Re: [Lcms-user] Mixing Lab color

[Armindo Da S. <tec...@wa...>]

What is the name of the profile I should use for CMYK?

  ----- Original Message -----=20
  From: Mart=ED Maria=20
  To: Armindo Da Silva ; Lcm...@li...=20
  Sent: Friday, January 25, 2002 7:33 PM
  Subject: Re: [Lcms-user] Mixing Lab color


  Hi,

  I'm not 100% sure, but I belive you cannot do that.

  Lab works as light, that is, additive. You add several=20
  lights and got a resulting one. No problem since there=20
  is no additional information needed.

  However, substractive colorspaces DOES need some=20
  additional information, since they are also reflective.=20

  You need to know illuminant (the source of light the inks
  are blocking off) and the media white of substrate (the=20
  chromaticity of paper) the gamut (the whitest you can get,=20
  that is no more that the substrate, and the darkest, the point
  at inks blocking all but ambience light) etc etc.

  So, a Lab color minus another lab color can result in a lot of=20
  colors, on depending on a lot of things. It *is* device dependent
  at all :-)

  In the other hand, assuming you have a 100% difusse perfect=20
  reflective substrate and same illuminant as Lab white point,=20
  Lab is an euclidean  space, and as a such should have=20
  substraction defined. Do you have tried a plain vector substraction?

  Please note that real world is far away of this latter. If you want =
this for
  anything remotely related with printing, your best move will be to =
convert
  to CMYK and operate in a real-word

  >One solution would be to convert all lab color to CMYK :
  >
  >and doing c:=3D (x1*c1 + x2*c2 + x3*c3) / (x1+x2+x3)
  >                    m:=3D(x1*m1 + x2*m2 +x3* m3) / 3 (x1+x2+x3)
  >                    y ....
  >                    k ....
  > ...
  > and then converting to Lab.

  Aha! This is what I was suggesting on first e-mail. I guess this is =
the best=20
  way at all. It handles ALL aspects I was talking off.

  > the only problem is that I will lose precision (double conversion)

  No too much, if you use 16 bits, that is enough. At least this is
  the maximum precission you can get from Photoshop, to say an
  example.

  Regards,
  Mart=ED.

    ----- Original Message -----=20
    From: Armindo Da Silva=20
    To: Lcm...@li...=20
    Sent: Friday, January 25, 2002 9:27 AM
    Subject: Re: [Lcms-user] Mixing Lab color


    No it's not really what I want to do :

    let's say Col 1 is (in Lab)  80 /10/35
                    Col 2 is (in Lab)  30 /40/65=20
                    Col 3 is (in Lab)  10 /10/-15=20

    I have to mix 10% of Col1 with 40% of col 2 and 90% of col 3
    So what is the formula to have Lab of the resulting color

    But as I said it's should work on soustractive maneer, because 0% =
mean no color and 100% mean full color