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IMCO: Describing color

Posted Sep 9, 2021 Updated Oct 3, 2021 5 min read views

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Describing a relative color

  • Need of a vocabulary to describe a color
  • 2 (or more) objects may be all blue, but different
    • Need additional descriptions within colors
  • How do we organize colors ?

Blue jacket ?

All of this is blue

This is navy blue

This is dark navy blue

How ? - Desert Island Experiment

A person with normal color vision is on an island and need to group pebbles

  1. Think about colo in terms of common color names (red, blue, green, etc.)
  2. How to arrange achromatic samples ?
    • Orders from darker to lighter
  3. How to arrange chromatics smaples ?
    • By color (hue)

    • Example: group all red pebbles together

  4. How to arrange chromatic samples of the same hue ?
    • order by lightness
    • order by how much color they contain (chroma)

Color attributes

We found color attributes

  • Pyschological attributes that describe colors:
    • Hue
    • Lightness
  • achromatic color: perceived with lacking hue
  • chromatic color: perceived color with hue

Elementary colors: white, black, red, green, yellow, blue

  • unique hues according to opponent color theory

Color Order Systems

Munsell Color system

Composition:

  • Hue
    • 10 hues (each divided into 10 subhues)
  • Lightness (called Value)
    • 11 steps (0: ideal black, 10: ideal white)
  • Chroma
    • Range depending on the hue

Notation: H V/C

  • H = Hue, V = Value, C = Chroma
  • e.g. 5Y 7/12 or 5R 1/4

Munsell Color Tree from Pantone

How to communicate color ?

Pantone Color-Naming System

  • Used in the printing/manufactuing industry
  • Swatches are used to specify colors
  • Printed using 14 inks
  • Useful for specifying communicating color
  • Patented ! Need a license to use the list

Describing Relative Color

Color Mixing Systems

Amounts give a specification, not the resulting color

  • RGB value ${100, 20,90}$ in your screen $\neq$ RGB ${100, 20,90}$ in my screen
  • CMY value ${90, 10,50}$ in your printer $\neq$ CMY ${90, 10,50}$ on my printer
  • RGB value ${100, 20,90}$ in my screen $\neq$ CMY ${90, 10,50}$ on my printer

HSV/HSL Spaces

  • Hue: color
  • Saturation: measure of chroma
  • Value or Lightness: measure of lightness

HSV/HSL difference:

  • HSL: maximum lightness = white
  • HSV: maximum Value = “intense” color

HSV $\leftrightarrow$ RGB

YCbCr Space

  • Y is Luminance $\simeq$ Brightness
  • Cb is related to blue Chrominance
  • Cr is related to red Chrominance

Used a lot in color compression

Decomposition

Divide image in RGB

Divide Image in HSV

Divide image in YCbCr

Use Case - Color Segmentation

However, as RGB and CMYK, resulting color is only relative to capture condition

Describing Standard Color

  • Use an universal way (i.e., numbers) to communicate color instead of using names
  • Need to standardize the color forming conditions
    • Illuminant (viewing lighting)
    • Observer (the human visual response to a given stimulus)
    • Object reflectance ($\lambda$-dependent spectral measurement)
  • Colorimetry

CIE Standard Observers

  • Light sources (primaries): $435.8nm$, $546.1nm$, $700nm$
  • CIE 1931 Standard Colorimetric Observer
    • $2^o$ visual angle ($2^o$ Observer)
    • $17$ color normal observers
  • CIE 1964 Standard Colorimetric Observer
    • $10^o$ visual angle ($10^o$ Observer)
    • 76 color normal observers

CIE Standard Iluminants

  • D: Different types of daylight
    • D50 ($\color{cyan}{5003K}$) (warm daylight)
      • printing/graphic arts
    • D65 ($\color{blue}{6504K}$) (natural daylight)
      • art/film/photography
  • A: incandescent lamp ($\color{orange}{2856K}$)
  • F: Fluorescent light
    • F2 (4230K), F11 (4000K)

Color Temperature

Temperature uses as a reference an ideal object called “Black Body”

CIE Standard Iluminants

  • Warm: $T \lt 3300K$
    • Dormitory, Restaurant, Hotel, Coffee Shop
  • Intermediate: $3300K\lt T\lt 5300K$
    • Stores, Shcool, Libraries
  • Cold: $T\gt 5300K$
    • Offices, Hospitals

Computing XYZ Tristimulus Values

\[\begin{aligned} X &= k\int_{\lambda}I(\lambda)R(\lambda)\bar x_{\lambda}d\lambda\\ Y &= k\int_{\lambda}I(\lambda)R(\lambda)\bar y_{\lambda}d\lambda\\ Z &= k\int_{\lambda}I(\lambda)R(\lambda)\bar z_{\lambda}d\lambda\\ \end{aligned} \quad k =\]

Chromaticity Diagrams: CIE 1931

All Hues are perceivable by the standard observer

MacAdam Ellipses Where are the grey ? Brown ? No light info !

Uniform color spaces

  • CIE XYZ Space is not perceptually uniform
    • equal perceptual differences between colors $\neq$ equal distances in the XYZ space
  • CIE recommenced uniform color spaces
    • CIE 1976 $L\times u\times v$
    • CIE 1976 $L\times a\times b$

Example

In matplotlib:

CIE $L\times a \times b$ Space

\(L^* = 116f(\frac{Y}{Y_n})-16 \begin{cases} a^* = 500[f(\frac{X}{X_n})-f(\frac{Y}{Y_n})]\\ b^* = 200[f(\frac{X}{X_n})-f(\frac{Y}{Y_n})] \end{cases}\)

$L\times C\times h$

What’s left ?

  • Modeling cognitive effects or phenomena
  • How to obtain absolute color attributes (brightness and colorfulness) ?
  • Debate about accuracy of $\bar x, \bar y, \bar z$ Color Matching Functionc
    • Representativeness of the population used for the experiments
    • Limitaations of equipment used for the experiments
    • How to understand color perception of “color anomalous” observers ?
Image S9, IMCO
Image S9 IMCO
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