Re: some thoughts on CIELAB was Dry Time for UltraChromeinks/Calibration
Re: some thoughts on CIELAB was Dry Time for UltraChromeinks/Calibration
- Subject: Re: some thoughts on CIELAB was Dry Time for UltraChromeinks/Calibration
- From: Robin Myers <email@hidden>
- Date: Thu, 06 Feb 2003 07:54:48 -0800
Graeme Gill wrote:
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>
tlianza wrote:
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> 6. Finally, when plotted in Lab space, most hardcopy devices, "hooked" which
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> implied that numerical mappings would not be single valued....very "uncool"
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> mathematically. If the devices included a pre linearization and limiting
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> step, then the hooking was minimized.
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>
Are you sure about this ? In all the messing about I've done
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with forward and reverse mappings between device and L*a*b* values,
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I don't seem to have stumbled across any instances of non-monotonicity
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that could be attributed to using L*a*b* space. XYZ space is generally
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a worse choice from many points of view unless you have a device model
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that particularly suits it (such as a CRT additive model or Neugenbauer
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printing model), and other appearance model spaces are generally more complicated
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and controversial. As a rough perceptual space, and the one that is used
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for as the basis for many color difference measures (including CIE94,
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CMC etc.), it doesn't seem that bad a choice, particularly when values
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are going to be quantized in some fashion.
I cannot count the number of profiles that I have had to tune to remove
the results of the extreme curvature and "hooking" of the L*a*b*
encoding. It has been especially prevalent in blue sky images on film.
You can easily discover this "hooking" encoding effect by taking values
for constant hue and lightness (I used Munsell values) and plot them on
an a*b* diagram.
As for XYZ being a worse choice I disagree. It is linear in color
mixing, all the coordinate values are positive, when projected onto xy
diagrams, lines of constant hue exhibit much less curvature. The only
objection I have heard that has some merit is that it is more difficult
to develop simple perceptual models for XYZ. However, I have
successfully used gamut compression/expansion, selective colorant
adjustment and other techniques used in perceptual transformations in
XYZ space. It requires using 3D XYZ vector calculations rather than
simple 2D a*b* calculations. Also, since many color matching
computations require conversion to and from XYZ space as an intermediary
(e.g. RGB to L*a*b* usually requires RGB to XYZ to L*a*b*), then there
are computational benefits to using XYZ as the PCS.
For a really interesting weird view of L*a*b* space and how it distorts
things, take a look at the spectral locus, including the purple line,
converted into L*a*b*. Do not use the diagram in Wyczecki and Stiles
since it is a monochrome image and difficult to visualize the effect.
Robin Myers
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