Right way of watermarking & storing & displaying images in PHP

Question

I'm building a web based system, which will host loads and loads of highres images, and they will be available for sale. Of course I will never display the highres image, instead when browsing people will only see a low resolution, watermarked image. Currently the workflow is as follows:

PHP script handles the highres image upload, when image is uploaded, it's automatically re-sized to a low res image and to a thumbnail image as well and both of the files are saved on the server, (no watermark is added).

When people are browsing, the page displays the thumbnail of the image, on click, it enlarges and displays the lowres image with watermark as well. At the time being I apply the watermark on the fly whenever the lowres image is opened.

My question is, what is the correct way:

1) Should I save a 2nd copy of the lowres image with thumbnail, only when it's access for the first time? I mean if somebody access the image, I add the watermark on the fly, then display the image & store it on the server. Next time the same image is accessed if a watermarked copy exist just display the wm copy, otherwise apply watermark on the fly. (in case watermark.png is changed, just delete the watermarked images and they will be recreated as accessed).

2) Should I keep applying watermarks on the fly like I'm doing now.

My biggest question is how big is the difference between a PHP file_exists(), and adding a watermark to an image, something like:

$image = new Imagick();
$image->readImage($workfolder.$event . DIRECTORY_SEPARATOR . $cat . DIRECTORY_SEPARATOR .$mit);
$watermark = new Imagick();
$watermark->readImage($workfolder.$event . DIRECTORY_SEPARATOR . "hires" . DIRECTORY_SEPARATOR ."WATERMARK.PNG");
$image->compositeImage($watermark, imagick::COMPOSITE_OVER, 0, 0);

All lowres images are 1024x1024, JPG with a quality setting of 45%, and all unnecessary filters removed, so the file size of a lowres image is about 40Kb-80Kb.

It is somehow related to this question, just the scale and the scenarios is a bit different.

I'm on a dedicated server (Xeon E3-1245v2) cpu, 32 GB ram, 2 TB storage), the site does not have a big traffic overall, but it has HUGE spikes from time to time. When images are released we get a few thousand hits per hours with people browsing trough the images, downloading, purchasing, etc. So while on normal usage I'm sure that generating on the fly is the right approach, I'm a bit worried about the spike period.

Need to mention that I'm using ImageMagick library for image processing, not GD.

Thanks for your input.

UPDATE

None of the answers where a full complete solution, but that is good since I never looked for that. It was a hard decision which one to accept and whom to accord the bounty.

@Ambroise-Maupate solution is good, but yet it's relay on the PHP to do the job.

@Hugo Delsing propose to use the web server for serving cached files, lowering the calls to PHP script, which will mean less resources used, on the other hand it's not really storage friendly.

I will use a mixed-merge solution of the 2 answers, relaying on a CRON job to remove the garbage.

Thanks for the directions.

Answer 1

Personally I would create a static/cookieless subdomain in a CDN kinda way to handle these kind of images. The main reasons are:

1. Images are only created once
2. Only accessed images are created
3. Once created, an image is served from cache and is a lot faster.

The first step would be to create a website on a subdomain that points to an empty folder. Use the settings for IIS/Apache or whatever to disable sessions for this new website. Also set some long caching headers on the site, because the content shouldn't change

The second step would be to create an .htaccess file containing the following.

RewriteEngine on
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^(.*)   /create.php?path=$1  [L]

This will make sure that if somebody would access an existing image, it will show the image directly without PHP interfering. Every non-existing request will be handled by the create.php script, which is the next thing you should add.

<?php
function NotFound()
{
    if (!headers_sent()) {
        $protocol = (isset($_SERVER['SERVER_PROTOCOL']) ? $_SERVER['SERVER_PROTOCOL'] : 'HTTP/1.0');
        header($protocol . ' 404 Not Found');
        echo '<h1>Not Found</h1>';
        exit;
    }
}

$p = $_GET['path'];

//has path
if (strlen($p)<=1)
    NotFound();

$clean = explode('?', $p);
$clean = explode('#', $clean[0]);
$params = explode('/', substr($clean[0], 1)); //drop first /

//I use a check for two, because I dont allow images in the root folder
//I also use the path to determine how it should look
//EG: thumb/125/90/imagecode.jpg
if (count($params)<2)
    NotFound();

$type = $params[0];

//I use the type to handle different methods. For this example I only used the full sized image
//You could use the same to handle thumbnails or cropped/watermarked
switch ($type) {
    //case "crop":if (Crop($params)) return; else break;
    //case "thumb":if (Thumb($params)) return; else break;
    case "image":if (Image($params)) return; else break;
}
NotFound();
?>
<?php
/*
Just some example to show how you could create a responds
Since you already know how to create thumbs, I'm not going into details

Array
(
    [0] => image
    [1] => imagecode.JPG
)
*/
function Image($params) {
    $tmp = explode('.', $params[1]);
    if (count($tmp)!=2)
        return false;

    $code = $tmp[0];


    //WARNING!! SQL INJECTION
    //USE PROPER DB METHODS TO GET REALPATH, THIS IS JUST EXAMPLE
    $query = "SELECT realpath FROM images WHERE Code='".$code."'";
    //exec query here to $row
    $realpath = $row['realpath'];


    $f = file_get_contents($realpath);

    if (strlen($f)<=0)
        return false;

    //create folder structure
    @mkdir($params[0]);

    //if you had more folders, continue creating the structure
    //@mkdir($params[0].'/'.$params[1]);

    //store the image, so a second request won't access this script
    file_put_contents($params[0].'/'.$params[1], $f);

    //you could directly optimize the image for web to make it even better
    //optimizeImage($params[0].'/'.$params[1]);

    //now serve the file to the browser, because even the first request needs to show the image
    $finfo = finfo_open(FILEINFO_MIME_TYPE);
    header('Content-Type: '.finfo_file($finfo, $params[0].'/'.$params[1]));

    echo $f;

    return true;
}
?>

Answer 2

I would suggest you to create watermarked images on-the-fly and to cache them at the same time as everybody suggested.

Then you could create a garbage-collector PHP script that will be executed every days (using cron). This script will browse your cache folder to read every image access time. This can done using fileatime() PHP method. Then when a cached wm image has not been accessed within 24 or 48 hours, just delete it.

With this method, you can handle spike periods as images are cached at the first request. AND you will save your HDD space as your garbage-collector script will delete unused images for you.

This method will only work if your server partition has atime updates enabled.

See http://php.net/manual/en/function.fileatime.php