[cairo] Spot colors (and CMYK)
spitzak at gmail.com
Wed Jan 20 12:15:34 PST 2010
What is needed is "this is a spot color with this id" along with "here
is the sRGB alternative". Devices that do not understand spot colors can
use the alternative. Drivers that do recognize the spot color can
instead allocate another channel (along with RGB) to accumulate it and
set RGB to zero. After compositing is done they convert the remaining
RGB into ink levels.
C,M,Y,K can themselves be spot colors, perhaps with predefined ids.
We do not need Lab or XYZ as the alternate. Because Cairo's api supports
negative floating point numbers it can cover any 3D volume of color
using any three primaries that are not on a straight line. Staying with
the sRGB primaries will make things easier for most back ends and most
users. (the real question is how to represent negative values in the
non-linear sRGB gamma curve. Microsoft with their scRGB chose to
continue the gamma curve past 1 and to mirror-image the positive curve
to make the curve for numbers less than 0, so that should be what is
If Pantone colors come with a CMYK "alternative", this is an
approximation already! I see no problem with further approximating it by
the simplistic conversion (r=1-c-b, etc in sRGB gamma space). The
advantage is that this is pretty much reversable (perfectly reversble if
the value for b is known). Not only that I would not be suprised if 50%
of those "approximations" were done by choosing the values so that this
simplistic conversion of sRGB produced the best screen result!
I previously suggested that if the mapping from spot colors to sRGB
values is unique (ie all spot colors map to different sRGB values) then
no id needs to be sent, just the "this is a spot color" flag. I still
think this may be a good idea, it would remove all Pantone IP from the
interface, allow a fixed-sized color structure (4 floats plus a single
enumeration), and would prevent malicious programs from, for instance,
asking for the "C" spot color but with 1,1,1 as the sRGB values.
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