lum.php

<?php

function resizeImage($image, $origWidth, $origHeight, $maxWidth, $maxHeight)
{
    if ($maxWidth === 0) {
        $maxWidth = $origWidth;
    }

    if ($maxHeight === 0) {
        $maxHeight = $origHeight;
    }

    $widthRatio = $maxWidth / $origWidth;
    $heightRatio = $maxHeight / $origHeight;

    $ratio = min($widthRatio, $heightRatio);

    $newWidth = (int)$origWidth * $ratio;
    $newHeight = (int)$origHeight * $ratio;

    $newImage = imagecreatetruecolor($newWidth, $newHeight);
    imagecopyresampled($newImage, $image, 0, 0, 0, 0, $newWidth, $newHeight, $origWidth, $origHeight);


    return [
        'image' => $newImage,
        'width' => $newWidth,
        'height' => $newHeight,
    ];
}

// http://stackoverflow.com/questions/596216/formula-to-determine-brightness-of-rgb-color

function getLum($filename)
{
    try {

        list($width, $height, $type) = getimagesize($filename);

        switch ($type) {
            case IMAGETYPE_PNG:
                $img = imagecreatefrompng($filename);
                break;
            default:
                $img = imagecreatefromjpeg($filename);
                break;
        }

        $height /= 2;

        $area = ["x" => 0, "y" => $height, "width" => $width, "height" => $height];
        $img = imagecrop($img, $area);

        $result = resizeImage($img, $width, $height, 500, 500);
        $img = $result['image'];
        $width = $result['width'];
        $height = $result['height'];

        $totalLum = 0;
        $sampleNo = 1;

        for ($x = 0; $x < $width; $x += 2) {
            for ($y = 0; $y < $height; $y += 2) {
                $rgb = imagecolorat($img, $x, $y);
                $r = ($rgb >> 16) & 0xFF;
                $g = ($rgb >> 8) & 0xFF;
                $b = $rgb & 0xFF;

                $lum = ($r + $r + $b + $g + $g + $g) / 6;

                $totalLum += $lum;
                $sampleNo++;
            }
        }

        $avgLum = $totalLum / $sampleNo;

        $res = ($avgLum / 255) * 100;

        return [
            'lum' => $res < 30 ? 0 : 1,
            'factor' => $res,
            'error' => '',
        ];

    } catch (Exception $exception) {
        return [
            'lum' => '',
            'factor' => '',
            'error' => $exception->getMessage(),
        ];
    }
}

$startTime = microtime(true);

print_r(getLum('./big.jpeg')) . PHP_EOL;

echo microtime(true) - $startTime . PHP_EOL;