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LCh Color Space

LCh (Lightness, Chroma, Hue) is a polar representation of Lab color space. It's particularly useful for perceptual color manipulation and creating color harmonies, as it separates lightness, color intensity, and color direction.

Channels​

LCh has three channels:

  • l (Lightness): Perceptual lightness, 0-100
  • c (Chroma): Color intensity or saturation, 0-100+
  • h (Hue): Color direction, 0-360 degrees

Channel Ranges​

  • Lightness (l): 0-100
    • 0 = Black
    • 50 = Middle gray
    • 100 = White
  • Chroma (c): 0-100+ (theoretical maximum depends on lightness and hue)
    • 0 = No color (gray)
    • Higher values = More intense color
  • Hue (h): 0-360 degrees
    • 0° = Red
    • 90° = Yellow
    • 180° = Green
    • 270° = Blue
    • 360° = Red (wraps around)

Default Values​

All channels default to 0 when not specified, resulting in black (lch(0, 0, 0)).

CIE Standard Support​

LCh supports CIE Standard Illuminants and Observers, just like Lab:

  • 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.

Creating LCh Colors​

use Negarity\Color\Color;

// Create an LCh color with default illuminant/observer
$color = Color::lch(50, 30, 210);

// Create with specific illuminant and observer
$color = Color::lch(50, 30, 210,
    CIEIlluminant::D50,
    CIEObserver::TenDegree
);

Working with LCh Channels​

Getting Channel Values​

$color = Color::lch(75, 45, 240);

$l = $color->getL();  // 75
$c = $color->getC();  // 45
$h = $color->getH();  // 240

Checking Channel Existence​

use Negarity\Color\ColorSpace\LCh;

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

Getting Channel Defaults​

use Negarity\Color\ColorSpace\LCh;

LCh::getChannelDefaultValue('l');  // 0
LCh::getChannelDefaultValue('c');  // 0
LCh::getChannelDefaultValue('h');  // 0

Getting All Channels​

use Negarity\Color\ColorSpace\LCh;

LCh::getChannels();  // ['l', 'c', 'h']

Checking CIE Support​

use Negarity\Color\ColorSpace\LCh;

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

Validation​

LCh validates channel values:

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

try {
    // This will throw InvalidColorValueException
    $color = Color::lch(150, 30, 210);  // Lightness exceeds 100
} catch (InvalidColorValueException $e) {
    // Handle error
}

try {
    // This will also throw InvalidColorValueException
    $color = Color::lch(50, 30, 400);  // Hue exceeds 360
} catch (InvalidColorValueException $e) {
    // Handle error
}

Understanding LCh​

Relationship to Lab​

LCh is a polar (cylindrical) representation of Lab:

  • L (Lightness): Same as Lab's L channel
  • C (Chroma): Distance from the neutral axis (calculated from a and b)
  • H (Hue): Angle around the neutral axis (calculated from a and b)

The conversion is:

  • C = sqrt(a² + b²)
  • H = atan2(b, a)

Advantages of LCh​

  1. Intuitive Manipulation: Easy to adjust lightness, saturation (chroma), and hue independently
  2. Color Harmonies: Simple to create complementary, triadic, or analogous colors
  3. Perceptual Uniformity: Inherits Lab's perceptual uniformity
  4. Separation of Concerns: Lightness, color intensity, and color direction are clearly separated

Chroma vs Saturation​

  • Chroma: Absolute color intensity (distance from gray)
  • Saturation: Relative color intensity (chroma relative to lightness)

In LCh, chroma is absolute, making it easier to work with than HSL's saturation.

Use Cases​

  • Color Harmonies: Creating complementary, triadic, or analogous color schemes
  • Perceptual Color Manipulation: Adjusting colors in a perceptually uniform way
  • Color Design: Intuitive color adjustments for design work
  • Color Analysis: Analyzing color relationships and differences
  • Accessibility: Adjusting colors while maintaining perceptual relationships

Color Space Information​

use Negarity\Color\ColorSpace\LCh;

LCh::getName();  // "lch"
LCh::getChannels();  // ['l', 'c', 'h']
LCh::supportsIlluminant();  // true
LCh::supportsObserver();  // true

See Also​