--omg-optimized

foo.php

<?php

    private function predictQualityForFilesize($width, $height, $bytes)
    {
        /*
         * There's apparently some overhead in the JPEG file format that
         * scales with the square root of the file's size. Accounting for it
         * causes the bpp-to-quality curves for the different dimensions to
         * line up better.
         *
         * I still don't know what it actually is.
         */
        $bits = $bytes*8;
        $adjustedBits = max($bits - 78.4*sqrt($bits), 0);
        $pixels = $width*$height;
        $adjustedBpp = $adjustedBits/$pixels;
        // Generalized logistic curve, fitted to the median bpp/quality curves
        // of 450 creatives (50/dimension) randomly sampled from production
        // and compressed to all 100 different quality settings
        list($lower, $upper) = [1, 100];
        $quality = $lower +
            ($upper - $lower) / pow(1 - 18.0*exp(-1.44*$adjustedBpp - 3.85), -6.31);
        /*
         * Err on the side of suggesting too much compression.
         *
         * This is because we want to minimize the number of times an image is
         * compressed: if we suggested too *little* compression, then STW would
         * send us an image that was too big, and we would have to compress it
         * again.
         *
         * Extra compressions are worth avoiding because each compression adds
         * noise to the image. In fact, a single compression to a lower quality
         * (e.g., 100% -> 65%) can look *better* than a double compression to
         * a higher quality (100% -> 75% -> 70%).
         */
        $quality -= 5;
        // Clean up the result
        $quality = (int) round($quality);
        $quality = min($quality, 95); // In case the math above fails: don't suggest a
        $quality = max(40, $quality); // quality setting that's too wasteful or too stingy
        return $quality;
    }