiar with signal processing, but at the time I got my engineering degree wavelets were not yet 'mainstream'. So my - leaking away - background is Fourier analysis. I did try and read some wavelet stuff and was desperately trying to match it with the code. Got close, but not to a level I understand fully. What would it be nice that - non trivial algorithms - in open-source software are documented the one or the other way ..
> For > what I know the basis is twofold: pure math and the human visual > perception. The math has probably been translated into the numerical > domain and has been optimized for that. So you can't easily see why it > became what it is by just looking at the code. > Close though ... I'm pretty sure it is an 'a trous algorithm'. Despite the 'hat_transform' name, I think it is a linear interpolation filter, but here's where I start feeling unsure... > The human perception comes into play because the algorithm should > do something which ultimately pleases the eye (or more precise: the > human brain). For example, the implementation contains a sqrt() which > I think is an approximation of a gamma compensation My understanding is that the sqrt is introduced to denoise shadows and highlights +/- equally. The sqrt is for instance not part of the derived gimp plugin. It's not essential. > (which again is > an approximation) done to "linearize" the effect for the eye. So the > algorithm is appplied after converting the samples to the appropriate > domain. See http://www.poynton.com/notes/colour_and_gamma/ for some > more basics. > > >> I understand it is an a trous algorithm with some linear interpolation >> scaling filter, but I don't grasp the latest detail of implementation >> (and sometimes I doubt if the signs are fully correct in the >> hat_transform function). Also the noise thresholding seems black magick >> (I mean the noise array , where is it coming from ?). >> > > I've had a look at the hat transform function: two out of three loops are > there for handling pixels close to the edge of the image. The function > looks ok to me from a logical/programmer point of view. No offence, but how can you say without knowing the underlying algorithm ? > The noise array > is based on math (exponential curve) but has likely been tweaked at > index 0 because of the visual perception. > The gimp plugin uses something along the lines : double THold = 5.0 / (1<<6) * exp (-2.6*sqrt(Level+1))*0.8002/exp(-2.6); It looks to give the same results, but doesn't make anything clearer for me. > This is what I can tell. Please correct me if I'm wrong anywhere here. > > > BTW, my plan is to use this algorithm for luma/chroma denoising in ufraw > (after develop() but before any conversion to 8 bit). The colorspace > conversion has been implemented (it took some time figuring out correct > matrix constants having the right precision). The algorithm itself is not > the challenge (it's there) but everything else is, actually. > Yes, in my hobbyproject I'm also trying and using wavelet denoising on L and a/b of Lab. That was the very first reason to try and understand it. Many thanks for your response. Jos ------------------------------------------------------------------------------ Let Crystal Reports handle the reporting - Free Crystal Reports 2008 30-Day trial. Simplify your report design, integration and deployment - and focus on what you do best, core application coding. Discover what's new with Crystal Reports now. http://p.sf.net/sfu/bobj-july _______________________________________________ ufraw-devel mailing list ufraw-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/ufraw-devel
