Re: [Lcms-user] How to create a gray profile with L-star grey response?

[Elle S. <ell...@ni...>]

On 05/24/2016 06:54 AM, Marco Freudenberger wrote:
> Hi!
>
> I'm trying to create a gray profile that has an L* (L-star) response
> curve, but I'm a little confused about the "parametric tone curve" syntax.
>
> Is there an example how to create the correct tone curve? I guess I need
> the inverted version of L* companding, right? Would that be a 4
> parametric curve, with:
>
> y(Gamma) = 3.0
>
> a=1.0/1.16
>
> b=0.16/1.16
>
> c=100.0/903.3
>
> d=0.008856*903.3/100.0
>
> So in code something like that:
>
> cmsFloat64Number lStarParams[5] = { 3.0, 1.0/1.16, 0.16/1.16,
> 100.0/903.3, 0.008856*903.3/100.0 };
>

FWIW (I'm no expert!), I use these parameters to make the Lab parametric 
curve:

cmsFloat64Number labl_parameters[5] =
{ 3.0, 1.0/1.16, 0.16/1.16, 2700.0/24389.0, 0.08000 };

The parameters are from these two pages:

http://brucelindbloom.com/index.html?Eqn_RGB_to_XYZ.html
http://brucelindbloom.com/index.html?LContinuity.html

Where does the equation for your last parameter come from? I've always 
used Lindbloom's values and he gives 0.08.

Best,
Elle
-- 
http://ninedegreesbelow.com
Color management and free/libre photography