Post by Gernot HoffmannDanny,
Post by Danny RichThis is again not true. This is a standard
**** Major typographic errors here ****
The sentence should have read: "This is again not true. There is a
standard - true but the lots are sorted
into those that are 1 unit away, 2 units away and so one. Garments may only
be cut from lots that are 3 units away from the target. They cannot afford
to throw that cloth away - unlike printing where the first 2 to 3 hours of
printing are scrapped"
The point being that the textile industry use CMC or CIE94 to acutually
assess absolute color difference and seems to achieve the goal of sorting
fabric into specific lots of absolute differences from the aim point
effectively.
Post by Gernot HoffmannWhat was it, in your opinion, that "again was not true"? Yes, of course
the dE formulas are standards. But they are incorrect/faulty standards,
they do not deliver what they are said to deliver. They do not, at all,
quantify visual color difference.
Post by Danny RichI never claimed that dE is perceptually uniform - across large distances
in color space.
That is so. But here we are discussing only about small color differences.
Post by Danny RichI merely disagreed with your assertion that dE is not useful for judging
color differences.
Here you say that we disagree. But at the end of your message you indeed
do agree that dE is not useful for judging (small) color differences.
Where did I say that color difference or color tolerance formulas are not
useful? They absolute are useful and used daily. Many psychophysics
studies have been conducted in which the visual differences are ranked or
magnitude scaled and those scales show strong correlations with the
numerical predictions of modern color difference or color tolerances
equations. Are the correlations perfect - no but then if I show the same
set of colors to the same observer a second or third time the observer will
not have perfect correlation with her own judgments. Self generated visual
errors can occur in a much as 40% of the judgments on small to moderate
color differences (<2 CIELAB deltaE units). CIEDE2000 has been reported to
have errors of only 32% to 38% so the equation is about as consistent as
normal human observer.
Post by Gernot HoffmannPost by Danny RichAnd the Coil Coaters Association issues a booklet with colored chips
mounted side by side showing differences of CMC dE at 1
and 5 that are visually uniform.
So now you claim that, instead of the CIE dE functions, it is the CMC
function that is visually uniform???
The CIE(1994) is derived from CMC. Could you please tell me what (l,c)
parameters of the CMC were used in that booklet? I will add the CMC
comparison to the evaluation page as soon as I find my copy of the
original spec (BS:6923 "Method for calculation of small colour
differences"). But it really does not notably differ from the CIE(1994).
The CIE94 equation was not derived from CMC. Both are derived from CIELAB
using different sets of observations and optimization criterion. Similarly
derived was the Bradford equation DeltaE(BFD). And these equations should
not be conceptualized as being more visually uniform but as distorting the
spacing of the CIELAB metric to better match the results of visual
judgements of small and moderate color differences.
The are two major differences between CMC and CIE94 and one major
similarity. Both equations use a hyperbolic weighting function to expand
the volume of the visual difference region. This region is a sphere in
CIELAB but is a spheroid - not quite an ellipse - in CMC and CIE94. The
main difference between CIE94 and CMC is that CMC has a hyperbolic weighting
on the lightness difference DeltaL* which expands the spheroid in the
lightness direction as the lightness increases. CIE94 keeps the CIELAB
spacing of lightness because in the studies that were done on smooth
surfaces and CRT images of color patches no lightness non-uniformity could
be detected in the visual observerations. The second area of difference
between the two tolerance equations in the weighting for the metric hue term
DeltaH*. CMC has both a chroma and hue function but CIE94 has only a chroma
function.
The (l,c) terms are additional weights to expand or contract the shape of
the ellipsoid for certain "parametric" effects. Parametric effects include
things like how close together the specimens are placed, how smooth the
surfaces are or whether there are goniochromic additives in the specimen
like metal flakes, pearlescent or fluorescent materials and so forth. For
textiles and plastics with a weave pattern or rough surface it is not
possible to judge lightness differences as effectively as when the surface
is smooth so the axis of the spheroid in the DeltaL* direction is expanded
by an amount (l,) and the a similar additional correction may be added to
the axis in the DeltaC* direction (,c). There is no weight in the DeltaH*
direction since it was known at the time that hue was always the most
critical parameter and its weights should always be 1 so there is an implied
ratio of L to C to H so only the L and C weights are shown as (l,c) or
sometimes as (l:c). New tolerance equations such as CIE94 and CIEDE2000
allow for non-proportional weights on each axis, resulting in a set (l:c:h)
of parametric weights.
Post by Gernot HoffmannPost by Danny Richwhat have we proved? That CIELAB dE is not adequate, that CMC is better,
the CIE94 is about as good and that CIEDE2000 is perhaps
slightly better for acceptance sampling of small but not neglibile
differences.
I have proved that the CIE color difference functions (CIELAB dE, CIE94
and CIEDE2000) are not good at all for any kind of quantification of
visual color difference, at least not on a color-managed monitor screen.
Post by Danny RichI continue to be fully puzzled by this since every other study that has
ever been reported has shown that L* is the only
component that is truly well mapped and uniform.
Why is it that you are puzzled? Or do you allude that the evaluation I
have provided is somehow faulty?
Post by Danny RichIt agrees quite well with the Munsell scale used by artists and the
scales used in NTSC and PAL b/w broadcast standards. Most
of these studies are not involved in judging acceptability differences
but in scaling perceived lightness of object
surfaces viewed against a 19% gray background and 2000 lux.of
simulated daylight of 6500K to 6700K correlated color temperature.
My evaluation has about 19% gray background. A patch on the CRT screen
that is at L*=50 has something like 20cd/m2 luminance. On a similar real
object surface the 2000 lux would translate to something like 130cd/m2
luminance so a rather large difference there. Do you allude that the CIE
color difference formulas fail because of such difference in absolute
luminance?
This could be one possible issue. It is known that in the mesopic region -
(luminances less than 50 and more than 5) there is both rod intrusion into
the cone responses as it seems that blue cones and the rods share the visual
pathways to the visual cortex.
Post by Gernot HoffmannPost by Danny RichIn your display - referenced above - you have a page of 8 unit differences
http://www.aim-dtp.net/aim/evaluation/cie_de/index.htm
you will find that there also are similar evaluations for dE=5, dE=3, dE=2
and dE=1.
Post by Danny Richin which the only differences perceptable, at least on my monitor, are
the lightness differences.
Then there is something very wrong with your monitor/system.
Post by Danny RichAt 8 units - we should not any longer be looking at color differences but
at different colors
This clearly is not so. dE=8 is rather small color difference in many
cases when that dE=8 is due to a difference in a* and/or b* only (when the
L* is the same).
The steps between chips in the Munsell or NCS system are about 8 to 10
CIELAB units - I do not call those small differences.
Post by Gernot HoffmannPost by Danny RichThis is in agreement with my 30 years of experience and the data in the
literature that lightness is well modeled but chromaticness and hue are
not.
Thank you very much. In other words: The CIE dE functions are not at all
usable functions for the task of quantification of visual color difference.
Post by Danny RichMore importantly - I find that perceived difference between the center of
the block and upper and lower lightness limits are far more consistent
from color center to color center than are the spreads of the other
attributes where sometimes they seem easily perceived and others they
are completely imperceptible.
In other words, those "lightness" differences are large (enormous, huge)
compared to those differences that are due to a change in a* and/or b*
only. That they are consistently huge, is not a good thing, is not
A Color Difference Function should reliably quantify
the visual difference between two colors.
Timo Autiokari