Assessing Grey Level of an Image with PHP

<?php


/**
 * Get gray level of some position of an image
 */
function get_gray_rgb($image, $x, $y) {
	$rgb  = imagecolorat($image, $x, $y);
	//level of red, green and blue
	$red    = ($rgb >> 16) & 0xFF;
	$green    = ($rgb >> 8 ) & 0xFF;
	$blue    = $rgb & 0xFF;

	//formula to calculate gray level
	$gray = round(.299*$red + .587*$green + .114*$blue);
	return $gray;
}

/**
 * Calculate the average gray level of an area in an image
 */
function get_avg_gray($image, $based_x, $based_y, $area_width, $area_height, $base_is_end=false) {
	//	whether the based position is the beginning or the end
	if ($base_is_end) {
		// when the based position is the end
		$ending_x    = $based_x;
		$ending_y    = $based_y;
		// re-calculate the beginning point
		$based_x = $based_x - $area_width; $based_y = $based_y - $area_height;
	} else {
		$ending_x    = $based_x+$area_width;
		$ending_y    = $based_y+$area_height;
	}
	/*
	 * Sum up all gray levels/the number of pixels
	 * */
	$total_gray_level = 0;
	for ($i = $based_x; $i <= $ending_x; $i++) {
		for ($j = $based_y; $j <= $ending_y; $j++) {
			$total_gray_level += digital_crew_theme_get_gray_rgb($image, $i, $j);
		}
	}

	return round($total_gray_level/(($area_width+1)*($area_height+1)));
}