Page 2 of 2

Re: Improving sigmoidal image resampling (enlargement)

Posted: 2014-06-29T12:17:23-07:00
by NicolasRobidoux
Another result (EWA Lanczos through gamma 2.5) here: http://www.dpreview.com/forums/post/53939045

Re: Improving sigmoidal image resampling (enlargement)

Posted: 2014-06-30T07:28:23-07:00
by NicolasRobidoux
I'm starting to think that the best way to improve sigmoidization is to replace by a blend of an enlargement through a gamma space and an enlargement through linear light.
In other words, the best way to improve sigmoidization >>for natural images<< is to use something else.

Re: Improving sigmoidal image resampling (enlargement)

Posted: 2014-07-15T00:36:24-07:00
by NicolasRobidoux
Preliminary findings
I am pretty sure that sigmoidization is dead as a general purpose resampling method.
The simple version of what works better is to use mapping through a gamma space (with a matching pair of -gamma commands, like

Code: Select all

-gamma 3 -distort Resize 800% -gamma .33333333333333
) to reduce the most offensive of the halos, namely the dark ones, varying gamma until the light and dark halos are equally offensive :)
Converting whatever into linear light before passing things through "gamma resizing" is almost certainly a very good idea. For example

Code: Select all

-set colorspace sRGB -colorspace RGB -gamma 3 -distort Resize 800% -gamma .33333333333333 -colorspace sRGB
if both your input image and your output image are sRGB.
(So far, I have tested with XYZ instead of RGB using a different software program.)
Generally, gamma 1 leads to offensive dark halos with filters that have negative lobes. On the other hand, large gammas lead to exaggerated light halos and, possibly, to color artifacts, esp. if a non-HDRI version of ImageMagick is used.
-----
I have developed a more robust way of doing the above, which I am investigating as time permits.

Re: Improving sigmoidal image resampling (enlargement)

Posted: 2014-07-15T11:25:55-07:00
by NicolasRobidoux

Re: Improving sigmoidal image resampling (enlargement)

Posted: 2014-07-16T05:07:14-07:00
by NicolasRobidoux