PHP GD: How to get imagedata as binary string?

Question

I'm using a solution for assembling image files to a zip and streaming it to browser/Flex application. (ZipStream by Paul Duncan, http://pablotron.org/software/zipstream-php/).

Just loading the image files and compressing them works fine. Here's the core for compressing a file:

// Reading the file and converting to string data
$stringdata = file_get_contents($imagefile);

// Compressing the string data
$zdata = gzdeflate($stringdata );

My problem is that I want to process the image using GD before compressing it. Therefore I need a solution for converting the image data (imagecreatefrompng) to string data format:

// Reading the file as GD image data
$imagedata = imagecreatefrompng($imagefile);
// Do some GD processing: Adding watermarks etc. No problem here...

// HOW TO DO THIS??? 
// convert the $imagedata to $stringdata - PROBLEM!

// Compressing the string data
$zdata = gzdeflate($stringdata );

Any clues?

Answer 1

One way is to tell GD to output the image, then use PHP buffering to capture it to a string:

$imagedata = imagecreatefrompng($imagefile);
ob_start();
imagepng($imagedata);
$stringdata = ob_get_contents(); // read from buffer
ob_end_clean(); // delete buffer
$zdata = gzdeflate($stringdata);

Answer 2

// ob_clean(); // optional
ob_start();
imagepng($imagedata);
$image = ob_get_clean();