Re: Is this a scum dot?
Re: Is this a scum dot?
- Subject: Re: Is this a scum dot?
- From: Lee Varis <email@hidden>
- Date: Tue, 19 Nov 2002 06:50:14 -0800
I just realized teat I originally sent this to Bruce Lindbloom (those
darn reply to commands!) so here it is again, properly addressed (Sorry
Bruce)
On Monday, November 18, 2002, at 02:49 PM, Bruce J. Lindbloom wrote:
c) The only recourse I can think of (short of custom profile editing
for
each original or scanning in AbsCol mode and adjusting the white point
manually afterwards) is to adjust the white point of the scanner when
scanning the special original so that its whitest white has the same
RGB
value as patch A16 of the IT8 (namely 240 in our example). This
prevents
both RGB clipping and Lab clipping. We are cheating of course,
because we
have changed the scanning setup. But we are forced to cheat since our
special original has colors outside the gamut of the IT8 target.
Bruce Fraser writes:
My compromise is a bit different from the one you suggested, and it's
one that's well-supported by most profiling tools, which generally
don't map the A16 patch to L*100 -- they do in fact look at the RGB
values and make sure that RGB 255 maps to L*100.
I profile the scanner's wide-open 16-bit/channel behavior, and scan
everything wide open. I experiment to find the tone curve that will
produce the best agreement between the actual values in the target
and the predicted ones I get by assigning the profile to the scan,
converting to LAB, and comparing with the target values.
The resulting scans come in a little flat and a little unsaturated,
but with accurate gray balance and hue. It then becomes very simple
to optimize the contrast and saturation by editing the images.
Wow... you guys are making my head hurt! Seriously, though, I think
this really points out how interpretive any input characterization is.
I've always had questions about just how useful input profiles really
are - I mean, there's no question that a printer profile helps and the
monitor needs to be profiled to make Photoshop previews work at all
but the scanner just seems to be a bit sticky for me. The IT8 target
is somewhat limited as a representation of the gamut limits of
possible source material. I've found that quite often you encounter
"special originals" that have RGB values outside of the end points of
the IT8. I'm not sure how you compensate for this using the current
profile building tools as Mr Linbloom points out they seem to be
designed to work within the limits of the IT8. If you limit the RGB
values of the input to hit the Lab white (L*=100) value of the A16
patch aren't you limiting the number of bits used to define the gamut
of the original? Even so this is just what I've habitually done in my
own limited scanning experience I never allow the white point of the
original to go beyond RGB= 245 and I place the blackest point at RGB =
5 or sometimes 10 - I'm not sure how this might be similar to Mr.
Fraser's method of scanning "wide open" other than I believe that I'm
leaving some fudge room for the scanner to capture detail that may
fall too close to the L value limits to make it through the inevitable
conversion into my working space (I'm not a big fan of wide gamut
spaces)
The best one can hope for is an approach to device characterization
that allows one to "place one's feet" somewhere firm before leaping
into a particular interpretation of the image. There are so many
emotional/psychological implications with a real photo-image that any
rigid way of placing input values is bound to create problems with
certain images. I like the idea of locking the scanner down in some
way that leaves plenty of room to maneuver after the fact and
profiling for gray balance and hue (and still not waste bits that
could be useful for encoding image detail - asking for too much?). I
like to do my own tone mapping, compressing or expanding tones to fit
the image and I'd prefer to do that in Photoshop ( as Mr. Fraser does)
rather than the scanner software. Is it even necessary to profile the
scanner to make this approach work if we're just going to take it into
a workspace and edit? Or maybe its just a kind of half-profile to
account for gray balance anomalies without regard for end point
mapping. I'm sure you two have some ideas about the usefulness of
input profiles within the limitations of the available technology.
Input profiles seem to be easy to build but something about the
automatic scan-measure procedure seems to guarantee dullness that I
end up editing out in Photoshop anyway is it a waste of time? or
does the profile really help? Perhaps Mr Fraser could elaborate on his
method of profiling the "wide-open 16-bit channel behavior" and
editing to produce the "best agreement" between actual values and
predicted ones? I'm particularly interested to see if I can find
something similar for digital camera captures.
regards,
Lee Varis
email@hidden
http://www.varis.com
888-964-0024
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