KoColor && CMYK -> RGB

[Boudewijn Rempt]

Well, my CMYK strategy works now without crashes -- and it gets called, too, 
so that's nice. But I fear that there might be a bug in KoColor. Since 
KoColor isn't used anywhere but on in Krita, but is still in the koffice lib, 
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 = (R < G) ? R : G;
  *K = (min < B) ? min : B;

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

The color conversion website (http://www.easyrgb.com/math.php?MATH=M11#text11) 
doesn't give a direct rgb-> cmyk, but advises:

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

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

followed by:

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

var_K = 1

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

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

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

-- 
Boudewijn Rempt | http://www.valdyas.org/index2.html
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