Script to determine the perceived luminosity of an image. The darker the image the lower the number returned, the lighter the image the higher the number.

image.php

<?php

    # Function to determine the average luminance of an image
    function image_avg_luminance($filename, $num_samples=10, $section="all")
    {

        # Variables required
        $img        = imagecreatefromjpeg($filename);
        $width      = imagesx($img);
        $height     = imagesy($img);
        $x_step     = intval($width/$num_samples);
        $y_step     = intval($height/$num_samples);
        $x_start    = 0;
        $y_start    = 0;
        $total_lum  = 0;
        $sample_no  = 1;
        $section    = strtolower(trim($section));

        # We need to be able to section the image out.
        switch($section)
        {
            # We need to be able to section the top of the image alone
            case "top":
                $height     = ceil($height / 2);
                break;

            # We also need to be able to section the bottom
            case "bottom":
                $y_start    = ceil($height / 2);
                break;

            # We need to be able to section the top of the image alone
            case "left":
                $width      = ceil($width / 2);
                break;

            # We also need to be able to section the bottom
            case "right":
                $y_start    = ceil($width / 2);
                break;

            # If there is not section, or it's all sections we will not alter the width or height
            case "all":
            default:
                break;
        }


        # Loop through the x axis
        for ($x=$x_start; $x<$width; $x+=$x_step)
        {
            # Loop through the y axis
            for ($y=$y_start; $y<$height; $y+=$y_step)
            {
                $rgb    = imagecolorat($img, $x, $y);
                $r      = ($rgb >> 16) & 0xFF;
                $g      = ($rgb >> 8) & 0xFF;
                $b      = $rgb & 0xFF;

                # Luminance formula
                // http://stackoverflow.com/questions/596216/formula-to-determine-brightness-of-rgb-color
                $lum    = ($r+$r+$b+$g+$g+$g)/6;

                $total_lum += $lum;

                # Debugging
                // echo "$sample_no - XY: $x,$y = $r, $g, $b = $lum<br />";
                $sample_no++;
            }
        }


        # Determine the average
        $avg_lum  = ceil($total_lum / $sample_no);
        return $avg_lum;
    }
?>

index.php

<?php 

include("image.php");

# Get the path and then open the directory
$path   = 'uploads/large/';
$dir    = opendir($path);

# Loop through the contents of the directory
while ($dir AND ($file = readdir($dir)) !== false)
{
    # Check for the . and .. directives
    if (($file == ".") OR ($file == "..")) continue;

    echo '<div>
            <h1>TOP: ' . image_avg_luminance($path . $file, 10, "top") . ', BTM: ' . image_avg_luminance($path . $file, 10, "bottom") . '</h1>
            <img src="' . $path . $file . '" width="350" />
        </div>';

    //var_dump($file);
    //echo "<hr />";
}


?>