Iterate over the pixels in the top left of the image, changing them to be black.
<?php
$im = new Imagick(realpath("./testImage.png"));
$areaIterator = $im->getPixelRegionIterator(0, 0, 10, 10);
foreach ($areaIterator as $rowIterator) {
foreach ($rowIterator as $pixel) {
// Paint every pixel black
$pixel->setColor("rgba(0, 0, 0, 0)");
}
$areaIterator->syncIterator();
}
$im->writeImage("./output.png");
?>