Example

function clear($image_path)
{
    $imagick = new \Imagick(realpath($image_path));
    $imageIterator = $imagick->getPixelRegionIterator(100, 100, 250, 200);

    /* Loop through pixel rows */
    foreach ($imageIterator as $pixels) {
        /** @var $pixel \ImagickPixel */
        /* Loop through the pixels in the row (columns) */
        foreach ($pixels as $column => $pixel) {
            if ($column % 2) {
                /* 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();
    }

    $imageIterator->clear();

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