[magick-users] Re: Curves in ImageMagick

[Mikolaj Machowski]

Anthony Thyssen scripsit:
> Mikolaj Machowski on  wrote...
>| Hello,
>| 
>| How to simulate Curves tool from GUI programs?
>| 
>| I suppose there is some smart meddling with fx option but I would be
>| grateful for any hints.
>| 
> Yes and with the help of an external script (which uses gnuplot
> underneath),
>     http://www.imagemagick.org/Usage/scripts/im_fx_curves

Thanks but something doesn't work. I tried to use it as explained to get
line going through 10,0 and 90,100 points but in with -d option I get
line going through 0,50 and 50,100 with output: 1,000*u+0,000

>| Specifically I am interested in effects like described here:
>| 
>| http://www.outbackphoto.com/workflow/wf_81/essay.html
>| 
>
> That is not a 'curve' but a linear change.  Though not a typical one as
> you are doing a contrast change rather than a color limit change.
>
> However given the two points the curves technique will still give you
> the linear equation for the adjustment.
>
> At this time there is no IM contrast option with specific arguments to
> control the contrast adjustment.  There are some automatic ones simular
> to normalize, but no non-normalizing linear contrasting functions.
>
> The closest would be a curved contrast adjustment known as
> -sigmodial-contrast.

Somewhere is typo: in my docs distributed with IM 6.3.0-3 in docs is
option called "sigmoidal-contrast". Also this whole trick is to *avoid*
contrast change. As explained in tutorial operating on HSB changes
contrast, but in L*a*b* it doesn't happen.

m.