A Duotone filter for Imagine PHP library.

Duotone.php

<?php

use Imagine\Image\Point;
use Imagine\Image\ImageInterface;
use Imagine\Filter\FilterInterface;
use Imagine\Filter\Advanced\OnPixelBased;
use Imagine\Image\Palette\Color\RGB;

/**
 * Duotone
 *
 * A filter class for creating duotone images with Imagine.
 *
 * @author    Nicholas Charbonneau
 * @license   http://opensource.org/licenses/MIT
 * @link      https://gist.github.com/nicholasc/e263d976d3aa0304537ea2f075ba7906
 */
class Duotone extends OnPixelBased implements FilterInterface
{
    /** @var array A matrix of all red duotone values based on a grayscale index. */
    protected $redPixels = array();

    /** @var array A matrix of all green duotone values based on a grayscale index. */
    protected $greenPixels = array();

    /** @var array A matrix of all blue duotone values based on a grayscale index. */
    protected $bluePixels = array();

    /**
     * Duotone filter class constructor.
     *
     * @param \Imagine\Image\Palette\Color\RGB $light The lightest color in the dual tone.
     * @param \Imagine\Image\Palette\Color\RGB $dark The darkest color in the dual tone.
     *
     * @return void
     */
    public function __construct($light, $dark)
    {
        // Calculate averages between light and dark colors
        $redAvg = $light->getRed() - $dark->getRed();
        $greenAvg = $light->getGreen() - $dark->getGreen();
        $blueAvg = $light->getBlue() - $dark->getBlue();

        // Create a matrix of all possible duotone colors based on gray values
        for($i = 0; $i <= 255; $i++) {
            $grayAvg = $i / 255;
            $this->redPixels[$i] = $dark->getRed() + $grayAvg * $redAvg;
            $this->greenPixels[$i] = $dark->getGreen() + $grayAvg * $greenAvg;
            $this->bluePixels[$i] = $dark->getBlue() + $grayAvg * $blueAvg;
        }

        parent::__construct(function(ImageInterface $image, Point $point) {

            // Retrieve pixel information
            $color = $image->getColorAt($point);

            // Get the grayscale value for this pixel
            $gray = min(255, round(0.299 * $color->getRed() + 0.114 * $color->getBlue() + 0.587 * $color->getGreen()));

            // Draw the pixels based on the duotone matrix
            $image->draw()->dot(
                $point,
                new RGB(
                    $color->getPalette(),
                    array(
                        $this->redPixels[$gray],
                        $this->greenPixels[$gray],
                        $this->bluePixels[$gray]
                    ),
                    100
                )
            );

        });
    }
}