Re: Is this a scum dot?
Re: Is this a scum dot?
- Subject: Re: Is this a scum dot?
- From: Graeme Gill <email@hidden>
- Date: Tue, 19 Nov 2002 14:53:25 +1100
"Bruce J. Lindbloom" wrote:
>
b) If the scanner is set up during profiling so that the RGB value of patch
>
A16 was less than 255 (say 240, but it could be something else), then
>
scanning the special original would produce RGB greater than 240 (let's say
>
250), so RGB clipping has not occurred. But since the profile maps 240 into
>
Lab 100, 0, 0, then our 250 would probably get mapped into Lab 100, 0, 0
>
also, so we still get clipping. This time on the Lab side of the profile
>
instead of the RGB from the scanner. This is still bad.
This clipping won't occur if the scanner profile is a matrix type,
or an XYZ Lut type (since XYZ values are allowed to range up
to 2.0), but in all cases extrapolation is involved in going
outside the gamut of the test chart. From a limited number of
experiments with IT8 profiles, I have had better accuracy from
Lab Lut based profiles, which do suffer from the clipping problem.
For this reason, I added a flag to my profiling software to allow
the creation of non-ICC standard "Absolute only" Lab LUT based
input profiles, which won't clip values outside the IT8 chart white
(but will clip above absolute L*100 of course). This would seem
to facilitate a viable "scan absolute, adjust to relative manually"
type workflow.
Graeme Gill.
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