Histogram of an image using PHP, from http://bubble.ro/How_to_create_the_histogram_of_an_image_using_PHP.html

histogram.php

<?php 
$source_file = "test_image.jpg";

// histogram options

$maxheight = 300;
$barwidth = 2;

$im = ImageCreateFromJpeg($source_file); 

$imgw = imagesx($im);
$imgh = imagesy($im);

// n = total number or pixels

$n = $imgw*$imgh;

$histo = array();

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
                
                $r = ($rgb >> 16) & 0xFF;
                $g = ($rgb >> 8) & 0xFF;
                $b = $rgb & 0xFF;
                
                // get the Value from the RGB value
                
                $V = round(($r + $g + $b) / 3);
                
                // add the point to the histogram
                
                $histo[$V] += $V / $n;
        
        }
}

// find the maximum in the histogram in order to display a normated graph

$max = 0;
for ($i=0; $i<255; $i++)
{
        if ($histo[$i] > $max)
        {
                $max = $histo[$i];
        }
}

echo "<div style='width: ".(256*$barwidth)."px; border: 1px solid'>";
for ($i=0; $i<255; $i++)
{
        $val += $histo[$i];
        
        $h = ( $histo[$i]/$max )*$maxheight;

        echo "<img src=\"img.gif\" width=\"".$barwidth."\"
height=\"".$h."\" border=\"0\">";
}
echo "</div>";
?>