"Chris Nellist" wrote in
message ...
I understand that the Dulux paint colour codes are based on
the NCS colour system. I'd like to convert between this
and the RGB/CMY systems, so as to be able, for example, to
identify the Dulux colour most closely matching a given 3-byte
hex code (e.g. in CMY, primary magenta as 00FF00).

I have been unable to find the mathematical specification of
the NCS.


This is because there is no mathematical specification - the NCS system is a
perceptual space.

I have read the material at the NCS website and would be
grateful if someone could tell me whether

a) the definitions of the elementary hues R, Y, etc. in NCS
are the same as in CMY and RGB?


No - RGB & CMY are device space coordinates and RYGB are perceptual, unique
hues.

b) the gradation between two neighbouring elementary
colours, e.g. R and Y, is linear (in CMY terms)?


No they are not since RGB/CMY are not linear in perceptual color space.

c) a colour such as S 2030-Y90R can be made by
subtractively mixing
- 20% black (255/255/255 in CMY)
- 30% of the hue defined by Y90R
- 50% white (0/0/0 in CMY)?


The NCS is not a substractive space - it is a perceptual space in which
blackness and whiteness are attributes and not phytsical quantities,

If the answer to each of these questions is yes, conversion is
simple, but I suspect there is more to it than this. I know
that some NCS codes do not exist, i.e. some spaces in
triangles in the NCS colour atlas for given hues are left
blank. I am not sure whether this is because the colours are
imaginary or just because the required pigments do not exist.


These spaces are left blank because these perceptual colors cannot be
reliably created in the paint medium used to create the example pages.

If there *is* more to it than this, could someone post
sufficient information to allow conversion between NCS and CMY
(and therefore RGB), given standard illumination conditions?
If the answer to c) above is 'yes', all I need is a formula
for converting between the two codes for a given hue.


The Scandinavian Color Institute offers a program that can generate standard
RGB values for a given input notation. They have a program which you can
license that contains a combination Look-Up Table and analytical model that
is based on instrumental readings of the example chips.

Obviously a formula for converting between NCS and any colour
system from which I can get to RGB will be just as good!

Failing this, does anyone know the RGB codes for the
elementary colours (red, green, blue, yellow) used in NCS?
Or the NCS codes for the primary colours in RGB or CMY?
I have looked on loads of colour conversion sites, but
haven't been able to find even this information, anywhere!


This is because the conversion between NCS and any other color order system
is considered commercially proprietary. Some things in life are
unfortunately, not free.

Many thanks in advance for help with this.

Yours sincerely,

Chris


Danny Rich