Example

function exportImagePixels($image_path)
{
    $imagick = new \Imagick(realpath($image_path));
    for ($y=0; $y< $imagick->getImageHeight(); $y++) {
        $redPixels = $imagick->exportImagePixels(0, $y, $imagick->getImageWidth(), 1, "R", \Imagick::PIXEL_CHAR);
        
        $average = array_sum($redPixels) / count($redPixels);

        
        //Make the pixels that are redder than average be 100% red.
        foreach ($redPixels as &$redPixel) {
            if ($redPixel > $average) {
                $redPixel = 255;
            }
        }
        
        $imagick->importImagePixels(0, $y, $imagick->getImageWidth(), 1, "R", \Imagick::PIXEL_CHAR, $redPixels);
    }
    header("Content-Type: image/jpeg");
    echo $imagick->getImageBlob();
}