[argyllcms] Re: targen options

Gerhard Fuernkranz wrote:
> sRGB should be pretty accurate in the grays, for a synthetic gray wedge 
> with R=B=G.

Since you are using sRGB, this suggestion wasn't applicable to
your situation.

> I don't remember exactly, since its a while ago. I think I supplmened 
> approx. 125 points near the gray axis, with [a*,b*] values of [0,0], 
> [3,0], [-3,0], [0,3], [0,-3], at 25 different lightness levels and 
> converted these PCS values to RGB with the old profile, printed and 
> measured the patches, and added the results to the .ti3 file. Then I 
> created a new profile. But I did not make further iterations (iterations 
> are time consuming due to one day drying time for inkjet prints, until 
> the color drift stabilizes). I also guess, that my supplemented points 
> were distributes too regularly.

I suspect with sRGB that a*b* values of 0,0 amount to the same thing,
but may not with different source profiles.  It sounds as if you
did a reasonable job of augmenting the test chart.

> Do you mean an (independent) linearization of each channel with 
> high-resolution 1D luts?
Yes, that's basically what most calibration systems do.

> I've no doubts that this is very helpful, if the device channels correspond to
> the actual device colorants, since the colorants usually mix rather smoothly.
 > This corresponds also to my experience. In my "successful" case, the CMYK 
 > channels
 > already did have a nearly linear response, when I plotted C,M,Y, or K vs. the
 > euclidian CIELAB distance to paper white.

 > of this mapping). Thus I could only linearize the RGB channels. But will 
 > linearized RGB
> channels also imply a smooth RGB->Lab mapping near the gray axis? Maybe, if 
> I'm lucky.
 > But maybe not.

If the RGB is not a real device RGB, then yes, just linearising the RGB channels
may not be enough.

> Another idea might be to linearize all three RGB channels not separately, but 
> together, in
 > order to optimize the grayscale accuracy?

That would be the logical thing to do. This is not unreasonable given how
critical grey tones are in judging results. This is the type of thing that
EIZO claim to have done with their LCD monitor linearisation.
> 
> Btw, I've also another CMYK example, which did not work as well as the 
> "successful" one.
 > Here the problem was obviously caused by course quantization due to a low 
 > number of screen
> levels, particularly in light areas. The profile simply outputs 4 values 
> (CMYK), which are
> quantized independently by the halfone threshold arrays, and this also 
> resulted in
> alternating color shifts into C, M, or Y direction along the grayscale (for 
> light grays),
> if -kh was used for profile generation (these color shifts of course 
> disappeared with -kx,
> at the cost of more visible lightness steps, than with -kh, so it's a 
> tread-off). A vector
> quantization in Lab space (to the percetually nearest discrete CMYK tuple) 
> would likely
> give better results in this case, than per channel quantization, but of 
> course this would
 > be a too great demand on a profile.

If your basic control over the device is limited (such as by limited screen 
levels),
then yes, there is not much a color correction system can to do correct the 
results.

Graeme Gill.

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