Re: [Gimp-print-devel] Wow!

[<sh...@al...>]

> Is a spectrometer really needed? Wouldn't a (more or less) calibrated scanner
> do?

The short answer is no.

This is from the Color FAQ:

  13.  DOES MY SCANNER USE THE CIE SPECTRAL CURVES?

  Probably not. Scanners are most often used to scan images such as color
  photographs and color offset prints that are already "records" of three
  components of color information. The usual task of a scanner is not
  spectral analysis but extraction of the values of the three components that
  have already been recorded. Narrowband filters are more suited to this task
  than filters that adhere to the principles of colorimetry.

  If you place on your scanner an original colored object that has
  "original" SPDs that are not already a record of three components, chances
  are your scanner will not very report accurate RGB values. This is because
  most scanners do not conform very closely to CIE standards.

> Of course it wouldn't yield full spectral information but how much of it is
> really needed for color management?

Excerpted from question 24:

  Practical photographic dyes and offset printing inks have spectral
  absorption curves that overlap significantly. Most magenta dyes absorb
  mediumwave (green) light as expected, but incidentally absorb about half
  that amount of shortwave (blue) light. If reproduction of a color, say
  brown, requires absorption of all shortwave light then the incidental
  absorption from the magenta dye is not noticed. But for other colors, the
  "one minus RGB" formula produces mixtures with much less blue than
  expected, and therefore produce pictures that have a yellow cast in the mid
  tones. Similar but less severe interactions are evident for the other pairs
  of practical inks and dyes.

  Due to the spectral overlap among the colorants, converting CMY using the
  "one-minus-RGB" method works for applications such as business graphics
  where accurate color need not be preserved, but the method fails to
  produce acceptable color images.

  Multiplicative mixture in a CMY system is mathematically nonlinear, and the
  effect of the unwanted absorptions cannot be easily analyzed or
  compensated. The colors that can be mixed from a particular set of CMY
  primaries cannot be determined from the colors of the primaries
  themselves, but are also a function of the colors of the sets of
  combinations of the primaries.

> Could someone please post some URLs where to
> get basic information?

Start here:

http://www.inforamp.net/~poynton/ColorFAQ.html

Eric