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
![Smile :)](./images/smilies/icon_smile.gif)
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.
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I have developed a more robust way of doing the above, which I am investigating as time permits.