Re: [Lcms-user] LUTs in littlecms

[Marti.Maria <mar...@li...>]

Ignacio,

Please note the weird v2-Lab encoding of Lab, where L=100 would never 
fall into a node unless
special input curves  are used.

See the function "CreateLabPrelinearization" in cmsgmt.c:

"v2 L=100 is supposed to be placed on 0xFF00. There is no reasonable
number of gridpoints that would make exact match. However, a
prelinearization of 258 entries, would map 0xFF00 on entry 257.
This is almost what we need, unfortunately, the rest of entries
should be scaled by (255*257/256) and this is not exact.
 
An intermediate solution would be to use 257 entries. This does not
map 0xFF00 exactly on a node, but so close that the dE induced is
negligible. AND the rest of curve is exact."

I wonder if this is the case
Regards
Marti


Ignacio Ruiz-de-Conejo wrote:
> Thanks, Kai-Uwe, I'll do that.
>
> I'm afraid, though, that visualizing the internals of the profiles 
> doesn't help.
>
> I have generated several profiles.
> - CLUT with 17, 33 and 65 nodes
> - CLUT plus prelinearization curves (17, or 33 or 65 steps, shifted to 
> center them around the nodes)
> - CLUT, plus prelinearization curves, plus postlinearization curves (to 
> clamp c,m,y,k values)
>
> The  profiles look perfect, as I expected them to be when I designed 
> them. Is its application wat puzzles me.
> My test consists in applying icctrans to an IT8 file with 7 256-step 
> ramps of RGB values (sRGB),as in
>    icctrans -o <profile> test_rgb.it8 test_cmyk.it8
> Most of the time, the result is the expected one. But in all the 
> different profiles I have built, the tests show glitches, and I can find 
> RGB values that do not map to the desired output.
> It is weird, for instance, that clamping does not produce a clean, 
> rounded number. And the presence of mis-behaving values here and there, 
> not only in the border of the regions.
> Maybe the rounding and interpolation is a limitation I can't avoid,and 
> with deeper effects than I expected.
>
> You can find sources in http://docs.google.com/Doc?id=dfscpqm8_45t4q5ffst
> It is a ZIP, but with a .txt extension, so that GoogleDocs allows me to 
> upload it (other suggestions?)
> It contains several profiles, the source program (with an explanation of 
> the goal to achieve) and the test file.
> There is also an spreadsheet with the results of the tests (see below), 
> highlighted to show my concerns.
>
> Best regards,
>     Ignacio
>
>
> Kai-Uwe Behrmann wrote:
>   
>> Possibly posting a link to your profile(s) would help.
>> For most OSes exist profile inspection tools to see a nLUT, at least 
>> for version 2.x. For the ICC version 4.x it may differ.
>>
>> kind regards
>> Kai-Uwe Behrmann
>>     
>
>   
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