CIE Standards Comparison Tool for IQColour.com web site

this Google Document is managed by Michael Jahn and is is related to the online CIE Standards Comparison Tool we have build for your inspection

This Google Document is a prototype and a 'work in progress" - so please excuse typos and feel free to email us at mjahn@iqcolour.com

- feel free to visit this site and use the tool by clicking the link below;

http://www.iqcolour.com/res_colorcompare.html

What is this thing ?
The IQColour CIE Standards Comparison Tool - or Calculator - is a tool for studying and comparing the historical iterations of the CIE standard formulas. 

What are CIE Standard formulas ?

Measured color differences are computed in several different ways, and this has changed significantly over the years as color scientist learn more.

CIE Delta E 76

This is the standard CIE color difference method, (circa 1976, hence the 76 moniker )which is simply the distance between the two colors, calculated in three-dimensional Lab color space.

The color difference, or ΔE, between two colors L₁a₁b₁ and L₂a₂b₂ is:

CIE Delta E 94

This is a more recent modification which has been recommended by CIE TC1-29 as the CIE94 color difference formula. For graphic arts applications, K₁ = 0.045 and K₂ = 0.015. For textile applications, K₁ = 0.048 and K₂ = 0.014.

The color difference, or ΔE, between a sample color L₂a₂b₂ and a reference color L₁a₁b₁ is:

where

CIE Delta E 2000

This is a even more recent modification. In this implementation K_L = K_C = K_H = 1.0.

The color difference, or ΔE, between a sample color L₂a₂b₂ and a reference color L₁a₁b₁ is:

where

Implementation Notes:

1. The angles supplied to the sin and cos functions are shown in degrees. Most math libraries expect radians, so don't forget to convert.
2. The inverse tangent is also expressed in degrees. In most math libraries, the inverse tangent returns radians, so don't forget to convert.
3. In computing hue angles, be careful with the inverse tangent since a could be zero. Instead, use special math functions to do this. In both the Standard C library and Java, this function is called atan2. In Microsoft Excel, it is called ATAN2. These special functions will compute the proper inverse tangents without needing to worry about "divide by zero" conditions.

CMC (1:1)

This method is drafted to become a new ISO standard (ISO 105-J03). This implementation uses a lightness weight of 1.0 and a chroma weight of 1.0 for use with perceptibility data.

The color difference method of the Color Measurement Committee (the CMC) is a model using two parameters l and c, typically expressed as CMC(l:c). Commonly used values for acceptability are CMC(2:1) and for perceptability are CMC(1:1).

The color difference, or ΔE, between a sample color L₂a₂b₂ and a reference color L₁a₁b₁ is:

where

Implementation Notes:

1. H₁ is in degrees, not radians.
2. If H₁ < 0°, add 360° to it.
3. If H₁ ≥ 360°, subtract 360° from it.
4. In computing H₁, be careful with the inverse tangent since a could be zero. Instead, use special math functions to do this. In both the Standard C library and Java, this function is called atan2. In Microsoft Excel, it is called ATAN2. These special functions will compute the proper inverse tangents without needing to worry about "divide by zero" conditions.

CMC (2:1)

As above, but using a lightness weight of 2.0 and a chroma weight of 1.0 for use with acceptability data.

Note that for some of the Delta E methods, the color differences are not commutative. That is, the difference between Color A and Color B may not be the same as the difference between Color B and Color A. In such cases, one color must be understood to be the reference or standard against which a sample color is compared.

to try this tool for yourself - go here --> http://www.iqcolour.com/res_colorcompare.html

Using the sliders, the user can quickly see how well (or how poorly) each formula report “differences” – or DeltaE – against selected reference (REF) colours at different levels of saturation, lightness and “tolerance”.

In the display area above the user interface (the sliders) in the large gray square area, there are 6 sets of smaller squares arranged in a circular fashion.

Each of these squares are entitled with the CIE formula they represent visually.

Within each of these 6 formula displays, you can see that they are divided into 12 smaller squares.

I have placed numbers inside each of the 12 squares as a “key” so you might see exactly that each square represents related to their change in position on the Chromaticty Diagram.

L+ is a positive change in Lightness, the two squares in the center are the "Reference color (they color we are using, selected in the top slider) and the L- is a negative change in Lightness for the reference colour.

When we change the bottom slider on the user interface to the right, each formula is processed against a selected hue in the top slider – moving the bottom slider to the left represents a small degree of difference and as we move this slider to the right the degree of difference increases and the squares change according to the diagram.

As you move to the right, what you are 'seeing' is a greater amount of "change" along to CIE hue lines.

In this bottom slider we use the word Tolerance in a similar manner as the term DeltaE ’difference’ but not ‘exactly’

As you know, in most cases an observer cannot perceive a ‘difference” at Tolerance:1 or a single DeltaE – where at At Tolearnce:4, most people can perceive a difference.

The data set used was obtained from Color Science: Concepts and Methods, Quantitative Data and Formulae, Wyszecki and Stiles pp. 840-852, where it is tabulated as xyY, relative to the Color Computer module currently uses D65 illuminant/2 degree observer - (we assume sRGB).

With the calculator we are actually displaying in our ATD or “IQRGB” color space.

So - as you move the slider from left to right, what you see is changes across the hue line, as demonstrated in the animation below

How do I use it ?

Move the sliders around – each formula is applied to the same color you select on the top (Hue) slider – this is the REF (or Reference) color – this color is displayed in the center two squares of each of the 6 formula displays. As you change saturation and darkness, you can see how well the different formuals predict ‘difference” – in particular, you can see how miserably thr CMC formula predicts darkness difference, and how changes in saturation fail to illicit an expected difference in CIE Delta E 2000.

What am I looking at ?

This tool is designed for color scientist, color management professionals and color measurement manufacturers to better explain the difference between the different standards where one can visually see where problems exist. We feel this clearly demonstrates that one of the biggest problems in the current formulas are predicting “difference” properly as we move along a Q line.

Whats the issue ?
In order to capture and reproduce color reliably and repeatably, we are dependent on measuring color, and accurate applications and instruments to tell us – in succinct numerical values – what that color is. Once we know what the numerical data of given color we can then use applications and tools that can helps knowing what we need to do to convert that into the desired color.

While this seems straight forward enough, it is not a simple thing to accomplish. Since the first initial studies of how the human eye perceives color – to 1931 where the Commission International de l’Eclairage (CIE) moved to define a standard system – to 1976 when Fortner and Meyer pointed out that there are some strange things about the 1931 CIE standard chromaticity diagram which “devotes an enormous amount of real estate to various green shades” and less space to colors like the reds and purples which are more differentiable to the eye – to several other studies, formula tinkering, adjustments and balloting – we are in somewhat of a mess – as this tool will help show.

The process of converting color, therefore – is fraught with complex and sometime “hidden” issues that are hid to the advantage of proprietary vendor solutions. Gamut mapping of colors generally involves finding substitute colors that “look like” the desired color. Profile creation software commonly performs these substitutions by moving the out-of-gamut color along a line of constant Lab hue angle. But experiments and experience have shown that colors sharing the same Lab hue angle do always not share the same apparent hue. The result is that a substituted color may have a completely different apparent hue than the original color had. A notorious example is the “blue turns purple” problem (special thanks to Bruce Lindbloom for this image!).

Many people have experienced the disappointment of printing an image, only to find that the rich blues seen on their monitor have turned purple on the print.

The problem starts with the observation that a typical full blue on a monitor simply cannot be reproduced on a printer. Since an accurate reproduction is physically impossible, a substitute color must be used in its place. It would be nice if this substitute color was the same apparent hue, although perhaps less saturated. A printer profile will typically choose the substitute color by selecting a color of similar hue angle, as measured in the CIE Lab color system. Unfortunately, the Lab color system is not perfect in its ability to match its hue angle with perceptual hue. This is easier to understand by looking at a picture:

This should not be blamed on Lab, since it was designed to measure color differences.

It was not designed to have the perceptual qualities needed for gamut mapping.

Using LAB for this purpose is really a misuse, resulting primarily from a lack of a better alternative. It was never intended or designed to be used as a 'color mode' - nor should it be expected to perform properly as a Profile Connection Space (PCS) - this is what if fundamentally wrong with most popular applications that use the Adobe Color Engine (ACE)

Now what do I do ?

IQColour mission is to encourage an alternative that is developed in the imaging industry by internationally color scientist employed by device manufacturers and software developers.

Please do visit our web site and read our white papers !

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