axeloz/Generate Image Placeholder

## function.php
<?php
function generateImagePreview(String $file):String {
	// Supported image formats
	$mimetypes = [
		'image/jpeg'		=> [
			'mime'			=> 'image/jpeg',
			'function' 		=> 'imagecreatefromjpeg',
			'extensions'	=> ['jpeg', 'jpg']
		],
		'image/png'		=> [
			'function' 		=> 'imagecreatefrompng',
			'extensions'	=> ['png']
		],
		'image/webp'		=> [
			'function' 		=> 'imagecreatefromwebp',
			'extensions'	=> ['webp']
		],
		'image/gif'		=> [
			'function' 		=> 'imagecreatefromgif',
			'extensions'	=> ['gif']
		]
	];

	// Checking that GD extension is enabled
	if (! extension_loaded('gd')) {
		throw new Exception('GD library is not installed on this instance');
	}

	// Reading EXIF data
	if ($exif = exif_read_data($file)) {
		$type = $exif['MimeType'] ?? null;
	}

	// Getting correct function to use
	$function = $mimetypes[$type]['function'] ?? null;
	if (empty($function) || ! is_callable($function) || ! function_exists($function)) {
		throw new Exception('This image format is not supported');
	}

	// Calling the function
	if (false === $gd = call_user_func($function, $file)) {
		throw new Exception('Unable to generate a GD image from source');
	}

	// Does this image needs to be rotated or flipped?
	if (! empty($exif['Orientation']) && $exif['Orientation'] !== 1) {
		// It's a rotate
		if (in_array($exif['Orientation'], [3, 6, 8])) {
			switch ($exif['Orientation']) {
				case 3:
					$angle = 180;
				break;
				case 6:
					$angle = 270;
				break;
				case 3:
					$angle = 90;
				break;
			}

			if (false === $gd = imagerotate($gd, $angle, 0)) {
				// Should we block the preview generation because this image is not rotated?
			}
		}
		// It's a flip
		else {
			switch ($exif['Orientation']) {
				case 2:
					$flip = IMG_FLIP_HORIZONTAL;
				break;
				case 4:
					$flip = IMG_FLIP_VERTICAL;
				break;
				case 5:
					$flip = IMG_FLIP_BOTH;
				break;
			}

			if (false === $gd = imageflip($gd, $flip)) {
				// Should we block the preview generation because this image is not rotated?
			}
		}
	}

	// First we generate the new image sizes
	$max_width 	= 4;
	$width		= imagesx($gd);
	$height		= imagesy($gd);
	$ratio		= $width / $height;
	$max_height	= max(round($max_width, 1) / $ratio, 1);

	// Then we create a new image with correct size and ratio
	if (false === $temp = imagecreatetruecolor($max_width, $max_height)) {
		throw new Exception('Cannot create a new image container');
	}

	// Finally we copy the original image to the new container image
	if (false === imagecopyresampled($temp, $gd, 0, 0, 0, 0, $max_width, $max_height, $width, $height)) {
		throw new Exception('Cannot resample image');
	}

	// Adding blur and smoothing
	imagefilter($temp, IMG_FILTER_GAUSSIAN_BLUR);
	imagefilter($temp, IMG_FILTER_SMOOTH, IMG_FILTER_SMOOTH);

	// Outputting image to GIF format
	ob_start();
	imagegif($temp);
	$image = ob_get_contents();
	ob_end_clean();

	return base64_encode($image);
}

## Generate Image Placeholder
<html>

  <body>
    	<img
           width="300"
           src="data:image/gif;base64,<?php echo generateImagePreview('./image/myimage.png'); ?>"
      />

      <!--
      THIS IS THE ACTUAL SIZE OF THE BASE64 USED TO DISPLAY THE IMAGE. SHORT ENOUGHT TO BE STORED INTO A DATABASE FIELD
      <img
           width="300"
           src="data:image/gif;base64,R0lGODdhBAACAKIAAJSanJyipKSenJSepJyWlKSipJSapKSmpCwAAAAABAACAAADBih1QTYgAQA7"
      />
      -->
  </body>

</html>
	<?php
	function generateImagePreview(String $file):String {
	// Supported image formats
	$mimetypes = [
	'image/jpeg' => [
	'mime' => 'image/jpeg',
	'function' => 'imagecreatefromjpeg',
	'extensions' => ['jpeg', 'jpg']
	],
	'image/png' => [
	'function' => 'imagecreatefrompng',
	'extensions' => ['png']
	],
	'image/webp' => [
	'function' => 'imagecreatefromwebp',
	'extensions' => ['webp']
	],
	'image/gif' => [
	'function' => 'imagecreatefromgif',
	'extensions' => ['gif']
	]
	];

	// Checking that GD extension is enabled
	if (! extension_loaded('gd')) {
	throw new Exception('GD library is not installed on this instance');
	}

	// Reading EXIF data
	if ($exif = exif_read_data($file)) {
	$type = $exif['MimeType'] ?? null;
	}

	// Getting correct function to use
	$function = $mimetypes[$type]['function'] ?? null;
	if (empty($function) \|\| ! is_callable($function) \|\| ! function_exists($function)) {
	throw new Exception('This image format is not supported');
	}

	// Calling the function
	if (false === $gd = call_user_func($function, $file)) {
	throw new Exception('Unable to generate a GD image from source');
	}

	// Does this image needs to be rotated or flipped?
	if (! empty($exif['Orientation']) && $exif['Orientation'] !== 1) {
	// It's a rotate
	if (in_array($exif['Orientation'], [3, 6, 8])) {
	switch ($exif['Orientation']) {
	case 3:
	$angle = 180;
	break;
	case 6:
	$angle = 270;
	break;
	case 3:
	$angle = 90;
	break;
	}

	if (false === $gd = imagerotate($gd, $angle, 0)) {
	// Should we block the preview generation because this image is not rotated?
	}
	}
	// It's a flip
	else {
	switch ($exif['Orientation']) {
	case 2:
	$flip = IMG_FLIP_HORIZONTAL;
	break;
	case 4:
	$flip = IMG_FLIP_VERTICAL;
	break;
	case 5:
	$flip = IMG_FLIP_BOTH;
	break;
	}

	if (false === $gd = imageflip($gd, $flip)) {
	// Should we block the preview generation because this image is not rotated?
	}
	}
	}

	// First we generate the new image sizes
	$max_width = 4;
	$width = imagesx($gd);
	$height = imagesy($gd);
	$ratio = $width / $height;
	$max_height = max(round($max_width, 1) / $ratio, 1);

	// Then we create a new image with correct size and ratio
	if (false === $temp = imagecreatetruecolor($max_width, $max_height)) {
	throw new Exception('Cannot create a new image container');
	}

	// Finally we copy the original image to the new container image
	if (false === imagecopyresampled($temp, $gd, 0, 0, 0, 0, $max_width, $max_height, $width, $height)) {
	throw new Exception('Cannot resample image');
	}

	// Adding blur and smoothing
	imagefilter($temp, IMG_FILTER_GAUSSIAN_BLUR);
	imagefilter($temp, IMG_FILTER_SMOOTH, IMG_FILTER_SMOOTH);

	// Outputting image to GIF format
	ob_start();
	imagegif($temp);
	$image = ob_get_contents();
	ob_end_clean();

	return base64_encode($image);
	}
	<html>

	<body>
	<img
	width="300"
	src="data:image/gif;base64,<?php echo generateImagePreview('./image/myimage.png'); ?>"
	/>

	<!--
	THIS IS THE ACTUAL SIZE OF THE BASE64 USED TO DISPLAY THE IMAGE. SHORT ENOUGHT TO BE STORED INTO A DATABASE FIELD
	<img
	width="300"
	src="data:image/gif;base64,R0lGODdhBAACAKIAAJSanJyipKSenJSepJyWlKSipJSapKSmpCwAAAAABAACAAADBih1QTYgAQA7"
	/>
	-->
	</body>

	</html>