Marti,
why a simple scaling is done? A chromatic adaptation
could be executed instead?
Giovanni Brembati
On Sat, 8 Sep 2007 11:38:42 +0200
"Marti Maria" <marti.maria littlecms.com> wrote:
>
> Hi,
>
> You are right; absolute colorimetric is not stored in
>the profile.
> Instead, BToA1 tag is used with a white point scaling.
>The algorithm
> is rather simple, it is performed on XYZ color space
and
>it is only
> a plain scaling.
>
> AbsCol = RelCol * (MediaWhite / D50)
>
>For each of the tristimulous:
>
> Xabs = Xrel * (Xwtpt/0.9642)
> Yabs = Yrel * (Ywtpt/1.0)
> Zabs = Zrel * (Zwtpt/0.8249)
>
> Latest revision of ICC spec, however, specifies a DToB3
>tag, which stores
> floating-point absolute colorimetric intent. But this
is
>an optional tag
> and AFAIK very few CMM (if any) actually supports this
>tag (lcms doesn't)
>
>
> Hope this helps
>
> Marti Maria
> The littleCMS project
> www.littlecms.com
>
>
> -----Original Message-----
>From: lcms-user-bounceslists.sourceforge.net
> [mailto:lcms-user-bounceslists.sourceforge.net] On
>Behalf Of John Weissberg
> Sent: Friday, September 07, 2007 2:14 PM
> To: lcms-userlists.sourceforge.net
> Subject: [Lcms-user] [Fwd: Using BtoA1 table with
>rendering intent absolute]
>
> Consider the following case: Converting from Lab ->
CMYK
>using the BtoA
> tables of an ICC profile with "Absolute
Colorimetric" as
>a rendering intent.
>
> As I understand it, the ICC profile does not contain an
>Absolute
> colorimetric BtoA table. Rather, it will use the
>relative colorimetric
> BtoA table, along with whitepoint data, for any
look-ups
>it makes.
>
> Can anyone explain what algorithm is used for these
>conversions between
> relative and absolute colorimetric?
>
> Is it simply adding/subtracting the DeltaE between the
>whitepoint and
> 100,0,0 ? Is this delta value added for all points in
>the color space?
> Only for points where L is above a certain limit?
>
> Any information is appreciated.
>
>
> --
> John Weissberg
> PerfectProof - Accuracy Made Easy
>
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>
> --
> John Weissberg
> PerfectProof - Accuracy Made Easy
>
> www.perfectproof.com
> www.printers-website.com
> www.proofmaster.net
> pakready.perfectproof.com
>
> USA
> (888) 623 3662 Tel.
> (888) 623 3662 FAX
>
> Europe
> +32 15 272502 Tel.
> +32 15 273740 FAX
>
>
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