[argyllcms] Re: R: Re: Neutralize Grey Axis
- From: Klaus Karcher <lists@xxxxxxxxxxxxxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Mon, 10 Jan 2011 09:50:23 +0100
Graeme Gill wrote:
So some clearer technical explanation of what it does is needed.
Quote from ProfileMaker Documentation:
"Perceptual Rendering Intent Option
Daily work shows that different aspects are frequently required for
ideal implementation of a gray axis. ProfileMaker therefore provides two
methods for defining the gray axis. These definitions affect only the
Perceptual Rendering Intent.
With Paper-Colored Gray, the substrate color of the scanner/camera
testchart or the paper color of the print run affects the entire color
space. This may shift the gray axis of the image. Colors in the image
may end up with a cast although they are in fact neutral. This means
that even a medium gray still has a yellow cast on a yellow paper. In
printing, this corresponds to the use of the same printing plates for
papers with different colors (paper white).
Note: From the technical point of view, this corresponds to relative
colorimetry with shadow compensation.
With the Neutral Gray option, the substrate color or paper color only
affects lighter areas. In the other color areas, the colors and gray
axis of the image are preserved to the best possible extent, and are not
influenced by the paper white.
Note: This corresponds to relative colorimetry in highlight areas and
absolute colorimetry with shadow compensation in the remaining colors.
Note: The default and recommended setting is Neutral Gray.
For example, if B/W and color images are positioned next to each other
in reproduction, the Paper Gray Axis option should be selected to
achieve the same impression in the neutral tones both in the B/W images
and in the color images. Conversely, if an ad is produced that must be
reproduced with identical results in offset printing and in gravure
(with the exception of highlights that are allowed to differ on account
of the different papers used), the Neutral Gray option should be used
when generating the offset and gravure profiles.
Note: The Perceptual Rendering Intent calculation options Paper Gray or
Neutral Gray don't affect the proofing process. In the proofing process
the Relative Colorimetric or Absolute Colorimetric Rendering Intent is
used."
I'm not really persuaded of the assumptions and recommendations made in
the PM documentation.
I guess we can roughly differentiate between two situations in practice:
a) adaptation to paper white which is probably the most common case in
commercial printing, where in general some plain paper is still visible
b) adaptation to some other white which might be the case when one views
e.g. a postcard, a borderless photo print or a framed picture (with
passe-partout)
PM's "Neutral Gray" option is -- if anything -- applicable to situation
(b), but there are two pitfalls
- the adopted white point is not essentially "neutral" (i.e. conditioned
only by the chromaticity of the light source, but might be affected e.g.
by the color of the passe-partout)
- if there is whatever transition of the neutral axis from "neutral" in
the shadows and mid-tones to paper color in the highlights, the
highlights might still exhibit a color cast.
Klaus
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