Hi ! Thanks for your response,
looks like I'm on the right path, your solution looks pretty similar - and BTW, I take your comment that you are no expert as an understatement, I've visited your website a couple of times and think it's a great reference and shows a lot of knowledge and experience :-) Regarding my last parameter value: I indirectly took it off Bruce Lindbloom's site as well :-) Unfortunately, I missed the page you linked with the "Inverse Companding" somehow, I started at the "(forward) Companding" page (http://brucelindbloom.com/index.html?Eqn_RGB_to_XYZ.html) and did the math to revert it. For the last parameter I started with: e(epsilon) = 0.008856, K(kappa) = 903.3 so I'm calculating: e' = e * K / 100.0 (as e <= e). It's very close to 0.08 anyways :-) However, I used the numbers that Bruce lists as "Actual CIE Standard". Thanks to your second link (and I've seen that page before, but didn't remember it when I needed it ...) I realized that the values given as "Intent of the CIE Standard" as you are using them are probably the better values... and with e = 216/23489 and K = 23489/27, e' turns out to be 216/2700 = 2/25 = exactly 0.08... Thanks and best regards, Marco PS - to the group: I'm still wondering what would happen if I'd simply set the TransferFunction to NULL... -----Original Message----- From: Elle Stone [mailto:ellest...@ninedegreesbelow.com] Sent: Dienstag, 24. Mai 2016 14:08 To: lcms-user@lists.sourceforge.net Subject: Re: [Lcms-user] How to create a gray profile with L-star grey response? 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 ------------------------------------------------------------------------------ Mobile security can be enabling, not merely restricting. Employees who bring their own devices (BYOD) to work are irked by the imposition of MDM restrictions. Mobile Device Manager Plus allows you to control only the apps on BYO-devices by containerizing them, leaving personal data untouched! https://ad.doubleclick.net/ddm/clk/304595813;131938128;j _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user ------------------------------------------------------------------------------ Mobile security can be enabling, not merely restricting. Employees who bring their own devices (BYOD) to work are irked by the imposition of MDM restrictions. Mobile Device Manager Plus allows you to control only the apps on BYO-devices by containerizing them, leaving personal data untouched! https://ad.doubleclick.net/ddm/clk/304595813;131938128;j _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
