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: Mon, 18 Nov 2002 14:12:37 +1100
bruce fraser wrote:
>
If Capture RGB 255 translates to something
>
other than L*100, you get a scum dot.
This is not true. If white of the original translates
to other than L*100, you get scum dots. White may
not be RGB 255. The whole point about characterizing
the input device is (amongst other things) to compensate
for white not being 255!
>
Absolute colorimetric capture generally produces hideous results.
Do you mean hideous in a technical or subjective sense ?
The point about running in absolute mode is that this
lets you color correct for the scanner characteristics,
while not restricting the capture gamut, or making
assumptions about original white. Of course you then
have to adjust the absolute scan to turn it into
a suitable customized perceptual image.
>
When we look at prints, we adapt to paper white. Translating white in
>
the capture to anything other than paper white clobbers the
>
already-limited dynamic range and introduces the appearance of a
>
color cast.
Given an arbitrary scanner, you can't expect it to be perfect
match to the desired dynamic range. You either have to be very
lucky, or in fact you've fiddled with the controls on the scanner
(assuming it has some) to adjust it so that white is exactly 255,
and black is 0. In general, you can't expect a scanner to have
this set of controls, nor the operator to have the necessary skill
and tools to do such an adjustment. Given the trade-offs, it seems to me
better to compromise the number of bits, rather than risk loosing
information off the ends. It's easy to get rid off stuff you
don't want, its harder to recover the information once it has
gone. If quantisation is an issue (amongst all the noise introduced
by real world media and the scanner), then use 16 bits on your input.
>
We don't get paid for producing colorimetrically-correct results. We
>
get paid for producing color that looks right.
It depends completely on your application. I would guess that
you're thinking expert scan operator with professional gear. I'm
thinking somewhat more generally. If you really don't
care about the color accuracy, then you probably won't bother with
trying to use ICC profiles on a scanner. Use whatever its
native RGB is, and and fiddle it till it looks right.
People without that expertise and gear want it to "just work",
and the only rational basis for creating profiles that just work
is to base them on sound colorimetric & color appearance principles.
Graeme Gill.
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