Drupal 7 snippet to determine if an image is mostly dark.

HOOK_is_dark_image.php

<?php
/**
 * Determine if an image is mostly dark.
 *
 * @param string $uri The uri of the image file.
 * @param array $options
 *   - resolution int The larger the number the more measurements will be taken
 *     and the longer it will take. Defaults to 10.
 *   - threshold int The luminance threshold.  170 is equal to #acacac.
 * @param array $info Pass an array to be filled with data used to calculate
 *   the answer.  The returned keys are:
 *   - r
 *   - g
 *   - b
 *   - total_luminance
 *   - average_luminance
 *
 * @return bool
 *
 * @see http://stackoverflow.com/questions/5842440/background-image-dark-or-light
 */
function MODULE_NAME_is_dark_image($uri, $options = array(), Array &$info = NULL) { 
  if (!array_key_exists('gd', image_get_available_toolkits())) {
    watchdog('MODULE_NAME', '%func has only been testing with the gd toolkit.  Image dark/light detection is not available.', array('%func' => __FUNCTION__), WATCHDOG_ERROR);
    return NULL;
  }

  $images = &drupal_static(__FUNCTION__, array());
  if (!array_key_exists($uri, $images)) {
    $options += array(
      'resolution' => 10,
      'threshold' => 170,
    );

    $image                   = image_load($uri, 'gd');
    $width                   = $image->info['width'];
    $height                  = $image->info['height'];
    
    $x_step                  = intval($width / $options['resolution']);
    $y_step                  = intval($height / $options['resolution']);
    
    $info['total_luminance'] = 0;
    $sample_no               = 1;

    for ($x = 0; $x < $width; $x += $x_step) {
      for ($y = 0; $y < $height; $y += $y_step) {
        $rgb = imagecolorat($image->resource, $x, $y);
        $info['r'] = ($rgb >> 16) & 0xFF;
        $info['g'] = ($rgb >> 8) & 0xFF;
        $info['b'] = $rgb & 0xFF;
        // choose a simple luminance formula from here
        // http://stackoverflow.com/questions/596216/formula-to-determine-brightness-of-rgb-color
        $lum = ($info['r'] + $info['r'] + $info['b'] + $info['g'] + $info['g'] + $info['g']) / 6;
        $info['total_luminance'] += $lum;
        $sample_no++;
      }
    }

    // work out the average
    $info['average_luminance']  = $info['total_luminance'] / $sample_no;
    $info['is_dark'] = $info['average_luminance'] < $options['threshold'];

    $images[$uri] = $info;
  }

  return $images[$uri]['is_dark'];
}