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: Wed, 20 Nov 2002 12:48:48 +1100
bruce fraser wrote:
>
Mostly I scan film, which fortunately doesn't fluoresce. I can't get
>
an ideal diffuser to fit in the film holder.
Yes, I wasn't thinking film. It is of course not a problem
to check how a transparency scanner responds with no
film in it, so, unlike the perfect diffuser, the perfect
transmitter is achievable in practice, and it would be relatively
easy to calibrate it to give a particular RGB output in this
state.
>
>I'm not sure I understand where the profile comes from, that you
>
>are using to get Lab values, to set up your tone curves. Seems
>
>like a chicken and egg problem. You can't create the scanner
>
>profile until you've decided on tone curves, yet you're using
>
>some profile to choose your tone curves. Can you clarify how
>
>this works ?
>
>
I scan the capture target at different gamma settings, and build a
>
profile from each one. Then I assign the corresponding profile to
>
each captured target and convert it to Lab. I compare these Lab
>
values with those in the target description file, and choose the one
>
that gives the lowest delta-e values, and use that gamma setting (or
>
tone curve in a drum scanner) for all subsequent scans.
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 ?
>
It's L*100 in the PCS. It's not a media white. It's the brightest
>
white that the scanner can capture. In practice, I never capture this
>
value during the scan.
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".
Graeme Gill.
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