> --- 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). Regards, Gerhard -- Gerhard Fuernkranz nospam456@xxxxxx DSL-Aktion wegen großer Nachfrage bis 28.2.2006 verlängert: GMX DSL-Flatrate 1 Jahr kostenlos* http://www.gmx.net/de/go/dsl