[argyllcms] Re: profile: black generation

  • From: "Gerhard Fuernkranz" <nospam456@xxxxxx>
  • To: argyllcms@xxxxxxxxxxxxx
  • Date: Tue, 3 Jan 2006 13:00:05 +0100 (MET)

> --- Ursprüngliche Nachricht ---
> Von: Graeme Gill <graeme@xxxxxxxxxxxxx>
> An: argyllcms@xxxxxxxxxxxxx
> Betreff: [argyllcms] Re: profile: black generation
> Datum: Tue, 03 Jan 2006 17:56:17 +1100
> It is possible to reformulate the B2A Lut such that its efficiency is
> much improved, by using the matrix element and input curves to map an XYZ
> PCS into a pseudo-RGB space that encompasses the CMYK space, and then
> use the CLUT to translate between them. This approach might give a result
> much closer to that of device link -G. The disadvantage is that the
> clipping
> behaviour for out of gamut colors won't be very good (you'll get
> per device channel clipping in XYZ/RGB space, rather than nearest color
> clipping), and (with ICCV2 profiles) the A2B will have to be a CLUT table
> in XYZ PCS, which may affect accuracy badly. This might be alleviated
> somewhat by mapping the input to a log encoded (or L* curve) XYZ,
> or by using ICCV4 to convert between L*a*b* and XYZ between the CLUT and
> the PCS.

Another simple method for improving the accuracy for in-gamut colors in
conjunction with a CIELAB-based B2A table might be the use of sigmoid-shaped
prelinearization tables for the a* and b* channels, such that the CLUT
operates on a distortet Lab space, while it still maps the complete Lab
space to device space, giving a higher accuracy for near neutral colors
(in-gamut) at the cost of a lower accuracy for highly cromatic
(out-of-gamut) colors. Thich would basically also be consistent with the way
how color differences are perceived (e.g. dE CIE94), i.e. near-neutral
colors must be rendered very accurately, while this is not so important for
highly crhomatic colors.

And of course, since the B2A table is only 3D, while a CMYK A2B table is 4D,
there is also more room for using an even finer grid resolution for the B2A
tables (than for A2B), which also may help a bit to compensate the
deficiencies of the B2A tables (i.e. the "wasted" space).


Gerhard Fuernkranz

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