Highlights and Shadows...

[dognose]

I'm trying to reproduce this effect from picasa.

As demo'd in youtube.
http://www.youtube.com/watch?v=beejFNmc_aI#t=1m45s

The highlights increases the brighter colors
the shadows makes darker colors darker..

it seems to do it somewhat evenly.. or should I say proprotionally.

[fmw42]

just use -sigmoidal-contrast Ax50% to equally adjust the darks darker and the brights brighter or +sigmiodal-contrast Ax50%to go the other way. where A is your adjustment amount.

If you want to adjust them separately, use +-sigmoidal-contrast Ax0% and +-sigmoidal-contrast Ax100%

You can see this interactively in the non-linear filter in the tone category (as brightness and contrast sliders) at jqmagick.imagemagick.org/V13

You can see examples of this in my web page for my bash script, sigmoidal, at the link below.

You can also do something with +-levels in three regions as in my script, balance. You can see that interactively in the linear filter in the tone category (as midtones, hightlights and shadows sliders) on http://jqmagick.imagemagick.org/V13

[dognose]

Oh, thank you, that's perfect.

I'm finally understanding that command!

It seems that everyone has different namings of these actions.

[fmw42]

Yes that is true. I also have a script called dualtonemap that does something similar, though a slightly different approach than the two others I mentioned. I am sure there are many variations to this kind of operation and each one may use a different approach be it linear or non-linear. Photoshop has another variation.