Re: Is this a scum dot?
Re: Is this a scum dot?
- Subject: Re: Is this a scum dot?
- From: bruce fraser <email@hidden>
- Date: Wed, 20 Nov 2002 08:19:12 -0800
At 12:48 PM +1100 11/20/02, Graeme Gill wrote:
<snip>
Interesting. Of course if the profiling system was perfect, you
shouldn't see much difference between the results with different
gamma settings, since it should be exactly compensating for
any changed you make. Do you think in doing this you're finding
the "sweet spot" of the profiling system, or are you optimizing
the quantization of the RGB encoding used between the output of
the scanner and the input of the CMM ?
I suspect the latter, because different profiling tools seem to agree
on the "ideal" gamma. The differences are pretty subtle -- on Don
Hutcheson's HCT target, with the Imacon 848 scanner, the default
gamma 2.2 has an average delta-e of 1.66 and a maximum delta-e of
5.84 compared to the target, while gamma 3 (the sweet spot) lowers
that to 1.44 and 4.34 respectively. The main difference between the
two is that the gamma 3 version captures dark saturated colors (of
which the CHT target has a better sampling than the IT8) better than
the default. Doing the same comparison with the IT8 gets different
numbers, but they move in the same direction.
For transparency, it's obvious what the brightest white can be.
For the general case (transparency and reflection), it is not
so clear, since the practical whitest white is determined by the
transparency/reflectivity of the substrate, and the theoretical
whitest white may not be measurable.
If someone can show me a more effective approach, I'm always willing
to learn, but it's important to realise that when we scan E6
transparencies, the goal is hardly ever to reproduce exactly what's
on the film, partly because in most output scenarios it's physically
impossible to do so.
Your workflow sounds very logical and practical. This seems very
close to what I mean by "scan absolute, adjust manually".
Damn. I was hoping you had a magic bullet!
I need to think about the reflective case. It tends to be a lot
messier than scanning film, where most everything you scan is made up
of CMY dyes. Reflective scanning often encounters metamerism and
fluorescence issues. Don't most flatbeds have a calibration strip,
usually white teflon, inside the scan bed? Do you know if that's
simply used to balance the output from each sensor, or is it setting
the absolute white response?
It's probably been 3 years since I last profiled a reflective
scanner, but I seem to remember that when I did so I followed Kodak's
recommendations for the captured RGB values from the IT8, which were
something like white around level 245 and black around level 10. It
worked pretty well scanning photographic prints, but other reflective
artwork sometimes failed dramatically, mostly, I suspect, from
scanner metamerism.
B
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