Transform an 24 bits color depth image file into 16 bits color depth raw file and back again

color depth 24 bits to 16 bits.php

<?php

// https://ee-programming-notepad.blogspot.com/2016/09/php-script-to-transform-24-bits-color.html
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));
}