Example

/**
 * Downsamples an image to be a lower resolution, while keeping the same canvas dimensions.
 * aka combines the pixels into 'blockier' pixels.
 *
 * @param Imagick $imagick The image to use
 * @param int $pixel_sample_rate dimensions of the pixel blocks.
 */
function downSampleImage(Imagick $imagick, int $pixel_sample_rate)
{
    $width = $imagick->getImageWidth();
    $height = $imagick->getImageHeight();

    // For each of the channels, downsample to reduce the image information
    // then resize back the the original image size.
    $imagick->resizeimage(
        $width / $pixel_sample_rate,
        $height / $pixel_sample_rate,
        Imagick::FILTER_LANCZOS,
        1.0
    );
    $imagick->resizeImage($width, $height, Imagick::FILTER_POINT, 1.0);
}

/**
 * Takes an image, converts to a new colorspace, separates the image
 * into into individual color channels, downsamples the
 * individual channels, then recombines the image to RGB color space.
 *
 * @param int $channel_1_sample
 * @param int $channel_2_sample
 * @param int $channel_3_sample
 * @param int $colorspace Which colorspace to do the downsampling in.
 * @param string $image_path Which image to use.
 */
function eyeColourResolution(
    int $channel_1_sample,
    int $channel_2_sample,
    int $channel_3_sample,
    int $colorspace,
    string $image_path
) {
    // Create the source image and get the dimension of it.
    $imagick = new \Imagick(realpath($image_path));

    // Make the image smaller to make easier to compare channels.
    $imagick->resizeimage(
        $imagick->getImageWidth() / 2,
        $imagick->getImageHeight() / 2,
        Imagick::FILTER_LANCZOS,
        1
    );

    $width = $imagick->getImageWidth();
    $height = $imagick->getImageHeight();

    // Transform the image to the color space that we're going to separate
    // the channel in.
    $imagick->transformImageColorspace($colorspace);

    // Separate the 3 channels to individual images.
    $channel1 = clone $imagick;
    $channel1->separateImageChannel(Imagick::CHANNEL_RED);
    $channel2 = clone $imagick;
    $channel2->separateImageChannel(Imagick::CHANNEL_GREEN);
    $channel3 = clone $imagick;
    $channel3->separateImageChannel(Imagick::CHANNEL_BLUE);

    // For technical reasons, the neutral color should be black for
    // some colour spaces where 0 = no signal, and gray for color spaces
    // where 0.5 = no signal
    if ($colorspace === Imagick::COLORSPACE_RGB) {
        $neutralColor = 'black';
    }
    else {
        $neutralColor = 'gray';
    }

    // Create an empty canvas that we will use to recombine the separate images.
    $canvas = new Imagick();
    $canvas->newPseudoImage(
        $imagick->getImageWidth() * 2,
        $imagick->getImageHeight() * 2,
        "canvas:" . $neutralColor
    );

    // Make the canvas image be the correct color space to copy the individual channels
    // back correctly.
    $canvas->transformImageColorspace($colorspace);

    // Downsample the individual channels
    downSampleImage($channel1, $channel_1_sample);
    downSampleImage($channel2, $channel_2_sample);
    downSampleImage($channel3, $channel_3_sample);

    // Copy the individual channels into the canvas.
    $canvas->compositeImage($channel1, Imagick::COMPOSITE_COPYRED, 0, 0);
    $canvas->compositeImage($channel1, Imagick::COMPOSITE_COPYRED, $width, 0);

    $canvas->compositeImage($channel2, Imagick::COMPOSITE_COPYGREEN, 0, 0);
    $canvas->compositeImage($channel2, Imagick::COMPOSITE_COPYGREEN, 0, $height);

    $canvas->compositeImage($channel3, Imagick::COMPOSITE_COPYBLUE, 0, 0);
    $canvas->compositeImage($channel3, Imagick::COMPOSITE_COPYBLUE, $width, $height);

    // Convert the final image back to RGB for display
    $canvas->transformImageColorspace(Imagick::COLORSPACE_SRGB);

    // make it bigger so it's easier to see.
    $canvas->resizeimage(
        $canvas->getImageWidth() * 2,
        $canvas->getImageHeight() * 2,
        Imagick::FILTER_POINT,
        1
    );

    // Output the image.
    $canvas->setImageFormat('jpg');
    header("Content-Type: image/jpeg");
    echo $canvas->getImageBlob();
}