Background image dark or light?

Question

I'm doing a ODP->HTML conversion with PHP. I have problems with the following:

Use the style:use-window-font-color property to specify whether or not the window foreground color should be as used as the foreground color for a light background color and white for a dark background color.

(OpenDocument specification version 1.0, 15.4.4)

If i have a background image, how do i check, if this image is light or dark?

Do you have any ideas?

Thanks in advance, Levu

Answer 1

I thought this was quite an interesting problem to solve, so i hacked up a quick script to do it. following the other suggestions provided

<?php

    // TODO supply your own filenames
    $filenames = array(
        'testpics/client-bella-vi.jpg',
        'testpics/istockphoto_8577991-concept-of-business-people-crowd.jpg',
        'testpics/medium-gray.jpg');

    // loop though each file
    foreach ($filenames as $filename) {

        echo "$filename<br/>";

        $luminance = get_avg_luminance($filename,10);
        echo "AVG LUMINANCE: $luminance<br />";

        // assume a medium gray is the threshold, #acacac or RGB(172, 172, 172)
        // this equates to a luminance of 170
        if ($luminance > 170) {
            echo "Black Text<br />";
        } else {
            echo 'White Text<br />';
        }

        echo "<br />";
    }
    exit;

    // get average luminance, by sampling $num_samples times in both x,y directions
    function get_avg_luminance($filename, $num_samples=10) {
        $img = imagecreatefromjpeg($filename);

        $width = imagesx($img);
        $height = imagesy($img);

        $x_step = intval($width/$num_samples);
        $y_step = intval($height/$num_samples);

        $total_lum = 0;

        $sample_no = 1;

        for ($x=0; $x<$width; $x+=$x_step) {
            for ($y=0; $y<$height; $y+=$y_step) {

                $rgb = imagecolorat($img, $x, $y);
                $r = ($rgb >> 16) & 0xFF;
                $g = ($rgb >> 8) & 0xFF;
                $b = $rgb & 0xFF;

                // choose a simple luminance formula from here
                // http://stackoverflow.com/questions/596216/formula-to-determine-brightness-of-rgb-color
                $lum = ($r+$r+$b+$g+$g+$g)/6;

                $total_lum += $lum;

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

        // work out the average
        $avg_lum  = $total_lum/$sample_no;

        return $avg_lum;
    }