Apart from the "and a sigmoidal (s) shape between perfectly straight and infinitely steep (essentially a step function)" (my English is not good enough to understand your meaning, so I can't say if this is correct or not), yes, this is correct. In the Gimp curves filter, the sigmoidal curve could be inflected as a printed "S", in which case the results are accentuated (by increasing the dynamic range in the middle tones and losing details in the extremes), or like a rounded "Z" and this results subjectively in a loss of contrast but should improve the details in the extremes.
2012/12/20 JohnPW <[email protected]> > I'm glad this is stimulating discussion. This is the kind of thing I enjoy > reading here (especially when somebody can explain things they know.) > > Anyway, I'd like to figure out some things about sigmoidal contrast. Is a > "sigmoidal contrast" simply an approach to setting the contrast curve we > all know from photography and digital imaging? In other words, is it a > subset of possible contrast curves that could be uses which follow a > sigmoidal shape as described in the formula? > From what I can understand, this means it is a typical contrast curve but: > • with the endpoints locked down at blackest black and and whitest white > • with a single inflection point somewhere in the middle > • and a sigmoidal (s) shape between perfectly straight and infinitely > steep (essentially a step function) > • presumably this function always has a positive slope and the 's' shape > is never inverted into a backward 's' shape. > Are my assumptions correct? > > > > On Wednesday, December 19, 2012 5:09:02 PM UTC-6, Bruno Postle wrote: >> >> On Wed 19-Dec-2012 at 12:13 +0100, Frederic Da Vitoria wrote: >> > >> > But now that I think of it more closely, I understand that >> > JohnPW's question is still unanswered and that my answers >> > completely missed the point. I expect the faux-bracketing to keep >> > the lightest parts of the darkest exposure and the darkest parts >> > of the lightest exposure. >> >> I'm not entirely sure what you are trying to do, but Hugin will >> extract 'bracketed' exposures from any photo. By default it uses a >> 'sigmoidal' camera response curve to map the data to linear space, >> multiply and then map back again - This curve will be quite accurate >> if you have calibrated your camera photometric parameters. >> >> All you have to do is load your photo and increment Eev for the >> input or output. >> >> You can then enfuse these brackets, but this process only really >> makes sense if you start with 16bit per channel data created from >> RAW. >> >> -- >> Bruno >> > -- > You received this message because you are subscribed to the Google Groups > "Hugin and other free panoramic software" group. > A list of frequently asked questions is available at: > http://wiki.panotools.org/Hugin_FAQ > To post to this group, send email to [email protected] > To unsubscribe from this group, send email to > [email protected] > For more options, visit this group at > http://groups.google.com/group/hugin-ptx > -- Frederic Da Vitoria (davitof) Membre de l'April - « promouvoir et défendre le logiciel libre » - http://www.april.org -- You received this message because you are subscribed to the Google Groups "Hugin and other free panoramic software" group. A list of frequently asked questions is available at: http://wiki.panotools.org/Hugin_FAQ To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/hugin-ptx
