check if image is NOT fully black

<?php
/*
check if image is NOT fully black -- start
*/
$source_file = $filename;
$im = ImageCreateFromJpeg($source_file);
$imgw = imagesx($im);
$imgh = imagesy($im);
$r = array();
$g = array();
$b = array();
$c = 0;
for ($i=0; $i<$imgw; $i++)
{
        for ($j=0; $j<$imgh; $j++)
        {
                // get the rgb value for current pixel
                $rgb = ImageColorAt($im, $i, $j);
                // extract each value for r, g, b
                $r[$i][$j] = ($rgb >> 16) & 0xFF;
                $g[$i][$j] = ($rgb >> 8) & 0xFF;
                $b[$i][$j] = $rgb & 0xFF;
                // count gray pixels (r=g=b)
                if ($r[$i][$j] == $g[$i][$j] && $r[$i][$j] == $b[$i][$j])
                {
                        $c++;
                }

        }
}
if ($c == ($imgw*$imgh))
{
        echo "The image is grayscale.";
}
else
{
        echo "The image is NOT grayscale.";
}
/*
check if image is NOT fully black -- end
*/