Grayscale

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20 Sep 2026 - Nicola Asuni

The grayscale model holds one component, 0.0 for black to 1.0 for white. DeviceGray takes a single operand per colour operator, so a monochrome document written in it is smaller than the same document in DeviceRGB.

Model

Item	Value
Type string	`GRAY`
Enum case	`ColorModelType::Gray`
Class	`\Com\Tecnick\Color\Model\Gray`
PDF colour space	DeviceGray
Notations	`g(128)`, `g(50%)`, `["G",0.5]`

g() is a notation of this library, not a CSS function. getCssColor() emits it and getColorObj() reads it back. A browser does not parse it.

Components

Component	Key	Range	Out of range
Gray	`gray`	0.0 to 1.0	clamped
Alpha	`alpha`	0.0 to 1.0, default 1.0	clamped

Construction

<?php

require_once __DIR__ . '/vendor/autoload.php';

$gray = new \Com\Tecnick\Color\Model\Gray(['gray' => 0.371920]);

echo $gray->getCssColor(), "\n";
echo $gray->getPdfColor();
echo $gray->getRgbHexColor(), "\n";

g(95)
0.371920 g
#5f5f5f

Output

Every accessor, against #336699:

Call	Result
`getCssColor()`	`g(95)`
`getRgbHexColor()`	`#5f5f5f`
`getRgbaHexColor()`	`#5f5f5fff`
`getComponentsString()`	`0.371920`
`getPdfColor()`	`0.371920 g`
`getPdfColor(true)`	`0.371920 G`
`getJsPdfColor()`	`["G",0.371920]`
`getArray()`	`['G' => 0.37192000000000003, 'A' => 1.0]`
`getNormalizedArray(255)`	`['G' => 95.0, 'A' => 1.0]`
`getPDFacArray()`	`[0.37192000000000003]`

The Weights

The conversion from RGB uses the Rec. 709 luma coefficients: 0.2126 red, 0.7152 green, 0.0722 blue. Green carries most of the weight, so full green greys to 182 and full blue to 18.

$web = new \Com\Tecnick\Color\Web();

foreach (['#ff0000', '#00ff00', '#0000ff', '#336699'] as $hex) {
    $gray = \Com\Tecnick\Color\Model::create('GRAY', $web->getColorObj($hex)->toGrayArray());
    printf("%-9s %s\n", $hex, $gray->getCssColor());
}

#ff0000   g(54)
#00ff00   g(182)
#0000ff   g(18)
#336699   g(95)

An unweighted mean of the three channels of #336699 is 102; the luma weights give 95.

toRgbArray() writes the grey value into all three channels.

Conversions

toHslArray() returns hue 0 and saturation 0, and a round trip through HSL returns the same grey. Keep the source colour where the hue is needed later.