Re: Is this a scum dot?
Re: Is this a scum dot?
- Subject: Re: Is this a scum dot?
- From: "Bruce J. Lindbloom" <email@hidden>
- Date: Mon, 18 Nov 2002 11:56:39 -0600
Profiles are used to establish the relationship between device values (e.g.
RGB or CMYK) and device-independent values (e.g. Lab). In the process of
creating a profile, we know one side of this relationship (for a selected
set of colors) and we empirically determine the other side. From this, the
entire profile behavior is generalized.
For printer profiles we know the device side (the RGB or CMYK values of the
profiling test pattern patches) and we discover the corresponding
device-independent side by measuring the colors of the printed test pattern.
The ICC specification states that for relative colorimetric rendering
intent, device white (R = G = B = 255, or C = M = Y = K = 0) should map into
perfect PCS white (Lab = 100, 0, 0).
For scanner profiles the opposite is true. We know the device-independent
side (the Lab values of the IT8 patches as found in the reference file) and
we discover the device values (how the scanner "sees" each of these patches,
expressed by its RGB values). The ICC specification does not say anything at
all about the relationship between any device value and device-independent
value for a scanner profile. So when Bruce Fraser wrote:
>
If the scanner profile translates RGB 255,255,255, relcol, to something less
>
than L*100, it is by definition broken.
I'm not sure what definition is being referred to, but I am not aware of any
definition like this. I would instead agree with Graeme Gill that the RGB
value of the whitest neutral patch of the IT8 should map onto PCS white (in
RelCol). One cannot safely make any further generalizations about what other
RGB values must map to Lab 100, 0, 0.
Since a scanner profile is made from a specific IT8 target, it only works
with colors that are within the gamut of that particular photographic
medium. If during use, it comes across colors that are outside this gamut
(as might happen with an overexposed piece of film), it should extrapolate
the known behavior into this unknown region. In any normal situation, the
needed extrapolation would be a small amount.
Considering an example where the entire gray scale of the IT8 shows scanner
values having R greater than G and B, then one would expect an extrapolation
to hold this same type of behavior. In this case, any scanner color whose R,
G and B components are equal to each other would be expected to have a
slightly cyan Lab color, that is, non-neutral.
If the IT8 white patch has RGB values that are quite different from R = G =
B = 255, then we shouldn't be preoccupied with what Lab value is assigned to
this hypothetical RGB 255 value because, if the scanner is operated in a
normal way, this RGB value could never be produced anyway.
The bottom line is that you should not assume any a priori characteristics
about any device values for a scanner profile. This is something that may be
counter intuitive and perhaps must be deliberately unlearned.
Just my two-cents worth...
--
Bruce J. Lindbloom
email@hidden
http://www.brucelindbloom.com
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