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Lab (CIELAB) Color Space

Lab (also known as CIELAB or L*a*b*) is a perceptually uniform color space designed to approximate human vision. It's widely used in color matching, color analysis, and applications where perceptual uniformity is important.

Channels​

Lab has three channels:

  • l (Lightness): Perceptual lightness, 0-100
  • a (Green-Red axis): Green (negative) to Red (positive)
  • b (Blue-Yellow axis): Blue (negative) to Yellow (positive)

Channel Ranges​

  • Lightness (l): 0-100
    • 0 = Black
    • 50 = Middle gray
    • 100 = White
  • a channel: Typically -128 to 127 (theoretical range is wider)
    • Negative values = Green
    • Positive values = Red
    • 0 = Neutral (neither green nor red)
  • b channel: Typically -128 to 127 (theoretical range is wider)
    • Negative values = Blue
    • Positive values = Yellow
    • 0 = Neutral (neither blue nor yellow)

Default Values​

All channels default to 0 when not specified, resulting in middle gray (lab(0, 0, 0)).

CIE Standard Support​

Lab supports CIE Standard Illuminants and Observers, which are essential for accurate color representation:

  • Illuminants: A, B, C, D50, D55, D65, D75, E, F1-F12
  • Observers: 2° (1931) and 10° (1964)

The default is D65 illuminant with 2° observer, which is standard for most applications.

Creating Lab Colors​

use Negarity\Color\Color;

// Create a Lab color with default illuminant/observer
$color = Color::lab(50, 20, -30);

// Create with specific illuminant and observer
$color = Color::lab(50, 20, -30, 
    CIEIlluminant::D50, 
    CIEObserver::TenDegree
);

Working with Lab Channels​

Getting Channel Values​

$color = Color::lab(75, 25, -40);

$l = $color->getL();  // 75
$a = $color->getA();  // 25
$b = $color->getB();  // -40

Checking Channel Existence​

use Negarity\Color\ColorSpace\Lab;

Lab::hasChannel('l');  // true
Lab::hasChannel('a');  // true
Lab::hasChannel('b');  // true
Lab::hasChannel('c');  // false (that's LCh)

Getting Channel Defaults​

use Negarity\Color\ColorSpace\Lab;

Lab::getChannelDefaultValue('l');  // 0
Lab::getChannelDefaultValue('a');  // 0
Lab::getChannelDefaultValue('b');  // 0

Getting All Channels​

use Negarity\Color\ColorSpace\Lab;

Lab::getChannels();  // ['l', 'a', 'b']

Checking CIE Support​

use Negarity\Color\ColorSpace\Lab;

Lab::supportsIlluminant();  // true
Lab::supportsObserver();  // true

Validation​

Lab validates channel values:

use Negarity\Color\Color;
use Negarity\Color\Exception\InvalidColorValueException;

try {
    // This will throw InvalidColorValueException
    $color = Color::lab(150, 20, -30);  // Lightness exceeds 100
} catch (InvalidColorValueException $e) {
    // Handle error
}

Understanding Lab​

Perceptual Uniformity​

Lab is perceptually uniform, meaning:

  • Equal changes in Lab values produce equal perceived color differences
  • This makes it ideal for color matching and color difference calculations
  • The distance between two colors in Lab space (ΔE) correlates with perceived difference

The a and b Axes​

  • a axis: Represents the green-red dimension
    • Negative = Green
    • Positive = Red
  • b axis: Represents the blue-yellow dimension
    • Negative = Blue
    • Positive = Yellow

Lightness​

The L channel represents perceptual lightness, which is more accurate than RGB brightness or HSL lightness for human perception.

Use Cases​

  • Color Matching: Matching colors across different devices
  • Color Analysis: Analyzing color differences and similarities
  • Quality Control: Ensuring color consistency in manufacturing
  • Color Science: Research and color management
  • Image Processing: Perceptually uniform color adjustments

Color Space Information​

use Negarity\Color\ColorSpace\Lab;

Lab::getName();  // "lab"
Lab::getChannels();  // ['l', 'a', 'b']
Lab::supportsIlluminant();  // true
Lab::supportsObserver();  // true

See Also​