Example

function setIteratorRow($image_path)
{
    $imagick = new \Imagick(realpath($image_path));
    $imageIterator = $imagick->getPixelRegionIterator(200, 100, 200, 200);

    for ($x = 0; $x < 20; $x++) {
        $imageIterator->setIteratorRow($x * 5);
        $pixels = $imageIterator->getCurrentIteratorRow();
        /* Loop through the pixels in the row (columns) */
        foreach ($pixels as $pixel) {
            /** @var $pixel \ImagickPixel */
            /* Paint every second pixel black*/
            $pixel->setColor("rgba(0, 0, 0, 0)");
        }

        /* Sync the iterator, this is important to do on each iteration */
        $imageIterator->syncIterator();
    }

    header("Content-Type: image/jpeg");
    echo $imagick;
}