[argyllcms] Re: Custom illumination
- From: Klaus Kompatscher <kkompatscher@xxxxxxxx>
- To: <argyllcms@xxxxxxxxxxxxx>
- Date: Wed, 20 Jan 2010 14:45:41 +0100
Ok this sounds all quite logic and I could simulate things you mentioned.
If printing by using relative Intend the print looks OK.
Initially I have been shocked because I did only a softproof by using Source
-> relative -> Printer -> absolut -> Monitor conversion and the Monitor
image has nothing to do with the print.
Perhaps this is the point you can't see to fit an option the way PM is doing
it?
Thanks
Klaus
On 20.01.2010 2:22 Uhr, "Graeme Gill" <graeme@xxxxxxxxxxxxx> wrote:
> Klaus Kompatscher wrote:
>> Did try to create profile for custom illumination:
>> - Measured light with I1 with special diffuser; Got measurements in cxf
>> format (file attached)
>> - Made a profile with Profile Maker White point looks good
>> - Copied spectral values to ref. D50_1.5.sp file and renamed DalTile...
>> (file attached)
>> - made a profile ./colprof -v -i
>> /Applications/Argyll_V1.1.0/ref/DalTileLichtI1.sp -kp 0 .3 1 1 .8 -r 2 -L
>> 100 -l 280 G60_G07_5c
>>
>> White point calculated by Argyll is fare off from the real.
>> Visually it does not correspond at all.
>> PM white point: Lab 87 0 2
>> Argyll white point: Lab 88 5 30
>
>> Why this big difference?
>
> What's going on I suspect is that you are asking Argyll to do
> a non-ICC standard thing and it is accommodating you, whereas
> Profile Maker is doing a sort-of standard ICC thing. See the note
> at the bottom of the paragraph here in the Argyll doco:
>
> <http://www.argyllcms.com/doc/colprof.html#i>:
>
> "Note that if an illuminant other than D50 is chosen, the resulting ICC
> profile will
> not be standard, and cannot be freely interchanged with other profiles that
> that use
> the standard D50 illuminant, particularly if the absolute rendering intent
> is used.
> Profiles should generally be linked with other profiles that have the same
> illuminant
> and observer."
>
> What I guess Profile Maker is doing is computing the media colors as if they
> are viewing in the given illuminant, and then creating a chromatic adaptation
> matrix to convert the white to D50 to make it (sort of) conform to the
> standard
> ICC conditions. I say sort-of because standard ICC is actually to measure
> under
> D50 illuminant where possible, and a non-D50 illuminant + chromatic matrix is
> not
> the same thing, and Profile Maker could have simply ignored the given
> illuminant
> and used actual D50 if it is really trying to create a standard ICC profile.
> [The option in the ICC spec. of using a chromatic adaptation matrix for the
> illuminant
> is intended for the situation where measuring under D50 is not possible.]
>
> I think this rather defeats the purpose of Absolute colorimetric rendering
> intent
> though, since a standard CMM will not reproduce the absolute appearance if the
> illuminant has been adapted to be D50 white point.
>
> No doubt I could do the same with Argyll as an option, but I'm sure it
> would increase the confusion factor, and I'm not sure I see the point. If
> you want ICC standard D50, then give it D50 spectrum. If you want absolute
> intent under an alternate illuminant, then that's exactly what Argyll
> provides, even though a non-standard ICC profile will result.
>
>> What means Spectral_Norm in the Spectral illumination file?
>
> It's a number to scale the spectral values. Used as an illuminant
> this shouldn't have any effect (An illuminant is always normalized
> to 1.0).
>
> Graeme Gill.
>
>
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