color depth transform from 24 bits to 16 bits

<?php

error_reporting(E_ALL);
ini_set('display_errors', 'On');
header('Content-type: text/html; charset=utf-8');
set_time_limit(0);

// DESCRIPTION:
// these functions will transform an 24 bits color depth image file into 16 bits color depth raw file and back again

// USEFULNESS:
// to be used on the ILI9340 IC based LCD screens (2.2" SPI TFT, 240x320)

// NOTES:
// - characters representation on 16 bits are made in 2 chars

define('IMAGE_WIDTH', 240);
define('IMAGE_HEIGHT', 320);

function jpeg_image_to_raw_file($filename)
{
    $im = imagecreatefromjpeg($filename);

    $output = '';

    for ($y = 0; $y < IMAGE_HEIGHT; $y++)
    {
        for ($x = 0; $x < IMAGE_WIDTH; $x++)
        {
            $rgb = imagecolorat($im, $x, $y);

            // split into color components (RGB)
            $r = ($rgb >> 16) & 0xFF;
            $g = ($rgb >> 8) & 0xFF;
            $b = $rgb & 0xFF;

            if (1) {
                // transform into procent of 8-8-8 bits
                $r_procent = round(($r * 100) / 255);
                $g_procent = round(($g * 100) / 255);
                $b_procent = round(($b * 100) / 255);

                // transform into procent of 5-6-5 bits
                $r_5bits = round(($r_procent * 31) / 100);
                $g_6bits = round(($g_procent * 63) / 100);
                $b_5bits = round(($b_procent * 31) / 100);
            } else {
                // loose 3-2-3 bits of color depth
                $r_5bits = $r >> 3;
                $g_6bits = $g >> 2;
                $b_5bits = $b >> 3;
            }

            // merge all color components into one
            $rgb_16bits = ($r_5bits << 11) ^ ($g_6bits << 5) ^ $b_5bits;

            // split into 2 chunks of 8 bits
            $char1 = ($rgb_16bits >> 8) & 0xFF;
            $char2 = $rgb_16bits & 0xFF;

            // make them ASCII char
            $rgb_16bits_char = chr($char1) . chr($char2);

            // join them
            $output .= $rgb_16bits_char;
        }
    }

    return $output;
}

function raw_file_to_jpeg_image($filename)
{
    $content = file_get_contents($filename);
    $content_len = strlen($content);
    $im = imagecreatetruecolor(IMAGE_WIDTH, IMAGE_HEIGHT);
    $increm = 1;
    $x = 0;
    $y = 0;

    for ($i = 0; $i < $content_len; $i += 2)
    {
        // get the first 2 chunks of 8 bits in reversed order and make them numbers again
        $colors = 0;
        $k = 1;
        for ($j = 0; $j < 2; $j++)
        {
            $colors ^= ord($content[$i + $j]) << ($k * 8);
            $k--;
        }

        // split into color components (RGB)
        $r_5bits_back = ($colors >> 11) & 0x1F;
        $g_6bits_back = ($colors >> 5) & 0x3F;
        $b_5bits_back = $colors & 0x1F;

        // convert from 5-6-5 bits to 8-8-8 bits
        if (1) {
            // transform into procent of 5-6-7 bits
            $r_proc = round(($r_5bits_back * 100) / 31);
            $g_proc = round(($g_6bits_back * 100) / 63);
            $b_proc = round(($b_5bits_back * 100) / 31);

            // transform into 8-8-8 bits of color from procent
            $r = round(($r_proc * 255) / 100);
            $g = round(($g_proc * 255) / 100);
            $b = round(($b_proc * 255) / 100);
        } else {
            // add some zeros
            $r = $r_5bits_back << 3;
            $g = $g_6bits_back << 2;
            $b = $b_5bits_back << 3;
        }

        // rebuild the image
        $color = imagecolorallocate($im, $r, $g, $b);
        imagesetpixel($im, $x, $y, $color);

        // lets calculate some image coordinates
        if ($increm % IMAGE_WIDTH == 0) {
            $y++;
            $x = 0;
        } else {
            $x++;
        }

        $increm++;
    }

    ob_start();
    imagejpeg($im);

    return ob_get_clean();
}

// run the script
{
    $original_image = 'nature.jpg';
    $raw_image = 'nature.raw';
    $output_image = 'nature-back.jpg';

    echo '<pre>';
    if (!file_exists($original_image)) {
        echo '<b>There is no '.$original_image.' image file</b>';
        exit;
    }

    // IMAGE TO RAW
    echo '<b>Transform to raw:</b> <br>';
    echo 'input original_image: '.$original_image.'<br>';
    echo 'input raw_image: '.$raw_image.'<br>';
    file_put_contents($raw_image, jpeg_image_to_raw_file($original_image));
    echo 'filesize raw_image: '.filesize($raw_image).'<br>';

    // RAW TO IMAGE
    echo '<br>';
    echo '<b>Transform from raw:</b> <br>';
    echo 'input raw_image: '.$raw_image.'<br>';
    echo 'input output_image: '.$output_image.'<br>';
    file_put_contents($output_image, raw_file_to_jpeg_image($raw_image));
}