[argyllcms] Re: Colprof feature request: amount of black pointhuecorrection control

Gerhard Fuernkranz wrote:

> I find for instance this huge divergence "interesting":
> $ colprof -v -qh -S sRGB.icm LSM2484
> $ xicclu -s255 -fif -ir LSM2484.icc
> 0.000000 0.000000 0.000000 [Lab] -> Lut -> 0.000000 0.000000 0.000000 [RGB] 
> (clip)
> 
> $ xicclu -s255 -fb -ir LSM2484.icc
> 0.000000 0.000000 0.000000 [Lab] -> Lut -> 30.935831 23.114864 0.000001 [RGB]

Gerhard, the xicclu uses "simple" gamut clipping by default. When You try 
xicclu -b -s255 -fb -ir LSM2484.icc, the clipping for relative intent will be 
by CAM Jab. The xicclu values with Jab clipping will be rather close to profile 
data. You'll not see such difference.
 
> $ xicclu -s255 -ff -ir LSM2484.icc
> 0.000000 0.000000 0.000000 [RGB] -> Lut -> 12.068596 -0.064363 -8.293796 [Lab]
> 30.935831 23.114864 0.000001 [RGB] -> Lut -> 19.881831 0.965791 -1.891457 
> [Lab]
> 
> => that's a difference of about 10 delta E !
> 

Yes. The mapping works properly. The lightness difference of blacks is less, 
than tone shift was compensated. The image is OK, with quality tones, but less 
contrast, than needed in the case.

> [Btw, looking at the measurements, I find this RGB printer driver not
> too well-behaved in dark regions - possibly the generation of CMYK
> separations from RGB is not optimally tuned for the used inks/paper. It
> seems to put down too much cyan and magenta ink (and/or not enough
> yellow ink) for RGB=[0,0,0]. Just wondering, is this the driver provided
> by the printer vendor, using original inks/paper from the printer
> vendor? Did you tweak any driver settings? (e.g. increase cyan/magenta
> density)]

Yes, there is native printer driver without any tweaks (with "no CM" mode the 
driver haven't any adjustments), with non-original inks and the specific, 
non-original media.
Unfortunately, the workflow prevents using of original materials or using of 
another RIP (the Gutenprint for example). I've not found any alternative RIP 
for "EPSON Stylus Photo 1410" with GDI-driver level - most of software at the 
site needed it.

> Regards,
> Gerhard

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