From koffice-devel  Thu Oct 23 21:09:19 2003
From: Boudewijn Rempt <boud () valdyas ! org>
Date: Thu, 23 Oct 2003 21:09:19 +0000
To: koffice-devel
Subject: KoColor && CMYK -> RGB
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Well, my CMYK strategy works now without crashes -- and it gets called, too=
,=20
so that's nice. But I fear that there might be a bug in KoColor. Since=20
KoColor isn't used anywhere but on in Krita, but is still in the koffice li=
b,=20
I crossposted.  Is there still anyone maintaining this useful class?

Anyway, given a nice, white RGB color (255, 255, 255), this function
gives (255, 255, 255, 255) -- which is as black as can be.

void KoColor::RGBtoCMYK(int R, int G, int B, int *C, int *M, int *Y, int *K)
{
  int min =3D (R < G) ? R : G;
  *K =3D (min < B) ? min : B;

  *C =3D 255 - (R - *K);
  *M =3D 255 - (G - *K);
  *Y =3D 255 - (B - *K);
}

The color conversion website (http://www.easyrgb.com/math.php?MATH=3DM11#te=
xt11)=20
doesn't give a direct rgb-> cmyk, but advises:

//inp_R =3D From 0 to 255
//inp_G =3D From 0 to 255
//inp_B =3D From 0 to 255

out_C =3D 1 - ( inp_R / 255 )
out_M =3D 1 - ( inp_G / 255 )
out_Y =3D 1 - ( inp_B / 255 )

followed by:

//inp_C =3D From 0 to 1
//inp_M =3D From 0 to 1
//inp_Y =3D From 0 to 1

var_K =3D 1

if ( inp_C < var_K )   var_K =3D inp_C
if ( inp_M < var_K )   var_K =3D inp_M
if ( inp_Y < var_K )   var_K =3D inp_Y

out_C =3D ( inp_C - var_K ) / (1 - var_K )
out_M =3D ( inp_M - var_K ) / (1 - var_K )
out_Y =3D ( inp_Y - var_K ) / (1 - var_K )
out_K =3D var_K

Which is obviously rather more costly, with all the divisions... I'm hackin=
g=20
this in for now, to see what it does, but does anyone have a nice bit of ma=
th=20
that makes this cheaper?

=2D-=20
Boudewijn Rempt | http://www.valdyas.org/index2.html

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