There was a question raised about whether to
include color bars while digitizing aged photo albums and Stanley
Smith posted a reply. I asked Ken Fleisher of my staff at the National
Gallery (who does not belong to this list) to prepare a comment to Stanley
to post. This is a core topic of an upcoming Mellon benchmark grant to the
RIT School of Printing and Dr. Franziska Frey who will work with some of the
museums in ImageMuse. Please come to the ImageMuse panel at MCN for more
discussion.
Thanks, Alan Newman. Stanley?s comments are preceded by and Ken?s replies
follow each of the comments.
Sent: Mon 10/20/2008 5:44 PM
Subject: Re: [MCN-L] photography, digitization, and a color/grey card?
A couple of comments regarding the use of grayscale and color bars:
- due to differences between dyes and/or pigments used to make the
color bars and the materials used to produce the actual artwork,
accurately rendering the grayscales in a particular image may NOT
produce the most accurate rendition of the artwork itself.
I think it's important to begin introducing the distinction into
common usage between scene-referred encoding and output-referred
encoding (see definitions at the end of this message). The idea of
accurately reproducing any target that was captured with the artwork
on a printed output, even if there were not issues associated with
metamerism (which there are), is in my opinion an incorrect goal to
begin with.
To accurately capture a target in the digital image implies a
scene-referred encoding and is a valid goal in artwork reproduction.
By recording the scene information, you have the opportunity to know
more about the object itself and to properly transform it as necessary
for various types of output.
On the other hand, to accurately reproduce that same target on a
printed output implies that the goal is to transfer the scene
colorimetry to the printed page. This is an incorrect goal because to
make the best possible reproduction, it will be necessary to alter the
tone reproduction curve (TRC), if nothing else, to reflect the
differences in viewing condition associated with a printed image on a
white page. For example, there is often a more limited gamut on the
printed image so a TRC adjustment will be needed to compensate for the
difference in appearance, the white page surrounding the image makes
further TRC adjustment necessary, the level of illumination for
viewing the printed image is likely different from how the artwork was
illuminated during photography which, once again, means additional TRC
adjustment is needed.
With that out of the way, I have one more comment which I hope to
bring into public discussion and awareness. That is the idea of
creating an accurate reproduction . I understand why everyone says
this, and I'm as guilty of it as anyone else, but the fact is that
there is no single accurate reproduction. There are many renderings
of the artwork which can be considered accurate for a given viewing
condition. The appearance will be different if you view the artwork in
gallery lighting versus 5000K lighting. Which is more accurate? They
both are equally accurate. The same holds true even if you keep the
same lighting and place the artwork in a white matte and hang it on a
white wall versus using a beige matte on a dark gray wall. The artwork
will have a different appearance, but both are accurate. So which
one do you reproduce? It is my hope that we will all start using the
term appearance preserving reproduction rather than accurate
reproduction and that we will have a defined viewing condition
associated with the appearance that we are trying to reproduce. This
may seem like a subtle point to some, but I feel it is an important
distinction in helping remove ambiguities from our discussions.
Further, and more related to the question at hand, it makes it more
clear that an appearance preserving reproduction of a target is more
important, and more appropriate, than a colorimetrically accurate one.
Back to the comment above. It is correct to state that due to
differences in pigments, the use of a color target to capture
scene-referred colorimetry is a bad idea and will only have limited
success. Therefore it is not recommended to use a color target for any
capture (Note we are talking about standard targets like the
ColorChecker--a case could be made for specialized targets that are
known to be spectrally relevant to the object being photographed). On
the other hand, an idea I adapted from the conservation guidelines is
to include a single color patch simply to indicate that the capture is
in color rather than grayscale. With regard to a grayscale target,
something like the Kodak Q-13 is not appropriate for capture because
it is not made from spectrally neutral materials (it is printed on
photographic paper). However a set of step wedges that are spectrally
neutral, such as the gray patches of the GretagMacbeth ColorChecker,
are very useful to include when capturing
